ITN100 Exam 2 End of Chapter Questions

28. Describe the anatomy of a router. How does a router differ from a computer?

A router consists of a CPU, memory, and ports or interfaces. A router differs from a computer because they are disk-less and they don't come with a monitor, keyboard or mouse.

30. Many of the wired and wireless LANs share the same or similar components (e.g., error control). Why?

Wired and wireless LANs share the same or similar components because they are both generally based on the same Ethernet protocol. Thus, although some hardware components are different, the underlying foundation is the similar.

5. What types of cables are commonly used in wired LANs?

It is very common to see LANs built using traditional twisted pair cables (e.g., Cat 5, Cat 5e).

15. Compare and contrast the two types of antennas.

Media access control uses Carrier Sense Multiple Access with Collision Avoidance, or CSMA/CA, which is similar to the media access control used in Ethernet LANs. The computers "listen" before they transmit, and if there is not a collision, all is well. Wi-Fi does attempt to avoid a collision more than regular Ethernet LANs do, however, by using two techniques called Distributed Coordination Function and Point Coordination Function (refer to questions 12 and 13 for detailed descriptions of these two access control methods).

20. Some experts are predicting that Ethernet will move into the WAN. What do you think?

The new Ethernet/IP packet networks have become dominant for high-traffic networks (2 Mbps to 1Gbps), even though SONET and ATM remain preferred for some requirements. Since WAN required a network with high network capacity, I believe that Ethernet will move into the WAN into the near future.

1. Define local area network.

A local area network is a group of microcomputers or other workstation devices located within a small or confined area and are connected by a common cable. A LAN can be part of a larger backbone network connected to other LANs, a host mainframe, or public networks.

10. How does a logical topology differ from a physical topology?

A logical topology illustrates how the network operates with the various protocols that may be running. A single network can have multiple protocols. A physical topology illustrates exactly where all the hardware and cabling are 'physically' located and connected.

2. Describe at least three types of servers.

A LAN can have many different types of dedicated servers. Four common types are file servers, database servers, print servers, and communication servers. File servers allow many users to share the same set of files on a common, shared disk drive. A database server usually is more powerful than a file server. It not only provides shared access to the files on the server, but also can perform database processing on those files associated with client-server computing. The key benefit of database servers is that they reduce the amount of data moved between the server and the client workstation. They can also minimize data loss and prevent widespread data inconsistencies if the system fails. Print servers handle print requests on the LAN. By offloading the management of printing from the main LAN file server or database server, print servers help reduce the load on them and increase network efficiency in much the same way that front end processors improve the efficiency of mainframe computers. Communications servers are dedicated to performing communication processing. There are three fundamental types: fax servers, modem servers, and access servers. Fax servers manage a pool of fax-boards that enable LAN users to send or receive faxes. Access servers and modem servers allow users to dial into and out of the LAN by telephone. Dialing into the LAN is accomplished with an access server, whereas dialing out is accomplished with a modem server.

8. Distinguish among T-1, T-2, T-3, and T-4 circuits.

A T-1 circuit (sometimes called a DS-1 circuit) provides a data rate of 1.544 Mbps. T-1 circuits can be used to transmit data, but often are used to transmit both data and voice. In this case, a time division multiplexer (TDM) provides 24 64 Kbps circuits. Digitized voice using pulse code modulation (PCM) requires a 64 Kbps circuit so a T-1 circuit enables 24 simultaneous voice channels. A T-2 circuit transmits data at a rate of 6.312 Mbps. Basically, it is a multiplexed bundle of four T-1 circuits. A T-3 circuit allows transmission at a rate of 44.376 Mbps, although most articles refer to this rate as 45 megabits per second. This is equal to the capacity of 28 T-1 circuits. T-3 circuits are becoming popular as the transmission medium for corporate WANs because of their higher data rates. At low speed, these T-3 circuits can be used as 672 different 64 Kbps channels or voice channels. A T-4 circuit transmits at 274.176 Mbps, which is equal to the capacity of 178 T-1 circuits. Obviously, an organization using either T-3 or T-4 circuits must have a tremendous need to transmit very large quantities of data.

6. Under what circumstances would you use a VLAN backbone?

A VLAN backbone is useful when you want to put computers that are in different geographic locations in the same subnet. In addition, VLANs make it much simpler to manage the broadcast traffic and provide a better opportunity to prioritize traffic on the network.

5. Is a WAN that uses dedicated circuits easier or harder to design than one that uses packet-switched circuits? Explain.

A WAN using dedicated circuits is harder to design than one that uses packet-switched circuits. With dedicated circuits, once you sign a contract, making changes can be expensive because it means rewiring the buildings and signing a new contract with the carrier. Dedicated circuits therefore require more care in network design than using packet-switched circuits. In addition, packet-switched networks enable packets from separate messages with different destinations to be interleaved for transmission, unlike dedicated circuits.

7. What is a cable plan and why would you want one?

A cable plan is a plan for the network layout, including how much cable is used, where the cables are, how many and where hubs are located, how many ports are available, what local city fire codes must be followed, and what are the identification labels of the cable. Most buildings under construction today have a separate LAN cable plan as they do for telephone cables and electrical cables. The same is true for older buildings in which new LAN cabling is being installed. It is common to install 20 to 50 percent more cable than you actually need to make future expansion simple. With today's explosion in LAN use, it is critical to plan for the effective installation and use of LAN cabling. The cheapest time to install network is during the construction of the building; adding cable to an existing building can cost significantly more. Indeed, the costs to install cable (i.e., paying those doing the installation and additional construction) are usually substantially more than the cost of the cable itself, making it expensive to re-install the cable if the cable plan does not meet the organization's needs.

1. What are common carriers, local exchange carriers, and interexchange carriers?

A common carrier is a private company that sells or leases communication services and facilities to the public. Common carriers are profit-oriented, and their primary products are services for voice and data transmissions, both over traditional wired circuits as well as cellular services. Common carriers often supply a broad range of computer-based services, such as the manufacturing and marketing of specialized communication hardware and software. Common carriers that provide local telephone services are commonly called local exchange carriers (LEC), while carriers that provide long distance services (e.g., Sprint) are commonly called interexchange carriers (IXC). As the LECs move into the long distance market and IXCs move into the local telephone market, this distinction may disappear.

12. What is a module and why are modules important?

A module is any of certain types of network devices that can be plugged directly into a chassis switch. Since a chassis switch must be able to support simultaneous activities of all connected module, each switch has an internal capacity (in Mbps) which limits the maximum number of modules that can be accepted by the switch. Modules can be switches, hubs, or routers.

14. Explain how multiswitch VLANs work.

A multi-switch VLAN works the same way as a single switch VLAN, except that several switches are used to build the VLANs. The switches must be able to send packets among themselves in a way that identifies the VLAN to which the packet belongs. There are two approaches to this: packet encapsulation and modifying the Ethernet packet. • In the encapaulation approach a proprietary protocol encapsulates the packet. When a packet needs to go from one VLAN switch to another VLAN switch, the first switch puts a new VLAN packet around the outside of the Ethernet packet. The VLAN packet contains the VLAN information and is used to move the packet from switch to switch within the VLAN network.. When the packet arrives at the final destination switch, the VLAN packet is stripped off and the unchanged Ethernet packet inside is sent to the destination computer. • In the modification approach the Ethernet packet itself is to modified to carry the VLAN information. 16-bytes of VLAN information (according to emerging standard IEEE 802.1q) are added to the standard Ethernet (IEEE 802.3) packet. The additional VLAN information is used to move the packet from switch to switch within the VLAN network. The original Ethernet packet is restored from the modified packet at the final destination switch and then sent to the destination computer.

6. Compare and contrast ring architecture, star architecture, and mesh architecture.

A ring architecture connects all computers in a closed loop, with each computer linked to the next. The circuits are full duplex circuits. Computers in the ring may send data in one direction or the other depending upon which direction is the shortest to the destination. Properties of the ring architecture (assuming a double ring structure): • Messages can take a long time to travel from the sender to the receiver. Considering there are only two routes from any one computer to another, if one part of the circuit or any one computer becomes overloaded, traffic delays can build up very quickly. • The double-ring structure offers redundancy. If a circuit fails, messages can travel to their destinations in the opposite direction (probably with a time and distance penalty). If the network is operating close to its capacity, this will dramatically increase transmission times because the traffic on the remaining part of the network may come close to doubling (because all traffic originally routed in the direction of the failed link will now be routed in the opposite direction through the longest way around the ring). A star architecture connects all computers to one central computer that routes messages to the appropriate computer. Properties of the star architecture: • It is easy to manage because the central computer receives and routes all messages in the network. • It can be faster than the ring network since, in general any message needs to travel through fewer circuits to reach its destination than is the case in the ring network). However, the star topology is the most susceptible to traffic problems because the central computer must process all messages on the network. The central computer must have sufficient capacity to handle traffic peaks or it may become overloaded and network performance will suffer. • In general, the failure of any one circuit or computer affects only the one computer on that circuit. • If the central computer fails, the entire network fails because all traffic must flow through it. It is critical that the central computer be extremely reliable. In a mesh architecture (usually implemented as partial mesh architecture), every computer may be connected to any other computer. Typically many, but not all, computers are connected. Properties of the mesh architecture: • The effects of the loss of computers or circuits in a mesh network depend entirely upon the circuits available in the network. If there are many possible routes through the network, the loss of one or even several circuits or computers may have few effects beyond the specific computers involved. However, if there are only few circuits in the network, the loss of even one circuit or computer may seriously impair the network. • In general, mesh networks combine the performance benefits of both ring networks and star networks. Mesh networks usually provide relative short routes through the network (compared to ring networks) and provide many possible routes through the network to prevent any one circuit or computer from becoming overloaded when there is a lot of traffic (compared to star networks in which all traffic goes through one computer). • Since mesh networks use decentralized routing, each computer in the network performs its own routing. This requires more processing by each computer in the network than in star or ring networks as well as the "overhead" transmission of network status information (e.g., how busy each computer is), reducing network capacity.

17. What is a session?

A session can be thought of as a conversation between two computers. When the sender wants to send a message, it first establishes a session with the destination computer. The sender then sends the data packets in order until all data has been sent. Then the session is ended.

15. How do VPN services differ from common carrier services?

A type of VAN, called a virtual private network (VPN) (or sometimes software defined network) has emerged. VPNs provide circuits that run over the Internet but appear to the user to be private networks. Internet access is inexpensive compared to the cost of leasing dedicated circuits, circuit switched services, or packet switched services from a common carrier. Different VPNs provide different services, but most offer packet switching hardware that will communicate via the Internet, or VPN services which you lease from the VPN in much the same way as leasing a service from a common carrier. These VPN hardware (or services) take your data, encrypt it, and send it through the Internet through a series of "tunnels" -- a virtual circuit through the Internet which constrains the source and destination to only those within the VPN.

16. Explain how VPN services work.

A virtual private network (VPN) provides the equivalent of private packet switched network over the public Internet. You establish a series of PVCs that run over the Internet, so that the network acts like a set of dedicated circuits over a private packet network. With a VPN, you first lease an Internet connection at whatever access rate and access technology you choose for each location you want to connect. For example, you might lease a T-1 circuit from a common carrier that runs from your office to your Internet service provider (ISP). You pay the common carrier for the circuit and the ISP for Internet access. Then you connect a VPN device (a specially designed router or switch) to each Internet access circuit to provide access from your networks to the VPN. The VPN devices enable you to create PVCs through the Internet that are called tunnels. The VPN device at the sender takes the outgoing packet and encapsulates it with a protocol that is used to move it through the tunnel to the VPN device on the other side (a technology focus box later in this chapter describes this process in more detail). The VPN device at the receiver, strips off the VPN packet and delivers the packet to the destination network. The VPN is transparent to the users; it appears as though a traditional packet switched network PVC is in used. The VPN is also transparent to the ISP and the Internet as a whole; there is a simply a stream of Internet packets moving across the Internet. The primary advantages of the VPNs is low cost and flexibility. Because they use the Internet to carry messages, the major cost is Internet access, which is inexpensive compared to the cost of circuit switched services, dedicated circuit services, or packet switched services from a common carrier. Likewise, anywhere you can establish Internet service, you can quickly put a VPN. There are two important disadvantages. • Since traffic on the Internet is unpredictable, sometimes packets travel quickly and at other times they take a long time to reach their destination. • Second, because the data travels on the Internet, security is always a concern. Most VPN networks encrypt the packet at the source VPN device before it enters the Internet and decrypt the packet at the destination VPN device.

16. What are the advantages and disadvantages of VLANs?

Advantages: • VLANs are often faster and provide greater opportunities to manage the flow of traffic on the LAN and BN than do the traditional LAN and routed BN architecture. • Allow the ability to prioritize traffic • They allow computers in separate geographic locations to be placed on the same LAN. Disadvantages: • However, VLANs are significantly more complex, so they usually are used only for large networks. • Cost

19. What are the preferred architectures used in each part of the backbone?

Answers can vary depending on preferences, but suggestions include: Access - switched backbones Distribution layer - Virtual LAN Core layer - routed backbones

11. Briefly describe how CSMA/CD works.

CSMA/CD, like all contention-based techniques, is very simple in concept: wait until the bus is free (sense for carrier) and then transmit. Computers wait until no other devices are transmitting, and then transmit their data. As long as no other computer attempts to transmit at the same time, everything is fine. However, it is possible that two computers located some distance from one another can both listen to the circuit, find it empty, and begin to simultaneously. This simultaneous transmission is called a collision. The two messages collide and destroy each other. The solution to this is to listen while transmitting, better known as collision detection (CD). If the NIC detects any signal other than its own, it presumes that a collision has occurred, and sends a jamming signal. All computers stop transmitting and wait for the circuit to become free before trying to retransmit. The problem is that the computers which caused the collision could attempt to retransmit at the same time. To prevent this, each computer waits a random amount of time after the colliding message disappears before attempting to retransmit.

29. Describe three ways to improve network performance on circuits.

Circuit performance can be improved by using faster technologies, by adding more circuits, and by segmenting the network into several separate LANs by adding more switches or access points.

20. Explain how multicasting works.

Computers wishing to participate in a multicast (e.g., for videoconferencing) send a message to the sending computer or some other computer performing routing along the way using a special type of TCP-level packet called Internet Group Management Protocol (IGMP). Each multicast group is assigned a special Class D IP address to identify the group. Any computer performing routing knows to route all multicast messages with this Class D IP address onto the subnet that contains the requesting computer. The routing computer sets the data link layer address on multicast messages to a matching multicast data link layer address. Each requesting computer must inform its data link layer software to process incoming messages with this multicast data link layer address. When the multicast session ends (e.g., the videoconference is over), the client computer sends another IGMP message to the organizing computer or the computer performing routing to remove it from the multicast group.

16. What are the differences between connectionless and connection-oriented routing?

Connectionless routing means each packet is treated separately and makes its own way through the network. It is possible that different packets will take different routes through the network depending upon the type of routing used and the amount of traffic. Because packets following different routes may travel at different speeds, they may arrive out of sequence at their destination. The sender's network layer therefore puts a sequence number on each packet, in addition to information about the message stream to which the packet belongs. The network layer must reassemble them in the correct order before passing the message to the application layer. Connection-oriented routing sets up a virtual circuit between the sender and receiver. In this case, a temporary virtual circuit is defined between the sender and receiver. The network layer makes one routing decision when the connection is established, and all packets follow the same route. All packets in the same message arrive at the destination in the same order in which they were sent. In this case, packets only need to contain information about the stream to which it belongs; sequence numbers are not needed, although many connection-oriented protocols include a sequence number to ensure that all packets are actually received. Connection-oriented routing has greater overhead than connectionless routing, because the sender must first "open" the circuit by sending a control packet that instructs all the intervening devices to establish the circuit routing. Likewise, when the transmission is complete, the sender must "close" the circuit. Connection-oriented protocols also tend to have more overhead bits in each packet.

25. How does the design of the data center differ from the design of the LANs intended to provide user access to the network?

Data centers are designed to house significant number of servers because this is where most of the data on a network either comes from or goes to. Thus, the data center needs significant physical space and a significant amount of circuit capacity added to handle the data flow. The data center must also be built with other devices like load balancers and virtual servers, which the LAN does not have. Due to the physical space requirements and the large amount of data transferred, the design of the data center is different than that of a LAN for user access.

15. What is IEEE 802.1q?

IEEE 802.1q is an emerging standard that inserts 16-bytes of VLAN information into the normal IEEE 802.3 Ethernet packet. When a packet needs to go from one VLAN switch to another VLAN switch, the first switch replaces the incoming Ethernet packet with an 802.1q packet that contains all the information in the original 802.3 Ethernet packet, plus 16-bytes of VLAN information. The additional VLAN information is used to move the packet from switch to switch within the VLAN network. When the packet arrives at the final destination switch, the IEEE 802.1q packet is stripped off and replaced with a new Ethernet packet that is identical to the one with which it entered the VLAN.

13. Which likely to be the longer term winner, IP, MPLS, or Ethernet services?

Each of these technologies has benefits and an argument could be made for each. As such, student answers will vary. IP seems to be a contender because the Internet utilizes TCP/IP. Ethernet has the advantage of being used heavily on LANs, so the protocol conversion doesn't have to occur. MPLS is a relatively new option and has the benefit of being able to work so closely with Layer 2 and Layer 3 protocols.

7. What is a subnet and why do networks need them?

Each organization must assign the IP addresses it has received to specific computers on its networks. In general, IP addresses are assigned so that all computers on the same local area network have a similar addresses. For example, suppose a university has just received a set of Class B addresses starting with 128.184.x.x. It is customary to assign all the computers in the same LAN numbers that start with the same first three digits, so the Business School LAN might be assigned 128.184.56.x while the Computer Science LAN might be assigned 128.184.55.x (see Figure 6-8). Likewise, all the other LANs at the university and the backbone network that connects them, would have a different set of numbers. Each of these LANs are called a TCP/IP subnet because they are logically grouped together by IP number. Knowing whether a computer is on your subnet or not it very important for message routing.

28. Describe three ways to improve network performance on the server.

Improving server performance can be approached from two directions simultaneously: software and hardware. Software methods include changing the NOS and fine-tuning the NOS. Hardware methods include adding a second server and upgrading the server's hardware.

18. How can you improve WAN performance?

Improving the performance of WANs is handled in the same way as improving LAN performance. You begin by checking the devices in the network, by upgrading the circuits between the computers, and by changing the demand placed on the network. Below is the performance checklist for improving WANs. Increase Computer and Device Performance • Upgrade devices • Change to a more appropriate routing protocol (either static or dynamic) Increase Circuit Capacity • Analyze message traffic and upgrade to faster circuits where needed • Check error rates Reduce Network Demand • Change user behavior • Analyze network needs of all new systems • Move data closer to users

9. How does wired Ethernet work?

Ethernet is the most commonly used LAN in the world, accounting for almost 70 percent of all LANs. Ethernet uses a bus topology and a contention-based technique media access technique called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). There are many different types of Ethernet that use different network cabling (e.g., 10Base-2, 10Base-5, 10Base-T, and 10Broad-36).

13. How do Ethernet switches know where to send the frames they receive? Describe how switches gather and use this knowledge.

Ethernet switches operate on the destination MAC address of each packet processed to determine which port to pass on each packet presented for transmission. Ethernet switches learn and store in memory in the form of a forwarding table, the specific port location of each MAC address for every device connected to any of its ports.

21. Are there any WAN technologies that you would avoid if you were building a network today? Explain.

FDDI because of bandwidth issues and high hardware costs, Integration of transport favors technologies using IP addressing. At this point, the avoidance of ATM in newer networks is suggested since so much new development is using Ethernet.

3. How does MPLS work?

It is sometimes called a layer 2.5 technology because it inserts four-byte header that contains its own information between the layer 2 frame and the layer 3 IP packet. With MPLS, the customer connects to the common carrier's network using any common layer 2 service (e.g., T carrier, SONET, ATM, frame relay, Ethernet). The carrier's switch at the network entry point examines the incoming frame and converts the incoming layer 2 or layer 3 address into an MPLS address label. This label and some other control information (e.g., quality of service (QoS)) form the MPLS header, which is inserted into the layer 2 frame for transmission inside the carrier's network.

19. Describe five important factors in selecting WAN services.

Five important factors in selecting WAN services are vendor capabilities, network capacity, flexibility, control, and reliability. The best vendors provide high quality service, quickly respond to network problems, adapt to changing customer needs, and provide useful network management services along with the data transmission services. There are a variety of services available at many different data transmission rates. Try to estimate the general capacity you need at each network site, and be aware that users' needs change. In general, dedicated circuits are much less flexible than switched services. Control is another important issue. With dedicated circuits, you have more control over how you messages get routed in the network because your computers do the routing. With switched services, the service provider is responsible for the routing, and, your messages get intermixed with those of other network users. The reliability of a network service both in terms of average error rates and any circuit failures is also important.

12. Compare and contrast frame relay, MPLS, SMDS, and Ethernet services.

Frame relay differs from traditional networks in three important ways. First, frame relay operates only at the data link layer. Frame relay, like other packet switched networks takes the incoming packets from the user network and converts them to its own packet structure for internal transmission. Frame relay uses variable length packets which adapt to the size of the incoming packet (up to 8K). Second, frame relay networks do not perform error control. Virtually all other types of networks perform error checking at each computer in the network. Any errors in transmission are corrected immediately, so that the network layer and application software can assume error-free transmission. However, this error control is one of the most time consuming processes in a network. Most networks today are relatively error-free, so frame relay networks do not ensure error-free delivery of the packets (they do perform error checking, but simply discard packets with errors; they do not generate NAKs and ask for retransmission). It is up to the software at the source and destination to perform error correction and to control for lost messages. Since the user's data link packet remains intact, it is simple for the devices at the edge of the frame relay network to check the error control information in the user's data link layer packet to ensure that no errors have occurred and to request transmission of damaged or lost packets. A third major difference is that frame relay defines two connection data rates that are negotiated per connection and for each virtual circuit as it is established. The committed information rate (CIR) is the data rate the circuit must guarantee to transmit. If the network accepts the connection, it guarantees to provide that level of service. Most connections also specify a maximum allowable rate (MAR), which is the maximum rate that the frame relay network will attempt to provide, over and above the CIR. The circuit will attempt to transmit all packets up to the MAR, but all packets that exceed the CIR are marked as discard eligible (DE). If the network becomes overloaded, DE packets are discarded. So while can transmit faster than the CIR, they do so at a risk of lost packets. Switched Multimegabit Data Service (SMDS) is an unreliable packet service like ATM and frame relay. SMDS encapsulates incoming packets from the user's network with ATM-like 53-byte cells, although the address is different than an ATM address. The user's data link layer address is mapped to the SMDS address, which is used for transmission through the SMDS network. The SMDS cell is stripped off at the destination and the user's data link layer packet reassembled. Like ATM and frame relay, SMDS does not perform error checking; the user is responsible for error checking. SMDS provides only a connectionless datagram service. Ethernet service networks bypass the PSTN; companies offering Ethernet/IP packet networks have laid their own gigabit Ethernet fiber optic networks in large cities. When an organization signs up for service, the packet network company installs new fiber optic cables from their city-wide WAN backbone into the organization's office complex and connect it to an Ethernet switch. The organization simply plus their network into their Ethernet switch and begins using the service. All traffic entering the packet network must be Ethernet using IP. Since most organizations today use Ethernet and IP in the LAN and BN environment, Ethernet/IP avoids the need to translate or encapsulate to generate addresses for LAN or BN traffic and gains in throughput. It avoids complexity, meaning that companies do not have to add staff knowledgeable in the different WAN protocols, software, and hardware these technologies require. This technology is offered by relatively new startup companies like Yipes.com. MPLS is different in that it is designed to work with a variety of commonly used layer-2 protocols. The customer connects to the common carrier's network using any common layer-2 service. MPLS offers a wide range of speeds because it can run on a variety of physical circuits such as T-carrier and SONET.

22. How does HTTP use TCP and DNS use UDP?

HTTP at the application layer would pass its message packet with overhead, including the Internet address of the destination, to the transport layer where TCP software would complete packetization at the Transport layer and hand it off to the Network layer. Domain Name Services, the Domain Name Server (aka DNS) is primarily responsible for translating IP Addresses into valid Domain Names and translating valid Domain Names into IP addresses. UDP is a connection-less transport layer protocol. DNS would pass either a Domain Name or an IP address along with its packet(s) to the Transport layer for forwarding to the Network layer to be routed to the destination.

12. Explain the terms 100Base-T, 1000Base-T, 100Base-F,10GbE, and 10/100/1000 Ethernet.

Historically, the original Ethernet specification was a 10 Mbps data rate using baseband signaling on thick coaxial cable, called 10Base5 (or "Thicknet"), capable of running 500 meters between hubs. Following 10Base5 was 10Base2 or thinnet as we used to say. Thinnet or RG-58 coaxial cable, similar to what is used for cable TV was considerably cheaper and easier to work with, although it was limited to 185 meters between hubs. The 10Base-2 standard was often called "Cheapnet." When twisted pair cabling was standardized for supporting Ethernet (app. 1988) the T replaced the 2 to represent "twisted-pair". Twisted pair is the most commonly used cable type for Ethernet. 10BaseT breaks down as 10 Mbps, baseband, and the "T" means it uses twisted pair wiring (actually unshielded twisted pair). It was the 10Base-T standard that revolutionized Ethernet, and made it the most popular type of LAN in the world. Eventually the 10BaseT standard was improved to support Fast Ethernet or 100BaseT that breaks down as 100Mbps baseband over twisted-pair cable, and 100BaseF over fiber. This eventually was improved even further to 1000BaseT or 1 Billion BITs per second baseband. There is currently a revised standard evolving which makes Ethernet even faster. It is known as the 10GbE or 10 Billion BITs per second Ethernet. Though proven to work it has yet to reach the marketplace. But it would be astute to consider that it will be here in the near future. Finally, 10/100Mbps Ethernet refers to the standard that can autosense which speed it needs to run at between the two speeds of 10Mbos or 100Mbps. It comes down to the type of NIC running at the individual node and the type of switch port that the node connects into. It is commonplace to run 10/100Mbps switches in LAN operating environments where there are older NICs already operating and no real business case requirements for upgrading these nodes.

17. How can you improve the performance of a BN?

Improving the performance of backbone networks is similar to improving LAN performance. First, find the bottleneck, and then solve it (or more accurately, move the bottleneck somewhere else). You can improve the performance of the network by improving the computers and other devices in the network, by upgrading the circuits between computers, and by changing the demand placed on the network. Network performance can be improved by upgrading the computers and other devices in the network, by using static rather than dynamic routing if there are few routes through the network, by reducing switch-to-switch traffic in networks without standard protocols, by using the same protocols in the backbone network as in the attached LANs, by encapsulating rather than translating between different protocols, and by increasing the memory in backbone devices. Performance can also be improved by adding additional circuits to increase capacity, by going to a switched network, and by increasing the circuits on high traffic circuits. In addition, performance can be enhanced by reducing demand or by restricting applications that use lots of network capacity, and by using switches that filter certain broadcast messages.

13. Explain how single-switch VLANs work.

In a single switch VLAN the VLAN operates only inside one switch. The computers on the VLAN are connected into the one switch and assigned by software into different VLANs. The network manager uses special software to assign the dozens or even hundreds of computers attached to the switch to different VLAN segments. The VLAN segments function in the same way as physical LAN segments; the computers in the same VLAN act as though they are connected to the same physical switch or hub.

27. What is a bottleneck and how can you locate one?

In order to improve performance, the administrator must locate the bottleneck, the part of the network that is restricting the data flow. Generally speaking, the bottleneck will lie in one of two places. The first is the network server. In this case, the client computers have no difficulty sending requests to the network server, but the server lacks sufficient capacity to process all the requests it receives in a timely manner. The second location is the network circuit. The network server can easily process all the client requests it receives, but the network circuit lacks enough capacity to transmit all the requests to server. It is also possible that the bottleneck could also lie in the client computers themselves (e.g., they are receiving data to fast for them to process it), but this is extremely unlikely.

11. What is address resolution?

In order to send a message, the sender must be able to translate the application layer address (or server name) of the destination into a network layer address and in turn translate that into a data link layer address. This process is called address resolution. There are many different approaches to address resolution that range from completely decentralized (each computer is responsible for knowing all addresses) to completely centralized (there is one computer that knows all addresses).

20. Are Ethernet services a major change in the future of networking or a technology blip?

It is reasonable to expect major changes in the future of networking with Ethernet services. Several carriers have announced that they intend to stop offering all services except Ethernet and Internet services. With the advent of Ethernet services such as YIPES, this offers a significant approach to networking because it offers a way to "slice" fiber services down to the 1 Mbit level. Thus the customer can dynamically allocate slices and pay for what is used without the costs of infrastructure change. Further, as organizational LANs heavily use Ethernet and IP in the LAN and BN environment, and the WAN packet network services (X.25, ATM, Frame Relay, and SMDS) use layer-2 protocols, it is likely that Ethernet services would be more attractive. Any LAN or BN traffic must be translated or encapsulated into a new protocol and destination addresses generated for the new protocol. This takes time, slowing network throughput. Thus, the advantage of Ethernet services in networking is that there is no translation prior to transmission, making the service appealing. Ethernet services will represent a new and very attractive WAN technology in the future.

1. How does a layer-2 switch differ from a router?

Layer 2 switches operate by using the data link layer address or MAC address to forward packets between network segments. They connect the same or different types of cable. Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use simultaneously by managing paired combinations of ports as separate point-to-point circuits. Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from which it was received) only for a packet with a destination address not already in the forwarding table. The resulting ACK from the destination computer (that recognized its address) is then used by the layer-2 switch to add the new port number and address to the forwarding table. Routers operate at the network layer. They connect two different TCP/IP subnets. Routers strip off the data link layer packet, process the network layer packet, and forward only those messages that need to go to other networks on the basis of their network layer address. In general, they perform more processing on each message than switches and therefore operate more slowly.

2. How does a layer-2 switch differ from a VLAN?

Layer 2 switches operate by using the data link layer address or MAC address to forward packets between network segments. They connect the same or different types of cable. Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use simultaneously by managing paired combinations of ports as separate point-to-point circuits. Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from which it was received) only for a packet with a destination address not already in the forwarding table. The resulting ACK from the destination computer (that recognized its address) is then used by the layer-2 switch to add the new port number and address to the forwarding table. VLAN switches work a little differently. When a VLAN switch receives a frame that is destined for another computer in the same subnet on the same VLAN switch, the switch acts as a traditional layer-2 switch: it forwards the frame unchanged to the correct computer. VLAN switches use Ethernets 802.1q's tagging to move frames from one switch to another. When a VLAN switch receives an Ethernet frame that needs to go to a computer on another VLAN switch, it changes the Ethernet frame by inserting the VLAN ID number and a priority code into the VLAN tag field.

6. Compare and contrast category 5 UTP, category 5e UTP, and category 5 STP.

Max. Data Category Type Rate (Mbps) Often Used By Cost ($/foot) 5e UTP 100 1,000Base-T Ethernet .10 5 UTP 100 100Base-T Ethernet .07 5 STP 100 100Base-T Ethernet .18

16. How does Wi-Fi perform media access control?

Media access control uses Carrier Sense Multiple Access with Collision Avoidance, or CSMA/CA, which is similar to the media access control used in Ethernet LANs. The computers "listen" before they transmit, and if there is not a collision, all is well. Wi-Fi does attempt to avoid a collision more than regular Ethernet LANs do, however, by using two techniques called Distributed Coordination Function and Point Coordination Function (refer to questions 12 and 13 for detailed descriptions of these two access control methods).

2. Who regulates common carriers and how is it done?

Most countries have a federal government agency that regulates data and voice communications. In the United States, the agency is the Federal Communications Commission (FCC); in Canada it is the Canadian Radio-Television and Telecommunications Commission (CRTC). The FCC regulates interstate and international communications to and from the United States. State Public Utilities Commissions (PUCs) regulate intrastate communications within their states. Regulation is achieved on the basis of tariffs filed and approved (or disapproved) by the FCC and PUCs. Most countries have a federal government agency that regulates data and voice communications. In the United States, the agency is the Federal Communications Commission (FCC); in Canada it is the Canadian Radio-Television and Telecommunications Commission (CRTC). Each state or province also has its own public utilities commission (PUC) to regulate communications within its borders. The FCC/CRTC differs from the state PUCs in the following way: • The FCC has regulatory powers to compel common carriers to conform to the Federal Communications Act of 1934 and its revisions. It regulates the tariffs for interLATA and international (calls to and from the United States) communications. These usually are referred to as long distance communications. • A state PUC sets the rules and regulates the tariffs for all communications in its individual state boundary. Note to the instructor: There may be some overlap between federal and state jurisdictions because you can have both intraLATA and interLATA communications take place entirely within one state.

31. As WLANs become more powerful, what are the implications for networks of the future? Will wired LANS still be common or will we eliminate wired offices?

Networks of the future will continue to become increasingly wireless due to the increased speed and portability wireless offers. Wired LANs will continue to be common partly due to better security and reliability. The best practice networks of the future will continue to be wired networks with added wireless capabilities.

18. What is Quality of Service routing and why is it useful?

Quality of service (QoS) routing is a special type of connection-oriented dynamic routing in which different messages or packets are assigned different priorities. For example, videoconferencing requires fast delivery of packet to ensure that the images and voices appear smooth and continuous; they are very time-dependent, because delays in routing will seriously affect the quality of the service provided. Email can have a low QoS, as it is not critical that it be delivered immediately to the destination.

10. How do packet-switching services differ from other wide area networks services?

Packet switched services are quite different from the other types of network services. For each of these three, dialed circuit services, dedicated circuit services, and circuit switched services, a physical circuit was established between the two communicating computers. This circuit provided a guaranteed data transmission capability that was available for use by only those two computers. In contrast, packet switched services enable multiple connections to exist simultaneously between computers. With packet switched services, the user again buys a connection into the common carrier network. The user pays a fixed fee for the connection into the network and charged for the number of packets transmitted.

5. Under what circumstances would you want to use a routed backbone?

Routed backbones are good for connecting different buildings on the same enterprise campus backbone network. The primary advantage of the routed backbone is that it clearly segments each part of the network connected to the backbone. Each segment has its own subnet addresses that can be managed by a different network manager. Broadcast messages stay within each subnet and do not move to other parts of the network.

7. Explain how routed backbones work.

Routed backbones move packets along the backbone based on their network layer address (i.e., layer 3 address). The most common form of routed backbone uses a bus topology (e.g., using Ethernet 100Base-T). Routed backbones can be used at the core or distribution layers. At the core layer routed backbones are sometimes called subnetted backbones or hierarchical backbones and are most commonly used to connect different buildings within the same campus network. At the distribution layer a routed backbone uses routers or layer 3 switches to connect a series of LANs (access layer) to a single shared media backbone network. Each of the LANs are a separate subnet. Message traffic stays within each subnet unless it specifically needs to leave the subnet to travel elsewhere on the network, in which case the network layer address (e.g., TCP/IP) is used to move the packet.

10. What are the key advantages and disadvantages among routed and switched backbones?

Routed backbones • Clear segmentation of parts of the network connected to the backbone as each network has a subnet address and can be managed separately. • Slower performance as routing takes more time than bridging or switching. • Management and/or software overhead costs due to need to establish subnet addressing and provide reconfiguration when computers are moved (or support dynamic addressing). Switched backbones • Performance is improved. With the traditional backbone network, the backbone circuit was shared among many LANs; each had to take turns sending messages. With the collapsed backbone, each connection into the switch is a separate point-to-point circuit. The switch enables simultaneous access, so that several LANs can send messages to other LANs at the same time. Throughput is increased significantly, often by 200% to 600%, depending upon the number of attached LANs and the traffic pattern. • Since there are far fewer networking devices in the network, this reduces costs and greatly simplifies network management. All the key backbone devices are in the same physical location, and all traffic must flow through the switch. If something goes wrong or if new cabling is needed, it can all be done in one place. • Software reconfiguration replaces hardware reconfiguration. • Because data link layer addresses are used to move packets, there is more broadcast traffic flowing through the network and it is harder to isolate and separately manage the individually attached LANs. Layer 3 switches can use the network layer address, so future collapsed backbones built with layer 3 will not suffer from this problem. • Collapsed backbones use more cable, and the cable must be run longer distances, which often means that fiber optic cables must be used. • If the switch fails, so does the entire backbone network. If the reliability of the switch has the same reliability as the reliability of the routers, then there is less chance of an failure (because there are fewer devices to fail). For most organizations, the relatively minor disadvantages of cable requirements and impacts of potential switch failure are outweighed by the benefits offered by collapsed backbones.

14. What is routing?

Routing is the process of determining the route or path through the network that a message will travel from the sending computer to the receiving computer. Every computer that performs routing has a routing table developed by the network manager that specifies how messages will travel through the network.

20. Explain how association works in WLAN.

Searching for an available AP is called scanning and NIC can engage in either active or passive scanning. During active scanning, a NIC transmits a special frame called probe frame on all active channels on its frequency range. When an AP receives a probe frame, it responds with a probe response that contains all the necessary information for a NIC to associate with it. A NIC can receive several probe responses from different APs. It is up to the NIC to choose with which AP to associate with. This usually depends on the speed rather than distance from an access point. Once a NIC associates with an access point they start exchanging packets over the channel that is specified by the access point. During passive scanning, the NIC listens on all channels for a special frame called beacon frame that is sent out by an access point. The beacon frame contains all the necessary information for a NIC to associate with it. Once a NIC detects this beacon frame it can decide to associate with it and start communication on the frequency channel set by the access point.

12. How does TCP/IP perform address resolution for network layer addresses?

Server name resolution is the translation of application layer addresses into network layer addresses (e.g., translating an Internet address such as www.cba.uga.edu into an IP address such as 128.192.98.3). This is done using the Domain Name Service (DNS). Throughout the Internet there are a series of computers called name servers that provide DNS services. These name servers run special address databases that store thousands of Internet addresses and their corresponding IP addresses. These name servers are in effect the "directory assistance" computers for the Internet. Any time a computer does not know the IP number for a computer, it sends a message to the name server requesting the IP number. When TCP/IP needs to translate an application layer address into an IP address, it sends a special TCP-level packet to the nearest DNS server. This packet asks the DNS server to send the requesting computer the IP address that matches the Internet address provided. If the DNS server has a matching name in its database, it sends back a special TCP packet with the correct IP address. If that DNS server does not have that Internet address in its database, it will issue the same request to another DNS server elsewhere on the Internet. Once your computer receives an IP address it is stored in a server address table. This way, if you ever need to access the same computer again, your computer does not need to contact a DNS server. Most server address tables are routinely deleted whenever you turn off your computer.

18. Why are broadcast messages important?

Some application software packages and network operating system modules written for use on LANs broadcast status messages to all computers on the LAN (but not necessarily all computers served by a BN). For example, broadcast messages inform users when printers are out of paper, or when the network manager is about to shut down the server. These types of messages require filtering in a backbone network if their broadcast scope should be restricted to a particular LAN or segment.

11. Where does packetizing take place?

Splitting messages into individual packets (packetizing) takes place at a packet assembly/disassembly device (PAD), which can be owned and operated by the customer or by the common carrier. The PAD converts the sender's data into the network layer and data link layer packets used by the packet network and sends them through the packet switched network. At the other end, another PAD reassembles the packets back into the network layer and data link layer protocols expected by the destination and delivers it to the appropriate computer. This "packetizing" and re-assembly is almost instantaneous, and data are transmitted continuously. The PAD can translate between different data link layer protocols between the sender and the destination (e.g., ethernet at the sender and token ring at the receiver). It may also provide conversion from one code to another (i.e., ASCII to EBCDIC).

9. Explain how switched backbones work.

Switched backbone networks use a star topology with one device, usually a switch, at its center. The traditional backbone circuit and set of routers or bridges is replaced by one switch and a set of circuits to each LAN. The collapsed backbone has more cable, but fewer devices. There is no backbone cable. The "backbone" exists only in the switch, which is why this is called a collapsed backbone. The original collapsed backbone technology uses layer-2 switches and suffers some disadvantage due to the load of data link layer overhead message traffic and limitations on network segmentation. As this weakness has been recognized, collapsed backbone technology is adapting by evolving to the use of layer-3 switches to overcome these problems. The result is better performance and improved network management capabilities for switched backbone networks. Collapsed backbones are probably the most common type of backbone network used in the distribution layer (i.e., within a building). Most new building backbone networks designed today use collapsed backbones. They also are making their way into the core layer as the campus backbone, but routed backbones still remain common.

4. Under what circumstances would you use a switched backbone?

Switched backbones can be used in situations where the network administrators wants to spread the traffic around the network more efficiently. In addition, it also provides an architecture where network capacity is no longer tied to the physical location of the computers, as computers in

7. What are the most commonly used T carrier services? What data rates do they provide?

T-Carrier Designation DS Designation Speed DS-0 64 Kbps T-1 DS-1 1.544 Mbps T-2 DS-2 6.312 Mbps T-3 DS-3 33.375 Mbps T-3 DS-4 274.176 Mbps

5. How is TCP different from UDP?

TCP is a connection-oriented protocol. UDP is a connection-less protocol. What are the differences between connectionless and connection-oriented routing? Connection-oriented routing sets up a virtual circuit between the sender and receiver. In this case, a temporary virtual circuit is defined between the sender and receiver. The network layer makes one routing decision when the connection is established, and all packets follow the same route. All packets in the same message arrive at the destination in the same order in which they were sent. In this case, packets only need to contain information about the stream to which it belongs; sequence numbers are not needed, although many connection-oriented protocols include a sequence number to ensure that all packets are actually received. Connection-oriented routing has greater overhead than connectionless routing, because the sender must first "open" the circuit by sending a control packet that instructs all the intervening devices to establish the circuit routing. Likewise, when the transmission is complete, the sender must "close" the circuit. Connection-oriented protocols also tend to have more overhead bits in each packet. Connectionless routing means each packet is treated separately and makes its own way through the network. It is possible that different packets will take different routes through the network depending upon the type of routing used and the amount of traffic. Because packets following different routes may travel at different speeds, they may arrive out of sequence at their destination. The sender's network layer therefore puts a sequence number on each packet, in addition to information about the message stream to which the packet belongs. The network layer must reassemble them in the correct order before passing the message to the application layer.

3. What are the parts of TCP/IP and what do they do? Who is the primary user of TCP/IP?

TCP performs packetizing: breaking the data into smaller packets, numbering them, ensuring each packet is reliably delivered, and putting them in the proper order at the destination. IP performs routing and addressing. IP software is used at each of the intervening computers through which the message passes; it is IP that routes the message to the final destination. The TCP software only needs to be active at the sender and the receiver, because TCP is only involved when data comes from or goes to the application layer. TCP/IP is the network layer protocol now used on the Internet. It is also the world's most popular network layer protocol, used by almost 70 percent of all backbone, metropolitan, and wide area networks. TCP/IP is commonly combined with Ethernet.

6. How does TCP establish a session?

TCP sets up a virtual circuit between the sender and the receiver. The transport layer software sends a special packet (called a SYN, or synchronization characters) to the receiver requesting that a connection be established. The receiver either accepts or rejects the connection, and together, they settle on the packet sizes the connection will use. Once the connection is established, the packets flow between the sender and the receiver, following the same route through the network.

24. What type of routing does a TCP/IP client use? What type of routing does a TCP/IP gateway use? Explain.

The TCP/IP client uses static routing because the client must always point to a single gateway router. The TCP/IP gateway router uses dynamic routing because typically it must process multiple requests for routing beyond the single segment that it physically supports.

3. Describe the basic components of a wired LAN.

The basic components of a wired LAN are the NICs, circuits, access points, and network operating system. The network interface card (NIC) allows the computer to be physically connected to the network cable, which provides the physical layer connection among the computers in the network. The circuits are the cables that connect devices together. In a LAN, these cables are generally twisted pair from the client to the hub or server. Outside the building, fiber optic is generally used. Network hubs and switches serve two purposes. First, they provide an easy way to connect network cables. In general, network cables can be directly connected by splicing two cables together. Second, many hubs and switches act as repeaters or amplifiers. Signals can travel only so far in a network cable before they attenuate and can no longer be recognized. The network operating system (NOS) is the software that controls the network. Every NOS provides two sets of software: one that runs on the network server(s), and one that runs on the network client(s). The server version of the NOS provides the software that performs the functions associated with the data link, network, and application layers and usually the computer's own operating system. The client version of the NOS provides the software that performs the functions associated with the data link and the network layers, and must interact with the application software and the computer's own operating system.

4. Describe the basic components of a wireless LAN.

The basic components of a wireless LAN are the NICs, circuits, access points, and network operating system. The network interface card (NIC) allows the computer to be physically connected to the network cable, which provides the physical layer connection among the computers in the network. The "circuit" is the air that connects the wireless clients to the access points. Between the access points and the switches or servers, twisted pair cable is typically utilized. A wireless access point performs the same functions as a hub or switch in a wired environment. The network operating system (NOS) is the software that controls the network. Every NOS provides two sets of software: one that runs on the network server(s), and one that runs on the network client(s). The server version of the NOS provides the software that performs the functions associated with the data link, network, and application layers and usually the computer's own operating system. The client version of the NOS provides the software that performs the functions associated with the data link and the network layers, and must interact with the application software and the computer's own operating system.

21. What is the best practice recommendation for wired LAN design?

The best recommendations are based primarily on evaluating the trade-off between effective data rates and costs. Sometimes it is also interesting to evaluate LAN vs. WLAN as part of the process.

22. What are the best practice recommendations for WLAN design?

The best recommendations are based primarily on evaluating the trade-off between effective data rates and costs. Sometimes it is also interesting to evaluate LAN vs. WLAN as part of the process.

10. What benefits and problems does dynamic addressing provide?

The bootp or DHCP server can be configured to assign the same network layer address to the computer each time it requests an address (based on its data link layer address), or it can lease the address to the computer by picking the "next available" network layer address from a list of authorized addresses. Addresses can be leased for as long as the computer is connected to the network or for a specified time limit (e.g., two hours). When the lease expires, the client computer must contact the bootp or DHCP server to get a new address. Address leasing is commonly used by Internet service providers (ISPs) for dial-up users. Dynamic addressing greatly simplifies network management in non-dial-up networks too. With dynamic addressing, address changes need to be done only to the bootp or DHCP server, not each individual computer. The next time each computer connects to the network or whenever the address lease expires, it automatically gets the new address.

14. Explain the differences between CIR and MAR.

The committed information rate (CIR) is the data rate the circuit must guarantee to transmit. If the network accepts the connection, it guarantees to provide that level of service. Most connections also specify a maximum allowable rate (MAR), which is the maximum rate that the frame relay network will attempt to provide, over and above the CIR. The circuit will attempt to transmit all packets up to the MAR, but all packets that exceed the CIR are marked as discard eligible (DE). If the network becomes overloaded, DE packets are discarded. So while can transmit faster than the CIR, they do so at a risk of lost packets.

19. Compare and contrast unicast, broadcast, and multicast messages.

The most common type of message in a network is the usual transmission between two computers. One computer sends a message to another computer (e.g., a client requesting a Web page). This is called a unicast message. In the situation of a broadcast message, the message is sent to all computers on a specific LAN or subnet. A third type of message called a multicast message is used to send the same message to a group of computers.

2. What does the network layer do?

The network layer performs three important functions: addressing, routing, and breaking long messages into smaller packets for transmission by the data link layer. The network layer sits between the application layer and the data link layer. The network layer accepts messages from the application layer and formats and addresses them for transmission by the data link layer. The network layer also accepts individual messages from the data link layer and organizes them into coherent messages that it passes to the application layer.

24. How do you decide how many APs are needed and where they should be placed for best performance?

The network manager will make a determination based off four factors: nominal data rates, error rates, efficiency of the data link layer protocols used, and efficiency of the media access control protocols.

8. What does a NOS do? What are the major software parts of a NOS?

The network operating system (NOS) is the software that controls the network. Every NOS provides two sets of software: one that runs on the network server(s), and one that runs on the network client(s). The server version of the NOS provides the software that performs the functions associated with the data link, network, and application layers and usually the computer's own operating system. The client version of the NOS provides the software that performs the functions associated with the data link and the network layers, and must interact with the application software and the computer's own operating system.

8. In Figure 8.5, would the network still work if we removed the routers in each building and just had one core router? What would be the advantages and disadvantages of doing this?

The network would still work, although the traffic on the network would be significantly increased due to the creation of one large LAN instead of three subnets. The advantages of this would be slightly lower costs due to only purchasing one router instead of four and less maintenance and management. Each of these advantages are minor, and certainly not work the additional traffic on the network.

11. Compare and contrast rack-based and chassis-switch based switched backbones.

The rack-based collapsed backbone has the advantage of placing all network equipment in one place for easy maintenance and upgrade, but does require more cable. In most cases, the cost of the cable itself is only a small part of the overall cost to install the network, so the cost is greatly outweighed by the simplicity of maintenance and the flexibility it provides for future upgrades. The room containing the rack of equipment is sometimes called the main distribution facility (MDF) or central distribution facility (CDF). The cables from all computers and devices in the area served by the MDF (often hundreds of cables) are run into the MDF room. Once in the run they are connected into the various devices. The devices in the rack are connected among themselves using very short cables called patch cables. With rack-based equipment, it becomes simple to move computers from one LAN to another. This convenience is used to spread the traffic around the network more efficiently so that network capacity is no longer tied to the physical location of the computers. Computers in the same physical area can be connected into very different network segments conveniently in the MDF. A chassis switch enables users to plug modules directly into the switch. Each module is a certain type of network device. The key advantage of chassis switches is their flexibility. It becomes simple to add new modules with additional ports as the LAN grows, and to upgrade the switch to use new technologies. For example, if you want to add gigabit Ethernet or ATM you simply lay the cable and insert the appropriate module into the switch.

23. What is a site survey, and why is it important?

The site survey determines the feasibility of the desired coverage, the potential source of interference, the current locations of the wired network into which the WSAN will connect, and an estimate of the number of APs required to provide coverage.

9. Describe SONET. How does it differ from SDH?

The synchronous optical network (SONET) has recently been accepted by the U.S. standards agency (ANSI), as a standard for optical transmission at gigabit per second speeds. The international telecommunications standards agency (ITU-T) also recently standardized a version of SONET under the name of synchronous digital hierarchy (SDH). SONET and SDH are very similar and can be easily interconnected. The SONET standard includes more data rates than the SDH standard. As with T carrier services, a CSU/DSU is needed to connect the user's network into the SONET/SDH circuit. SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of 51.84 Mbps. Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1, with SONET data rates defined as high as OC-192 or about 10 Gbps. Each level above OC-1 is created by multiplexing. Although not yet available in all locations, SONET/SDH is available in most large cities worldwide.

1. What does the transport layer do?

The transport layer links application software in the application layer with the network and is responsible for the end-to-end delivery of the message. The transport layer sits between the application layer and the network layer. The transport layer accepts messages from the application layer and packetizes them. Packetizing means to take one outgoing message from the application layer and break it into a set of smaller packets for transmission through the network. Conversely, it also means to take the incoming set of smaller packets form the network layer and reassemble them into one message for the application layer.

23. It is said that frame relay services and dedicated-circuit services are somewhat similar from the perspective of the network designer. Why?

They are both based on a single connection to the common carrier and provide similar transmission speed and reliability.

22. Suppose you joined a company that had a WAN composed of SONET, T carrier and frame relay services, each selected to match a specific network need for a certain set of circuits. Would you say this was a well-designed network? Explain.

This depends on how suitable the various technologies are for the applications supports. Today, increasingly, network designers make decisions to reduce costs, gain reliability, and increase performance through network integration, dynamic allocation so that unused capacity much not be paid for, and failover capabilities.

18. Explain how CSMA/CA DCF works.

This technology relies on the ability of computers to physically listen before they transmit. With DCF, each frame in CSMA/CA is sent using stop and wait ARQ, and it is designed in such a way so that no other computer begins transmitting while the waiting period is going on.

26. What are three special purpose devices you might find in a data center and what do they do?

Three special purpose devices that the data center may contain include a load balancer, virtual servers, and storage area networks. The load balancer acts as a router at the front of the server farm to distribute any processing to an appropriate server. Logical servers are logically separate servers (e.g., a Web server, an email server, and a file server) on the same physical computer. The virtual servers run on the same physical computer but appear completely separate to the network. Lastly, the storage area network are LANs devoted solely to data storage.

17. Compare the three types of VPN.

Three types of VPN are in common use: intranet VPN, extranet VPN and access VPN. • An intranet VPN provides virtual circuits between organization offices over the Internet. Each location has a VPN device that connects the location to another location through the Internet. • An extranet VPN is the same as an intranet VPN except that the VPN connects several different organizations, often customers and suppliers, over the Internet. • An access VPN enables employees to access an organization's networks from a remote location. Employees have access to the network and all the resources on it in the same way as employees physically located on network. The user connects to a local ISP that supports the VPN service via POTS, ISDN, or other circuit. The VPN device at the ISP accepts the user's login, establishes the tunnel to the VPN device at the organization's office, and begins forwarding packets over the Internet. An access VPN provides a more less expensive connection than having a national toll-free 800 number that connects directly into large sets of modems at the organization's office. Compared to a typical ISP-based remote connection, the access VPN is secure connection than simply sending packets over the Internet.

13. How does TCP/IP perform address resolution for data link layer addresses?

To send a message to a computer in its network, a computer must know the correct data link layer address. In this case, the TCP/IP software sends a broadcast message to all computers in its subnet. A broadcast message, as the name suggests, is received and processed by all computers in the same LAN (which is usually designed to match the IP subnet). The message is a specially formatted TCP-level request using Address Resolution Protocol (ARP) that says "Whoever is IP address xxx.xxx.xxx.xxx, please send me your data link layer address." The TCP software in the computer with that IP address then responds with its data link layer address. The sender transmits its message using that data link layer address. The sender also stores the data link layer address in its address table for future use.

25. What is the transmission efficiency of a 10-byte Web request sent using HTTP, TCP/IP, and Ethernet? Assume the HTTP packet has 100 bytes in addition to the 10-byte URL. Hint: Remember from Chapter 4 that efficiency = user data/total transmission size.

Total user data = 10 bytes Total transmission size = 192 bytes Efficiency = User Data / Total transmission size 10/192 = .05

26. What is the transmission efficiency of a 1000 byte file sent in response to a web request HTTP, TCP/IP, and Ethernet? Assume the HTTP packet has 100 bytes in addition to the 1000-byte file. Hint: Remember from Chapter 4, that efficiency = user data / total transmission size.

Total user data = 1000 bytes Total transmission size = 1171 bytes Efficiency = User Data / Total transmission size 1000/1171 = .85

17. How does Wi-Fi differ from shared Ethernet in terms of topology, media access control, and error control, Ethernet frame?

Wi-Fi is very similar to shared Ethernet in terms of the logical and physical topologies. The Wi-Fi approach uses a logical bus and a physical star arrangement, just like shared Ethernet. On the shared bus in Wi-Fi, the computers must take turns transmitting, which is not always so in shared Ethernet. For error control, Wi-Fi has a hidden node problem, where some computers may not sense contention, and may therefore transmit when they should not, so Wi-Fi uses a slightly different technique for contention to try and cut down on collisions.

27. What is the transmission efficiency of a 5000 byte file sent in response to a web request HTTP, TCP/IP, and Ethernet? Assume the HTTP packet has 100 bytes in addition to the 5000-byte file. Assume that the maximum packet size is 1200 bytes Hint: Remember from Chapter 4, that efficiency = user data / total transmission size.

User data = 1200 bytes Transmission size = 1223 bytes Efficiency = 1200/1223 = .98 (.98) * 4 = 3.92 User data = 271 bytes Transmission size = 1223 bytes Efficiency = 271/1223 = .22 Total transmission efficiency = (3.92 + .22) / 5 = .828 .83

19. Explain how CSMA/CA PCF works.

Using PCF (also called the virtual carrier sense method), works in traditional Ethernet, and because every computer on thee shared circuit receives every transmission on the shared circuit. There can be a "hidden node problem" with CSMA/CA PCF because some computers at the edge of the network may not sense every transmission, increasing the likelihood of collisions.

3. How does a router differ from a VLAN?

VLAN switches can create multiple subnets, so they act like routers, except the subnets are inside the switch, not between switches. Therefore, broadcast messages sent by computers in one VLAN segment are sent only to the computers on the same VLAN.

21. Explain how the client computer in Figure 5.14 (128.192.98.xx) would obtain the data link layer address of its subnet gateway.

When a computer is installed on a TCP/IP network (or dials into a TCP/IP network), it knows the IP address of its subnet gateway. This information can be provided by a configuration file or via a bootp or DHCP server. However, the computer does not know the subnet gateway's Ethernet address (data link layer address). Therefore, TCP would broadcast an ARP request to all computers on its subnet, requesting that the computer whose IP address is 128.192.98.1 to respond with its Ethernet address. All computers on the subnet would process this request, but only the subnet gateway would respond with an ARP packet giving its Ethernet address. The network layer software on the client would then store this address in its data link layer address table.

4. Compare and contrast the three types of addresses used in a network.

When users work with application software, they typically use the application layer address (e.g., entering an Internet address into a browsers, such as www.cba.uga.edu). When a user types an Internet address into a Web browser, the request is passed to the network layer as part of an application layer packet formatted using the HTTP standard. The network layer software translates this application layer address into a network layer address. The network layer protocol used on the Internet is TCP/IP, so this Web address (www.cba.uga.edu) is translated into an TCP/IP address (usually just called an IP address for short) which is four bytes long when using IPv4 (e.g., 128.192.78.5). The network layer then determines the best route through the network to the final destination. Based on this routing, the network layer identifies the data link layer address of the next computer to which the message should be sent. If the data link layer is running Ethernet, then the network layer IP address would be translated into an Ethernet address (e.g., 00-0F-00-81-14-00).

8. What is a subnet mask?

While it is customary to use the last byte of the IP address to indicate different subnets, it is not required. Any portion of the IP address can be designated as a subnet by using a subnet mask. Every computer in a TCP/IP network is given a subnet mask to enable it to determine which computers are on the same subnet (i.e., LAN) as it is, and which computers are outside of its subnet. The subnet mask is a number that each routing device on a network can use to compare the routing packet to in order to determine if the packet stays in that portion or the network, or should be routed on to the next portion of the network. For example, a network could be configured so that the first two bytes indicated a subnet (e.g., 128.184.x.x) so all computers would be given a subnet mask giving the first two bytes as the subnet indicator. This would mean that a computer with an IP address of 128.184.22.33 would be on the same subnet as 128.184.78.90, and that the message stays within that network. On the other hand, if the subnet comparison shows that the message is not on that particular subnet, then the routing device will move the packets on in their journey.

15. How does decentralized routing differ from centralized routing?

With centralized routing, all routing decisions are made by one central host computer. Centralized routing is used typically only in host-based networks and in this case, routing decisions are rather simple. All computers are connected to the central computer by individual point-to-point circuits, so any message received is simply retransmitted on the point-to-point circuit connected to the destination. Decentralized routing allows all computers in the network make their own routing decisions following a formal routing protocol. In MANs and WANs, the routing table for each computer is developed by its individual network manager. Most decentralized routing protocols are self-adjusting, meaning that they can automatically adapt to changes in the network configuration (e.g., adding and deleting computers and circuits).

14. Compare and contrast cut-through, store and forward, and fragment-free switching.

With cut through switching, the switch begins to transmit the incoming packet on the proper outgoing circuit as soon as it has read the destination address in the packet. With store and forward switching the switch does not begin transmitting the outgoing packet until it has received the entire incoming packet and has checked to make sure it contains no errors. Fragment-free switching lies between the extremes of cut through and store and forward switching. With fragment-free switching, the first 64 bytes and if all the header data appears correct, the switch presumes the rest of the packet is error free and begins transmitting.

4. Compare and contrast dedicated-circuit services, and packet-switched services.

With dedicated circuit networks, a circuit is established between the two communicating computers. This circuit provides a guaranteed data transmission capability that was available for use by only those two computers and is assigned solely to that transmission. No other transmission is possible until the circuit is closed. In contrast, packet switched services enable multiple connections to exist simultaneously between computers over the same physical circuit or even over different physical circuits. With a dedicated circuit network, you lease circuits from common carriers for their exclusive use twenty-four hours per day, seven days per week. All connections are point to point, from one building in one city to another building in the same or a different city. The carrier installs the circuit connections at the two end points of the circuit and makes the connection between them. The circuits still run through the common carrier's cloud, but the network behaves as if you have your own physical circuits running from one point to another: • Dedicated circuits are billed at a flat fee per month and the user has unlimited use of the circuit. • Once you sign a contract, making changes can be expensive because it means rewiring the buildings and signing a new contract with the carrier. Therefore, dedicated circuits require more care in network design than switched circuits both in terms of locations and the amount of capacity you purchase. With packet switched services, the user again buys a connection into the common carrier cloud). The user pays a fixed fee for the connection into the network (depending upon the type and capacity of the service) and is charged for the number of packets transmitted.

9. How does dynamic addressing work?

With dynamic addressing, a server is designated to supply a network layer address to a computer each time the computer connects to the network. This is commonly done for client computers, but usually not done for servers. Instead of providing a network layer address in a configuration file, a special software package is installed on the client that instructs it to contact bootp or DHCP servers using data link layer addresses. This message asks the servers to assign the requesting computer a unique network layer address. The server runs a corresponding bootp or DHCP software package that responds to these requests and sends a message back to the client giving it its network layer address (and its subnet mask).

23. How does static routing differ from dynamic routing? When would you use static routing? When would you use dynamic routing?

With static routing, the routing table is developed by the network manager, and changes only when computers are added to or removed from the network. For example, if the computer recognizes that a circuit is broken or unusable (e.g., after the data link layer retry limit has been exceeded without receiving an acknowledgment), the computer will update the routing table to indicate the failed circuit. If an alternate route is available, it will be used for all subsequent messages. Otherwise, messages will be stored until the circuit is repaired. When new computers are added to the network, they announce their presence to the other computers, who automatically add them into their routing tables. Static routing is commonly used in networks that have few routing options. For example, most LANs are connected to the backbone network in only one place. There is only one route from the LAN to the backbone, so static routing is used. Dynamic routing (or adaptive routing) is used when there are multiple routes through a network and it is important to select the best route. Dynamic routing attempts to improve network performance by routing messages over the fastest possible route, away from busy circuits and busy computers. An initial routing table is developed by the network manager, but is continuously updated by the computers themselves to reflect changing network conditions, such as network traffic. Routers can monitor outgoing messages to see how long they take to transmit and how long it takes for the receiving computer to acknowledge them. Based on this monitoring the router can effectuate table updating.