MI361 Ch7 Questions

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.

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.

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.

Compare and contrast the two types of antennas.

A directional antenna projects a signal only in one direction. Because the signal is concentrated in a narrower, focused area, it is a stronger signal and carries further. More popular is the omnidirectional antenna, which broadcasts in all directions except directly above itself.

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.

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.

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

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

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.

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.

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).

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.

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.

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.

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.

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).

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).

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.

plain 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.