MIDTERM

Why is multiplexing useful in data communication?

Drastically reduced need for cabling. Cabling costs are one of the biggest fixed cost components of data communication.

What are the common data transfer speeds in Ethernet?

Ethernet is commonly available at speeds of 10/100/1000 Mbps, called 10BaseT, 100BaseT or 1GbE.

1. Briefly describe routing — the primary function of the network layer.

1. It's the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called router. Routing is a key feature of the internet because it enables message to pass from one computer to another and eventually reach the target machine.

8. What is the need for the time-to-live field in the IP header?

1. The Time-To-Live field in the IP header specifies a 128 or 256 value when it is created. At each hop, the router will decrement the value by one. This allows "lost" packets to not bounce around the internet indefinitely. Without TTL, sections of the internet could be brought down by DDOS attacks easily.

What is UTP? Why is the copper cable commonly used in computer networks called UTP?

UTP is 8 individual strands of copper wire are organized as four pairs; Copper is a very good conductor of electricity and is relatively abundantly available

What are the properties of a good signal?

• Easy detection by receiver (simply tune in) • Immunity from noise (compare FM vs. AM) • Efficient use of bandwidth • Multiplexing

18. In what way are the 32 bits of an IP address organized similarly to the 10 digits of phone numbers? In what way are they different?

12. Your phone number is a unique set of numbers that identifies your phone so that other people can call you. Similarly, an IP address is a unique set of numbers that identifies your computer so that it can send and receive data with other computers. Broadly speaking, IP addressed are broken down into two parts. The left part of the IP address represents the network to which the IP address belongs. This called the network part of the IP address. The remaining bits of the IP address identify the computer within this network. This is often called the host part of the IP address. The phone network uses a three-part addressing scheme where the first part identifies the geographical area; the second part identifies the local exchange within the geographical area, and the remaining digits identify the specific user. Unlike phone numbers, IP addresses have limited geographic significance. (IP network addressed identify networks (organizations), but not geographies.)

14. What is dotted decimal notation?

13. While computers work with IP addresses as 32-bit binary values, humans normally use the dotted-decimal notation (a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop) to convert IP address numbers from binary format so can easily read and remember. A binary address and its dotted-decimal equivalent are shown below. Note that the 32-bit address is divided into four 8-bit fields called octets.

15. What information is conveyed by each part of a three-part IP address?

15. The leftmost part of the IP address is the network part, the subnet, and the rightmost part is the host.

16. How are the 32 bits of an IP address organized in a typical large network?

16. IP addresses are organized in network Classes A, B, and C depending on the size of the organization. In Class A the first octet is the network part while the rest are the hosts. In Class B, the first and second octets are the network and the rest are hosts. In Class C the first through third octets are the network and the last is the individual host computer.

19. What were the three address classes in early IP networks? How many hosts (computers) could be accommodated in a network in each address class?

18. Class A: first 8 bits identified the network ID and the remaining 24 bits identified hosts within the network. 16,777,216 hosts. Class B: first 16 bits identified the network ID and the remaining 16 bits identified hosts within each network. 65,536. Class C: the first 24 bits are the network part leaving 8 bits for host part within the network. 2,097,152 are possible.

20. What are the disadvantages of using address classes? How does CIDR overcome these disadvantages?

19. The A, B, C class system suffers from a problem of allocative efficiency. Half of the available I addresses on the internet could be allocated to 128 networks. Worse more are that there are many unallocated IP addresses. CIDR solves these problems by allowing the network part to be whatever length an organization would need to allocate the right amount of IP addresses. It is a much more flexible scheme for allocation.

2. How are IP addresses similar to MAC addresses? In what ways are the two addresses different?

2. IP addresses and Mac addresses are both key components to network. They provide unique addressed for computers to send information to each other. Mac is actually a physical address of device which is constant and it is used at a data link layer and IP address is an address of a device which is used at network layer. Throughout the packet's journey from the source to the destination, the destination IP address don't change but the data link layer addressed keep changing as the packet gets transferred from network to network.

21. What is registry in the context of IP addresses?

20. A database where IP addressed are assigned to different networks

22. What are regional registries? What is the need for regional registries?

21. There are five regional registries. ARIN for North American, RIPE for Europe, the Middle-East, and Central Asia. APNIC for the Far East. LACNIC for Latin America and AfriNIC for Africa. For administrative convenience, the registries have adopted a policy of allocating addressed only to lager telecom carriers. This limits the number of organizations the registries have to deal with. It also provides the registries with some assurance that organizations with address allocations will have suitable technical expertise to manage these address allocations. Allocating IP address in large blocks also simplifies the global routing of packets by limiting the growth in the size of routing tables.

23. What requirements must an organization satisfy in order to obtain IP addresses directly from a registry?

22. Only networks that are connected to two or more networks are allowed to apply to the registries to obtain a network address. Most organizations that need IP addresses with therefore contract with their local ISPs to obtain network addresses. The registries actively try not to allocate IP addresses directly to individuals and very small organizations.

3. What is the need for a computer address at the IP layer when computers also have a MAC address?

3. Since many of the networks on the Internet use technology that was developed independently of IP, both addresses are necessary. The data-link address allows the packet to be delivered correctly within each individual network on the path of the destination. The IP address retains the address of the actual source and the final destination. Additionally, having addressing handled on two different layers creates great flexibility. The IP protocol could be upgraded without requiring changes to the Data-Link layer and vice versa.

4. What are the advantages of designing the Internet in such a way that specialized devices called routers handle all the details of routing? What may be the possible disadvantages?

4. Routers are the glue that holds the Internet together. They are placed at the interface between two networks and guide packets across networks in the right direction to their respective destination. It allows even simple devices, such as handheld computers and inexpensive security cameras with limited processing capability, to be connected to the Internet. Each router is only responsible for passing the packet on to a suitable neighboring router. As long as the neighbors are correctly chosen, the packet will reach the destination. Once a router dispatches a data packet on to a neighboring router, the packet is left to its own fate and the router does not worry about the packet anymore. If the neighboring router is unable to process the data packet for any reason, the packet can get lost, but routers do now worry about recovering lost packets. Instead, routers focus on routing the next packet.

5. What are the advantages of designing IP as a best-effort protocol?

5. Best-effort delivery describes a network service in which the network does not provide any special features that recover lost or corrupted packets. These services are instead provided by end systems. By removing the need to provide these services, the network operates more efficiently. In the TCP/IP protocol suite, TCP provides guaranteed services while IP provides best-effort delivery. TCP performs the equivalent of obtaining a delivery confirmation from the recipient and returning it to the sender. Because IP provided basic packet delivery services without guarantees, it is called a best-effort delivery service. It does its best to deliver packets to the destination, but takes no steps to recover packets that are lost or misdirected. With best-effort services, packet discard is acceptable because recovery is handled by other services.

6. What were the primary motivations for the development of IP version 6?

6. It's the predicted shortage of available I addresses under IPv4. With IPv4, there are more than 4 billion addresses available, which is about one for every two people on earth, only a fraction of those addresses are available under real-world conditions. While there is no shortage today, in the future as more devices connect (especially household "smart" appliances), a scheme like IPv6 will become desirable due to its ability to provide 340 undecillion) IP addresses.

7. Which, in your opinion, are the three most important fields in the IP header? Briefly describe the functions of these fields.

7. Header Length- this field specifies the headers length of the IP. The IP allows the header to include various options, which means the length of the header can be variable. Time to live- this field makes sure no packet can live forever on the Internet. It acts as a safety valve on the Internet. Header Checksum- this field carries error detection information for the packet header.

11. You wish to assign unique labels to 200 objects using binary numbers. What is the minimum number of bits needed?

8

9. What is the size of the largest possible IP packet?

8. 65,535

10. How many objects can be uniquely labeled with 10-bit address labels?

9. 2 ^10 = 1024 objects.

What is federal standard 1037C?

A Glossary of Telecommunication Terms, was initially published in 1980 to provide Federal departments and agencies a comprehensive source of definitions of terms used in telecommunications and directly related fields by international and U.S. Government telecommunications specialists.

What devices does a packet typically encounter in its journey from source to destination?

A packet typically encounters network adapters, switches and routers.

What is the hexadecimal notation? How is the number 14 represented in hexadecimal?

A way of representing binary numbers in 4 bit blocks; E

What are the advantages of broadcasting data in Ethernet? What are some other examples of communication in day-to-day life that use broadcast?

Advantages: simple and great for small network. Example: satellite, radio.

24. The packets used to transmit voice on the Internet are similar to the packets that are used to send email. What are some of the advantages of this approach?

All packets are forwarded as they arrive at the routers

What are hubs?

An old technology that sends data out to all computers on the network.

What are the components of optical fiber? What is the role of each component?

Core has high refractive index through which light travels. Cladding has low reflective index to propagate. The high refractive index of the core together with the low refractive index of the cladding restricts the light signal so that it stays within the core and moves along the fiber. The coating and strengthening fibers protect and cushion the glass. The jacket makes the fiber easy to handle, adds weatherproofing and encloses the fibers.

Why is binary representation preferred in computers over common representations such as decimal?

Binary representations are the most reliable form of representing data. It helps reliable reception in the presence of noise.

Provide a lay person's overview of CRC

CRC is an error checking technique that uses division to detect errors in data sent over a network.

What is carrier sensing in Ethernet? What is multiple access? What is a collision and what is collision detection? How are collisions detected in Ethernet?

Carrier sensing: an ongoing activity of a data station in a multiple access network to detect whether another station is transmitting. Multiple access: a scheme that gives more than one computer access to the network for the purpose of transmitting information. ¬the situation that occurs when 2 or more demands are made simultaneously on a system that can handle only one demand at any given instant. Collision detection: the requirement which a transmitting computer that detects another signal while transmitting data, stops transmitting that data. It immediately stops the transmission when computer detects a collision.

What are the common categories of copper cable used in networks? Under what conditions would you prefer to use each category of cable?

Categories range from Cat-1 to Cat-6. The higher the category, the higher the maximum possible data rate. When you need to support higher speeds, you'll need to use a higher category, such as categories Cat5e through category 7.

What is the need to convert data to signals?

Data cannot be transmitted over wires.

What factors favor the use of optical fiber as a physical medium over copper?

Data rates, distance, installation and costs. Easier to access, weighs much less, and carries large volumes of data over long distances.

What is the structure of a MAC address? What information can be gathered from a MAC address?

Ethernet defines the address field to be 48 bits in length. The 48 bits are split into two parts of 24 bits. The first 24 bits determine the organizationally unique identifier (OUI) or manufacturer ID. The remaining 24 bits are assigned by the manufacturer such that no two NICs made by the same manufacturer have the same ID.

What are the two categories of multiplexing?

Frequency division multiplexing (FDM) and division multiplexing (TDM). FDM is used with analog signals while TDM is used with digital signals.

What are some of the ways in which computer networks are used in large businesses?

In large businesses, computer networks are used to store and retrieve data from databases. Retail chains depend on computer networks to get real-time inventory in its stores.

Why is Internet traffic considered "bursty" compared to voice traffic?

Internet traffic is considered "bursty" compared to voice traffic because a user doesn't always have to be using data. The user will click on a link to download a web page, and then spend considerable time reading the page. While reading the page, there is no data being sent or received by the computer.

What are the limitations of broadcast as a method of sending data to the intended receiver of communication?

It can only handle one transmission at a time. Privacy of data

Describe the major trends in the growth of Internet traffic.

It is generally observed that different users have different traffic patterns and when all these patterns are aggregated together, the average traffic volume is less bursty than the individual traffic patterns.

Ethernet is the most popular end-user technology at the data-link layer. What is ether in the context of computer networking?

It's a cable available throughout a building and is also the core of Ethernet.

What is broadcast in the context of Ethernet?

It's the transmission of signals that may be simultaneously received by stations that usually make no acknowledgment.

What is CRC used for in data communication?

It's to help the receiver determine on its own that the data it has received has errors, without seeking confirmation from the sender.

What is layering?

Layering is the practice of arranging functionality of components in a system in a hierarchical manner such that the lower layers provide functions and services that support the functions and services of higher layers.

Why is layering useful in computer networking?

Layering is useful in computer networking because if there is new technology, everything does not have to be changed, for example, change from IPv4 to IPv6 does not require change in email client or browsers.

Why is layering useful in organizations?

Layering is useful in organizations because if one worker, manager, etc needs replaced it can be done so without having to change the whole company. If one person ran the whole organization and they failed, the company would too.

What properties are required for a material to be suitable for use as a physical medium in computer networks?

Media are chosen for their special properties that enable efficient transmission (movement) of signals. They have to efficiently conduct electricity, light, heat or other forms of energy.

What are the two categories of optical fiber? Under what conditions is each category preferred?

Multimode and single mode. Multimode is used when you want to transmit data over short-to-medium sources while signal mode is used to transmit data over very long distances.

Define data.

Numbers, letters, or other representation of information that can be processed by people or machines.

What are some of the factors that make packet switching more complex technologically than traditional phone circuits (circuit switching)?

Packet switching handle packets from multiple senders addressed to different receivers. It is very efficient at handling data bursts because no new connections need to be established on a per-packet basis. Routers are always connected to other neighboring routers through connections that can be shared by all the users of the router.

Why is packetization useful in business data communication?

Packetization eliminates single points of failure in data communication systems and they are easy to transport.

What is packetization?

Packetization is the process of breaking down user data into small segments and packaging these segments appropriately so that they can be delivered and reassembled across the network.

What is the primary responsibility of the physical layer in data communication?

Provide transparent transmission of a bit stream over a circuit built from some physical communications medium

What are some examples of multiplexing in day-to-day life?

Radio and television broadcasting, cable television, and telephone systems.

What is total internal reflection? How does it help optical fiber transmit light signals efficiently?

Refraction, light bends inward when it passes from air to water. Total internal reflection makes glass very efficient at carrying signals over long distances because energy (in the form of light) sent into the glass medium is tightly contained within the glass. Fiber can transmit signals over huge distances without repeaters.

How does noise affect signals? What happens if the level of noise becomes too high relative to the strength of the signal?

Signal detection over long distances over any medium is constrained by noise. As the signal travels through the medium, noise keeps getting added to the signal. Noise distorts the signal; eventually, the noise becomes too loud to receive the signal.

What are some of the ways in which small businesses can benefit from computer networks?

Small businesses use computer networks for communication, such as email and chat. They also use networks to share resources such as printers and files.

What is the size of the smallest Ethernet frame? The largest frame?

Smallest:64 bytes ; Largest: 1518 bytes

list the fields in the Ethernet header. What are the roles of each of these fields?

Source MAC addresses and destination address. The MAC address is a unique identifier that shows where the information is coming from, while the destination address show us where the data is being sent.

What is the ASCII code? Why is it useful in data communication? What is the ASCII code for the letter a? For the letter A?

Standard method to convert characters to binary; There is a binary representation for every letter; 01100001; 01000001.

What are the advantages of switches over hubs in Ethernets? Under what conditions may you prefer to use a hub instead of a switch?

Switched speed up the network at an extremely low cost. They greatly increase the data transmission efficiency of hub based CDMA/CD Ethernets. They reduce collisions by limiting broadcast. They can also simultaneously transfer data between multiple pairs of ports. Hubs are useful when performing network diagnostics.

TCP is often considered the most important layer of the TCP/ IP model. What are the primary responsibilities of TCP?

TCP operates at layer 4 and performs segmentation and reassembly of the packets.

How has the OSI model been useful in the development of computer networks?

The OSI model helped the development of computer networking because technology developers were able to define their technologies in terms of layers served by their technology.

What is the OSI model? What was the motivation for the development of the OSI model?

The OSI model is a logical structure for communication networks standardized by the International Organization for Standardization. The OSI model was motivated by difficulties in interoperation between networking technologies that created communication islands within organizations.

Consider a typical office memo as a data packet. What information in the memo would be characterized as header information? What information would be characterized as the body of the packet?

The addressee and destination address are included in the header of the packet. The useful information is in the body.

What are the primary functions of each of the seven layers of the OSI model?

The application layer is a request-reply mechanism for remote operations across a network. The presentation layer deals with syntax conversion from host-specific syntax to syntax for network transfer. The session layer creates and terminates connection; it establishes synchronization points for recovery in case of failure. The transport layer is the segmentation, reassembly of packets in one connection, multiplexing connections on one machine. The network layer is the routing and network addressing. The Data-link layer is error free data transmission over a single link. The physical layer converts data to signals for transmission over physical media.

Provide a high-level overview of the structure of a typical data packet. What are the kinds of information you are likely to find in the header of a typical packet?

The body of the packet is the segment from the original data carried out by the packet. The header is the information added during packetization to aid the delivery of the packet to the correct destination and for the body of the packet to be correctly reassembled with the bodies delivered by other packets. The header information added during packetization is also called packet overhead because it is the unavoidable information cost for packet delivery.

What is modulation? How does modulation help in data transmission?

The changes of one or more properties of the carrier wave in response to data to transfer information using the carrier wave.

What are the two design principles behind Internet protocols?

The first is that at the very outset, the designers of TCP/IP visualized a federation of networks, where each network could adopt arbitrary design or technology. The second principle is that multiple applications, even those that have not yet been conceived, can use the internet for network connectivity.

What are the five layers of the TCP/ IP model?

The five layers of the TCP/IP model are: the application layer, transport layer, network layer, data-link layer and physical layer.

What are the primary functions of each layer in the TCP/ IP model?

The function of the application layer handles user needs, the transportation layer deals with the segmentation and reassembly of packets, the network layer identifies and locates the destination, the data-link layer does an error check and the physical layer sends data as signals over media.

Why is CRC preferred over simpler computational techniques?

The overall detection rate is 99.9999998% of all errors that affect 33 or more bits in the data. This is considered satisfactory for commercial use.

Define physical medium in the context of computer networking. What are the common physical media used in computer networks?

The physical medium is the transmission path along which a signal propagates; copper wires, optical fiber.

What are the primary functions of the data-link layer?

The primary functions of the data-link layer are addressing and error detection. Addressing ensures the signals from the physical layer reach the correct device on the network and error detection detects if any errors were introduced during signal transmission.

What are the seven layers of the OSI model?

The seven layers of the OSI model are: Application, presentation, session, transport, network, data-link, and physical.

Describe how the interstate system may be seen as a multiplexed transportation system.

There are multiple lanes on an interstate. If there is more frequency, the cars need to share lanes.

The SFD field alerts the receiver about the beginning of a data frame. How does the receiver know when the frame ends?

There is no field that explicitly defines the end of the frame. Instead the length field helps the receiver identify the end of the frame. The receiver counts bytes till it gets "length" number of bytes.

What are the components of a typical Ethernet? What are the functions of each component?

They are 2 or more computers, network interface cards in every computer, Ethernet Cable Cat5, router and Hub or switch, computer network software. NIC is assigned unique address for unique identification. 2 computers are either connected with communication medium Ethernet Cable or directly attached to each other with hub/switch which enable networked computers to communicate with each other. Switch and hubs works as relay.

The start-of-frame and preamble fields are unique to the data-link layer in that they do not carry any useful information. What is the role of these fields?

They help the receiver locate the beginning of an incoming data-link layer frame. These fields are necessary because in normal operation, there are stray signals in the medium even if there is no network traffic.

What are switches?

They read the destination addresses of incoming frames and send the frames only to the correct targets.

What is amplitude modulation?

When a data bit of 1 may be sent as a wave of high amplitude and a 0 may be sent as a low amplitude wave. It's Simple, efficient bandwidth utilization, but poor noise immunity and wastes energy

Briefly describe the standard procedure used by the physical layer to send and receive data as a signal.

a. Sender side i. Step 1: Encode letters as binary numbers ii. Step 2: Modulate carrier signal with binary numbers b. Receiver side i. Step 3: Demodulate signal into binary numbers ii. Step 4: Decode binary numbers as letters

What is business data communications

the movement of information from one computer application on one computer to another computer by means of electrical or optical transmission systems.

55. What is sliding window in the context of flow control? Describe its operation.

• A sliding window is created at the sender so that the receiver can regulate the amount of data the sender can send. Packets that have received permission for transmission are within the window. Window slides as receiver acknowledges packets or modifies window size.

54. How is flow control implemented?

• A window is returned with every ACK which indicates how many bytes of data the sender may transmit before receiving any more ACK.

What is multiplexing?

• Ability to combine multiple channels of information on a common transmission medium. • Combine many signals at sending end and separate them at the receiving end

48. Why is segmentation useful?

• Because errors are less likely when using smaller segments. There is less retransmission if error is introduced in a segment. It is easier for routers to hold segments in memory if onward route is busy.

47. What is segmentation?

• Breaking down data into segments to transfer arbitrarily large data blocks. A sequence number is assigned to each segment which help receiver order the segments even when received out of order.

46. Why are two protocols, TCP and UDP, defined at the transport layer instead of just TCP?

• Companies who use real-time data prefer to use UDP. UDP is used when applications send very small amounts of data. Prefer speed to reliability. If we can avoid TCP, we eliminate the overhead of keeping track of sequence numbers, window sizes, etc.

51. How does TCP provide reliability?

• Damage is corrected by adding a checksum to each segment and comparing it at the receiver. Damaged segments are discarded and retransmitted by sender since no ACK is received. Also, sequence numbers help identify when there are duplicated segments.

define signal.

• Detectable transmitted energy that can be used to carry information. • A time-dependent variation of a characteristic of a physical phenomenon, used to convey information

45. What are the functions of the transport layer?

• End-to-end flow control, recovering from crash, ensuring ordered delivery of data • Makes sure that data is broken down into packets that can be carried by the IP layer. Makes sure if there is any application that requires internet connection that it connects to the internet and tracks who is connecting to make sure each application gets the correct data.

53. Why is it more useful to allow the receiver to control flow speed, rather than the sender?

• If the receiver is slow, it will not be able to process the packets reaching it. Therefore, TCP enables the receiver to limit the sender's data rate. If the sender is slow, it only tests the patience of the receiver.

49. What is reliability in the context of the transport layer?

• It makes sure the packet is delivered and that TCP recovers from network damage to data.

52. What is flow control?

• Making sure that applications at both ends are sending data to each other at a pace they can communicate comfortably in.

56. What is multiplexing in the context of the transport layer?

• Provides multiple port addresses within each host with one communication channel per host.

50. What are the important potential problems with reliability that are handled by the transport layer?

• Segments may get seriously damaged during transmission, which cannot be detected by Ethernet. If segments are further fragmented by IP, some IP fragments may get lost during transmission. Also, segments may get duplicated.