CN-I

COMPUTER NETWORKS – I

Subject Code
10CS55               
IA Marks
25
No. of Lecture Hrs./ Week
04
Exam Hours
03
Total No. of Lecture Hrs.
52
Exam Marks
100

PART – A
UNIT - 1                                                                                                         7 Hours
Introduction: Data Communications, Networks, The Internet, Protocols &Standards, Layered Tasks,
The OSI model, Layers in OSI model, TCP/IP Protocol suite, Addressing

UNIT- 2                                                                                                          7 Hours
Physical Layer-1: Analog & Digital Signals, Transmission Impairment, Data Rate limits, Performance, Digital-digital conversion (Only Line coding: Polar, Bipolar and Manchester coding), Analog-to-digital conversion (only PCM), Transmission Modes, Digital-to-analog conversion

UNIT- 3                                                                                                          6 Hours
Physical Layer-2 and Switching: Multiplexing, Spread Spectrum, Introduction to switching, Circuit Switched Networks, Datagram Networks, Virtual Circuit Networks

UNIT- 4                                                                                                          6 Hours
Data Link Layer-1: Error Detection & Correction: Introduction, Block coding, Linear block codes, Cyclic codes, Checksum.
PART -B
UNIT- 5                                                                                                          6 Hours
Data Link Layer-2: Framing, Flow and Error Control, Protocols, Noiseless Channels, Noisy channels, HDLC, PPP (Framing, Transition phases only)

UNIT- 6                                                                                                          7 Hours
Multiple Access & Ethernet: Random access, Controlled Access, Channelization, Ethernet: IEEE standards, Standard Ethernet, Changes in the standard, Fast Ethernet, Gigabit Ethernet

UNIT – 7                                                                                                         6 Hours
Wireless LANs and Cellular Networks: Introduction, IEEE 802.11, Bluetooth, Connecting devices, Cellular Telephony

UNIT - 8                                                                                                         7 Hours
Network Layer: Introduction, Logical addressing, IPv4 addresses, IPv6addresses, Internetworking basics, IPv4, IPv6, Comparison of IPv4 and IPv6 Headers.








Text Books:

1. Behrouz A. Forouzan: Data Communication and Networking, 4th  Edition Tata McGraw-Hill, 2006.
(Chapters 1.1 to 1.4, 2.1 to 2.5, 3.1 To 3.6, 4.1 to 4.3, 5.1, 6.1, 6.2, 8.1to 8.3, 10.1 to 10.5, 11.1 to 11.7, 12.1 to 12.3, 13.1 to 13.5, 14.1, 14.2,15.1, 16.1, 19.1, 19.2, 20.1 to 20.3)

Reference Books:

  1. Alberto Leon-Garcia and Indra Widjaja: Communication Networks -Fundamental Concepts and Key architectures, 2nd Edition Tata McGraw-Hill, 2004.

  1. 2. William Stallings: Data and Computer Communication, 8th Edition, Pearson Education, 2007.

  1. Larry L. Peterson and Bruce S. Davie: Computer Networks – A Systems Approach, 4th Edition, Elsevier,     2007.


  1. 4. Nader F. Mir: Computer and Communication Networks, Pearson Education, 2007.







OBJECTIVES



   Data Communication and Networking are changing the way we do business and we live. Business today relay on computer network and internet and therefore we need to know how network operates ,what types of technology available, which design best fills which set of needs. Technological advances are making it possible for communication links to carry more and faster signals.

             Network exists so that data may be sent from one place to another. Therefore it becomes important to understand the data communication components, how data can be represented and how to create a data flow. Data communication between remote parties can be achieved through network involving the connection of computers, media and network devices. Protocols and standards are vital to the implementation of data communication and networking. Network model serve to organize unify and control hardware and software components of data communication and networking. 
Upon completing the course, the student will:
- be familiar with the basics of data communication;
- be familiar with various types of computer networks;
- have experience in understanding communication protocols at data link and network layer;
- be exposed to the TCP/IP protocol suite.
- Understand state-of-the-art in network protocols, architectures, and applications.

Assignment Questions
UNIT 1
INTRODUCTION
             
Q 1      What is data communication? What are its four fundamental characteristics? Explain the          components of communication system?
Q 2        Describe Simplex, half-duplex and full duplex methods of data flow
Q3       Explain the various ways how the data is represented?
Q 4      What is a physical topology? Describe the four basic topologies?
Q 5      Assume that 60 devices are connected in mesh topology? How many inks are needed? How many ports are needed for each device?
Q6       Explain how the networks are categorized?
Q7       Differentiate between LAN, MAN and WAN
Q 8        What are ISPS? List different types of ISP’S and functions in brief?
Q 9     What is protocol? Explain its key elements
Q 10     What do you mean by De facto and De jure Standards? Explain the various standards organizations?
Q 11    Explain the OSI reference model with a neat figure?
Q 12      Explain the duties of Physical layer and Data Link layer in OSI model
Q 13      List the responsibilities of the Network layer and Transport layer.]
Q 14      What is the difference between a port address, a logical address and a physical address
Q 15 Draw a neat figure of TCP/IP protocol suite and explain the various protocols
Q 16      What are the functions of IP, ARP, RARP, ICMP, and IGMP Protocols at network layer?
Q 17   Explain the role of each layer in OSI reference model also compare with TCP/IP model?
Q 18 Explain four levels of addresses used in internet employing TCP/IP protocols
UNIT 2
Physical Layer-1
Q 1      Define the following
a.       Analog and Digital signals
b.      Amplitude, Frequency, phase and Wavelength
c.       Bandwidth
d.      Bit Rate
Q 2       Explain the different causes for transmission impairments during the signal transmission through the media            or
          List and explain the three causes of transmission impairments
Q 3      What are the factors on which data rate depends in data communication?
Q 4      Define the Nyquist bit rate and Shannon capacity
Q 5      What are the propagation time and transmission time for a 10 Mbyte message if bandwidth of the network is 1 Mbps? Assume that the distance between sender and receiver is 12000 Km and light travels at 2.4*108 m/s?
Q 5        Calculate the Shannon channel capacity in following cases
i)                    Bandwidth 20 KHZ    SNRdb = 40
ii)                  Bandwidth 200 KHZ  SNRdb = 6
Q 6      Explain the following
i)                     Bandwidth
ii)                   Throughput
iii)                 Latency
iv)                 Transmission time
v)                   Jitter
vi)                 Queuing Time
Q 7      Define line coding Describe Unipolar NRZ, Polar, Bipolar encoding by applying on the information sequence “10011101”
Q 8      Represent the bit sequence “ 01101100 ” using Uniploar, Polar and Bipolar schemes
Q 9      Explain with a neat diagram the components of PCM encoder
Q 10    What are the various data transmission modes? Explain
OR
Write the definition of the following transmission mode with diagrams i) parallel ii)serial iii) synchronous iv)asynchronous transmissions.
Q 11   With the help of a neat diagram explain binary ASK, FSK and PSK system
Q 12      Suppose we wish to transmit at a rate of 64kbps over a 3 KHz telephone channel, what is the minimum SNR required to accomplish this,
Q 13   A 10Khz baseband channel is used by a digital transmission                  system. Ideal pulses are sent at the Nyquist rate and pulses take 16levels. What is the data rate of the system.
Q 14 What is line coding? Why is it necessary? Line code the stream 101011 using different schemes
Q 15 Using Shannon’s theorem , compute the maximum bit rate for a channel having bandwidth of 3100Hz and SNR of 20db.
Q 16 Calculate the number of levels (signals) required to transmit the maximum bit rate. What         is the baud rate?
Q 17 Sketch the signal waveforms when 00110101 is transmitted in the following signal codes i) NRZ-L ii) Manchester code iii)Bi Phase-M
Q 18 Give data rate formula suggested by Nyquist and Shannon. Low pass communication has BW of 1 Mhz. What is Shannon’s capacity of channel if SNR is 40db? What bit rate is attainable using 8-level pulses?
Q 19      What does Shannon capacity has to do with communication? Suppose that a low pass communication system has a 1MHZ bandwidth, what bit rate is attainable using 8 level pulses? What is the Shannon capacity of this channel if the SNR is 20db? 40db?
Q 20  Define Nyquist theorem. What is the bit rate required to digitize human voice assuming 8 bits per sample at a 3,000 Hz frequency?

UNIT 4
Data Link Layer -1
1.     What are the different types of errors?
2.     Explain with an example  block coding method for error detection and correction
3.     Explain the terms Hamming Distance and minimum hamming distance? What is the error detection and correction capability of coding scheme which has d min = 5
4.     What are the linear block codes? Explain with a neat figure of encoder and decoder the simple parity check code?
5.     What are the limitations of simple parity check code? Explain how it can be overcome in two dimensional parity check ?
6.     With a neat figure of encoder and decoder generate the hamming code
C ( 7,4) with d min = 3
7.     We need a data word of at least 7 bits calculate the values of k and n that satisfies this requirement
8.     With a neat figure explain the CRC encoder and decoder
Find the code word using CRC given data word “ 1010” and the generator “1011”                     or      What is CRC explain with a suitable example
9.      Explain the hardware implementation of CRC code
10.   Find the code word C(x) for the information  d(x) = x3 + 1 with the generator polynomial  t(x) = x3 + x +1
11.   What is internet checksum?  With an example list the steps taken by sender and receiver for error detection
12.  What is internet check sum? With an example of word “Computer” calculate the check sum at sender and receiver side
14.       Explain how CRC is used in detecting errors for the following polynomial, g(x) = x3          +x+1.  Consider the following information sequence 1101011011
            i)         Find the codeword corresponding to this sequence.
            ii)        If the codeword has an error in third bit, what does receiver obtain when it does its error checking?
15.       Suppose we want to transmit the message 11001001 and protect it from errors using the CRC polynomial X3 +1.
            i) Use polynomial long division to determine the message that should be transmitted.
            ii) Suppose the leftmost bit of the message is inverted due to noise on the transmission link. What is the result of the receivers CRC calculations? How does the receiver know that an error has occurred?
16.       Write short notes on polynomial codes.
17.       What is CRC? If the generating polynomial for CRC code is x3 +x +1 the message word is 11110000.Determine the check bits  and the encoded word.
18.       Find the code word for G(X)= x3 +x +1 and the information bits 
( 1,1,0,1,1,0)
19.       Give a brief description of the cyclic redundancy check in error detection.
20.       Explain internet checksum algorithm along with an example.
21.       Let G(p)= x3 +x+1 .Consider the information sequence 1001.
            i) Find the code word corresponding to the preceding information sequence.
            ii) Suppose that the codeword has a transmission error in the  first bit. What does that receiver obtain when it does its  error checking?
22.       Generate CRC code for the data word 110101010 using the divisor  10101.

UNIT 5
Data Link Layer -2
1.         Define flow control .Describe stop- and – wait flow control.          
2.         Name the types of HDLC frames give a brief description of each.
3.         Explain the conditions for stop-and-wait, Go-back-N and selective repeat protocols.            
4.         Explain in detail HDLC frame format.         
5.         What is the disadvantage of Go-Back-N ARQ protocol? Show how it is overcome in Selective Repeat protocol. What is the maximum window size of protocol?         
6.         Explain any two farming methods?  
7.         Define framing and two types of framing with examples.   
8.         Explain character oriented protocol. What is the problem encountered and explain how it is solved?        
9.         Explain           the concept of byte stuffing and un stuffing with example.
10        Explain bit oriented protocol. Explain bit stuffing and unstuffing with example.    
11.       Define flow control and explain its mechanism.        
12.       In what kind of channels Go-Back-N ARQ Protocol is inefficient?  What are the features of selective repeat? Explain the working of  selective repeat ARQ showing the details of send and receive windows and buffer states.
13.       Discuss the performance of selective repeat and the effect of error  rate.  
14.       With a neat diagram explain the working of sliding window  protocol.     
15.       With a neat diagram explain the working of stop-and-wait  protocol. What is the need for sequence numbers? What is the  reason for its inefficiency and how is it addressed in Go-Back-N ARQ protocol.      
16.       Discuss the effect of error rate on the efficiency of Stop-and-Wait,  Go-Back-N and Selective-Repeat protocols. Graphically compare  the 3 methods along with their equations of efficiency.           
17.       What is framing? How the beginning and end of the frame are  marked? What is byte stuffing? Illustrate with an example.    
18.       What is point-to-point protocol? With a neat schematic explain the  frame structure of PPP protocol.        
19.       Write a short notes HDLC Data link control.           
20.       Explain the salient features of i) Stop and wait protocol.  ii)Stop and wait ARQ protocol.
21.       Explain point-to-point protocol frame format. Also briefly describe Different transition phases of PPP in establishing a connection from Home PC to ISP.         Or
Explain frame format and transitional phases of point to point Protocol.  
22.       What is bit stuffing and unstuffing?  Apply bit stuffing to the sequence 0110111111111100
Apply unstuffing : 01111110000111011111011111011001111110



NOTES
UNIT – I
Introduction
1.1 DATA COMMUNICATIONS
Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter.
1. Delivery. The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 30 ms. If some of the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the result.
Components
A data communications system has five components:
1. Message. The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.
4. Transmission medium. The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
5. Protocol. A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.
Data Representation
Information today comes in different forms such as text, numbers, images, audio, and
video.
Text
In data communications, text is represented as a bit pattern, a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or character used in any language in the world.
Numbers
Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations.
Images
Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution. For example, an image can be divided into 1000 pixels or 10,000 pixels. In the second case, there is a better representation of the image (better resolution), but more memory is needed to store the image.
After an image is divided into pixels, each pixel is assigned a bit pattern. The size and the value of the pattern depend on the image. For an image made of only black- and-white dots (e.g., a chessboard), a 1-bit pattern is enough to represent a pixel.
There are several methods to represent color images. One method is called RGB, so called because each color is made of a combination of three primary colors: red, green, and blue.
Audio
Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal.
Video
Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex as shown in figure.
Simplex
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.
Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems.
The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.
Full-Duplex
In full-duplex made (also, called duplex), both stations can transmit and receive simultaneously. In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals travelling in both directions.
One common example of full-duplex communication is the telephone network. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.
NETWORKS
A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.
Distributed Processing
Most networks use distributed processing, in which a task is divided among multiple computers. Instead of one single large machine being responsible for all aspects of a process, separate computers (usually a personal computer or workstation) handle a subset.
Network Criteria
A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security.
Performance
Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software. Performance is often evaluated by two networking metrics: throughput and delay.
We often need more throughput and less delay.
Reliability
In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe.
Security
Network security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses.
Physical Structures
Type of Connection
A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections: point-to-point and multipoint.
Point-to-Point A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible.
Multipoint A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection.
Physical Topology
The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring.
Mesh In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n-1 nodes, node 2 must be connected to n-1 nodes, and finally node n must be connected to n-1 nodes. We need n(n-1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need
n(n - 1) / 2
duplex-mode links.
A mesh offers several advantages over other network topologies.
1. The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.
2. A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.
3. There is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages.
4. Point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution.
The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required.
1. Because every device must be connected to every other device, installation and reconnection are difficult.
2. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.
3. The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.
For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a backbone connecting the main computers of a hybrid network that can include several other topologies.
Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device.
Advantages:
1. A star topology is less expensive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub.
2. Other advantages include robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links.
Disadvantages:
1. One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.
2. Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies.
Bus Topology The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network.
Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps
Advantages:
Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies.
2. In a bus, redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone.
Disadvantages:
1. Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices.
2. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone.
3. a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions.
Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.
Advantages:
1. A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors.
2. To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices).
3. In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location.
Disadvantages:
1. Unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break
Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology as shown:
Network Models
Computer networks are created by different entities. Standards are needed so that these heterogeneous networks can communicate with one another. The two best-known standards are the OSI model and the Internet model. The OSI (Open Systems Interconnection) model defines a seven-layer network; the Internet model defines a five-layer network.
Categories of Networks
Local Area Network
A local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus. Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two PCs and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers.
LANs are designed to allow resources to be shared between personal computers or workstations. The resources to be shared can include hardware (e.g., a printer), software (e.g., an application program), or data. A common example of a LAN, found in many business environments, links a workgroup of task-related computers, for example, engineering workstations or accounting PCs. One of the computers may be given a large capacity disk drive and may become a server to clients. Software can be stored on this central server and used as needed by the whole group.
In addition to size, LANs are distinguished from other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are bus, ring, and star.
Wide Area Network
A wide area network (WAN) provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or even the whole world.
A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the Internet. We normally refer to the first as a switched WAN and to the second as a point-to-point WAN. The switched WAN connects the end systems, which usually comprise a router (internet-working connecting device) that connects to another LAN or WAN. The point-to-point
WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (ISP). This type of WAN is often used to provide Internet access.
Metropolitan Area Networks
A metropolitan area network (MAN) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example of a MAN is the part of the telephone company network that can provide a high-speed DSL line to the customer.
Interconnection of Networks: Internetwork
Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are connected to one another. When two or more networks are connected, they become an internetwork, or internet.
As an example, assume that an organization has two offices in separate cities. One established office has a bus topology LAN; the other office has a star topology LAN. The president lives in some other city and needs to have control over the company from her home. To create a backbone WAN for connecting these three entities (two LANs and the president's computer), a switched WAN (operated by a service provider such as a telecom company) has been leased. To connect the LANs to this switched WAN, however, three point-to-point WANs are required. These point-to-point WANs can be a high-speed DSL line offered by a telephone company or a cable modem line offered by a cable TV provider as shown:
1.3 THE INTERNET
The Internet is a structured, organized system. We begin with a brief history of the Internet.
A Brief History
n the mid-1960s, mainframe computers in research organizations were stand-alone devices. Computers from different manufacturers were unable to communicate with one another. The Advanced Research Projects Agency (ARPA) in the Department of Defense (DoD) was interested in finding a way to connect computers so that the researchers they funded could share their findings, thereby reducing costs and eliminating duplication of effort.
In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented its ideas for ARPANET, a small network of connected computers. The idea was that each host computer (not necessarily from the same manufacturer) would be attached to a specialized computer, called an interface message processor (IMP). The IMPs, in turn, would be connected to one another. Each IMP had to be able to communicate with other IMPs as well as with its own attached host.
By 1969, ARPANET was a reality. Four nodes, at the University of California at Los Angeles (UCLA), the University of California at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the University of Utah, were connected via the IMPs to form a network. Software called the Network Control Protocol (NCP) provided communication between the hosts.
In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group, collaborated on what they called the Internetting Project. Cerf and Kahn's land-mark 1973 paper outlined the protocols to achieve end-to-end delivery of packets. This paper on Transmission Control Protocol (TCP) included concepts such as encapsulation, the datagram, and the functions of a gateway.
Shortly thereafter, authorities made a decision to split TCP into two protocols: Transmission Control Protocol (TCP) and Internetworking Protocol (IP). IP would handle datagram routing while TCP would be responsible for higher-level functions such as segmentation, reassembly, and error detection. The internetworking protocol became known as TCP/IP.
The Internet Today
The Internet today is not a simple hierarchical structure. It is made up of many wide- and local-area networks joined by connecting devices and switching stations. It is difficult to give an accurate representation of the Internet because it is continually changing--new networks are being added, existing networks are adding addresses, and networks of defunct companies are being removed. Today most end users who want Internet connection use the services of Internet service providers (ISPs). There are international service providers, national service providers, regional service providers, and local service providers. The Internet today is run by private companies, not the government. The figure shows a conceptual (not geographic) view of the Internet.
International Internet Service Providers
At the top of the hierarchy are the international service providers that connect nations together.
National Internet Service Providers
The national Internet service providers are backbone networks created and maintained by specialized companies. To provide connectivity between the end users, these backbone networks are connected by complex switching stations (normally run by a third party) called network access points (NAPs). Some national ISP networks are also connected to one another by private switching stations called peering points. These normally operate at a high data rate.
Regional Internet Service Providers
Regional intemet service providers or regional ISPs are smaller ISPs that are connected to one or more national ISPs. They are at the third level of the hierarchy with a smaller data rate.
Local Internet Service Providers
Local Internet service providers provide direct service to the end users. The local ISPs can be connected to regional ISPs or directly to national ISPs. Most end users are connected to the local ISPs.

1.4 PROTOCOLS AND STANDARDS
Protocols
In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. However, two entities cannot simply send bit streams to each other and expect to be understood. For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines what is communicated, how k is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics, and timing.
•Syntax. The term syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.
•Semantics. The word semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted, and what action is to be taken based on that interpretation?
•Timing. The term timing refers to two characteristics: when data should be sent and how fast they can be sent. For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost.
Standards
Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes. Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of interconnectivity necessary in today's marketplace and in international communications. Data communication standards fall into two categories: de facto (meaning
"by fact" or "by convention") and de jure (meaning "by law" or "by regulation").
•De facto. Standards that have not been approved by an organized body but have been adopted as standards through widespread use are de facto standards. De facto standards are often established originally by manufacturers who seek to define the functionality of a new product or technology.
•De jure. Those standards that have been legislated by an officially recognized body are de jure standards.
Standards Organizations
Standards are developed through the cooperation of standards creation committees, forums, and government regulatory agencies.
Standards Creation Committees
While many organizations are dedicated to the establishment of standards, data telecommunications in North America rely primarily on those published by the following:
•International Organization for Standardization (ISO). The ISO is a multinational body whose membership is drawn mainly from the standards creation committees of various governments throughout the world. The ISO is active in developing cooperation in the realms of scientific, technological, and economic activity.
•International Telecommunication Elnion Telecommunication Standards Sector (ITEl-T). By the early 1970s, a number of countries were defining national standards for telecommunications, but there was still little international compatibility. The United Nations responded by forming, as part of its International Telecommunication Union (ITU), a committee, the Consultative Committee for International Telegraphy and Telephony (CCITT). This committee was devoted to the research and establishment of standards for telecommunications in general and for phone and data systems in particular. On March 1, 1993, the name
of this committee was changed to the International Telecommunication Union - Telecommunication Standards Sector (ITU-T).
•American National Standards Institute (ANSI). Despite its name, the American National Standards Institute is a completely private, nonprofit corporation not affiliated with the U.S. federal government. However, all ANSI activities are undertaken with the welfare of the United States and its citizens occupying primary importance.
• Institute of Electrical and Electronics Engineers (IEEE). The Institute of Electrical and Electronics Engineers is the largest professional engineering society in the world. International in scope, it aims to advance theory, creativity, and product quality in the fields of electrical engineering, electronics, and radio as well as in all related branches of engineering. As one of its goals, the IEEE oversees the development and adoption of international standards for computing and communications.
• Electronic Industries Association (EIA). Aligned with ANSI, the Electronic Industries Association is a nonprofit organization devoted to the promotion of electronics manufacturing concerns. Its activities include public awareness education and lobbying efforts in addition to standards development. In the field of information technology, the EIA has made significant contributions by defining physical connection interfaces and electronic signaling specifications for data communication.
Forums
Telecommunications technology development is moving faster than the ability of stan-dards committees to ratify standards. Standards committees are procedural bodies and by nature slow-moving. To accommodate the need for working models and agreements and to facilitate the standardization process, many special-interest groups have developed forums made up of representatives from interested corporations. The forums
work with universities and users to test, evaluate, and standardize new technologies. By concentrating their efforts on a particular technology, the forums are able to speed acceptance and use of those technologies in the telecommunications community. The forums present their conclusions to the standards bodies.
Regulatory Agencies
All communications technology is subject to regulation by government agencies such as the Federal Communications Commission (FCC) in the United States. The purpose of these agencies is to protect the public interest by regulating radio, television, and wire/cable communications.
Internet Standards
An Internet standard is a thoroughly tested specification that is useful to and adhered to by those who work with the Internet. It is a formalized regulation that must be followed. There is a strict procedure by which a specification attains Internet standard status. A specification begins as an Internet draft. An Internet draft is a working document (a work in progress) with no official status and a 6-month lifetime. Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC). Each RFC is edited, assigned a number, and made available to all interested parties. RFCs go through maturity levels and are categorized according to their requirement level.
Network Models
2.1 LAYERED TASKS
As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. The figure shows the steps in this task.
Sender, Receiver, and Carrier
In Figure 2.1 we have a sender, a receiver, and a carrier that transports the letter. There is a hierarchy of tasks.
At the Sender Site
The activities that take place at the sender site, in order, are:
Higher layer. The sender writes the letter, inserts the letter in an envelope, writes the sender and receiver addresses, and drops the letter in a mailbox.
 Middle layer. The letter is picked up by a letter carrier and delivered to the post office.
 Lower layer. The letter is sorted at the post office; a carder transports the letter
On the Way
The letter is then on its way to the recipient. On the way to the recipient's local post office, the letter may actually go through a central office. In addition, it may be trans- ported by truck, train, airplane, boat, or a combination of these.
At the Receiver Site
 Lower layer. The carrier transports the letter to the post office.
 Middle layer. The letter is sorted and delivered to the recipient's mailbox.
 Higher layer. The receiver picks up the letter, opens the envelope, and reads it.
Hierarchy
According to our analysis, there are three different activities at the sender site and another three activities at the receiver site. The task of transporting the letter between the sender and the receiver is done by the carrier. Something that is not obvious immediately is that the tasks must be done in the order given in the hierarchy. At the sender site, the letter must be written and dropped in the mailbox before being picked up by the letter carrier and delivered to the post office. At the receiver site, the letter must be dropped in the recipient mailbox before being picked up and read by the recipient.
Services
Each layer at the sending site uses the services of the layer immediately below it. The sender at the higher layer uses the services of the middle layer. The middle layer uses the services of the lower layer. The lower layer uses the services of the carrier. The layered model that dominated data communications and networking literature before 1990 was the Open Systems Interconnection (OSI) model.

THE OSI MODEL
The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network.
Layered Architecture
The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2), network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and application (layer 7). The following figure shows the layers involved when a message is sent from device A to device B. As the message travels from A to B, it may pass through many intermediate nodes. These intermediate nodes usually involve only the first three layers of the OSI model.
Within a single machine, each layer calls upon the services of the layer just below it. Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4. Between machines, layer x on one machine communicates with layer x on another machine. This communication is governed by an agreed-upon series of rules and conventions called protocols. The processes on each machine that communicate at a given layer are called peer-to-peer processes. Communication between machines is therefore a peer-to-peer process using the protocols appropriate to a given layer.
Peer-to-Peer Processes
At the physical layer, communication is direct: In the figure below, device A sends a stream of bits to device B (through intermediate nodes). At the higher layers, however, communication must move down through the layers on device A, over to device B, and then back up through the layers. Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it.
At layer 1 the entire package is converted to a form that can be transmitted to the receiving device. At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. For example, layer 2 removes the data meant for it, then passes the rest to layer 3. Layer 3 then removes the data meant for it and passes the rest to layer 4, and so on.

Interfaces Between Layers
The passing of the data and network information down through the layers of the sending device and back up through the layers of the receiving device is made possible by an interface between each pair of adjacent layers. Each interface defines the information and services a layer must provide for the layer above it. Well-defined interfaces and layer functions provide modularity to a network. As long as a layer provides the
expected services to the layer above it, the specific implementation of its functions can be modified or replaced without requiring changes to the surrounding layers.
Organization of the Layers
The seven layers can be thought of as belonging to three subgroups. Layers 1, 2, and 3 - physical, data link, and network - are the network support layers; they deal with the physical aspects of moving data from one device to another (such as electrical specifications, physical connections, physical addressing, and transport timing and reliability). Layers 5, 6, and 7 - session, presentation, and application - can be thought of as the user support layers; they allow interoperability among unrelated software systems. Layer 4, the transport layer, links the two subgroups and ensures that what the lower layers have transmitted is in a form that the upper layers can use. The upper OSI layers are almost always implemented in software; lower layers are a combination of hardware and software, except for the physical layer, which is mostly hardware.
The following figure gives an overall view of the OSI layers, D7 means the data unit at layer 7, D6 means the data unit at layer 6, and so on. The process starts at layer 7 (the application layer), then moves from layer to layer in descending, sequential order. At each layer, a header, or possibly a trailer, can be added to the data unit. Commonly, the trailer is added only at layer 2. When the formatted data unit passes through the physical layer (layer 1), it is changed into an electromagnetic signal and transported along a physical link.
Upon reaching its destination, the signal passes into layer 1 and is transformed back into digital form. The data units then move back up through the OSI layers. As each block of data reaches the next higher layer, the headers and trailers attached to it at the corresponding sending layer are removed, and actions appropriate to that layer are taken. By the time it reaches layer 7, the message is again in a form appropriate to the application and is made available to the recipient.
Encapsulation
Figure 2.3 reveals another aspect of data communications in the OSI model: encapsulation. A packet (header and data) at level 7 is encapsulated in a packet at level 6. The whole packet at level 6 is encapsulated in a packet at level 5, and so on. In other words, the data portion of a packet at level N - 1 carries the whole packet
(data and header and maybe trailer) from level N. The concept is called encapsulation; level N- 1 is not aware of which part of the encapsulated packet is data and which part is the header or trailer. For level N- 1, the whole packet coming from level N is treated as one integral unit.

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