UNIT -I

1.    INTRODUCTION

This chapter provides an introduction to Computer networks and covers fundamental topics like data, information to the definition of communication and computer networks.

The main objective of data communication and networking is to enable seamless exchange of data between any two points in the world.

This exchange of data takes place over a computer network.

Data refers to the raw facts that are collected while information refers to processed data that enables us to take decisions.

Ex. When result of a particular test is declared it contains data of all students, when you find the marks you have scored you have the information that lets you know whether you have passed or failed.

The word data refers to any information which is presented in a form that is agreed and accepted upon by creators and users.

1.1INTRODUCTION TO DATA COMMUNICATIONS

Data communications refers to 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 30ms. If some of the packets arrive with 30ms delay and others with 40ms delay, an uneven quality in the video is the result.

COMPONENTS OF A DATA COMMUNICATIONS SYSTEM

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

It is the path by which the message travels from sender to receiver. It can be wired or wireless and many subtypes in both.

5. Protocol

It is an agreed upon set or rules used by the sender and receiver to communicate data.

A protocol is a set of rules that governs data communication.

A Protocol is a necessity in data communications without which the communicating entities are like two persons trying to talk to each other in a different language without know the other language.

DATA REPRESENTATION

Data is collection of raw facts which is processed to deduce information. There may be different forms in which data may be represented. Some of the forms of data used in communications are as follows:

1. Text :Text includes combination of alphabets in small case as well as upper case. It is stored as a pattern of bits. Prevalent encoding system : ASCII, Unicode .

2. Numbers:

Numbers include combination of digits from 0 to 9. It is stored as a pattern of bits. Prevalent encoding system : ASCII, Unicode.

3. Images:

An image is worth a thousand words‖ is a very famous saying. In computers images are digitally stored.

A Pixel is the smallest element of an image. To put it in simple terms, a picture or image is a matrix of pixel elements.

The pixels are represented in the form of bits. Depending upon the type of image (black n white or color) each pixel would require different number of bits to represent the value of a pixel.

The size of an image depends upon the number of pixels (also called resolution) and the bit pattern used to indicate the value of each pixel.

Example: if an image is purely black and white (two color) each pixel can be represented by a value either 0 or 1, so an image made up of 10 x 10 pixel elements would require only 100 bits in memory to be stored.

On the other hand an image that includes gray may require 2 bits to represent every pixel value (00 - black, 01 – dark gray, 10– light gray, 11 –white). So the same 10 x 10 pixel image would now require 200 bits of memory to be stored. Commonly used Image formats : jpg, png, bmp, etc

4. Audio:

Data can also be in the form of sound which can be recorded and broadcasted.

Example: What we hear on the radio is a source of data or information.

Audio data is continuous, not discrete.

5. Video:

Video refers to broadcasting of data in form of picture or movie.

DATA FLOW

The data can flow between the two devices in the following ways.

1. Simplex

2. Half Duplex

3. Full Duplex

1. Simplex

Ø  In Simplex, communication is unidirectional

Ø  Only one of the devices sends the data and the other one only receives the data.

Ø  Example: in the above diagram: a cpu send data while a monitor only receives data.

2. Half Duplex

Ø  In half duplex both the stations can transmit as well as receive but not at the same time.

Ø  When one device is sending other can only receive and vice-versa (as shown in figure above.)

Ø  Example: A walkie-talkie.

3. Full Duplex

Ø  In Full duplex mode, both stations can transmit and receive at the same time.

Ø  Example: mobile phones

1.2. A COMMUNICATIONS MODEL

The fundamental purpose of a communications system is the exchange of data between two parties. Figure 1.2b presents one particular example, which is communication between a workstation and a server over a public telephone network.

Another example is the exchange of voice signals between two telephones over the same network. The key elements of the model are as follows:

Source: This device generates the data to be transmitted; examples are telephones and personal computers.

Figure: Simplified Communications Model

Transmitter: Usually, the data generated by a source system are not transmitted directly in the form in which they were generated. Rather, a transmitter transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system. For example, a modem takes a digital bit stream from an attached device such as a personal computer and transforms that bit stream into an analog signal that can be handled by the telephone network.

Transmission system: This can be a single transmission line or a complex network connecting source and destination.

Receiver: The receiver accepts the signal from the transmission system and converts it into a form that can be handled by the destination device. For example, a modem will accept an analog signal coming from a network or transmission line and convert it into a digital bit stream.

 Destination: Takes the incoming data from the receiver.

The communications tasks are as follows:

1.3 DATA COMMUNICATIONS NETWORKING

Computer Network

            Computer Networks are used for data communications

Definition: A computer network can be defined as a collection of nodes. A node can be any device capable of transmitting or receiving data. The communicating nodes have to be connected by communication links.

A Compute network should ensure

reliability of the data communication process, should see

security of the data

performance by achieving higher throughput and smaller delay times

Categories of Network

Networks are categorized on the basis of their size. The three basic categories of computer networks are:

A. Local Area Networks (LAN) is usually limited to a few kilometers of area. It may be privately owned and could be a network inside an office on one of the floor of a building or a LAN could be a network consisting of the computers in a entire building.

B. Wide Area Network (WAN) is made of all the networks in a (geographically) large area. The network in the entire state of Maharashtra could be a WAN

C. Metropolitan Area Network (MAN) is of size between LAN & WAN. It is larger than LAN but smaller than WAN. It may comprise the entire network in a city like Mumbai.

PROTOCOL

A Protocol is one of the components of a data communications system. Without protocol communication cannot occur. The sending device cannot just send the data and expect the receiving device to receive and further interpret it correctly.

When the sender sends a message it may consist of text, number, images, etc. which are converted into bits and grouped into blocks to be transmitted and often certain additional information called control information is also added to help the receiver interpret the data.

For successful communication to occur, the sender and receiver must agree upon certain rules called protocol.

A Protocol is defined as a set of rules that governs data communications.

A protocol defines what is to be communicated, how it is to be communicated and when it is to be communicated.

NETWORK TOPOLOGY

Networks can be classified by their topology, which is the basic geometric arrangement of the network. Different types of network configurations exist for network designers to choose from. Communications channels can be connected in different arrangements using several different topologies. This arrangement allows users to exchange information and share resources (software and hardware).

Four basic types of network configurations are star, bus, ring, hierarchical and mesh. Ring, bus, and star topologies are commonly used in LANs and BNs. Star and mesh topologies are commonly used in MANs and WANS. The networks are usually built using a combination of several different topologies.

• Bus
• Ring
• Hierarchical
• Mesh

Star Topology

A star topology is one in which a central unit provides a link through which a group of smaller computers and devices is connected. The central computer is commonly called a host computer. A host computer is usually a large computer such as a minicomputer or a mainframe. A file server is a large storage device that provides volumes of data and programs to the other units in the network.

In the star network, all interactions between different computers in the network travel through the host computer. The central unit will poll each to decide whether a unit has a message to send. If so, the central computer will carry the message to the receiving computer.

Star networks represent a very popular form of configuration for time-sharing systems in which a central computer makes available resources and databases for several "client" computers to share. As such, the star network is appropriate for systems that demand centralized control. The disadvantage of the star network is that a processing problem in the central computer can be paralyzing to the entire system.

In a star network, the central unit may be a host computer or a file server. The host computer is a large centralized computer, usually a minicomputer or a mainframe. In contrast, the file server is a large-capacity hard-disk storage device. It stores data and programs files shared by the users on the network. Also, called a network server.

Bus Topology

In a bus configuration, each computer in the network is responsible for carrying out its own communications without the aid of a central unit. A common communications cable (the bus) connects all of the computers in the network. As data travels along the path of the cable, each unit performs a query to determine if it is the intended recipient of the message. The bus network is less expensive than the star configuration and is thus widely in use for systems that connect only a few microcomputers and systems that do not emphasize the sharing of common resources.

The problem in a computer on a bus topology does not frustrate the operation of the network, but a crack in the central cable will stop the whole network. Bus topology is popular because many computers can be connected to a single central cable. In a bus topology, each end user computer in the network handles its own communications control. There is no host computer or file server. As the information passes along the bus, it is examined by each terminal to see if the data is for it.

Ring Network

A ring configuration features a network in which each computer is connected to the next two other computers in a closed loop. Like the bus network, no single central computer exists in the ring configuration. Messages are simply transferred from one computer to the next until they arrive at their intended destinations. Each computer on the ring topology has a particular address. As the messages pass around the ring, the computers validate the address. If the message is not addressed to it, the node transmits the message to the next computer on the ring.

This type of network is commonly used in systems that connect widely dispersed mainframe computers. A ring network allows organizations to engage in distributed data processing system in which computers can share certain resources with other units while maintaining control over their own processing functions. However, a failure in any of the linked computers can greatly affect the entire network.

The ring arrangement is the least frequently used with microcomputers. However, as stated above, it often is used to link mainframes over wide geographical areas to build distributed data processing system. The loss of a mainframe usually does not restrain the operation of the network, but a cable problem will stop the network altogether.

Hierarchical Topology

A hierarchical network (or a tree network) resembles a star network in that several computers are connected to a central host computer (usually a mainframe). However, these "client" computers also serve as host computers to next level units. Thus, the hierarchical network can theoretically be compared to a standard organizational chart or a large corporation. Typically, the host computer at the top of the hierarchy is a mainframe computer. Lower levels in the hierarchy could consist of minicomputers and microcomputers. It should be noted that a system can sometimes have characteristics of more than one of the above topologies.

This topology is effective in a centralized corporation. For example, different divisions within a corporation may have individual microcomputers connected to divisional minicomputers. The minicomputers in turn may be connected to the corporation's mainframe, which contains data and programs.

Mesh Topology

This is a net-like communications network in which there are at least two pathways to each node. In a mesh topology, computers are connected to each other by point-to-point circuits. In the topology, one or more computers usually become switching centers, interlinking computers with others.

Although a computer or cable is lost, if there are other possible routes through the network, the damage of one or several cables or computers may not have vital impact except the involved computers. However, if there are only few cables in the network, the loss of even one cable or device may damage the network seriously.

1.4.COMPUTER COMMUNICATION ARCHITECTURE

1.4.1        Need for a Protocol Architecture

1.4.2        TCP/IP Protocol Architecture

1.4.3        OSI Model

1.4.1  Need for a Protocol Architecture

            Typical tasks to be performed are as follow:

1. The source system must either activate the direct data communication path or inform the communication network of the identity of the desired destination system.

2. The source system must ascertain that the destination system is prepared to receive data.

3. The file transfer application on the source system must ascertain that the file management program on the destination system is prepared to accept and store the file for this particular user.

4. If the file formats used on the two systems are different, one or the other system must perform a format translation function.

The peer layers communicate by means of formatted blocks of data that obey a set of rules or conventions known as a protocol. The key features of a protocol are as follows:

• Syntax: Concerns the format of the data blocks

• Semantics: Includes control information for coordination and error handling

• Timing:Includes speed matching and sequencing

1.4.2  TCP/IP Protocol Architecture

            In general terms, communications can be said to involve three agents: applications, computers, and networks. Examples of applications include file transfer and electronic mail. The applications that we are concerned with here are distributed applications that involve the exchange of data between two computer systems.

These applications, and others, execute on computers that can often support multiple simultaneous applications. Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another. Thus, the transfer of data from one application to another involves first getting the data to the computer in which the application resides and then getting the data to the intended application within the computer. With these concepts in mind, we can organize the communication task into five relatively independent layers.

Ø  Physical layer

Ø  Network access layer

Ø  Internet layer

Ø  Host-to-host, or transport layer

Ø  Application layer

The physical layer covers the physical interface between a data transmission device (e.g., workstation, computer) and a transmission medium or network. This layer is concerned with specifying the characteristics of the transmission medium, the nature of the signals, the data rate, and related matters.

The network access layer is concerned with the exchange of data between an end system (server, workstation, etc.) and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network.

The network access layer is concerned with access to and routing data across a network for two end systems attached to the same network. In those cases where two devices are attached to different networks, procedures are needed to allow data to traverse multiple interconnected networks. This is the function of the internet layer. The Internet Protocol (IP) is used at this layer to provide the routing function across multiple networks. This protocol is implemented not only in the end systems but also in routers. A router is a processor that connects two networks and whose primary function is to relay data from one network to the other on its route from the

source to the destination end system.

Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. As we shall see, the mechanisms for providing reliability are essentially independent of the nature of the applications.Thus, it makes sense to collect those mechanisms in a common layer shared by all applications; this is referred to as the host-to-host layer, or transport layer. The Transmission Control Protocol (TCP) is the most commonly used protocol to provide this functionality.

Finally, the application layer contains the logic needed to support the various

user applications. For each different type of application, such as file transfer, a separate module is needed that is peculiar to that application.

Operation of TCP and IP

Figure 2.1 indicates how these protocols are configured for communications. To make clear that the total communications facility may consist of multiple networks, the constituent networks are usually referred to as subnetworks. Some sort of network access protocol, such as the Ethernet logic, is used to connect a computer to a subnetwork. This protocol enables the host to send data across the subnetwork to another host or, if the target host is on another subnetwork, to a router that will forward the data. IP is implemented in all of the end systems and the routers. It acts as

a relay to move a block of data from one host, through one or more routers, to another host. TCP is implemented only in the end systems; it keeps track of the blocks of data to assure that all are delivered reliably to the appropriate application.

Each host on a subnetwork must have a unique global internet address; this allows the data to be delivered to the proper host. Each process with a host must have an address that is unique within the host; this allows the host-to-host protocol (TCP) to deliver data to the proper process. These latter addresses are known as ports.

Let us trace a simple operation. Suppose that a process, associated with port 3 at host A, wishes to send a message to another process, associated with port 2 at host B. The process at A hands the message down to TCP with instructions to send it to host B, port 2.TCP hands the message down to IP with instructions to send it to host B. Note that IP need not be told the identity of the destination port. All it needs to know is that the data are intended for host B. Next, IP hands the message down to the network access layer (e.g., Ethernet logic) with instructions to send it to router J (the first hop on the way to B).

To control this operation, control information as well as user data must be transmitted, as suggested in Figure 2.2. Let us say that the sending process generates a block of data and passes this to TCP. TCP may break this block into smaller pieces to make it more manageable.To each of these pieces,TCP appends control information known as the TCP header, forming a TCP segment. The control information is to be used by the peer TCP protocol entity at host B. Examples of items in this header include:

• Destination port: When the TCP entity at B receives the segment, it must know to whom the data are to be delivered.

• Sequence number: TCP numbers the segments that it sends to a particular destination port sequentially, so that if they arrive out of order, the TCP entity at B can reorder them.

·                  Checksum: The sending TCP includes a code that is a function of the contents of the remainder of the segment. The receiving TCP performs the same calculation and compares the result with the incoming code.A discrepancy results if there has been some error in transmission.

Next, TCP hands each segment over to IP, with instructions to transmit it to B. These segments must be transmitted across one or more subnetworks and relayed through one or more intermediate routers. This operation, too, requires the use of control information. Thus IP appends a header of control information to each segment to form an IP datagram. An example of an item stored in the IP header is the destination host address (in this example, B).

Finally, each IP datagram is presented to the network access layer for transmission across the first subnetwork in its journey to the destination. The network access layer appends its own header, creating a packet, or frame.The packet is transmitted across the subnetwork to router J. The packet header contains the information that the subnetwork needs to transfer the data across the subnetwork.

Examples of items that may be contained in this header include:

• Destination subnetwork address:The subnetwork must know to which attached device the packet is to be delivered.

• Facilities requests:The network access protocol might request the use of certain subnetwork facilities, such as priority.

At router J, the packet header is stripped off and the IP header examined. On the basis of the destination address information in the IP header, the IP module in the router directs the datagram out across subnetwork 2 to B. To do this, the datagram is again augmented with a network access header.

When the data are received at B, the reverse process occurs. At each layer, the corresponding header is removed, and the remainder is passed on to the next higher layer, until the original user data are delivered to the destination process.

TCP and UDP

For most applications running as part of the TCP/IP protocol architecture, the transport layer protocol is TCP. TCP provides a reliable connection for the transfer of data between applications. A connection is simply a temporary logical association between two entities in different systems. A logical connection refers to a given pair of port values. For the duration of the connection each entity keeps track of TCP segments coming and going to the other entity, in order to regulate the flow of segments and to recover from lost or damaged segments.

Figure 2.3a shows the header format for TCP, which is a minimum of 20 octets, or 160 bits. The Source Port and Destination Port fields identify the applications at the source and destination systems that are using this connection. The Sequence Number, Acknowledgment Number, and Window fields provide flow control and error control. The checksum is a 16-bit frame check sequence used to detect errors in the TCP segment. Chapter 20 provides more details.

In addition to TCP, there is one other transport-level protocol that is in common use as part of the TCP/IP protocol suite: the User Datagram Protocol (UDP). UDP does not guarantee delivery, preservation of sequence, or protection against duplication. UDP enables a procedure to send messages to other procedures with a minimum of protocol mechanism. Some transaction-oriented applications make use of UDP; one example is SNMP (Simple Network Management Protocol), the standard network management protocol for TCP/IP networks. Because it is connectionless, UDP has very little to do. Essentially, it adds a port addressing capability to IP. This is best seen by examining the UDP header, shown in Figure 2.3b. UDP also includes a checksum to verify that no error occurs in the data; the use of the checksum is optional.

1.4.3        OSI MODEL

The Open Systems Interconnection (OSI) reference model was developed by the International Organization for Standardization (ISO)2 as a model for a computer protocol architecture and as a framework for developing protocol standards.

The OSI model consists of seven layers:

Ø  Application

Ø  Presentation

Ø  Session

Ø  Transport

Ø  Network

Ø  Data link

Ø  Physical

Figure 2.6 illustrates the OSI model and provides a brief definition of the functions performed at each layer. The intent of the OSI model is that protocols be developed to perform the functions of each layer.

The designers of OSI assumed that this model and the protocols developed within this model would come to dominate computer communications, eventually replacing proprietary protocol implementations and rival multivendor models such as TCP/IP.This has not happened.Although many useful protocols have been developed in the context of OSI, the overall seven-layer model has not flourished. Instead, the TCP/IP architecture has come to dominate.There are a number of reasons for this outcome. Perhaps the most important is that the key TCP/IP protocols were mature and well tested at a time when similar OSI protocols were in the development stage. When businesses began to recognize the need for interoperability across networks, only TCP/IP was available and ready to go.Another reason is that the OSI model is unnecessarily complex, with seven layers to accomplish what TCP/IP does with fewer layers.

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. The physical layer is also concerned with the following:

 Physical characteristics of interfaces and medium: The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium.

Representation of bits: The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how Os and I s are changed to signals).

 Data rate: The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long

it lasts.

Synchronization of bits: The sender and receiver not only must use the same bit rate but also

must be synchronized at the bit level. In other words, the sender and the receiver clocks must

be synchronized.

Line configuration: The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a

multipoint configuration, a link is shared among several devices.

Physical topology: The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies).

Transmission mode: The physical layer also defines the direction of transmission between two

devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the

other can only receive. The simplex mode is a one-way communication. In the half-duplex mode,

two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex)

mode, two devices can send and receive at the same time.

The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). Other responsibilities of the data link layer include the following:

Framing: The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Physical addressing: If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

Flow control: If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

Access control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination. Other responsibilities of the network layer include the following:

Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

Routing. When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. Other responsibilities of the transport layer include the following:

Service-point addressing: Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

Segmentation and reassembly: A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

Connection control: The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

Flow control: Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

Error control: Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.

The services provided by the first three layers (physical, data link, and network) are not  ufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems. The session layer is responsible for dialog control and synchronization. Specific responsibilities of the session layer include the following:

Dialog control: The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half duplex (one way at a time) or full-duplex (two ways at a time) mode.

Synchronization: The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is  received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent.

The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following:

Translation: The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

Encryption: To carry sensitive information, a system must be able to ensure privacy. Encryption

means that the sender transforms the original information another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

Compression: Data compression reduces the number of bits contained in the information. Data

compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

TCP/IP and OSI Comparison

Figure 2.7 illustrates the layers of the TCP/IP and OSI architectures, showing roughly the correspondence in functionality between the two.

1.5  STANDARD DATA TRANSMISSION

1.5.1 Concepts and Terminology

Data transmission refers to computer-mediated communication among system users, and also with other systems. Data might be transmitted by transferring a data file from one user to another.

In a designed information system, data transmission by file transfer may be largely automatic, accomplished with little user involvement.

Coding

In all digital communications channels, computers transmit data and information in forms of binary codes. Both sender and receiver of the data and information should have a standard for both to understand them.

A coding scheme for communications is a binary system, as in the computer systems. The system consists of groups of bits (0 or 1) that represent characters. In computer systems, a byte is a group of bits and represents a character. In data communications, a byte is the same, but some codes use different number of bits such as 5, 7, 8 or 9.

Two predominant coding schemes ASCII and EBCDIC. ASCII refers America Standard Code for Information Interchange. It is the most popular code for data communications and is the standard code on most communications terminals. Among two types of ASCII, a 7-bit code can make 128 character combinations, and an 8-bit can do 256 combinations.

EBCDIC refers Extended Binary Coded Decimal Interchange Code. It is IBM's standard information code, and has 8 bits for a character.

Bandwidth

Each types of communications media has different transmission speed. The bandwidth is a measure of the transmission rate of communications channels.

§  Baseband: Digital signals are commonly called baseband signals. Baseband is a communications technique in which digital signals are placed onto the transmission line without change in modulation. It transmits up to a couple of miles, and does not require the complex modems. Typical Token Ring and Ethernet use baseband signals.

§  Broadband: Broadband is a technique for transmitting large amounts of data, voice and video over long distances simultaneously by modulating each signal onto a different frequency. Using the FDM (Frequency division multiplexing) technique, several streams of data can be transmitted simultaneously.
Broadband is the bandwidth used for direct communication between very high-speed computers (e.g., large mainframe computers). This bandwidth includes microwave, satellite, coaxial cable, and fiber-optic media.

Transmission Mode

Two forms of data movement exist: parallel data transmission and serial data transmission.

§  Parallel Transmission: Parallel data transmission involves the concurrent flow of bits of data through separate communications lines. This pattern resembles the flow of automobile traffic on a multilane highway. Internal transfer of binary data in a computer uses a parallel mode. If the computer uses a 32-bit internal structure, all the 32 bits of data are transferred simultaneously on 32 lane connections. \ Parallel data transmission is commonly used for interactions between a computer and its printing unit. The printer usually located close to the computer, because parallel cables need many wires and may not work stably in long distance.

§  Serial Data Transmission: Most data transmitted over telephone lines use a serial pattern. That is, each individual bit of information travels along its own communications path; the bits flow in a continuous stream along the communications channel. This pattern is analogous to the flow of traffic down a one-lane residential street.
Serial transmission is typically slower than parallel transmission, because data are sent sequentially in a bit-by-bit fashion.

Direction of Data Transmission

Besides the previously mentioned ways in which data may travel, there are three directional modes of travel in data transmission.

§  Simplex Communication: Simplex communication is a mode in which data only flows in one direction. Because most modern communications require a two-way interchange of data and information, this mode of transmission is not as popular as it once was. However, one current usage of simplex communications in business involves certain point-of-sale terminals in which sales data is entered without a corresponding reply.

§  Half-duplex Communication: Half-duplex communication adds an ability for a two-way flow of data between computer terminals. In this directional mode, data travels in two directions, but not simultaneously. Data can only move in one direction when data is not being received from the other direction. This mode is commonly used for linking computers together over telephone lines.

§  Full-duplex Communication: The fastest directional mode of communication is full-duplex communication. Here, data is transmitted in both directions simultaneously on the same channel. Thus, this type of communication can be thought of as similar to automobile traffic on a two-lane road. Full-duplex communication is made possible by devices called multiplexers. Full-duplex communication is primarily limited to mainframe computers because of the expensive hardware required to support this directional mode.

Modes of Transmitting Data

Another way of classifying data communications flow is as synchronous or asynchronous.

§  Synchronous Transmission: Large volumes of information can be transmitted at a single time with synchronous transmission. This type of transmission involves the simultaneous flow of several bytes of data. Because a large block of data being sent synchronously cannot be interrupted, a synchronized clock is necessary to carefully schedule the transmission of information. This special communications equipment is expensive; but this cost can be made up in part by faster, less expensive transmission of information.

§  Asynchronous Transmission: Conversely, asynchronous transmission involves the sending and receiving of one byte of data at a time. This type of transmission is most often used by microcomputers and other systems characterized by slow speeds.

1.5.2 Transmission Terminology

Data transmission occurs between transmitter and receiver over some transmission medium.Transmission media may be classified as guided or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and optical fiber. Unguided media, also called wireless, provide a means for transmitting electromagnetic waves but do not guide them; examples are propagation through air, vacuum, and seawater.

The term direct link is used to refer to the transmission path between two devices in which signals propagate directly from transmitter to receiver with no intermediate devices, other than amplifiers or repeaters used to increase signal strength. Note that this term can apply to both guided and unguided media.

A guided transmission medium is point to point if it provides a direct link between two devices and those are the only two devices sharing the medium. In a multipoint guided configuration, more than two devices share the same medium.

The signal is a function of time, but it can also be expressed as a function of frequency; that is, the signal consists of components of different frequencies. It turns out that the frequency domain view of a signal is more important to an understanding of data transmission than a time domain view. Both views are introduced here.

Time Domain Concepts Viewed as a function of time, an electromagnetic signal can be either analog or digital. An analog signal is one in which the signal intensity

Analog and Digital Waveforms

varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal.1 A digital signal is one in which the signal intensity maintains a constant level for some period of time and then abruptly changes to another constant level.2. Figure 3.1 shows an example of each kind of signal.The continuous signal might represent speech, and the discrete signal might represent binary 1s and 0s.

The simplest sort of signal is a periodic signal, in which the same signal pattern repeats over time. Figure 3.2 shows an example of a periodic continuous signal (sine wave) and a periodic discrete signal (square wave). Mathematically, a signal s(t) is defined to be periodic if and only if where the constant Tis the period of the signal (Tis the smallest value that satisfies the equation). Otherwise, a signal is aperiodic. The sine wave is the fundamental periodic signal. A general sine wave can be represented by three parameters: peak amplitude (A), frequency (f), and phase The peak amplitude is the maximum value or strength of the signal over time; typically, this value is measured in volts. The frequency is the rate [in cycles per

Examples of Periodic Signals

second, or Hertz (Hz)] at which the signal repeats. An equivalent parameter is the period (T) of a signal, which is the amount of time it takes for one repetition; therefore, Phase is a measure of the relative position in time within a single period of a signal, as is illustrated subsequently. More formally, for a periodic signal f(t), phase is the fractional part t/T of the period T through which t has advanced relative to an arbitrary origin. The origin is usually taken as the last previous passage through zero from the negative to the positive direction.

The general sine wave can be written

A function with the form of the preceding equation is known as a sinusoid.

1.6 DIGITAL AND ANALOG SIGNALS

One of the major functions of the physical layer is to move data in the form of electromagnetic

signals across a transmission medium.

Both data and the signals that represent them can be either analog or digital in form.

Analog and Digital Data

Data can be analog or digital. The term analog data refers to information that is continuous;

Digital data refers to information that has discrete states.

For example, an analog clock that has hour, minute, and second hands gives information in a continuous form; the movements of the hands are continuous. On the other hand, a digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06.

Analog data, such as the sounds made by a human voice, take on continuous values. When someone speaks, an analog wave is created in the air. This can be captured by a microphone and converted to an analog signal or sampled and converted to a digital signal.

Digital data take on discrete values. For example, data are stored in computer memory in the form of Os and 1s. They can be converted to a digital signal or modulated into an analog signal

for transmission across a medium.

Analog and Digital Signals

Like the data they represent, signals can be either analog or digital. An analog signal has

infinitely many levels of intensity over a period of time.

A digital signal, on the other hand, can have only a limited number of defined values. Although each value can be any number, it is often as simple as 1 and O.

Periodic and Non periodic Signals

Both analog and digital signals can take one of two forms: periodic or non periodic.

A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one full pattern is called a cycle.

A non periodic signal changes without exhibiting a pattern or cycle that repeats over time.

Periodic Analog Signals

Periodic analog signals can be classified as simple or composite.

A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves.

Sine Wave

The sine wave is the most fundamental form of a periodic analog signal. When we visualize it as

a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a

continuous, rolling flow.

A sine wave can be represented by three parameters: the peak amplitude, the frequency, and the phase. These three parameters fully describe a sine wave.

Peak Amplitude

The peak amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. For electric signals, peak amplitude is normally measured in volts.

Period and Frequency

Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. Frequency refers to the number of periods in 1s.

Period is the inverse of frequency, and frequency is the inverse of period, as the following

formulas show.

f = 1/T        and            T = 1/f

Unit                               Equivalent                    Unit                               Equivalent

Seconds (s)                     1s                                  Hertz (Hz)                     1 Hz

Milliseconds (ms)            10-3 s                            Kilohertz (kHz) 103 Hz

Microseconds (µs)           10-6 s                            Megahertz (MHz)           106 Hz

Nanoseconds (ns)            10-9 s                            Gigahertz (GHz)             109 Hz Picoseconds (ps)     10-12 s                               Terahertz (THz) 1012 Hz

Composite Signals

Simple sine waves have many applications in daily life. We can send a single sine wave to carry

electric energy from one place to another. For example, the power company sends a single sine wave with a frequency of 60 Hz to distribute electric energy to houses and businesses.

A single frequency sine wave is not useful in data communications; we need to send a composite signal, a signal made of many simple sine waves.

According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases.

Bandwidth

The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is

normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is 5000 - 1000, or 4000.

The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal.

Bit Rate

Most digital signals are non periodic, and thus period and frequency are not appropriate

characteristics. Another term-bit rate is used to describe digital signals.

The bit rate is the number of bits sent in 1s, expressed in bits per second (bps).

Bit Length

We discussed the concept of the wavelength for an analog signal: the distance one cycle occupies on the

transmission medium. We can define something similar for a digital signal: the bit length. The bit length is the distance one bit occupies on the transmission medium.

Bit length=propagation speed x bit duration

 Data Rate Limits

A very important consideration in data communications is how fast we can send data, in bits per second

over a channel. Data rate depends on three factors:

1. The bandwidth available

2. The level of the signals we use

3. The quality of the channel (the level of noise)

Two theoretical formulas were developed to calculate the data rate: one by Nyquist for a noiseless channel, another by Shannon for a noisy channel.

Noiseless Channel: Nyquist Bit Rate

For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate

BitRate = 2 x bandwidth x l0g2 L

In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used to

represent data, and Bit Rate is the bit rate in bits per second.

Noisy Channel: Shannon Capacity

In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude Shannon

introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a

noisy channel:

Capacity =bandwidth X log2 (1 +SNR)

In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and capacity

is the capacity of the channel in bits per second. Note that in the Shannon formula there is no indication of the signal level, which means that no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel. In other words, the formula defines a characteristic of the channel, not the method of transmission.

1.7.  TRANSMISSION IMPAIRMENT

Signals travel through transmission media, which are not perfect. The imperfection causes signal

impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.

Attenuation

Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it

loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal.

 

                Original                                              Attenuated                                                             Amplitude                                    

Decibel

To show that a signal has lost or gained strength, engineers use the unit of the decibel.

The decibel (dB) measures the relative strengths of two signals or one signal at two different points.

Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified.

Distortion

Distortion means that the signal changes its form or shape.

Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed (see the next section) through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if

the delay is not exactly the same as the period duration.

In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same.

Noise

Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise,

crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances.

To find the theoretical bit rate limit, we need to know the ratio of the signal power to the noise power.

The signal-to-noise ratio is defined as:

SNR = average signal power/ average noise power

Because SNR is the ratio of two powers, it is often described in decibel units, SNRdB, defined as

1.8. TRANSMISSION  MEDIA

Transmission media is a pathway that carries the information from sender to receiver. We use different types of cables or waves to transmit data. Data is transmitted normally through electrical or electromagnetic signals.

An electrical signal is in the form of current. An electromagnetic signal is series of electromagnetic energy pulses at various frequencies. These signals can be transmitted through copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different properties like bandwidth, delay, cost and ease of installation and maintenance. Transmission media is also called Communication channel.

Types of Transmission Media

Transmission media is broadly classified into two groups.

1. Wired or Guided Media or Bound Transmission Media

2. Wireless or Unguided Media or Unbound Transmission Media

 Wired or Guided Media or Bound Transmission Media: Bound transmission media are the cables that are tangible or have physical existence and are limited by the physical geography. Popular bound transmission media in use are twisted pair cable, co-axial cable and fiber optical cable. Each of them has its own characteristics like transmission speed, effect of noise, physical appearance, cost etc.

Magnetic Media

A common way to transfer data from one computer to another is to write them into magnetic tape or floppy disk and physically transport the tapes or disks to the destination machines and read them again using hardware parts

Twisted pair

• It is the oldest and still the most common transmission medium as far as networking is concerned.
• Twisted pair consists of 2 insulated copper wires typically about 1 mm in thickness.
• The wires are twisted together in a helical form just like a DNA molecule.
• He purpose of the twisting the copper wire is to reduce electrical interference from similar pairs close by. The most common application of a twisted pair is a telephone system.
• The twisted pair can run several kilometers without amplification but for longer distances repeaters may be needed.
• Twisted pair can be used either for analog or digital transmission.
• The bandwidth depends on the thickness of the wires its length and the distance traveled.
• But several megabytes can be achieved for a few kilometers in many cases.

Base band coaxial cable

• It is better than the twisted pair and hence can span longer distance at higher speed.
• It is a  cable which is commonly used for digital transmission.
• A coaxial cable consists of a stiff copper wire as the core surrounded by an insulated material.
• The insulator is encased by a cylindrical conductor. The outer conductor is covered in a protective shield

Broadband coaxial cable

It is a cable used for analog transmission. It is called broadband. It means any cable network using analog transmission.

**The major difference between the base band and the broadband coaxial cable is that the later covers a bigger area and involves analog transmission. Hence requires analog amplifiers to strengthen this signal periodically.

This amplifier only transmits signals in one direction. So a computer sending a “packet” will not be able to reach computers upstream from it if an amplifier lies between them.

For minimizing this error there are 2 types of broadband system

• Dual cable system
• Single cable system

Dual Cable system

It has two identical wires running parallel to each other.

A computer sends the data into cable 1 which runs to a device called the head-end at the root of the cable tree. The head end then transfer the signal to the cable for transmission back down the tree. All computers transmit through cable1 and receive through cable 2.

Single cable system

In this case, there is just one cable through which the sending and receiving of the data is done. The differentiating factor is the bandwidth. The bandwidth through which the packet is sent is not the same in which it is received.

Generally low frequency bandwidth is used for communication from the computer to the head end which then shifts the signal to the higher frequency bandwidth for broadcasting.

There are basically 2 categories in single cable system that is the exchange of packets through the bandwidth range is done in two ways

• sub split system

In this case the inbound traffic bandwidth rang: 5-30 mhz

Outbound traffic range: 40-300 Mhz

• Mid split system

In this case the inbound traffic bandwidth rang: 5-116 mhz

Outbound traffic range: 168-300 Mhz

Wireless or Unguided Media or Unbound Transmission Media: Unbound transmission media are the ways of transmitting data without using any cables. These media are not bounded by physical geography. This type of transmission is called Wirelesscommunication. Nowadays wireless communication is becoming popular. Wireless LANs are being installed in office and college campuses. This transmission usesMicrowave, Radio wave, Infra red are some of popular unbound transmission media.

                       

The data transmission capabilities of various Medias vary differently depending upon the various factors. These factors are:

 1. Bandwidth. It refers to the data carrying capacity of a channel or medium. Higher bandwidth communication channels support higher data rates.

2. Radiation. It refers to the leakage of signal from the medium due to undesirable electrical characteristics of the medium.

3. Noise Absorption. It refers to the susceptibility of the media to external electrical noise that can cause distortion of data signal.

4. Attenuation. It refers to loss of energy as signal propagates outwards. The amount of energy lost depends on frequency. Radiations and physical characteristics of media contribute to attenuation.