networking chap 1

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Chapter 1

Introduction to Computer Networks

Introduction

18th

Century – Mechanical Systems

19th

Century – Age of steam engine

20th

Century – Information gathering processing and distribution

New Inventions

1. World wide telephone networks

2. Invention of radio and television

3. Launching of communication & satellites.

With rapid growth in technology, these areas are coming nearer and the difference between collecting,

transporting, storing and processing information are disappearing. In a big organization, where there

are many offices located in different cities, it was tried to achieve such that they can exchange data at a push of button. Hence, there is fast need to collect process and distribute information. For this purpose,

there is need of highly sophisticated computers. Computer industry is not older as other industries but

it has done tremendous progress in the technology. In first two decades they were centralized and the

computer was kept in a centralized glass room. All the users use to submit their jobs to this centrally

located computers. In Universities there were hardly one or two computers whereas in a big institution

there were 10-12 computers. But after so many inventions in computer technology the computer size is

drastically reduced and they come in with minimum size of less than postage stamp. Their speed has

increased and cost also has tremendously reduced with the invention of powerful computers and

communication technology the old model of centralized computer system model is becoming obsolete.

1.1 Computer Network

The system in which a large number of computers are interconnected and fulfill the organizations

computational needs such a system is called as Computer Network . The computers are said to be

interconnected if they are able to exchange information. The connection can be via

1. Copper wire

2. Fiber wire

3. Microwave

4. Satellites, etc.

Autonomous Computer

If one computer cannot forcibly start, stop, or control another computers are autonomous i.e. there

should be no master /slave relationship between the computers to be autonomous.

“Computer network is a interconnected collection of autonomous computers”.

Note: A system with one control unit and may contain many slaves is not a network nor is a large

computer with remote printers and terminals.

Goals of Computer Network

1) Sharing Resources: To make all programs, equipment and especially data available to anyone on

the network without regard to physical location of the resource and the user .



2) High Reliability: Provides high reliability by giving alternative sources of supply. For example, all

fields could be replaced on two or more machines. So if one of them is unavailable (due to

hardware failure) the other copies could be used. With presence of multiple CPU, if one goes

down the other may be able to take over its work although at reduced performance. Eg. Military,

Air , Banking, etc. are supposed to be kept continuously operating even in case of hardware failure.

3) Saving Money: Small computers have a much better price/ performance ratio than large

computers. Mainframes are faster than personnel computers but their cost is very high. Nearly

thousand times the cost of personnel computers. Due to this imbalance many networks are made

of large number of personnel computers, one per user and data is kept on one more large capacity

computer called file server . The users are called as clients and the whole model in which such type

of arrangement is done is called as client-server model. In this model the communication takes

place by client giving request message to the server for the work to be done. The server then does

the work and sends back the reply.

4) Scalability: The ability to increase system performance gradually as the workload grows just by

adding more processors. In client-server model new clients and servers can be added as needed.

5) Power of Communication media: A computer network provides a powerful communication

medium among widely separated employees. With the help of computer network , it is easy for two

or more people who live far apart to write a report together . When user makes changes to an

online document, the others can see the change immediately, instead of waiting for several days.

Applications of Computer Network

1. Access to remote information: Person communicating with remote database servers.

a. Access to financial institutions – People pay bills, manage their accounts, and handle

investments electronically. eg. E-com, credit card system, etc.

b. Online shopping – People can view the online catalogs of products and view the picture along

with prices of the product. People can order the product and pay through credit card on just

click of button.

c. Digital libraries are available – You can view the whole book and read or order book online.

d. Access to information system like the WWW (World Wide Web). Information on arts, science,

commerce, medicine, Engineering, movies pictures, sports, etc.

2. Person-to-person Communication: One person communicating with one or more person

a. Electronic mail or email used to send or receive mails along with voice and pictures

b. Real time email allows remote users to communicate with no delay with possibility of seeing

and hearing each other .

c. Possible to have virtual meeting school, medical through video conferencing like Virtual school, medical transcription, etc.

Request

Reply

Client

process

Server process

Server

machine



3. Entertainment:

a. Video on demand. Possible to select any movie or television program ever made in any

country, and have it displayed on your screen instantly.

b. Multi- person real-time simulation games.

c. Viewers can participate in the live television programs.

1.2 Network Types

Networks are based on Transmission technology:

1. Broadcast network

a. There is a single communication channel that is shared by all the machine on the network .

b. Message sent by one machine usually called as packet is received by all the other machines in

the network .

c. There is an address field present in each packet specifying for whom the packet is intended for .

d.

When the packet is received its address field is checked by the machine. If the packet is for it then it is received and processed otherwise the packet is ignored by that machine and just send

to next machine.

i. In a crowd ‘Ajit’ is calling to ‘Vinish’. Only Vinish responds to the call whereas all other

simply ignore it though they have received they don’t process the message because it is

not meant for them.

ii. Railway announcement asking all train 1234 on Bhusaval Express passengers to report to

platform no 6.

e. Broadcast also allow the possibility of addressing a packet to all destination by using a special

code in the address field when a packet with this code is transmitted it is received and

processed by every machine on the network .

eg. 1. Network supervisor sending a message to all the users to logout.

2. Addressing a speech or lecture to everyone.

f. Some broadcast systems also support transmission to a subset of the machine known as

multicasting .

eg. Delivering a message to a specific group only and not everyone, like delivering a message

only the students of “Third year of BCS students” and not everyone who comes to the institute.

g. Smaller geographical system uses this system i.e. broadcasting.

2. Point-to-Point Network 1) It consists of many connections between individual pairs of machines.

2) To go from the source to destination a packet on this type of network may have to first visit

one or more intermediate machines.

3) Often there are many routes possible, so routing algorithms are to be implemented.

4) Large networks use point-to- point terminology.

Classification of network based on scale:

Inter-processor distance Processors located in same Example

0.1m Circuit board Data flow m/c

1m System Multi-computer

10m Room



100m Building Local Area Network

1km Campus

10km City Metropolitan Area Network

100km Country

Wide Area Network 1000km Continent 10000km Planet Internet

1. Data flow machines: Based on parallel architecture with many CPUs working on same program.

2. Multi-Computers: These systems communicate by sending messages over very short, very fast

buses.

3. Computers that communicate by exchanging messages over longer cables are called as true

networks. They are local area, metropolitan area and wide area network .

4. Interconnection of two or more networks is called as Internet.

Local Area Network (LAN)

1. They are privately owned networks within a single building or campus upto few kilometers in size.

2. Used to connect different personnel computers in the company offices to exchange information

and to share resources like printer , etc.

3. LANs are characterized based on

a. Size

b. Transmission technology

c. Topology

4. LANs usually use broadcast transmission technology.

5. Usually LANs run at 10-100 Mbps, having less delay and less error but these are new LANs, which

are much faster running at several hundreds of mega bits per sec(mbps).

6. In bus topology at any time only one machine is allowed to transmit. All other machines are not

allowed to transmit.7. When two or more computers transmit simultaneously then there is a conflict. To resolve this

conflict some bus arbitration mechanism is needed.

8. To resolve conflict IEEE802.3 also called Ethernet network is used in which computer can

transmit whenever they want. If packets collide, each computer just waits random time and tries

again later .

9. Ring is also a broadcast system in which each bit propagates around on its own, not waiting for the

rest of the packet to which it belongs.

10. Broadcast network are further divided into static and dynamic based on the way in which channel

is allocated.

11. In static allocation time is divided into discrete interval and run a Round Robin algorithm,

allowing each machine to broadcast only when its time slot comes up.

Server

Bus topology Ring topology

Server

Star topology



12. The disadvantage of static allocation is it wastes channel capacity when machine has nothing to

say during its allocated slot.

13. Dynamic allocation methods are either centralized or decentralized.

14. In centralized method, there is a single entity. Eg. bus arbitration unit, which determines who goes

next. It does this by using some internal algorithms.

15. In decentralized method, each machine must decide for itself whether or not to transmit.

16. In other type of LAN uses point-to- point technology in which line is connected to each machine.

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. A fully

connected mesh therefore has n(n-1)/2 physical channels to link n devices.

A mesh offers several advantages over other network topologies. First, 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.Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire

system.

Another advantage is privacy or security. When every message sent travels along a dedicated line, only

the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages.

Finally, 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. First, because every device must be connected to every other device, installation and

reconfiguration are difficult. Second, the sheer bulk of the writing can be greater than the available

space (in walls, ceilings, or floors) can accommodate. And, finally, the hardware required to connect

each link (I/O ports and cable) can be prohibitively expensive.

Tree

A tree topology is a variation of a star . As in a star , nodes in a tree are linked to a central hub that

control to the network . However , not every device plugs directly into the central hub. The majority of

devices connect to a secondary hub that in turn is connected to the central hub.



The central hub in the tree is an active hub. An active hub contains a repeater , which is a hardware

device that regenerates the received bit patterns before sending them out. Repeating strengthens

transmissions and increases the distance a signal can travel.

The secondary hubs may be active or passive hubs. A passive hub provides a simple physical

connection between the attached devices.

The addition of secondary hubs, however , brings two further advantages. First, it allows more devices

to be attached to a single central hub and can therefore increase the distance a signal can travel between

devices. Second, it allows the network to isolate and prioritize communications from different

computers.

For example, the computers attached to one secondary hub can be given priority over computers

attached to another secondary hub. In this way, the network designers and operator can guarantee that

time-sensitive data will not have to wait for access to the network .

A good example of tree topology can be seen in cable TV technology where the main cable from the

main office is divided into main branches and each branch is divided into smaller branches and so on.

The hubs are used when a cable is divided.

Hybrid Topologies

Often a network combines several topologies as subnetworks linked together in a larger topology. For

instance, one department of a business may have decided to use a bus topology while another

department has a ring. The two can be connected to each other via a central controller in a star

topology.

Machine Area Network (MAN)

1. It is bigger version of LAN and uses similar technology.

Hub

Hub

Hub

Hub

Hub

Star

Ring

Hub

Bus

Star



2. Covers a group of nearby offices or a city and can be either private or public.

3. Supports both data and voice and be connected to local cable television network

4. DQDB (Distributed Queue Dual Bus) or 802.6 is standard used for LAN.

5. DQDB consists of two unidirectional buses to which all computers are connected.

6. Each bus has a head-end, a device that indicates transmission activity.

7. Traffic that is destined for a computer to the sender uses the upper bus. Traffic to the left uses the

lower one.

8. MAN uses broadcast technology.

Wide Area Network (WAN)

1. It covers large geographical area, often country or continent.

2. It contains a collection of machine which run user programs. These machines are called host/end

system.

3. These hosts are connected by communication subnet, which carries messages from host to host.

4. The subnet consists of two distinct components

a.

Transmission lines: Also called as channels/trunks/circuits. b. Switching elements: Also called as data switching exchanges/routers/ packet switching

nodes/intermediate system

5. Transmission lines move bits between machines.

6. Switching elements are specialized computers used to connect two or more transmission lines

when data arrive on an incoming line, the switching element must choose an outgoing line to

forward them on.

7. Subnet is collection of routers and communication lines that move packets from the source host to

the destination host.

8. WANs contain numerous cables or telephone lines, each one connecting a pair of routers. If two

routers that do not share a cable want to communicate can easily do it by intermediate router .

9. When a packet is sent from one router to another via one or more intermediate routers, the packet

is received at each routers, the packet is received at each intermediate route in its entirety, stored

there until required output line is free, and then forwarded.

1 2 3 N…

Computer

Bus A

Direction of flow on bus A →

Direction of flow on bus BBus B

Subnet Routers

Host



10. A subnet using this principle is called a point-to- point, store-and-forward, or packet-switched

subnet.

11. When a point-to- point subnet is used, router connection topology can be of the following:

12. LAN uses symmetric topology, while WAN uses irregular .

Peer-to-peer networks

A peer LAN is a network where no computer is set-aside as a server . In a Peer -to- peer LAN, all the

machines can act as workstations and can be file servers at the same time. Such a configuration is often

used on smaller LAN’s Figure depicts a Peer LAN of three PCs in which each acts as a workstation

and a file server .

1. To connect a small group of computers permanently, which can be done by a simple file transfer

utility.

2. When you tie computers together on more than a temporary basis, you set up to the first level of

local area network . At this first level, you don’t need to sit aside a personal computer to act as a file

server .

3. All the computers can be file servers and workstations at the same time, this is called Peer LAN.

4. In a Peer LAN the disk space and files on your computer becomes communal property.

5. Peer LANs, however are cost-effective for small, lightly loaded networks.

6. The computer can be slower than the others while it is sharing resources with the other computer .

7. Peer LANs, have the advantage that users don’t have to remember to copy files from their

computer to a separate file server for other people to access.8. A list of peer LAN products are 10NetPlus, EasyNet, AppleTalk , GV LAN Os, etc.

Characteristics

1. In a peer LAN, all can share the disks and files on computers, which are workstations as well as

servers.

2. The computer may become slower because its resources are being shared

Star Ring Tree

Complete Intersecting Ring Irregular



3. It’s a cost-effective solution for lightly loaded networks.

4. As against the server based LAN, the users do not have to copy their files to the central location so

that they can be shared.

5. In peer LAN, the disk space and files on your computer becomes a communal property.

Peer networks are good for :

• File sharing,

• Printer sharing,

• E-mail,

• Easy installation

They are not good for :

• Security,

• Organization of data,

• Database applications,

• Large networks

Server Based LAN’s

In this network , one computer is dedicated as file server . The Network Operating system on the file

server replaces DOS entirely.

By using the computer completely as a file server and organizing the disk in such a way that it

performs well for large files as well (which DOS cannot do), server - based LANs enable LANs to be

much larger , do more work , increase reliability and give a better performance. Figure below illustrates

a server based LAN.

Server Networks are good for

• Centralized file services,

• Security,

• Archiving,

• Organizing data,

• Database applications.

They are not good for :

• Distributed organizations

• Tight budgets

Workstation

Workstation

Workstation

Workstation

Workstation

Workstation

Workstation

Workstation

Workstation

Workstation

File Server



Components of Data Communication System

A data communication system is made up of five components:

1. Message: The message is the information (data) to be communicated. It can consist of text,

numbers, pictures, sound, or video- or any combination of these.

2. Sender: The sender is the device that sends the data message. It can be a computer workstation,

telephone, 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. Medium: The transmission medium is the physical path by which a message travels from sender

to receiver . It can consist of twisted pair wire, coaxial cable, fiber -optic cable, laser , or radio waves

(terrestrial or satellite microwave).

5. Protocol: A protocol is a set of rules that govern data communication. It represents an agreement

between the communicating devices. Without a protocol, two devices may be connected but not

communicating, just as a person specking French cannot be understood by a person who speaks

only Japanese.

1.3 Network Software

Protocol Hierarchies

To reduce design complexity, most networks are organized as a series of layers or levels, each one built upon the one below it. The number of layers, the name of each layer , the contents of each layer , and the

function of each layer differ from network to network . In all networks, the purpose of each layer is to

offer certain services to the higher layer , shielding those layers from the details of how the offered

services are actually implemented.

Layer ‘n’ on one machine carries on conversation with layer n protocol . Protocol is an agreement

between the communicating parties on how communication is to proceed.

eg. when a woman is introduced to a man she takes her hand ahead. He, in turn may decide either to

shake it or kiss it, depending on whether she has come for a party or for an official meeting.

Violating the rules makes the communication difficult. The entities comprising the corresponding

layers on different machine are called peers. Peers communicate using the protocol.

Step 1:

Step 2:Step 3:

…………………………

Step 1:Step 2:Step3:

…………………………

Protocol Protocol Message

Medium

Sender Receiver



In reality, no data are directly transferred from layer n on one machine to layer n on other machine.

Instead each layer passes data and control information to the layer immediately below it, until the

lowest layer is reached. Below layer 1 is the physical medium through which actual communication

occurs. Virtual communication is shown by dotted lines and physical communication by solid lines.

Between each pair of adjacent layers there is an interface. The interface defines which primitive

operations and services the lower offers to the upper one.

A set of layers and protocols is called as network architecture.

The specifications of architecture must contain enough information to allow an implementer to write

the program or build the hardware for each layer so that it will correctly obey the appropriate protocol.

A list of protocols used by a certain system, one protocol per layer , is called a protocol stack .

eg. Imagine two philosophers ( peer processes in layer 3), one of who speaks Urdu and English and one

of who speaks Hindi and Marathi. Since they have no common language, they each engage a translator

( peer processes in layer 3), each of who in turn contacts a secretary ( peer processes in layer 2).

Philosopher 1 wishes to convey his affection to his peer . To do so, he passes a message in English

across the 2/3, to his translator , saying “How are you?”. The translators have agreed on a neutral

language, German, so the message is converted to “xyz lmn pqr ”. The choice of language is the layer 2

protocol and is up to the layer 2 peer processes.

The translator then gives the message to a secretary for transmission, by, for example fax (the layer 1

protocol). When the message arrives, it is translated into Hindi and passed across the 2/3 interface to philosopher 2. Note that each protocol is completely independent of the other as long as the interfaces

are not changed. The translators can switch from German to say, Hindi, at will, provided that they both

agree, and neither changes this interface with either layer 1 or layer 3. Similarly the secretaries can

switch from fax to email, or telephone without disturbing the other layer . Each process may add some

information intended only for its peer . This information not passed upward to the layer above.

How communication takes place between the layers

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Physical Medium

Layer 5 Protocols

Layer 4 Protocols

Layer 3 Protocols

Layer 2 Protocols

Layer 1 Protocols

Layer 4/5 interfaces

Layer 3/4 interfaces

Layer 2/3 interface

Layer 1/2 interface

C1 C2

Layer 5 protocol

M

MH4z

M2 H3 M1 H4 H3 M2 H3 M1 H4 H3

Layer 4 protocol

Layer 3 protocol

M

MH4z



A message M is produced by an application process running in layer 5 and given to layer 4 for

transmission. Layer 4 puts a header in front of the message to identify the message and passes the

result the layer 3. The header includes control information, such as sequence number , to allow layer 4

on the destination machine to deliver messages in the right order if the lower layer do not maintain

sequences. It also contains information like size, time etc and control field.

The layer 3 protocols imposes limit on the size of the messages transmitted. Hence layer 2 breaks the

messages into smaller units called packets, putting header of layer 3 in front of each packet.

Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2. The Layer 2 also

adds its own header to each piece along with the trailer and gives it to layer 1 for doing physical

transmission. At the receiving machine the message moves upward, from layer to layer , with headers

being stripped off as it progresses. None of the headers for layers below n are passed up to layer n.

Design Issues for the layers

• Every layer needs a mechanism for identifying senders & receivers. Since network is made up of many computers they can communicate with all the other computers. There should be some means

to specify with whom it wants to talk . Hence, some form of addressing is needed to specify

destination.

• Design issue concerned with rules of data transfer :

a. Simplex communication: Data can be traveled only in one direction.

eg. Television, Keyboard uses Simplex communication.

b. Half -duplex communication: Data can be traveled in either direction, but not simultaneously.

eg. Walky Talky uses half duplex communication

c. Full-duplex communication: Data can be traveled in both directions at once.

eg. Telephone system uses full duplex system.

• The protocol must determine how many channels are needed for connection. For example, some

networks have two channels: one for normal data & one for control information.

• Design issue concerned with error control: Many error -detecting and error -correcting codes are

there, but both ends of connection must agree on which one is being used. There is also need of

some mechanism of acknowledgement.

• All the messages are to be numbered to maintain the sequence. So that any of the messages is lost

it can be retransmitted.

• There is need of synchronization of fast sender with slow receiver .

• All the process cannot accept long messages. There should be some mechanism of breaking long

messages into smaller , transmitting and reassembling back .



• Sometimes it is possible that several processes transmit messages into units, which are very small.

Transmitting such small messages on a separate communication line is very expensive. Here all

such small messages are combined together to form a single message, which is heading to a

common destination and it is again broken into smaller messages at the other side.

Interfaces and services

1. The function of each layer is to provide services to the layer above it.

2. The active elements in each layer are called entities. Eg. entities can be software ( process) or

hardware (I/O chip).

3. Entities in the same layer on different m/c s are called peer entities.

4. The entities in layer n implement a service used by layer n+1. Layer n is called the service provider

and layer n+1 is called the service user .

5. Services are available at SAPs. The layer n SAPs are places where layer n+1 can access the service

offered. Each SAP has an address that uniquely identifies it.

a. The SAPs in the telephone system are the sockets into which modular telephones can be plugged, and the SAP addresses are the telephone numbers of these sockets. To call someone

you must know callee’s SAP address.

b. In the postal system, the SAP addresses are street addresses and the post office box number . To

send a letter , you must know the addressee’s SAP address.

6. In order for two layers to exchange information, there has to be an agreed upon set of rules about

the interfaces.

7. The layer n+1 entity passes an IDU (Interface Data Unit) to the layer n entity through the SAP.

8. The IDU consists of an SDU (Service Data Unit) and some control information.

9. The SDU is the information passed across the network to the peer entity and then up to layer n+1.

The control information is needed to help the lower layer do its job (eg., the number of bytes in the

SDU) but it is not the part of the data itself .

10. In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of

which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet.

Services

Services offered by each layer to the layer above them are of two types:

SDUICI

SDUICI

SAP

IDU

Layer N+1

Interface

Layer N

SAP-Service Access Point

IDU- Interface Data Unit

SDU-Service Data Unit

PDU-Protocol Data Unit

Layer N entities

exchange N PDUs

in their layer N protocol

Header

SDU

N-PDU



1. Connection Oriented – It is modeled after the telephone system. To talk to someone you pick up

the phone, dial the number , talk , and then hang up. Similarly, to use a connection-oriented network

service, the service user first establishes a connection, uses the connection, and then releases the

connection. The connection acts like a tube; the sender pushes bits at one end, and the receiver

takes out in the same order at the other end.

2. Connection less Service- It is modeled after postal system. Each message carries the full

destination address, and each one is routed through the system independent of the other . Normally,

when two messages are sent to the same destination, the first one sent is the first one to arrive.

However , it is possible that the first one sent can be delayed so that the second one arrives first.

This is impossible in connection-oriented services.

Reliable Service- They never loose the data. Reliable service is implemented by having the receiver

acknowledge the recipient of each message, so the sender is sure that it is arrived. The

acknowledgement process introduces overhead and delays, which are often worth it but sometimes

undesirable.eg. File transfer . The owner of the file wants to be sure that all the bits arrive correctly and in the same

order they were sent.

Unreliable Service: Not acknowledged, also called as data-gram services. eg. telegram services do not

sent acknowledgement to the sender . In some situation connection is not necessary but

acknowledgement is necessary like registered post.

Request-reply service: In this service the sender transmits a single data-gram containing a request; the

reply contains the answer . For example, a query to the local library asking for list of books written by

Prof . XYZ. This service is often needed in client-server method.

Service Example

Connection Oriented Reliable message stream Sequence of pages

Reliable byte stream Remote login

Unreliable connection Digitized voice

Connection Less Unreliable data gram Electronic mail

Acknowledged data gram Registered mail

Request – Reply Database query

Service Primitives

A service is formally specified by a set of primitives (operations) available to a user or other entity to

access the service. These primitives tell the service to perform some action or report on an action taken

by a peer .

Primitive Meaning

Request An entity wants the service to do some work

Indication An entity is to be informed about an event

Response An entity wants to respond to an event

Confirm The response to an earlier request has come back

The initiating entity does a CONNECT.request, which results in a packet being sent. The receiver then

gets a CONNECT.indication announcing that an entity somewhere wants to set up a connection to it.

The entity getting the CONNECT.indication then uses the proposed connection. Either way, the entity

issuing the initial CONNECT.request finds out what happened via a CONNECT.confirm primitive.



CONNECT is a confirmed service because explicit response is required, where as DISCONNECT is

unconfirmed service because no response is required.

Consider the example of a simple connection-oriented service with eight service primitives as follows:

1. CONNECT.request – Request a connection to be established.

2. CONNECT.indication – Signal the called party.

3. CONNECT.response – Used by the callee to accept/reject calls.

4. CONNECT.confirm – Tell the caller whether the call was accepted.

5. DATA.request – Request that data be sent.

6. DATA.indication – Signal the arrival of data.

7. DISCONNECT.request – Request that a connection be released.

8. DISCONNECT.indication – Signal the peer about the request.

Consider the steps required to call “Vinish” on the telephone and invite him to your house for tea.

1. CONNECT.request – Dial Vinish’s phone number .

2. CONNECT.indication – His phone rings.

3. CONNECT.response – He picks up the phone.

4. CONNECT.confirm – You hear the ringing stop.

5. DATA.request – You invite him to tea.

6. DATA.indication – He hears your invitation.

7. DATA.request – He says he would be delighted to come.

8. DATA.indication – You hear his acceptance.

9. DISCONNECT.request – You hang up the phone.

10. DISCONNECT.indication – He hears it and hangs up too.

Protocols

In computer networks, communications occurs between entities in different systems. An entity is

anything capable of sending or receiving information. Examples include application programs, file

transfer packages, browsers, and database management systems, and electronic mail software. A

system is a physical object that contains one or more entities. Examples include computers and

terminals.

But two entities cannot just send bit streams to each other and except to be understood. For

communication to occur , the entities must agree on a protocol.

In data communications , a protocol is a set of rules (conventions) that govern all aspects of

information communication.

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

key elements of a protocol are syntax, semantics, and timing.

• Syntax : Syntax refers to the structure of the data, meaning the order in which they are presented.

For example, a simple protocol might expect the first eight bits of data to be the address of the

sender , the second eight bits to be address of the receiver , and the rest of the stream to be the

message itself .

• Semantics: Semantics refers to the meaning of each section of bits. How is a particular pattern to

be interpreted, and when action is to be based on that interpretation? For example, does an address

identify the route to be taken or the final destination of the message?



• Timing : Timing refers to two characteristics: when data should be sent and how fast they can be

set. 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 data will be largely lost.

Standards

With so many factors to synchronize, a great deal of coordination across the nodes of a network is

necessary if communication is to occur at all, let alone accurately or efficiently. Where there are no

standards, difficulties arise. Automobiles are an example of non-standardized products. A steering

wheel from one make or model of car will not fit into another model without modification. A standard

provides a model for development that makes it possible for a product to work regardless of the

individual manufacturer .

Standards are essential in creating and maintaining an open and competitive market for equipment

manufactures and in guaranteeing national and international interoperability of data and

telecommunications technology and processes. They provide guidelines to manufacturers, vendors,

government agencies, and other services 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 jure standards are those that have been legislated by an officially recognized body. Standards that have not been approved by an organized body but have been adopted through widespread use

are de facto standards. De facto standards are often established originally by manufactures seeking

to define the functionality of a new product or technology.

• De facto standards can be further subdivided into two classes: proprietary and nonproprietary.

Proprietary standards are those originally invented by a commercial organization as a basis for the

operation of its products. They are called proprietary because the company that invented them

wholly owns them. These standards are also called closed standards because they close off

communications between systems produced by different vendors. Nonproprietary standards are

those originally developed by groups or committees that have passed them into the public domain;

they are also called open standards because they open communications between different systems.

Standards

De facto (by fact) De jure (by law)

Categories of standards

networking chap 1

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