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Computer Networks Unit 2

Sikkim Manipal University Page No. 22

Unit 2 Network Software &

Network Standardization

Structure

2.1 Introduction

Objectives

2.2 Networks Software

2.2.1 Protocol hierarchy

2.2.2 Design issues for the layers

2.2.3 Merits and De-merits of Layered Architecture2.2.4 Service Primitives

Self Assessment Questions

2.3 Reference models

2.3.1 The OSI Reference Model

2.3.2 The TCP/IP Reference Model

2.3.3 Comparison of the OSI & the TCP/IP Reference Models


2.4 Network standardization2.4.1 Who's who in the telecommunication world?

2.4.2 Who's who in the standards world

2.4.3 Who's who in the Internet standards world?


2.5 Summary

2.6 Terminal questions

2.7 Answer to Self Assessment Questions

2.8 Answer to Terminal Questions

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2.1 Introduction

With a network the user must explicitly log into a machine, explicitly submit jobs remotely, explicitly move files around and generally handle all network

management personally. To reduce the design complexity, most networks

are organized a series of layers or levels. Each layer built one above the

other. The number of layers, name of each layer, functions of each layer

differs from network to network. The purpose of each layer is to offer certain

set of services to the higher layers, shielding those layers from the details of

how the offered services are actually implemented.

Here in this unit we will study two models and discuss the different layers

and the tasks performed by these layers of the model. We compare these

two standard reference models and also discuss their critics. Finally we well

see the network standardization.

Objectives

By the end of Unit 2, the learners should be able to:

1. Design issues of layered architecture.

2. Explain the ISO-OSI Reference model

3. Explain the TCP/IP Reference model

4. List different IEEE Standards used for networks.

5. Discuss network standardization

2.2 Networks Software

Network software is highly structured. In this section we examine the

software techniques. In the following sections we examine the software

structuring technique in some detail. The method described here forms the

keystone of the entire book and will occur repeatedly later on.

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2.2.1 Protocol Hierarchy

A protocol is an agreement between the communicating parties on howcommunication is to proceed. To reduce their design complexity, most

networks are organized as a stack 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.

The purpose of each layer is to offer certain services to the higher layers,

shielding those layers from the details of how the offered services are

actually implemented. In a sense, each layer is a kind of virtual machine,

offering certain services to the layer above it. That is the rules andconventions used in the conversations collectively known as a protocol.

This concept is actually a familiar one and used throughout computer

science, where it is variously known as information hiding, abstract data

types, data encapsulation, and object-oriented programming. The

fundamental idea is that a particular piece of software (or hardware)

provides a service to its users but keeps the details of its internal state and

algorithms hidden from them. Layer n on one machine carries on a

conversation with layer n on another machine. The rules and conventions

used in this conversation are collectively known as the layer n protocol.

Basically, a protocol is an agreement between the communicating parties on

how communication is to proceed. Violating the protocol will make

communication more difficult, if not completely impossible.

A five-layer network is illustrated in figure 2.1. The entities comprising the

corresponding layers on different machines are called peers. It is the peers

that communicate using the protocol. In reality, no data are directly

transferred from layer n on one machine to layer n on another machine.

Instead the data and control information is passed to the layer immediately

below it, until it reaches the lowest layer. This lowest layer is usually referred

as physical layer, which interfaces directly with the physical medium. The

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virtual communication is indicated by dotted lines and physical

communication by solid lines in figure 2.1

Figure 2.1: Layers, protocols and interfaces

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

defines which primitive operations and services the lower layer offers to the

upper one. When network designers decide how many layers to include in a

network and what each one should do, one of the most important

considerations is defining clean interfaces between the layers. Doing so, in

turn, requires that each layer perform a specific collection of well-understood

functions. In addition to minimizing the amount of information that must be

passed between layers, clearcut interfaces also make it simpler to replace

the implementation of one layer with a completely different implementation

(e.g., all the telephone lines are replaced by satellite channels) because all

that is required of the new implementation is that it offer exactly the same

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set of services to its upstairs neighbor as the old implementation did. In fact,

it is common that different hosts use different implementations.

The set of layers and protocols is called Network architecture. A list of

protocols used by a system is called a protocol stack. The subjects of

network architectures, protocol stacks, and the protocols themselves are the

principal topics of this book.

Figure 2.2: Communication of information in a five-layer network.

Consider the communication between two hosts using a five-layer network.

Let M be the source message produced by the application process runn ing

at layer 5. This message is to be transmitted to the layer 5 of the destination

machine.

This message is given to layer 4 for transmission as shown in Figure 2.2.Layer 4 puts a header for identification in front of the message and passes it

to lower layer 3. The header includes control information, such as sequence

numbers, to allow layer 4 on the destination machine to deliver messages in

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the right order if the lower layers do not maintain sequence. In some layers,

headers can also contain sizes, times, and other control fields. There mightbe limit on the size of the message and hence messages can also be

segmented.

In many networks, there is no limit to the size of messages transmitted in

the layer 4 protocol, but there is nearly always a limit imposed by the layer 3

protocol. Consequently, layer 3 must break up the incoming messages into

smaller units, packets, prepending layer 3 headers to each packet. In this

example, M is split into two parts, M1 and M2.

Layer 3 decides which of the outgoing lines to use and passes the packets

to layer 2. Layer 2 adds not only a header to each piece, but also a trailer,

and gives the resulting unit to layer 1 for physical transmission. Thus the

message reaches the lowest layer where it is transmitted through the

physical medium. The actual flow of the message from the top layer of

source machine to the top layer of the destination machine is illustrated in

figure 2.2. The message has to be delivered in proper sequence to the

layers of the destination machine.

At the receiving machine the message moves upward, from layer to layer,

with the headers being stripped off as it progresses by the appropriate

layers. Note that none of the headers for layers below n are passed up to

layer n.

The important thing is to see the relation of actual flow and virtual flow, the

different protocols and interfaces. Even though we refer network software

for the design of all layers, the lower layers are implemented in hardware or

firmware.

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2.2.2 Design Issues for the Layers

There are some key design issues that are to be considered in computer networks. Every layer needs a mechanism for identifying senders and

receivers. As many computers are normally connected in networks, few of

which have multiple processes. A means for a process on one machine is

needed to specify with whom it wants to communicate to. Thus some form

of addressing scheme is to be devised.

Another design issue is data transmission modes. It concerns the rules for

the data transfer. The systems can use serial or parallel transmission,

synchronous or Asynchronous transmission, simplex or duplex

transmission. The protocol also must determine how many logical channels

the connection corresponds to and what their priorities are.

Another major design issue is Error Control techniques as physical circuits

are not perfect. Some of the error detecting or correcting codes are to be

used at both the ends of the connection. At the same time we need to

consider Flow Control techniques is necessary to keep a fast sender from

swamping a slow receiver. Some systems use some kind of feedback from

receiver, which is useful to limit the transmission rate.

It is inconvenient or expensive to set up separate connection for each pair of

communicating processes. Same connection can be used by multiple &

unrelated conversation. Thus we need to focus on Multiplexing and de-

multiplexing techniques as one of the design issue. Multiplexing is needed in

the physical layer, where all the traffic for all connections has to be sent over

at most a few physical circuits.

When there are multiple paths between the source and destination thecomplexity lies in finding the best, optimum and shortest path. Hence to find

optimum path we need Routing schemes.

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Apart from these some of the design issues can be related to security,

compression techniques and so on.

2.2.3 Merits and de-merits of Layered Architecture

Advantages of Layered Architecture

Any given layer can be modified or upgraded without affecting the other

layers.

Modulazition by means of layering simplifies the overall design.

Different layers can be assigned to different standards, committees, and

design teams.

Mechanisms like packet-switching, circuit-switching may be used without

effecting more than one layer.

Different machines may be plugged in at different layers.

The relation between different control functions can be better

understood.

Common lower levels may be shared by different higher levels.

Functions (especially at lower levels) may be removed from software to

hardware and micro-codes.

Increases the compatibility of different machines.

Disadvantages of Layered Architecture

Total overhead is higher.

Two communicating machines may have to use certain functions which

they could do without layers.

As technology changes, the functions may not be in the most cost-

effective layer.

Connection-Oriented and Connectionless Services

Layers can offer two types of services to the layers above them. They are

Connection oriented and Connection less. Connection oriented service is

modeled after telephone system. To use this service, the service user first

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establishes a connection, uses the connection and then releases the

connection. In most of the cases the order is preserved so that bits arrive atreceiver in the same order as they were sent by the transmitter. In some

cases when a connection is established the source, the subnet, and the

receiver conduct negotiation of certain parameters like the maximum size of

the message, quality of service (QoS) required and other issues.

We have another type of service called Connection less service. This is

modeled after the postal system. Here each message carries the full

destination address, and each one is routed through the system

independent of each others. Here messages may not arrive at the receiver

in the same order as they were sent, as it depends on the route each

message takes on the way to the destination. Six different types of services

are summarized in table 2.1.

Table 2.1: Comparisons of different services

2.2.4 Service Primitives

A service is formally specified by a set of primitives or operations available

to the user to access the service. These primitives tell the service to

perform some action or report an action taken by the peer entity. The

primitives for the connection-oriented service are given in table 2.2.

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Table 2.2: Service primitives for a connection oriented service

Communication in a simple client server model using the above service

primitives is illustrated in figure 2.3. First the server executes the LISTEN toindicate that is ready to accept incoming connections. The client executes

CONNECT (1) to establish the connection with the server. The server now

unblocks the listener and sends back an acknowledgement (2). Thus the

connection is established.

Figure 2.3: Simple client server model on a connection oriented network

The next step for a server is to executes a RECEIVE (3) to prepare to

accept the first request. The arrival of the request packet unblocks the

server so that it can process the request. After it has done the work it uses

SEND (4) to answer to the client. It all the data transfer is done then it canuse DISCONNECT (5) suspending the client. When the server gets this

packet, it also issues a DISCONNECT (6) and when it reaches the client,

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the client process is releases and the connection is broken. In the process

packets may get lost, timings may be wrong, many other complex issues.

The Relationship of Services to Protocols

Figure 2.4: Relationship between the service and protocols

A service is a set of primitives that a layer provides to the layer above it. The

service defines what operation the layer is prepared to perform on behalf of

its users. It says nothing about the implementation of these operations.

A protocol is a set of rules governing the format and meaning of the

packets, or messages that are exchanged by the peer entities within a layer.

Figure 2.4 illustrates the relationship of services to protocols. Entities use

protocols to implement their service primitives. Protocols relate to the

packets sent between entities.


1. Define a protocol

2. Discuss network architecture with an example

3. List the merits and demerits of layered architecture4. Discuss the types of services

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2.3 Reference models

There are two important network architectures. They are ISO-OSI referencemodel and TCP/IP reference model. These two are discussed below.

In 1977, the International Organization for Standardization (ISO) began to

develop its OSI networking suite. OSI has two major components: an

abstract model of networking (the Basic Reference Model, or seven-layer

model), and a set of concrete protocols. The standard documents that

describe OSI are for sale and not currently available online.

Parts of OSI have influenced Internet protocol development, but none more

than the abstract model itself, documented in ISO 7498 and its various

addenda. In this model, a networking system is divided into layers. Within

each layer, one or more entities implement its functionality. Each entity

interacts directly only with the layer immediately beneath it, and provides

facilities for use by the layer above it.

In particular, Internet protocols are deliberately not as rigorously architected

as the OSI model, but a common version of the TCP/IP model splits it into

four layers. The Internet Application Layer includes the OSI ApplicationLayer, Presentation Layer, and most of the Session Layer. Its End-to-End

Layer includes the graceful close function of the OSI Session Layer as well

as the Transport Layer. Its Internet work Layer is equivalent to the OSI

Network Layer, while its Interface layer includes the OSI Data Link and

Physical Layers. These comparisons are based on the original seven-layer

protocol model as defined in ISO 7498, rather than refinements in such

things as the Internal Organization of the Network Layer document.

Protocols enable an entity in one host to interact with a corresponding entity

at the same layer in a remote host. Service definitions abstractly describe

the functionality provided to a (N)-layer by an (N-1) layer, where N is one of

the seven layers inside the local host.
http://en.wikipedia.org/wiki/TCP/IP_modelhttp://en.wikipedia.org/wiki/TCP/IP_model

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2.3.1 The OSI Reference Model

This reference model is proposed by International standard organization(ISO) as a a first step towards standardization of the protocols used in

various layers in 1983 by Day and Zimmermann. This model is called Open

system Interconnection (OSI) reference model. It is referred OSI as it deals

with connection open systems. That is the systems are open for

communication with other systems. It consists of seven layers.

Layers of OSI Model

The principles that were applied to arrive at 7 layers:

1. A layer should be created where a different level of abstraction is

needed.

2. Each layer should perform a well defined task.

3. The function of each layer should define internationally standardized

protocols

4. Layer boundaries should be chosen to minimize the information flow

across the interface.

5. The number of layers should not be high or too small.

Figure 2.5: ISO - OSI Reference Model

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The ISO-OSI reference model is as shown in figure 2.5. As such this model

is not a network architecture as it does not specify exact services andprotocols. It just tells what each layer should do and where it lies. The

bottom most layer is referred as physical layer. ISO has produced standards

for each layers and are published separately.

Each layer of the ISO-OSI reference model are discussed below:

1. Physical Layer

This layer is the bottom most layer that is concerned with transmitting raw

bits over the communication channel (physical medium). The design issues

have to do with making sure that when one side sends a 1 bit, it is received

by other side as a 1 bit, and not as a 0 bit. It performs direct transmission of

logical information that is digital bit streams into physical phenomena in the

form of electronic pulses. Modulators/demodulators are used at this layer.

The design issue here largely deals with mechanical, electrical, and

procedural interfaces, and the physical transmission medium, which lies

below this physical layer.

In particular, it defines the relationship between a device and a physicalmedium. This includes the layout of pins, voltages, and cable specifications.

Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in

Storage Area Networks) are physical-layer devices. The major functions and

services performed by the physical layer are:

Establishment and termination of a connection to a communications

medium.

Participation in the process whereby the communication resources are

effectively shared among multiple users. For example, contentionresolution and flow control.

Modulation, is a technique of conversion between the representation of

digital data in user equipment and the corresponding signals transmitted
http://en.wikipedia.org/wiki/Pinshttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Ethernet_hubhttp://en.wikipedia.org/wiki/Host_adapterhttp://en.wikipedia.org/wiki/Electrical_connectorhttp://en.wikipedia.org/wiki/Communicationhttp://en.wikipedia.org/wiki/Transmission_mediumhttp://en.wikipedia.org/wiki/Contentionhttp://en.wikipedia.org/wiki/Flow_controlhttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Digital_datahttp://en.wikipedia.org/wiki/Digital_datahttp://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Flow_controlhttp://en.wikipedia.org/wiki/Contentionhttp://en.wikipedia.org/wiki/Transmission_mediumhttp://en.wikipedia.org/wiki/Communicationhttp://en.wikipedia.org/wiki/Electrical_connectorhttp://en.wikipedia.org/wiki/Host_adapterhttp://en.wikipedia.org/wiki/Ethernet_hubhttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Pins

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over a communications channel. These are signals operating over the

physical cabling (such as copper and fiber optic) or over a radio link.

Parallel SCSI buses operate in this layer. Various physical-layer Ethernet

standards are also in this layer; Ethernet incorporates both this layer and the

data-link layer. The same applies to other local-area networks, such as

Token ring, FDDI, and IEEE 802.11, as well as personal area networks such

as Bluetooth and IEEE 802.15.4.

2. Data Link Layer

The Data Link layer provides the functional and procedural means to

transfer data between network entities and to detect and possibly correct

errors that may occur in the Physical layer. That is it makes sure that the

message indeed reach the other end without corruption or without signal

distortion and noise. It accomplishes this task by having the sender break

the input data up into the frames called data frames. The DLL of transmitter,

then transmits the frames sequentially, and processes acknowledgement

frames sent back by the receiver. After processing acknowledgement frame,

may be the transmitter needs to re-transmit a copy of the frame. So

therefore the DLL at receiver is required to detect duplications of frames.

The best known example of this is Ethernet. This layer manages the

interaction of devices with a shared medium. Other examples of data link

protocols are HDLC and ADCCP for point-to-point or packet- switched

networks and Aloha for local area networks. On IEEE 802 local area

networks, and some non-IEEE 802 networks such as FDDI, this layer may

be split into a Media Access Control (MAC) layer and the IEEE 802.2

Logical Link Control (LLC) layer. It arranges bits from the physical layer intological chunks of data, known as frames.

This is the layer at which the bridges and switches operate. Connectivity is

provided only among locally attached network nodes forming layer 2
http://en.wikipedia.org/wiki/Channel_%28communications%29http://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Parallel_SCSIhttp://en.wikipedia.org/wiki/IBM_token_ringhttp://en.wikipedia.org/wiki/Fiber_distributed_data_interfacehttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/Data_link_layerhttp://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/HDLChttp://en.wikipedia.org/wiki/ADCCPhttp://en.wikipedia.org/wiki/Point-to-pointhttp://en.wikipedia.org/wiki/Packethttp://en.wikipedia.org/wiki/Aloha_protocolhttp://en.wikipedia.org/wiki/Local_area_networkhttp://en.wikipedia.org/wiki/IEEE_802http://en.wikipedia.org/wiki/FDDIhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/IEEE_802.2http://en.wikipedia.org/wiki/Logical_Link_Controlhttp://en.wikipedia.org/wiki/Network_bridgehttp://en.wikipedia.org/wiki/Network_switchhttp://en.wikipedia.org/wiki/Network_switchhttp://en.wikipedia.org/wiki/Network_bridgehttp://en.wikipedia.org/wiki/Logical_Link_Controlhttp://en.wikipedia.org/wiki/IEEE_802.2http://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/FDDIhttp://en.wikipedia.org/wiki/IEEE_802http://en.wikipedia.org/wiki/Local_area_networkhttp://en.wikipedia.org/wiki/Aloha_protocolhttp://en.wikipedia.org/wiki/Packethttp://en.wikipedia.org/wiki/Point-to-pointhttp://en.wikipedia.org/wiki/ADCCPhttp://en.wikipedia.org/wiki/HDLChttp://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/Data_link_layerhttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/Fiber_distributed_data_interfacehttp://en.wikipedia.org/wiki/IBM_token_ringhttp://en.wikipedia.org/wiki/Parallel_SCSIhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Channel_%28communications%29

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domains for unicast or broadcast forwarding. Other protocols may be

imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.

The data link layer might implement a sliding window flow control and

acknowledgment mechanism to provide reliable delivery of frames; that is

the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and

LAPD. In modern practice, only error detection, not flow control using sliding

window, is present in modern data link protocols such as Point-to-Point

Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is

not used for most protocols on Ethernet, and, on other local area networks,

its flow control and acknowledgment mechanisms are rarely used. Sliding

window flow control and acknowledgment is used at the transport layers by

protocols such as TCP.

3. Network Layer

The Network layer provides the functional and procedural means of

transferring variable length data sequences from a source to a destination

via one or more networks while maintaining the quality of service requested

by the Transport layer. The Network layer performs network routing

functions, and might also perform fragmentation and reassembly, and report

delivery errors. Routers operate at this layer sending data throughout the

extended network and making the Internet possible. This is a logical

addressing scheme values are chosen by the network engineer. The

addressing scheme is hierarchical.

The best known example of a layer 3 protocol is the Internet Protocol (IP).

Perhaps it's easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks

that carry sacks of mail to other post offices or airports, airplanes that carry

airmail between major cities, trucks that distribute mail sacks in a city, and
http://en.wikipedia.org/wiki/Sliding_windowhttp://en.wikipedia.org/wiki/Synchronous_Data_Link_Controlhttp://en.wikipedia.org/wiki/LAPBhttp://en.wikipedia.org/wiki/Link_Access_Procedures%2C_D_channelhttp://en.wikipedia.org/wiki/Point-to-Point_Protocolhttp://en.wikipedia.org/wiki/Point-to-Point_Protocolhttp://en.wikipedia.org/wiki/TCPhttp://en.wikipedia.org/wiki/Network_layerhttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Routerhttp://en.wikipedia.org/wiki/Internet_Protocolhttp://en.wikipedia.org/wiki/Internet_Protocolhttp://en.wikipedia.org/wiki/Routerhttp://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Network_layerhttp://en.wikipedia.org/wiki/TCPhttp://en.wikipedia.org/wiki/Point-to-Point_Protocolhttp://en.wikipedia.org/wiki/Point-to-Point_Protocolhttp://en.wikipedia.org/wiki/Link_Access_Procedures%2C_D_channelhttp://en.wikipedia.org/wiki/LAPBhttp://en.wikipedia.org/wiki/Synchronous_Data_Link_Controlhttp://en.wikipedia.org/wiki/Sliding_window

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carriers that take a letter to its destinations. Think of fragmentation as

splitting a large document into smaller envelopes for shipping, or, in thecase of the network layer, splitting an application or transport record into

packets.

The major tasks of network layer are listed

It controls routes for individual message through the actual topology.

Finds the best route.

Finds alternate routes.

It accomplishes buffering and deadlock handling.

4. Transport Layer

The Transport layer provides transparent transfer of data between end

users, providing reliable data transfer while relieving the upper layers of it.

The transport layer controls the reliability of a given link through flow control,

segmentation/de-segmentation, and error control. Some protocols are state

and connection oriented. This means that the transport layer can keep track

of the segments and retransmit those that fail. The best known example of a

layer 4 protocol is the Transmission Control Protocol (TCP).

The transport layer is the layer that converts messages into TCP segments

or User Datagram Protocol (UDP), Stream Control Transmission Protocol

(SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer

is to compare it with a Post Office, which deals with the dispatch and

classification of mail and parcels sent. Do remember, however, that a post

office manages the outer envelope of mail. Higher layers may have the

equivalent of double envelopes, such as cryptographic Presentation

services that can be read by the addressee only.

Roughly speaking, tunneling protocols operate at the transport layer, such

as carrying non-IP protocols such as IBM's SNA or Novell' s IPX over an IP

network, or end-to-end encryption with IP security (IP sec). While Generic
http://en.wikipedia.org/wiki/Transport_layerhttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Transmission_Control_Protocolhttp://en.wikipedia.org/wiki/User_Datagram_Protocolhttp://en.wikipedia.org/wiki/Stream_Control_Transmission_Protocolhttp://en.wikipedia.org/wiki/IBMhttp://en.wikipedia.org/wiki/Systems_Network_Architecturehttp://en.wikipedia.org/wiki/Novellhttp://en.wikipedia.org/wiki/IPXhttp://en.wikipedia.org/wiki/Generic_Routing_Encapsulationhttp://en.wikipedia.org/wiki/Generic_Routing_Encapsulationhttp://en.wikipedia.org/wiki/IPXhttp://en.wikipedia.org/wiki/Novellhttp://en.wikipedia.org/wiki/Systems_Network_Architecturehttp://en.wikipedia.org/wiki/IBMhttp://en.wikipedia.org/wiki/Stream_Control_Transmission_Protocolhttp://en.wikipedia.org/wiki/User_Datagram_Protocolhttp://en.wikipedia.org/wiki/Transmission_Control_Protocolhttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Transport_layer

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Routing Encapsulation (GRE) might seem to be a network layer protocol, if

the encapsulation of the payload takes place only at endpoint, GREbecomes closer to a transport protocol that uses IP headers but contains

complete frames or packets to deliver to an endpoint.

The major tasks of Transport layer are listed below:

It locates the other party

It creates a transport pipe between both end-users.

It breaks the message into packets and reassembles them at the

destination.

It applies flow control to the packet stream.

5. Session Layer

The Session layer controls the dialogues/connections (sessions) between

computers. It establishes, manages and terminates the connections

between the local and remote application. It provides for either full-duplex or

half-duplex operation, and establishes check pointing, adjournment,

termination, and restart procedures. The OSI model made this layer

responsible for "graceful close" of sessions, which is a property of TCP, and

also for session check pointing and recovery, which is not usually used in

the Internet protocols suite.

The major tasks of session layer are listed

It is responsible for the relation between two end-users.

It maintains the integrity and controls the data exchanged between the

end-users.

The end-users are aware of each other when the relation is established

(synchronization).It uses naming and addressing to identify a particular user.

It makes sure that the lower layer guarantees delivering the message

(flow control).
http://en.wikipedia.org/wiki/Session_layerhttp://en.wikipedia.org/wiki/Duplex_%28telecommunications%29http://en.wikipedia.org/wiki/Half-duplexhttp://en.wikipedia.org/wiki/Transmission_Control_Protocolhttp://en.wikipedia.org/wiki/Transmission_Control_Protocolhttp://en.wikipedia.org/wiki/Half-duplexhttp://en.wikipedia.org/wiki/Duplex_%28telecommunications%29http://en.wikipedia.org/wiki/Session_layer

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6. Presentation Layer

The Presentation layer transforms the data to provide a standard interfacefor the Application layer. MIME encoding, data encryption and similar

manipulation of the presentation are done at this layer to present the data as

a service or protocol developer sees fit. Examples of this layer are

converting an EBCDIC- coded text file to an ASCII- coded file, or serializing

objects and other data structures into and out of XML.

The major tasks of presentation layer are listed below:

It translates the language used by the application layer.

It makes the users as independent as possible, and then they can

concentrate on conversation.

7. Application Layer (end users)

The application layer is the seventh level of the seven-layer OSI model. It

interfaces directly to the users and performs common application services

for the application processes. It also issues requests to the presentation

layer. Note carefully that this layer provides services to user-defined

application processes, and not to the end user. For example, it defines a file

transfer protocol, but the end user must go through an application process

to invoke file transfer. The OSI model does not include human interfaces.

The common application services sub layer provides functional elements

including the Remote Operations Service Element (comparable to Internet

Remote Procedure Call), Association Control, and Transaction Processing

(according to the ACID requirements). Above the common application

service sub layer are functions meaningful to user application programs,

such as messaging (X.400), directory (X.500), file transfer (FTAM), virtualterminal (VTAM), and batch job manipulation (JTAM).
http://en.wikipedia.org/wiki/Presentation_layerhttp://en.wikipedia.org/wiki/MIMEhttp://en.wikipedia.org/wiki/Data_encryptionhttp://en.wikipedia.org/wiki/EBCDIChttp://en.wikipedia.org/wiki/Computer_filehttp://en.wikipedia.org/wiki/ASCIIhttp://en.wikipedia.org/wiki/Serializationhttp://en.wikipedia.org/wiki/Object_%28computer_science%29http://en.wikipedia.org/wiki/Data_structurehttp://en.wikipedia.org/wiki/XMLhttp://en.wikipedia.org/wiki/ACIDhttp://en.wikipedia.org/wiki/ACIDhttp://en.wikipedia.org/wiki/XMLhttp://en.wikipedia.org/wiki/Data_structurehttp://en.wikipedia.org/wiki/Object_%28computer_science%29http://en.wikipedia.org/wiki/Serializationhttp://en.wikipedia.org/wiki/ASCIIhttp://en.wikipedia.org/wiki/Computer_filehttp://en.wikipedia.org/wiki/EBCDIChttp://en.wikipedia.org/wiki/Data_encryptionhttp://en.wikipedia.org/wiki/MIMEhttp://en.wikipedia.org/wiki/Presentation_layer

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Information Exchange among the Layers

The seven OSI layers use various forms of control information tocommunicate with their peer layers in other computer systems. This control

information consists of specific requests and instructions that are exchanged

between peer OSI layers.

Control information typically takes one of two forms: headers and trailers.

Headers are prepended to data that has been passed down from upper

layers. Trailers are appended to data that has been passed down from

upper layers. An OSI layer is not required to attach a header or a trailer to

data from upper layers.

Headers, trailers, and data are relative concepts, depending on the layer

that analyzes the information unit. As illustrated in figure 2.2, at the network

layer, for example, an information unit consists of a Layer 3 header called

Network header (NH) and data. At the data link layer, however, all the

information passed down by the network layer (the Layer 3 header and the

data) is treated as data.

Similar to Network layer now attaches its header (DH) and Trailer (DT) tothe data that received from network layer. In other words, the data portion of

an information unit at a given OSI layer potentially can contain headers,

trailers, and data from all the higher layers. This is known as encapsulation.

Figure 2.6 shows how the header and data from one layer are encapsulated

into the header of the next lowest layer. In figure AH, PH, SH, TH, NH, refer

to the header of application layer to Network layer respectively. DT & DH

refer to Data link layer Trailer & Header.

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Figure 2.6: Encapsulation of Data in ISO-OSI Reference model

2.3.2 The TCP/IP Reference Model

The TCP/IP reference model is the network model used in the current

Internet architecture. It was created in the 1970s by DARPA for use in

developing the Internet's protocols, and the structure of the Internet is still

closely reflected by the TCP/IP model. It has fewer, less rigidly defined

layers than the commonly referenced OSI model, and thus provides an

easier fit for real world protocols. It is considered as the grandfather of the

Internet, the ARPANET. This was a research network sponsored by the

Department of Defense in the United States.

A goal was of continuing the conversation between source and destination

even if transmission went out of operation. The reference model was named

after two of its main protocols, TCP (Transmission Control Protocol) and IP

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(Internet Protocol). No document officially specifies the model. Different

names are given to the layers by different documents, and different numbersof layers are shown by different documents. There are versions of this

model with four layers and with five layers.

The original four-layer version of the model has layers as shown in figure

2.7. It consists of the following four layers

Layer 4 Process Layer or Application Layer:

This is where the "higher level" protocols such as FTP, HTTP, etc.

operate. The original TCP/IP specification described a number of

different applications that fit into the top layer of the protocol stack.

These applications include Telnet, FTP, SMTP and DNS. These are

illustrated in figure 2.10.

Telnet is a program that supports the TELNET protocol over TCP.

TELNET is a general two-way communication protocol that can be used

to connect to another host and run applications on that host remotely.

FTP (File Transfer Protocol) is a protocol that was originally designed to

promote the sharing of files among computer users. It shields the user

from the variations of file storage on different architectures and allows

for a reliable and efficient transfer of data.

SMTP (Simple Mail Transport Protocol) is the protocol used to transport

electronic mail from one computer to another through a series of other

computers along the route.

DNS (Domain Name System) resolves the numerical address of a

network node into its textual name or vice-versa. It would translatewww.yahoo.com to 204.71.177.71 to allow the routing protocols to find

the host that the packet is destined for.

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Layer 3 Host-To-Host (Transport) Layer:

This is where flow-control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring

that packets are in fact received. The transport layer is the interface

between the application layer and the complex hardware of the network.

It is designed to allow peer entities on the source and destination hosts

to carry on conversations. Data may be user data or control data. Two

modes are available, full-duplex and half duplex. In full-duplex operation,

both sides can transmit and receive data simultaneously, whereas in half

duplex, a side can only send or receive at one time. Layer 2 Internet or Internetworking Layer:

This layer defines IP addresses, with many routing schemes for

navigating packets from one IP address to another. The job of the

network layer is to inject packets into any network and have them travel

independently to the destination. The layer defines IP (Internet Protocol)

for its official packet format and protocol. Packet routing is a major job of

this protocol.

Layer 1 Network Access Layer:

This layer describes the physical equipment necessary for

communications, such as twisted pair cables, the signalling used on that

equipment, and the low-level protocols using that signalling. The Host-

to-Network layer interfaces the TCP/IP protocol stack to the physical

network. The TCP/IP reference model does not specify in any great

detail the operation of this layer, except that the host has to connect to

the network using some protocol so it can send IP packets over it. As it

is not officially defined, it varies from implementation to implementation,

with vendors supplying their own version.
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Figure 2.7: TCP/IP Network Protocol

The basic idea of the networking system is to allow one application on a

host computer to talk to another application on a different host computer.

The application forms its request, then passes the packet down to the lower

layers, which add their own control information, either a header or a footer,

onto the packet. Finally the packet reaches the physical layer and is

transmitted through the cable onto the destination host.

The packet then travels up through the different layers, with each layer

reading, deciphering, and removing the header or footer that was attached

by its counterpart on the originating computer. Finally the packet arrives at

the application it was destined for. Even though technically each layer

communicates with the layer above or below it, the process can be viewed

as one layer talking to its partner on the host.

Interaction with Application, Transport and Internet LayersInteraction between the transport layer and the other layers immediately

above and below is shown in figure 2.8.

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Figure 2.8: Interactions with Application, Transport and Internet Layers

Any program running in the application layer has the ability to send a

message using TCP or UDP, which are the two protocols defined for the

transport layer. The application can communicate with the TCP or the UDP

service, whichever it requires. Both the TCP and UDP communicate with theInternet Protocol in the internet layer. In all cases communication is a two

way process. The applications can read and write to the transport layer. The

diagram only shows two protocols in the transport layer.

A message to be sent originates in the application layer. This is then passed

down onto the appropriate protocol in the transport layer. These protocols

add a header to the message for the corresponding transport layer in the

destination machine for purposes of reassembling the message. The

segment is then passed onto the internet layer where the Internet Protocol

adds a further header. Finally the segment is passed onto the physical layer,

a header and a trailer are added at this stage. Figure 2.9 shows the

structure of the final segment being sent.

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LAN/WAN

Header

IP

Header

TCP/UDP

Header

User data LAN/WAN

Trailer

Figure 2.9: Transmitted Segment from TCP/IP Network

The relations of all protocols that reside in corresponding layers are as

shown in figure 2.10.

Figure 2.10: Protocols in TCP/IP reference model

2.3.3 A Comparison of OSI and TCP/IP Reference Models

Concepts central to the OSI model are:

Services: It tells what the layer does.

Interfaces: It tells the processes above it how to access it. It specifies

what parameters are and what result to expect.

Protocols: It provides the offered service. It is used in a layer and are

layers own business.

The TCP/IP did not originally distinguish between the service, interface &

protocols. The only real services offered by the internet layer are SEND IP

packets and RECEIVE IP packets.

The OSI model was devised before the protocols were invented. Data link

layer originally dealt only with point-to-point networks. When broadcast

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networks came around, a new sub-layer had to be hacked into the model.

With TCP/IP the reverse was true, the protocols came first and the modelwas really just a description of the existing protocols. This TCP/IP model did

fit any other protocol stack.

Then OSI model has seven layers and TCP/IP has four layers as shown in

figure 2.11

Figure 2.11: Comparisons of the two reference models

Another difference is in the area of connectionless and connection oriented

services. The OSI model supports both these services in the network layer

but supports only connection oriented communication in the transport layer.

Where as the TCP/IP has supports only connection less communication in

the network layer, and supports both these services in the transport layer.

A Critique of the OSI Model and Protocols

Why OSI did not take over the world

Bad timing

Bad technology

Bad implementations

Bad politics

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A Critique of the TCP/IP Reference Model

Problems:Service, interface, and protocol not distinguished

Not a general model

Host-to- network layer not really a layer

No mention of physical and data link layers

Minor protocols deeply entrenched, hard to replace


1. Name the layers of OSI in sequence from bottom.

2. List few protocols that are used at layer 2

3. List the major task of network layer

4. Name the two major protocols of TCP/IP reference model

5. Give the comparison of the two reference models

2.4 Network standardization

Network standardization is a definition that has been approved by a

recognized standards organization. Standards exist for programming

languages, operating systems, data formats, communications protocols, and

electrical interfaces.

Two categories of standards:

De facto (Latin for from the fact'') standards:

These are those that have just happened without any formal plan. These

are formats that have become standard simply because a large number

of companies have agreed to use them. They have not been formally

approved as standards E.g., IBM PC for small office computers, UNIXfor operating systems in CS departments. PostScript is a good example

of a de facto standard.
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De jure (Latin for by law'') standards:

These are formal legal standards adopted by some authorizedstandardization body.

Two classes of standard organizations

Organizations established by treaty among national governments.

Voluntary, nontreaty organizations.

From a user's standpoint, standards are extremely important in the

computer industry because they allow the combination of products from

different manufacturers to create a customized system. Without standards,

only hardware and software from the same company could be used

together. In addition, standard user interfaces can make it much easier to

learn how to use new applications.

Most official computer standards are set by one of the following

organizations:

ANSI (American National Standards Institute)

ITU (International Telecommunication Union)

IEEE (Institute of Electrical and Electronic Engineers)ISO (International Standards Organization)

VESA (Video Electronics Standards Association)

Benefits of standardization:

Allow different computers to communicate.

Increase the market for products adhering to the standard.

2.4.1 Who's who in the telecommunication world?

Common carriers: private telephone companies (e.g., AT&T, USA).PTT (Post, Telegraph & Telephone) administration: nationalized

telecommunication companies (most of the world).
http://www.webopedia.com/TERM/S/user.htmlhttp://www.webopedia.com/TERM/S/computer.htmlhttp://www.webopedia.com/TERM/S/user_interface.htmlhttp://www.webopedia.com/TERM/S/VESA.htmlhttp://www.webopedia.com/TERM/S/VESA.htmlhttp://www.webopedia.com/TERM/S/user_interface.htmlhttp://www.webopedia.com/TERM/S/computer.htmlhttp://www.webopedia.com/TERM/S/user.html

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ITU (International Telecommunication Union): an agency of the UN for

international telecommunication coordination.CCITT (an acronym for its French name): one of the organs of ITU (i.e.,

ITU-T), specialized for telephone and data communication systems.

2.4.2 Who's who in the standards world

ISO is a voluntary, nontreaty organization founded in 1946, with members

from 89 member countries. The procedure for ISO to adopt standards:

First, one of the national standards organizations feels the need for an

international standard in some area.

A working group is then formed to come up with a CD (Committee Draft).

The CD is then circulated to all the member bodies, which get six

months to criticize it.

If a substantial majority approves, a revised document, called a DIS

(Draft International Standard) is produced and circulated for comments

and voting.

Based on the results of this round, the final text of the IS (International

Standard) is prepared, approved, and published.IEEE (Institute of Electrical and Electronics Engineers) is the largest

professional organization in the world, is another major player in the

standards world, e.g., IEEE's 802 standard for LANs has been taken over by

ISO as the basis for ISO 8802.

2.4.3 Who's who in the Internet standards world?

The world wide Internet has its own standardization mechanism, different

from those of ITU-T and ISO. When ARPANET was set up, DoD created an

informal committee. In 1983 the committee was renamed the IAB (Internet

Activities Board). Then the meaning was changed as to Internet Architecture

Board. Communication was done by a series of technical reports called

RFCs (Request for comments). RFCs are stored on-line and can be fetched

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by anyone interested in them. They are numbered in chronological order of

creation.

There was another group called IRTF (Internet Research Task Force),

which was made subsidiary to IAB along with the IETF (Internet Engineering

Task Force). Later, the Internet society was created. IRTF concentrated on

long term research and IETF dealt with short term engineering issues and

was divided into working groups, each with specific problem to solve.


1. List two classes of standard organizations

2. Name few standard organizations

3. Discuss the procedure for ISO to adopt standards

2.5 Summary

In this unit we have discussed the architecture of a network that is the entire

task is been spread into layers and different task or design issues that are to

be taken by the layers. We have also seen the different services and service

primitives and the relation of services and protocols. Then we have seen the

two existing reference models ISO-OSI and TCP/IP. We have also given the

comparison of these two models. Finally we glanced on Network

Standardization, its benefits and listed various standards.

2.6 Terminal Questions

1. Discuss the design issues necessary for the layered architecture.

2. Define an interface

3. Describe the ISO-OSI reference model and discuss the importance of

every layer.4. List the layers that are not there in TCP/IP reference model

5. Discuss the procedure for ISO to adopt standards

6. List the service primitives for connection oriented services

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2.7 Answers to Self Assessment Questions

Answer for Section 2.21. It is an agreement between the communicating parties on how is

communication to proceed.

2. Give a five layer example and explain

3. Refer section 2.2.3

4. Connection oriented and connectionless services explain (refer 2.2.4)

Answer for Section 2.3

1. Refer 2.3.1 start from physical layer to application layer.

2. SDLC, HDLC, LAPB, LAPD etc

3. Routing is the major task along with few others refer 3 rd point in 2.3.1

4. TCP and IP


Answer for Section 2.4

1. One that is established by treaty among national governments and the

other Voluntary, nontreaty organizations.

2. ANSI, ITU, IEEE, ISO, VESA

2.8 Answer for Terminal Questions

1. Here discuss the different tasks that are to be taken care for a reliable

communication Refer section 2.2.2

2. Here the interface defines which primitive operations and services the

lower layer offers to the upper one

3. List all the seven layers along with at least two functions of each layer

Refer section 2.3.1 and refer figure 2.5

4. Session layer and presentation layer.Refer figure 2.5 & 2.7 or 2.11


6. LISTEN, CONNECT, RECEIVE, SEND, DISCONNECT
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