Computer Networks Chapter 2 – Network Models. Summer 2006Computer Networks2 Communication Tasks ...

Computer Networks

Chapter 2 – Network Models

Summer 2006 Computer Networks 2

Communication Tasks

The tasks given below need to be taken care of in any kind of communication system.

Signal generation Transmission Interfacing Synchronization Error detection

and correction Flow control

Addressing and routing

Recovery from malfunctioning

Message formating Security Network management Applications


Dealing with Communication Tasks

Communication system is complicated and need to solve a large number of tasks

Solution Use the devide and conqure technique This technique splits the problem in

managable pieces and is therefore convinient for dealing with complex systems


Design Principle

Communication tasks are divided into series of layers or levels Each layer is responsible for particular

task and act on them by using one or more protocols

Each layer is built upon one bellow it The number and name of the layers

differ from network to network


The Concept of Layers

Layer n on one machine communicates with layer n on the other machine via layer n protocol. The communication is virtual

Peers are entities comprising the corresponding layers on different machines.

There is an interface between each pair of adjacent layers for communication with the layer above and the layer below.


An Example of Five Layers Network

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Physical medium

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Layer 5 protocol

Layer 4 protocol

Layer 3 protocol

Layer 2 protocol

Layer 1 protocol

Layer 4/5 interface

Layer 3/4 interface

Layer 2/3 interface

Layer 1/2 interface

Machine 1 Machine 2

The path through which the actual transmission take place


Peer-to-peer Processes

The processes on the two machines that communicate at a given layer are called peer-to-peer processes

At the physical layer communication is direct At the upper layers the communication has

to go down through the layers on the sender machine, than to be transmited through the physical layer and than to go back up to the same layer at the receiving machine


ProtocolsConstruction versus Reduction

H – header (control data added at the front end of the data unit)T – trailer (control data added at the back end of the data unit)Trailers are usually added only at layer 2

Layer 5

Layer 4

Layer 3

Layer 2

Physical BITS

H2 DATA UNIT T2

DATA UNIT

H4 DATA UNIT

H5 DATA

H3

DATA

Construction

Reduction

Layer 5

Layer 4

Layer 3

Layer 2

Physical


Illustration of the Construction and Reduction Process

Observe how headers and trailer are added at the sender and removed at the receiver

Annimation of Figure 2.4 in the book


Messages and Protocol Stacks

On the sender machine, each layer: Accepts an outgoing message from the layer above Adds a header and does other processing Passes resulting message to next lower layer

On the receiver, each layer: Receives an incoming message from the layer below Removes the header for that layer and performs other

processing Passes the resulting message to the next higher layer


Drawbacks in the layered architecture

The resources necessary for the user’s data (payload) are also used for transfering information in the headers and trailers. This cannot be avided. However, well designed protocols should

have as litle as possible additional information carried, yet provide in the best way the service required.


Service vs. Protocol

Service Treat the protocol as a black box. what service

is provided to the upper layer

Protocol How do the peer entities cooperate with each

other and the lower layer to provide the service to the upper layer

Service

Lower layer Lower layer

InterfaceInterface

Upper layer Uper layerUpper layer protocol

Lower layer protocol


Network Architecture

A set of layers and protocols Enough information to allow

implementers to write a program or build the hardware for each layer, so that it obeys the appropriate protocol

Protocol stack or protocol suit – a list of protocols used by a certain system


Protocol Suites

Open Sysytem Interconnection (OSI) Used mostly as a reference model

Internet (TCP/IP) Most popular suite today

Xerox Networking Sysytems (XNS) System Network Architecture (SNA –

IBM) Digital Network Architecture (DNA –

DEC) NetBIOS (Software interface)


Open System Interconnection (OSI)

Developed by International Standard Organization (ISO) as a first step towards international standardization De jure protocol

Deals with interconnecting systems that are open for communication with other systems Open protocol suite

Good as theoretical model, but not widely implemented in practice


The Layers of OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

Data Link

Physical

IntermediateSystem

EndSystem

Application

Presentation

Session

Transport

Network

Data Link

Physical

R


The OSI layers

Physical layer Transmission of unstructured bit stream

Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium

Data link layer Provides reliable transfer across the physical

link between two ends connected via single link Sends blocks of data (frames) with the necessary

synchronization, error control and flow control Can add header and trailer


The OSI layers

Network layer Provides upper layers with independence from

the data transmission and switching technologies accross internetwork Responsible for source-to-destination delivery,

addressing and routing in the internetwork

Transport layer Provides transparent transport of data

between end points that might not be connected via single link Provides source-to-destination connection, error

recovery and flow control


The OSI layers

Session layer Provides the control structure for communication

between applications (dialog control) Establishes, manages and terminate connections

(sessions) between cooperating applications

Presentation layer Provides independence to the application

processes from differences in data representation Application layer

Provides access to the OSI environment for users and provides distributed information services


Summary of OSI Layer Functions

Application

Presentation

Session

Transport

Network

Data Link

Physical

To allow access to network resources

To establish, manage and terminate sessions

To move packets from source to destination; to provide internetworking

To transmit bits over a medium; to provide mechanical and electrical specifications

To translate, encrypt and compress data

To provide reliable end-to-end message delivery and error recovery

To organize bits into frames, to provide node-to-node delivery


Illustration of the Source-to-end Delivery at the Network Layer

Observe how data are sent from node to node to reach the final destination.

Animation of Figure 2.11 in the book


TCP/IP Protocol Suite

De facto (and after that de jure) standards Open (All modification and newly proposed

protocols are published in a form of RFC (Request for Comments)

RFC as well as drafts are published on the Internet can be found on many URL (one is

www.rfc-editor.org) RFC becomes a standard when it is:

Stable and well understood Technically competent Implemented on multiple independent places


The TCP/IP Protocol Suite (Cont.)

Allows computers of many sizes, vendors and operating systems to communicate with each other

History: Developed as de facto standard before OSI 1960’s: started as goverment financed research

project 1990’s: most widely used form of networking

Forms the basis for the Internet (capital “I”)(a WAN that spans the globe)


TCP/IP Layers and Relation to OSI

Transport

Network

Application

Physical

Network

SMTP FTP

TELNET DNS

HTTP Application

Presentation

Session

TransportTCP UDP

NetworkIGMPICMP

IP

RARPARP

Data Link

Physical

Protocols defined by the underlying networks


Some Standard Organizations

ITU: International Telecommunication Union

CCITT: International Telegraph and Telephone Consultative Committee

ISO: International Standards Organization

IEEE: Institute of Electrical and Electronic Engineering

IRTF: Internet Research Task Force IETF: Internet Engineering Task Force

Computer Networks Chapter 2 – Network Models. Summer 2006Computer Networks2 Communication Tasks ...

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