January 2000Communication Networks1 Computer Networks: Architecture & Concepts Abdulaziz Almulhem

January 2000 Communication Networks 1

Computer Networks: Architecture & Concepts

Abdulaziz Almulhem


Communication

People need to communicate with each other.

Why? To exchange information. What? Voice, Sound, Graphics,

Pictures, Text, or Data One way is to use the postal

service.


Communication (cont.)

Better, pull wires between each two people. (point-to-point communication)» cost is high and resources are wasted» complexity (mesh, drop and add)

We need to build a shared communication media (links)» Efficient» needs management (data may wait)


Communication (cont.)

We build communication networks. These are arrangements of

hardware and software that allow users to exchange information. (computer net)

Thus we have two types of nodes:» terminal nodes» communication nodes


Computer Network?

An interconnected collection of autonomous computers and computer resources

Terminal node

communication node

Shared medium


Simple Data Communication Model

TransceiverTransceiverTransport System

DigitalAnalog/Digital

Digital

001101

001101

Data Network

Public Telephone Network


Network Services

The objective is to provide services to the users.» Information transportation» signaling» billing

A service is the execution of a sequence of basic actions on network resources. (how to place a phone call?)


Network Services (cont.)

Scripts are executed by the communicating entities only.

These scripts could be very complex.

It is more efficient to have a modular construction of scripts:» action it performs» interaction to other modules


Network Services (Cont.)

Advantages of such modular:» reusability» upgradability» interoperability

The general organization of services into simpler services is called the network architecture.


Communication Protocols To provide error-free and convenient

information transfers, the network communication is regulated by a set of rules and conventions called network protocols.

Protocols define connectors, cables, signals, data formats, error control techniques, and algorithms for message preparation, analysis and transfer.


Protocol Data Units (PDU)

Protocol entities exchange PDUs» Each PDU must contain two major parts:

– Header: Identifies how the following parts are to be

handled and routed.

– Message: This is the message body itself. This is where the protocol is determined to be

character oriented or bit oriented.

Header Message Trailer


OSI Reference Model of ISO

Architecture/structure that defines communication tasks and which would:» Serve as a reference model for international

standards» would facilitate efficient internetworking among

systems from different technologies, manufacturers, administrations, nationalities, and enterprises.


Reference Model


Most Important Standards Organizations

ITU-T: International Telecommunication Union (a United Nations specialized agency, was created on March 1, 1993)

ISO: International Organization for Standardization (an international voluntary, nontreaty organization, founded in 1946)

IETF: Internet Engineering Task Force (responsible for publishing RFCs (Requests For Comments))

IEEE: Institute of Electrical and Electronic Engineers (ATM Forum: This organization is not a standard organization. After ITU defined

the ATM concept in Nov 1990, ATM Forum was initiated in October 1991 to accelerate the deployment of ATM products and services. ATM Forum develops implementation agreements and publishes them as “specifications” on its web site: www.atmforum.com)


ISO OSI Reference Architecture

The architecture is layered to reduce complexity.» Each layer offers certain services to the

layer immediately above it.» Each layer shields the higher layer from

the details of implementation of how the services are offered.

» Layer "n" on one station carries on a conversation with layer "n" on another network station.


7 Application ftp, telnet, email, www, etc.

6 Presentation Data representation

5 Session Negotiation and connection

4 Transport End-to-end delivery

3 Network Addresses and best path (routing)

2 Data Link Access to media (transfer of frames) 1 Physical Binary transmission and cabling

Layer Functions


Application Application Presentation Presentation Session Session Transport Transport NetworkNetwork Data Link Data Link Physical Physical

Layer Functions

bits

frames

packets

segments

Host A Host B


Data Encapsulation

Router Router

RouterRouter

Frames

Bits

Packets Data

DataSalamsSalams

7. application6. presentation5. session4. transport


Data Encapsulation Example

segment data header

network segment data header header

data

Frame Network Segment Data Frame header header header trailer

Data

Segment

Packet

Frame

Bits01111111010101101000100100010110101

Router


Summary Internetworking evolved to support current and

future applications The OSI reference model organizes network

functions into seven layers Data flows from upper-level user applications to

lower-level bits transmitted over network media Peer-to-peer functions use encapsulation and de-

encapsulation at layer interfaces Most network manager tasks configure the lower

three layers


Interfaces and Services

Each layer in the OSI layered architecture has a function (service) to be provided to the upper layer through some interface (service access point).

The upper layer can request a service from the lower layer through SAP.


Interface and services (cont.)

When layer n+1 requests a service from layer n, Layer n will encapsulate the PDU of layer n+1 into a new PDU to be handed to layer n-1.

Services provided by each layer:» Connection-oriented (telephone)» Connectionless (post)


Service Primitives

Layer services are specified by a set of operations available to the layer above it.

These operations either:» Ask for the service» report the status of the service


Service Primitives (cont.)

Layer n+1

Layer n

Request

Indication Response

Confirm


Services and Protocols

Services are set of operations provided to upper layers or user.

Protocols are sets of rules governing how the same peers should be communicating.

Protocols are transparent from user and may change without changing services.


Protocols and Services

Session

Transport

Network

Session

Transport

NetworkServ

ices

Protocols


Application, Presentation,

and Session Layers


Application Layer

Computer Applications

Word Processing Presentation Graphics Spreadsheet Database Design/Manufacturing Project Planning Others

Network Applications

Electronic mail File Transfer Remote Access Client/Server Process Information Location Network Management Others


Application Layer (cont.)

Network Applications

(For enterprise communication)

Electronic mail File Transfer Remote Access Client/Server Process Information Location Network Management Others

Internetwork Applications(Extend beyond the enterprise)

Electronic Data Interchange World Wide Web E-mail Gateways Special-Interest Bulletin Boards Financial Transaction Services Internet Navigation Utilities Conferencing (Video, Voice,

Data)


Presentation Layer Text Data

» ASCII» EBCDIC» Encrypted

Sound Video

» MIDI (Musical Instrument Digital Interface)» MPEG (Motion Picture Experts Group)» QuickTime


Presentation Layer

Graphics Visual Images

» PICT(format to transfer QuickDraw graphics between Macintosh or PowerPC programs)

» TIFF (Tagged Image File Format)» JPEG (Joint Photographic Experts Group)» GIF

Provides code formatting and conversion for applications


Session Layer

Coordinates applications as they interact on different hosts (dialogue control and synchronization)

Service Request

Service Reply


Session Layer (contd.)

Network File System (NFS)– Allows transparent access to remote network resources

Structured Query Language (SQL) Remote-Procedure Call (RPC)

– RPC procedures are built on clients and executed on servers

X Window System– Allows intelligent terminals to communicate with remote

UNIX machines AppleTalk Session Protocol (ASP)

– Establishes and maintains sessions between an AppleTalk client and server

DNA Session Control Protocol (SCP)


Transport & NetworkLayers


Transport Layer Overview

Segments upper-layer PDUs Establishes an end-to-end connection Sends segments from one end host to

another Ensures end-to-end data reliability


Segment Upper-Layer PDUs

Transport segments share traffic stream

Application Data Application Data port port

Electronic mail

File transfer

Terminal session

ApplicationPresentationSession

Transport


Establishes Connection

sender

receiver

synchronize

Negotiate connection

synchronize

Acknowledge

Connection established

Data transfer (send segments)


Establishes Connection

sender

receiver

transmit

not ready

ready

Resume Transmission

Buffer full process segments

Buffer OK


Reliability with Windowing

In the most basic form of reliable connection-oriented transfer, data segments must be delivered to the recipient in the same sequence that they were transmitted.

Windowing is a method to control the amount of information transferred end-to-end. Some protocols measure information in terms of number of packets


Reliability with Windowing

sender

receiver

sender

receiver

Send 1

Send 2

Send 1

Send 2

Send 3

Window size 1

Window size 3

Receive 1

Receive 2

Receive 3

Receive 1

Receive 2

ACK 2

ACK 3

ACK 4

Send 4


PAR Technique Reliable delivery guarantees that a stream of

data sent from one machine will be delivered through a functioning data link to another machine without duplication or data loss. Positive acknowledgement with retransmission is one technique that guarantees reliable delivery of data streams.

The sender keeps the record of each segment it sends and waits for an acknowledgement.

The sender also starts a timer when it sends a segment, and it retransmits a segment it the timer expires before an acknowledgement arrives.


PAR Technique (contd.)

send 1 send 2 send 3 Ack 4 send 4 send 5

send 6Ack 5

send 5Ack 7

sender receiver

1 2 3 4 5 61 2 3 4 5 6

X


Transport to Network Layer

End-to-end segments

Routed packetsRouter

Router

Router

RouterRouter


Network Layer

Controlling the operation of the network.

How to route data from source to destination? » Static routing tables» Dynamic routing tables

Datagram is similar to a postal letter.


Summary Presentation layer formats and converts network

application data to represent text, graphics, images, video, and audio.

Session-layer functions coordinate communication interactions between applications.

Reliable transport-layer functions include» Multiplexing» Flow control» Error recovery» Reliability through windowing


Physical and Data Link Layers


Physical and Data-Link Standards

The data link layer provides data transport across a physical link. To do so, the data link layer handles physical addressing, network topology, line discipline, error notification, orderly delivery of frames , and optional flow control.

The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link between end systems.

These requirements and characteristics are codified into standards.


LAN Data-Link Sublayers

Logical Link Control

Media Access Control

MAC Frame 802.2 LLC Packet or datagram

Network

Physical

Data Link

LLC

MAC



LLC refers upward to higher-layer software functions.

MAC refers downward to lower-layer hardware functions.

LAN protocols occupy the bottom two layers of OSI reference model: the physical layer and data link layer.



The IEEE 802 committee subdivided the data link layer into two sublayers: » The logical link control (LLC) sublayer» The media access control (MAC) sublayer

The LLC sublayer provides for environments that need connectionless or connection-oriented services and the data link layer.

The MAC sublayer provides access to the LAN medium in an orderly manner.


LLC Sublayer Functions

Enable upper layers to gain independence over LAN media access.

Allow service access points (SAPs) from interface sublayers to upper-layer functions.

Provide optional connection, flow control, and sequencing services.


Client-Server Model


Client Server Model

Client-Server paradigm is the primary pattern of interactions among cooperating applications.

This model constitutes the foundation on which distributed algorithms are built.


What is the Client-Server Paradigm?

The paradigm divides communicating applications into 2 broad categories, depending on whether the application waits for communication or initiates it.» An application that initiates a

communication is called a client.» End users usually invoke a client software

when they use a network service.


Client Server Model (cont.)

Server: Any program that offers a service reachable over the network» If a machine’s primary purpose is to

support a particular server program, the term server is usually applied to both, the machine and the server program

Client: An executing program becomes a client when it sends a request to a server and waits for a response



A server is any program that waits for incoming communication requests from a client.» Each time a client application needs to contact a

server, it sends a request and awaits a response.

» The server receives a client’s request, performs the necessary computation, and returns the result to the client.

» When the response arrives at the client, the client continues processing.



Request

Reply

Machine Running Client Application

Machine Running Server Application

ClientProgram

ServerProgram



A Misconception:» Technically, a server is a program and

not a piece of hardware.» However, computer users frequently

(mis)apply the term to the computer responsible for running a particular server program.– For example, Web Server, is usually a

computer running the http server program.


Summary Internetworking evolved to support current and

future applications. The OSI reference model organizes network

functions into seven layers. Data flows from upper-level user applications to

lower-level bits transmitted over network media. Peer-to-peer functions use encapsulation and de-

encapsulation at layer interfaces. Client-Server paradigm constitutes the foundation

on which distributed algorithms are built.

Documents

January 2000Communication Networks1 Computer Networks: Architecture & Concepts Abdulaziz Almulhem