Computer Networks Chapter 1 : Communications Networks and Services.

Computer Networks

Chapter 1 : Communications Networks and Services

Introduction

● Computer networs are today ubiquitous● We use network applications to introduce

networks● We see how services dictate designs● This is meant to give you the “big picture”

on network designs and uses.● Examples: Telephone system, T.V. & Radio

The caller picks up the phone triggering the flow of current in wires that connect to the telephone office.

The current is detected and a dial tone is transmitted by the telephone office to indicate that it is ready to receive the destination number.

The caller sends this number by pushing the keys on the telephone set. Each key generates a pair of tones that specify a number. (In the older phone sets the user dials a number which in turn generates a corresponding number of pulses.)

The equipment in the telephone office then uses the telephone network to attempt a connection. If the destination telephone busy, then a busy tone is returned to the caller. If the destination telephone is idle, then ringing signals are sent to both the originating and destination telephones.

The ringing signals are discontinued when the destination phone is picked up and communication can then proceed.

Either of the users terminate the call by putting down a receiver.

Telephone Office

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Telephone Office

2.

Telephone Office

3.

4.

Telephone Office

5.

6.

Telephone Office

Telephone Office

Telephone services● Connection-oriented: prelude before comm.● Beyond the simple connection, telephone

system provides: – Tool free calls, Credit card billed calls, – reliability, high availability, low delay,– high quality (more thanjust intelligible voice), – security, privacy, – Caller ID, Call return, Call waiting, Voice

mail...etc

● Cellular telephony extends to mobile users

Other services

● Email is very fault tolerant as an application.● WWW is interactive and introduces some

interactive use of networks.● End user specifies URL, and protocol (http)

http:\\www.alakhawayn.ma● Protocol? Set of rules that govern interaction

(chapter 2)

The user clicks on a link to indicate which document is to be retrieved.

The browser must determine the address that contains the document. It does this by sending a query to its local name server.

Once the address is known the browser establishes a connection to the specified machine, usually a TCP connection. In order for the connection to be successful, the specified machine must be ready to accept TCP connections.

The browser runs a client version of HTTP, which issues a request specifying both the name of the document and the possible document formats it can handle.

The machine that contains the requested document runs a server version of HTTP. It reacts to the HTTP request by sending an HTTP response which contains the desired document in the appropriate format.

The TCP connection is then closed and the user may view the document.

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Ever more demanding applications

● Video on demand...the future of video rental: Little tolerance to Jitter, Huge amount of data for good quality audio/video

● Audio-visual conferencing (videoconferencing), all of the above plus interactivity and little tolerance to delay.

Realplayer example

Copyright © 1995-2000, RealNetworks, Inc. All rights reserved. RealPlayer is a trademark of RealNetworks, Inc.

Network Design

● Different applications>Different requirements>Diff. network design approach

● Requirements? Transfer delay, Reliability, Accuracy, Volume of info., Convenience, Speed, Cost.

● Switching approaches: Message switching, circuit switching and packet switching.

Minimum functionality● A network needs to provide:

– Connectivity (transmission system=cables)● Adapted to the type of data (bits, analog,...)

– Routing (switches – axchanges on a highway)– Forwarding (retransmission in th right direction)– Multiplexing (to connect access networks =

trunks)– Traffic control, congestion/overload control– Network management: Monitoring of

performance, cost, billing,

t0t1

Network

Information transmitted in a single block

Information transmitted in multiple blocks

Full mesh NetworkN(N-1) connections

(a) A switch provides the network to a cluster of users

(b) A multiplexer connects two access networks

Access network using switch =N connections

Metropolitan network A consists of access subnetworks a, b, c, d.

Hierarchical network topology

National network consists of regional subnetworks .

Metropolitan network A is part of regional subnetwork .

Backbone to connect MANs

A

A

Metropolitan

1*

a

c

b

d

(a)

(b)

2

34

Addressing

● Same routes shared among several users => Necessity of addresing.

● Two types of addressing:– Hierarchical for WANs: ß.A.1– Flat for LANs: Broadcast packets for all to see

Telegraph Networks

● All text represented using dots and dashes● Transmitted over copper wire using

electrical current.● First binary digital transmission system. The

more frequent the letter the shorter the message: data rate 20 bps

● Routing by operator. Message stored until forwarded: Message switching

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nsm

issi

on c

apac

ity

(bit

s/se

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1.0E+10

1.0E+12

1.0E+14

1850 1875 1900 1925 1950 1975 2000

Printing telegraph

Baudot multiplex

T-1 carrier

T-4 carrier

SONET OC-48

DWDM

Circuit Switching

● Dedicated phone line for every pair unsustainable > Introduce switch

● Connection setup and kept for duration of call = Circuit switching

● Addressing used at setup only● PCM to digitize voice. Multiplex 24

channels to a 1.5 Mbps T1 (US)● Today a DWDM carries 1600 Gigbps

(a) A switch in the form of an operator with a patch cord panel (not shown)

(b) Cords interconnect user sockets providing end-to-end connection

Figure 1.10

Tandem

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Toll

CO COCO

CO

Tandem

Packet Switching

● First for dumb terminals using polling● A more efficient line sharing: Statistical

multiplexing● Transmission over faulty lines>Error

protection: Check bits● Connectionless/datagrams or virtual

connection (as in ATM)

(a) Time-Shared Computers & Cables for Input Devices

(b) Dial In

C...T T

C...

T

T

T

ModemPool

PSTN Modem T

T = terminal

C

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Response from terminal

Poll to terminal

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Host...

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Address Info

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New York City

San Francisco

Chicago Atlanta

Host

UCLA RAND TINKER

USC

NBS

UCSB

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SCD

BBN

STAN

AMES

AMES McCLELLAN UTAH BOULDER GWC CASE

CARN

MITRE

ETAC

MIT

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LINC

RADC

ARPANET: WAN, Distributed Routing , End-to-end Flow control)

(a) (b)

transceivers

Ethernet: LAN, Broadcasting MAC,

G

G

G

G

G

net 1

net 2

net 3

net 4

net 5

G = gateway

G

Internet: Connects heterogenous networks, TCP/IP

Domain Name Service

● Routing and identification is based on IP address: meaningless to humans

● To give some meaning to IP numbers, we associate them with domain names.

Technology

Regulation Market

Standards

time

Capability

Initial class ofinvention

Second type ofinvention

Third type ofinvention

time

Capability

(a) (b)