Communication concepts(Continued)

Week 2 Lecture 2

Agenda Yesterday, we covered

Media Analogue & digital transmission Multiplexing

Today we will cover Circuit & packet switching Virtual circuits & datagrams Latency & bandwidth

Multiplexing Transmission capacity of the media is

often much greater than the needs of any one user

This capacity can be shared by allowing simultaneous transmission of multiple signals on a single data link

This technique is known as multiplexing There is one device to combine the

signals – a mux and one to separate them again – a demux – at each end of the link

Wide Area Network

Local Area Network

Home ISP

ISP & Telco (IBP)

The last kilometre

Business

WiFiNetwork

Home computer

ISPPC is in Sydney

Web server is inNew York

How do our messagesget through to New York?

Hong Kong

LondonNew York

Sydney

How do we connect two hosts across the world?

Circuit switching Packet switching

Circuit switching The resources required for a connection are

reserved for the duration of the connection. Good for voice, with low bandwidth and

relatively constant usage Set up time to establish a circuit While the circuit is reserved this does not

imply physical resources totally reserved (multiplexing)

When used by data, transfer rate is constant

Packet switching Data is bursty, often high data rates for

short periods, and low usage for other parts of the connection. Inefficient use of circuits

Packet switching breaks the message into small packets, wraps an electronic envelope with address and sends it through the network with other packets from other users

Circuit switching

Packet switching

Packet switching

Packets can be routedthrough the network

Packet switching like public road usage

Driveway Many vehicles share

the road Junctions allow cars to

change direction Map determines

which roads to take to get to destination

Telephone connects to PSTN

Data packets share the link with data from other connects

Switches allow data also to change direction

Packet address instructs switch as to which links should be taken

Multiplexing is not the same as Packet switching Multiplexing allows a physical link to

carry multiple circuits Any one circuit can carry packets for a

number of user “connections” Think of how a road can be divided into

multiple lanes One lane can carry a large number of cars

going to different destinations Another lane may be only available to buses

going from A to B.

Two types of addressing Virtual circuit –

the path is established at the beginning defining each segment the data is to be passed along for the period of the connection

Each of the switches must maintain state information about the connection

X.25, Frame relay and ATM use this form of addressing

Two types of addressing (cont) Datagram

The packet has a final address in a hierarchical form similar to that on an envelope

Switches maintain tables that map destination addresses to outbound links and use this to send the message along

IP uses datagram addressing IP can be carried over virtual circuit protocol

links like X.25 & ATM

Packets can go via anypathPackets thus can get out of sequenceAssembled in right order at end

Store & Forward Packets are routed through a number of

switches in their journey Each switch uses a store & forward mechanism The packet must be totally received and

checked before being sent on the outward link Sometimes the outward link is busy, thus the

packet must be buffered Sometimes buffers are full and the packet is

dropped, and must be sent again This delay is called latency, and the uncertain

nature of this latency is one of the problems with packet switching

Packet V Message switching Why break the message into

packets? Messages can be quite long – eg on

a file transfer In packets you get a parallel

processing through each switch Many messages fit into a packet But packets have an overhead of

data and processing

Quick quiz

You have a 10 second message to transmit through two switches. Will sending it asone whole message, or as 10x1 second packetstake longer, end to end?

10 second file transfer

A B C D

10 second file transfer

A B C D

Message switchA to B 10B to C 10C to D 10

Total transmit time is 30

Packet switch (10 x 1)

A-B B-C C-D

1 sec 1 sec 1 secA-B B-C C-D

1 sec 1 sec 1 secA-B B-C C-D

1 sec 1 sec 1 sec

P1

P2

P3

Total transmit time is 10+2= 12 seconds

Latency & Bandwidth Bandwidth is the speed at which data is

passed down a specific link. Usually expressed in bits per second – bps

Latency is the collective delay from all of the switches in the trip to the final destination

As bandwidth increases, then latency becomes more of an issue

Read Kurose pages 41 to 49 for a more complete discussion