CC Notes Lecture 03

7/29/2019 CC Notes Lecture 03

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Computer Communication &

Networks

Lecture # 03

Circuit Switching, Packet Switching

Course Instructor:

Engr. Sana Ziafat


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Communication Network

Communication networks

Broadcast networks

End nodes share a common channel

(TV, radio)

Switched networksEnd nodes send to one (or more) end nodes

Packet switchingData sent in discrete portions

(the Internet)

Circuit switchingDedicated circuit per call

(telephone, ISDN)

(physical)


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Switching Networks

Long distance transmission is typically done

over a network of switched nodes

A collection of nodes and connections is a

communications network

Nodes not concerned with content of data

End devices are stations

Computer, terminal, phone, etc.

Data routed by being switched from node to

node


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Nodes

Nodes may connect to other nodes only, or to

stations and other nodes

Node to node links usually multiplexed

Network is usually partially connected

Some redundant connections are desirable for

reliability

Two different switching technologies Circuit switching

Packet switching


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Simple Switched Network


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Switching Activities

Some nodes connect only to other nodes

(intermediary nodes). Sole purpose is to

switch data

Some nodes have one or more stationsattached. They accept from and deliver data

to the attached station.

Node-to-node links are usually multiplexed Multiple paths enhance reliability


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Circuit Switched Networks

A circuit-switched network consists of a setof switches connected by physical links.A connection between two stations is a

dedicated path made of one or more links.However, each connection uses only onededicated channel on each link. Each link

is normally divided into n channels byusing FDM or TDM.


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Circuit switching (cntd)

Three phases involved in the communication process:1. Establish the circuit

2. Transmit data

3. Terminate the circuit

If circuit not available: busy signal (congestion)


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In circuit switching, the resources needto be reserved during the setup phase;the resources remain dedicated for theentire duration of data transfer until the

circuit is terminated.

Note


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Circuit switching

A dedicated communication path (sequence of links-circuit)is established between the two end nodesthrough the nodes of the network

Bandwidth: A circuit occupies a fixed capacityof

each link for the entire lifetime of the connection.

Capacity unused by the circuit cannot be used by

other circuits.

Latency: Data is not delayed at switches


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Circuit Switching- Applications

Developed for voice traffic (phone)

Inefficient

Channel capacity dedicated for duration of

connection

If no data, capacity wasted

Set up (connection) takes time

Once connected, transfer is transparent


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Telecom Components Subscriber

Devices attached to network Subscriber line

Link between subscriber and network

Also called Local Loop or Subscriber Loop

Almost all Local Loops are TPW

Range from Few km up to tens of km

Exchange

Switching center in the network

End office specific switching center that supports

subscribers Trunks

Branches between exchanges

Multiplexed


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Circuit Establishment


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Time diagram of circuit switching

circuit

establishment

data

transmission

host 1node 1 node 2

host 2

Delay

host 1- node 1

time

Processing

delay node 1

DATA

Delay

host 2- host 1

switch


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Circuit Switching Concepts

Digital Switch Provide transparent signal path between devices

Typically allows full duplex transmission

Network Interface Control Unit

Establish connections - Generally on demand, Handle andacknowledge requests, Determine if destination isfree,construct path

Maintain connection

Disconnect


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Blocking or Non-blocking Circuit

Switching Blocking

A network may not be able to connect stations

because all paths are in use (more stations than

path) Used on voice systems

Short duration calls

Non-blocking

Permits all stations to connect (in pairs) at once

(at least as many paths as stations)

Used for some data connections


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Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 usersExample:


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Assume that a voice channel occupies a bandwidth of 4

kHz. We need to combine three voice channels into a linkwith a bandwidth of 12 kHz, from 20 to 32 kHz. Show the

configuration, using the frequency domain. Assume there

are no guard bands.

Solution

We shift (modulate) each of the three voice channels to a

different bandwidth, as shown in Figure on next Slide. We

use the 20- to 24-kHz bandwidth for the first channel, the24- to 28-kHz bandwidth for the second channel, and the

28- to 32-kHz bandwidth for the third one. Then we

combine them.

Example


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Example (contd.)


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Five channels, each with a 100-kHz bandwidth, are to be

multiplexed together. What is the minimum bandwidth of

the link if there is a need for a guard band of 10 kHz

between the channels to prevent interference?

SolutionFor five channels, we need at least four guard bands. This

means that the required bandwidth is at least

5 100 + 4 10 = 540 kHz

Example


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Applications

AM Radio

Band 530-1700KHz

Each AM Station needs 10KHz

FM Radio

Band 88-108MHz

Each FM Station needs 200KHz

TV Each Channel needs 6MHz


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Switching Technique

Station breaks long message into packets

Packets sent one at a time to the network

Packets handled in two ways

Datagram

Virtual circuit


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Packet Switching

each end-end data stream

divided into packets

user A, B packets sharenetwork resources

each packet uses full link

bandwidth resources used as needed

resource contention:

aggregate resource

demand can exceed

amount available

congestion: packetsqueue, wait for link

use

store and forward:

packets move one hopat a time Node receives complete

packet before forwarding

Bandwidth division into

pieces

Dedicated allocation

Resource reservation


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Packet switching

- Why not message switching?-

Store-and-Forward

host 1 node 1 node 2 host 2

propagation delayhost 1 node1

processing &set-up delayof a message at

node 1

time

message

message

message


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Use of Packets


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Datagram

Each packet treated independently

Packets can take any practical route

Packets may arrive out of order

Packets may go missing

Up to receiver to re-order packets and

recover from missing packets


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Datagram

Diagram


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Virtual Circuit

Preplanned route established before any

packets sent

Call request and call accept packets establish

connection (handshake)

Each packet contains a virtual circuit identifier

instead of destination address

No routing decisions required for each packet Clear request to drop circuit

Not a dedicated path


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Virtual

CircuitDiagram


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Source-to-destination data transfer in a virtual-circuit network


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Virtual Circuits vs Datagram

Virtual circuits Network can provide sequencing and error control

Packets are forwarded more quickly No routing decisions to make

Less reliable Loss of a node loses all circuits through that node

Datagram

No call setup phase

Better if few packets More flexible

Routing can be used to avoid congested parts of thenetwork


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Circuit vs. Packet Switching

Bandwidthguaranteed

Circuit capacity notreduced by othernetwork traffic

Circuit costs

independent ofamount of datatransmitted, resultingin wasted bandwidth

Bandwidthdynamically allocatedon as-needed basis

May have concurrenttransmissions overphysical channel

May have delays and

congestion More cost-effective,

offer betterperformance


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How do loss and delay occur?

packets queuein router buffers packet arrival rate to link exceeds output link capacity

packets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing(delay)

free (available) buffers: arriving packets

dropped (loss) if no free buffers


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1. Store and forward delay

store-and-forwardpacket switches introduced store andforward delay

delay is proportional to the packet's length in bits.

If a packet consists ofL bits, and the packet is to beforwarded onto an outbound link ofRbps, then the store-and-forward delay at the switch is L/Rseconds.


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2. Queuing Delay

Within each router there are multiple buffers (also calledqueues), with each link having an input buffer (to store packetsthat have just arrived to that link) and an output buffer.

If packet has to wait in output buffer packets suffer output buffer

queuing delays

These delays are variable and depend on the level of congestionin the network.

Since the amount of buffer space is finite, an arriving packet mayfind that the buffer is completely filled with other packets waitingfor transmission packet loss will occur


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Assignment # 01

Q1)Solve the following exercise problems. (Chapter #2)

15, 17, 20, 24

Q2)Solve the following exercise problems. (Chapter #8)

13, 17

Note: Deadline of the assignment is 9thMarch 2011( instart of class)


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Readings

Chapter 8 (B. A Forouzan)

Section 8.1, 8.2, 8.3


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References

Chapter 8 (Data & computer Communication by Behroz A.Forozun)

Chapter 10 ( Computer Communication by William

Stallings) Chapter 1 (Computer Networking by James K. Kurose)


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Q & A

CC Notes Lecture 03

Documents

Transcript of CC Notes Lecture 03