Lecture # 03Switching

Course Instructor:Engr. Sana Ziafat

Communication NetworkCommunication networks

Broadcast networksEnd nodes share a common channel

(TV, radio…)

Switched networks End nodes send to one (or more) end nodes

Packet switchingData sent in discrete portions

(the Internet)

Circuit switchingDedicated circuit per call

(telephone, ISDN)

(physical)

Figure 8.2 Taxonomy of switched networks

Switching NetworksLong distance transmission is typically done

over a network of switched nodesA collection of nodes and connections is a

communications networkNodes not concerned with content of dataEnd devices are stations

Computer, terminal, phone, etc.Data routed by being switched from node to

node

NodesNodes may connect to other nodes only, or to

stations and other nodesNode to node links usually multiplexedNetwork is usually partially connected

Some redundant connections are desirable for reliability

Two different switching technologiesCircuit switchingPacket switching

Simple Switched Network

Switching ActivitiesSome nodes connect only to other nodes

(intermediary nodes). Sole purpose is to switch data

Some nodes have one or more stations attached. They accept from and deliver data to the attached station.

Node-to-node links are usually multiplexedMultiple paths enhance reliability

Circuit Switched NetworksA circuit-switched network consists of a A circuit-switched network consists of a

set of switches connected by physical set of switches connected by physical links. A connection between two stations links. A connection between two stations is a is a dedicated pathdedicated path made of one or more made of one or more links. However, each connection uses links. However, each connection uses only one dedicated channel on each link. only one dedicated channel on each link. Each link is normally divided into n Each link is normally divided into n channels by using FDM or TDM.channels by using FDM or TDM.

Circuit switching (cnt’d)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)

8.10

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

circuit is terminated.

Note

Circuit switching

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

Bandwidth: A circuit occupies a fixed capacity of 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

Circuit Switching- ApplicationsDeveloped for voice traffic (phone)Inefficient

Channel capacity dedicated for duration of connection

If no data, capacity wastedSet up (connection) takes timeOnce connected, transfer is transparent

Telecom ComponentsSubscriber

Devices attached to networkSubscriber line

Link between subscriber and network Also called Local Loop or Subscriber Loop

Almost all Local Loops are TPWRange from Few km up to tens of km

ExchangeSwitching center in the networkEnd office specific switching center that supports

subscribersTrunks

Branches between exchangesMultiplexed

Circuit Establishment

Time diagram of circuit switching

circuit establishment

data transmission

host 1 node 1 node 2 host 2

Delay host 1- node 1

time

Processing delay node 1

DATA

Delay host 2- host 1

switch

Circuit Switching: FDM and TDMFDM

frequency

time

TDM

frequency

time

4 users

Example:

Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.

SolutionWe 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, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them.

Example

Example (contd.)

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

ApplicationsAM Radio

Band 530-1700KHzEach AM Station needs 10KHz

FM RadioBand 88-108MHzEach FM Station needs 200KHz

TVEach Channel needs 6MHz

Switching TechniqueStation breaks long message into packetsPackets sent one at a time to the networkPackets handled in two ways

DatagramVirtual circuit

Figure 8.7 A datagram network with four switches (routers)

8.23

Figure 8.8 Routing table in a datagram network

8.24

A switch in a datagram network uses a routing table that is based on the destination address.

Note

Packet Switchingeach end-end data stream

divided into packetsuser A, B packets share

network resources each packet uses full link

bandwidth resources used as needed

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time Node receives complete

packet before forwarding

Bandwidth division into “pieces”

Dedicated allocation

Resource reservation

Packet switching- Why not message switching?-

Store-and-Forward

host 1 node 1 node 2 host 2

propagation delay host 1 – node1

processing & set-up delay of a message at node 1

time

message

message

message

Use of Packets

DatagramEach packet treated independentlyPackets can take any practical routePackets may arrive out of orderPackets may go missingUp to receiver to re-order packets and

recover from missing packets

DatagramDiagram

Virtual CircuitPreplanned route established before any

packets sentCall request and call accept packets establish

connection (handshake)Each packet contains a virtual circuit

identifier instead of destination addressNo routing decisions required for each

packetClear request to drop circuitNot a dedicated path

VirtualCircuitDiagram

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

Virtual Circuits vs DatagramVirtual circuits

Network can provide sequencing and error controlPackets are forwarded more quickly

No routing decisions to makeLess reliable

Loss of a node loses all circuits through that nodeDatagram

No call setup phase Better if few packets

More flexible Routing can be used to avoid congested parts of the

network

Circuit vs. Packet SwitchingCircuit Switched

Bandwidth guaranteed

Circuit capacity not reduced by other network traffic

Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth

Packet SwitchedBandwidth

dynamically allocated on as-needed basis

May have concurrent transmissions over physical channel

May have delays and congestion

More cost-effective, offer better performance

How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link

capacitypackets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

1. Store and forward delaystore-and-forward packet switches introduced store

and forward delay

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

If a packet consists of L bits, and the packet is to be forwarded onto an outbound link of R bps, then the store-and-forward delay at the switch is L/R seconds.

2. Queuing DelayWithin each router there are multiple buffers (also called

queues), with each link having an input buffer (to store packets that 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 congestion in the network.

Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely filled with other packets waiting for transmission packet loss will occur

38

ReadingsChapter 8 (B. A Forouzan)

Section 8.1, 8.2, 8.3

ReferencesChapter 8 (Data & computer Communication by

Behroz A. Forozun)Chapter 10 ( Computer Communication by William

Stallings)Chapter 1 (Computer Networking by James K.

Kurose)

Q & A