MSIS 4523 Ch8.Switching

8/3/2019 MSIS 4523 Ch8.Switching

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Data Communications SystemsCh 8: Switching

JinKyu Lee, Ph.D.

[email protected]

Include the course code (MSIS4523) in every email subject!!



Topics

Switched Networks

Circuit Switching

Three phases

Performance characteristics (sources for delays) Packet Switching

Datagram Network

Routing Table, Performance characteristics

Virtual Circuit Network VCI, Connection process, Performance characteristics



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Switching

Switched network is a series of interlinked nodes Switches are devices that create temporary

connections between two or more devices linkedto the switch

Most switching we see today is either circuit orpacket based



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Figure 8.2 Taxonomy of switched networks



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Figure 8.1 Switched network



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• A circuit-switched network is made of a set of switches connected by physical links, in which

each link is divided into n channels by using

FDM or TDM. Circuit switching takes place atthe physical layer

• Need to reserve resources at setup; then

resources are dedicated through the duration of

data transfer until teardown

• Data is not packetized

• No addressing involved

CIRCUIT-SWITCHED NETWORKS



Figure 8.3 A trivial circuit-switched network



As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area.

Communication is through 4-kHz voice channels. We

assume that each link uses FDM to connect a

maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure 8.4 shows the situation.

Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8;

and 4 to 6. Of course the situation may change when

new connections are made. The switch controls the connections.

Example 8.1



Figure 8.4 Circuit-switched network used in Example 8.1



As another example, consider a circuit-switched

network that connects computers in two remote offices of a private company. The offices are connected using

a T-1 line leased from a communication service

provider. There are two 4 × 8 (4 inputs and 8 outputs)

switches in this network. For each switch, four output

ports are folded into the input ports to allow

communication between computers in the same office.

Four other output ports allow communication between the two offices. Figure 8.5 shows the

situation.

Example 8.2



Figure 8.5 Circuit-switched network used in Example 8.2



More on Circuit Switching

Three PhasesSet up

Data Transfer

TeardownEfficiency: Not very good as resources

are tied up for the duration of the

connectionDelay: Minimal delay; circuit-switching is

good for telephone use



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Figure 8.6 Delay in a circuit-switched network



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DATAGRAM NETWORKS

In data communications, we need to send

messages from one end system to another. If the

message is going to pass through a packet- switched network, it needs to be divided into

packets of fixed or variable size. The size of the

packet is determined by the network and the

governing protocol.



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Packet-Switched Datagram Network

In a packet-switched network, there is no resource

reservation; resources are allocated on demand

Datagram networks treat each packet independently

of others

Connectionless network

A switch in a datagram network uses a routing table

that is based on the destination address

The destination address in the header of a packet in adatagram network remains the same during the entire

journey of the packet



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Figure 8.7 A datagram network with four switches (routers)



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Figure 8.8 Routing table in a datagram network



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More on Packet Switching

No set up or tear down phasesEfficiency: Very good as resources are

used on demand; resources are used onlywhen the packets are to be transferred

Delay: Additional delay over circuitswitching; the packets may have to wait atthe switch



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Figure 8.9 Delay in a datagram network



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VIRTUAL-CIRCUIT NETWORKS

A virtual-circuit network is a cross between a circuit-

switched network and a datagram network.

It has some characteristics of both.

Virtual-Circuit Networks has nothing to do withVirtual Private Network (VPN)!!



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VCN Characteristics

Set-up and Tear-down phaseResources can be allocated at set-up or

on demand

All packets follow the same path

Special addressing is needed.



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VCN Addressing

Two types of addressing are used: globaland local

Global address unique in the scope of thenetwork

Virtual Circuit Identifier (VCI) is the localaddress to get from one switch to the next

Each switch has a table of addresses



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Figure 8.11 Virtual-circuit identifier



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Figure 8.12 Switch and tables in a virtual-circuit network



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Set-up and Acknowledgement

Need to have a mechanism to initiate thecommunication

This is done through Set-up andAcknowledgement

Set-up sends a frame from sender toreceiver to reserve the resources

Acknowledgement frame replies to

complete the set-up.



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Figure 8.14 Setup request in a virtual-circuit network

A B

Src Dst

Global ID

A B

Src Dst

Global ID

A B

Src Dst

Global ID

B A



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Figure 8.15 Setup acknowledgment in a virtual-circuit network

A B

Src Dst

Global ID

A B

Src Dst

Global ID

A B

Src Dst

Global ID

B A7722

66

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Figure 8.13 Source-to-destination data transfer after a VC is setup



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More on V-C Switching

Efficiency: Good - resources are used ondemand. The source can check theavailability of resources

Delay: A little delay due to initial set-up



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Figure 8.16 Delay in a virtual-circuit network



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STRUCTURE & TYPES OF A SWITCH

Circuit Switches

Space Division Switches: Separates the path ofthe circuits spatial

Crossbar and Multistage

Time Division Switching

Time-slot interchange

Time and Space Combination Time-Space-Time Switch



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Packet Switches

Input Ports: Doorway into the switchBits constructed from received signalDecapsulated from the frameErrors detected and corrected

Output Port: Same functions as input portbut in reverse

Routing Processor: Uses routing tables tofind the address of the next hop based on

destination addressSwitching Fabric: Cross-bar, Banyan,

Batcher-Banyan



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Figure 8.21 Packet switch components

MSIS 4523 Ch8.Switching

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