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8/3/2019 MSIS 4523 Ch8.Switching
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Data Communications SystemsCh 8: Switching
JinKyu Lee, Ph.D.
Include the course code (MSIS4523) in every email subject!!
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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
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Figure 8.3 A trivial circuit-switched network
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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
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Figure 8.4 Circuit-switched network used in Example 8.1
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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
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Figure 8.5 Circuit-switched network used in Example 8.2
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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
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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