Ch 8. Switching

Switch Devices that interconnected with each other

Connecting all nodes (like mesh network) is not cost-effective

Some topology like bus has limitation on distance Switched network, where end systems (e.g., A, B,

… J) are connected through switches (e.g., I, II, … V)

Taxonomy of Switched Networks

Telephone system

The Internet (IP)

Circuit-Switched Networks A set of switches are connected by physical

links Each link is divided into n channels using FDM or

TDM Each connection has one dedicated channel

Example (n = 3)

Circuit Switching Resources (channels, switch buffer, switch

processing time, switch ports, etc) must be reserved before communication, and released after communication

Three phases Setup – establish a connection and reserve the

resources for communication Data transfer – do communication Teardown – finishing communication, release the

resources

Setup

Teardown

Example of Circuit-Switched Network Three switches are used to “route” 4

connections

Properties of Circuit-Switching Require resource reservation and release Data do not needed to be packetized No addressing is involved during data transfer Low efficiency

Reserved resources are unavailable to others Low delay

Except the setup delay for the resource reservation

Datagram Networks Messages pass through packet-switched

networks, without reserving resources Resources are allocated on demand

Data should be divided into small pieces, called packet or datagram

Each packet is treated independently of others Network has no idea about data stream

Often called, connectionless networks, since the switch does not keep information about the connection state

Example of Datagram Network

Routing How the switches route packets without

reserving resources? Each packet carries its destination address Each switch keep routing table, which is

dynamic and updated periodically Routing table

Specifies the output port of the switch for each destination address

Properties of Packet-Switching Resources are allocated on demand Data should be packetized, and each packet

should include its destination address High efficiency – more multiplexing High delay

Recall the Taxonomy

Telephone system

The Internet (IP)

Virtual-Circuit Networks Can be regarded as a blend of both a circuit-

switched network and a datagram network Three phases: setup, transfer, and teardown Resources can be reserved at setup, or allocated

on demand Data are packetized and each packet carries an

address Normally, switches are implemented at

physical layer - Circuit-switched networks network layer - Packet-switched networks data link layer - Virtual-circuit networks

Addressing Two-level addressing

Global addressing – address is unique over networks

Virtual-circuit identifier (VCI, local addressing) – used by a frame between two switches

Routing Table Routing using (port, VCI)

End-to-end Data Transfer

Connection Setup (1) Request (source destination)

How does switch 1 know it should go to port 3? This will be covered later

Connection Setup (2) Acknowledgement (destination source)

Properties of VC Switching Three phases

Setup Data transfer – all packets belonging to the same

source and destination travel the same path Teardown – the similar method as setup (i.e.,

request and confirm) Efficiency and delay

Depends on whether resources are either reserved during the setup, or allocated on demand

Advantage The source can check availability of the resources,

without actually reserving it.

Structure of a Switch Switches are used in both circuit-switched and

packet-switched networks Circuit switch

Space-division switch: paths in the circuit are separated from one another spatially

Time-division switch: internally uses time-division multiplexing (TDM)

Packet Switch

Space-Division Switch (1) Crossbar Switch

Connect n inputs to m outputs in a grid Switch with too many crosspoints is impractical

and inefficient

Space-Division Switch (2) Multi-stage Switch

Combine crossbar switches in several stages (usually three)

Ex: number of crosspoints? N/n (n x k) + k (N/n x N/n) + N/n (k x n) = 2kN + k(N/n)2

This is much smaller than single-stage crossbar: N2

Space-Division Switch (3) Blocking – problem of multistage switch

Under heavy traffic, resources (i.e., crosspoints) are limited, if many users want a connection at the same time

Blocking refers to times when one input cannot be connected to an output due to no available path

Can we avoid blocking? Clos criterion: n = (N/2)1/2, k > 2n-1 Number of crosspoitns ≥ 4N ((2N)1/2 – 1) This is still huge, though less than N2

Time-Division Switch Time-Slot Interchange (TSI)

TDM muxer, demuxer TSI with Random Access Memory (RAM)

To support inputs continuously, TSI should operate at a faster rate – speed-up

Time- and Space-Division Switch Space-division requires many cross-points Time-division requires speed-up (or delay if

store in the switch) Time-space-time (TST) switch

Packet Switches Components:

Input port, routing processor, switching fabric, output port

Packet Switch Structure (1) Input port performs the physical and data

link functions: decapsulates packet from the frame, detects/corrects errors, and store packets at its queue

Output port performs the same function of the input port, but in the reverse order

Packet Switch Structure (2) Routing processor performs the functions of

the network layer: finds the output port number by looking up the routing table (table lookup)

Switching fabrics move packets from the input queue to the output queue Crossbar Banyan Batch-Banyan

Banyan Switch

Multistage switch with many 2x2 micro-switches log2 n stages, n/2 micro-switches at each stage Packets are automatically routed to the destination

using the binary expression of the destination address

Banyan tree from dailycognition.co

m

Banyan Switch Micro-switch (2x2)

A packet has a control bit, 0 or 1 The packet goes up if the control bit is 0 The packet goes down if the control bit is 1

0

01

1

0

1

0 or 1

0 or 1

Banyan Switch Two examples

Left figure: packet to output 6 (= 110) Right figure: packet to output 2 (= 010)

Control bit at 1st stageControl bit at 2nd stageControl bit at 3rd

stage

Banyan Switch First bit determines the block of the next stage

Two blocks are separated First bit indicates which block the packet should go Procedure repeats at the next stage with the next bit

Batcher-Banyan Switch Collision of packets even for a different dest.

At port 0, to dest. 4 (100) At port 6, to dest. 5 (101)

Pre-sorting can solve the problem E.g., the second packet arrives at port 1

Batcherswitch doesthe sorting

Collision

Homework Exercise in Chap. 8

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