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Packet Switching

Outline Space switch - crossbar Add multiplexers and demultiplexers

Packet multiplexing Practice: Ethernet switch

Malathi VeeraraghavanUniversity of Virginia

Some figures: Courtesy of Leon-Garcia & Widjaja’s textbook web site

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Host 1

SwitchLink

Host R

Host R – 1

Controller

123

P

123

P

Connectionof inputs to outputs… …

Network: Links & switches A switch redirects (forwards) data units (bits or packets)

from one link to another

Network

Switch

Port or interface

Size of switch fabric = P x P

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Purpose of a switch

Why are switches needed? To directly connect P hosts with each other, we

need P(P-1)/2 links By connecting P hosts to a switch, we only

need P links: Make connections between links through the

switch to create host-to-host communication paths Cheaper to dig up the ground and lay fiber or

copper wires from host locations to one central location where the switch is located rather than on every pairwise route

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Different views of a switch(example switch: 12 input by 12 output)

123

4 5 6 7

9101112

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1

23

12

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1

23

12

Folded view of a switch

Inputinterfaces

Outputinterfaces

Unfolded view of a switch

Bidirectional links

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P

1 2

1

P

2

P –1

…

…

Crossbar Space Switch Size of switch:

written as P x P read as P by P switch

Number of crosspoints = P2

Connect an input to an output by closing a crosspoint

…

Inputinterfaces

OutputinterfacesNote: this is the same as the

unfolded view with the output interfacesmoved to the bottom

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Status check

Outline• Space switches – crossbar• Add multiplexers and

demultiplexers Packet switch: packet mux/demux

• Practice: Ethernet switch and SONET switch

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Add multiplexers/demultiplexers (mux/demux) to interfaces

The links connected to the switch are typically shared this means the

multiplexing function of the data-link layer is implementated at the end of the link

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23

Q

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Inputinterfaces

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1

23

Q

Outputinterfaces

Unfolded view of a switch

M

M

M

M

Spaceswitch

D

D

D

D

D: DemultiplexerM: Multiplexer

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Types of switches The type of switch is determined by

the type of multiplexing used on its links Circuit switch:

Position-based multiplexing TDM, FDM/WDM

Packet switch: Packet-based multiplexing

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Controller

1

2

3

P

Line card

Line card

Line card

Line card

Spa

ce s

witc

h

Line card

Line card

Line card

Line card

1

2

3

P

Input ports Output ports

Data path Control path (a)

…………Packet switch: header based

“Unfolded” View of Switch Input line card functions

Packet-based demultiplexing Header processing

Output line card functions Packet-based multiplexing Buffering packets

Space switch Transfer packets between

line cards Close crosspoints on a

packet-by-packet basis Controller

Routing slides

Folded view: line card has both input and output functions

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Packet-based multiplexing

Host 1 Host 2

Switch

Switch

Host 3 Host 4Packet-basedmultiplexer

inputlink

inputlink

outputlink

Scheduling algorithms: FCFS, priority

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Example of a packet switch: an Ethernet switch

Headers of incoming packets are processed to extract destination address Forwarding table is consulted to find output port, O, corresponding to destination Corresponding space switch crosspoint connection is closed Packet is sent from input port to the output port, O If multiple packets destined to the same output link arrive at the same time,

they are held in queues (either on the input line card or output line card) and scheduling algorithms are applied

Host 1

Host 4Host 2

Host 3

Ethernetswitch

Destination MAC address: 05:a1:08:10:a4:3e

Destination MAC address: 09:a5:08:10:a4:3da

b

c

d

Destination Output Port

05:a1:08:10:a4:3e

09:a5:08:10:a4:3d

05:a1:08:10:a4:3e c

b09:a5:08:10:a4:3d

Forwarding table

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Recall Ethernet frame format

CRCSrc.Addr. Data

IPdatagram

Type

66 46-1500

0800

Type

2

2

46-1500

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Dst.Addr.

Example MAC address:04-3C-5A-11-26-78Each four-bit half of eachbyte is expressed inhexa-decimal notation

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Insides of an Ethernet switch – unfolded view (assuming crossbar)

Controller

1

2

3

4

Line card

Line card

Line card

Line card

Line cardLine cardLine cardLine card

1 2 3 4

Input ports

Output ports1. Physical layer2. Data link layer3. Once a frame is received correctly, consult forwarding table with destination MAC address4. Make appropriate crosspoint connection in fabric5. Send frame to appropriate outgoing line card

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Analogy for switch: road intersection

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Analogy for a forwarding table

Road signs