Swiching

8.1

Chapter 8

SwitchingModified by Allam AbuMwais

8.2

Introduction

Whenever we have multiple devices, we have the

problem of how to connect them to make one-to-one

communication possible.

Mesh network (point-point) and Star network (to

central device) connection. But these impractical in large

networks.

A better solution is switching. A switched network

consists of a series of interlinked nodes, called switches.

Switches are devices capable of creating temporary

connections between two or more devices linked to the

switch.

8.3

Figure 8.1 Switched network

8.4

Figure 8.2 Taxonomy of switched networks

8.5

8-1 CIRCUIT-SWITCHED NETWORKS

A circuit-switched network consists of a set of

switches connected by physical links. A connection

between two stations is a dedicated path made of one

or more links. However, each connection uses only

one dedicated channel on each link. Each link is

normally divided into n channels by using FDM or

TDM.

Three Phases

Efficiency

Delay

Circuit-Switched Technology in Telephone Networks

Topics discussed in this section:

8.6

A circuit-switched network is made of a

set of switches connected by physical

links, in which each link is

divided into n channels.

Note

8.7

Figure 8.3 A trivial circuit-switched network

circuit-switched network with four switches

and four links. Each link is divided into n=3.

8.8


When end system A needs to communicate with end

system M:

1. the setup phase: system A needs to request a

connection to M that must be accepted by all switches

as well as by M itself.

2. A dedicated path: a circuit (channel) is reserved on

each link, and the combination of circuits or channels.

3. Data transfer can take place. After all data have been

transferred.

8.9


1. Circuit switching takes place at the physical layer.

2. The stations must make a reservation for the resources

to be used during the communication.

3. Data transferred between the two stations are not

packetized.

4. There is no addressing involved during data transfer.

The switches route the data based on their occupied

band (FDM) or time slot (TDM).

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

teardown phase.

Note

8.11

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

8.12

Figure 8.4 Circuit-switched network used in Example 8.1

8.13

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

8.14

Figure 8.5 Circuit-switched network used in Example 8.2

8.15

3 phases:

1. Setup Phase2. Data Transfer Phase3. Teardown Phase

Note

8.16

Efficiency

It can be argued that circuit-switched

networks are not as efficient as the other

two types of networks because resources

are allocated during the entire duration of

the connection.

8.17

Delay

Figure 8.6 Delay in a circuit-switched network

the delay in this type of network is minimal. The total delay is due to the

time needed to create the connection, transfer data, and disconnect the

circuit

8.18

Switching at the physical layer in the

traditional telephone network uses

the circuit-switching approach.

Note

8.19

8-2 DATAGRAM NETWORKS

1. 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.

2. In a datagram network, each packet is treated independently

of all others.

3. Datagram switching is normally done at the network layer.

Routing Table

Efficiency

Delay

Datagram Networks in the Internet


8.20

In a packet-switched network, there

is no resource reservation;

resources are allocated on demand.

Note

8.21

Figure 8.7 A datagram network with four switches (routers)

Figure 8.7 shows how the datagram approach is used to deliver four

packets from station A to station X.

The datagram networks are sometimes referred to as connectionless networks. The term connectionless here means that the switch (packet switch) does not keep information about the connection state.

8.22

Figure 8.8 Routing table in a datagram network

If there are no setup or teardown phases,

how are the packets routed to their

destinations in a datagram network?

each switch (or packet switch) has a

routing table which is based on the

destination address.

The routing tables are dynamic and

are updated periodically.

8.23

A switch in a datagram network uses a

routing table that is based on the

destination address.

Note

8.24

The destination address in the header of

a packet in a datagram network remains

the same during the entire journey of the

packet.

Note

8.25

Figure 8.9 Delay in a datagram network

There may be greater delay in a datagram network than in a virtual-

circuit network and delay is not uniform

8.26

Switching in the Internet is done by

using the datagram approach to packet

switching at the network layer.

Note

Datagram Networks in the Internet

8.27

8-3 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.

1. As in a circuit-switched network, there are setup and teardown

phases in addition to the data transfer phase.

2. Resources can be allocated during the setup phase, as in a

circuit-switched network, or on demand, as in a datagram

network.

3. As in a datagram network, data are packetized and each packet

carries an address in the header.

4. As in a circuit-switched network, all packets follow the same

path established during the connection.

5. virtual-circuit network is normally implemented in the data link

layer,

8.28

8-3 VIRTUAL-CIRCUIT NETWORKS

Addressing

Three Phases

Efficiency

Delay

Circuit-Switched Technology in WANs


8.29

Figure 8.10 Virtual-circuit network

8.30

Figure 8.11 Virtual-circuit identifier

Addressing

two types of addressing:

1. Global Address

2. Local (Virtual-circuit identifier): The identifier

that is actually used for data transfer (switch scope)

When a frame arrives at a switch, it has VCI; when it leaves, it has a

different VCl.

8.31

3 phases

setup, data transfer, and teardown.1. Setup: the source and destination use their global addresses

to help switches make table entries for the connection.

2. the source and destination inform the switches to delete the

corresponding entry.

3. occurs between these two phases.

8.32

Figure 8.12 Switch and tables in a virtual-circuit network

To transfer a frame from a source to its

destination, all switches need to have a

table entry for this virtual circuit. The

table, in its simplest form, has four

columns.

8.33

Figure 8.13 Source A-to-destination Bdata transfer in a virtual-circuit network

8.34

Figure 8.14 Setup request in a virtual-circuit network

In the setup phase, a switch creates an

entry for a virtual circuit. For example,

suppose source A needs to create a

virtual circuit to B. Two steps are

required:

1. the setup request

2. the acknowledgment.

8.35

Figure 8.15 Setup acknowledgment in a virtual-circuit network

8.36

Teardowil Phase: In this phase, source A, after

sending all frames to B, sends a special frame

called a teardown request.

Teardown Phase

8.37

In virtual-circuit switching, all packets

belonging to the same source and

destination travel the same path;

but the packets may arrive at the

destination with different delays

if resource allocation is on demand.

Note

Efficiency

8.38

Figure 8.16 Delay in a virtual-circuit network

8.39

Switching at the data link layer in a

switched WAN is normally implemented

by using virtual-circuit techniques.

Note

8.40

8-4 STRUCTURE OF A SWITCH

We use switches in circuit-switched and packet-

switched networks. In this section, we discuss the

structures of the switches used in each type of

network.

Structure of Circuit Switches

Structure of Packet Switches


8.41 Figure 8.17 Crossbar switch with three inputs and four outputs

Structure of Circuit Switches

Circuit switching today can use either of two technologies: the

space-division switch or the time-division switch.

In space-division switching, the paths in the circuit are separated

from one another spatially.

1-crossbar switch

8.42

Figure 8.18 Multistage switch

8.43

In a three-stage switch, the total

number of crosspoints is

2kN + k(N/n)2

which is much smaller than the number of

crosspoints in a single-stage switch (N2).

Note

8.44

Design a three-stage, 200 × 200 switch (N = 200) with

k = 4 and n = 20.

Solution

In the first stage we have N/n or 10 crossbars,each of size 20 × 4. In the second stage, we have4 crossbars, each of size 10 × 10. In the thirdstage, we have 10 crossbars, each of size 4 × 20.The total number of crosspoints is 2kN + k(N/n)2,or 2000 crosspoints. This is 5 percent of thenumber of crosspoints in a single-stage switch(200 × 200 = 40,000).

Example 8.3

8.45

According to the Clos criterion:

n = (N/2)1/2

k > 2n – 1

Crosspoints ≥ 4N [(2N)1/2 – 1]

Note

8.46

Redesign the previous three-stage, 200 × 200 switch,

using the Clos criteria with a minimum number of

crosspoints.

Solution

We let n = (200/2)1/2, or n = 10. We calculate k = 2n− 1 = 19. In the first stage, we have 200/10, or 20,crossbars, each with 10 × 19 crosspoints. In thesecond stage, we have 19 crossbars, each with 10× 10 crosspoints. In the third stage, we have 20crossbars each with 19 × 10 crosspoints. Thetotal number of crosspoints is 20(10 × 19) + 19(10× 10) + 20(19 ×10) = 9500.

Example 8.4

8.47 Figure 8.19 Time-slot interchange

Time-Division Switch

Time-division switching uses time-division multiplexing

(TDM) inside a switch. The most popular technology is

called the time-slot interchange (TSI).

8.48

Time-space-time switch

space-division adv. The advantage of space-division switching is that it

is instantaneous

Dis. the number of cross-points required

Time –division

Adv. No need of cross-points.

Dis. is that processing each connection creates delays.

Each time slot must be stored by the RAM, then

retrieved and passed on.

8.49

Figure 8.20 Time-space-time switch

8.50

Figure 8.21 Packet switch components

a packet switch has four components:

input ports, output ports, the routing processor, and

the switching fabric

8.51

Figure 8.22 Input port

An input port performs the physical and data link functions of

the packet switch. The bits are constructed from the received

signal. The packet is de-capsulated from the frame. Errors are

detected and corrected. The packet is now ready to be routed by

the network

8.52

ROllting Processor

The routing processor performs the functions of the network layer.

The destination address is used to find the address of the next hop

and, at the same time, the output port number from which the

packet is sent out.

Switching "Fabrics

The most difficult task in a packet switch is to move the packet

from the input queue to the output queue. The speed with which

this is done affects the size of the input/output queue and the

overall delay in packet delivery. As crossbar switch and banyan

switch

8.53

Figure 8.23 Output port

The output port performs the same functions

as the input port, but in the reverse order.

First the outgoing packets are queued, then

the packet is encapsulated in a frame, and

finally the physical layer functions are

applied

8.54 Figure 8.24 A banyan switch

banyan switch

A banyan switch is a multistage switch with micro-switches at

each stage that route the packets based on the output port

represented as a binary string. For n inputs and n outputs, we

have log2 n stages with nl2 micro-switches at each stage.

#of stages = Iog2(8) = 3.

8.55

Figure 8.25 Examples of routing in a banyan switch

8.56

Figure 8.26 Batcher-banyan switch

The problem with the banyan switch is the possibility of internal

collision even when two packets are not heading for the same

output port.

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