Swiching

network switching

1. 8.1
Chapter 8
Switching
Modified by Allam AbuMwais

2. 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.

3. 8.3
Figure 8.1 Switched network

4. 8.4
Figure 8.2 Taxonomy of switched networks

5. 8.5
8-1 CIRCUIT-SWITCHED NETWORKS
Acircuit-switchednetworkconsistsofasetof
switchesconnectedbyphysicallinks.Aconnection
betweentwostationsisadedicatedpathmadeofone
ormorelinks.However,eachconnectionusesonly
onededicatedchanneloneachlink.Eachlinkis
normallydividedintonchannelsbyusingFDMor
TDM.
Three Phases
Efficiency
Delay
Circuit-Switched Technology in Telephone Networks
Topics discussed in this section:

6. 8.6
A circuit-switched network is made of a
set of switches connected by physical
links, in which each link is
divided into nchannels.
Note

7. 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. 8.8
Figure 8.3 A trivial circuit-switched network
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.

9. 8.9
Figure 8.3 A trivial circuit-switched network
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).

10. 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

11. 8.11
Asatrivialexample,letususeacircuit-switched
networktoconnecteighttelephonesinasmallarea.
Communicationisthrough4-kHzvoicechannels.We
assumethateachlinkusesFDMtoconnectamaximum
oftwovoicechannels.Thebandwidthofeachlinkis
then8kHz.Figure8.4showsthesituation.
Telephone1isconnectedtotelephone7;2to5;3to8;
and4to6.Ofcoursethesituationmaychangewhen
newconnectionsaremade.Theswitchcontrolsthe
connections.
Example 8.1

12. 8.12
Figure 8.4 Circuit-switched network used in Example 8.1

13. 8.13
Asanotherexample,consideracircuit-switched
networkthatconnectscomputersintworemoteoffices
ofaprivatecompany.Theofficesareconnectedusinga
T-1lineleasedfromacommunicationserviceprovider.
Therearetwo4×8(4inputsand8outputs)switchesin
thisnetwork.Foreachswitch,fouroutputportsare
foldedintotheinputportstoallowcommunication
betweencomputersinthesameoffice.Fourother
outputportsallowcommunicationbetweenthetwo
offices.Figure8.5showsthesituation.
Example 8.2

14. 8.14
Figure 8.5 Circuit-switched network used in Example 8.2

15. 8.15
3 phases:
1.Setup Phase
2.Data Transfer Phase
3.Teardown Phase
Note

16. 8.16
Efficiency
Itcanbearguedthatcircuit-switched
networksarenotasefficientastheother
twotypesofnetworksbecauseresources
areallocatedduringtheentiredurationof
theconnection.

17. 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

18. 8.18
Switching at the physical layer in the
traditional telephone network uses
the circuit-switching approach.
Note

19. 8.19
8-2 DATAGRAM NETWORKS
1.Indatacommunications,weneedtosendmessagesfromone
endsystemtoanother.Ifthemessageisgoingtopass
throughapacket-switchednetwork,itneedstobedivided
intopacketsoffixedorvariablesize.Thesizeofthepacketis
determinedbythenetworkandthegoverningprotocol.
2.Inadatagramnetwork,eachpacketistreatedindependently
ofallothers.
3.Datagramswitchingisnormallydoneatthenetworklayer.
Routing Table
Efficiency
Delay
Datagram Networks in the Internet
Topics discussed in this section:

20. 8.20
In a packet-switched network, there
is no resource reservation;
resources are allocated on demand.
Note

21. 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.

22. 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.

23. 8.23
A switch in a datagram network uses a
routing table that is based on the
destination address.
Note

24. 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

25. 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

26. 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

27. 8.27
8-3 VIRTUAL-CIRCUIT NETWORKS
Avirtual-circuitnetworkisacrossbetweenacircuit-
switchednetworkandadatagramnetwork.Ithas
somecharacteristicsofboth.
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,

28. 8.28
8-3 VIRTUAL-CIRCUIT NETWORKS
Addressing
Three Phases
Efficiency
Delay
Circuit-Switched Technology in WANs
Topics discussed in this section:

29. 8.29
Figure 8.10 Virtual-circuit network

30. 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.

31. 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.

32. 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.

33. 8.33
Figure 8.13 Source A-to-destination Bdata transfer in a virtual-circuit network

34. 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.

35. 8.35
Figure 8.15 Setup acknowledgment in a virtual-circuit network

36. 8.36
TeardowilPhase: In this phase, source A, after
sending all frames to B, sends a special frame
called a teardown request.
Teardown Phase

37. 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

38. 8.38
Figure 8.16 Delay in a virtual-circuit network

39. 8.39
Switching at the data link layer in a
switched WAN is normally implemented
by using virtual-circuit techniques.
Note

40. 8.40
8-4 STRUCTURE OF A SWITCH
Weuseswitchesincircuit-switchedandpacket-
switchednetworks.Inthissection,wediscussthe
structuresoftheswitchesusedineachtypeof
network.
Structure of Circuit Switches
Structure of Packet Switches
Topics discussed in this section:

41. 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

42. 8.42
Figure 8.18 Multistage switch

43. 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 (N
2
).
Note

44. 8.44
Design a three-stage, 200 ×200 switch (N = 200) with
k = 4 and n = 20.
Solution
InthefirststagewehaveN/nor10crossbars,
eachofsize20×4.Inthesecondstage,wehave
4crossbars,eachofsize10×10.Inthethird
stage,wehave10crossbars,eachofsize4×20.
Thetotalnumberofcrosspointsis2kN+k(N/n)
2
,
or2000crosspoints.Thisis5percentofthe
numberofcrosspointsinasingle-stageswitch
(200×200=40,000).
Example 8.3

45. 8.45
According to the Clos criterion:
n= (N/2)
1/2
k> 2n–1
Crosspoints ≥ 4N [(2N)
1/2
–1]
Note

46. 8.46
Redesignthepreviousthree-stage,200×200switch,
usingtheCloscriteriawithaminimumnumberof
crosspoints.
Solution
Weletn=(200/2)
1/2
,orn=10.Wecalculatek=2n
−1=19.Inthefirststage,wehave200/10,or20,
crossbars,eachwith10×19crosspoints.Inthe
secondstage,wehave19crossbars,eachwith10
×10crosspoints.Inthethirdstage,wehave20
crossbarseachwith19×10crosspoints.The
totalnumberofcrosspointsis20(10×19)+19(10
×10)+20(19×10)=9500.
Example 8.4

47. 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).

48. 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.

49. 8.49
Figure 8.20 Time-space-time switch

50. 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

51. 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

52. 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

53. 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

54. 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 log2n stages with nl2 micro-switches at each stage.
#of stages =Iog2(8) = 3.

55. 8.55
Figure 8.25 Examples of routing in a banyan switch

56. 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.