1. Chapter 9. Circuit Switching
Switching Networks
Circuit-Switching Networks
Switching Concepts
Routing in Circuit-Switched Networks
Control Signaling

2. Switching Networks

3. Circuit-Switching Networks
Dedicated communication path btw two stations
Three phases of communications
Circuit establishment
Data transfer
Circuit disconnect
Application of circuit switching
Private Branch Exchange (PBX)

4. Telecommunications Network
Subscribers
The devices that attach to the network
e.g. telephones
Local loop
The link between the subscriber and the network
also referred to as the subscriber loop
Exchange
The switching centers in the network
End office
Trunks
The branches between exchanges

5. Public Circuit-Switching Network

6. Circuit-Switching Technology
Driven by applications that handle voice
no transmission delay
no variation in delay
i.e. constant signal transmission rate
Transparency
once a circuit is established, it appears as a direct connection to the two attached stations; no special networking logic is needed at either point

7. Switching Concepts

8. Space-division Switching

9. Space-division Switching (cont)

10. Space-division Switching (cont)
Limitations of Crossbar
Costly for a large switch
loss of a crosspoint prevents connection between two devices
inefficiently utilized
Multiple-stage switches
The number of crosspoints is reduced
more than one path through the network to connection two endpoints
it may be blocking

11. E.g. Banyan Switch (ATM) 1 1
100 2 2 3 3 4 4
011 5 5 6 6 7 7 1 1

12. Time-division Switching

13. Time-division Switching (cont)
Control of a TDM bus switch 1 X 2 X 3 X 4 X 5 X 6 X
1  3
2  5
Control logic
Control memory
4  6
3  1
5  2
6  4

14. Routing in Circuit-Switched Networks
Traditionally
The switches of a network were organized into a tree structure, or hierarchy
Dynamic routing
routing decisions are influenced by current traffic conditions
Two broad classes of dynamic routing
alternate routing
adaptive routing

15. Alternate Routing
Possible routes to be used between two end offices are predefined
The routing sequences reflects an analysis based on historical traffic patterns
Fixed alternate-routing scheme
only one routing sequence
Dynamic alternate-routing scheme
a different set of preplanned routes is used for different time periods

16. Alternate Routing (e.g.)

17. Alternate Routing (e.g.)
Alternate routes from end office X to end office Y
If only one route sequence  Fixed alternate-routing

18. Adaptive Routing Enable switches to react to changing traffic patterns
Require greater management overhead
Switches must exchange information to learn of network conditions
More effectively optimizing the use of network resources
E.g. Dynamic traffic management (DTM)
by Northern Telecom
DTM uses a central controller to find the best alternate route choices depending on congestion in the network

19. Control Signaling Control Signals Management Common Channel Signaling
the means by which the network is managed and by which calls are established, maintained, and terminated
Management
requires that information be exchanged between subscriber and switch, among switch, and between switch and network management center
Common Channel Signaling

20. Signaling Functions Audible communication with the subscriber
Dial tone, ringing tone, busy signal, etc.
Transmission of the number dialed to s/w offices
Information between switches
Indicating that a call cannot be completed
Indicating that a call has ended and the path can be disconnected
A signal to make a telephone ring
Information for billing purpose
Information for routing and maintenance purpose
Information for diagnosing and isolating failures
Control of special equipment

21. Control Signaling (e.g.)
Circuit-switched telephone network

22. In-Channel Signaling
The same channel is used to carry control signals as is used to carry the call
Two forms of in-channel signaling
in-band
out-of-band
Drawbacks
The information transfer is quite limited
amount of delay from the time a subscriber enter an address to when the connection is established

23. Common Channel Signaling
Signal path is physically separate from the path for calls (voice)
Two modes of operation
associated mode
non-associated mode
used in ISDN

24. Common Channel Signaling Modes

25. Common Channel Signaling Modes

26. Discussion

27. Discussion (cont) Common Channel Signaling In-Channel Signaling
control signals are transfer directly from one processor to another, w/o being tied to a voice signal
call setup time is reduced
complexity of the technique
In-Channel Signaling
is still needed from some of the communication with the subscriber
e.g. dial tone, ringback, and busy signals