1. 1 9. Circuit Switching

2. Prof. Sang-Jo Yoo 2 Contents  Switching Networks  Circuit-Switching Networks  Switching Concepts  Routing in Circuit-Switching Networks  Control Signaling

3. Prof. Sang-Jo Yoo 3 Switching Networks  Long distance transmission is typically done over a network of switched nodes  Nodes not concerned with content of data  End devices are stations  Computer, terminal, phone, etc.  A collection of nodes and connections is a communications network  Data routed by being switched from node to node

4. Prof. Sang-Jo Yoo 4 Nodes  Nodes may connect to other nodes only, or to stations and other nodes  Node to node links usually multiplexed  Network is usually partially connected  Some redundant connections are desirable for reliability  Two different switching technologies  Circuit switching  Packet switching

5. Prof. Sang-Jo Yoo 5 Simple Switched Network

6. Prof. Sang-Jo Yoo 6 Circuit Switching  Dedicated communication path between two stations  Three phases  Establish  Transfer  Disconnect  Must have switching capacity and channel capacity to establish connection  Must have intelligence to work out routing

7. Prof. Sang-Jo Yoo 7 Circuit Switching - Applications  Inefficient  Channel capacity dedicated for duration of connection  If no data, capacity wasted  Set up (connection) takes time  Once connected, transfer is transparent  Developed for voice traffic (phone)

8. Prof. Sang-Jo Yoo 8 Circuit-Switching Networks  Public telecommunications network  Subscribers  Devices that attach to the network  Local loop  Link between the subscriber and the network  Also called subscriber loop  Exchanges  Switching centers in the network  End office, Long-distance office  Trunk  Branches between exchanges  Using FDM or synchronous TDM

9. Prof. Sang-Jo Yoo 9 Public Circuit Switched Network

10. Prof. Sang-Jo Yoo 10 Circuit-Switching Networks

11. Prof. Sang-Jo Yoo 11 Switching Concepts  Single circuit-switched node  A collection of stations attached to a central switching node  Digital switch  Establishes a dedicated path between any two devices  Network-interface  Control unit  Establishes, maintain, and tear down the connection

12. Prof. Sang-Jo Yoo 12 Switching Concepts  Blocking  A network is unable to connect stations because all paths are in use  A blocking network allows this  Used on voice systems  Short duration calls  Non-blocking  Permits all stations to connect (in pairs) at once  Used for some data connections

13. Prof. Sang-Jo Yoo 13 Switching Concepts  Space-division Switching  Signal paths between pairs of devices are divided in space  Metallic crosspoint or semiconductor gate.  Crossbar switch  High cost : O(n 2 )  Inefficiency  Non-blocking

14. Prof. Sang-Jo Yoo 14 Switching Concepts  Multistage network  Reduced numbers of crosspoints  Blocking example

15. Prof. Sang-Jo Yoo 15 Switching Concepts  Time-division Switching  Partition low speed bit stream into pieces that share higher speed stream  e.g. TDM bus switching  based on synchronous time division multiplexing  Each station connects through controlled gates to high speed bus  Time slot allows small amount of data onto bus (input)  Another line ’ s gate is enabled for output at the same time (output)

16. Prof. Sang-Jo Yoo 16 Switching Concepts  Control of a TDM bus switch

17. Prof. Sang-Jo Yoo 17 Routing in Circuit-Switched Networks  Reasons for routing in circuit switching  connections require a path through more than one switch  Requirements of network architecture for routing  efficiency, resilience  Efficiency  busy-hour traffic load : the average load expected over the course of the busiest hour of use during the course of a day.  should be able to handle busy-hour traffic load with minimum equipment  Resilience  should be able to control the network in the case of traffic congestion and failures in switch and trunk.

18. Prof. Sang-Jo Yoo 18 Routing in Circuit-Switched Network  Traditional routing technique  static approach- quite simple  organized into a tree structures, or hierarchy  cannot adapt to changing conditions  Dynamic approach is needed  alternate routing  adaptive routing Calling subscriber Called subscriber

19. Prof. Sang-Jo Yoo 19 Alternate Routing  Possible routes are predefined  routing decision is based on current traffic status and historical traffic patterns X Y I J K End Office Intermediate Switching node Route A Route B Route C Route D

20. Prof. Sang-Jo Yoo 20 Control Signaling  Network management, call establishment, call maintenance, call termination  signaling functions  audible communication : dial tone, ringing tone, busy  transmission of the dial number  transmission of information  call connection, call ending, call disconnection  billing purpose  status of equipment and trunk  diagnosing and isolating system failures  supervisory,address,call-information, network management

21. Prof. Sang-Jo Yoo 21 Categories of Control Signalings  Supervisory  binary character(true/false, on/off)  request for service, answer, alerting, return to idle  deal with availability of the called subscriber and of the needed network resources  Address  identify a subscriber  phone number  call-information  provides information to the subscriber about the status of a call  audible tones  Network management  maintenance, trouble-shooting

22. Prof. Sang-Jo Yoo 22 Control Signaling : example on hook hang up switch dial tone called address ringing signal ringing tone off hook ringing signal termination ringing tone termination connection establishment connection released hang up off hook

23. Prof. Sang-Jo Yoo 23 Inchannel Signaling  The same channel is used to carry control signals  no additional transmission facilities are needed  Inband signaling  uses the same frequency band as voice signals that are carried  no limit on the use of inband signaling anywhere in the network  channel for control signal is only available when no voice signals  Out-of-band signaling  not full use of 4k-Hz bandwidth by the voice signal  a separate narrow signaling band within the 4k-Hz is used for control signaling  very narrow bandwidth -> slower signaling rate

24. Prof. Sang-Jo Yoo 24 Inchannel Signaling Office AOffice B SIG Trunks Inchannel signaling Office AOffice B Trunks CC-SIG Processor Common Channel signaling

25. Prof. Sang-Jo Yoo 25 Common Channel Signaling  Control signals’ path is independent of voice signal  call-setup time is reduced  signaling protocols and network control are more complex  associate mode  common channel closely tracks along its entire length the interswitch trunk groups switching point signaling link speech

26. Prof. Sang-Jo Yoo 26 Common Channel Signaling  Non associate mode  network is augmented by additional nodes(signal transfer points)  control requests can be processed with a more global view of the network conditions switching point(speech) signaling network speech network switching point (signal transfer point)

27. Prof. Sang-Jo Yoo 27 Signaling System Number 7  SS7  Common channel signaling scheme  ISDN  Optimized for 64k digital channel network  Call control, remote control, management and maintenance  Reliable means of transfer of info in sequence  Will operate over analog and below 64k  Point to point terrestrial and satellite links