Circuit Switching

1. Circuit Switching by Dr. Nguyen Minh Hoang hoang.dhbk@gmail.com March 2007

2. References • Reference books: • Data Communications and Networking, 4/e, by Behrouz A. Forouzan, DeAnza College (http://highered.mcgraw-hill.com/sites/0072967757/information_center_view0/) • Data & Computer Communicationsby William Stallings (http://williamstallings.com/DCC/DCC7e.html)

3. Introduction • How can we connect multiple devices? • Point-to-point vs. • Multipoint • Better solution  switching • Switches: hardware and/or software devices allowing temporary connections between two or more devices • Three methods: circuit, packet, and message

4. Example of a Switched Network

5. Circuit Switching • Creates a direct physical connection between two devices • Originally developed for voice traffic • Does not use point-to-point connections between every device to every other device • Instead, devices link to a switch which may in turn link to other switches • Allows for a dedicated connection for duration of transmission

6. Circuit Switching • Space-division switching • Time-division switching • TDM Bus • Combinations Circuit switching: create a direct physical connection between 2 devices such as phones or computers

7. Circuit Switch vs. Folded Switch n inputs, m outputs n lines in full duplex Folded Switch: An n-by-n folded switch allows every device to connect to every other device in full-duplex mode

8. Space-Division Switch • The paths in the circuit are separated from each other spatially • Originally for analog networks, but now for both analog and digital • Crossbar switch • Multistage switch

9. Crossbar Switch Inputs and outputs connected in a grid using electronic microswitches (transistors) at crosspoints Disadvantage??? Too many crosspoints, only a few used at any given time Inefficient; fewer than 25% of switches are in used at a given time

10. Multistage Switches • Combine crossbar switches in several stages • Devices are linked to switches that are linked to a hierarchy of other switches • Middle stages usually have fewer switches than first and last stages • Fewer crosspoints but still allowing multiple paths through the network  increased reliability

11. Multistage Switch (1) A set of crossbar switches in several stages Use a 15x15 crossbar => 225 crosspoints Use the above multistage =>78 crosspoints

12. Multistage Switch Multiple paths available for each pair of devices => reliable Disadvantage??? Blocking problem when heavy traffic, experienced in public phone systems

13. Blocking • Reduced number of crosspoints may mean that at times of heavy traffic, an input may not be able to connect to an output if there is no path available (i.e. all switches are occupied) • Does not occur in single-stage switch; a non-blocking path is always available

14. Time-Division Switches • Uses time-division multiplexing to achieve switching • Used by most modern switches • TD switching involves partitioning low speed bit stream into pieces that share higher speed stream • Two methods: • Time-slot interchange (TSI) • TDM bus • Based on synchronous time division multiplexing (TDM) • Each station connects through controlled gates to high speed bus • Time slot allows small amount of data onto bus • Another line’s gate is enabled for output at the same time

15. TDM with a Time Slot Interchange Without a TSI, order is unchanged 1->3, 2->4, 3->1, 4->2

16. Time Slot Interchange (TSI) • Consists of RAM with several memory locations • Fills up with incoming data from time slots in order received • Slots are sent out in order based on control unit logic #memory locations = # inputs size of mem. loc. = size of a time slot Cannot handle bursty traffic

17. TDM Bus • Input and output lines are connected to a high-speed bus through input and output gates • Control unit opens and closes gates as needed

18. Time vs. Space Division Switch • Time division • No crosspoints • Delay • Space division • Many crosspoints • Zero delay

19. Space- and Time-Division Switching Combinations • Advantage of space-division switching is it is instantaneous; however, requires too many crosspoints • Advantage of time-division switching is it needs no crosspoints; however, processing connections creates delays • Combining them optimizes the number of crosspoints and reduces the amount of delay • TSS: Time-Space-Space • TSST: Time-Space-Space-Time • STTS: Space-Time-Time-Space

20. TST Switch

21. Telephone Networks 1876: Alexander Graham Bell patented the telephone 1878: Bell Telephone Company first operated 1984: AT&T breakup => AT&T Long Lines and 23 Bell Operating Companies, and a few other pieces Since 1996: Any company can provide local or long-distance services

22. Telephone Network Structure ~22,000 end offices in US, each having up to 10,000 local loops Local loop: analog 1-10km => how far is it if we stretch out all the local loops in the world? Trunks between switching offices: coax, microwave, fiber optics All trunks and switches are now digital

23. Major Components • Local loops – twisted-pair cable connecting subscriber phone to nearest end office or local central office • Bandwidth of 4000 Hz (4 KHz) • Trunks – transmission media that handles communication between offices • Muxing through optical fibers or satellite links • Switching Office – connects local loops or trunks allowing connections between subscribers

24. Major Components

25. LATAs • Local Access and Transport Area – defines the boundary of a service area • Intra-LATA services are provided by local exchange carriers (LEC) • Incumbent local exchange carriers • Competitive local exchange carriers • Communication is handled by end switches and tandem switches

26. Services • Inter-LATA services are handled by interexchange carriers (IXCs) – long-distance companies • To provide these services, each IXC must have a POP (Point of Presence) inside each LATA

27. Local Access Transport Areas (LATAs) • 200+ LATAs in the US • Intra-LATA services • Services offered by the common carriers (telephone companies) inside a LATA • Such a carrier = local exchange carrier (LEC) • Before 1996: 1 LEC in a LATA => monopoly • After 1996: 1+ LEC inside a LATA • The LEC that owns the local loops system is called incumbent LEC (ILEC) => provide main services • Extra carrier = competitive LEC (CLEC) => extra services: mobile, toll calls, etc.

28. Switching Offices in a LATA Calls that go through a tandem office is charged Calls that are completed by using only end offices are toll-free

29. Inter-LATA Services • IXCs: inter-exchange carriers, a. k. a. long-distance companies, provide communication between 2 customers in different LATAs • AT&T, MCI, WorldCom , Sprint, Verizon • IXCs provide general data communications services including telephone. Phone service is normally digitized • How do IXCs interact with LECs? Use POP

30. Point of Presence (POP)

31. Example PSTN Network

32. Example PSTN Network Representative Voice Network Hierarchy

33. Making Connections • Subscriber telephones are connected through local loops to end offices • Accessing the switching station at the end offices is accomplished via dialing • Dialing • Before: use Rotary or pulse dialing • for each number dialed, a digital signal is sent to end office • Prone to errors due to inconsistency of humans during dialing • Today: touchtone dialing

34. Rotary and Touchtone Dialing Touchtone: Number 8 = 2 bursts of signals with f1=1336Hz and f2=852Hz

35. Analog Switched Telephone Services • Familiar dial-up service, analog local loop • Local calls • Toll call • intra- (when LATA is large) or inter-LATA • 800 (888, 877, 866) services: • organization pays for call • Wide area telephone service (WATS): • Organization pays for outbound call at rate cheaper than long distance • 900 services • Caller pays for call, much more expensive than long-distance

36. Analog Leased Lines • A line is dedicated permanently between 2 customers • This line goes through a switch, but seen as a single line because the switch for it is always closed. • No dialing is needed

37. Digital Telephone Services • Less sensitive to noise and other interference • Switch/56 • digital version of analog switched line, data up to 56Kbps • Line is digital, no need for modem, but need a digital service unit (DSU) • Support bandwidth on demand, can use more lines for more bandwidth => can use video conferencing, fast fax, fast data transfer, among other services • Digital Data Service (DDS) • Digital version of analog leased line, up to 64Kbps • High-speed digital: T-lines and home networks

38. Question?