1. EE 489 Telecommunication Systems Engineering University of Alberta Dept. of Electrical and Computer Engineering Switching Systems Wayne Grover slides based on material developed by W. Grover for EE589, (1998-2002) set in powerpoint with a few additions by J. Doucette 2002

2. Material prepared by W. Grover (1998-2002) 2 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Switching Circuit SwitchingCircuit Switching –A path is established between the caller and destination. –Real-time connection formed. –Example: PSTN Message SwitchingMessage Switching store and forward –Also called store and forward. –A message is first stored in a buffer and then sent on in its entirety step by step as resources become available. connectionless –No real-time connection (i.e. connectionless). –Example: E-mail Packet SwitchingPacket Switching –A message is broken down into parts and each part is sent separately (possibly via different routes). –Example: Internet UDP protocol

3. Material prepared by W. Grover (1998-2002) 3 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Separating Circuits Four technologies for separating circuits: –Space, RF frequency, time, optical wavelength We want to logically connect circuits coming into a switch with circuits at the output. Example “space division” equivalent interconnection pattern : Input 1 Input 2 Input 3 Output 1 Output 2 Output 3

4. Material prepared by W. Grover (1998-2002) 4 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Space Division Switching Connecting two channels that are separated in space. Can be mechanical and/or electronic. Several problems: –Slow –Bulky with lots of interconnect wiring –Subject to cross-talk Input 1 Input 2 Input 3 Output 1 Output 2 Output 3 Input 1 Input 2 Input 3 Output 1 Output 2 Output 3 Equals :

5. Material prepared by W. Grover (1998-2002) 5 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Strowger Switching Patented 12/March/1889 and in some places still in use today. First widely-used automatic exchange system. contact arm contact bankA wiper assembly (contact arm) moves across a fixed set of switch contacts (contact bank). –Each contact is connected to an outgoing channel. Uni-selector: Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2 nd Edition, John Wiley & Sons Ltd, pp. 90, 1997. Strowger uni-selector

6. Material prepared by W. Grover (1998-2002) 6 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Strowger Switching (2) Several uni-selectors can be “graded” together so multiple incoming circuits can connect to multiple outgoing circuits. Graded uni-selectors: Unless there is heavy traffic, it is inefficient and uneconomical to provide each incoming circuit with a uni-selector. line-findersOr two uni-selectors can be wired back-to-back (line-finders). –1 st uni-selector chooses the incoming circuit, the 2 nd chooses the outgoing circuit. Line-finder (hunter): Incoming Circuits Outgoing Circuits or other uni-selectors Line-finders can be graded together as well to form large switches.

7. Material prepared by W. Grover (1998-2002) 7 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Strowger Switching (3) In general, multiple uni-selectors, line-finders, and two-motion selectors (movable in two planes) can be connected in series. These switches respond to dialled digits, automatically switching an incoming circuit to the correct outgoing trunk. –Step-by-step switching will respond to each digit individually. Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2 nd Edition, John Wiley & Sons Ltd, pp. 93, 1997. Strowger two-motion selector

8. Material prepared by W. Grover (1998-2002) 8 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Crossbar Switching Crossbar switching became popular in the 1940’s and is still used in some places today. Uses a simple rectangular matrix. –Actuators are operated at incoming circuits and outgoing circuits to make metallic contact and form the desired connection. 1234 A B C D Incoming Circuits Outgoing Circuits Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2 nd Edition, John Wiley & Sons Ltd, pp. 96, 1997. Crossbar switch

9. Material prepared by W. Grover (1998-2002) 9 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Division Switching In digital TDM systems (e.g. DS1), channels are divided by time slot, but switching is still possible. time-slot interchangerTSISwitching is by a time-slot interchanger (TSI) and is accomplished by rearranging the order in which data is read out of the buffer. speech store speech address memorySAMIncoming data enters a speech store while the outgoing channels indicate to the speech address memory (SAM) which incoming timeslot it is assigned to. During each time-slot, the outgoing circuit reads the speech store slot corresponding to the SAM. 1 A 2 B 3 C 4 D 1 A 2 B 3 C 4 D 1 A 2 B 3 C 4 D TSI 1 B 2 D 3 A 4 C 1 B 2 D 3 A 4 C 1 B 2 D 3 A 4 C

10. Material prepared by W. Grover (1998-2002) 10 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Optical Switching One wavelength (or colour) can be turned into another. wavelength conversiontranslation –Called wavelength conversion or translation. RWA –Important in reducing blocking due to wavelength contention in routing and wavelength assignment (RWA) problem. O/E E/O –Optoelectronic conversion consists of optical receiver, conversion to electronic signal (O/E), and then transmitter generates optical signal at the desired new wavelength (E/O).

11. Material prepared by W. Grover (1998-2002) 11 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Optical Switching (2) One (or several wavelengths) are switched from one fibre into another. –Can use splitters and tunable filters, or Micro-Electro-Mechanical SwitchesMEMS –More recently - Micro-Electro-Mechanical Switches (MEMS) On the scale of a human hair (100 microns) MEMS Mirror (Lucent Technolgies) WaveStar TM MEMS Mirror (Lucent Technolgies) WaveStar TM LambdaRouter Optical Cross-Connect (Lucent Technolgies) Source: Lucent Technologies - Bell Labs Web-site: http://www.bell-labs.com/news/1999/november/10/1.html http://www.bell-labs.com/org/physicalsciences/timeline/1999_mems_expansion.html

12. Material prepared by W. Grover (1998-2002) 12 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Switching Network Design Several Points to Consider –Blocking versus non-blocking switches –Number of cross-points (i.e. size of the switch) –Reliability –Overload –Growth –Cost and technology “Trunk Switch” traffic switch“Trunk Switch” (aka traffic switch) –One-to-one connection. –One specific inlet must connect to one specific outlet. “Access Switch”“Access Switch” –One-to-any connection. –One specific inlet must connect to any free outlet.

13. Material prepared by W. Grover (1998-2002) 13 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics Consider a switch with a 100 x 100 interconnect function. (a) - Full Matrix Switch 1 100 1 Need 10 000 cross-points. E.g. Example: 4x4 = 16 cross-points 1234 1 2 3 4 If bi-directional transmission, then connection from A to B is equivalent to a connection from B to A (and connection from A to A is meaningless). 1 100 1 (b) - Folded Matrix Switch Need 4950 cross-points. E.g.

14. Material prepared by W. Grover (1998-2002) 14 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics (2) Full matrix and even folded matrix switches may be inefficient since they scale as O(n 2 ). A third method of achieving a 100 x 100 interconnect function is by splitting the switch into two stages using smaller square matrices as building blocks. xpts –Then to form a connection, two xpts are operated, one in each stage, but: (i) fewer xpts needed in total (ii) we may have introduced some blocking probability Example: 100 x 100 in 2 stages: 10 x 10 1 10 1 10 x 10 1 10 1 (10) 10 x 10 1 10 1 10 x 10 1 10 1 (10) How many xpts? Each block is 10 x 10 = 100 xpts. Each stage is 10 blocks = 1000 xpts. 2000 xpts Whole switch has 2 stages = 2000 xpts.

15. Material prepared by W. Grover (1998-2002) 15 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics (3) How does it work? –Divide the 100 inlets into groups of 10. –1 st outlet of each Stage 1 block is connected to an inlet of the 1 st Stage 2 block. –2 nd outlet of each Stage 1 block is connected to an inlet of the 2 nd Stage 2 block. –3 rd outlet of each Stage 1 block is connected to an inlet of the 3 rd Stage 2 block… –i th outlet of each Stage 1 block is connected to an inlet of the i th Stage 2 block. 10 x 10 1 10 1 10 x 10 1 10 1 (10) 10 x 10 1 10 1 10 x 10 1 10 1 (10) 100 Inlets100 Outlets

16. Material prepared by W. Grover (1998-2002) 16 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics (4) Example: 16x16 2-stage switch using 4x4 non-blocking full matrices: 4 x 4 Using this example, we can see every path through the switch to connect any inlet in the 1 st stage to any outlet in the 2 nd stage. Again, notice the connection pattern: j th k th k th j th The j th outlet of the k th Stage 1 block is connected to the k th inlet of the j th Stage 2 block. Using any size of n x n blocks, you can make an n 2 x n 2 2-stage switch. We can also add a 3 rd stage to the switch to get an n 3 x n 3 3-stage switch… How?

17. Material prepared by W. Grover (1998-2002) 17 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics (5) Adding a 3 rd stage to a 2-stage switch: Treat the original n 2 x n 2 2-stage switch as it’s own block, attach it to n 2 new blocks of n x n and use the same connection pattern: j th k th k th j th The j th outlet of the k th Stage 1 block is connected to the k th inlet of the j th Stage 2 block. Then copy the original n 2 x n 2 2-stage switch n times and repeat. 3 x 3 How many xpts? 27 x 27 3-stage switch: 243 27 x 27 1-stage full matrix: 729

18. Material prepared by W. Grover (1998-2002) 18 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multi-Stage Switch Fabrics (6) 10 x 10 (10) 10 x 10 (10) 10 x 10 (10) 10 x 10 (10) 10 x 10 (10) 10 x 10 (10) How many xpts? 1000 x 1000 3-stage switch: 30 000 1000 x 1000 1-stage full matrix: 1 million distribution Connection pattern used is called distribution, and in general: nkj Stage n - Module k - Outlet j connects to… n+1jk Stage n+1 - Module j - Inlet k k j j k Example: 2191 Stage 2 - Module 1 - Outlet 91 connects to… 3911 Stage 3 - Module 91 - Inlet 1

19. Material prepared by W. Grover (1998-2002) 19 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Link Blocking Because of the single link between each module and the modules in the next stage, there’s a possibility of blocking. –Consider an inlet in the 1 st block of stage 1 connected to an outlet in the 3 rd block of stage 2. –Now what happens if we want to connect another inlet the 1 st block of stage 1 to another outlet of the 3 rd block of stage 2? 4 x 4 only a single route available A problem arises because there is only a single route available through a switch with only distribution-type of stages. we can still encounter blocking  Even though the entire switch is made up of non- blocking square matrices, we can still encounter blocking.

20. Material prepared by W. Grover (1998-2002) 20 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Estimating Blocking “Distribution” stages increase the overall inlet/outlet size of the switch but introduce increasing probability of blocking. –there is only a single path between any specific 1 st stage inlet and any specific final stage outlet. –Mechanism of blockage is when an inter-stage link on required path is in use. –The greater the number of links in the path, the greater the probability that one of them is in use. –Therefore, the more distribution stages we have, the greater the probability of blocking (but the larger the total switch size is). Stage 1Stage 2 Link 1 Stage 3 Link 2 Stage 4 Link 3 Stage 5 Link 4 Stage 6 Link 5

21. Material prepared by W. Grover (1998-2002) 21 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Estimating Blocking (2) For a Pure “Distribution” switch: a aSay we have a Erlangs of traffic on an inlet, then the proportion of time it is used is also a, and… aassume that all connections are random, and so the probability of any one link being occupied is also a in any stage (if we use square blocks), so… 1- aProbability of any specific link being free is 1- a. (1- a) k-1But we need all links in the path to be free so probability that the path is available is (1- a) k-1.

22. Material prepared by W. Grover (1998-2002) 22 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Mixing Stages We’ve seen that we can add distribution stages to increase the switch size n k x n k (where n is the size of each square matrix block, and k is the number of distribution stages), but… –We need a way of reducing blocking. mixing stagecollection stage adding multiple paths through the switchThe solution is to add a mixing stage (also called collection stage) that keeps the overall switch size the same (in terms of n k inlets and outlets), but can reduce blocking by adding multiple paths through the switch. n x n (n) n x n (n) Distribution n x n (n) Mixing

23. Material prepared by W. Grover (1998-2002) 23 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Mixing Stages (2) distribution stage Adding a 3 rd distribution stage to a 2-stage switch: 3 x 3 mixing stage Adding a mixing stage to a 2-stage switch: 3 x 3 Connection pattern is the same as for distribution: nkj Stage n - Module k - Outlet j connects to… n+1jk Stage n+1 - Module j - Inlet k The difference is that we don’t replicate the 2-stage switch n times.

24. Material prepared by W. Grover (1998-2002) 24 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Mixing Stages (3) By how much does a mixing stage reduce blocking? –Adding a mixing stage will provide n alternate paths through the switch. Example (n = 3): 3 x 3 each path Recall that probability of blocking of each path is: But for blocking to occur, we must have all n paths blocked:

25. Material prepared by W. Grover (1998-2002) 25 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Call Packing Analyze how blocking in a network occurs: –There are generally free links in each stage. –Problem is that they are mismatched from stage to stage. For instance: brown connection Even though there are free links throughout the switch, there is a conflict for specific links required for the brown connection. 5 x 5

26. Material prepared by W. Grover (1998-2002) 26 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Call Packing (2) Call packing is a strategy of organizing new calls so that they use free links corresponding to other busy links in the next stage if possible. 5 x 5 brown connection By appropriately packing the other connections, the brown connection can now find an available path.

27. Material prepared by W. Grover (1998-2002) 27 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Consider the call blocking mechanism: Clos Non-Blocking Switches brown connection The brown connection can’t find a path through the switch. Is there a way of designing the switch with appropriately sized modules and stages so that it’s impossible for there to be blocking, even if without call packing? 5 x 5

28. Material prepared by W. Grover (1998-2002) 28 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Clos Non-Blocking Switches (2) Consider the worst possible case: n-1 of its outlets are already in use n-1 inlets are already in use –Connect from an inlet in a first stage module (with n inlets) where n-1 of its outlets are already in use to an outlet in a final stage module where (with n outlets) where n-1 inlets are already in use, and none of the busy links are matched. n n 1 1 N/n N n 1 N n-1 Need one extra module to connect through.

29. Material prepared by W. Grover (1998-2002) 29 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Clos Non-Blocking Switches (3) n n 1 1 N/n N n 1 N n-1 2n-1  For a guarantee of a free path through the switch, we need (n-1)+(n-1)+1 = 2n-1 modules in the 2 nd stage, and… 2n-1 each 1 st stage module needs 2n-1 outlets, and N/n each 2 nd stage module needs N/n outlets and inlets, and 2n-1 each 3 rd stage module needs 2n-1 inlets.

30. Material prepared by W. Grover (1998-2002) 30 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Clos Non-Blocking Switches (4) Clos switchA (non-blocking) Clos switch will have the following structure: N ? ? ? ? ? ? ? ? ? N n x 2n-1 N/n n x 2n-1 N/n x N/n 2n-1 N/n x N/n 2n-1 x n N/n 2n-1 x n Can also show that to minimize number of xpts:

31. Material prepared by W. Grover (1998-2002) 31 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering To Minimize Number of Cross-Points To minimize the number of xpts:

32. Material prepared by W. Grover (1998-2002) 32 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 5-, 7-, 9-Stage Clos Switches Clos switches can be nested together. –Middle stage modules themselves are appropriately-sized 3-stage Clos switches. NN n x 2n-1 N/n n x 2n-1 3-stage Clos N/n x N/n 2n-1 3-stage Clos N/n x N/n 2n-1 x n N/n 2n-1 x n Why would we want to do this? –Each module is non-blocking (whether full matrix or Clos network). –If we use Clos networks, we have fewer xpts.

33. Material prepared by W. Grover (1998-2002) 33 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Digital Switching TSITime Slot Interchanger (TSI). –A TSI is a time switch. –Switches one time slot channel in a single physical input to another time slot channel on a single physical output. –Functionally equivalent to an n x n space-divided switch where n is the number of time slots per frame. 1 A 2 B 3 C 4 D 1 A 2 B 3 C 4 D 1 A 2 B 3 C 4 D TSI 1 B 2 D 3 A 4 C 1 B 2 D 3 A 4 C 1 B 2 D 3 A 4 C TMSSTime multiplexed space switch (TMSS) –A space switch (multiple physical inputs and outputs) that is potentially reconfigured entirely in every time slot of each frame. –Data is switched such that for each time slot, specific inlets are connected (switched) to specific outlets. –Data does not switch timeslots.

34. Material prepared by W. Grover (1998-2002) 34 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Slot Interchanger TSIIn a TSI, one time slot is switched to another. Performed through use of two memory stores: –Speech store –Speech store is RAM with capacity to store one full frame of data. For DS1 (1.544 Mbps) with 24 channels of 8 bits, the speech store is 24 bytes long. For E1 (2.048 Mbps) with 32 channels of 8 bits, the speech store is 32 bytes long. –Speech address memorySAM –Speech address memory (SAM) or Time Switch Connection Store is RAM with capacity to store a “word” for each time slot, each word being a number identifying a specific time slot. For DS1, the SAM has capacity to store 24 words of 5 bits per word (need 5 bits to store a number between 1 and 24) for a total of 24x5 bits. For E1, the SAM has capacity to store 32 words of 5 bits per word (need 5 bits to store a number between 1 and 32) for a total of 32x5 bits.

35. Material prepared by W. Grover (1998-2002) 35 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Slot Interchanger (2) How does a TSI system work? –Data is written to the speech store cyclically as it comes in (i.e. sequentially, one time slot at a time). –Path set-up control signalling tells the SAM to store the name of the input time slot in the appropriate location corresponding to the output time slot it must be switched to. For example, if input time slot 7 is to be switched to output time slot 15, then location 15 of the SAM will store the number “7”. –Data is read a-cyclically from the speech store in the order of the output time slots as stored in the SAM. Note that this means there could be a delay of up to nearly a full frame.

36. Material prepared by W. Grover (1998-2002) 36 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 1 2 3 4 Speech Store RAM = 24 x 8 bits 23 24 Data Out (contents of timeslots rearranged) 1 2 3 4 SAM RAM = 24 x 5 bits 23 24 Data In (cyclic frame timeslot order) Time Slot Interchanger (3) Timing Write Address Counter Speech Store Speech Store: time slot xlocation x Stores the data of time slot x in location x. Control Signalling SAM Data In SAM SAM: time slot y Stores the name of the input time slot being switched to output time slot y. time slot y i.e. “In output time slot y, which speech store location do I read?” Timing Read Address Counter 1 24 1 Space switch equivalent: 24 x 24 full matrix

37. Material prepared by W. Grover (1998-2002) 37 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Multiplexed Space Switch TMSSA TMSS is a space switch (with multiple physical inputs and outputs) that is potentially reconfigured entirely in every time slot of each frame. For instance, say we have 3 time slots on each of 4 physical inlets and 4 physical outlets (also called I/P highways and O/P highways): TMSS TS1TS2TS3TS1 TS2 TS3 Space switch equivalent: Three 4 x 4 full matrices (one for each time slot)

38. Material prepared by W. Grover (1998-2002) 38 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Multiplexed Space Switch (2) How does a TMSS system work? cross-point address memoryXAM –A memory structure called cross-point address memory (XAM) is used to control switching. XAM is a RAM with capacity to store a “word” for each time slot, each word being a number identifying a specific physical input to connect to during each time slot. –Control signalling tells the XAM to store the name of the physical input in the appropriate time slot location. For example, if input 6 must be connected to output 9 during time slot 7, the the XAM for output 9 will store the number “6” in location 7. –The space switch is rapidly reconfigured at each time slot to affect the proper connections. Note that data is switched across physical inputs/outputs, but not across time slots.

39. Material prepared by W. Grover (1998-2002) 39 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Multiplexed Space Switch (3) Column Oriented Control – “Who do I get it from?” I/P 1 I/P 2 I/P 3 I/P 4 O/P 1O/P 2O/P 3O/P 4 XAM #1 XAM #2 XAM #3 XAM #4 1 2 3 4 XAM RAM = 24 x 5 bits 23 24 Each XAM stores the name of the I/P to which its O/P is connected to in each time slot. Example: To switch I/P 2 to O/P 4 in time slot 18, then XAM #4 stores the value “2” in location 18.

40. Material prepared by W. Grover (1998-2002) 40 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Multiplexed Space Switch (4) Row Oriented Control – “Who do I give it to?” I/P 1 I/P 2 I/P 3 I/P 4 O/P 1O/P 2O/P 3O/P 4 XAM #1 XAM #2 XAM #3 XAM #4 1 2 3 4 XAM RAM = 24 x 5 bits 23 24 Each XAM stores the name of the O/P to which its I/P is connected to in each time slot. Example: To switch I/P 2 to O/P 4 in time slot 18, then XAM #2 stores the value “4” in location 18.

41. Material prepared by W. Grover (1998-2002) 41 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time-Space-Time Switching beam me up Scotty :-) Space Switch: Physical inputs are connected to physical outputs but data does not cross time slots. Time Switch: TSI ABCDABCDBDACBDAC Data is switched between time slots but remains on the same physical connection. Time-Space-Time Switch: TST A A A B B B C C C D D D A A A B B B C C C D D D B D C D C B A A D A B C B D C D C B A A D A B C Data is switched between time slots and physical connections.

42. Material prepared by W. Grover (1998-2002) 42 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time-Space-Time Switching (2) Time-Space-Time switching is when data is switched across time slots and physical connections. Affected by a combination of TSI and TMSS. DS1 I/P TSI #1 DS1 I/P TSI #2 DS1 I/P TSI #3 DS1 I/P TSI #4 DS1 I/P TSI #5 DS1 O/P TSI #1 DS1 O/P TSI #2 DS1 O/P TSI #3 DS1 O/P TSI #4 DS1 O/P TSI #5 TMSS

43. Material prepared by W. Grover (1998-2002) 43 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time-Space-Time Switching (3) What is the space division equivalent of a TST switch? Each input highway is a DS1 line. Each output highway is a DS1 line. 24 x 24 One inlet for each time slot. 5 24 x 24 One module for each I/P. 5 x 5 24 5 x 5 One module for each time slot in TMSS. 5 24 x 24 One module for each O/P. One outlet for each time slot.

44. Material prepared by W. Grover (1998-2002) 44 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time-Space-Time Switching (4) How does a time-space-time switch work? –First, we find a time slot that is free from the input TSI to the TMSS and from the TMSS to the output TSI we wish to connect to. –Next, switch the input channel’s time slot in question to the free time slot. –Then at the TMSS, connect the proper input line to the proper output line during free time slot. –Finally, at the output line’s TSI, switch the free time slot to the time slot we wish to switch to. Input TSI Switch to free TS TMSS Switch to desired O/P TSI Switch to desired TS Output

45. Material prepared by W. Grover (1998-2002) 45 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multiplexing TSI Stages MultiplexingMultiplexing will increase the number of time slots into a TSI. Example: I/P #1 24 TS I/P #8 24 TS Data 8:1 Channel # 8:1 Data Channel # Data Speech Store 192 Bytes Write Address Control Signalling SAM 192 Slots - each slot big enough to store number as big as 192 Control Signalling Read Address Output to TMSS (8 x 1.544 Mbps) 192 TS

46. Material prepared by W. Grover (1998-2002) 46 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Multiplexing TSI Stages (2) What benefits do we get by multiplexing? 16 x 16 TMSS TSI 32 TS TSI 32 TS 16 32 Inputs multiplexed together in groups of four. 4 x 4 TMSS TSI 128 TS TSI 128 TS 44 128 Smaller P(B)