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EE 489 Telecommunication Systems Engineering University of Alberta

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. Switching.

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EE 489 Telecommunication Systems Engineering University of Alberta

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  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. Switching • Circuit Switching • A path is established between the caller and destination. • Real-time connection formed. • Example: PSTN • Message Switching • 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. • No real-time connection (i.e. connectionless). • Example: E-mail • Packet Switching • A message is broken down into parts and each part is sent separately (possibly via different routes). • Example: Internet

  3. Input 1 Output 1 Input 2 Output 2 Input 3 Output 3 Separating Circuits • Four physical ways of separating circuits: • Space, frequency, time, optical • We want to logically connect circuits coming into a switch with circuits at the output. • Eight basic functions for a conventional switch: • Interconnection • Control • Alerting • Attending • Information receiving • Information transmitting • Busy testing • Supervising

  4. Output 1 Input 1 Output 2 Output 3 Input 2 Input 3 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

  5. Uni-selector: Strowger uni-selector Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2nd Edition, John Wiley & Sons Ltd, pp. 90, 1997. Strowger Switching • Patented 12/March/1889 and in some places still in use today. • First widely-used automatic exchange system. • A wiper assembly (contact arm) moves across a fixed set of switch contacts (contact bank). • Each contact is connected to an outgoing channel.

  6. Graded uni-selectors: Line-finder (hunter): Outgoing Circuits or other uni-selectors Incoming Circuits Strowger Switching (2) • Several uni-selectors can be graded together so multiple incoming circuits can connect to multiple outgoing circuits. • Or two uni-selectors can be wired back-to-back (line-finders). • 1st uni-selector chooses the incoming circuit, the 2nd chooses the outgoing circuit. Unless there is heavy traffic, it is inefficient and uneconomical to provide each incoming circuit with a uni-selector. Line-finders can be graded together as well to form large switches.

  7. Strowger two-motion selector Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2nd Edition, John Wiley & Sons Ltd, pp. 93, 1997. 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.

  8. A B Incoming Circuits C D Crossbar switch Source: M. P. Clark, Networks and Telecommunications Design and Operation – 2nd Edition, John Wiley & Sons Ltd, pp. 96, 1997. 1 2 3 4 Outgoing Circuits 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.

  9. TSI D C B A D C B A D C B A C A D B C A D B C A D B 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 Time Division Switching • In digital TDM systems (e.g. DS1), channels are divided by time slot, but switching is still possible. • Switching is by a time-slot interchanger (TSI) and is accomplished by rearranging the order in which data is read out of the buffer. • Incoming 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.

  10. Optical Switching • One wavelength (or colour) can be turned into another. • Called wavelength conversion or translation. • Important in reducing blocking due to wavelength contention in routing and wavelength assignment (RWA) problem. • Optoelectronic conversion consists of optical receiver, conversion to electronic signal (O/E), and then transmitter generates optical signal at compliant wavelength (E/O). • Optical gating with semiconductor optical amplifiers (SOA) can also be used. • An SOA will amplify the strongest of two wavelengths, so a tunable probe laser generates a weak but constant signal which is combined with original signal. • At output of SOA, we filter off the original signal frequency and what remains is an inverse of the original signal at the probe laser’s frequency.

  11. MEMS Mirror (Lucent Technolgies) WaveStarTM MEMS Mirror (Lucent Technolgies) WaveStarTM LambdaRouter Optical Cross-Connect (Lucent Technolgies) Optical Switching (2) • One (or several wavelengths) are switched from one fibre into another. • Can use splitters and tunable filters, or • More recently - Micro-Electro-Mechanical Switches (MEMS) • On the scale of a human hair (100 microns) 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. 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 Network (speech switch or traffic switch) • One-to-one connection. • One specific inlet must connect to one specific outlet. • Access Switch Network • One-to-any connection. • One specific inlet must connect to any free outlet.

  13. Example: 4x4 = 16 cross-points (a) - Full Matrix Switch 1 2 1 1 3 100 100 4 1 2 3 4 (b) - Folded Matrix Switch 1 100 1 100 Multi-Stage Switch Fabrics • Consider a switch with a 100 x 100 interconnect function. Need 10 000 cross-points. E.g. 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). Need 4950 cross-points. E.g.

  14. 1 1 1 1 10 x 10 10 x 10 10 10 10 10 (10) (10) 1 1 1 1 10 x 10 10 x 10 10 10 10 10 Multi-Stage Switch Fabrics (2) • Full matrix and even folded matrix switches are inefficient since they provide for all possible connections. • Particularly with low trunk utilization, most connections will only be formed very infrequently, and not many at a time. • A third method of achieving a 100 x 100 interconnect function is by splitting the switch into two stages using smaller matrices or blocks. • To form a connection, two xpts are operated How many xpts? Each block is 10 x 10 = 100 xpts. Each stage is 10 blocks = 1000 xpts. Whole switch has 2 stages = 2000 xpts.

  15. 1 1 1 1 10 x 10 10 x 10 10 10 10 10 (10) (10) 100 Inlets 100 Outlets 1 1 1 1 10 x 10 10 x 10 10 10 10 10 Multi-Stage Switch Fabrics (3) • How does it work? • Divide the 100 inlets into groups of 10. • 1st outlet of each Stage 1 block is connected to an inlet of the 1st Stage 2 block. • 2nd outlet of each Stage 1 block is connected to an inlet of the 2nd Stage 2 block. • 3rd outlet of each Stage 1 block is connected to an inlet of the 3rd Stage 2 block… • ith outlet of each Stage 1 block is connected to an inlet of the ith Stage 2 block.

  16. 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 Multi-Stage Switch Fabrics (4) 16x16 2-stage switch using 4x4 non-blocking full matrices: Using this switch, we can clearly follow a path through the switch to connect any inlet in the 1st stage to any outlet in the 2nd stage. Again, notice the connection pattern: The jth outlet of the kth Stage 1 block is connected to the kth inlet of the jth Stage 2 block. Using any size of n x n blocks, you can make an n2 x n2 2-stage switch. We can also add a 3rd stage to the switch to get an n3 x n3 3-stage switch… How?

  17. 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 Multi-Stage Switch Fabrics (5) Adding a 3rd stage to a 2-stage switch: Treat the original n2 x n2 2-stage switch as it’s own block, attach it to n2 new blocks of n x n and use the same connection pattern: The jth outlet of the kth Stage 1 block is connected to the kth inlet of the jth Stage 2 block. Then copy the original n2 x n2 2-stage switch n times and repeat. How many xpts? 27 x 27 3-stage switch: 243 27 x 27 1-stage full matrix: 729

  18. 10 x 10 10 x 10 10 x 10 10 x 10 10 x 10 (10) (10) (10) (10) (10) 10 x 10 10 x 10 10 x 10 10 x 10 10 x 10 (10) 10 x 10 k j (10) j 10 x 10 k Multi-Stage Switch Fabrics (6) How many xpts? 1000 x 1000 3-stage switch: 30 000 1000 x 1000 1-stage full matrix: 1 million Connection pattern used is called distribution, and in general: Stage n - Module k - Outlet j connects to… Stage n+1 - Module j - Inlet k Example: Stage 2 - Module 1 - Outlet 91 connects to… Stage 3 - Module 91 - Inlet 1

  19. 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 4 x 4 Link Blocking • Because of the connection pattern used, there’s a possibility of blocking. • Consider an inlet in the 1st block of stage 1 connected to an outlet in the 3rd block of stage 2. • Now what happens if we want to connect another inlet the 1st block of stage 1 to another outlet of the 3rd block of stage 2? A problem arises because there is only a single route available through a switch with only distribution-type of stages.  Even though the entire switch is made up of non-blocking square matrices, we can still encounter blocking.

  20. Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Link 1 Link 5 Link 4 Link 2 Link 3 Estimating Blocking • “Distribution” stages reduce the number of xpts but introduce a probability of blocking. • In a one-to-one multi-stage switch, there is only a single path between any 1st stage inlet and any final stage outlet. • Mechanism of blockage is when a 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).

  21. Estimating Blocking (2) For a Pure “Distribution” switch: • Say we have a Erlangs of traffic on an inlet, then the proportion of time it is used is also a, and… • assume 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… • Probability of any specific link being free is 1- a. • But we need all links in the path to be free so probability that the path is available is (1- a)k-1.

  22. n x n n x n n x n (n) (n) (n) n x n n x n n x n Mixing Stages • We’ve seen that we can add distribution stages to increase the switch size nk x nk (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. • The solution is to add a mixing stage (also called collection stage) that keeps the overall switch size the same (in terms of nk inlets and outlets), but can reduce blocking by adding multiple paths through the switch. Distribution Distribution Mixing

  23. 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 Mixing Stages (2) Adding a 3rddistribution stage to a 2-stage switch: Adding a mixing stage to a 2-stage switch: Connection pattern is the same as for distribution: Stage n - Module k - Outlet j connects to… Stage n+1 - Module j - Inlet k The difference is that we don’t replicate the 2-stage switch n times.

  24. 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 3 x 3 Mixing Stages (3) • By how much does a mixing stage reduce blocking? • Adding a mixing stage will provide a total of n alternate paths through the switch. Example (n = 3): Recall that probability of blocking of each path is: But for blocking to occur, we must have all n paths blocked:

  25. 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 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: Even though there are free links throughout the switch, there is a conflict for specific links required for the brown connection.

  26. 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 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. By appropriately packing the other connections, the brown connection can now find an available path.

  27. 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 5 x 5 Clos Non-Blocking Switches • Consider the call blocking mechanism: 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?

  28. 1 n-1 n n-1 1 N N N/n n-1 n n-1 1 n Clos Non-Blocking Switches (2) • Consider the worst possible case: • 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. Need one extra module to connect through.

  29. 1 n-1 n n-1 1 N N N/n n-1 n n-1 1 n Clos Non-Blocking Switches (3)  For a guarantee of a free path through the switch, we need (n-1)+(n-1)+1 = 2n-1 modules in the 2nd stage, and… each 1st stage module needs 2n-1 outlets, and each 2nd stage module needs N/n outlets and inlets, and each 3rd stage module needs 2n-1 inlets.

  30. n x 2n-1 ? N/n x N/n ? ? 2n-1 x n N ? N/n N ? N/n 2n-1 ? n x 2n-1 ? ? N/n x N/n 2n-1 x n ? Clos Non-Blocking Switches (4) • A (non-blocking) Clos switch will have the following structure: Can also show that to minimize number of xpts:

  31. To Minimize Number of Cross-Points To minimize the number of xpts:

  32. n x 2n-1 3-stage Clos N/n x N/n 2n-1 x n N N/n N N/n 2n-1 n x 2n-1 3-stage Clos N/n x N/n 2n-1 x n 5-, 7-, 9-Stage Clos Switches • Clos switches can be nested together. • Middle stage modules themselves are appropriately-sized 3-stage Clos switches. • 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. TSI D C B A D C B A D C B A C A D B C A D B C A D B 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 Digital Switching • Time 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. • Time 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. Time Slot Interchanger • In a TSI, one time slot is switched to another. • Performed through use of two memory stores: • 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 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. 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. Data In 1 2 3 4 Speech Store RAM = 24 x 8 bits Data Out (contents rearranged) 23 24 1 2 3 4 SAM Data In SAM RAM = 24 x 5 bits Space switch equivalent: 1 1 23 24 24 x 24 full matrix 24 24 Time Slot Interchanger (3) Speech Store: Stores the data of time slot x in location x. Timing Write Address Counter Timing Read Address Counter Control Signalling SAM: Stores the name of the input time slot being switched to output time slot y. i.e. “In output time slot y, which speech store location do I read?”

  37. TS1 TS2 TS3 TS1 TS2 TS3 Space switch equivalent: TMSS Three 4 x 4 full matrices (one for each time slot) Time Multiplexed Space Switch • A 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):

  38. Time Multiplexed Space Switch (2) • How does a TMSS system work? • 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. I/P 1 I/P 2 I/P 3 I/P 4 1 2 3 4 XAM RAM = 24 x 5 bits O/P 1 O/P 2 O/P 3 23 O/P 4 24 Time Multiplexed Space Switch (3) • Column Oriented Control – “Who do I get it from?” 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. XAM #1 XAM #2 XAM #3 XAM #4

  40. I/P 1 I/P 2 I/P 3 1 2 3 4 XAM RAM = 24 x 5 bits I/P 4 23 24 O/P 1 O/P 2 O/P 3 O/P 4 Time Multiplexed Space Switch (4) • Row Oriented Control – “Who do I give it to?” Each XAM stores the name of the O/P to which its I/P is connected to in each time slot. XAM #1 XAM #2 XAM #3 Example: To switch I/P 2 to O/P 4 in time slot 18, then XAM #2 stores the value “4” in location 18. XAM #4

  41. D C B A D C B A A A D B A A D B D C B A D C B A B A C D B A C D D C B A D C B A C D B C C D B C Time-Space-Time Switching Space Switch: Physical inputs are connected to physical outputs but data does not cross time slots. Time Switch: TSI Data is switched between time slots but remains on the same physical connection. D C B A D C B A C A D B C A D B Time-Space-Time Switch: TST Data is switched between time slots and physical connections.

  42. DS1 I/P TSI #1 DS1 O/P TSI #1 DS1 I/P TSI #2 DS1 O/P TSI #2 TMSS DS1 I/P TSI #3 DS1 O/P TSI #3 DS1 I/P TSI #4 DS1 O/P TSI #4 DS1 I/P TSI #5 DS1 O/P TSI #5 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.

  43. Each input highway is a DS1 line. Each output highway is a DS1 line. One module for each time slot in TMSS. 24 x 24 5 x 5 24 x 24 One outlet for each time slot. One inlet for each time slot. 5 24 5 One module for each I/P. One module for each O/P. 24 x 24 5 x 5 24 x 24 Time-Space-Time Switching (3) • What is the space division equivalent of a TST switch?

  44. Switch to free TS Switch to desired O/P Switch to desired TS 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. TSI TMSS TSI Input Output

  45. Data I/P #1 Output to TMSS (8 x 1.544 Mbps) 24 TS Data Channel # 192 TS Write Address Read Address Control Signalling Data I/P #8 24 TS Channel # Control Signalling Multiplexing TSI Stages • Multiplexing will increase the number of time slots into a TSI. • Example: Speech Store 192 Bytes 8:1 SAM 192 Slots - each slot big enough to store number as big as 192 8:1

  46. TSI TSI 32 TS 32 TS 16 x 16 TMSS 16 16 32 TSI TSI 32 TS 32 TS Inputs multiplexed together in groups of four. TSI TSI 128 TS 128 TS 4 x 4 TMSS 128 4 4 TSI TSI 128 TS 128 TS Multiplexing TSI Stages (2) • What benefits do we get by multiplexing? Smaller P(B)

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