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Digital Switching

Digital Switching. Switching Functions:. To set up and release connection. Line Switching: Direct connections between subscriber loops at an end office or between station loops at a pBX. Each loop must be accessible to every other loop . Transit Switching:

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Digital Switching

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  1. Digital Switching

  2. Switching Functions: To set up and release connection. • Line Switching: • Direct connections between subscriber loops at an end office or between station loops at a pBX. • Eachloopmustbeaccessibletoeveryotherloop. • Transit Switching: • setting up path from any incoming line to outgoing line or trunk group • alternatives exist as to which outgoing line is selected • it is not necessary that every outgoing line be accessible from every incoming line. • Calldistributors : • samebasic equipmentaspBXs. Lecture 5

  3. Lecture 5

  4. 1 2 N 1 2 N De-Mux 1 2 N MUX Multiplexors and demultiplexors • Multiplexor: aggregates sessions • N input lines • Output runs N times as fast as input • Demultiplexor: distributes sessions • one input line and N outputs that run N times slower • Can cascade multiplexors Lecture 5

  5. D E M U X M U X TSI Time division switching • Key idea: when demultiplexing, position in frame determines output link • Time division switching interchanges sample position within a frame: • Time slot interchange (TSI) Lecture 5

  6. 4 3 2 1 1 2 3 4 3 1 4 2 Time Slot Interchange (TSI) : example sessions: (1,3) (2,1) (3,4) (4,2) Read and write to shared memory in different order Lecture 5

  7. TSI • Simple to build. • Multicast: easy (why?) • Limit is the time taken to read and write to memory • For 120,000 telephone circuits • Each circuit reads and writes memory once every 125 ms. • Number of operations per second : 120,000 x 8000 x2 • each operation takes around 0.5 ns => impossible with current technology • Need to look to other techniques Lecture 5

  8. i n p u t s outputs Space division switching • Each sample takes a different path through the switch, depending on its destination • Crossbar: Simplest possible space-division switch • Crosspoints can be turned on or off Lecture 5

  9. 1 2 3 4 4 1 2 3 Crossbar - example sessions: (1,2) (2,4) (3,1) (4,3) Lecture 5

  10. Crossbar • Advantages: • simple to implement • simple control • strict sense non-blocking • Multicast • Drawbacks • number of crosspoints, N2 • large VLSI space • vulnerable to single faults Lecture 5

  11. MUX 1 2 1 TSI 1 2 2 3 MUX 4 3 4 3 TSI 4 DeMux DeMux Time-space switching • Precede each input trunk in a crossbar with a TSI • Delay samples so that they arrive at the right time for the space division switch’s schedule Crosspoint: 4 (not 16) memory speed : x2 (not x4) Lecture 5

  12. Analog Time Division Switching Lecture 5

  13. 1 2 1 2 3 4 3 4 Finding the schedule • Build a routing graph • nodes - input links • session connects an input and output nodes. • Feasible schedule • Computing a schedule • compute perfect matching. Lecture 5

  14. 2 1 2 1 time 2 time 1 TSI 4 3 3 4 3 1 2 4 Time-Space: Example Internal speed = double link speed Lecture 5

  15. TSI TSI TSI TSI TSI TSI TSI TSI Time-space-time (TST) switching • Allowed to TSI both on input and output • Gives more flexibility => lowers call blocking probability Lecture 5

  16. Circuit switching - Space division • graph representation • transmitter nodes • receiver nodes • internal nodes • Feasible schedule • edge disjoint paths. • cost function • number of crosspoints (complexity of AxB is AB) • internal nodes Lecture 5

  17. Crossbar - example 1 2 3 4 4 1 2 3 Lecture 5

  18. Another Example sessions: (1,3) (2,6) (3,1) (4,4) (5,2) (6,5) Lecture 5

  19. 2x2 2x2 2x2 Clos Network Clos(N, n , k) : N - inputs/outputs; cross-points: 2 (N/n)nk + k(N/n)2 kxn nxk (N/n)x(N/n) 2x2 3x3 N=6 n=2 k=2 2x2 N 3x3 2x2 k Lecture 5

  20. n-1 k x n n-1 n x k Clos Network - strict sense non-blocking • Holds for k  2n-1 • Proof: • Consider an idle input and output • Input box connected to at most n-1 middle layer switches • output box connected to at most n-1 middle layer switches • There exists an ”unused" middle switch good for both. Lecture 5

  21. 2x3 4x4 3x2 2x3 4x4 3x2 N=8 n=2 k=3 2x3 3x2 4x4 2x3 3x2 Example Clos(8,2,3) Need to route a new call Lecture 5

  22. kxn nxk (N/n)x(N/n) 2x2 3x3 2x2 N=6 n=2 k=2 2x2 2x2 3x3 2x2 2x2 Clos Network Why is k=n internally blocking? Lecture 5

  23. 1 2 1 2 3 4 3 4 Clos Network - re-arrangable • Holds for k  n • Proof: • Consider the routing graph. • find a perfect matching. • route the perfect matching through a single middle switch! • remaining network is Clos(N-N/n,n-1,k-1) • summary: • smaller circuit • weaker guarantee • Multicast ? Lecture 5

  24. Recursive Construction: basis The basic element: The dimension: r=0 The two states: Lecture 5

  25. Recursive Construction: Benes Network r-1 dimension N/2 size r-1 dimension N/2 size Lecture 5

  26. Benes Networks • Symmetry • Size: • F(N) = 2(N/2)*4 + 2F(N/2) = O(N log N) • Rearrangable • Clos network with k=2 n=2 • Proof I: • Build routing graph. • Find 2 matchings • route one in the upper Benes and the other in the lower. Lecture 5

  27. Greedy permutation routing • Start with an arbitrary node i1 • set i1 to upper. • At the output, o1 , a new constraint, • set o2 to lower. • Continue until no new constraint. • Completing a cycle. • Continue until done. • Solve for the upper and lower Benes recursively. Lecture 5

  28. Example 1 2 3 4 5 6 7 8 1 5 6 8 4 2 3 7 ) ( I1 1 2 3 4 5 6 7 8 I2 level 0 switches level 2r switches Lecture 5

  29. Example 1 2 3 4 5 6 7 8 1 5 6 8 4 2 3 7 ) ( I1 1 2 3 4 5 6 7 8 I2 level 0 switches level 2r switches Lecture 5

  30. Example 1 2 3 4 5 6 7 8 1 5 6 8 4 2 3 7 ) ( I1 1 2 3 4 5 6 7 8 I2 level 0 switches level 2r switches Lecture 5

  31. Example 1 2 3 4 5 6 7 8 1 5 6 8 4 2 3 7 ) ( I1 1 2 3 4 5 6 7 8 I2 level 0 switches level 2r switches Lecture 5

  32. Digital Cross Connect Systems • Automatic record keeping • Remote and rapid provisioning • Automated Test access Lecture 5

  33. I]Consolidationand segregation Lecture 5

  34. II] DCS Hierarchy Lecture 5

  35. iii] IntegratedCrossConnectEquipment Lecture 5

  36. Digital Switching in an Analog Environment • Zero-LossSwitching Lecture 5

  37. BORSCHT B: Battery Feed O:Overvoltage protection R: Ringing S: Supervision C: Coding H: Hybrid T: Test • Conferencing Lecture 5

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