Switching system

1. TSN Lectures
ETE 310
Prof. A.H.M. Asadul Huq, Ph.D.
http://asadul.drivehq.com/students.htm
asadul@univdhaka.edu
5 June 2015 A.H. 1
Digital Switching System

2. Digital Signal
A digital signal has following characteristics
• Holds a fixed value for a fixed length of time
• Has sharp abrupt changes
• A present number of values allowed
Note: Each pulse is known as the binary digit (bit). The number
of bits transmitted is the bit rate of the signal.
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3. The process of digitization [P. GNA 90]
1. Filtering
2. Sampling
3. Quantization
4. Encoding
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The process has four steps

4. Quantization
Quantization with number of levels 8
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5. Advantages and disadvantages of Digital
Transmission
Advantages
• Satisfactory transmission
• Signal regeneration
• Lower signal to line noise ratio
• Possibility of time division multiplexing
Disadvantages
• Greater bandwidth
• Need synchronization
• Multiplexing difficulties.
• Incompatibilities with analog facilities.
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6. Data transmission
The transmission of binary data across a link can be accomplished
in either parallel or serial mode. In parallel mode, multiple bits
are sent with each clock tick. In serial mode, 1 bit is sent with
each clock tick. While there is only one way to send parallel data,
there are two subclasses of serial transmission: asynchronous,
synchronous.
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7. Parallel transmission
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8. Serial transmission
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9. Asynchronous Transmission
• In asynchronous transmission, we send 1 start bit (0) at the beginning and
1 or more stop bits (1s) at the end of each byte. There may be a gap
between each byte.
• Asynchronous here means “asynchronous at the byte level”, but the bits
are still synchronized; their durations are the same.
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10. Synchronous Transmission
• In synchronous transmission, we send bits one after another
without start or stop bits or gaps. It is the responsibility of the
receiver to group the bits.
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11. Space-Division Switching
• Transfer signal from a given input to a specific output
(same for any switch).
• Provide a separate physical connection between
inputs and outputs.
• Signal paths are physically separate from one
another (divided in space).
• Like a mechanical switch, or semi-conductor gate
that can be enabled or disabled by a control unit.
• Popular implementation is Crosspoint switch.
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12. 6x6 Crosspoint Switch
• Crosspoint switches are simplest possible space-division switch.
• Crosspoints can be turned on or off.
• Internally nonblocking but need N2 crosspoints.
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13. Crosspoint Switch
 The basic building block of the switch is a metallic
cross point or semiconductor gate that can be
enabled or disabled by a control unit.
 Xilink crossbar switch using FPGAs
Based on reconfigurable routing infrastructure.
High speed capacity non-blocking switches.
Sizes varying from 64×64 to 1024×1024 with data
rate of 200 Mbps.
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14. Limitations of crossbar switch
 The number of cross-points grows with the square of the
number of attached stations. (For single stage Nx(SS) = N2).
 Costly for a large switching system.
 The failure of a cross-point prevents connection between the
two devices whose lines intersect at that cross-point.
 The cross-points are inefficiently utilized.
Only a small fraction are engaged even if all of the
attached devices are active (use N switches instead of N2,
even full connection).
 Solution is to build multistage space division switches.
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15. A three stage space division crossbar switch
By splitting the crossbar switch into smaller units
and interconnection them, it is possible to build
multistage switches with fewer cross-points.

16. Three stage Space Switching Example
• Total no. of
subscribers N = 64.
• n=16 (around 10%)
• k = N/n = 64/16 = 4
• 1st stage: 16×4
• 2nd stage: 4×4
• 3rd stage: 4×16
• Multi stage Cross
points Nx =576
• Then single stage
cross points Nx(SS) =
4096
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17. Introduction to Time Division Switching
 Both voice and data can be transmitted using digital
signals.
 All modern circuit switches use digital time-division
multiplexing (TDM) technique for establishing and
maintaining circuits.
Synchronous TDM allows multiple low-speed bit
streams to share a high-speed line.
A set of inputs is sampled in a round robin manner. The
samples are organized serially into slots (channels) to form
a recurring frame of slots.
During successive time slots, different I/O pairings are
enabled, allowing a number of connections to be carried
over the shared bus.
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18. Introduction to Time Division Switching 2
 To keep up with the input lines, the data rate on the bus must
be high enough.
 For 100 full-duplex lines at 19.200 Kbps, the data rate on the
bus must be greater than 1.92 Mbps.
The source-destination pairs corresponding to all
active connections are stored in the control memory.
Thus the slots need not specify the source and
destination addresses.
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19. Figure Time-division multiplexing, without and with a time-slot interchange
• Time-division switching uses time-division multiplexing to
achieve switching. Two methods used are:
– Time-slot interchange (TSI) changes the order of the slots based on
the desired connection.
– TDM bus
Time-Division Switch

20. Time-slot interchange
• TSI consists of random access memory (RAM) with several memory
locations. The size of each location is the same as the size of a single time
slot.
• The number of locations is the same as the number of inputs.
• The RAM fills up with incoming data from time slots in the order received.
Slots are then sent out in an order based on the decisions of a control unit.

21. TDM bus
• Input and output lines are connected to a high-speed bus through input and
output gates (microswitches)
• Each input gate is closed during one of the four slots.
• During the same time slot, only one output gate is also closed. This pair of
gates allows a burst of data to be transferred from one specific input line to
one specific output line using the bus.
• The control unit opens and closes the gates according to switching need.

22. 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.

23. 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.

24. 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:
Stores the data of time
slot x in location x.
Control
Signalling
SAM Data In 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?”
Timing
Read
Address
Counter
1
24
1
24
Space switch equivalent:
24 x 24
full matrix

25. Comparison of SDM and
TDM
• SDM
– Advantage:
• Instantaneous.
– Disadvantage:
• Number of cross points required.
• TDM
– Advantage:
• No cross points.
– Disadvantage:
• Processing delay.

26. Time-Space Switching -1
• This switch contains a time stage T followed by a space switch S.
• The space array have N inlets and N outlets.
• For each inlet line, a TSI is connected
• The time switch delays samples so that they arrive at the right time for the
space division switch’s schedule.
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27. Time-Space Switching -2
• In the figure, communication takes place between subscriber A and B
• A is assigned TS (time slot) 2 and line 1 and B is assigned TS 16 and line 11
• The signal moved from TS2 to TS 16 by the TSI-1 and is transferred from
line 1 to line 11 in space switch.
• Similarly, the signal originated by B is moved from slot 16 to slot 2 through
line 11 to 1.
• If there are T time slots and space array is NxN, then the simultaneous
connections possible is NT.
• If T=128 and N=16, NT=2048 connections can be supported.
• The structure is a blocking system.
• Example of TS system –DMS -100 (Digital Multiplex System) telephone
exchange developed by NORTEL in 1979. It supported 100 000 subscribers.
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28. TST Switch
• The space stage is sandwiched between two time stage switches.
• This results in switches that are optimized both physically (the number of
crosspoints) and temporally (the amount of delay).
• TST structure is popular.
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29. TST Switch - 2
• The incoming TS (time slots) are delayed at the inlet time stage unless an
appropriate path through the space stage is available.
• Then the TS is transferred through the space stage to the appropriate
outlet line at output time stage.
• At the output time stage the TS is held until the desired outgoing time slot
occurs.
• Any space stage time slot can be used to establish connection.
• The space stage operates independent of the external TDM links.
• There are many alternative paths between a prescribed input and output
(unlike the TS stage described before which has only one fixed path)
• Relatively low blocking probability than the same of a TS switch.
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30. Time-Space-Time Switching Concept
Space Switch: Physical inputs are connected
to physical outputs but data
does not cross time slots.
Time Switch:
TSIABCDABCD BDACBDAC
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.

31. TST Switch Features
• Low blocking probability
• Space stage is independent of external TDM links
• For large switches with heavy traffic loads, the TST has good
implementation advantage.
• More cost effective. Time expansion of TST can be achieved at
less cost. [Page. 114 (P. GNA)]
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32. TSN Lecture
Volume-3
THE END
THANK YOU
This ppt may be downloaded from my web site:
http://asadul.drivehq.com/students.htm
Password (email address): tsn.ete@ulab.edu.bd
This password does not live long !
5 June 2015 A.H. 32