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TSN LecturesETE 310
Prof. A.H.M. Asadul Huq, Ph.D.http://asadul.drivehq.com/students.htm
April 18, 2023 A.H. 1
Digital Switching System
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 allowedNote: Each pulse is known as the binary digit (bit). The number of bits transmitted is the bit rate of the signal.
April 18, 2023 A.H. 2
The process of digitization [P. GNA 90]
1. Filtering2. Sampling
3. Quantization4. Encoding
April 18, 2023 A.H. 3
The process has four steps
Quantization
Quantization with number of levels 8
April 18, 2023 A.H. 4
Advantages and disadvantages of Digital Transmission
Advantages• Satisfactory transmission• Signal regeneration• Lower signal to line noise ratio• Possibility of time division multiplexingDisadvantages• Greater bandwidth• Need synchronization• Multiplexing difficulties.• Incompatibilities with analog facilities.
April 18, 2023 A.H. 5
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.
April 18, 2023 A.H. 6
Parallel transmission
April 18, 2023 A.H. 7
Serial transmission
April 18, 2023 A.H. 8
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.
April 18, 2023 A.H. 9
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.
April 18, 2023 A.H. 10
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.
April 18, 2023 A.H. 11
6x6 Crosspoint Switch
• Crosspoint switches are simplest possible space-division switch.• Crosspoints can be turned on or off.• Internally nonblocking but need N2 crosspoints.
April 18, 2023 A.H. 12
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.
April 18, 2023 A.H. 13
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.
April 18, 2023 A.H. 14
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.
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
April 18, 2023 A.H. 16
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.
April 18, 2023 A.H. 17
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.
April 18, 2023 A.H. 18
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
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.
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.
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.
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.
1234
Speech Store
RAM = 24 x 8 bits
2324
Data Out
(contents of timeslots rearranged)
1234
SAMRAM = 24 x 5
bits
2324
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 Signallin
g
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?”
TimingRead
Address Counter
1
24
1
24
Space switch equivalent:
24 x 24full
matrix
Comparison of SDM and TDM
• SDM– Advantage:
• Instantaneous.
– Disadvantage:• Number of cross points required.
• TDM– Advantage:
• No cross points.
– Disadvantage:• Processing delay.
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.
April 18, 2023 A.H. 26
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.
April 18, 2023 A.H. 27
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.
April 18, 2023 A.H. 28
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.
April 18, 2023 A.H. 29
Time-Space-Time Switching Concept
Space Switch: Physical inputs are connected to physical outputs but data does not cross time slots.
Time Switch:
TSIABCDABCD BDACBDACData 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.
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)]
April 18, 2023 A.H. 31
TSN LectureVolume-3
THE END
THANK YOU
This ppt may be downloaded from my web site:http://asadul.drivehq.com/students.htm
Password (email address): [email protected]
This password does not live long !
April 18, 2023 A.H. 32