1c- communication networks - circuit switching - copy
TRANSCRIPT
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
1/141
COMMUNICATION SYSTEMREVIEW
CIRCUIT SWITCHING
The N 2 ProblemFor N users to be fully connected directly Requires N (N 1)/2 connectionsRequires too much space for cablesInefficient & costly since connections not always on
N = 1000N (N 1)/2 = 499500
1
2
34
N
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
2/14
2
Circuit SwitchingPatchcord panel switch invented in 1877Operators connect users on demand
Establish circuit to allow electrical current to flowfrom inlet to outlet
Only N connections required to central office1
23
N 1
N
Manual Switching
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
3/14
3
Switch Network A switched network consists of a series of inter-linked nodes, called switches.Switches are hardware and/or software devicescapable of creating temporary connectionsbetween two or more devices linked to theswitch but not to each other.
Methods of switchingCircuit switching, packet switching, and messageswitching
5
Circuit Switch
Circuit switching creates a direct physicalconnection between two devices such asphones or computers.
We can use switches to reduce the number andlength of links.
6
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
4/14
4
A Circuit Switch A circuit switch is a device with n inputs and moutputs that creates a temporary connectionbetween an input link and an output link.The number of inputs does not have to matchthe number of outputs.
7
A Folded Switch
An n-by-n folded switch can connect n lines in full-duplexmode. For example, it can connect n telephones in sucha way that each phone can be connected to every otherphone.Circuit switching uses space-division switch [paths in thecircuit are separated from each other spatially] or time-division switch.
8
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
5/14
5
Circuit Switch Types
Space-Division switchesProvide separate physical connection betweeninputs and outputsCrossbar switchesMultistage switches
Time-Division switchesTime-slot interchange techniqueTime-space-time switches
Hybrids combine Time & Space switching
Space Division Switch
Paths in the circuit are separated from each other spatially.Crossbar SwitchCrossbar switch connects n inputs to m outputs in a grid,using electronic micro-switches (transistors) at each cross-
point.Limitation is the number of cross-points required
10
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
6/14
6
Multistage SwitchMultistage switch combines crossbar switches in severalstages.Design of a multistage switch depends on the number ofstages and the number of switches required (or desired) ineach stage.Normally, the middle stages have fewer switches than do thefirst and last stages.
11
Multiple Switching Paths
Multiple paths are available in multistage switches.Blocking refers to times when two inputs are looking forthe same output. The output port is blocked.
12
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
7/14
7
n k
n k
n k
n k
N/n N/n
N/n N/n
N/n N/n
k n1
2
N/n
N inputs
1
2
3 3
N/n
N outputs
1
2
k
2(N/n)nk + k (N/n)2 crosspoints
k n
k n
k n
Multistage Space Switch
Large switch built from multiple stages of small switchesThe n inputs to a first-stage switch share k paths through intermediatecrossbar switchesLarger k (more intermediate switches) means more paths to outputIn 1950s, Clos asked, How many intermediate switches required tomake switch nonblocking?
Clos Non-Blocking Condition:k=2n-1
Request connection from last input to input switch j to last output in output switch mWorst Case: All other inputs have seized top n-1
middle switches AND all other outputs haveseized next n-1 middle switchesIf k=2n-1 , there is another path left to connectdesired input to desired output
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
8/14
8
nxk
nxk
nxk
N/nx N/n
N/nx N/n
N/nx N/n
k xn1
N/n
Desiredinput
1
j m
N/n
Desiredoutput
1
2n -1
k xn
k xn
n-1
N/nx N/nn+1
N/nx N/n2n-2
Free path Free path
n-1busy
n-1busy
Clos Non-Blocking Condition:
k=2n-1
# internal links =2x # external links
C(n) = number of crosspoints in Clos switch
= 2Nk + k ( )2 = 2 N (2n 1)+(2 n 1)( ) 2
Differentiate with respect to n:
0 = = 4 N + 4 N ==> n
The minimized number of crosspoints is then:
C* =(2N + )(2( ) 1/2 1) 4 N 2N = 4 2N 1.5
This is lower than N 2 for large N
Minimum Complexity Clos Switch
N 2N /2
2N 2n2
2N 2n3
2N 2n2
N 2
d Cd n
N n
N n
N 2
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
9/14
9
Example: Clos Switch Design
Circa 2002, Mindspeed offered a Crossbarchip with the following specs:
144 inputs x 144 outputs, 3.125Gbps/line
Aggregate Crossbar chip throughput:450 Gbps
Clos Nonblocking Design for 1152x1152switch
N=1152, n=8, k=16
N/n=144 8x16 switches in first stage16 144x144 in centre stage144 16x8 in third stage
Aggregate Throughput: 3.6 Tbps!
Note: the 144x144 crossbar can bepartitioned into multiple smaller switches
8x16
8x16
8x16
8x16
144 144
144x144
144x144
16x8 1
2
144
1 1 5 2 i n p u
t s
1
2
3 3
N/n
1 1
5 2 o u t p u t s
1
2
16
16x8
16x8
16x8
Time Division Switch
Time-division switching uses time-division multiplexing toachieve switching. Two methods used are:
Time-slot interchange (TSI) changes the order of the slots based on thedesired connection.
TDM bus
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
10/14
10
Time-Slot Interchange (TSI)TSI consists of random access memory (RAM) with severalmemory locations. The size of each location is the same asthe 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 theorder received. Slots are then sent out in an order based onthe decisions of a control unit.
19
1
2
3
22
23
24
Writeslots inorder ofarrival
Read slotsaccording toconnectionpermutation
24 23 12
Time-slot interchange
24 23 12abcd b a d c
a
b
c
d
Time-Slot Interchange (TSI)Switching
Write bytes from arriving TDM stream into memoryRead bytes in permuted order into outgoing TDM streamMax # slots = 125 msec / (2 x memory cycle time)
IncomingTDMstream
OutgoingTDMstream
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
11/14
11
TDM BusInput and output lines are connected to a high-speed bus through input andoutput 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 ofgates allows a burst of data to be transferred from one specific input line toone specific output line using the bus.The control unit opens and closes the gates according to switching need.
21
Comparison of SDM and TDM
SDM Advantage:
Instantaneous.
Disadvantage:Number of cross points required.
TDM Advantage:
No cross points.
Disadvantage:Processing delay.
22
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
12/14
12
TST SwitchCombine Space division and time division switching.This results in switches that are optimized both physically (thenumber of crosspoints) and temporally (the amount of delay).Various types are: time-space-time (TST), time-space-space-time (TSST), space-time-time-space (STTS), etc.
23
nxk
nxk
nxk
nxk
N/nx N/n k xn1
2
N/n
N inputs
1
3
1
12
n
Time-slot interchange
Input TDMframe withn slots
Output TDMframe with k slots
n 2 1 k 2 1
Time-Space-Time Hybrid Switch
Use TSI in first & third stage; Use crossbar in middleReplace n input x k output space switch by TSI switch that takes n-slotinput frame and switches it to k-slot output frame
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
13/14
13
n k N/n N/n
N/n N/n
N/n N/n
k n1 1
2
N/n
1
2
k
k n
k n
n k 2
n k N/n
First slot
k th slot
First slot
k th slot
Flow of time slots betweenswitches
Only one space switch active in each time slot
nxk
nxk
nxk
nxk
N/nx N/nTime-sharedspace switch
k xn1
2
N/n
N inputs
1
2
3 3
N/n
N outputs
TDMn slots
n slots
n slots
n slots
k xn
k xn
k xn
TDMk slots
TDMk slots
TSI stage TSI stageSpace stage
Time-Share the Crossbar Switch
Interconnection pattern of space switch is reconfiguredevery time slotVery compact design: fewer lines because of TDM &less space because of time-shared crossbar
-
8/14/2019 1C- Communication Networks - Circuit Switching - Copy
14/14
2x3
2x3
3x21
2
1
23x2D1
B1 A1B 2 A2
C 1D2 C2
B 1 A1
C1D1
A1
B 1
C 1
D1
A1 C 1
B1 D1
(b)
AB
CD
(a)C
A
D
B
Example: A3, B4, C1, D3
3-stageSpaceSwitch
EquivalentTST Switch
Example: T-S-T Switch DesignFor N = 960
Single stage space switch ~ 1 million crosspointsT-S-T
Let n = 120 N/n = 8 TSIsk = 2 n 1 = 239 for non-blockingPick k = 240 time slotsNeed 8x8 time-multiplexed space switch
For N = 96,000T-S-T
Let n = 120 k = 239N / n = 800Need 800x800 space switch