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8/2/2019 Switching L3 Slides
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L3 - 1 P. Raatikainen Switching Technology / 2004
Switch Fabrics
Switching Technology S38.165http://www.netlab.hut.fi/opetus/s38165
L3 - 2 P. Raatikainen Switching Technology / 2004
Switch fabrics
Basic concepts
Time and space switching Two stage switches
Three stage switches
Cost criteria
Multi-stage switches and path search
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Accessibility
A network has full accessibility (=connectivity)when each inlet can be connected to each outlet (incase there are no other I/O connections in thenetwork)
A network has a limited accessibility when theabove given property does not exist
Interconnection networks applied in todays switchfabrics usually have full accessibility
L3 - 6 P. Raatikainen Switching Technology / 2004
Blocking
Blocking is defined as failure to satisfy a connection requirementand it depends strongly on the combinatorial properties of theswitching networks
Blocking Others
Network class Network type Network state
Non-blocking
With
blocking
state
Without blockingstates
Strict-sensenon-blocking
Wide-sensenon-blocking
Rearrangeablynon-blocking
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Blocking (cont.)
Non-blocking - a path between an arbitrary idle inlet and arbitrary idleoutlet can always be established independent of network state at set-uptime
Blocking - a path between an arbitrary idle inlet and arbitrary idle outletcannot be established owing to internal congestion due to the alreadyestablished connections
Strict-sense non-blocking - a path can always be set up between anyidle inlet and any idle outlet without disturbing paths already set up
Wide-sense non-blocking - a path can be set up between any idleinlet and any idle outlet without disturbing existing connections,provided that certain rules are followed. These rules prevent networkfrom entering a state for which new connections cannot be made
Rearrangeably non-blocking - when establishing a path between anidle inlet and an idle outlet, paths of existing connections may have tobe changed (rearranged) to set up that connection
L3 - 8 P. Raatikainen Switching Technology / 2004
Complexity
Complexity of an interconnection network is expressed bycost index
Traditional definition of cost index gives the number of cross-points in a network
used to be a reasonable measure of space division switchingsystems
Nowadays cost index alone does not characterize cost of aninterconnection network for broadband applications
VLSIs and their integration degree has changed the way how
cost of a switch fabric is formed (number of ICs, powerconsumption)
management and control of a switching system has a significantcontribution to cost
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L3 - 9 P. Raatikainen Switching Technology / 2004
Scalability
Due to constant increase of transport links and data rates onlinks, scalability of a switching system has become a keyparameter in choosing a switch fabric architecture
Scalability describes ability of a system to evolve withincreasing requirements
Issues that are usually matter of scalability
number of switching nodes
number of interconnection links between nodes
bandwidth of interconnection links and inlets/outlets throughput of switch fabric
buffering requirements
number of inlets/outlets supported by switch fabric
L3 - 10 P. Raatikainen Switching Technology / 2004
Reliability
Reliability and fault tolerance are system measures that have animpact on all functions of a switching system
Reliability defines probability that a system does not fail within agiven time interval provided that it functions correctly at the start
of the interval Availability defines probability that a system will function at a
given time instant
Fault tolerance is the capability of a system to continue itsintended function in spite of having a fault(s)
Reliability measures:
MTTF (Mean Time To Failure)
MTTR (Mean Time To Repair)
MTB (Mean Time Between Failures)
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Throughput
Throughput gives forwarding/switching speed/efficiency of aswitch fabric
It is measured in bits/s, octets/s, cells/s, packet/s, etc.
Quite often throughput is given in the range (0 ... 1.0], i.e. theobtained forwarding speed is normalized to the theoreticalmaximum throughput
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Switch fabrics
Basic concepts
Time and space switching Two stage switches
Three stage switches
Cost criteria
Multi-stage switches and path search
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L3 - 13 P. Raatikainen Switching Technology / 2004
Switching mechanisms
A switched connection requires a mechanism thatattaches the right information streams to each other
Switching takes place in the switching fabric, thestructure of which depends on networks mode ofoperation, available technology and required capacity
Communicating terminals may use different physicallinks and different time-slots, so there is an obviousneed to switch both in time and in space domain
Time and space switching are basic functions of a
switch fabric
L3 - 14 P. Raatikainen Switching Technology / 2004
Space division switching
1
2
3
4
5
6
7
8
1
2
3
4
5
6
INPUTSOUTPUTS
m INPUT LINKS n OUTPUT LINKSINTERCONNECTION
NETWORK
A space switch directs traffic from input links to output links An input may set up one connection (1, 3, 6 and 7), multiple
connections (4) or no connection (2, 5 and 8)
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L3 - 15 P. Raatikainen Switching Technology / 2004
Crossbar switch matrix
Crossbar matrix introduces the basic structure of a space switch Information flows are controlled (switched) by opening and closing
cross-points m inputs and n outputs => mncross-points (connection points) Only one input can be connected to an output at a time, but an input
can be connected to multiple outputs (multi-cast) at a time
1234
5678
1 2 3 4 5 6
m
INPUTLINKS
n OUTPUT LINKS
A CLOSED CROSS-POINT
MULTI-CAST
L3 - 16 P. Raatikainen Switching Technology / 2004
An example space switch
mx1 -multiplexer used to implement a space switch Every input is fed to every output mux and mux control signals
are used to select which input signal is connected througheach mux
mx1
12
m
1 2 m
mx1 mx1
mux/connection control
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L3 - 17 P. Raatikainen Switching Technology / 2004
Time division multiplexing
Time-slot interchanger is a device, which buffers mincoming time-slots, e.g. 30 time-slots of an E1 frame, arranges new transmitorder and transmits ntime-slots
Time-slots are stored in buffer memory usually in the order theyarrive or in the order they leave the switch - additional control logicis needed to decide respective output order or the memory slotwhere an input slot is stored
123456123456
INPUT CHANNELS OUTPUT CHANNELS
TIME-SLOT INTERCHANGER
123456
BUFFER SPACE FOR TIME-SLOTS
Time-slot 1
Time-slot 2
Time-slot 3
Time-slot 4
Time-slot 5
Time-slot 6
1 23 45 6
L3 - 18 P. Raatikainen Switching Technology / 2004
Time-slot interchange
12345678 123456
1
2
3
4
5
6
7
8
BUFFER FOR mINPUT/OUTPUT SLOTS
m
INPUTLINKS
n
OUTPUTLINKS(3) (2) (4) (1,6) (5)
DESTINATION OUTPUT #
51 162 434 2567 38
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L3 - 19 P. Raatikainen Switching Technology / 2004
Time switch implementation example 1
Incoming time-slots are written cyclically into switch memory Output logic reads cyclically control memory, which contains a pointer for
each output time-slot Pointer indicates which input time-slot to insert into each output time-slot
1
2
3
...
Time-slot counter & R/W controlTime-slot counter & R/W control
...k
m
Switch
memory
1
2
3
n
Control
memory
...
..
.j (k)
123m
Incoming frame buffer
12jn
Outgoing frame buffer
Cyclic read
write
address(3)
read/write
address(j)read
address (k)
Cyclic write
L3 - 20 P. Raatikainen Switching Technology / 2004
Time switch implementation example 2
Incoming time-slots are written into switch memory by using write-addressesread from control memory
A write address points to an output slot to which the input slot is addressed Output time-slots are read cyclically from switch memory
1
2
3
...
Time-slot counter & R/W controlTime-slot counter & R/W control
...k
n
Switch
memory
1
2
3 (k)
m
Control
memory
...
123m
Incoming frame buffer
12jn
Outgoing frame buffer
read
address(3)
writeaddress (k)
Cyclic read Cyclic write
read/write
address(2)
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L3 - 21 P. Raatikainen Switching Technology / 2004
Properties of time switches
Input and output frame buffers are read and written at wire-speed,i.e. mR/Ws for input and nR/Ws for output
Interchange buffer (switch memory) serves all inputs and outputsand thus it is read and written at the aggregate speed of all inputsand outputs=> speed of an interchange buffer is a critical parameter in timeswitches and limits performance of a switch
Utilizing parallel to serial conversion memory speed requirement
can be cut Speed requirement of control memory is half of that of switchmemory (in fact a little moor than that to allow new control data tobe updated)
L3 - 22 P. Raatikainen Switching Technology / 2004
Time-Space analogy
A time switch can be logically converted into a space switch bysetting time-slot buffers into vertical position => time-slots can beconsidered to correspond to input/output links of a space switch
But is this logical conversion fair ?
123m 123n
Time switch
1
2
3
m
1
2
3
m
Space switch
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L3 - 23 P. Raatikainen Switching Technology / 2004
Space-Space analogy
A space switch carrying time multiplexed input and output signals can belogically converted into a pure space switch (without cyclic control) bydistributing each time-slot into its own space switch
Inputs and outputs aretime multiplexed signals
(K time-slots)
1
2
m
1
2
n
1
1
2
m
1
2
n
2
1
2
m
1
2
n
K
1
2
m
1
2
n
To switch a time-slot, it sufficesto control one of the K boxes
L3 - 24 P. Raatikainen Switching Technology / 2004
An example conversion
1
2
K
1
2
n
mxn KxK
1
2
K
nxm
12
m
12
m
12
m
12
m
12
m
12
m
nKmultiplexed
inputsignals
oneachlink
2
11
2
m
1
2
m
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L3 - 25 P. Raatikainen Switching Technology / 2004
Properties of space and time switches
number of cross-points (e.g.AND-gates)- minput x noutput = mn- when m=n=> n2
output bit rate determines thespeed requirement for theswitch components
both input and output linesdeploy bus structure=> fault location difficult
Space switches
size of switch memory (SM)and control memory (CM)grows linearly as long asmemory speed is sufficient, i.e.SM + CM = 2 x 2 x number oftime-slots
a simple and cost effectivestructure when memory speedis sufficient
speed of available memorydetermines the maximumswitching capacity
Time switches
L3 - 26 P. Raatikainen Switching Technology / 2004
Switch fabrics
Basic concepts
Time and space switching Two stage switches
Three stage switches
Cost criteria
Multi-stage switches and path search
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L3 - 27 P. Raatikainen Switching Technology / 2004
A switch fabric as a combination ofspace and time switches
Two stage switches Time-Time (TT) switch Time-Space (TS) switch Space-Time (SP) switch Space-Space (SS) switch
TT-switch gives no advantage compared to a single
stage T-switch SS-switch increases blocking probability
L3 - 28 P. Raatikainen Switching Technology / 2004
A switch fabric as a combination ofspace and time switches (cont.)
ST-switch gives high blocking probability (S-switch candevelop blocking on an arbitrary bus, e.g. slots from twodifferent buses attempting to flow to a common output)
TS-switch has low blocking probability, because T-switchallows rearrangement of time-slots so that S-switching canbe done blocking free
12
n
12
n
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 2
ST-switch
12
n
12
n
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS-switch
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L3 - 29 P. Raatikainen Switching Technology / 2004
Time multiplexed space (TMS) switch
Space divided inputs and each of themcarry a frame of three time-slots
Input frames on each link are
synchronized to the crossbar A switching plane for each
time-slot to direct incomingslots to destined outputlinks of thecorrespondingtime-slot
Incom
ingfra
mes TS3 TS2 TS1
Output link address
4 1
2 3 2
3 2 1
1 4
1 2 3 4
Ti
me
Spac
e
4x4 plane for slot 1
4x4 plane for slot 2
4x4 plane for slot 3
1
2
3
4
1
2
3
4
1
2
3
4
Outputs
Cross-point closed
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Connection conflicts in a TMS switch
Space divided inputs and each of themcarry a frame of three time-slots
Input frames on each link aresynchronized to the crossbar
A switching plane for each
time-slot to direct incomingslots to destined outputlinks of thecorrespondingtime-slot
Cross-point closed
1 2 3 4
Ti
m
e
Spac
e
4x4 plane for slot 1
4x4 plane for slot 2
4x4 plane for slot 3
1
2
3
4
1
2
3
4
1
2
3
4
Incom
ingfra
mes
Outputs
TS3 TS2 TS1
4 1 4
2 3 2
3 2 1
1 4
Connectionconflict
Conflict solved
by time-slotinterchange
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TS switch interconnecting TDM links
1
2
3
3x3 TSI
SPACE
Time division switchingapplied prior to spaceswitching
Incoming time-slots canalways be rearranged suchthat output requests becomeconflict free for each slot ofa frame, provided that thenumber of requests for eachoutput is no more than thenumber of slots in a frame
TIME
OUTPUTS OF 4x4 TMS
PLA
NE
FOR
SLO
T2
PLA
NE
FOR
SLO
T3
PLA
NE
FOR
SLOT
1
L3 - 32 P. Raatikainen Switching Technology / 2004
SS equivalent of a TS-switch
3 PLANES
3x3S-SWITCH
PLANES
4 PLANES
4x4S-SWITCH
PLANES
3INPUTS
4OUTP
UTS
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Three stage switches
Basic TS-switch sufficient for switching time-slots onto addressedoutputs, but slots can appear in any order in the output frame
If a specific input slot is to carry data of a specific output slot then atime-slot interchanger is needed at each output=> any time-slot on any input can be connected to any time-slot on any output=> blocking probability minimized
Such a 3-stage configuration is named TST-switching(equivalent to 3-stage SSS-switching)
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 21
2
n
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 2 1
2
n
TST-switch:
L3 - 36 P. Raatikainen Switching Technology / 2004
SSS presentation of TST-switch
INPUTS
3x3
T- or S-SWITCHPLANES
4 PLANES
4x4
S-SWITCHPLANES
3x3
T- or S-SWITCHPLANES
OUTPUTS3 HORIZONTAL
PLANES
4 PLANES
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L3 - 37 P. Raatikainen Switching Technology / 2004
Three stage switch combinations
Possible three stage switch combinations: Time-Time-Time (TTT) ( not significant, no connection from
PCM to PCM) Time-Time-Space (TTS) (=TS) Time-Space-Time (TST) Time-Space-Space (TSS) Space-Time-Time (STT) (=ST) Space-Time-Space (STS)
Space-Space-Time (SST) (=ST) Space-Space-Space (SSS) (not significant, high probabilityof blocking)
Three interesting combinations TST, TSS and STS
L3 - 38 P. Raatikainen Switching Technology / 2004
Time-Space-Space switch
Time-Space-Space switch can be applied to increase switchingcapacity
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 21
2
n
1
2
n
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 21
2
n
1
2
n
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L3 - 39 P. Raatikainen Switching Technology / 2004
Space-Time-Space switch
Space-Time-Space switch has a high blocking probability (likeST-switch) - not a desired feature in public networks
1
2
n
1
2
n
TS n
TS 1
TS 2
TS n
TS 1
TS 2
TS n
TS 1
TS 2
L3 - 40 P. Raatikainen Switching Technology / 2004
Graph presentation of space switch
A space division switch can be presented by a graph G= (V, E)
- Vis the set of switching nodes
- Eis the set of edges in the graph
An edge eEis an ordered pair (u,v) V
- more than one edge can exist between uand v
- edges can be considered to be bi-directional
Vincludes two special sets (Tand R) of nodes not considered
part of switching network
- Tis a set of transmitting nodes having only outgoing edges
(input nodes to switch)
- Ris a set of receiving node having only incoming edges (output
nodes from switch)
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L3 - 41 P. Raatikainen Switching Technology / 2004
Graph presentation of spaceswitch (cont.)
A connection requirement is specified for each tTby subset RtR
to which tmust be connected
- subsets Rtare disjoint for different t
- in case of multi-cast Rtcontains more than one element for each t
A path is a sequence of edges (t,a), (a,b), (b,c), ,(f,g), (g,r) E,
tT, rRand a,b,c,,f,g are distinct elements of V- (T+R)
Paths originating from different tmay not use the same edge Paths originating from the same tmay use the same edges
L3 - 42 P. Raatikainen Switching Technology / 2004
Graph presentation example
INPUT NODES t OUTPUT NODES r
t1
t2
t3
t15
.
.
.
r1
r2
r3
r15
.
.
.
s1
s2
s5
v1
v2
v5
u1
u2
u3
v3
v4
s3
s4
V= (t1, t2,... t15, s1, s2,... s5, u1, u2, u3, v1, v2,... v5, r1, r2,... r15)
E= {(t1, s1), ...(t15, s5), (s1, u1), (s1, u2) ,... (s5, u3), (u1, v1 ), (u1, v2), ... (u3, v5),(v1, r1 ), (v1, r2),... (v5, r15)}
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SSS-switch and its graph presentation
INPUTS
3x3S-SWITCHPLANES
5PLANE
S
5x5S-SWITCHPLANES
3x3S-SWITCHPLANES
OUTPUTS3 HORIZONTALPLANES
5PLANES
INPUTS t OUTPUTS r
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Graph presentation of connections
INPUTS t OUTPUTS r
A PATH
A TREE