switching l3 slides

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


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


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


4/22


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


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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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


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)


6/22


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


Switch fabrics

Basic concepts



Cost criteria



7/22


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


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)


8/22


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


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


9/22


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


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


10/22


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


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)


11/22


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)


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


12/22


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


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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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


Switch fabrics

Basic concepts



Cost criteria



14/22


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


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


15/22


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



1

2

3

4

1

2

3

4

1

2

3

4

Outputs

Cross-point closed


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




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


16/22


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


SS equivalent of a TS-switch

3 PLANES

3x3S-SWITCH

PLANES

4 PLANES

4x4S-SWITCH

PLANES

3INPUTS

4OUTP

UTS


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18/22



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:


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


19/22


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


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


20/22


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


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)


21/22


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


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)}


22/22


SSS-switch and its graph presentation

INPUTS

3x3S-SWITCHPLANES

5PLANE

S

5x5S-SWITCHPLANES

3x3S-SWITCHPLANES

OUTPUTS3 HORIZONTALPLANES

5PLANES

INPUTS t OUTPUTS r


Graph presentation of connections

INPUTS t OUTPUTS r

A PATH

A TREE

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