Digital Switching System

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Digital SwitchingDigital SwitchingSystemSystem

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=~`Ó�Ó=~`Ó�Ó

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��Digital Switching System ArchitectureDigital Switching System Architecture

SwitchingNetwork

Controller

SystemConsole

DSS

ServiceCircuits

Interface

256Line

Trunk1:1

8:1 or 5:1

STDMT1:24T3

��Terminal InterfaceTerminal Interface

MP

TrunkInterface

SwitchingNetwork

T1

BORSCHT functions for line circuit

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1. 1. Basic Switching SystemBasic Switching System

Analog

Analogor Digital

PCM

highways

(4.096Mbps)

P P

S/P

Central ProcessorCommon Control

I/O

SystemConsole

Duplication/TriplicationDuplex/Triplex

P : ProcessorBHCA : Busy Hour Call Attempt

E.O. T.C. P.C. T.C. E.O.

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2. 2. Terminal Interface TechniquesTerminal Interface Techniques

2W2WRx

2WTx

PCM-256PCM-512

BORSCHT

Line Scanning Program Call Processing Program

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BORSCHT FunctionsBORSCHT Functions

■■ Line Circuit Functions are BORSCHTLine Circuit Functions are BORSCHTFunctions.Functions.

TestLine

ProtectBatteryFeed

LineSupervi-

sion

Ringingcontrol

Hybrid Filter CODECNetworkInterface

T O CHB

R

S

B: Battery Feeding

O: Overvoltage Protection

R: Ringing (20Hz, 90 volt) 1 sec on, 2sec off

S: Supervision

C: Codec and channel Filtering

H: Hybrid

T: Testing

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Example of Line CircuitExample of Line Circuit

From mux/Network

To mux/Network4W

T: TipR: Ring

T

R

2W

90Vp-p

NorthernTelecomm(Nortel)

Balance Network : Impedance match for decreasing echo.

Functional Block Diagram for a LineFunctional Block Diagram for a LineModule (LM)Module (LM)

Timeswitch

Spaceswitch

µP

µP

PCM signals2.56Mbps= 10 bits × 32 × 8K

Serial

8-bit PCM2-bit signaling andchannel control

NorthernTelecom(Nortel)

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8

Analog Trunk InterfaceAnalog Trunk Interface

■■ Similar to Subscribe LineSimilar to Subscribe Line

■■ Inband Signaling and Common-channelInband Signaling and Common-channelSignalingSignaling

■■ Per-channel CODECPer-channel CODEC

■■ Without ConcentrationWithout Concentration

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Digital Trunk InterfaceDigital Trunk Interface

■■ GAZPACHO FunctionsGAZPACHO Functions–– G: Generation of outgoing frame codeG: Generation of outgoing frame code

–– A: Alignment of incoming frameA: Alignment of incoming frame

–– Z: Zero string suppressionZ: Zero string suppression

–– P: Polar conversionP: Polar conversion

–– A: Alarm processingA: Alarm processing

–– C: Clock recoveryC: Clock recovery

–– H: Hunting during reframeH: Hunting during reframe

–– O: Office signal extraction and insertionO: Office signal extraction and insertion

■■ Digital Terminal FunctionDigital Terminal Function

B-U

U-B

FrameSynchronization

FrameAlignment

Clockrecovery

FrameSynchronization

AlarmProcessing Zero

Suppression

SignalExtractor

SignalInserter

C

OZA

AH

G

P4W

PCM-24

PCM-24

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3. 3. Switching Network ConsiderationsSwitching Network Considerations

■■ Time Division SwitchingTime Division Switching–– Time switching in memoryTime switching in memory

Randomly

–– Time switching in spaceTime switching in space

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3. 3. Switching Network Considerations (cont.)Switching Network Considerations (cont.)

■■ Solid-state XptSolid-state Xpt

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■■ Multistate Digital SwitchingMultistate Digital Switching–– T-S-T Switching NetworkT-S-T Switching Network

�Sequential write-in�Random read-out

�Random write-in�Sequential read-out

TSI

TMLλ

(λ, m)(l, m)

n(l, n)

),(),(),(),(:

),(),(),(),(:

νλ→λ→→

→→λ→νλTST

TST

mmlnlB

nlmlmA

Link numberTime slot number

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■■ Multistate Digital SwitchingMultistate Digital Switching–– S-T-S Switching NetworkS-T-S Switching Network

),(),(),(),(

),(),(),(),(

νλ→ν→→

→→ν→νλSTS

STS

knknl

nlnkk

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■■ Time-Multiplexed SwitchingTime-Multiplexed Switching

For high speed, it can assignmore slots to it.

Time Divisionspace switch

λ→l

l th Link

λ th Link

Link #, Slot #

T

S

T Randomly write-inSequentially read-outSequentially write-in

Randomly read-out

nm

CM

νm

CM

),(),(),(),(

),(),(),(),(

nlmlm

mmlnlTST

TST

→→λ→νλ

νλ→λ→→

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4. 4. Service Circuit TechniquesService Circuit Techniques

■■ Tone Generation : ROM ImplementedTone Generation : ROM Implemented

■■ Tone Reception : Digital Filters (DSP)Tone Reception : Digital Filters (DSP)

■■ Digital Conference :Digital Conference :–– Switching-type ConferenceSwitching-type Conference

–– Summing -type ConferenceSumming -type Conference

■■ Hybrid method (Combination method)Hybrid method (Combination method)–– Digital PAD : Implemented by ROMDigital PAD : Implemented by ROM

–– Digital echo suppressorDigital echo suppressor

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5. 5. Control ArchitecturesControl Architectures

■■ Central Control SystemsCentral Control Systems–– small or medium-sized PABXsmall or medium-sized PABX

■■ Shared Control System (load-shared)Shared Control System (load-shared)

■■ Distributed Control SystemsDistributed Control Systems–– by functionby function

–– by block sizeby block size

6. 6. Maintenance Diagnostics andMaintenance Diagnostics andAdministrationAdministration

■■ System maintenanceSystem maintenance–– fault detection, fault analysis, faultfault detection, fault analysis, fault

isolation, fault reporting, fault localization,isolation, fault reporting, fault localization,fault clearance, and service restoration.fault clearance, and service restoration.

■■ Maintenance strategiesMaintenance strategies–– N+1 redundancyN+1 redundancy

–– periodic check (on-line maintenance)periodic check (on-line maintenance)

–– notification, alarmnotification, alarm

■■ AdministrationAdministration–– Database management (recent change)Database management (recent change)

–– Generic program change (programGeneric program change (programpatch/program retrofit)patch/program retrofit)

–– Data collection (billing)Data collection (billing)

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6. 6. Maintenance Diagnostics andMaintenance Diagnostics andAdministration (cont)Administration (cont)

■■ Administration (Traffic)Administration (Traffic)–– Blocked call cleared assumptionBlocked call cleared assumption

–– Blocked call held assumptionBlocked call held assumption

–– Blocked call delayed assumptionBlocked call delayed assumption

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∑ =

= s

k

k

s

aska

saB

0

,)!/(

!/Erlang B formulaErlang Loss formula

∑∞

=

−=sj

ja

as j

aeP

!,

)1(1)0(

,

,

as

as

Bsa

BP

−−=> Erlang C formula

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■■ Call Control ProcedureCall Control Procedure–– Call Processing in Digital Circuit-Call Processing in Digital Circuit-

Switching SystemsSwitching Systems

Enblock oroverlap

(DTMF)

(RBT) (RBT)

Subscriberloop

T1

MF/SS7

SL

71 79

erer

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■■ Common control functions:Common control functions:–– A. Call processingA. Call processing

■■ 1. The peripheral I/O scans the lines/trunks,1. The peripheral I/O scans the lines/trunks,and on detecting that a call is arriving,and on detecting that a call is arriving,requests creation of a terminal-handlingrequests creation of a terminal-handlingprocess (labeled A in Figure 11-4).process (labeled A in Figure 11-4).

■■ 2. Terminal process A applies dial tone via2. Terminal process A applies dial tone viathe peripheral I/O process and then waitsthe peripheral I/O process and then waitsfor the digits.for the digits.

■■ 3. On receiving the first digit, terminal3. On receiving the first digit, terminalprocess A removes dial tone and collectsprocess A removes dial tone and collectsthe digits from the peripheral I/O process.the digits from the peripheral I/O process.

■■ 4. Terminal process A sends a message to4. Terminal process A sends a message tothe routing and terminal-allocation processthe routing and terminal-allocation processto locate the called party.to locate the called party.

■■ 5. The routing and terminal-allocation5. The routing and terminal-allocationprocess, after locating the called party,process, after locating the called party,notifies the switch-allocation process to setnotifies the switch-allocation process to setup the speech path through the time-up the speech path through the time-multiplexed switch. The routing-terminalmultiplexed switch. The routing-terminalallocation process creates a terminal-allocation process creates a terminal-handling process B in the called interfacehandling process B in the called interfacemodule, to handle the called line (telephone)module, to handle the called line (telephone)or trunk.or trunk.

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–– A. Call processing (cont)A. Call processing (cont)■■ 6. Terminal process B communications with6. Terminal process B communications with

terminal process A: It sends either a busyterminal process A: It sends either a busysignal or a signal or a ““ setup completesetup complete”” message, message,depending on conditions at the calleddepending on conditions at the calledinterface. If the telephone at the calledinterface. If the telephone at the calledinterface is on-hook, terminal process Binterface is on-hook, terminal process Bapplies a ringing tone.applies a ringing tone.

■■ 7. Terminal process A, on receiving the7. Terminal process A, on receiving the““ setup completesetup complete”” message from B, applies message from B, appliesaudible ringing to the calling terminal.audible ringing to the calling terminal.

■■ 8. When the called terminal goes off-hook,8. When the called terminal goes off-hook,terminal process B removes the ringing andterminal process B removes the ringing andsends an answer message to A.sends an answer message to A.

■■ 9. Terminal process A suspends audible9. Terminal process A suspends audibleringing.ringing.

–– At this point the calling and calledAt this point the calling and calledpart ies can begin their two-waypart ies can begin their two-wayconversation. Note how the variousconversation. Note how the varioussteps in setting up the connectionsteps in setting up the connectionbetween the two parties correspond tobetween the two parties correspond tothe sequence of signals described in thethe sequence of signals described in theearlier discussion, as portrayed in Fig.earlier discussion, as portrayed in Fig.11-1.11-1.

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–– A. Call processing (cont)A. Call processing (cont)■■ 10. When either terminal goes on-hook, the10. When either terminal goes on-hook, the

terminal process involved sends anterminal process involved sends anappropriate release signal to the other sideappropriate release signal to the other sideand notifies the switching-path-allocationand notifies the switching-path-allocationprocess to release the speech path betweenprocess to release the speech path betweenthe two interface modules.the two interface modules.

–– B. MaintenanceB. Maintenance

–– C. AdministrationC. Administration

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■■ Switching softwareSwitching software–– Call Processing in Digital Circuit-SwitchingCall Processing in Digital Circuit-Switching

SystemsSystems

OMAP

scheduling

O.S. Kernel

Billing

Maintenance

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–– Call Processing in Digital Circuit-SwitchingCall Processing in Digital Circuit-SwitchingSystemsSystems

Time MultiplexedSwitch

Digital Switching System

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