Chapter 9 Telephone Network - course.sdu.edu.cn€¦ · Subscriber loop system ... MDF:...
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Chapter 9Telephone Network
School of Information Science and Engineering, Shandong UniversityAssociate Prof., Deqiang Wang
Outline
Subscriber loop system 用户环路系统
Switching hierarchy and Routing 交换体系与路由
Transmission Plan 传输方案
Transmission Systems 传输系统
Signaling Techniques 信令技术
Introduction
Major systems involved in telecommunication networks
Subscriber end equipmentsSubscriber loop systemsSwitching systemsTransmission systemsSignaling systems
9.1 Subscriber Loop Systems
Cable hierarchy for subscriber loops
MDF:主配线架MF:主馈送电缆FP: 馈送点BF: 分支馈送电缆DP: 分配点DC: 分配电缆DW:入户线
Issues on Subscriber loop
How long the subscriber loop could be?Signaling limits: Current delivered should be high enough.Attenuation limits: The resistance of the cables increase in proportion to length.
Issues on Subscriber loop
How to cover subscribers too far away?
Use of higher diameter wire.Use of equalized telephone set.Use of higher supply voltage.
Issues on Subscriber loop
How to cover disperse subscribers?Party lines: two subscriber share one line.Carrier systems: (FDM or TDM)Concentrators: A large number of subscribers share a small number of lines.
C/E C/ESubscribersN
ExchangeM
Power feed& control
Junction lines
C/E:concentrator / expander 集中器/扩展器
Issues on Subscriber loop
Subscriber loop interfaceFunctions: BORSCHTB=Battery feed 馈电功能
O=Overvoltage protection 过压保护
R=Ringing 振铃
S=Supervision 监控
C=Coding 编码(PCM)
H=Hybrid 2/4线混合
T=Test 测试
9.2 Switching Hierarchy and Routing
Interconnection of switching exchangesTrunk group
Topologies adopted in Telephone networksMesh network (网状网)
Fully connected networkNumber of Trunk groups is proportional to square of the exchanges interconnectedSuitable for heavy traffic among exchanges
E E
E E
Topologies adopted in Telephone networks
Star network (星型网络)A tandem exchange (汇接交换机) is employed.All other exchanges communicate through tandem exchange.Suitable for low traffic applications.
Hierarchical networkMultilevel star connection.The number of trunk groups can be minimized.
Star & hierarchy
四级中心
三级中心
二级中心
初级中心
本地交换
用户
高效路有
Routing Methods
Right-through routingThe original exchange determines the complete route from source to destination.There are a number of predefined routes.A route is selected based on certain criteria, such as time of the day, distribution of the traffic etc.. No routing decisions are taken at the intermediate exchanges/nodes.
Right-through routing
Source:A
Destination:B
C
D E
Routing Decision Right-through
Right-through
Right-through
Routing Methods
Own-exchange/distributed routingAlternative routes can be chosen at the intermediate nodes.Capable of responding to changes in traffic loads and network configurations.Minimal modifications are required when new exchanges are added.
Own-exchange/distributed routing
A
B
C
D E
Routing Decision Routing Decision
Routing Decision
Routing Decision
Routing Methods
Computer-controlled routingBased on the use of common channel signaling (CCS) systems.In CCS, there is a separate computer-controlled signaling network.A number of routing methods can be implemented.
Computer-controlled routing
A B
C D
STP
Routing decisions are made by an independent signaling network.
9.3 Transmission PlanTransmission quality and efficiency of operating of signaling impose limit on number of circuits connected in tandem.CCITT Q40:
The maximum number of circuit to be used in an international call is 12.No more than four international circuits be used in tandem between the originating and the terminating international switching centres.In excepted cases and for a low number of calls, the total number of circuits may be 14, but even in this case, the international circuits are limited to a maximum of four.
9.3 Transmission Plan
Factors in Transmission loss budgetLine loss (线路损耗)Switch loss (开关损耗)Echo level (回声电平)Singing (啸叫)
Echo & Measures taken
Echo: talker is disturbed
Measures takenAttenuator: short delay echos (<50ms)Echo suppressor: long delay echos (>50ms)Echo canceller: long delay echos (>50ms)
A BEcho of A
Amplifier
2/4Hybrid
Echo
Attenuator: short delay echos (<50ms)
A BEcho of A
Echo of BAttenuator
Attenuator
Echo
Echo suppressor: long delay echos(>50ms)
A BEcho of A
Echo of BControlledAttenuator
ControlledAttenuator
Echo
Echo canceller: long delay echos(>50ms)
A B
Store and delay
Canceller
Singing & Control
Singing: both talker and listener are disturbed.
ControlCCITT: a minimum loss of 10dB
A BSinging
Amplifier
2/4Hybrid
9.7 Signaling Techniques
TerminologySubscriber loop signalingIntraexchange or register signalingInterexchange or interregister signaling
Signaling techniquesInchannel signaling (信道内信令方式)
Uses the speech or data path for signaling.Common channel signaling (公共信道信令)
Uses a separate common channel for passing control signals for a group of trunks or information paths.
Signaling techniquesSignaling
Inchannel Common Channel
D.C. LowFrequency
VoiceFrequency
PCM Associated Nonassociated
Inband Outband
Inchannel vs. CCSInchannel
Trunks are held up during signalingSignal repertorie is limitedInterference between voice and control signalsMisuse by customersSlowDifficult to changeReliability is not critical
CCSTrunks are not required for signalingPossible to be expandedNo interference between voice and control signalsNo misuseFastFlexibleReliability is critical
Modes of CCS
Channel associated mode (信道关联模式)The signaling path passes through the same set of switches as does the speech path.Topologies of the signaling network are the same as that of speech network.
A B
C D
STP: Signaling transfer point;SP: Signaling point
Modes of CCS
Channel nonassociated mode (~非关联模式)The signaling information may follow a different route from that of speech.The topologies of signaling network are different from that of speech network.
A B
C D
STP
CCS Network Nodes
Types of node: SP & STPSignaling Points (SP) 信令点
A SP is capable of handling control messages directly addressed to it, but is incapable of routing messages.
Signaling Transfer Points (STPs) 信令转接点
A STP capable of routing messages and could also perform the functions of a SP.
9.8 In-channel Signaling CCITT Inchannel Signaling Systems
SS1: 500/20Hz signalingSS2: 600/750Hz signalingSS3: 2280Hz single voice frequency
signalingSS4: 2040 and 2400Hz two voice
frequency compound end-to-end analog signaling
SS5: 2400 and 2600Hz two VF compound analog line signaling and 2/6 multifrequencyinband analog interregister signaling with TASI
TASI: Time assigned speech interpolation
SS4 (inband signaling)
SS4 adopts inband signaling using a combination of two voice frequencies or a single voice frequency.Timings for SS4 signaling elements
200±40350±70Single-long40±10100±20Single-short80±20150±30Compound
Recognition(ms)Duration (ms)Element
SS4
Sample control signals
PXlClear forward
PYlForward transfer
PYsTransit seizure
PXsTerminal seizure
CodeControl signal
P=prefix element (2-VF compound)Xs=2040Hz shortYs=2400Hz short
Xl=2040Hz longYl=2400Hz long
SS4
Digits of the dialed numberTransmitted as binary codes of four elements.Binary ‘1’: 2040HzBinary ‘0’: 2400HzPulse duration: 35±7msGap between neighbor digits: 35±7ms
SS5 (inband signaling)Line signaling
Compound of the two voice frequencies or a continuous single frequency.
Interregister signaling:2-out-of-6 MF (multiple frequency) code.
TASIAttempt to improve trunk utilization by assigning a circuit to a speech channel only when there is speech activity.A technique to support more speech channels with a number of trunks.Leads to speech/signaling clipping.
SS5
Techniques used to maintain trunkchannelassociation during the signaling period:
The address information is transmitted as a block after gathering all the address digits, and the gaps are ensured to be less than the speech detector hangover time.Address digits are transmitted as and when they arrive and a lock tone is transmitted during the gaps.
E and M signaling control
A standard method of transferring signaling information between the switching equipment and the signaling equipment.
M-lead: carries signals from the switching equipment to the signaling equipment.E-lead: carries signals from the signaling equipment to the switching equipment.
E and M signaling control
SwitchingEquipment
A
SignalingTerminal
A
SwitchingEquipment
B
SignalingTerminal
B
M
E M
E
M: mouth E: ear
Outband signaling
Outband signaling typesd.c. signalingLow frequency a.c. signalinga.c. signaling above speech bandInslot PCM
UsageDone on link-by-link basisEnd-to-end signaling is precluded
Outband signaling with E and M control
LPF
LPF
d.c.-a.c.
M
D
a.c.-d.c.
FLPF
LPF
d.c.-a.c.
M
D
a.c.-d.c.
F
Built-in PCM signaling
In-slot signalingThe signaling information pertaining to a particular speech channel is carried in the same time slot as the speech.Example: Bell 24-channel system.
Out-slot signalingThe signaling information pertaining to a particular speech channel is carried in a separate time slot.Example: CEPT 30-channel system.
Built-in PCM signaling
Bell D2 24-channel multiframe PCM signaling structure
Bit 1 unused
Bit 1 used for signaling as in Frame 1
Bit 1 used for frame synchronization
Frame 1
Frame 2
Frame 3
Frame 4
12345678 12345678
signalingSpeech sample
TS 1 TS 24
Built-in PCM signaling
CEPT 30-channel system outslotsignaling
Totally 32 time slots per frameTime slot 0 is used for synchronizationTime slot 16 is used for signaling, carrying signaling information for two speech channels each time.A multiframe structure (16 frames) is adopted for signaling purpose.
CEPT 30-channel system outslot signaling
TS0 TS1 TS2 TS16 TS30 TS31
TS0 TS1 TS2 TS16 TS30 TS31
TS0 TS1 TS2 TS16 TS30 TS31
TS0 TS1 TS2 TS16 TS30 TS31
Frame0
Frame1
Frame2
Frame15
SYN SIG
0-15
1-16
14-29
9.9 Common Channel Signaling
CCSSignaling is completely separate from switching and speech transmission.Dedicated channels for signaling are used to support a group of circuits.The CCS network is basically a store and forward (S&F) network where signaling information travels on a link-by-link basis along the route.
Basic scheme for CCS
Switchingnetwork
Switchingnetwork
Speech circuitsgroup
SPCprocessor
SPCprocessorST STM M
Voicechannel
Data channel
Signaling channel
CCS system
ST: Signaling Terminal M: Modem
CCS signaling message formats
SU: signaling unit of fixed length.SUM: single unit message
A message of one signal unit length.
MUM: multiunit messageA message with multiple signal units.
Header SignalingInformation Circuit label Error Check
Header SignalingInformation Circuit label Error Check
Subheader Length Other sig~ Error Check
Subheader Length Address digits Error Check
SS7
First defined in1980, revised in1984 and 1988.Can be used over a variety of digital circuit switched networks.The functions in SS7 are defined assuming packet switched operation.Primarily optimized to work with digital SPC exchanges utilizing 64kbps digital channels.
Architecture of SS7
Signaling datalink
Signaling link
Signaling network
SCCP
ISUP TUPOA&M
4
3
2
1
MTP NSP
levels
MTP: message transfer part NSP: network service partTUP: telephone user part ISU: ISDN user partSCCP: signaling connection control partOA&M: operation, administration and maintenance
Signaling units of SS7
Three types of signaling unitsMSU: message signal unitLSSU: link status signal unitFISU: fill-in signal unit
Flag bit pattern: ‘01111110’Used as delimiter for synchronizationOnly flags can contain six 1’s
Signaling units of SS7
F Control SER SIF CRC F
F Control Status CRC F
F Control CRC F
BSN Bl FSN FI LI U
MSU
LSSU
FISU
Controlsubfield
8
8
8
8 8
8
8
8
24 16
24 16
24 16
7 1 7 1 6 2
F=flag CRC=cyclic redundancy code SIF=signaling informationSER=service information field BSN=backward sequence numberBI=backward indicator FSN=forward sequence numberFI=forward indicator LI=length indicator U=unused
16~496
Assignments
Ex.17Ex.18