Chapter 9
Telephone Network
School of Information Science and
Engineering, Shandong University
Associate 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 equipments
Subscriber loop systems
Switching systems
Transmission systems
Signaling 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
Junction lines
lines.
Subscribers
N
C/E
C/E
Power feed
& control
C/E：concentrator / expander 集中器/扩展器
Exchange
M
Issues on Subscriber loop
† Subscriber loop interface
„
„
„
„
„
„
„
„
Functions: BORSCHT
B=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 exchanges
„ Trunk group
† Topologies adopted in Telephone networks
„ Mesh network (网状网)
¾ Fully connected network
¾ Number of Trunk groups is proportional to
square of the exchanges interconnected
¾ Suitable for heavy traffic among exchanges
E
E
E
E
Topologies adopted in Telephone networks
„ Star network (星型网络)
¾ A tandem exchange (汇接交换机) is employed.
¾ All other exchanges communicate through
tandem exchange.
¾ Suitable for low traffic applications.
„ Hierarchical network
¾ Multilevel star connection.
¾ The number of trunk groups can be minimized.
Star & hierarchy
四级中心
三级中心
高效路有
二级中心
初级中心
本地交换
用户
Routing Methods
† Right-through routing
„ The 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
Routing Decision
Right-through
Source:A
C
D
Right-through
E
Right-through
Destination:B
Routing Methods
† Own-exchange/distributed routing
„ Alternative 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
Routing Decision
A
Routing Decision
C
Routing Decision
E
D
Routing Decision
B
Routing Methods
† Computer-controlled routing
„ Based on the use of common channel
signaling (CCS) systems.
„ In CCS, there is a separate computercontrolled signaling network.
„ A number of routing methods can be
implemented.
Computer-controlled routing
Routing decisions are made by an independent signaling network.
STP
B
A
C
D
9.3 Transmission Plan
† Transmission 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 budget
„
„
„
„
Line 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
Amplifier
A
B
Echo of A
† Measures taken
2/4
Hybrid
„ Attenuator: short delay echos (<50ms)
„ Echo suppressor: long delay echos (>50ms)
„ Echo canceller: long delay echos (>50ms)
Echo
† Attenuator: short delay echos (<50ms)
Attenuator
A
Echo of B
Echo of A
Attenuator
B
Echo
† Echo suppressor: long delay echos
(>50ms)
Controlled Echo of B
Attenuator
A
Echo of A
Controlled
Attenuator
B
Echo
† Echo canceller: long delay echos
(>50ms)
Store and delay
A
B
Canceller
Singing & Control
† Singing: both talker and listener are
disturbed.
Amplifier
A
B
Singing
† Control
2/4
Hybrid
„ CCITT: a minimum loss of 10dB
9.7 Signaling Techniques
† Terminology
„ Subscriber loop signaling
„ Intraexchange or register signaling
„ Interexchange or interregister signaling
† Signaling techniques
„ Inchannel signaling (信道内信令方式)
¾ Uses the speech or data path for signaling.
„ Common channel signaling (公共信道信令)
¾ Uses a separate common channel for passing
control signals for a group of trunks or
information paths.
Signaling techniques
Signaling
Inchannel
D.C.
Low
Voice
PCM
Frequency Frequency
Inband Outband
Common Channel
Associated Nonassociated
Inchannel vs. CCS
† Inchannel
„ Trunks are held up
during signaling
„ Signal repertorie is
limited
„ Interference between
voice and control
signals
„ Misuse by customers
„ Slow
„ Difficult to change
„ Reliability is not
critical
† CCS
„ Trunks are not
required for signaling
„ Possible to be
expanded
„ No interference
between voice and
control signals
„ No misuse
„ Fast
„ Flexible
„ Reliability 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.
STP: Signaling transfer point;
SP: Signaling point
B
A
C
D
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.
STP
B
A
C
D
CCS Network Nodes
† Types of node: SP & STP
† Signaling 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 signaling
„ SS2: 600/750Hz signaling
„ SS3: 2280Hz single voice frequency
signaling
„ SS4: 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 multifrequency
inband 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
Element
Compound
Single-short
Single-long
Duration (ms) Recognition(ms)
150±30
80±20
100±20
40±10
350±70
200±40
SS4
† Sample control signals
Control signal
Code
Terminal seizure
PXs
Transit seizure
PYs
Clear forward
PXl
Forward transfer
PYl
P=prefix element (2-VF compound)
Xl=2040Hz long
Xs=2040Hz short
Yl=2400Hz long
Ys=2400Hz short
SS4
† Digits of the dialed number
„ Transmitted as binary codes of four
elements.
„ Binary ‘1’: 2040Hz
„ Binary ‘0’: 2400Hz
„ Pulse duration: 35±7ms
„ Gap 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.
† TASI
„ Attempt 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 trunkchannel
association 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
Switching
Equipment
A
Signaling
Terminal
A
Signaling
Terminal
B
Switching
Equipment
B
M
E
E
M
M: mouth
E: ear
Outband signaling
† Outband signaling types
„
„
„
„
d.c. signaling
Low frequency a.c. signaling
a.c. signaling above speech band
Inslot PCM
† Usage
„ Done on link-by-link basis
„ End-to-end signaling is precluded
Outband signaling with E and M control
d.c.-a.c.
LPF
a.c.-d.c.
F
M
LPF
D
LPF
M
LPF
D
F
a.c.-d.c.
d.c.-a.c.
Built-in PCM signaling
† In-slot signaling
„ The 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 signaling
„ The 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
signaling
Speech sample
Frame 1 12345678
TS 1
12345678
TS 24
Frame 2
Bit 1 unused
Frame 3
Bit 1 used for signaling as in Frame 1
Frame 4
Bit 1 used for frame synchronization
Built-in PCM signaling
† CEPT 30-channel system outslot
signaling
„ Totally 32 time slots per frame
„ Time slot 0 is used for synchronization
„ Time 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
SIG
SYN
Frame0
Frame1
Frame2
Frame15
TS0
TS0
TS0
TS0
TS1
TS1
TS1
TS1
TS2
TS16
TS30 TS31
TS2
0-15
TS16
TS30 TS31
TS2
1-16
TS16
TS30 TS31
TS2
14-29
TS16
TS30 TS31
9.9 Common Channel Signaling
† CCS
„ Signaling 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
Switching
network
SPC
processor
ST
Speech circuits
group
M
Switching
network
M
Voice
channel
Data channel
Signaling channel
CCS system
ST: Signaling Terminal
M: Modem
ST
SPC
processor
CCS signaling message formats
† SU: signaling unit of fixed length.
† SUM: single unit message
„ A message of one signal unit length.
Signaling
Header Information Circuit label Error Check
† MUM: multiunit message
„ A message with multiple signal units.
Signaling
Header Information Circuit label Error Check
Sub
header
Sub
header
Length
Length
Other sig~
Error Check
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
levels
OA&M
ISUP
TUP
4
SCCP
3
Signaling network
2
Signaling link
1
Signaling datalink
MTP
NSP
MTP: message transfer part
NSP: network service part
TUP: telephone user part
ISU: ISDN user part
SCCP: signaling connection control part
OA&M: operation, administration and maintenance
Signaling units of SS7
† Three types of signaling units
„ MSU: message signal unit
„ LSSU: link status signal unit
„ FISU: fill-in signal unit
† Flag bit pattern: ‘01111110’
„ Used as delimiter for synchronization
„ Only flags can contain six 1’s
Signaling units of SS7
MSU
F
8
Control
24
LSSU
F
8
Control Status
24
8
FISU
F
8
Control
24
CRC
16
F
8
BSN
7
Bl
1
FSN
7
FI
1
Control
subfield
SER
8
SIF
16~496
CRC
16
CRC
16
F
8
LI
6
F
8
U
2
F=flag CRC=cyclic redundancy code SIF=signaling information
SER=service information field BSN=backward sequence number
BI=backward indicator
FSN=forward sequence number
FI=forward indicator
LI=length indicator
U=unused
Assignments
† Ex.17
† Ex.18