Components, architecture, and
numbering in the Telephone
network(PSTN),
Raimo Kantola
raimo.kantola@hut.fi
SG210, 4512471
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Telecommunications studies in 38.xxx courses
Services, Products
Service Design, $£
001 TF, L-courses, 105
Management and parametrization
Networks and
Network elements
- exchanges
- routers
118, 110, 188
Architectures and Network topologies
Mechanisms
- signaling
- routing
- technologies
ATM,
TCP/IP,
802.xx
- queuing methods
- signal processing
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122, 164, Traffic Theories, 188, 110
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Components in a Wireline Telephone
Network
•
•
•
•
•
•
•
•
•
access network cabling
the line card in an exchange
concentrator
data link to a main exchange
switching matrix in a main exchange
data links to a transit exchange
switching matrix in a transit exchange
data link to an international station
switching matrix in an international station
By Ilkka Veuro/Hgin Puhelin
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A Network Structure
Building
Street
Distribution
Concentrator
House
Distribution
Local Exchange
Building
Concentrator
International
Exchange
Transit exchange
By Ilkka Veuro/Hgin Puhelin
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Hypothetical Reference Connection in the
Telecommunications Network
27 500 km
National Network
International Network
Local
LE
PC
SC
TC
ISC
LE - Local Exchange
PC - Primary Center
TC -Tertiary Center
ISC - International Switching Center
ISC
ISC
ISC
ISC
TC
SC
PC
LE
Digital Exchange
Digital link
The basic rule: End to end -connection passes at most through 13 exchanges in telephone networks.
There are exceptions due to private networks.
End to end delay budget can be managed:
-- delay over 150ms disturbs the quality of speech, delay over 20 ms demands echo suppression.
-- A center poses delay << 1ms , the speed of light is the main limiting factor in data links.
-- Satellite connections increase delay significantly.
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Telecommunications Networks can be
Reviewed in Layers
OSI
7
3
2
1
Service Systems
Components of intelligent networks ,
voice mail, ...
Exchanges, Concentrators,
Switching Systems
Private Branch Exchanges (PBXs)
Transmission
Systems
PDH, SDH, WDM, xDSL, BSS/GSM,
radio link, cross-connection equipment ...
Transmission Lines Copper cables, optical fibers, radio line, ...
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Key Questions on different Layers:
Service systems
Switching systems
• Differentiation, fast development of services and
deployment, new service architectures
• Interoperability, billing
• Dimensioning and design of networks, routing/routeing,
• interworking (signaling), charging, mobility
• circuit switching and packet switching
Transmission
systems
• Geographic coverage, large capacity (multiplexing),
• Efficient use of the radio band, radio network design
Transmission lines
• Right of way, long operating life, more efficient use
of existing lines
• Local market competition/natural monopoly
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Interpretation of the System Hierarchy
• Our comparison to the OSI template refers to the
subscriber’s/user’s point of view. The systems on various layers
include protocols on many layers of the OSI model.
• The interface between the service layer and the switching layers
is not well defined. The service layer is dependent on the
switching layer (call control) concerning many supplementary
services.
• Three higher layers consist of computer managed equipment
having plenty of software.
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In the Enterprise viewpoint, telecommunications is
decomposed into roles and stakeholders
PABX, private automatic branch
exchange
Service Provider
+ Service nodes
PBX, private branch exchange
Subscriber:
+ Terminal equipment
+ PABX,PBX
Network Provider/Network Operator:
+ transmission equipment,
cross-connect systems
+ exchanges
The figure above describes the target model. In reality, there are
no service providers (if not the ISPs) independent of network operators in
telecommunications networks. Also, teleoperators are fiercely trying to retake
their positions lost to ISPs.
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Topology describes the telecommunications
network from the switching layer’s point of view
• Mesh topology, every exchange has
been coupled to others with a route or
routes consisting of circuit groups.
• Star topology, two exchanges have been
coupled to each other with a third
exchange.
Nodes are switching systems and connecting lines
are transmission systems implementing the circuit groups.
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The general structure of telecommunications
networks consists of several star topology
networks and tie lines.
524xxx
345xxx
411xxx
Initially the numbering
is aligned with the topology:
Whole Number blocks are
allocated to exchanges
=>Routing is easy
to implement.
Rkantola, (translation: A.Paju)/12.10.20/s38.118
412xxx
544xxxx
602xxxx
a tie line
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Network topology can be presented as a
graph for processing it with software.
G = (V, E),
V - nodes (set of vertices or nodes, non-empty, finite set)
E = {ej | j = 1, 2, … M} - data links(set of edges or links)
ej = (vi, vk) = (i, k )
“Data structures and algorithm analysis” -course presents many ways to handle
graphs with software. In the course s-38.122 some graph algorithms are
applied in routing.
Examples:
- Find the shortest path from node a to node b.
- Join two graphs.
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Locality of Traffic Determines the Structure and
Dimensioning of Telecommunications Networks
• The structure is determined by the number of exchanges and traffic
transmitted by them in different directions of the network.
– High traffic --> mesh topology preferred
– Low outgoing traffic --> star topology preferred
• Networks are structured using all the preceding alternatives:
– Star topology can be seen at the subscriber end where subscriber concentrators
(1) and the local exchange (2) are located.
– 1 and 2 are connected with a higher level exchange or directly to the nearest
parallel exchanges.
– Higher level city exchanges mostly implement mesh topology due to high traffic.
– There is typically a restricted number of outgoing links from the cities. The
outgoing links connect to the next level of hierarchy, called telecommunications
area.
– Correspondingly, telecommunications areas are connected with a restricted
number of links. So, there is a clear hierarchy in the network.
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The trend is towards bigger capacity
exchanges
• For instance, the number of exchanges in Sonera’s wireline
telephone network dropped from ca. 500 to 40 in the 1990’s.
• Local exchanges have been replaced with concentrators in order
to reduce costs.
Concentrator can be:
- static: compresses PCMs with low usage into PCMs with almost full usage
The time slot allocation for subscribers is independent of traffic.
- dynamic: the time slot to/from the exchange is reserved only for the purpose of a call
In both cases:
- a call between two subscribers of the concentrator is always switched via the
exchange
- an exception may be direct support of emergency calls in dynamic concentrators
This might be necessary in big concentrators having over 1000 subscribers.
- the amount of software in concentrators is just a fraction compared with exchanges
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Exchanges switch calls independently or
under the control of intelligent networks
• Functions in exchanges:
– call control: number analysis, routing/ routeing, call management, handling of supplementary services, subscriber database
– accounting, statistics and charging
– signaling, connectivity with a wide set of different interfaces
– operation and maintenance
• The number analysis function of an exchange is able to deal with all
kind of E.164 numbers. However, for administrative reasons number
translations are made at control points of the Intelligent Network. As
a result, the software of services can be concentrated IN nodes.
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The number of exchange types is
decreasing in the wireline network
• Subscribers are connected to local exchanges.
• In general, transit exchanges have no subscribers ( => no
subscriber signaling and no subscriber database). Transit
exchanges are vanishing due to growing capacity of other
exchanges.
• International switching stations understand international signaling
and also take care of special characteristic of international interadministrative accounting.
• Mobile networks mostly have their own exchanges with the
characteristic functions of mobility management and mobile
network signaling.
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Intelligent networks concentrate software
of services
SCE
SMS
SDP
SCP
SSP
call control
exchange
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• SSP= Service Switching Point = an
exchange having additionally the SSF
• SSF - service switching function =
ability to transfer call processing to
SCP on demand
• SCP = Service Control Point = a
control point having the service logic
• SDP = service data point = database
• SMS = management, SCE = Service
Creation Environment
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Numbering (basend on E.164)
• The primary function of exchanges is to switch the call in accordance with the dialled
number to an appropriate outgoing link to the next exchange and finally to a
subscriber= number analysis => route
• Sub-areas in local telecommunications area network have their own area numbers
(destination number prefix)
– For instance: 09-325xxxx Mellunmäki & Vartiokylä
• length of numbers varies between 4…8 numbers (09-region)
• Access to Finland with code 358 and + international prefix (00 or an operator specific
prefix)
Syntax
approximately:
1
00
44
990
994 _ 34 _
358
+
...
….
9
50
40
...
_ ABCdefgh
0
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Semantics of phone numbers (E.164)
• A directory number can represent a subscriber or a service
• A directory number indicating a subscriber is both a routeing
number and a “logical” number (in a phone book)
• Portability of numbers breaks this conjunction
• Service numbers are always “logical” and must be translated into
routeing numbers
• A caller should be able to deduce costs of a call based on the
directory number. Therefore allocation of numbers is aligned with
geography and the topology of the network.
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Numbering in mobile networks
• A MSISDN number (e.g. 040-7501636) has been
allocated in some HLR (home location register) based
on a prefix part of the number.
• GMSC (gateway MSC) exchanges know this
allocation.
• When a number is called, the call is transferred to
some GMSC that knows which one of the HLRs has
information about the location of the subscriber.
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Properties of number analysis in exchanges
• Analysis can be affected by
– dialled number
– direction of incoming call (a set of junction lines defines the
direction), origin or category (e.g. an operator assisted call)
• Analysis can return
– a set of routing options
– instruction for translating the number (e.g. 0800-number):
In this case it might be necessary to make the analysis again.
• The operator uses MML-commands to build the Analysis
trees based on route plans.
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The number analysis tree of an exchange
associates routeing with signaling information
From call signaling we get:
A
ABC - area
Buckets/ route alternatives
ABCd - the shortest subscriber number
ABCdefgh - the longest subscriber number
B
C
Bucket-file describes routing
options of which one is selected
taking usage of the links
into account.
d
e
In addition, the incoming direction can
impact on the choice of the start point for
the analysis. It is also possible to make number
translations before choosing the route.
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f
g
h
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Memory requirements for the
analysis
tree
Assumptions:
- the analysis is made for the first 4 numbers of max. 8
- 800 000 subscribers (e.g. Helsinki telecom area).
Calculation:
- 8 numbers give max. 10^8 - 1 subscriber number space
- usage of this number space 0.8/100% = 0.8%
- average usage of numbers = n ==>
x
n8 = 800 000 => n = 5.47.
- analyzing 4 numbers demands on the average ca. 1+6+36+216=259 nodes
- the size of a node e.g. 16 bit x 16 = 32 bytes.
- space needed for nodes is 32 x 259 = 8 288 bytes, of which for real is used
ignoring characters * and #
5.47 x 2 x 259 = 2834 bytes = ca. 34%.
By continuing the calculation it’s easy to show that analyzing all the 8 numbers causes no
problem concerning memory requirements (instead, it is a management problem).
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