REGIONAL TELECOM TRAINING CENTRE
OVERVIEW OF TELECOMMUNICATION
NETWORKS - 1
OVERVIEW OF TELECOMMUNICATION NETWORKS
The telephone is a telecommunication device that is used to transmit and receive
electronically or digitally encoded speech between two or more people conversing. It is
one of the most common household appliances in the world today. Most telephones
operate through transmission of electric signals over a complex telephone network which
allows almost any phone user to communicate with almost any other user.
Telecommunication networks carry information signals among entities, which are
geographically far apart. An entity may be a computer or human being, a facsimile
machine, a teleprinter, a data terminal and so on. The entities are involved in the process
of information transfer that may be in the form of a telephone conversation (telephony) or
a file transfer between two computers or message transfer between two terminals etc.
With the rapidly growing traffic and untargeted growth of cyberspace,
telecommunication becomes a fabric of our life. The future challenges are enormous as
we anticipate rapid growth items of new services and number of users. What comes with
the challenge is a genuine need for more advanced methodology supporting analysis and
design of telecommunication architectures. Telecommunication has evaluated and growth
at an explosive rate in recent years and will undoubtedly continue to do so.
The communication switching system enables the universal connectivity. The
universal connectivity is realized when any entity in one part of the world can
communicate with any other entity in another part of the world. In many ways
telecommunication will acts as a substitute for the increasingly expensive physical
The telecommunication links and switching were mainly designed for voice
communication. With the appropriate attachments/equipments, they can be used to
transmit data. A modern society, therefore needs new facilities including very high
bandwidth switched data networks, and large communication satellites with small, cheap
Voice Signal Characteristics
Telecommunication is mainly concerned with the transmission of messages
between two distant points. The signal that contains the messages is usually converted
into electrical waves before transmission. Our voice is an analog signal, which has
amplitude and frequency characteristics.
Voice frequencies: - The range of frequencies used by a communication device
determines the communication channel, communicating devices, and bandwidth or
information carrying capacity. The most commonly used parameter that characterizes an
electrical signal is its bandwidth of analog signal or bit rate if it is a digital signal. In
telephone system, the frequencies it passes are restricted to between 300 to 3400 Hz.
In the field of telecommunications, a Telephone exchange or a Telephone switch
is a system of electronic components that connects telephone calls. A central office is the
physical building used to house inside plant equipment including telephone switches,
which make telephone calls "work" in the sense of making connections and relaying the
Switching system fundamentals
Telecommunications switching systems generally perform three basic functions:
they transmit signals over the connection or over separate channels to convey the identity
of the called (and sometimes the calling) address (for example, the telephone number),
and alert (ring) the called station; they establish connections through a switching network
for conversational use during the entire call; and they process the signal information to
control and supervise the establishment and disconnection of the switching network
In some data or message switching when real-time communication is not needed,
the switching network is replaced by a temporary memory for the storage of messages.
This type of switching is known as store-and-forward switching.
Signaling and control
The control of circuit switching systems is accomplished remotely by a specific
form of data communication known as signaling. Switching systems are connected with
one another by telecommunication channels known as trunks. They are connected with
the served stations or terminals by lines.
In some switching systems the signals for a call directly control the switching
devices over the same path for which transmission is established. For most modern
switching systems the signals for identifying or addressing the called station are received
by a central control that processes calls on a time-shared basis. Central controls receive
and interpret signals, select and establish communication paths, and prepare signals for
transmission. These signals include addresses for use at succeeding nodes or for alerting
(ringing) the called station.
Most electronic controls are designed to process calls not only by complex logic
but also by logic tables or a program of instructions stored in bulk electronic memory.
The tabular technique is known as translator. The electronic memory is now the most
accepted technique and is known as stored program control (SPC). Either type of control
may be distributed among the switching devices rather than residing centrally.
Microprocessors on integrated circuit chips are a popular form of distributed stored
Space and time division are the two basic techniques used in establishing
connections. When an individual conductor path is established through a switch for the
duration of a call, the system is known as space division. When the transmitted speech
signals are sampled and the samples multiplexed in time so that high-speed electronic
devices may be used simultaneously by several calls, the switch is known as time
In the early stages of development in telecommunication, manual switching
methods were deployed. But later on to overcome the limitations of manual switching;
automatic exchanges, having Electro-mechanical components, were developed. Strowger
exchange, the first automatic exchange having direct control feature, appeared in 1892 in
La Porte (Indiana). Though it improved upon the performance of a manual exchange it
still had a number of disadvantages, viz., a large number of mechanical parts, limited
availability, inflexibility, bulky in size etc. As a result of further research and
development, Crossbar exchanges,having an indirect control system, appeared in 1926 in
The Crossbar exchange improved upon many short- comings of the Strowger
system. However, much more improvement was expected and the revolutionary change
in field of electronics provided it. A large number of moving parts in Register, marker,
Translator, etc., were replaced en-block by a single computer. This made the exchange
smaller in size, volume and weight, faster and reliable, highly flexible, noise-free, easily
manageable with no preventive maintenance etc.
When electronic devices were introduced in the switching systems, a new concept
of switching evolved as a consequence of their extremely high operating speed compared
to their former counter-parts, i.e., the Electro-mechanical systems, where relays, the logic
elements in the electromechanical systems, have to operate and release several times
which is roughly equal to the duration of telephone signals to maintain required accuracy.
Research on electronic switching started soon after the Second World War, but
commercial fully electronic exchange began to emerge only about 30 years later.
However, electronic techniques proved economic for common control systems much
earlier. In electromechanical exchanges, common control systems mainly used switches
and relays, which were originally designed for use in switching networks. In common
controls, they are operated frequently and so wear out earlier. In contrast, the life of an
electronic device is almost independent of its frequency of operation. This gave a
motivation for developing electronic common controls and resulted in electronic
replacements for registers, markers, translators etc. having much greater reliability than
their electromechanical predecessors.
In electromechanical switching, the various functions of the exchange are
achieved by the operation and release of relays and switch (rotary or crossbar) contacts,
under the direction of a Control Sub-System. These contracts are hard - wired in a
predetermined way. The exchange dependent data, such as subscriber’s class of service,
translation and routing, combination signaling characteristics are achieved by hard-ware
and logic, by a of relay sets, grouping of same type of lines, strapping on Main or
Intermediate Distribution Frame or translation fields, etc. When the data is to be
modified, for introduction of a new service, or change in services already available to a
subscriber, the hardware change ranging from inconvenient to near impossible, are
In an SPC exchange, a processor similar to a general-purpose computer is used to
control the functions of the exchange. All the control functions, represented by a series of
various instructions, are stored in the memory. Therefore the processor memories hold all
exchange dependent data. such as subscriber date, translation tables, routing and charging
information and call records. For each call processing step. e.g. for taking a decision
according to class of service, the stored data is referred to, Hence, this concept of
switching. The memories are modifiable and the control program can always be rewritten
if the behavior or the use of system is to be modified. This imparts and enormous
flexibility in overall working of the exchange.
Digital computers have the capability of handling many tens of thousands of
instructions every second, Hence, in addition to controlling the switching functions the
same processor can handle other functions also. The immediate effect of holding both the
control programme and the exchange data, in easily alterable memories, is that the
administration can become much more responsive to subscriber requirements. both in
terms of introducing new services and modifying general services, or in responding to the
demands of individual subscriber. For example, to restore service on payment of an
overdue bill or to permit change from a dial instrument to a multi frequency sender,
simply the appropriate entries in the subscriber data-file are to be amended. This can be
done by typing- in simple instructions from a teletypewriter or visual display unit. The
ability of the administration to respond rapidly and effectively to subscriber requirements
is likely to become increasingly important in the future.
The modifications and changes in services which were previously impossible be
achieved very simply in SPC exchange, by modifying the stored data suitably. In some
cases, the subscribers can also be given the facility to modify their own data entries for
supplementary services, such as on-demand call transfer, short code (abbreviated)
The use of a central processor also makes possible the connection of local and
remote terminals to carry out man-machine dialogue with each exchange. Thus, the
maintenance and administrative operations of all the SPC exchanges in a network can be
performed from a single centralized place. The processor sends the information on the
performance of the network, such as, traffic flow, billing information, faults, to the
centre, which carries out remedial measures with the help of commands. Similarly, other
modifications in services can also be carried out from the remote centre. This allows a
better control on the overall performance of the network.
As the processor is capable of performing operations at a very high speed, it has
got sufficient time to run routine test programmes to detect faults, automatically. Hence,
there is no need to carry out time consuming manual routine tests.
In an SPC exchange, all control equipment can be replaced by a single processor.
The processor must therefore be quite powerful, typically it must process hundreds of
calls per second, in addition to performing other administrative and maintenance tasks.
However, totally centralized control has drawbacks. The software for such a central
processor will be voluminous, complex, and difficult to develop reliably. Moreover, it is
not a good arrangement from the point of view of system security, as the entire system
will collapse with the failure of the processor. These difficulties can be overcome by
decentralizing the control. Some routine functions such as scanning, signal distributing,
marking, which are independent of call processing, can be delegated to auxiliary or
Stored program control (SPC) has become the principal type of control for all types
of new switching systems throughout the world, including private branch exchanges, data
and Telex systems. Two types of data are stored in the memories of electronic switching
systems. One type is the data associated with the progress of the call, such as the dialed
address of the called line.
Another type, known as the translation data, contains infrequently changing information,
such as the type of service subscribed to by the calling line and the information required
for routing calls to called numbers. These translation data, like the program, are stored in
a memory, which is easily read but protected to avoid accidental erasure. This
information may be readily changed, however, to meet service needs. The flexibility of a
stored program also aids in the administration and maintenance of the service so that
system faults may be located quickly.
SPC exchanges can offer a wider range of facilities than earlier systems. In
addition, the facilities provided to an individual customer can be readily altered by
changing the customer’s class-of-service data stored in memory. Moreover, since the
processor’s stored data can be altered electronically,some of these facilities can be
controlled by customers. Examples include:-
1. Call barring (outgoing or incoming): The customer can prevent unauthorized
calls being made and can prevent incoming calls when wishing to be left in peace.
2. Call waiting: The ‘Call waiting’ service notifies the already busy subscriber of a
third party calling him.
3. Alarm calls: The exchange can be instructed to call the customer at a pre-arranged
time (e.g. morning alarm).
4. Call Forwarding: The subscriber having such a feature can enable the incoming
calls coming to his telephone to be transferred to another number during his
5. Conference calls: Subscriber can set up connections to more than one subscriber
and conduct telephone conferences under the provision of this facility.
6. Dynamic Barring Facility: Subscriber having STD/ISD facilities can dynamically
lock such features in their telephone to avoid misuse. Registering and dialing a
secret code will extend such such a facility.
7. Abbreviated Dialing: Most subscribers very often call only limited group of
telephone numbers. By dialing only prefix digit followed by two selection digits,
subscribers can call up to 100 predetermined subscribers connected to any
automatic exchange. This shortens the process of dialing all the digits.
8. Malicious call Identification: Malicious call identification is done immediately
and the information is obtained in the print out form either automatically or by
dialing an identification code.
9. Do Not Disturb: This facility enables the subscriber to free himself from attending
his incoming calls. Using this facility the calls coming to the subscriber can be
routed to an operator position or to an answering machine. The operator position
or the machine can inform the calling subscriber that the called subscriber is
temporarily inaccessible. Today SPC is a standard feature in all the electronic
Implementation of Switching Network.
In an electronic exchange, the switching network is one of the largest sub-system
in terms of size of the equipment. Its main functions are Switching (setting up temporary
connection between two or more exchange terminations), Transmission of speech and
signals between these terminations, with reliable accuracy.
There are two types of electronic switching system. viz. Space division and Time
Space Division switching System
In a space Division Switching system, a continuous physical path is set up
between input and output terminations. This path is separate for each connection and is
held for the entire duration of the call. Path for different connections is independent of
each other. Once a continuous path has been established., Signals are interchanged
between the two terminations. Such a switching network can employ either metallic or
electronic cross points. Previously, usage of metallic cross-points using reed relays and
all were favored. They have the advantage of compatibility with the existing line and
trunk signaling conditions in the network.
Time Division Switching System
In Time Division Switching, a number of calls share the same path on time
division sharing basis. The path is not separate for each connection, rather, is shared
sequentially for a fraction of a time by different calls. This process is repeated
periodically at a suitable high rate. The repetition rate is 8 KHz, i.e. once every 125
microseconds for transmitting speech on telephone network, without any appreciable
distortion. These samples are time multiplexed with staggered samples of other speech
channels, to enable sharing of one path by many calls. The Time Division Switching was
initially accomplished by Pulse Amplitude
Modulation (PAM) Switching. However, it still could not overcome the
performance limitations of signal distortion noise, cross-talk etc. With the advent of Pulse
Code Modulation (PCM), the PAM signals were converted into a digital format
overcoming the limitations of analog and PAM signals. PCM signals are suitable for both
transmission and switching. The PCM switching is popularly called Digital Switching.
Digital Switching Systems
A Digital switching system, in general, is one in which signals are switched in
digital form. These signals may represent speech or data. The digital signals of several
speech samples are time multiplexed on a common media before being switched through
To connect any two subscribers, it is necessary to interconnect the time-slots of
the two speech samples, which may be on same or different PCM highways. The
digitalized speech samples are switched in two modes, viz., Time Switching and Space
Switching. This Time Division Multiplex Digital Switching System is popularly known
as Digital Switching System.
The ESS No.1 system was the first fully electronic switching system but not
digital. But later came ESS No.4 system which was digital for trunk portion only. When
designed, the cost of A/D conversion (CODEC) on each subscriber line was seen as
prohibitive. So the ESS No.4 system was acting as a Trunk/Tandem exchange but not as
a local exchange. So the main difficulty for implementing a digital local exchange was
the implementation of the subscriber line interface. This was solved by the introduction
of Integrated Circuits, which made the digital local exchange economically feasible. This
implementation handles the following functions:
O-Over-voltage protection (from lightning and accidental power line contact)
C-Coding (A/D inter conversion & low pass filtering)
H-Hybrid (2W to 4W conversion)
T-Testing the connectivity of Subscriber
Examples of digital exchanges (switching systems) include CDOT, OCB, AXE, EWSD,
The general architecture of a Digital Switching System is depicted in
fig2General architecture of Digital Switching System
Trunks interface Digital Switch
Other auxiliary inter faces
(a) Tone generator
(b) Frequency receives
(c) Conference call facility
(d) CCS# 7 Protocol
Manager Operation &
(e) V 5.2 access manager Maintenance
The next evolutionary step was to move the PCM codec from the
exchange end of the customer’s line to the customer’s end. This provides digital
transmission over the customer’s line, which can have a number of advantages. Consider
data transmission. If there is an analog customer’s line, a modem must be added and data
can only be transmitted at relatively slow speeds. If the line is digital, data can be
transmitted by removing the codec (instead of adding a modem). Moreover, data can be
transmitted at 64 kbit/s instead of at, say, 2.4 kbit/s. Indeed, any form of digital signal
can be transmitted whose rate does not exceed 64 kbit/s. This can include high-speed
fax, in addition to speech and data.
This concept had led to the evolution of Integrated services digital
network (ISDN), in which the customer’s terminal equipment and the local digital
exchange can be used to provide many different services, all using 64 kbit/s digital
streams. In simple terms, we can say ISDN provides end-to-end digital connectivity.
Access to an ISDN is provided in two forms:
1. Basic-Rate Access (BRA)
The customer’s line carries two 64 kbit/s “B” channels plus a 16 kbit/s
“D” channel (a common signaling channel) in each direction.
2. Primary Rate Access (PRA)
The line carries a complete PCM frame at 2 Mbit/s in each direction.
This gives the customer 30 circuits at 64 kbit/s plus a common signaling channel, also at
Control of switching systems
Switching systems have evolved from being manually controlled to being
controlled by relays and then electronically. The change from the manual system to the
Strowger step-by-step system brought about a change from centralized to distributed
control. However, as systems developed and offered more services to customers, it
became economic to perform particular functions in specialized equipments that were
associated with connections only when required, thus, common control was introduced.
Later, the development of digital computer technology enabled different functions
to be performed by the same hardware by using different programs; thus switching
system entered the era of stored-program control (SPC).
There are basically two approaches to organizing stored program control:
centralized and distributed. Early electronic switching systems (ESS) developed during
the period 1970-75 almost invariably used centralized control. Although many present
day exchange designs continue to use centralized SPC, with the advent of low cost
powerful microprocessors and very large scale integration (VLSI) chips such as
programmable logic arrays (PLA) and programmable logic controllers (PLC), distributed
SPC is gaining popularity.
The figure below shows the evolution of electronic switching systems from the
manual switching systems. The figure also depicts the changing scenario from digital
switching to Broadband where the focus will be for high bit rate data transmissions.
Development of exchanges
Local and trunk Network
The term Trunk Line in telecommunications refers to the high-speed connection
between telephone central offices in the Public Switched Telephone Network (PSTN).
Trunk lines are always digital. The wiring between central offices was originally just
pairs of twisted copper wire (the twists in the wiring prevented things known as crosstalk
and noise). Because it is expensive to string up (or lay trenches for buried cables), the
phone company researched ways in which to carry more data over the existing copper
lines. This was achieved by using time-division multiplexing. Later, when fiber-optic
technology became available, phone companies upgraded their trunk lines to fiber optics
and used statistical time-division multiplexing, synchronous digital heirarchy, coarse or
dense wave division multiplexing and optical switching to further improve transmission
The signaling information exchanged between different exchanges via inter
exchange trunks for the routing of calls is termed as Inter exchange Signaling. Earlier in
band /out of band frequencies were used for transmitting signaling information. Later on,
with the emergence of PCM systems, it was possible to segregate the signaling from the
speech channel. A trunk line is a circuit connecting telephone switchboards (or other
switching equipment), as distinguished from local loop circuit which extends from
telephone exchange switching equipment to individual telephones or information
When dealing with a private branch exchange (PBX), trunk lines are the phone
lines coming into the PBX from the telephone provider. This differentiates these
incoming lines from extension lines that connect the PBX to (usually) individual phone
sets. Trunking saves cost, because there are usually fewer trunk lines than extension lines,
since it is unusual in most offices to have all extension lines in use for external calls at
once. Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN or
PRI. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office)
A signal travelling over a trunk line is not actually flowing any faster. The
electrical signal on a voice line takes the same amount of time to traverse the wire as a
similar length trunk line. What makes trunk lines faster is that the signal has been altered
to carry more data in less time using more advanced multiplexing and modulation
techniques. If you compared a voice line and a trunk line and put them side by side and
observed them, the first pieces of information arrive simultaneously on both the voice and
trunk line. However, the last piece of information would arrive sooner on the trunk line.
No matter what, you can't break the laws of physics. Electricity over copper or laser light
over fiber optics, you cannot break the speed of light--though that has rarely stopped
uneducated IT or IS managers from demanding that cabling perform faster instead of
Trunk lines can contain thousands of simultaneous calls that have been combined
using time-division multiplexing. These thousands of calls are carried from one central
office to another where they can be connected to a de-multiplexing device and switched
through digital access cross connecting switches to reach the proper exchange and local
Local and trunk Network
s L TR L S
TR TR L S
S : Remote line unit
L : Local subscriber exchange
TR : Transit exchange
CID : Outgoing international exchange
CIA : Incoming international exchange
CTI : International transit exchange
What is Trunking?
In telecommunications systems, trunking is the aggregation of multiple user circuits into
a single channel. The aggregation is achieved using some form of multiplexing. Trunking
theory was developed by Agner Krarup Erlang, Erlang based his studies of the statistical
nature of the arrival and the length of calls. The Erlang B formula allows for the
calculation of the number of circuits required in a trunk based on the Grade of Service
and the amount of traffic in Erlangs the trunk needs cater for.
In order to provide connectivity between all users on the network one solution is to build
a full mesh network between all endpoints. A full mesh solution is however impractical, a
far better approach is to provide a pool of resources that end points can make use of in
order to connect to foreign exchanges. The diagram below illustrates the where in a
telecommunication network trunks are used.
A Modern Telephone Network Indicating where trunks are used. SLC - Subscriber
LE – Local Exchange
TDM TAX –II – Level II Tax
TDM TAX –I – Level –I Tax
Level I Taxs are connected to the Gateway.
Routing in the PSTN is the process used to route telephone calls across the public
switched telephone network. This process is the same whether the call is made between
two phones in the same locality, or across two different continents.
Relationship between exchanges and operators
Telephone calls must be routed across a network of multiple exchanges, potentially
owned by different telephone operators. The exchanges are all are inter-connected
together using trunks. Each exchange has many "neighbours", some of which are also
owned by the same telephone operator, and some of which are owned by different
operators. When neighbouring exchanges are owned by different operators, they are
known as interconnect points.
This means that there is really only one virtual network in the world that enables any
phone to call any other phone. This virtual network comprises many interconnected
operators, each with their own exchange network. Every operator can then route calls
directly to their own customers, or pass them on to another operator if the call is not for
one of their customers.
The PSTN is not a fully meshed network with every operator connected to every other -
that would be both impractical and inefficient. Therefore calls may be routed through
intermediate operator networks before they reach their final destination. One of the major
problems in PSTN routing is determining how to route this call in the most cost effective
and timely manner.
Each time a call is placed for routing, the destination number (also known as the called
party) is entered by the calling party into their terminal. The destination number generally
has two parts, a prefix which generally identifies the geographical location of the
destination telephone, and a number unique within that prefix that determines the specific
destination terminal. Sometimes if the call is between two terminals in the same local
area (that is, both terminals are on the same telephone exchange), then the prefix may be
When a call is received by an exchange, there are two treatments that may be applied:
Either the destination terminal is directly connected to that exchange, in which
case the call is placed down that connection and the destination terminal rings.
Or the call must be placed to one of the neighbouring exchanges through a
connecting trunk for onward routing.
Each exchange in the chain uses pre-computed routing tables to determine which
connected exchange the onward call should be routed to. There may be several
alternative routes to any given destination, and the exchange can select dynamically
between these in the event of link failure or congestion.
The routing tables are generated centrally based on the known topology of the network,
the numbering plan, and analysis of traffic data. These are then downloaded to each
exchange in the telephone operators network. Because of the hierarchical nature of the
numbering plan, and its geographical basis, most calls can be routed based only on their
prefix using these routing tables.
Some calls however cannot be routed on the basis of prefix alone, for example non-
geographical numbers, such as toll-free or freephone calling. In these cases the Intelligent
Network is used to route the call instead of using the pre-computed routing tables.
In determining routing plans, special attention is paid for example to ensure that two
routes do not mutually overflow to each other, otherwise congestion will cause a
destination to be completely blocked.
According to Braess' paradox, the addition of a new, shorter, and lower cost route can
lead to an increase overall congestion[. The network planner must take this into account
when designing routing paths.
One approach to routing involves the use of Dynamic Alternative Routing (DAR). DAR
makes use of the distributed nature of a telecommunications network and its inherent
randomness to dynamically determine optimal routing paths. This method generates a
distributed, random, parallel computing platform that minimises congestion across the
network, and is able to adapt to take changing traffic patterns and demands into account.
Routing can be loosely described as the process of getting from here to there. Routing
may be discussed in the context of telephone networks or computer networks. In
telephone networks, routing is facilitated by switches in the network, whereby in
computer networks routing is performed by routers in the network.
Definition: Routing in telephone networks
Routing in the context of telephone networks is the selection of a specific circiut group,
for a given call or traffic stream, at an exchange in the network . "The objective of
routing is to establish a successful connection between any two exchangesin the network"
. By selecting routes that meet the constraints set by the user traffic and the network,
routing determines which network resources (circuit group) should be used to transport
which user traffic.
Different networks employ different routing techniques, but all communication networks
share a basic routing functionality based on three core routing functions
Assembling and distributing information on the state of the network and user traffic that
is used to generate and select routes.
Generating and selecting feasible and optimal routes based on network and user traffic
Forwarding user traffic along the selected routes.
The public switched telephone network (PSTN) architecture is made up of a hierarchy of
exchanges (e.g local and regoinal exchanges) with each level of the hierarchy performing
different functions . Two adjacent exchanges in the network may be connected by several
direct routes consisting of one or more circuits .
In circuit-switched networks, such as the PSTN, switching and transmission resources are
dedicated to a call along the path from source to destination for the complete duration of
the call. Routing decisions are imperative in facilitating this process as they determine the
most efficient links to use to connect users for a call . Routing in the PSTN is done using
a hop-by-hop approach . When a user wants to make a call, they dial the destination
number to which the call should be routed. This destination number is made up of a
prefix (area code or national destination network), which identifies the geographical
location of the called party, and a unique number (the subscriber number) linked to the
prefix that identifies the exact destination to which the call should be routed The end
exchange to which the calling party is connected (the originating exchange) uses the area
code to identify the outgoing circuit group connecting to the first choice adjacent
exchange en-route This circuit group is called the first choice route and is obtained using
a routing table at the originating switch . The function of the switch at the originating end
exchange is to connect the switch input port to which the calling user is connected to a
free outgoing circuit group in the first choice group . If all the circuits along the first
choice route are fully occupied, the switch then attempts to use an alternative route circuit
group to route the call to the destination exchange . The originating exchange then
forwards the address to the adjacent exchange (first choice or alternate route), and the
procedure is repeated at the adjacent exchange in order to reach the destination end
exchange to which the called party is connected . When the address reaches the
destination exchange, it only needs to process the last part of the address to identify the
switch input port that the called party is connected .
Routing directs forwarding . Forwarding of traffic can be done using connection-oriented
or connectionless approaches . In connection-oriented forwrding, forwarding instructions
are installed in all the switches along a designated route before the route can be used to
transport traffic . Traffic forwarded using the connectionless approach carries its own
forwarding information either as precise routing commands for each switch along a route
or as hints that may be autonomously interpreted by any switch in the network .
In PSTN, forwarding of traffic is based on the connection-oriented approach. Call routing
is achieved using pre-computed routing tables, containing all the possible pre-defined
routes for a connection, at each switch .The pre-defined routes specified in the routing
table include information of a direct route (or routes) to be used under normal traffic and
network conditions (e.g no link failure or network congestion) as well as alternative
routes that should be used in the event that all circuits along the direct route are fully
occupied . An alternative route may be an indirect route consisting of several circuit
groups connecting two exchanges via other exchanges . The following example illustrates
the use of an alternative route to connect two exchanges in the event of the direct route
A Typical Telephone Exchange -OCB-283
The Alcatel E10 system is located at the heart of the telecommunication networks
concerned. It is made up of three independent functional units:
- The “Subscriber Access Subsystem” which carries out connection of analogue and
digital subscriber lines,
- “Connection and Control” which carries out connections and processing of calls,
- “Operation and Maintenance” which is responsible for all functions needed by the
network operating authority.
Each functional unit is equipped with softwares which are appropriate for handling the
functions for which it is responsible.
Synchronization and Time Base Station STS
Time base (BT)
The BT ensures times distribution for LR and PCM to provide the synchronization, and
also for working out the exchange clock.Time distribution is tripled.
Time generation can be either autonomous or slaved to an external rhythm with a view to
synchronise the system with the network
Auxiliary Equipment Control Station SMA
Auxiliary equipment manager (ETA)
The ETA Supports:
- The tone generators (GT).
- The frequency receiving and generation (RGF) devices,
- Conference circuits (CCF),
- The exchange clock
CCS7 protocol handler (PUPE) and CCS7 controller (PC): CCITT No. 7 protocol
For connection of 64 kbit/s signaling channels, semi- permanent connections are
established via the connection matrix, to the PUPE which processes the CCITT No. 7
More precisely, the PUPE function carries out the following:
- “signaling channel” Level 2 processing,
- the “message routing” function
(Part of Level 3). The PC carries out:
- the “network management” function (part of Level 3),
- PUPE defence,
- Various observation tasks which are not directly linked to CCITT No. 7.
CCITT N 7 SIGNALLING
ALCATEL 1000 E10
Host switching matrix (SMX)
The SMX is a square connection matrix with a single time stage, T, duplicated in full,
which enables up to 2048 matrix links (LR) to be connected.
A matrix link LR is an internal PCM, with 16 bits per channel (32 channels). The MCX
can execute the following:
1) an unidirectional connection between any incoming channel and any out going
channel. There can be as many simultaneous connections as there are outgoing
channels. It should be remembered that a connection consists of allocating the
information contained within an incoming channel to an outgoing channel,
2) connection between any incoming channel and any M outgoing channels,
3) connection of N incoming channels belonging to one frame structure of any
multiplex onto N outgoing channels which belong to the same frame structure,
abiding to the integrity and sequencing of the frame received. This function is
referred to as “connection with N x 64 kbit/s”.
The MCX is controlled by the COM function (matrix switch controller) to ensure the:
- set up and breakdown of the connections by access to the matrix command memory.
This access is used to write at the output T.S. address the incoming T.S. address
- defense of the connections. Security of the connections in order to assure a good data
Truck Control Station SMT
PCM controller (URM)
The URM provides the interface between external PCMs and the OCB283. These PCM
come from either:
- a remote subscriber digital access unit (CSN) or from a remote electronic satellite
- another switching centre, on channel-associated signalling or CCITT No.7,
- the digital recorded announcement equipment
In particular, the URM carries out the following functions:
- HDB3 conversion to binary (PCM matrix link),
- binary conversion to HDB3 (matrix link” PCM),
- extraction and pre-processing of the channel-associated signalling of T.S.16 (PCM
- transmission of channel-associated signalling in T.S.16 (command PCM).
Main Control Station SMC
Call handler (MR)
The MR is responsible for the establishment and breaking off of communications.
The call handler takes the decisions necessary for processing of communications in terms
of the signaling received, after consultation of the subscriber and analysis database
manager (TR) if necessary. The call handler processes new calls and handling-up
operations, releases equipment, commands switching on and switching off etc.
In addition, the call handler is responsible for different management tasks (control of tests
of circuits, sundry observations).
Operation and maintenance function (OM) SMM
The functions of the operation and maintenance subsystem are carried out by the
operation and maintenance software OM).
The operating authority accesses all hardware and software equipment of the Alcatel
1000 E10 system via computer terminals belonging to the operation and maintenance
subsystem: consoles, magnetic media, intelligent terminal. These functions can be
grouped into 2 categories:
- operation of the telephone application,
- operation and maintenance of the system.
In addition, the operation and maintenance subsystem carries out:
- loading of softwares and of data for connection and command and for the
subscriber digital access units,
- temporary backup of detailed billing information,
- centralisation of alarm data coming from connection and control stations, via
- central defence of the system.
Finally, the operation and maintenance subsystem permits two-way communication with
operation and maintenance networks, at regional or national level (TMN).
CSN - digital satellite center
The digital satellite center [CSN center satellite numerique) is a subscriber connection
unit on which both analogue and digital subscribers can be connected.
Its design and composition enable the CSN to fit into an existing network and can be
connected to time-based systems using the CCITT N° 7 type of semaphore signalling.
The CSN is a connection unit designed to adapt to a variety of geographical situation: it
can be either local [CSNL] or distant [CSND] with respect to the connecting switch.
A Typical Telephone Exchange -OCB-283
NN SMX 1x3
( 1 TO 28) X 2 L
D SMA L
( 2 TO 37)
t machine 1 TO 4 MAS
2 TO 14
CSN : Digital satellite center
SMC : Main Control Station
SMA : Auxiliary Equipment Control Station
SMT : Truck Control Station
SMX : Matrix Control Station
SMM : Maintenance Station
STS : Synchronization and Time Base Station