Telephone Networks
PSTN or POTS is perhaps the most stupendous telecommunication network
in existence today.
A unique feature of the telephone network is that every piece of
equipment, technique and procedure which has evolved in the last 100 years from a number of different giant corporations, is capable of working with
each other.
The enormous complexity of the telephone network is managed by using a
hierarchical structure, operation and maintenance.
Any telecommunication network may be viewed as consisting of the
following major systems:
Subscriber end instruments or equipments Subscriber loop systems
Switching systems Transmission systems Signalling systems
Every subscriber in a telephone network is connected generally to the
nearest switching office by means of a dedicated pair of wires known as subscriber loop.
It is easier to lay cables containing a number of pairs of wires for different
geographical locations and run individual pairs as required by the subscriber
Subscriber loop systems
Generally, four levels of cabling are usedas shown in figure.
At the subscriber end, the drop wires
are taken to a distribution point.
At the DP, the drop wires are connected
to wire pairs in the distribution cables.
Many DCs from nearby geographical
locations are terminated on a feeder point where they are connected to
branch feeder cables which, in turn, are connected to the main feeder cable.
The main feeder cables carry typically
100-2000 wire pairs whereas the DCs carry typically 10-500 pairs.
The feeder cables are terminated on a
main distribution frame (MDF) at the exchange. The subscriber cable pairs emanating from the exchange are also terminated on the MDF, thus, provides a flexible interconnection mechanism
useful for reallocation of cable pairs and subscriber numbers.
Distribution & feeder points also have
flexible crosspoint connection capability which permits efficient utilization of the cable pairs as well as helps in cable management during faults
Subscriber loop systems
From the point of view of economy, it is desirable that the subscriber
loop lengths are as large as possible so that a single exchange can serve a large area. But two factors limit their length:
Signalling limits Attenuation limits
As d.c. signalling is used for subscriber lines, e.g. off-hook signal and dial
pulses, a certain minimum current is required to perform these signalling functions properly.
Exchanges are designed to accept a maximum loop resistance of 1300
ohm and the microphone in the phone set requires about 25 mA as bias current and this puts a limit on the total loop resistance which, in turn, limits the loop length for a given gauge of wire.
The d.c. loop resistance Rdc for copper conductors can be calculated from
the following formula:
Rdc=21.96/d2 ohms/km, where d is the diameter of the conductor in mm.
Subscriber instruments are usually connected to the exchanges using
copper conductors of sizes AWG 19 to AWG 26 American wire gauge.
Smaller gauge wires use thicker conductors and offer less d.c. resistance
per unit length. They are used to connect subscribers located at far away distances and are more expensive.
Subscriber loop systems
Attenuation limits arise from the a.c.response of the loop and refers to loop loss in decibels. The criterion here is to ensure that the quality of reception at the
subscriber end is satisfactory.
A rating system standardized by CCITT to
grade customer satisfaction is known as the reference equivalent (RE).
In this system, RE of a telephone set or a
subscriber loop is arrived at by comparing its performance with a standard setup
established in the ITU lab in Geneva known as NOSFER.
Tests are conducted for both transmit and
receive qualities and the corresponding REs are known as TRE and RRE respectively.
REs for cables are measured at a standard
frequency of 800 Hz as spscified by CCITT.
Trained listeners and talkers are engaged to
judge the quality of reception and to
transmit speech signal using a standard test language which is made up of logatoms.
A logatom is a 1-syllable word comprising a
consonant, a vowel and another consonant in that sequence.
Subscriber loop systems
A need often arises to connect to an existing exchange,
subscribers who are located beyond the maximum prescribed
distance.
In such cases, the d.c. resistance constraint is met by
Use of higher diameter (lower gauge) wire
Use of equalized telephone sets ( requires 8-12 mA bias current)
Unigauge design or use of higher supply voltage.
The attenuation constraint is usually overcome by the use of
loading coils.
Subscriber loop systems
In rural areas subscribers are generallydispersed. It is both unnecessary and expensive to provide a dedicated pair for every subscriber. Three techniques are used to gain on the number of
pairs:
Party lines ( two or more subscribers
are connected to one line)
Concentrators Carrier systems
Signalling and voice transmission on
the subscriber lines requires that the exchange performs a complete set of functions known by an acronym
BORSCHT which stands for:
B= battery feed
O= overvoltage protection R= ringing
S= supervision C= coding
H= hybrid T= test
Switching Hierarchy
Telephone networks require some formof interconnection of switching
exchanges to route traffic effectively & economically.
Exchanges are interconnected by
groups of trunk lines, known as trunk groups that carry traffic in one
direction. Two trunk groups are
required between any two exchanges.
Three basic topologies are adopted for
interconnecting exchanges: mesh, star, and hierarchy.
A 5-level switching hierarchy is
recommended by CCITT.
In a strictly HN, traffic from A to B
flows through the highest level known as the final route.
Direct trunk groups known as high
usage routes may be established in case of high traffic intensity between any pair of exchanges.
No overflow is permitted from the final
Numbering Plan
Generally, a large centrally located exchange called the main exchange
serving the main business centre of the town, and a number of smaller exchanges known as satellite exchanges serving different residential
localities, were used to cope with the growing traffic in a large area. The area containing the complete network of the main exchange, and the satellites is known as multiexchange area.
For calls originating from a location outside the MA, there is a need to
identify the area by a common code.
The introduction of subscriber trunk dialling (STD) for intercity
connections and international subscriber dialling (ISD) for international calling makes it necessary to have an national numbering plan as well as international numbering plan.
A numbering plan may be open, semi-open or closed.
An international numbering plan has been defined by CCITT in its
recommendations E.160-E.163.
For numbering purposes, the world is divided into 9 zones. Each zone is
given a single digit code.
The number of digits in an international subscriber number is limited to
Numbering Plan
Signalling Techniques
Three forms of signalling are involved in a telecommunication
network:
Subscriber loop signalling
Intraexchange or register signalling
Interexchange or interregister signalling
CCS
In channel association mode
, the common signalling
channel closely tracks the
trunk groups along the entire
length of a connection.
In channel nonassociated
mode, there is no close or
simple assignment of control
channels to trunk groups.
A CCS network consists of
two types of nodes:
Signalling transfer points
(STPs)-capable of routing
messages.
Signalling points
CCS vs Inchannel signalling
In channel signalling
CCS
Since signalling in CCS is digital innature, modems are used for carrying digital data over analog lines.
Since the channels are dedicated
for signalling they are capable of carrying signalling information for a group of circuits.
The group size is determined by
the capacity of a signalling channel.
A phase-equalized voice channel is
capable of supporting a bit rate of 2.4 kbps with acceptable error
rates for signalling. At this bit rate, one CCS link can carry signals for 1500-2000 speech circuits.
The CCS network is basically a
store and forward network where signalling information travels on a link-by-link basis along the route.
SS7
SS7 has been designed to be
an open ended CCS standard
that can be used over a
variety of digital circuit
switched networks.
While the network being
controlled is CS, the control
signalling itself uses PS.
Although SS7 is suitable for
operation over analog
channels and at speeds less
than 64 kbps, it is primarily
optimized to work with digital
SPC exchanges utilizing
64-kbps digital channels.
SS7 is suitable for operation
over both terrestrial and
satellite links.
SS7
The protocol architecture of SS7 has four levels.
The three lower levels referred to as the message transfer part
(MTP) provide a reliable service for routing messages through the
SS7 network.
The lowest layer, signalling data link, is concerned with the
physical & electrical characteristics of the signal links.
All signalling data links in SS7 are full duplex links dedicated to
SS7 traffic.
The main purpose of the second layer is to turn a potentially
unreliable physical link into a reliable data link.
There are three types of signalling units defined in SS7: MSU,
LSSU & FISU.
The level 3 signalling network functions relate to message
handling and network management. Message handling involves
discrimination, routing and distribution of messages.
The main purpose of SCCP is to enhance the limited routing,
distribution and addressing capabilities of the third layer.
