Unit I: Introduction
1.1 Public-switched Telephone Network (PSTN)
The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the
wired phone system over which landline telephone calls are made. The PSTN relies on circuit switching. To
connect one phone to another, the phone call is routed through numerous switches operating on a local,
regional, national or international level. The connection established between the two phones is called a circuit.
Originally POTS was analog system using analog signal to transmit voice but with the invention of computer
nowadays it can carry data in addition to voice. The network is now digital as well as analog. Parts of the PSTN
are also utilized for DSL, VoIP and other Internet-based network technologies.
One PSTN link supports 64 kilobits per second (Kbps) of bandwidth for data. In residences, the PSTN phone line
typically carries this bandwidth over a copper cable. Traditional dial-up network modems utilize nearly 56 Kbps
of this bandwidth when connected to a phone line.
Telephones are connected to a Central Office (CO), which supplies the necessary switching gear, voice battery,
wire cable facility terminations, etc. The telephone line (a single twisted-wire pair) connecting each telephone to
the central office is often called the local, or subscriber, loop.
Major Components
The telephone network, as shown in Figure 1.1, is made of three major components: local loops, trunks, and
switching offices. The telephone network has several levels of switching offices such as end offices, tandem
offices, and regional offices.
Fig 1.1
Local Loops
One component of the telephone network is the local loop, a twisted-pair cable that connects the subscriber
telephone to the nearest end office or local central office. The local loop, when used for voice, has a bandwidth
of 4000 Hz (4 kHz). It is interesting to examine the telephone number associated with each local loop. The first
three digits of a local telephone number define the office, and the next four digits define the local loop number.
Trunks
Trunks are transmission media that handle the communication between offices. A trunk normally handles
hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or
satellite links.
Switching Offices
To avoid having a permanent physical link between any two subscribers, the telephone company has switches
located in a switching office. A switch connects several local loops or trunks and allows a connection between
different subscribers.
Circuit Switching & Packet Switching
Circuit switching
A type of communications in which a dedicated channel (or circuit) is established for the duration of a
transmission. The most ubiquitous circuit-switching network is the telephone system, which links together wire
segments to create a single unbroken line for each telephone call.
Packet Switching
The other common communications method is packet switching, which divides messages into packets and sends
each packet individually. The Internet is based on a packet-switching protocol, TCP/IP.
Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packetswitching networks are more efficient if some amount of delay is acceptable.
Circuit-switching networks are sometimes called connection-oriented networks. Note, however, that although
packet switching is essentially connectionless, a packet switching network can be made connection-oriented by
using a higher-level protocol. TCP, for example, makes IP networks connection-oriented.
1.2 Network Topology
The PSTN network topology describes the particular network topology that was necessary to support the Public
switched telephone network (PSTN). In communication networks, a topology is a usually schematic description
of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining
network geometry: the physical topology and the logical (or signal) topology. The PSTN network topology
describes the particular network topology that was necessary to support the Public switched telephone network
(PSTN).
Central Office switch
Central office switch is the generic name for the network element in the PSTN that establishes temporary
connections between two subscribers. It is a voice switch that resides in the central office. Central office can
mean any of the following: Central office (CO) switch: Provides services to the devices on the local loop. These
services include signaling, digit collection, call routing, setup, and teardown.
A telephone exchange is a telecommunications system used in the public switched telephone network or in
large enterprises. An exchange consists of electronic components and in older systems also human operators
that interconnect (switch) telephone subscriber lines or virtual circuits of digital systems to establish telephone
calls between subscribers.
In the public telecommunication networks a telephone exchange is located in a central office (CO), typically a
building used to house the inside plant equipment of potentially several telephone exchanges, each serving a
certain geographical exchange area.
1.3 Subscriber Telephone
A subscriber is anyone who purchases telephone services or access to the public switched telephone network
(PSTN) and pays a monthly fee for this access. A subscriber uses a terminating device of some type to
communicate with the telephone network. This 'terminating device' can be a telephone, a computer modem or
a CSU/DSU.
The subscriber's phone line is referred to as a subscriber line. A subscriber line is not always a voice circuit.
Local Loop/ Subscriber Loop
The local loop is the connection (wireless or wired) between a customers’ telephone or data equipment and a
LEC (Local Exchange Carrier) or other telephone service provider. Traditionally, the local loop (also called
“outside plant” or the “last mile”) has been composed of copper wires that extend from the EO (End Office)
switch. The EO is the last switching office in the telephone network that connects customers to the telephone
network.
In telephony, the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from
the demarcation point of the customer premises to the edge of the Common carrier or telecommunications
service provider's network.
At the edge of the carrier access network in a traditional public telephone network, the local loop terminates in
a circuit switch housed in an incumbent local exchange carrier or telephone exchange.
1.4 Hierarchical Networks
The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls,
connections to other countries, direct dialing and so on. The model developed by the United States and Canada
was adopted by other nations, with adaptations for local markets.
The original concept was that the telephone exchanges are arranged into hierarchies, so that if a call cannot be
handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of
connecting trunks required between operators over long distances and also kept local traffic separate.
However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still
exist, they are much flatter, with perhaps only two layers.
Class 1 (regional center)
The class 1 office was the Regional Center (RC).
Class 2 (sectional center)
The class 2 office is the Sectional Center (SC). The sectional center typically connected major toll centers within
one or two states or provinces, or a significant portion of a large state or province, to provide interstate or
interprovincial connections for long-distance calls. At various times, there were between 50 and 75 active class
two offices in the network.
Class 3 (primary center)
The class 3 office is the Primary Center (PC). Calls being made beyond the limits of a small geographical area
where circuits are not connected directly between class 4 toll offices would be passed from the toll center to the
primary center. These locations use high usage trunks to complete connection between toll centers. The primary
center never served dial tone to the user. The number of primary centers in the network fluctuated from time to
time, ranging between 150 and 230.
Class 4 (toll center)
The class 4 office is the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP). A call going between two end
offices not directly connected, or whose direct trunks are busy, is routed through the toll center. The toll center
is also used to connect to the long-distance network for calls where added costs are incurred, such as operator
handled services. This toll center may also be called the tandem office because calls have to pass through this
location to get to another part of the network. Toll centers might have been operated either as interstate
facilities, under the operation of AT&T Long Lines (GTE in a few cases), or by local telephone companies,
handling long distance traffic to points within a particular operating company territory. Class 4 offices continue
to exist, although with considerable changes, as they handle local exchange company interconnections, locally
charged or long distance rated, or provide facilities for connection to long distance company points of presence.
Class 5 (local exchange)
The class 5 office is the local exchange or end office. It delivers dial tone to the customer. The end office, also
called a branch exchange, is the closest connection to the end customer. Over 19,000 end offices in the United
States alone provide basic dial tone services.
In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class
4. This change was prompted in great part by changes in the power of switches and the relative cost of
transmission, both of which tended to flatten the switch hierarchy. The breakup of the Bell System, and the
need for each of the surviving regional operating companies to handle long distance interconnections, also
promoted the inclusion of inter-regional and international processing through larger Class 4 offices.
Subscriber trunk dialing
The dialing codes used by trunk operators to connect calls were originally assigned and established to ensure
speed with pulse dialing equipment. With the advent of subscriber dialed calls, numbering patterns were
reassigned to provide for mnemonic methods of improving customer performance. Subscriber trunk dialing
(STD) codes all began with 0. The largest cities, which had seven digit local numbers, were allocated special
codes - London, 01; Birmingham, 021; etc. Smaller towns were typically allocated a code based on the first
letters of their name, translated into digits on the telephone dial. For example, Oxford translated into 09 on the
British phone dial, so the original STD code for Oxford was 0096. However, because of subscriber dialing errors,
there was an early decision to eliminate codes that began with 00 and Oxford soon became 0865, the sequence
86 designating the first two letter of university.
Some of the smallest towns connected to the trunk network only through nearby switches. In those cases, STD
codes were composed of combination of the code for the nearby switch, plus some additional digits that were
unused in that nearby switch, but which served two purposes (1) to identify the end location, and allow the
nearby switch to complete the call (2) to "pad out" the overall length of the dialing string, since a small town
might only have a three-digit telephone number, and allow the network to move to a more-standard number
length.