1.

0 Introduction
A phone call is selectively routed to a public switched telephone network or to a packet switched
network based on a called number to which the phone call is directed. A phone call may be made
as a Voice-Over-Internet-Protocol call through, for example, the Internet except when a user dials
an emergency number, in which case the call is made to the public switched telephone network.

1.1 Public Switched Telephone Network (PSTN)

PSTN is based on the principles of circuit switching Therefore when a call is made a particular
dedicated circuit activates which eventually deactivates when the call ends [ CITATION Wil07 \l
1033 ]. The public switched telephone network refers to the international telephone system that
uses copper wires to carry analog voice data. It consists of a collection of individual telephones
that are hardwired to a public exchange.

Figure 1.1: PSTN Network

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1.2 The Concept of Public Switched Telephone Network
Telecommunication networks rely heavily on the Public Switch Telephone Network (PSTN), which
was originally created many decades ago for the transmission of human speech.
PSTN uses circuit switching to allow users to make landline telephone calls to one another. The
call is placed after it is routed through multiple switches. These switches are found in a central
office (CO) or in a wire center. Ultimately, voice signals are able to travel over the connected
phone lines.[ CITATION Mar171 \l 1033 ]
When two telephones are connected, analog voice data is transmitted over the copper wires of the
PSTN. The voice data is then converted into electrical signals which are eventually routed in the
switching centers. Finally, a connection is made and communication is possible.

Figure 1.2 Public Switched Telephone Network

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1.3 Routing
Routing is the process of selecting a path for traffic in a network or between or across multiple
networks. Broadly, routing is performed in many types of networks, including circuit-switched
networks, such as the public switched telephone network (PSTN), and computer networks, such
as the Internet.
In packet switching networks, routing is the higher-level decision making that directs network
packets from their source toward their destination through intermediate network nodes by specific
packet forwarding mechanisms. Packet forwarding is the transit of network packets from
one network interface to another. Intermediate nodes are typically network hardware devices such
as routers, gateways, firewalls, or switches [ CITATION Val06 \l 1033 ].
The routing process usually directs forwarding on the basis of routing tables, which maintain a
record of the routes to various network destinations. Routing tables may be specified by an
administrator, learned by observing network traffic or built with the assistance of routing
protocols

1.4 Types of Routing

Routing can be classified into three categories:
1.4.1 Static Routing
Static Routing is also known as Non adaptive Routing. It is a technique in which the administrator
manually adds the routes in a routing table. A Router can send the packets for the destination
along the route defined by the administrator. In this technique, routing decisions are not made
based on the condition or topology of the networks

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 Figure 1.3 Static Routing
Advantages of Static Routing
i. No Overhead: It has ho overhead on the CPU usage of the router. Therefore, the cheaper
router can be used to obtain static routing.
ii. Bandwidth: It has not bandwidth usage between the routers.
iii. Security: It provides security as the system administrator is allowed only to have control
over the routing to a particular network.
Disadvantages of Static Routing
i. For a large network, it becomes a very difficult task to add each route manually to the
routing table.
ii. The system administrator should have a good knowledge of a topology as he has to add
each route manually.
1.4.2 Default Routing
Default Routing is a technique in which a router is configured to send all the packets to the same
hop device, and it doesn't matter whether it belongs to a particular network or not. A Packet is
transmitted to the device for which it is configured in default routing. Default Routing is used
when networks deal with the single exit point. It is also useful when the bulk of transmission
networks have to transmit the data to the same device.
1.4.3 Dynamic Routing
It is also known as Adaptive Routing. It is a technique in which a router adds a new route in the
routing table for each packet in response to the changes in the condition or topology of the
network. Dynamic protocols are used to discover the new routes to reach the destination. If any
route goes down, then the automatic adjustment will be made to reach the destination.
1.4.4 Features of Dynamic Protocol
All the routers must have the same dynamic routing protocol in order to exchange the routes.
If the router discovers any change in the condition or topology, then router broadcast this
information to all other routers.

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 Fig 1.4 Dynamic Routing

Advantages of Dynamic Routing:

i. It is easier to configure.
ii. It is more effective in selecting the best route in response to the changes in the condition
or topology.
Disadvantages of Dynamic Routing:
i. It is more expensive in terms of CPU and bandwidth usage.
ii. It is less secure as compared to default and static routing

1.5 Routing Mechanism in PSTN

Routing in PSTN is the process used to route telephone calls across the public switched

telephone network.
Telephone calls are routed across a network of potentially many switching systems, often owned
by different telephone carriers. Switching systems are connected with trunks. Each switch may
have many neighbors. Neighboring switches owned by different operators are connected at
interconnect points. 
The PSTN is not a full mesh network with the nodes of every operator directly connected to those
of every other, which would be impractical and inefficient. Therefore, calls may be routed
through intermediate operator networks before they reach their final destination.
A routing mechanism determines the path a message takes through the network to get from the
source to the destination node. It takes as input a message's source and destination nodes. It may
also use information about the state of the network. It returns one or more paths through the
network from the source to the destination node.

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Routing mechanisms can be classified as minimal or non-minimal.
i. A minimal routing mechanism always selects one of the shortest paths between the source
and the destination. In a minimal routing scheme, each link brings a message closer to its
destination, but the scheme can lead to congestion in parts of the network.
ii. A non-minimal routing scheme, in contrast, may route the message along a longer path to
avoid network congestion. Routing mechanisms can also be classified on the basis of how
they use information regarding the state of the network. A deterministic routing scheme
determines a unique path for a message, based on its source and destination. It does not use
any information regarding the state of the network. Deterministic schemes may result in
uneven use of the communication resources in a network. In contrast, an adaptive
routing scheme uses information regarding the current state of the network to determine
the path of the message. Adaptive routing detects congestion in the network and routes
messages around it.

1.6 Call Routing

Each time a call is placed for routing, the destination number 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.
When a call is received by an exchange, there are two treatments that may be applied:
a. Both the destination terminal is directly connected to that exchange, in which the call is
placed down that connection and the destination terminal rings.
b. The call must be placed to one of the neighboring 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.

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 Figure 1.5: Call Routing
1.7 The Art of Switching in PSTN
When a call is made, switches create a wire circuit between two telephones, with this particular
connection lasting as long as the duration of the call and we have four types of switching which
take place at different levels as following

Figure 1.6: Public Switch Telephone Network

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 1.7.1 A Local Exchange
Which may consist of one or more exchanges – hooks up subscribers to a PSTN line also known
as a central office or a switching exchange.
All telephones are connected to the local exchange in a specific area. Interestingly, if you were to
dial the number of your supplier located in the building next to yours, the call won’t leave your
local exchange and will be routed to the supplier as soon as it reaches the exchange.
The exchange then identifies the number dialed so it can route the call towards the correct end
destination. This process works as follows:
The first three digits of a phone number represent the exchange (the local switch), while the last
four digits identify the individual subscriber within that exchange. This means that when you dial
a number and it reaches your local exchange, your call is immediately linked to the subscriber
without the need for any further routing.

Figure 1.7: Local Exchange in a PSTN Network

1.7.2 The Tandem Office- A tandem office serves a large geographical area comprising several
local exchanges while managing switches between local exchanges. It is also known as a junction.
When you dialed the number of a client who lives in the same city but in another suburb. In this
case your call will be routed to a tandem office from your local exchange, and the tandem office
will route the signal on to the local exchange near your client’s location.
1.7.3 The Toll Office - A toll office is connected to all the tandem offices. This is where any
national long-distance switching takes place. For instance, if you have an office in another city
you’ll find that, whenever you dial that branch’s number, your call will be switched through a toll
office.

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1.74 The International Gateway- International gateways manage international call switching,
routing domestic calls to the appropriate countries.

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References
Mark. (2017, July 19th). different-types-telephone-lines/. Retrieved from Local Telecom Service (LTS) Ltd:
https://www.lts.co.uk

Stallings, W. (2007). Data and Computer Communication. Boston: Pearson.

Tekanyi, A. (2018). wireless Communication. Nigeria: Thunderbold Printers and Publishers.

Valdar, A. (2006). Understanding Telecommunications Network. London, United Kingdom: The Institution
of Engineering and Technology.

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