2.1

Network Layer

INTRODUCTION

2.2

SWITCHING

2.3

One solution to the switching is referred to as circuit switching, in which a physical

circuit (or channel) is established between the source and destination of the message

before the delivery of the message. After the circuit is established, the entire message, is

transformed from the source to the destination. The source can then inform the

network that the transmission is complete, which allows the network to open all

switches and use the links and connecting devices for another connection. The circuit

switching was never implemented at the network layer; it is mostly used at the physical

layer

Packet Switching

Circuit Switching

The network layer in the Internet today is a packet-switched network. In this type of

network, a message from the upper layer is divided into manageable packets and each

packet is sent through the network. The source of the message sends the packets one by

one; the destination of the message receives the packets one by one. The destination

waits for all packets belonging to the same message to arrive before delivering the

message to the upper layer. The connecting devices in a packet-switching network still

need to decide how to route the packets to the final destination.

8.4

8-1 CIRCUIT-SWITCHED NETWORKS

A circuit-switched network consists of a set of switches

connected by physical links. A connection between two

stations is a dedicated path made of one or more links.

However, each connection uses only one dedicated

channel on each link. Each link is normally divided

into n channels by using FDM or TDM.

Three Phases

Efficiency

Delay

Circuit-Switched Technology in Telephone Networks

Topics discussed in this section:

8.5

A circuit-switched network is made of a set of switches

connected by physical links, in which each link is

divided into n channels.

Note

8.6

In circuit switching, the resources need

to be reserved during the setup phase;

the resources remain dedicated for the

entire duration of data transfer until the

teardown phase.

Note

8.7

Switching at the physical layer in the

traditional telephone network uses

the circuit-switching approach.

Note

PACKET SWITCHING AT NETWORK LAYER

2.8

The network layer is designed as a packet-switched network. This means that the

packet at the source is divided into manageable packets, normally called datagrams.

Individual datagrams are then transferred from the source to the destination. The

received datagrams are assembled at the destination before recreating the original

message. The packet-switched network layer of the Internet was originally designed

as a connectionless service, but recently there is a tendency to change this to a

connection- oriented service.

• connectionless service

• connection- oriented service

8.9

In a packet-switched network, there

is no resource reservation;

resources are allocated on demand.

Note

8.10

A switch in a datagram network uses a

routing table that is based on the

destination address.

Note

8.11

The destination address in the header of

a packet in a datagram network

remains the same during the entire

journey of the packet.

Note

Connectionless Service

2.12

When the Internet started, the network layer was designed to provide a

connectionless service, in which the network layer protocol treats each packet

independently, with each packet having no relationship to any other packet. The

packets in a message may or may not travel the same path to their destination.

When the Internet started, it was decided to make the network layer a

connectionless service to make it simple. The idea was that the network layer is

only responsible for delivery of packets from the source to the destination.

Delay In Connectionless Network

2.13

8.14

Switching in the Internet is done by

using the datagram approach

to packet switching at

the network layer.

Note

Connection-Oriented Service

2.15

In a connection-oriented service, there is a relation between all packets belonging

to a message. Before all datagrams in a message can be sent, a virtual connection

should be set up to define the path for the datagrams. After connection setup, the

datagrams can follow the same path. In this type of service, not only must the

packet contain the source and destination addresses, it must also contain a flow

label, a virtual circuit identifier that defines the virtual path the packet should

follow.

Delay In Connection-Oriented Network

2.16

2.17