This lecture include some of the basic information about WAN.

1. COMPUTER NETWORKS
(Index)
Wide Area Network (WAN)
SWITCHING
Switched communication network
Packet Switching
Modes of Packet Switching
Types of Packet Switching
Packet switching Principles
Packet switching Advantages
Packet switching Disadvantages
Circuit Switching
Circuit switching Advantages
Circuit switching Disadvantages

2. Wide Area Network (WAN)
➢ A wide area network (WAN) is a network that exists
over a large-scale geographical area.
➢ A WAN connects different smaller networks, including
local area networks (LANs) and metro area networks
(MANs).
This ensures that computers and users in one
location can communicate with computers and users
in other locations.
➢ WAN implementation can be done either with the
help of the public transmission system or a private
network.
SWITCHING
➢ Switching is a method in which communication
devices are connected to one another efficiently.
➢ A switch is intermediary hardware or software that
links devices together temporarily.
➢ There are three fundamental switching methods:
circuit switching, packet switching, and message
switching.

3. Switched communication network:
➢ A circuit-switched communication system involves
three phases:
o Circuit establishment (setting up dedicated
links between the source and destination).
o Data transfer (transmitting the data
between the source and destination).
o Circuit disconnects (removing the dedicated
links).
➢ Consists of an interconnected collection of nodes
➢ Data are transmitted from source to destination by
being routed through the nodes
➢ The switching method describes how data are
processed and routed in the network.

4. Packet Switching :
➢ Packet switching is a digital network transmission process
in which data is broken into suitably-sized pieces or blocks
for fast and efficient transfer via different network
devices.
➢ When a computer attempts to send a file to another
computer, the file is broken into packets so that it can be
sent across the network in the most efficient way.
➢ These packets are then routed by network devices to the
destination.
➢ A small header containing signaling/addressing
information about the source and destination nodes is
added to each such small data unit, to form packets.
➢ Each packet is then routed from the source to the
destination by intermediate data exchange devices, using
the signaling information present in each packet.
➢ Packet switching is the switching method used in data
networks for computer communication.

5. ➢ In packet switching,
o There is no end to end circuit reservation
o A packet consists of a header and data
o Each packet has signaling information in the form of
source and destination addresses in the packet header.
o Signaling information is used by intermediate data
exchange devices to route packets.
o Exchange devices like routers and switches use a store and
forward approach for transmitting packets from an input
port to an output port
o Link utilization is efficient because there is no end to end
reservation of telecommunication links and multiple
connections can simultaneously share the link.
o Virtual Circuit and Datagram are two types of packet
switching, both of which are used in Computer
Communication at different layers. While VC based packet
switching is used primarily at the data link layer, datagram
based packet switching is used both at the data link and
network layers.
Modes of Packet switching:
There are two major modes of packet switching:
1. Connectionless Packet Switching: Each packet contains
complete addressing or routing information and is routed
individually. This can result in out-of-order delivery and

6. different paths of transmission, depending on the variable
loads on different network nodes (adapters, switches and
routers) at any given time. Also known as datagram
switching.
In connectionless packet switching, each packet has the
following information written in its header section:
-The destination address
-The source address
-Total number of pieces
-The sequence number (Seq#) needed to enable
reassembly after reaching the destination through
different routes, the packets are rearranged to form the
original message.
2.Connection-Oriented Packet Switching: Data packets are
sent sequentially over a predefined route. Packets are
assembled, given a sequence number and then
transported over the network to a destination in order. In
this mode, address information is not required. Also
known as virtual circuit switching.
Types of packet switching:
There are 2 types of packet switching:
➢ VC based packet switching/Connection oriented
packet switching

7. ➢ datagram based packet switching/connectionless
packet switching
VC based packet switching/Connection oriented
packet switching:
➢ A virtual circuit is a circuit or path between points in a
network that appears to be a discrete, physical path
but is actually a managed pool of circuit resources
from which specific circuits are allocated as needed to
meet traffic requirements.
➢ VC based switching is also called as connection-
oriented packet switching because a logical (not
physical) connection is first established between
sender and receiver, before data transfer starts.
➢ In VC based switching:
o Whenever a new session/flow of data transfer needs to
happen between a source and a destination node, a
connection establishment phase is executed in the
switched WAN network, using a signaling protocol.
o The signaling protocol (e.g. Label Distriution Protocol (LDP)
in MPLS) is first used to identify an end to end path
between the source and destination nodes. Once such a
path is identified, all packets of the data session/flow
MUST follow the same path. But this path is not reserved
for this session alone and multiple sessions can share the
links in this path.

8. o After identifying the path, packet switches/routers
allocate a set of VC IDs or labels to uniquely identify this
connection. These labels do not have end-to-end
significance and only have local significance at each
intermediate device.
o Data is split into small units and appended with a packet
header (containing labels) to form packets.
o Packets are switched within the WAN network through
labels. Usually the length of the label is shorter than IP and
layer 2 addresses thereby enabling faster lookup.
o At each intermediate device/packet switch, the device
looks at the incoming label and link. Based on this, it
identifies a suitable outgoing link and an outgoing label, by
consulting a local VC table. The label inside the packet
header is modified accordingly and the packet is switched
out of the outgoing link.
o At the end of the data transfer phase, the labels are
released from the VC table.
o VC based switching is very useful in Switched WAN Core
Backbone Networks for fast switching and better traffic
Engineering
Types of VC:
➢ Permanent Virtual Circuit (PVC)
➢ Switched Virtual Circuit (SVC)

9. Permanent Virtual Circuit (PVC):
➢ A permanent virtual circuit (PVC) is a virtual circuit that is
permanently available to the user just as though it were a
dedicated or leased line continuously reserved for that
user.
➢ A permanent virtual circuit (PVC) is a software-defined
logical connection in a network such as a frame
relay network.
➢ In frame relay, the end points and a stated bandwidth
called a Committed Information Rate (CIR) constitute PVC,
which is defined to the frame relay network devices. The
bandwidth may not exceed the possible physical
bandwidth. Typically, multiple PVCs share the same
physical paths at the same time. To manage the variation
in bandwidth requirements expressed in the CIRs, the
frame relay devices use a technique called statistical
multiplexing.
Switched Virtual Circuit (SVC):
➢ A switched virtual circuit (SVC) is a temporary virtual
circuit that is established and maintained only for the
duration of a data transfer session.
Datagram based packet switching/connectionless packet
switching:

10. ➢ Datagram based packet switching is the fundamental
method/technique used in the Internet for end-to-end
routing of packets at the Network Layer, between remote
communicating computers
➢ In datagram based switching, data is split into smaller units
and a header containing source and destination addresses
is added to each small unit to form packets. The packets of
a flow are given to the network and the network treats
each packet independently and routes the packet using
the packet header alone.
➢ It is also called connectionless packet switching
➢ It has no connection establishment and termination
phases.
➢ Each packet has header information and data
➢ Each packet of a data session/flow is independently
handled by the network, with no correlation between
other packets in the flow
➢ Packets of each session may take different routes
➢ Packets may also arrive out of order at the receiver
➢ Very simple to implement and scales well, as there are no
signaling protocols and VC tables required at intermediate
nodes
➢ IP and Ethernet are example protocols using the principle
of Datagram based Packet Switching

11. Datagram packet switching vs.
VC packet switching
Issue Datagram packet
switching
VC packet switching
Circuit setup No needed Required
Addressing Each packet contain full source
and destination address
Each packet contains a short VC
number
State
information
Subnet does not hold state
information
Each VC requires subnet table space
Routing Each packet is routed
independently
Route chosen when a VC is set up all
packets follow this route
Effect of router
failures
None: expect for packets lost
during the crash
All VCs that passed through the failed
router are terminated
Congestion
control
Difficult Easy if enough buffers can be allocated
in advance for each VC
Packet Switching Principles:
➢ Circuit switching designed for voice.
➢ Key characteristic is that resources within the network
are dedicated to a particular call..
➢ Much of the time a data connection is idle.
➢ Inefficient.
➢ Data rate is fixed.
Both ends must operate at the same rate, this limiting the
utility of the network in interconnecting a variety of computers.
Packet Switching Advantages:
i. Statistical Multiplexing.
ii. Every end point can operate on its own rate.

12. iii. Flexible to support various types of service.
iv. More adoptable to network condition changes.
Packet Switching Disadvantages:
i. Larger overhead.
ii. Data may not arrive in sequence.
iii. Unpredictable quality of service.
Circuit switching:
➢ Circuit-switched is a type of network in which a
physical path is obtained for and dedicated to a single
connection between two end-points in the network
for the duration of the connection.
➢ Ordinary voice phone service is circuit-switched. The
telephone company reserves a specific physical path
to the number you are calling for the duration of your
call. During that time, no one else can use the physical
lines involved.
➢ An End to end circuit (path) is first reserved using a
separate signaling protocol
➢ Data transfer proceeds only after the circuit establishment
phase
➢ All data of that session passes through the same circuit
➢ No other user can use this circuit till this session is
completed
➢ No signaling information is sent along with the data

13. ➢ Circuit is released after data transfer using the signaling
protocol
Circuit Switching Advantages:
i. Little Overhead.
ii. Data arriving in sequence.
iii. Predictable quality of service.
Circuit Switching Disadvantages:
i. Capacity during idle time is wasted.
ii. Source and destination must operate on same rate.
iii. Cannot support dynamic bandwidth request change.
COMPARISON
Circuit switching Datagram packet
switching
VC packet
switching
Dedicated transmission
path
No Dedicated
transmission path
No Dedicated
transmission path
Continuous transmission Transmission of packets Transmission of packets

14. Path stays fixed for entire
connection
Route of each packet is
independent
Path stays fixed for entire
connection
Call setup delay No setup delay Call setup delay
Negligible transmission
delay
Transmission delay for
each packet
Transmission delay for
each packet
No queuing delay Queuing delays at
switches
Queuing delays at
switches
Busy signal overloaded
network
Delays increase in
overloaded network
Delays increase in
overloaded network
Fixed bandwidth for each
circuit
Bandwidth is shared by all
packets
Bandwidth is shared by all
packets
No overhead after call
setup
Overhead in each packet Overhead in each packet