Computer Communication & Networks
Lecture # 03
Circuit Switching, Packet Switching
Nadeem Majeed Choudhary
nadeem.majeed@uettaxila.edu.pk
Communication Network
Communication networks
Switched networks
Broadcast networks
End nodes send to one (or more) end nodes
End nodes share a common channel
(TV, radio…)
Circuit switching
Packet switching
Dedicated circuit per call
(telephone, ISDN)
(physical)
Data sent in discrete portions
(the Internet)
Switching Networks
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Long distance transmission is typically done
over a network of switched nodes
A collection of nodes and connections is a
communications network
Nodes not concerned with content of data
End devices are stations

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Computer, terminal, phone, etc.
Data routed by being switched from node to
node
Nodes
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Nodes may connect to other nodes only, or to
stations and other nodes
Node to node links usually multiplexed
Network is usually partially connected
Some redundant connections are desirable for
reliability
Two different switching technologies
 Circuit switching
 Packet switching

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Simple Switched Network
Switching Activities
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Some nodes connect only to other nodes
(intermediary nodes). Sole purpose is to
switch data
Some nodes have one or more stations
attached. They accept from and deliver data
to the attached station.
Node-to-node links are usually multiplexed
Multiple paths enhance reliability
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.
Circuit switching (cnt’d)
Three phases involved in the communication process:
1. Establish the circuit
2. Transmit data
3. Terminate the circuit
If circuit not available: busy signal (congestion)
Note
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
circuit is terminated.
8.9
Circuit switching

A dedicated communication path (sequence of linkscircuit) is established between the two end nodes
through the nodes of the network

Bandwidth: A circuit occupies a fixed capacity of
each link for the entire lifetime of the connection.
Capacity unused by the circuit cannot be used by
other circuits.

Latency: Data is not delayed at switches
Circuit Switching- Applications
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Developed for voice traffic (phone)
Inefficient

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Channel capacity dedicated for duration of
connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Telecom Components
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Subscriber
 Devices attached to network
Subscriber line
 Link between subscriber and network
 Also called Local Loop or Subscriber Loop
 Range from Few km up to tens of km
Exchange
 Switching center in the network
 End office specific switching center that supports subscribers
Trunks
 Branches between exchanges
 Multiplexed
Circuit Establishment
Time diagram of circuit switching
switch
host 1
node 1
node 2
host 2
Delay
host 1- node 1
Processing
delay node 1
circuit
establishment
Delay
host 2- host 1
data
transmission
time
DATA
Circuit Switching Concepts
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Digital Switch
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Provide transparent signal path between devices
Typically allows full duplex transmission
Network Interface
Control Unit
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Establish connections - Generally on demand, Handle and
acknowledge requests, Determine if destination is
free,construct path
Maintain connection
Disconnect
Blocking or Non-blocking Circuit
Switching

Blocking
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A network may not be able to connect stations
because all paths are in use (more stations than
path)
Used on voice systems
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Short duration calls
Non-blocking
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Permits all stations to connect (in pairs) at once
(at least as many paths as stations)
Used for some data connections
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
Example
Assume that a voice channel occupies a bandwidth of 4
kHz. We need to combine three voice channels into a link
with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the
configuration, using the frequency domain. Assume there
are no guard bands.
Solution
We shift (modulate) each of the three voice channels to a
different bandwidth. We use the 20- to 24-kHz bandwidth
for the first channel, the 24- to 28-kHz bandwidth for the
second channel, and the 28- to 32-kHz bandwidth for the
third one. Then we combine them.
Example (contd.)
Example
Five channels, each with a 100-kHz bandwidth, are to be
multiplexed together. What is the minimum bandwidth of
the link if there is a need for a guard band of 10 kHz
between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This
means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz
Applications
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AM Radio
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FM Radio
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Band 530-1700KHz
Each AM Station needs 10KHz
Band 88-108MHz
Each FM Station needs 200KHz
TV
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Each Channel needs 6MHz
Switching Technique
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Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways
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Datagram
Virtual circuit
Packet Switching
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Each end-end data stream
divided into packets
user A, B packets share
network resources
each packet uses full link
bandwidth
resources used as needed
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
 aggregate resource
demand can exceed
amount available
 congestion: packets
queue, wait for link
use
 store and forward:
packets move one hop
at a time

Node receives complete
packet before forwarding
Packet switching
- Why not message switching?host 1
node 1
node 2
host 2
propagation delay
host 1 – node1
message
message
processing &
set-up delay
of a message at
node 1
message
time
Store-and-Forward
Use of Packets
Datagram
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Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and
recover from missing packets
Datagram
Diagram
Virtual Circuit
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Preplanned route established before any
packets sent
Call request and call accept packets establish
connection (handshake)
Each packet contains a virtual circuit identifier
instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path
Virtual
Circuit
Diagram
Source-to-destination data transfer in a virtual-circuit network
Virtual Circuits vs Datagram
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Virtual circuits
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Network can provide sequencing and error control
Packets are forwarded more quickly
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Less reliable
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No routing decisions to make
Loss of a node loses all circuits through that node
Datagram
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No call setup phase
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Better if few packets
More flexible
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Routing can be used to avoid congested parts of the
network
Circuit vs. Packet Switching
Circuit Switched
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Bandwidth
guaranteed
Circuit capacity not
reduced by other
network traffic
Circuit costs
independent of
amount of data
transmitted, resulting
in wasted bandwidth
Packet Switched
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Bandwidth
dynamically allocated
on as-needed basis
May have concurrent
transmissions over
physical channel
May have delays and
congestion
More cost-effective,
offer better
performance
How do loss and delay occur?
packets queue in router buffers
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packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
1. Store and forward delay

store-and-forward packet switches introduced store and
forward delay
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delay is proportional to the packet's length in bits.
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If a packet consists of L bits, and the packet is to be
forwarded onto an outbound link of R bps, then the store-andforward delay at the switch is L/R seconds.
2. Queuing Delay
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Within each router there are multiple buffers (also called
queues), with each link having an input buffer (to store packets
that have just arrived to that link) and an output buffer.
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If packet has to wait in output buffer packets suffer output buffer
queuing delays
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These delays are variable and depend on the level of congestion
in the network.
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Since the amount of buffer space is finite, an arriving packet may
find that the buffer is completely filled with other packets waiting
for transmission packet loss will occur
Assignment # 01
Q1) Solve the following exercise problems. (Chapter #
2)
 15, 17, 20, 24
Q2) Solve the following exercise problems. (Chapter #
8)
 13, 17
Readings
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Chapter 8 (B. A Forouzan)

Section 8.1, 8.2, 8.3
37
References
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Chapter 8 (Data & computer Communication by Behroz A.
Forozun)
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Chapter 10 ( Computer Communication by William
Stallings)
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Chapter 1 (Computer Networking by James K. Kurose)
Q&A