Network Layer 10 - Circuit Switching and Packet Switching
Computer Communication & Networking— CE - 402 Department of Computer Engineering, SSUET CE - 402
Instructor: M. Rehan Rasheed
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Introduction
Packet Switching refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination.
Once all the packets forming a message arrive at the destination, they are recompiled into the original message.
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Chapter # 10
Packet Switching Application
Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video.
Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.
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Packet Switching Operation
Data are transmitted in short packets. Typically an upper bound on packet size is 1000 octets.
If a station has a longer message to send it breaks it up into a series of small packets. Each packet now contains part of the user's data and some control information. The control information should at least contain: Destination Address Source Address Store and forward - Packets are received, stored briefly (buffered) and past on to the next node.
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Chapter # 10
Use of Packets
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Advantages
Line efficiency Single node to node link can be dynamically shared by many packets over time. Packets queued and transmitted as fast as possible.
Data rate conversion Two stations of different data rates can exchange packets because each connects to its node at its proper data rate.
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Chapter # 10
Switching Technique - Virtual Circuits and Datagrams
Packets handled in two ways
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Datagram approach Virtual circuit approach
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Datagram Packet Switching
In datagram approach each packet is treated independently with no reference to packets that have gone before. No connection is set up. 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 More processing time per packet per node Robust in the face of link or node failures.
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Packet Switching Datagram Approach
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Virtual Circuit Packet Switching
In the Virtual Circuit approach a pre-planned route is established before any packets are sent. There is a call set up before the exchange of data (handshake). All packets follow the same route and therefore arrive in sequence. Each packet contains a virtual circuit identifier instead of destination address More set up time No routing decisions required for each packet - Less routing or processing time Susceptible to data loss in the face of link or node failure Clear request to drop circuit Not a dedicated path
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Chapter # 10
Packet Switching Virtual Circuit Approach
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Virtual Circuits vs. Datagram
Virtual circuits
Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node looses all circuits through that node
Datagram
No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network
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Chapter # 10
One Station Can Have Many Virtual Circuit Connections
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Packet Size
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Chapter # 10
Packet Size
In this example it is assumed that there is a virtual circuit from station X through nodes a and b to station Y.
The message to be sent comprises 40 octets and 3 octets of control information called header.
If the entire message is sent the packet first transmitted from station X to node a, when the entire packet is received, it can be transmitted from a to b and then transmitted to Y. ignoring switching time, total transmission time is 129 octettime(43octets x 3 packet transmission).
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Instructor: M. Rehan Rasheed
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Circuit vs. Packet Switching
Performance Propagation delay The time it takes a signal to propagate from one node to the next. This time generally negligible. Typically on a wire medium 2x108 Transmission time The time it takes for a transmitter to send out a block of data, e.g it takes 1s to transmit 10,000 bit block of data onto a 10kbps line. Node delay The time it takes for a node to perform the necessary processing as it switches data.
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Chapter # 10
Comparison with Circuit Switching - Event Timing
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Circuit Switching
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Packet Switching Datagram Packet switching
Virtual-circuit Packet Switching
Dedicated transmission path
No dedicated path
No dedicated path
Continuous transmission of data
Transmission of packets
Transmission of packets
Fast enough for interactive
Fast enough for interactive
Fast enough for interactive
Messages are not stored
Packets may be stored until delivered
Packets stored until delivered
The path is established for entire conversation
Route established for each packet
Route established for entire conversation
Call setup delay; negligible transmission delay
Packet transmission delay
Call setup delay; packet transmission delay
Busy signal if called party busy
Sender may be notified if packet not delivered
Sender notified of connection denial
Overload may block call setup; no delay for established calls
Overload increases packet delay
Overload may block call setup; increase packet delay
Susceptible to data loss in the face of link or node failure. â« Clear request to drop circuit. â« Not a dedicated path. CE - 402. Instructor: M. Rehan Rasheed ...
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