Packet switching network

•Data is divided into packets.•Transfer of information as payload in data packets•Packets undergo random delays & possible loss

Packet Switching Principles :Basic Operation

• Data transmitted in small packets A typical upper bound on packet length is 1000

bytes.—Longer messages split into series of packets—Each packet contains a portion of user data plus

some control info

• Control information—Routing (addressing) information

• Packets are received, stored briefly (buffered) and past on to the next node—Store and forward

Advantages• Line efficiency

—Single node to node link can be shared by many packets over time

—Packets queued and transmitted as fast as possible

• Data rate conversion—Each station connects to the local node at its own speed—Nodes buffer data if required to equalize rates

• Packets are accepted even when network is busy—Delivery may slow down

• Priorities can be used

End system

βPhysicallayer

Data linklayer

Physicallayer

Data linklayerEnd

systemα

Networklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Transportlayer

Transportlayer

MessagesMessages

Segments

Networkservice

Networkservice

Network Service

• Network layer (provider of services) offers services to transport layer (user of services).

• It can be Connection-oriented service or connectionless service

Network Service vs. OperationNetwork Service• Connectionless: only two interactions are

required, A request for sending packets & indication of arrived packet. Any time user can request transmission of packet with out prior information to network layer.

• Connection-Oriented: Transport layer can’t initiate if end system is not connected. Network layer must be informed about the various parameters related with the usage & quality of service. The network layer maintains state information about the flows it is handling. This service entails greater complexity.

1

13 3 21 22

3 2 11 2

21

21

Medium

A B

3 2 11 221

C

21

21

2 14 1 2 3 4

End systemα

End systemβ

Network1

2

Physical layer entity

Data link layer entity 3 Network layer entity

3 Network layer entity

Transport layer entity4

Complexity at the Edge or in the Core?

Network layer can provide a choice of services to the user of network :

Best effort connectionless service

Low delay connectionless service

Reliable Connection oriented transfer of packets

Connection oriented transfer of packets with delay & bandwidth guarantee

Network Internal operation• Network operation can be connectionless or

connection oriented depending on the switching technique used.

• Various combinations of service & operation are possible

Connection-oriented service over Connectionless operation

Connectionless service over Connection-Oriented operation

The End-to-End Argument for System Design

An end-to-end function is best implemented at a higher level than at a lower level—End-to-end service requires all

intermediate components to work properly—Higher-level better positioned to ensure

correct operation

“MEANS CONNECTION ORIENTED SERVICE IS BETTER TO ENSURE THE RELIABLE COMMUNICATION”

Packet Network Topology

End-to-End Packet Network

• Packet networks very different than telephone networks

• User demand can undergo dramatic change• Internet structure highly decentralized

—Paths traversed by packets can go through many networks controlled by different organizations

—No single entity responsible for end-to-end service

—Oversubscription handling is the main issue.

Interdomain level

Intradomain level

Autonomoussystem

Border routers

Border routers

Internet service provider

s

ss

LAN

Example of packet network topology : Connecting to Internet Service Provider

CampusNetwork

network administeredby single organization

Domain & Autonomous system

• The term domain indicates that the router run the same routing protocol

• The term autonomous system used for one or more domains under single administration.

Switching Technique• Station breaks long message into packets• Packets sent one at a time to the network• Packets handled in two ways

—Datagram—Virtual circuit

DATAGRAM NETWORKS DATAGRAM NETWORKS

If the message is going to pass through a packet-If the message is going to pass through a packet-switched network, it needs to be divided into packets of switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is fixed or variable size. The size of the packet is determined by the network and the governing protocol. determined by the network and the governing protocol. In datagram networks packet is free to move in the In datagram networks packet is free to move in the network.network.

Datagram• 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

In a packet-switched network, there is no resource reservation;

resources are allocated on demand.

Note

A datagram network with four switches (routers)

DatagramDiagram

Routing table in a datagram network

A switch in a datagram network uses a routing table that is based on the

destination address.

Note

The destination address in the header of a packet in a datagram network

remains the same during the entire journey of the packet.

Note

Delay in a datagram network

Switching in the Internet is done by using the datagram approach

to packet switching at the network layer.

Note