1. Packet Switching Datagram Approach Virtual Circuit Approach
Packet Switching Principles
Switching Techniques
Datagram Approach
Virtual Circuit Approach

2. Introduction
Switching is process to forward packets coming in from one port to a port leading towards the destination.
When data comes on a port it is called ingress, and when data leaves a port or goes out it is called egress.
A communication system may include number of switches and nodes.
At broad level, switching can be divided into two major categories:
Connectionless: Data is forwarded on behalf of forwarding tables. No previous handshaking is required and acknowledgements are optional.
Connection Oriented: Before switching data to be forwarded to destination, there is a need to pre-establish circuit along the path between both endpoints. Data is then forwarded on that circuit. After the transfer is completed, circuits can be kept for future use or can be turned down immediately.

3. Types of Network Structures
Message Switching
Circuit Switching
Packet Switching
Datagram Packet Switching
Virtual Circuits Packet Switching

4. Message Switching
In message switching, the whole message is treated as a data unit and is switching / transferred in its entirety.
A switch working on message switching, first receives the whole message and buffers it until there are resources available to transfer it to the next hop. If the next hop is not having enough resource to accommodate large size message, the message is stored and switch waits. So this method is also known as store and forward technology.

5. Contd..

6. Advantage and disadvantage
Message switching has some drawbacks:
Every switch in transit path needs enough storage to accommodate entire message.
Because of store-and-forward technique and waits included until resources available, message switching is very slow.
Message switching was not a solution for streaming media and real-time applications.
Advantage of Message switching
Line efficiency is greater
Data rate conversion is possible
Even under heavy traffic, packets are accepted, possibly with greater delay in delivery
Message priorities can be used, to satisfy the requirement, if any.

7. Circuit Switching
When two nodes communicate with each other over a dedicated (physical or electrical) communication path, it is called circuit switching.
There’s a need of pre-specified route from which data will travel and no other data will permitted.
Circuits can be permanent or temporary.
Applications which use circuit switching may have to go through three phases:
Establish a circuit: Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established
Transfer of data: Data can be transmitted from source to destination over the pre-established dedicated path. The transmission may be analog or digital, depending on the nature of the net-work
Disconnect the circuit: After completion of data transfer (session), the connection is terminated, usually by the action of one of the two stations.

8. Contd..

9. Contd..
Circuit switching was designed for voice applications. Telephone is the best suitable example of circuit switching. Before a user can make a call, a virtual path between caller and callee is established over the network.
Advantage
Provide continuous transfer without the overhead associated with packets making maximal use of available bandwidth for that communication.
Disadvantage
Channel capacity dedicated for duration of connection
If no data, capacity wasted
Set up (connection) takes time

10. Packet Switching
The entire message is broken down into smaller chunks called packets.
The switching information is added in the header of each packet and transmitted independently.
Packets are received briefly (buffered) and passed on to the next node. Since the size of packets are smaller, it is easier for intermediate networking devices to store and they do not take much resources either on carrier path or in the switches’ internal memory.
The packets may use different routes to reach destination depending on the type of packet switching used. At the destination packets are reassembled into the original form.
The internet uses packet switching technique.

11. Contd..

12. Contd.. Advantage Disadvantage Line efficiency is greater
Packet switching enables the user to differentiate data streams based on priorities. Packets are stored and forward according to their priority to provide Quality of Service.
Packet switching can perform data rate conversion
Two stations of different data rates can exchange packets because each connects to the node at its proper data rate.
Disadvantage
Useless for real time transmission
Inefficient for high congestion.

13. Types of packet switching (Switching Techniques)
Datagram packet switching
Each packet treated independently
Call set up phase is avoided
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

15. Contd.. Advantage Disadvantage
Call setup phase is avoided (for transmission of a few packets, datagram will be faster)
Because it is more primitive, it is more flexible
Congestion/failed link can be avoided (more reliable)
Disadvantage
Packets may be delivered out of order
If a node crashes momentarily, all of its queued packets are lost.

16. Virtual circuit packet switching
Pre-planned route (logical link) 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.

18. Virtual circuit transmission is implemented in two formats
Switched virtual circuit (SVC)
Permanent virtual circuit (PVC)

19. SVC
In this method, a virtual circuit is created whenever it is needed and exists only for the duration of the specific exchange.
For example,
Imagine that station A wants to send four packets to station X.
First, A requests the establishment of a connection to X.
Once the connection is in place, the packets are sent one after another and in sequential order.
When the last packet has been received and, if necessary, acknowledged, the connection is released and that virtual circuit ceases to exist.
Only one single route exists for the duration of transmission, although the network could pick an alternate route in response to failure or congestion.
Each time that A wishes to communicate with X, a new route is established. The route may be same each time, or it may differ in response to varying network conditions.

20. PVC
In this method, the same virtual circuit is provided between two users on a continuous basis.
The circuit is dedicated to the specific users.
No one else can use it and, because it is always in place, it can be used without connection establishment and connection termination.
Whereas two SVC users may get a different route every time they request a connection, two PVC users always get the same route.

21. Virtual Circuits vs Datagram
Network can provide sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node loses 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

22. Comparison of Circuit switching, Datagram packet virtual circuit packet