Packet Switching Explain With types

Explain Packet Switching With Types And Advantages And Disadvantages. What Is Packet Switching Explain With types

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Packet switching
There are no dedicated circuits. 
Messages are broken into small pieces called packets. 
Each packet has a header containing source and destination addresses and well as ordering information
Individual packets take different routes to reach the destination.
When the packets arrive at the destination host, they must be reassembled to form the original message.
In this, the packet length is restricted to a maximum length. 

Types of packet switching
1.Datagram packet switching:
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

2.Virtual circuit packet switching
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.

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

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.  

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.


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 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