1. Data Communication and Computer Networks

2. Virtual Circuits
In this model in figure , the network layer provides the transport layer with a perfect channel and all packets delivered in order. A virtual path or circuit is set up so that packets can pass through over this connection. This connection can be a permanent virtual circuit or switched virtual circuit analog to leased or switched line.
The characteristics are:
Easier for the user host to use as the data is already in correct sequence.
Circuit setup and disconnection is required each time.
Sophisticated user may want to do their own error and flow control schemes.

3. Virtual Circuits

4. Datagram
Each message in the network is not related to any other messages.
There is no connection between the sender and receiver.
Transport layer of the receiver must handle error and flow control on its own.
No dedicated path between the sender and receiver,
The sub network accepts packets (often called datagrams) which contain sufficient addressing information so that the packets can be individually routed within he network.
Basically, the user supplies the packets and the subnet transports them to the destination.

5. Datagram
The packets are routed individually and there are usually no delivery assurance between the sender and receiver.
The characteristics of datagram service are given below:
Datagrams are individually routed within the subnet
No delivery assurance relating to the packets as the packets can be lost, out-of-sequence, contaminated, duplicated etc.
Transaction (Sending a short message), connectionless oriented
(No need to establish call prior to sending data.)
X.25 uses virtual circuit approach while TCP/IP Internet Protocol uses datagram.

6. Datagram
X.25 uses virtual circuit approach while TCP/IP Internet Protocol uses datagram.

7. Datagram
X.25 uses virtual circuit approach while TCP/IP Internet Protocol uses datagram.

8. X.25
X.25 is a standard suite of protocols used for packet switching across computer networks.
The X.25 protocols works at the physical, data link, and network layers (Layers 1 to 3) of the OSI model.
Each X.25 packets contains up to 128 bytes of data.
The X.25 network handles packet assembly at the source device, delivery, and then dis-assembly at the destination.
X.25 packet delivery technology includes not only switching and network-layer routing, but also error checking and re-transmission logic should delivery failures occur.

9. X.25
X.25 supports multiple simultaneous conversations by multiplexing packets and using virtual communication channels.
X.25 was originally designed more than 25 years ago to carry voice over analog telephone lines (dialup networks).
Typical applications of X.25 today include automatic teller machine networks and credit card verification networks.

10. Virtual Circuit Datagram
Host to host address is needed in link setup only
Host to host address is always needed in sending the datagram (Embedded in the datagram itself)
Errors is handled by subnetwork. Host will receive the packets in correct sequence.
Error checking is required by host to resemble the packet and find out the missing packets.
Messages passed in order to the network.
messages may be out of order in the communication sub-network
Connection setup is initially required prior to sending data
Connection setup is not required
Network component failure in path may affect the result
Is a flexible foundation to support a range of higher level protocols which can provide for additional network services
Less overhead in addressing embedded in the packet
Overhead in addressing
Example is X.25 Level 3
Example is Internet Protocol of (TCP/IP)