Sunday, July 21, 2013
- In virtual circuits the packets are allowed to contain in them the circuit number rather than storing the full address of the destination.
- This reduces the requirement for a much larger memory and bandwidth.
- This also makes it cheaper in cost.
- On the other hand, the data-grams have to contain the full destination address rather than a single circuit number.
- This causes a significant overhead in the data-gram sub nets.
- Also, this leads to wastage of the bandwidth.
- All this implies that the data-gram sub nets are more costly when compared to the virtual circuits.
- A set up phase is required for the virtual circuits.
- For establishing this phase a lot of resources are required along with a lot of time.
- Data-gram sub net in contrast does not require establishment of set up phase.
- Hence, there is no requirement of resources.
- In virtual circuits, for indexing purpose the circuit numbers are used by the router.
- These numbers are stored in a table and are used for finding out the destination of the packet.
- This procedure is quite simple when compared with the one used by the data-gram sub nets.
- The procedure used in data-gram sub nets for determining the destination of the packet is quite complex.
- Virtual circuits allow for reserving the resources in advance on the establishment of the resources.
- This has a great advantage which is that the congestion is avoided in the sub net.
- However, in the data-gram sub nets, it is quite difficult to avoid congestion.
- If a crash occurs in a router, then it will lose its memory.
- Even if it backs up after sometime, all the virtual circuits that pass via it must be aborted.
- This is not a major problem in the case of the data-gram sub nets.
- Here, if the router crashes, the only packets that will have to suffer will be the ones that were queued for that router at that instant of time.
- The virtual circuits can vanish as a result of the loss or fault on the current communication line.
- In data-gram sub nets, it is comparatively easy to compensate for the fault or loss on the communication line.
- In virtual circuits there is one more cause for the traffic congestion.
- This cause in the use of the fixed routes for the transmission of the data packets throughout the network.
- This also leads to the problem of unbalanced traffic.
- In data gram sub nets the routers are given the responsibility of balancing the traffic over the entire traffic.
- This has been made possible because it is allowed to change the routers halfway between the connections.
- Virtual circuits are one way of implementing the connection-oriented services.
- For various types of data gram sub nets, a number of protocols are defined by the internet protocol.
- Internet protocol provides the data-gram service at the internet layer.
- In contrast with the virtual circuits, data gram sub nets are connection-less service.
- It is the best effort message delivery service but at the same time is very unreliable.
- There are a number of high level protocols such as TCP that are dependent up on the data gram service of the internet protocol.
- This calls for additional functionality.
- The data gram service of IP is even used by the UDP.
- The fragments of a data gram might be referred to as the data packets.
- The IP and UDP both provide unreliable services and this is why both of them are termed as data grams.
- The fragments of TCP are referred to as TCP fragments to distinguish it from data-grams.