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Friday, September 20, 2013

Differentiate between transparent and nontransparent fragmentation?

A number of problems are encountered because of the size of the data packets. There is no ability in the data link layer by means of which it could handle these problems and so the bridges also don’t work here. 
The Ethernet also experiences a number of problems because of the following:
Ø  Different way in which the maximum packet size is defined.
Ø  Maximum packet size that can be handled by a router.
Ø  The maximum length slot that are used for transmission
Ø  Errors due to the packet length
Ø  Standards

The data packets can be fragmented in two ways namely:
  1. Transparent and
  2. Non – transparent
Both these ways can be followed based on a network by network basis. We can also say that no such end – to – end agreement exists based up on which it can be decided which process is to be used.

Transparent Fragmentation: 
- This type of fragmentation is followed when a packet is split in to smaller fragments by a router.
- These fragments are sent to the next router which does just the opposite i.e., it reassembles the fragments and combine them to form original packet. 
- Here, the next network does not come to know whether any fragmentation has taken place. 
- Transparency is maintained between the small packet networks when compared to the other subsequent networks.
- For example, transparent fragmentation is used by the ATM networks by means of some special hardware. 
- There are some issues with this type of fragmentation. 
- It puts some burden on the performance of the network since all the fragments have to be transmitted through the same gateway. 
- Also, sometimes the repeated fragmentation and reassembling has to be done for small packet network in series. 
- Whenever an over-sized packet reaches a router, it is broken up in to small fragments. 
- These fragments are transported to the next exit router. 
- The fragments are assembled by this exit router which then forwards them to the next router.
- Awareness regarding this fragmentation is not maintained for the subsequent networks. 
- For a single packet fragmentation is done many times before the destination is finally reached. 
- This of course consumes a lot of time because the repeated fragmentation and assembling has to be carried out. 
- Sometimes, it also presents the reason of corrupting the packet’s integrity.

Non-Transparent Fragmentation: 
- In this type, the packet is split in to fragments by one router. 
- But the difference is that these fragments are not reassembled until the fragments reach their destination. 
- They remain split till then. 
- Since in this type of fragmentation the fragments are assembled only at the destination host, the fragments can be routed independent of each other. 
- This type of fragmentation also experiences some problems such as header has to be carried by each of the fragments till they reach their destination. 
Numbering has to be done for all the fragments so that no problem is experienced in reconstructing the data stream.

Whichever type of fragmentation we use, one thing has to be made sure which is that later we should be able to form the original packets using the fragments. This insists on having some type of labeling for the fragments. 

Segmentation is another name for the fragmentation. A packet is injected in to the data link layer by the IP layer but it is not responsible for reliable transmission of the packets. Some maximum value on the size of the packets is imposed by each layer for their reasons. For a large packet that travels through the network for which the MTU is small, fragmentation is very much needed. 

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