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Saturday, July 20, 2013

What are data gram sub-nets?

- A data gram is defined as the basic transfer unit used in the networks that operate with the help of packet switching network. 
- In such networks, the time of the arrival and delivery is not guaranteed. 
- Also, the network services do not guarantee that whether it will be an ordered delivery or not. 
- The first project to use the data grams was the CYCLADES which was again a packet switching network. 
- The hosts in this network were responsible for making a reliable delivery rather than relying on the network for doing so. 
- This they did using the data grams that were themselves so unreliable and by associating the mechanisms of the end to end protocols. 
- According to Louis Pouzin, there are two sources from which came the inspiration for the data grams namely the Donal Davie’s studies and simplicity of the things. 
- The concept of the data gram sub-net was eventually adopted for the formulation of the protocols such as apple talk, Xerox network systems and of course the internet protocol.
- Data grams are used at the first 4 layers of the OSI model. 
- Each layer has its own name for the data grams as we mention below:
  1. Layer 1: chip (CDMA)
  2. Layer 2: frames (IEEE 802.3 and IEEE 802.11), cell (ATM)
  3. Layer 3: data packet
  4. Layer 4: data segment
- A data gram is a data packet that is self-reliant. 
- This means it does not rely on any of the exchanges made earlier since the fixed connection between the two points of communication has no connection such as in a majority of the telephonic conversations. 
- Virtual circuits and data gram sub-nets are two equally opposite things. 

Data gram is defined as an independent and self-contained data entity by the RFC 1594 that carries sufficient information required for routing from one source to another without relying on the transporting network and the earlier exchanges between the two same hosts.

- The services offered by the data gram sub nets can be compared to the mail delivery services. 
- This is so because the user needs to mention only the destination address.
- However, this service does not give any guarantee of whether the data gram will be delivered or not and also does not provide any confirmation upon successful delivery of the packet. 
- These are of course two major disadvantages of the data gram sub nets. 

- In data gram sub nets, the data grams or the data packets are routed along a route that is created at the same time. 
- In data gram sub nets the routes are not predetermined. 
- This again has its disadvantages. 
- Also, the order in which the data grams have to be sent or received is not considered. 
- In some cases, a number of data grams having same destination might travel along various different routes.

- There are two components of every data gram namely the header and the data payload.
- The former consists of all the information that is enough for the routing purpose from source to the destination without being dependent on the exchanges that were made before between the network and the equipment. 
The source as well the destination address might be included in the header as a kind of a field. 
- The data that is to be transmitted is stored in the latter part of the data gram. 
- In some cases the data payloads might be nested in to the tagged header. 
This process is commonly known as the encapsulation. 
- There are various types of data grams for which various standards are defined by the internet protocol or IP. 

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