- 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:
- Layer
1: chip (CDMA)
- Layer
2: frames (IEEE 802.3 and IEEE 802.11), cell (ATM)
- Layer
3: data packet
- 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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