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Old 11-27-2008, 12:55 AM   #1
sulekha
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Question difference between Packets, Frames, Datagrams and Cells


what exactly is the difference between Packets, Frames, Datagrams and Cells ? to what extent the description given here

http://www.tcpipguide.com/free/t_Mes...andCells-2.htm

is correct ?

# Packet: This term is considered by many to most correctly refer to a message sent by protocols operating at the network layer of the OSI Reference Model. So, you will commonly see people refer to “IP packets”. However, this term is commonly also used to refer generically to any type of message, as I mentioned at the start of this topic.

# Datagram: This term is basically synonymous with “packet” and is also used to refer to network layer technologies. It is also often used to refer to a message that is sent at a higher level of the OSI Reference Model (more often than “packet” is).

# Frame: This term is most commonly associated with messages that travel at low levels of the OSI Reference Model. In particular, it is most commonly seen used in reference to data link layer messages. It is occasionally also used to refer to physical layer messages, when message formatting is performed by a layer one technology. A frame gets its name from the fact that it is created by taking higher-level packets or datagrams and “framing” them with additional header information needed at the lower level.

# Cell: Frames and packets, in general, can be of variable length, depending on their contents; in contrast, a cell is most often a message that is fixed in size. For example, the fixed-length, 53-byte messages sent in Asynchronous Transfer Mode (ATM) are called cells. Like frames, cells usually are used by technologies operating at the lower layers of the OSI model.
 
Old 11-27-2008, 11:05 AM   #2
ugge
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I must agree that all the terms and word used in networking is a bit confusing and sometimes intimidating.
As you might already know the network communication is structured in layers, each layer responsible for certain tasks.
Each layer referes to the incoming information units by a different name.

Data is divided into segments, which are then "put inside" datagrams by the next level, these are then "put inside" frames.
Each layer add extra information relevant for the transmission.
 
Old 12-02-2008, 02:10 PM   #3
baldy3105
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The tcp guide is correct. The first four layers deal with data in "chunks" the reason we use different terms is to make it clear what layer we are referring to. If you refer to everything as a packet it causes all sorts of confusion.
 
Old 12-02-2008, 03:55 PM   #4
rg.viza
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Quote:
Originally Posted by sulekha View Post
# Datagram: This term is basically synonymous with “packet” and is also used to refer to network layer technologies. It is also often used to refer to a message that is sent at a higher level of the OSI Reference Model (more often than “packet” is).
Datagram packets are a type of packet which is not usually ACKed. They are spewed out without expecting a response, a la UDP(user _datagram_ protocol). This makes UDP inherently stateless.

Add TCP to IP and you get TCP/IP packets (which are delivery guaranteed). TCP and UDP are layer 4 protocols, IP is layer 3, and is used by both UDP and TCP.

The description of datagrams doesn't mention lack of delivery guarantee or frame receipt order guarantee, which is a distinction between datagram and other types of packets. With TCP/IP if the sending host doesn't recieve an ACK within the timeout window, it resends the packet. UDP applications don't wait for acks. It's fire and forget networking.

Frames happen at Layer 2, and carry TCP/IP packets, UDP datagrams etc from the host to switch, switch to router, etc. They don't care what the upper layers are doing with guarantees as frames are logically separated from the packets they contain. The frame headers of ethernet contain a CRC for error checking. The difference between layer 4 and 2 is that layer 4 guarantees delivery (when applicable) which makes the internet possible, and at layer 2 the integrity of the packet is tested though this can be done in the applications as well.

Frames are an abstraction for what's happening at layer 1, which is clock synchronized transmission of bits. Layer 1 transmits the frame in the form of clock regulated electrical pulses or light pulses.

Hope this helps...

-Viz

Last edited by rg.viza; 12-02-2008 at 06:28 PM.
 
  


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