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Messages: Packets, Frames, Datagrams and Cells
(Page 2 of 2)
Common Names For Messages
The most common terms that are used
for messages are the following:
- Packet: This term is considered by many
to most correctly refer to a message sent by protocols operating at
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
- 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.
- Protocol Data Unit (PDU) and Service Data
Unit (SDU): These are the formal terms used in the OSI Reference
to describe protocol messages. A PDU at layer N is a message sent between
protocols at layer N. It consists of layer N header information and
an encapsulated message from layer N+1, which is called both the layer
N SDU and the layer N+1 PDU. After you stop scratching your
head, see the
topic on OSI model data encapsulation
for a discussion of this that may actually make sense. J
I should also point out that there
are certain protocols that use unusual names to refer to their messages,
which arent used elsewhere in the world of networking. One prominent
example is the Transmission
Control Protocol (TCP), which calls its
Key Concept: Communication between devices on packet-switched networks is based on in items most generically called messages. These pieces of information also go by other names such as packets, datagrams, frames and cells, which often correspond to protocols at particular layers of the OSI Reference Model. The formal OSI terms for messages are protocol data unit (PDU) and service data unit (SDU).
Message Terminology in this Guide
As for this Guide and its use of
these terms, I have made a specific effort not to imply anything about
the nature of a message solely based on the name it uses, but I do try
to follow the most common name used for a particular technology. For
example, messages sent over Ethernet are almost always called Ethernet
framesthey are not generally called Ethernet datagrams, for example.
However, I do not structure discussions so that the type of name used
for a message is the only way to determine what sort of message it is.
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Version 3.0 - Version Date: September 20, 2005
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