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6.1 | |
Ethernet Fundamentals | |
| 6.1.5 | |
Layer 2 framing |
Encoded bit streams (data) on physical
media represent a tremendous technological accomplishment, but they,
alone, are not enough to make communication happen. Framing helps
obtain essential information that could not, otherwise, be obtained
with coded bit streams alone. Examples of such information are:
Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit. A voltage vs. time graph could be used
to visualize bits. However, when dealing with larger units of data,
addressing and control information, a voltage vs. time graph could
become large and confusing. Another type of diagram that could be used
is the frame format diagram, which is based on voltage versus
time graphs. Frame format diagrams are read from left to right,
just like an oscilloscope graph. The frame format diagram shows
different groupings of bits (fields) that perform other functions.
There are many different types of
frames described by various standards. A single generic frame has
sections called fields, and each field is composed of bytes.
When computers are connected to a physical medium, there must be a way they can grab the attention of other computers to broadcast the message, "Here comes a frame!" Various technologies have different ways of doing this process, but all frames, regardless of technology, have a beginning signaling sequence of bytes. All frames contain naming information, such as the name of the source node (MAC address) and the name of the destination node (MAC address). Most frames have some specialized fields. In some technologies, a length field specifies the exact length of a frame in bytes. Some frames have a type field, which specifies the Layer 3 protocol making the sending request. The reason for sending frames is to get upper layer data, ultimately the user application data, from the source to the destination. The data package has two parts, the user application data and the encapsulated bytes to be sent to the destination computer. Padding bytes may be added so frames have a minimum length for timing purposes. Logical link control (LLC) bytes are also included with the data field in the IEEE standard frames. The LLC sub-layer takes the network protocol data, an IP packet, and adds control information to help deliver that IP packet to the destination node. Layer 2 communicates with the upper-level layers through LLC. All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The Frame Check Sequence (FCS) field contains a number that is calculated by the source node based on the data in the frame. This FCS is then added to the end of the frame that is being sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded, and the source is asked to retransmit. There are three primary ways to calculate the Frame Check Sequence number:
The node that transmits data must get the attention of other devices, in order to start a frame, and to end the frame. The length field implies the end, and the frame is considered ended after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter.
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The names
of the fields are as follows: 
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