=[ x25-box.com ]=

Fig. 1.5 Multiple conversations in a single network; conversations between A and P, and B and Q, share the M to N link

Layer two sends the data in a defined structure called the frame.

• The frame is the unit of transmission for layer two.
• The frame is contiguous—that is, once the sender starts to send a
frame it will carry on to the end or give up completely and try
again.

There is a further feature imposed by most hardware implementations
that is not actually imposed by the standard:

• The frame is organised in bytes and always contains a whole
number of bytes.

Since layer one requires that a bit is sent on every beat of the clock, layer
two requires something to send between frames. This is the flag byte
which is a set sequence of bits which the receiver must essentially ignore.
Remembering that layer one is full duplex we can look at our example in
more detail. See Fig. 1.6.

Notice that there must always be a flag byte to delimit one frame from

Fig. 1.6 Flag bytes and frame delimiting

the next. It acts as a delimiter as well as an idle condition. Notice also
that the frames are of arbitrary length. The layout of the frame is fixed
and is as shown in Fig. 1.7.

The first byte of the frame is the Address byte, and simply indicates
whether the frame is a command or response. This is explained in more
detail in Chapter 4.

НАЗАД