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Fig. 1.7 Format of the layer two frame

The final two bytes are called the Frame Check Sequence (FCS) and
are a value related to the contents of the frame. The field is like a checksum,
but is actually a Cyclic Redundancy Check. Using the FCS the receiver of
the frame can determine whether or not the frame has been corrupted
during transmission.

Since the frame is of variable length with no length indication, the
receiver must capture all of the data up to the terminating flag byte,
before it can determine the length of the frame and where the FCS is
located. It then calculates the FCS itself, and checks whether this agrees
with the FCS in the frame to determine whether corruption has occurred.

The FCS does not catch all errors. Being 16 bits long it has 64000
possible values. Thus if corruption occurs during transmission there is
some chance that it will not be detected if the FCS happens to be correct.

The second byte of the frame is the Control byte and indicates what
type of frame this is. The control byte therefore indicates the format of
the frame, different frames having different formats. One of the frame
types is the Info frame and is used to carry data from one node to another.
The layout of the Info frame is shown in Fig. 1.8.

Fig. 1.8 Conceptual layout of the Info frame

The third and fourth fields of the Info frame are the Send and Receive
sequence numbers which are explained below. They are followed by the
data. Note that there is no length indication. The length is determined by
the position of the terminating flag byte. The sequence numbers are
actually included in the Control byte, but in the early part of this book
they will be shown separately for clarity.

The Send Sequence Number, depicted N(S), indicates which number
this is in a series of Info frames. The first Info frame sent will have an N
(S) of 0, the next will have an N(S) of 1, and so on. This mechanism
allows the receiver to verify that it has received all of the frames and that
none have been lost in transit. The N(S) can rise to a value of seven and
will then cycle back to zero. This limits the length of the N(S) field to
three bits in the frame. There is a problem here in that if the N(S) can
cycle, and if eight frames are lost in transit and the next frame is sent
correctly, then the receiver cannot detect the loss. To prevent this, the

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