o All the above information is included in
a special field called Header
o Header begins with start of the header
(SOH) character
o The header comes after the SYNs and
before the STX character
o Everything received
after the SOH
field but before
STX character is
the Header
information
Multiblock
Frame
 |
The
probability of an error in the block of text increases with the length of the
frame
o The more bits in a frame, the more are
the chances of an error
o For this reason, text in a message is
often divided b/w several blocks
o Each block starts with STX and ends with
ITB, intermediate text block except the last
one
o Last block ends with ETX character
o After
the ETX has
been reached, and
the last BCC
checked, the receiver
sends a
single
ACK for the entire frame
o The figure includes only 2 blocks but
actual frames can have more than two
o Some messages may be too long to fit in a
frame
o Several frames can carry continuation of
a single message
o To
let the Rx
know that the
end of frame
is not the
end of transmission, the
ETX
character in
all the frames
but the last
one is replaced
by an End
of Transmission
block
(ETB)
o The receiver must acknowledge each frame
separately
|
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