ADDRESSING
o For point-to-point configuration, there
is no need for addressing
o Any TX put onto the link by one device
can be intended only for the other
o For
the primary device
in a a
multipoint topology to
be able to
identify and
communicate with
a specific secondary
device, there must
be some addressing
convention
o For
this reason, every
device on the
link has an address that
can be used
for
identification
o In
any transmission, this
address will appear
in a specified
portion of each
frame,
called
the Address Field or Header depending upon the protocol
o If the TX comes from a secondary device,
the address indicates the originator of the
data
SELECT
o The select mode is used whenever the
primary device has something to send
o Primary control the link and if primary
is not sending or receiving data, it knows that
the
link is available
o If it has something to send, it sends it
o As a frame makes its way to the intended
device, each of the other devices check the
address
field
o Only when the device recognizes its own
address, does it open the frame and read the
data
o In case of a SEL frame, the enclosed data
consists of an alert that data is forthcoming
o What it does not know, however is if the
target device it ready to receive (ON)
o So
the primary must
alert the secondary
to the upcoming
TX and wait
for an
acknowledgement
of the secondary ready status
o Primary send a SEL frame, one field of
which includes the address of the intended RX
o If the secondary is awake and running, it
returns an ACK frame to the primary
o The
primary then sends
one or more
data frames ,
each addressed to
the intended
secondary
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