Layer 1 & Layer 2 Protocols• 7-4 •
The H.110 Basic Rate ISDN Board
Supervisory Frames
RR Receive Ready, used to acknowledge I frames
RNR Receiver Not Ready, used to suspend transmission
REJ Reject, reject an I frame due to an error
Unnumbered Frames
SABME Set Asynchronous Balanced Mode Extended
Command
DISC Disconnect Command
UA Unnumbered Acknowledgment Response
DM Disconnect Mode Response
UI Unnumbered Information Command
FRMR Frame Reject Response
To exchange Information frames, a data link must first be established. This
is done by a terminal requesting a TEI using a UI frame. The network can
grant the TEI also using a UI frame. There are a number of possible states
a data link can have once a TEI has been assigned. To exchange I frames,
the link must be in the multi-frame established state. This is achieved by
one end or the other sending an SABME frame and receiving a UA
response. Once the data link is in the multi-frame state it can remain there
indefinitely unless an error is detected or a request is made to disconnect
from the multi-frame state.
Each I frame contains two state variables N(S) and N(R) which are
sequence numbers for the I frame sent and the next I frame that is expected
to be received. The supervisory frame that acknowledges the I frame has
corresponding variables. These are used to insure that no I frames are lost.
They can also be used to request the retransmission of an I frame should
an error in transmission occur. Each frame also includes Frame Check
Sequence or FCS field to insure the integrity of the I frame contents.
The H.110 Basic Rate ISDN Board manages all the details of the Layer 2
protocol. For ports set as TE’s, a SABME frame is automatically sent
upon assignment of a TEI to put the data link into the multi-frame
established state. Acknowledgments and supervisory frames are sent as
needed and recovery procedures initiated without requiring the intervention
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