Telecom Tutorials by Samir Amberkar

LAPD Frames [Under Data Link Layer (LAPD) concepts]

Frame types are:

o Information
o Supervisory
o Unnumbered

Bit 0 and 1 of control field distinguish between frame types:
For Information frame type, bit 0 is 0,
For Supervisory frame type, bit 1 and 0 are 0 and 1 respectively, and
For Supervisory frame type, bit 1 and 0 are 1 and 1 respectively.

Control field for frame types are shown below:

Information frame    N(R)     (7 bits)     P bit     N(S)     (7 bits)     0 bit

Supervisory frames    N(R)     (7 bits)     P/F bit     6 bits     0 bit     1 bit

Unnumbered frames    8 bits     do not care     3 bits     P/F bit     2 bits     1 bit     1 bit

Poll (P/F) bit is set when sender expect response from receiver; this is useful while establishing connection or as a periodic check during idle period when connection is already established and present.

Information frame

I Information

Used to carry Layer 3 data.

Supervisory Frames

RR Receive Ready	This frame is used to acknowledg reception of I-frame. This frame along with RNR-frame is used for flow control. During idle period of no L3 data transfer, RR-frame is used periodically from both sides.
RNR Receive Not Ready	This frame is used for flow control. RNR indicate that the side is not ready to accept any more I-frames (may be due to storage restrictions). Once it is ready, it sends RR-frame to indicate that it is now ready to accept further I-frames.
REJ REJect	This frame indicate that error has been detected (through FCS calculations) in last received I-frame. Other side then need to re-transmit the frame which can identified by N(R) value).

Unnumbered Frames

SABME Set Asynchronous Balance Mode Extended	This frame is used to establish L2 connection as discussed earlier.
DM Disconnected Mode	DM-frame indicate to receiver that L2 connection is being taken down by sender due to some (may be L1 problem) error (without requiring acknowledgement). DISC-frame also disconnects L2 connection but this is normal scenario or graceful tearing down of L2 connection.
UI Unnumbered Information	UI-frame are similar to I-frame except that UI do not need acknowledgement. This is in a way one time transmission.
DISC DISConnect	DISC-frame initiates tearing down on L2 connection as shown in diagram below for MS-BTS (during release procedure)
UA Unnumbered Acknowledgement	UA frame is used to acknowledg L2 establishment or tearing down
FRMR FRaMe Reject	FRMR-frame is used to indicate that received frame is wrong (e.g. protocol error) such that retranmission will not be able to correct the error.
XID eXchane IDentification	XID-frame is for correcting various transmission parameters (like timers)

MS

BTS

- 1) SDCCH, RR - Channel Release 2) L2 DISC frame 3) L2 UA frame -

This complete the brief on Data Link layer concepts.

References: GSM Networks book by Heine, 3GPP 04.06 (LAPDm specification), LAPD specification - Q.920, Q.921.

Copyright © Samir Amberkar 2010

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