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Data Link layer concepts (LAPD frames) - 2

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

Used to carry Layer 3 data.

Supervisory Frames

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.
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.
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

Set Asynchronous Balance Mode Extended
This frame is used to establish L2 connection as discussed earlier.
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.
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-frame initiates tearing down on L2 connection as shown in diagram below for MS-BTS (during release procedure)
Unnumbered Acknowledgement
UA frame is used to acknowledg L2 establishment or tearing down
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.
eXchane IDentification
XID-frame is for correcting various transmission parameters (like timers)



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§ §

Part 1 - LAPD « Data Link layer concepts Index » Retransmission