Get started !
online LTE test
online C test

Updated or New
CDF explained
5G Page
5G Intent (a presentation)
5G Intent (article)
CV2X Page
A Look at CV2X (a presentation)

Information Theory
Multiple Access
OSI Model
Data Link layer
Word about ATM
Standard Reference
Reference books
Resources on Web
Mind Map
Perl resources
Magic MSC tool
Bar graph tool
C programming
ASCII table
Project Management
Simple Google box
HTML characters
Site view counter
7 5 0 , 5 4 0
(May-2010 till Mar-2019)
another knowledge site

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