ETSI ETS 300 938 ed.4 (1999-12)

SLOVENSKI STANDARD
01-december-2003
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Digital cellular telecommunications system (Phase 2+) (GSM); Mobile Station - Base
Station System (MS - BSS) interface; Data Link (DL) layer specification (GSM 04.06
version 5.3.1 Release 1996)
Ta slovenski standard je istoveten z: ETS 300 938 Edition 4
ICS:
33.070.50 Globalni sistem za mobilno Global System for Mobile
telekomunikacijo (GSM) Communication (GSM)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 938
TELECOMMUNICATION December 1999
STANDARD Fourth Edition
Source: SMG Reference: RE/SMG-030406QR2
ICS: 33.020
Key words: Digital cellular telecommunications system, Global System for Mobile communications (GSM)
R
GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS
Digital cellular telecommunications system (Phase 2+);
Mobile Station - Base Station System (MS - BSS) interface;
Data Link (DL) layer specification
(GSM 04.06 version 5.3.1 Release 1996)
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Internet: secretariat@etsi.fr - http://www.etsi.org
Tel.:+33492944200- Fax:+33493654716
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1999. All rights reserved.

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
Whilst every care has been taken in the preparation and publication of the present document, errors in
content, typographical or otherwise, may occur. If you have comments concerning its accuracy, please
write to "ETSI Standards Making Support Dept." at the address shown on the title page.

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
Contents
Intellectual Property Rights.7
Foreword.7
0 Scope .9
0.1 Normative references.9
0.2 Abbreviations .9
1 General.10
1.1 Options.10
2 Frame structure for peer-to-peer communication .11
2.1 General .11
2.2 Frame delimitation and fill bits .13
2.3 Address field .14
2.4 Control field.14
2.5 Length indicator field.14
2.5a Short L2 header type 1.14
2.6 Information field .14
2.7 Transparency.14
2.8 Format convention .14
2.8.1 Numbering convention.14
2.8.2 Order of bit transmission .15
2.8.3 Field mapping convention.15
3 Elements of procedures and formats of fields for Data Link Layer peer-to-peer communication.15
3.1 General .15
3.2 Address field format.15
3.3 Address field variables.16
3.3.1 Address field extension bit (EA) .16
3.3.2 Command/response field bit (C/R).16
3.3.3 Service access point identifier (SAPI) .16
3.4 Control field formats.17
3.4.1 Information transfer format - I.17
3.4.2 Supervisory format - S.17
3.4.3 Unnumbered format - U.17
3.4a Short L2 header type 1.18
3.5 Control field parameters and associated state variables .18
3.5.1 Poll/Final bit .18
3.5.2 Multiple frame operation - variables and sequence numbers.18
3.5.2.1 Modulus .18
3.5.2.2 Send state variable V(S).18
3.5.2.3 Acknowledge state variable V(A).18
3.5.2.4 Send sequence number N(S) .19
3.5.2.5 Receive state variable V(R) .19
3.5.2.6 Receive sequence number N(R) .19
3.5.2.7 Other parameters and variables .19
3.5.3 Unacknowledged operation variables and parameters .19
3.6 Length indicator field format.19
3.7 Length indicator field variables.20
3.7.1 Length indicator field extension bit (EL) .20
3.7.2 More data bit (M) .20
3.7.3 Length indicator (L).20
3.8 Commands and responses.20
3.8.1 Information (I) commands .20
3.8.2 Set asynchronous balanced mode (SABM) command.21
3.8.3 Disconnect (DISC) command.22

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3.8.4 Unnumbered information (UI) command . 22
3.8.5 Receive ready (RR) command/response. 22
3.8.6 Reject (REJ) command/response. 23
3.8.7 Receive not ready (RNR) command/response. 23
3.8.8 Unnumbered acknowledgement (UA) response. 23
3.8.9 Disconnected mode (DM) response . 23
4 Elements for layer-to-layer communication . 24
4.1 Definition of primitives and parameters. 24
4.1.1 Generic names . 24
4.1.1.1 DL-ESTABLISH. 24
4.1.1.2 DL-RELEASE. 24
4.1.1.3 DL-DATA. 24
4.1.1.4 DL-UNIT DATA . 24
4.1.1.5 DL-SUSPEND . 24
4.1.1.6 DL-RESUME . 24
4.1.1.7 DL-RECONNECT. 25
4.1.1.8 DL-RANDOM ACCESS. 25
4.1.1.9 MDL-RELEASE. 25
4.1.1.10 MDL-ERROR . 25
4.1.1.11 PH-DATA . 25
4.1.1.12 PH-RANDOM ACCESS . 25
4.1.1.13 PH-CONNECT . 25
4.1.1.14 PH-READY-TO-SEND . 25
4.1.1.15 PH-EMPTY-FRAME. 25
4.1.2 Primitives types. 25
4.1.2.1 REQUEST. 25
4.1.2.2 INDICATION . 26
4.1.2.3 RESPONSE . 26
4.1.2.4 CONFIRM . 26
4.1.3 Parameter definition. 26
4.1.3.1 Message unit. 26
4.1.3.2 Channel type . 27
4.1.3.3 Service Access Point . 27
4.1.3.4 Release mode. 27
4.1.3.5 Error cause. 27
4.1.3.6 Establish mode. 27
4.1.3.7 L2 header type . 27
4.2 Primitive procedures.31
5 Definition of the peer-to-peer protocol LAPDm. 32
5.1 General. 32
5.2 General Protocol Procedures . 33
5.2.1 Unacknowledged information transfer . 33
5.2.2 Acknowledged multiple frame information transfer. 33
5.3 Procedures for unacknowledged information transfer. 33
5.3.1 General . 33
5.3.2 Transmission of unacknowledged information . 33
5.3.3 Receipt of unacknowledged information. 33
5.4 Procedures for establishment and release of multiple frame operation. 34
5.4.1 Establishment of multiple frame operation . 34
5.4.1.1 General . 34
5.4.1.2 Normal establishment procedures . 34
5.4.1.3 Procedure on expiry of timer T200: Normal establishment. 35
5.4.1.4 Contention resolution establishment procedure. 35
5.4.1.5 Procedure on expiry of timer T200: contention resolution
(MS only). 37
5.4.2 Information transfer. 37
5.4.2.1 General requirements . 37
5.4.2.2 Error conditions . 38
5.4.2.3 Fill frames. 39
5.4.3 Suspension and resumption of multiple frame operation . 39
5.4.3.1 General . 39

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5.4.3.2 Suspension .40
5.4.3.3 Resumption.40
5.4.3.3.1 Procedure after channel change.40
5.4.3.3.2 Procedure after returning to the old
channel (MS only) .41
5.4.4 Termination of multiple frame operation.42
5.4.4.1 General .42
5.4.4.2 Normal release procedure .42
5.4.4.3 Procedure on expiry of timer T200 for normal release .42
5.4.4.4 Local end release procedure .43
5.4.5 Idle state.43
5.4.6 Collision of unnumbered commands and responses .43
5.4.6.1 Identical transmitted and received commands .43
5.4.6.2 Different transmitted and received commands.44
5.4.6.3 Unsolicited DM response and SABM or DISC command.44
5.5 Procedures for information transfer in multiple frame operation.44
5.5.1 Transmitting I frames .44
5.5.2 Receiving I frames.45
5.5.2.1 P bit of the received I frame set to "1" .45
5.5.2.2 P bit of the received I frame set to "0" .45
5.5.3 Receiving acknowledgement.45
5.5.3.1 On receipt of a valid I frame.45
5.5.3.2 Receiving supervisory command frames with the P bit set
to "1" .46
5.5.4 Receiving REJ frames.46
5.5.4.1 Receipt of a valid REJ frame .46
5.5.4.2 Transmitting frames.47
5.5.5 Receiving RNR frame.47
5.5.6 Data link layer own receiver busy condition.49
5.5.7 Waiting acknowledgement .49
5.6 Abnormal release and re-establishment of multiple frame operation .50
5.6.1 Criteria for re-establishment.50
5.6.2 Criteria for abnormal release.50
5.6.3 Procedures for re-establishment .50
5.6.4 Procedures for abnormal release.50
5.7 Exception condition reporting and recovery for multiple frame operation.51
5.7.1 N(S) sequence error.51
5.7.2 Timer recovery.51
5.7.3 Invalid frame condition.52
5.7.4 N(R) sequence error.52
5.8 List of system parameters.52
5.8.1 Timer T200.52
5.8.1.1 For SAPI=0 and SAPI=3.52
5.8.1.2 For SAPIs other than 0 or 3 .53
5.8.2 Maximum number of retransmissions (N200) .53
5.8.2.1 For SAPI=0 and 3 .53
5.8.2.2 For SAPIs other than 0 or 3 .53
5.8.3 Maximum number of octets in an I, UI, SABM and UA frame partially or
entirely available for the information field (N201) .53
5.8.4 Maximum number of outstanding I frames (k) .53
5.8.5 Maximum number of octets in a Layer 3 message .54
5.9 System performance requirements.54
6 Special protocol operation on SAPI=0 and SAPI=3 .55
Annex A (normative): Random access procedures.56
A.1 Description of the procedure .56
A.1.1 Procedure in the MS .56
A.1.2 Procedure in the BS.56
A.2 Format .56

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Annex G (normative): Handling of frames with parameter errors in the address, control and length
indicator fields . 57
G.1 General . 57
G.2 Parameter errors in the address field . 57
G.2.1 Unallocated SAPI . 57
G.2.2 Wrong value of the C/R bit . 57
G.2.3 EA bit set to "0". 57
G.3 Parameter errors in the control field . 57
G.3.1 Supervisory frames . 57
G.3.2 Unnumbered frames . 58
G.4 Parameter errors in the length indicator field. 58
G.4.1 EL bit error. 58
G.4.2 Information frames . 58
G.4.3 Supervisory frames . 58
G.4.4 DISC and DM frames . 58
G.4.5 SABM UA and UI frames. 58
Annex H (informative): Document change history . 59
History. 60

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Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The
information pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-
members, and can be found in SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially
Essential, IPRs notified to ETSI in respect of ETSI standards", which is available from the ETSI
Secretariat. Latest updates are available on the ETSI Web server (http://www.etsi.org/ipr).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI.
No guarantee can be given as to the existence of other IPRs not referenced in SR 000 314 (or the
updates on the ETSI Web server) which are, or may be, or may become, essential to the present
document.
Foreword
This European Telecommunication Standard (ETS) has been produced by the Special Mobile Group
(SMG) of the European Telecommunications Standards Institute (ETSI).
This ETS defines the data link layer protocol of the Mobile Station - Base Station System (MS - BSS)
interface within the digital cellular telecommunications system.
The contents of this ETS is subject to continuing work within SMG and may change following formal SMG
approval. Should SMG modify the contents of this ETS, it will be resubmitted for OAP by ETSI with an
identifying change of release date and an increase in version number as follows:
Version 5.x.y
where:
5 Indicates GSM Phase 2+
y the third digit is incremented when editorial only changes have been incorporated in the
specification;
x the second digit is incremented for all other types of changes, i.e. technical enhancements,
corrections, updates, etc.
The specification from which this ETS has been derived was originally based on CEPT documentation,
hence the presentation of this ETS may not be entirely in accordance with the ETSI drafting rules.
Annexes B to F were deleted from the original specification, however, for referencing purposes the annex
numbering has been maintained.
Transposition dates
Date of adoption of this ETS: 03 December 1999
Date of latest announcement of this ETS (doa): 31 March 2000
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 30 September 2000
Date of withdrawal of any conflicting National Standard (dow): 30 September 2000

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
0 Scope
This European Telecommunication Standard (ETS) defines a data link layer protocol to be used for
signalling, and possibly also for other applications, on the MS-BS interface.
0.1 Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this ETS only when incorporated in it by amendment or revision. For undated references, the
latest edition of the publication referred to applies.
[1] GSM 01.04 (ETR 350): "Digital cellular telecommunications system (Phase 2+);
Abbreviations and acronyms".
[2] GSM 04.01: "Digital cellular telecommunications system; Mobile Station - Base
Station System (MS - BSS) interface; General aspects and principles".
[3] GSM 04.03: "Digital cellular telecommunications system (Phase 2+); Mobile
Station - Base Station System (MS - BSS) interface Channel structures and
access capabilities".
[4] GSM 04.04 (ETS 300 936): "Digital cellular telecommunications system; Layer 1
General requirements".
[5] GSM 04.05 (ETS 300 937): "Digital cellular telecommunications system; Data
Link (DL) layer General aspects".
[6] GSM 04.07 (ETS 300 939): "Digital cellular telecommunications system
(Phase 2+); Mobile radio interface signalling layer 3; General aspects".
[7] GSM 04.08 (ETS 300 940): "Digital cellular telecommunications system
(Phase 2+); Mobile radio interface layer 3 specification".
[8] GSM 04.10 (ETS 300 941): "Digital cellular telecommunications system; Mobile
radio interface layer 3 Supplementary services specification; General aspects".
[9] GSM 04.12 (ETS 300 943): "Digital cellular telecommunications system
(Phase 2+); Short Message Service Cell Broadcast (SMSCB) support on the
mobile radio interface".
[10] GSM 05.02 (ETS 300 908): "Digital cellular telecommunications system
(Phase 2+); Multiplexing and multiple access on the radio path".
[11] GSM 08.56: "Digital cellular telecommunications system; Base Station Controller
- Base Transceiver Station (BSC - BTS) interface; Layer 2 specification".
[12] GSM 08.58: "Digital cellular telecommunications system (Phase 2+); Base
Station Controller - Base Transceiver Station (BSC - BTS) interface; Layer 3
specification".
[13] CCITT Recommendation Z.100: "Specification and description language (SDL)".
0.2 Abbreviations
Abbreviations used in this ETS are listed in GSM 01.04.

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1 General
This ETS describes the frame structure, elements of procedure, format of fields and procedures for the
proper operation of the Link Access Procedure on the Dm channel, LAPDm.
NOTE 1: The term Dm channel is used for convenience to designate the collection of all the
various signalling channels required in the GSM system. See also GSM 04.03.
The concepts, terminology, overview description of LAPDm functions and procedures, and the relationship
with other Technical Specifications are described in general terms in GSM 04.05.
The frame formats defined for LAPDm are based on those defined for LAPD. However, there are
important differences between LAPDm and LAPD, in particular with regard to frame delimitation methods
and transparency mechanisms. These differences are necessary for operation within the constraints set
by the radio path.
LAPDm supports two modes of operation:
- unacknowledged operation using UI frames;
- acknowledged operation using the multiple frame procedure.
As a choice of implementation, the two modes of operation may be implemented independently of each
other. This is possible since there is no interactions between the two modes, other than queuing at the
transmitter, even when they coexist on the same physical channel. For BCCHs and CCCHs only the
unacknowledged mode of operation needs to be implemented.
LAPDm is used for information sent on the control channels BCCH, AGCH, NCH, PCH, FACCH, SACCH
and SDCCH as defined in GSM 04.03.
NOTE 2: AGCH, NCH and PCH are sometimes referred to by the collective name CCCH and
FACCH, SACCH and SDCCH are, similarly, referred to by the collective name DCCH.
LAPDm may also be used on other types of channel.
NOTE 3: As stated in GSM 04.05, the term "data link layer" is used in the main text of this
Technical Specification. However, mainly in figures and tables, the terms "layer 2" and
"L2" are used as abbreviations. Furthermore, in accordance with GSM 04.07 and
GSM 04.08, the term "layer 3" is used to indicate the layer above the data link layer.
This Technical Specification is organized as follows:
The frame structure for peer-to-peer communication is given in clause 2. The elements of procedure and
formats of fields are given in clause 3. The elements of layer-to-layer communication are contained in
clause 4. The details of the peer-to-peer procedures are given in clause 5. Section 6 summarizes the
special protocol operations used mandatorily with SAPI=0 and SAP = 3.
The specification for the random access channel is contained in annex A, even though it is not a LAPDm
function. This part of the ETS is descriptive and does not constrain the implementation of the random
access function. The procedure is used for CHANNEL REQUEST on the RACH and HANDOVER
ACCESS on the main DCCH.
(Annexes B to F are deleted).
Annex G gives an overview of actions taken on frames containing parameter errors.
1.1 Options
Support of short L2 header type 1 is an option in both the mobile station and the network; under certain
conditions the support is mandatory, as specified in other Specifications. A layer 2 protocol entity not
implementing short L2 header type 1 shall diagnose an E/A bit error and proceed as defined in
annex G.2.3.
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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
2 Frame structure for peer-to-peer communication
2.1 General
All data link layer peer-to-peer exchanges are in frames conforming to one of the formats shown in
figure 1. Several format types are shown in the figure:
- Format A is used on DCCHs for frames where there is no information field.
- Formats B, Bter and B4 are used on DCCHs for frames containing an information field:
- format Bter is used on request of higher layers if and only if short L2 header type 1 is
supported and a UI command is to be transmitted on SAPI 0;
- format B4 is used for UI frames transmitted by the network on SACCH;
- format B is applied in all other cases.
- Format Bbis is used only on BCCH, PCH, NCH, and AGCH.
- In addition there is a Format C for transmission of random access signals.
Format C frames are described in annex A. Format A, B, Bbis, Bter and B4 frames are described in the
remainder of this ETS.
876 5 4 321
Bit
Octet No
: Address field :
k
Control field k+1
k+2
: Length indicator field :
n
n+1
: Fill bits :
N201+n
Format type A
Figure 1/GSM 04.06 (sheet 1 of 3): General frame formats

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876 5 4 321
Bit
Octet No
: Address field :
k
Control field k+1
k+2
Length indicator field
: :
n
n+1
: Information field :
N
N+1
: Fill bits :
N201+n
Format type B
Bit 876 5 4 321
Octet No
: :
: Information field :
: :
N201
Format type Bbis
Figure 1/GSM 04.06 (sheet 2 of 3): General frame formats

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876 5 4 321
Bit
Octet No
: Address field :
k
Control field k+1
k+2
: Information field :
N201 +
k+1
Format type B4
Bit 876 5 4 321
Octet No
short L2 header
type 1
:
Information field :
: :
N201
Format type Bter
Figure 1/GSM 04.06 (sheet 3 of 3): General frame formats
The parameter N201 is the maximum number of octets which are partially or entirely available for the
information field of a frame. It depends on the type of channel and the format, see subclause 5.8.3.
2.2 Frame delimitation and fill bits
Frame delimitation is provided by the physical layer:
- in format type A, B and B4 frames at the beginning of the frame for determining the start of the
first octet in the address field, in format type Bter frames for determining the start of the octet
containing the short L2 header type 1 (which is also the first octet used for the information field),
and in format type Bbis frames for determining the start of the first octet for the information field;
- at the end of the frame for determining the last bit of the frame.
The end of the useful part of the frame, i.e. the end of the length indicator field in type A frames and
the end of the information field in type B frames, is determined by a length indicator contained in the
length indicator field. The useful part of a Bbis frames takes all N201 octets of that frame. The
useful part of a Bter frame takes all N201 octets of that frame except those bits of octet 1 which

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
contain the short L2 header type 1. The useful part of a B4 frame takes all N201 octets of that
frame except those octets which contain the address field and the control field.
If a frame contains a length indicator has a value less than N201, the frame contains fill bits. Octets
containing fill bits shall take the binary value "00101011", when sent by the network. Octets
containing fill bits shall take the value "00101011" or "11111111", when sent by the mobile station.
NOTE: The value "00101011" is chosen due to the modulation and interleaving scheme used
in the GSM system.
2.3 Address field
The address field may consist of a variable number of octets. However, for applications on control
channels the field consists of only one octet. The address field identifies the SAP for which a command
frame is intended and the SAP transmitting a response frame. The format of the address field is defined in
subclause 3.2.
2.4 Control field
The control field consists of one octet. The format of the control field is defined in subclause 3.4.
2.5 Length indicator field
The length indicator field may consist of a variable number of octets. However, for applications on control
channels the field consists of only one octet. The format of the field is defined in subclause 3.6.
2.5a Short L2 header type 1
The short L2 header type 1 consists of two bits. Its contents are defined in subclause 3.4a.
2.6 Information field
The information field of a frame, when present, has the position in the frame defined in 2.1.
The maximum number of octets in the information field (N201) is defined in subclause 5.8.3.
2.7 Transparency
Because of the frame delimitation technique used (see subclause 2.2), the frame can include any possible
sequence of bits without the need for additional transparency mechanisms.
2.8 Format convention
2.8.1 Numbering convention
The basic convention used in this Technical Specification is illustrated in figure 2. The bits are grouped
into octets.
The bits of an octet are shown horizontally and are numbered from 1 to 8. Multiple octets are shown
vertically and are numbered from 1 to n.

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
Bit 8 76 54 3 2 1
Octet no
;
;
n
Figure 2/GSM 04.06: Format convention
2.8.2 Order of bit transmission
The order of bit transmission is defined in GSM 04.04.
2.8.3 Field mapping convention
When a field is contained within a single octet, the lowest bit number of the field represents the lowest
order value.
When a field spans more than one octet, the order of bit values within each octet progressively decreases
as the octet number increases. In that part of the field contained in a given octet the lowest bit number
represents the lowest order value.
For example, a bit number can be identified as a couple (o, b) where o is the octet number and b is the
relative bit number within the octet. Figure 3 illustrates a field that spans from bit (1, 3) to bit (2, 7). The
high order bit of the field is mapped on bit (1, 3) and the low order bit is mapped on bit (2, 7).
Bit 8 76 54 3 2 1
st
43 2 1 octet field
22 2
nd
10 2 octet field
Figure 3/GSM 04.06: Field mapping convention
3 Elements of procedures and formats of fields for Data Link Layer
peer-to-peer communication
3.1 General
The elements of procedures define the commands and responses that are used on the data link
connections carried on the Dm channel.
Procedures are derived from these elements of procedures and are described in clause 5.
If a bit position is marked as "spare", it shall be coded as "0". For future compatibility reasons, an entity
receiving frames, where spare bit positions are coded otherwise, shall ignore those values without
notification of any error.
3.2 Address field format
The field consists of:
- the address field extension bit EA;

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ETS 300 938 (GSM 04.06 version 5.3.1 Release 1996): December 1999
- the command/response bit C/R;
- the SAPI;
- and the Link Protocol Discriminator LPD.
The format of the address field for control channels is shown in figure 4.
The address field extension bit, EA, enables extension of the field to span more than one octet. The value
"0" is used to indicate an extension, the value "1" shall be used for the final address field octet.
The Link Protocol Discriminator (LPD) corresponding to the use in this Technical Specification shall take
the value "0 0", all other values are reserved.
NOTE: LPD = "0 1" corresponds to the data link protocol used for SMSCB (see GSM 04.12).
Bit 8 765 43 21
Spare LPD SAPI C/R EA
=1
Figure 4/GSM 04.06: Address field format
3.3 Address field variables
3.3.1 Address field extension bit (EA)
The address field range is extended by reserving the first transmitted bit of the address field octets to
indicate the final octet of the address field. The presence of a "1" in the first bit of an address field octet
signals that it is the final octet of the address field. Figure 4 shows the case where the field consists of one
octet.
3.3.2 Command/response field bit (C/R)
The C/R bit identifies a frame as either a command or a response. The MS side shall send commands
with the C/R bit set to "0", and responses with the C/R bit set to "1". The BS side shall do the opposite;
that is commands are sent with C/R set to "1", and responses are sent with C/R set to "0". The
combinations for the BS side and MS side are shown in table 1.
Table 1/GSM 04.06: C/R field bit usage
Type Direction C/R value
Command BS side to MS side 1
MS side to BS side 0
Response BS side to MS side 0
MS side to BS side 1
3.3.3 Service access point identifier (SAPI)
The service access point identifier (SAPI) identifies a point at which data link layer services are provided
by the data link layer to a layer 3 entity (see GSM 04.05). The SAPI allows 8 service access points to be
specified initially, where bit 3 of the address field octet containing the SAPI is the least significant binary
digit and bit 5 is the most significant.
The SAPI values are allocated as shown in table 2.

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Table 2/GSM 04.06: Allocation of SAPI values
SAPI value Related entity
0 Call control signalling, mobility
management signalling and radio
resource management signalling
(see GSM 04.08 and 04.10)
3 Short message service
All others Reserved for future standardization
3.4 Control field formats
The control field identifies the type of frame, which will be either a command or a response. The control
field will contain sequence numbers, where applicable.
Three types of control field formats are specified: numbered information transfer (I format), supervisory
functions (S format), and unnumbered information transfer and control functions (U format). The control
field formats for LAPDm are shown in table 3.
3.4.1 Information transfer format - I
The I format shall be used to perform an information transfer between layer 3 entities. The functions of
N(S), N(R) and P (defined in 3.5) are independent; that is, each I frame has an N(S) sequence number, an
N(R) sequence number which may or may not acknowledge additional I frames received by the data link
layer entity, and a P bit that may be set to "0" or "1". The use of N(S), N(R) and P is defined in clause 5.
3.4.2 Supervisory format - S
The S format shall be used to perform data link supervisory control functions such as: acknowledge
I frames, request retransmission of I frames, and request a temporary suspension of transmission of
I frames. The functions of N(R) and P/F are independent; that is, each supervisory frame has an N(R)
sequence number which may or may not acknowledge additional I frames received by the data link layer
entity, and a P/F bit that may be set to "0" or "1".
The use of N(R) and the P/F bit is described in clause 5.
3.4.3 Unnumbered format - U
The U format shall be used to provide additional data link control functions and unacknowledged
information transfer.
This format does not contain sequence numbers. It does include a P/F bit that may be set to "0" or "1".
Table 3/GSM 04.06: Control field format
Controlfieldbits 87 654 321
Iformat N(R) P N(S) 0
Sformat N(R) P/F S S 0 1
Uformat U U U P/F U U 1 1
N(S) Transmitter send sequence number
N(R) Transmitter receive sequence number
S Supervisory function bit
U Unnumbered function bit
P/F Poll bit, when issued as a command, final bit, when issued as a response

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For definition of values for supervisory function bits and unnumbered function bits, see table 4.
3.4a Short L2 header type 1
The short L2 header type 1 field is used in format Bter frames, see subclause 2.1. It consists of 2 bits,
both shall take value 0.
NOTE: Introduction of further short L2 header types (longer th
...