SIST TBR 034:2000

SLOVENSKI STANDARD
3SIST TBR 034:
01DYJXVW
'LJLWDOQRRPUHåMH]LQWHJULUDQLPLVWRULWYDPL,6'1=DKWHYH]DSULNOMXþLWHY
WHUPLQDOVNHRSUHPHYSDNHWQHPQDþLQXQD,6'1]XSRUDERSULPDUQHJDGRVWRSD
Integrated Services Digital Network (ISDN); Attachment requirements for packet mode
terminal equipment to connect to an ISDN using ISDN primary rate access
Ta slovenski standard je istoveten z: TBR 034 Edition 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
3SIST TBR 034: en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL TBR 34
BASIS for December 1997
REGULATION
Source: SPS Reference: DTBR/SPS-05120
ICS: 33.020
Key words: ISDN, packet mode, terminal, primary, access
Integrated Services Digital Network (ISDN);
Attachment requirements for packet mode terminal equipment
to connect to an ISDN using ISDN primary rate access
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
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.fr
Tel.: +33 4 92 94 42 00 - Fax: +33 4 93 65 47 16
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 1997. All rights reserved.

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TBR 34: December 1997
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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TBR 34: December 1997
Contents
Foreword .9
1 Scope .11
2 Normative references.11
3 Definitions and abbreviations .12
3.1 Definitions .12
3.2 Abbreviations .13
4 General.14
4.1 General requirements for the access of the PH .14
4.1.1 Channel used for the access of the PH.14
4.1.2 Requirements for layer 1.15
4.1.2.1 Requirements for complete layer 1 (comprising D-channel
and B-channels as a whole) .15
4.1.2.2 Requirements for layer 1 with respect to the selected
channel .15
4.1.3 Requirements for layer 2 of the D-channel.15
4.1.3.1 Requirements for data link connections with SAPI 0.15
4.1.3.2 Requirements for data link connections with SAPI 16.16
4.1.4 Notification classes.16
4.1.5 Requirements for layer 3 of the D-channel.16
4.1.5.1 Outgoing calls.16
4.1.5.2 Incoming calls.17
4.1.5.3 Use of outgoing and incoming calls to support access of
the PH.17
5 Definitions of primitives .17
6 Safety requirements .17
7 Electro-Magnetic Compatibility (EMC) requirements .17
8 Overvoltage protection requirements .18
8.1 Impulse transfer from mains, common mode.18
8.2 Impulse transfer from mains, transverse mode.18
8.3 Conversion of common mode to transverse mode.18
9 Layer 1 requirements .18
9.1 Physical characteristics (on-demand access).18
9.2 Specification at the output port .18
9.3 Specification at the input port.18
9.4 Frame structure .18
9.5 Operational functions.18
9.6 Semi-permanent B-channel access.18
10 Layer 2 D-channel requirements.19
10.1 Frame structure for peer-to-peer communication.19
10.1.1 General.19
10.1.2 Flag sequence.19
10.1.3 Address field.19
10.1.4 Control field .19
10.1.5 Information field.20
10.1.6 Transparency .20
10.1.7 Frame Check Sequence (FCS) field .20

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TBR 34: December 1997
10.1.8 Format convention. 20
10.1.8.1 Numbering convention . 20
10.1.8.2 Order of bit transmission. 20
10.1.8.3 Field mapping convention . 20
10.1.9 Invalid or incompatible frames. 20
10.2 Elements of procedures and formats of fields for data link layer peer-to-peer
communication. 20
10.2.1 General . 20
10.2.2 Address field format. 20
10.2.3 Address field variables. 20
10.2.3.1 Address field extension bit (EA). 20
10.2.3.2 Command/Response field bit (C/R) . 21
10.2.3.3 Service Access Point Identifier (SAPI). 21
10.2.3.4 Terminal Endpoint Identifier (TEI). 21
10.2.3.4.1 TEI for broadcast data link connection. 21
10.2.3.4.2 TEI for point-to-point data link
connection. 21
10.2.4 Control field formats . 21
10.2.4.1 Information transfer (I) format. 21
10.2.4.2 Supervisory (S) format . 21
10.2.4.3 Unnumbered (U) format. 21
10.2.5 Control field parameters and associated state variables. 22
10.2.5.1 Poll/Final bit. 22
10.2.5.2 Multiple frame operation - variables and sequence
numbers . 22
10.2.5.2.1 Modulus. 22
10.2.5.2.2 Send state variable V(S) . 22
10.2.5.2.3 Acknowledge state variable V(A) . 22
10.2.5.2.4 Send sequence number N(S). 22
10.2.5.2.5 Receive state variable V(R). 22
10.2.5.2.6 Receive sequence number N(R). 22
10.2.6 Frame types. 22
10.2.6.1 Commands and responses. 22
10.2.6.2 Information (I) command. 23
10.2.6.3 Set Asynchronous Balanced Mode Extended (SABME)
command . 23
10.2.6.4 DISConnect (DISC) command. 23
10.2.6.5 Unnumbered Information (UI) command . 23
10.2.6.6 Receive Ready (RR) command/response . 24
10.2.6.7 REJect (REJ) command/response . 24
10.2.6.8 Receive Not Ready (RNR) command/response . 24
10.2.6.9 Unnumbered Acknowledgement (UA) response . 24
10.2.6.10 Disconnected Mode (DM) response . 24
10.3 Provision of point-to-point signalling connections . 24
10.4 Procedures for unacknowledged information transfer . 24
10.4.1 Transmission of unacknowledged information . 24
10.4.2 Receipt of unacknowledged information. 24
10.5 Terminal Endpoint Identifier (TEI) management procedures. 25
10.5.1 General . 25
10.5.2 TEI assignment procedure . 25
10.5.2.1 Expiry of timer T202. 25
10.5.3 TEI check procedure . 25
10.5.4 TEI removal procedure . 25
10.5.4.1 Action taken by the data link layer entity receiving the
MDL-REMOVE-REQUEST primitive. 25
10.5.4.2 Conditions for TEI removal . 25
10.5.5 TEI identity verify procedure . 25
10.5.5.1 General . 25
10.5.5.2 Operation of the TEI identity verify procedure. 25
10.5.6 Formats and codes. 25
10.6 Procedures for establishment and release of multiple frame operation. 26
10.6.1 Establishment of multiple frame operation . 26
10.6.1.1 General . 26

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TBR 34: December 1997
10.6.1.2 Establishment procedures.26
10.6.1.2.1 Establishment initiation .26
10.6.1.2.2 Establishment response .26
10.6.1.3 Procedure on expiry of timer T200 .27
10.6.2 Termination of multiple frame operation.27
10.7 Procedures for information transfer in multiple frame operation .27
10.7.1 Transmitting I-frames .27
10.7.2 Receiving I-frames .27
10.7.2.1 P bit set to 1.27
10.7.2.2 P bit set to 0.27
10.7.3 Receiving acknowledgements.28
10.7.4 Receiving REJ frames.28
10.7.5 Receiving RNR frames.29
10.7.5.1 Receiving a valid RNR command or response.29
10.7.5.2 Expiry of timer T200 during "peer receiver busy" .29
10.7.5.3 Receiving a valid RNR command or response during
"peer receiver busy".30
10.7.5.4 Receiving a valid RR or REJ command or response
during "peer receiver busy".30
10.7.5.5 Appropriate supervisory response frame.30
10.7.6 Waiting acknowledgement .31
10.8 Re-establishment of multiple frame operation .31
10.9 Exception condition reporting and recovery.32
10.9.1 N(S) sequence error.32
10.9.2 Invalid frame condition .32
10.9.3 Frame rejection condition.32
10.9.4 Multiple-assignment of TEI value .32
10.10 List of system parameters.33
10.10.1 Timer T200.33
10.10.2 Maximum number of retransmissions (N200).33
10.10.3 Maximum number of octets in an information field (N201) .33
10.10.4 Maximum number of transmission of the TEI identity request message
(N202) .33
10.10.5 Maximum number of outstanding I-frames (k) .33
10.10.6 Timer T202.33
10.10.7 Layer 2 response time.33
11 Layer 3 D-channel requirements.34
11.1 Overview of call control.34
11.1.1 Call states at the user side of the interface .34
11.1.1.1 Null state (U0).34
11.1.1.2 Call initiated (U1) .34
11.1.1.3 Outgoing call proceeding (U3).34
11.1.1.4 Call present (U6).34
11.1.1.5 Call received (U7).35
11.1.1.6 Connect request (U8) .35
11.1.1.7 Incoming call proceeding (U9).35
11.1.1.8 Active (U10).35
11.1.1.9 Disconnect request (U11).35
11.1.1.10 Disconnect indication (U12).35
11.1.1.11 Release request (U19) .35
11.1.2 Network call states .35
11.1.2.1 Null state (N0).35
11.1.2.2 Call initiated (N1) .35
11.1.2.3 Outgoing call proceeding (N3).35
11.1.2.4 Call present (N6).36
11.1.2.5 Call received (N7).36
11.1.2.6 Connect request (N8) .36
11.1.2.7 Incoming call proceeding (N9).36
11.1.2.8 Active (N10).36
11.1.2.9 Disconnect request (N11).36
11.1.2.10 Disconnect indication (N12).36
11.1.2.11 Release request (N19) .36

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TBR 34: December 1997
11.1.3 States associated with the global call reference. 37
11.1.3.1 Call states at the user side of the interface. 37
11.1.3.1.1 Null (Rest 0) . 37
11.1.3.1.2 Restart (Rest 2). 37
11.1.3.2 Call states at the network side of the interface . 37
11.1.3.2.1 Null (Rest 0) . 37
11.1.3.2.2 Restart request (Rest 1). 37
11.2 Message functional definitions and content . 37
11.2.1 Alerting. 38
11.2.2 Call proceeding. 39
11.2.3 Connect . 39
11.2.4 Connect acknowledge . 40
11.2.5 Disconnect. 40
11.2.6 Release. 41
11.2.7 Release complete. 41
11.2.8 Setup . 42
11.2.9 Status. 43
11.2.10 Status enquiry. 43
11.2.11 Messages used with the global call reference. 43
11.2.11.1 Restart. 43
11.2.11.2 Restart acknowledge . 43
11.3 General message format and information elements coding . 44
11.3.1 Overview. 44
11.3.2 Protocol discriminator . 44
11.3.3 Call reference . 44
11.3.4 Message type. 44
11.3.5 Other information elements . 46
11.3.5.1 Coding rules. 46
11.3.5.1.1 Codeset 0. 46
11.3.5.2 Extensions of codesets. 47
11.3.5.3 Locking shift procedure. 47
11.3.5.4 Non-locking shift procedure . 47
11.3.5.5 Bearer capability . 47
11.3.5.6 Call state . 49
11.3.5.7 Called party number. 49
11.3.5.8 Called party subaddress . 49
11.3.5.9 Calling party number. 49
11.3.5.10 Calling party subaddress. 51
11.3.5.11 Cause. 52
11.3.5.12 Channel identification. 52
11.3.5.13 Restart indicator. 52
11.4 Call control procedures for the control of packet-mode access of the PH. 53
11.4.1 Call establishment at the originating interface. 53
11.4.1.1 Call request. 53
11.4.1.2 Overlap sending. 53
11.4.1.3 Call proceeding . 53
11.4.1.4 Call confirmation indication . 54
11.4.1.5 Call connected . 54
11.4.2 Call establishment at the destination interface . 54
11.4.2.1 Incoming call . 54
11.4.2.2 Compatibility checking . 54
11.4.2.3 Channel selection-destination. 55
11.4.2.4 Overlap receiving . 55
11.4.2.5 Call confirmation . 55
11.4.2.6 Call accept . 56
11.4.2.7 Active indication . 56
11.4.3 Call clearing . 56
11.4.3.1 Terminology . 56
11.4.3.2 Exception conditions . 56
11.4.3.3 Clearing initiated by the user. 56
11.4.3.4 Clearing initiated by the network . 57
11.4.3.4.1 Clearing when tones/announcements
provided. 57

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TBR 34: December 1997
11.4.3.4.2 Clearing when tones/announcements
not provided .57
11.4.3.4.3 Completion of clearing .57
11.4.3.5 Clear collision .57
11.4.4 Call rearrangements.57
11.4.5 Call collisions.58
11.4.6 Handling of error conditions .58
11.4.6.1 Call reference procedural errors.58
11.4.6.2 Message type or message sequence errors .58
11.4.6.3 Duplicated information elements .58
11.4.6.4 Mandatory information element errors.58
11.4.6.4.1 Mandatory information element
missing.58
11.4.6.5 Non-mandatory information element errors.59
11.4.6.5.1 Unrecognized information element.59
11.4.6.5.2 Non-mandatory information element
content error .59
11.4.6.6 Status enquiry procedure.60
11.4.6.7 Receiving a STATUS message.60
11.4.7 User notification procedure .61
11.4.8 Restart procedure.61
11.5 TE timers .61
12 X.25.62
12.1 X.25 requirements .62
12.2 X.25 procedures .62
12.2.1 Data link protocol supporting the X.25 packet level protocol .62
12.2.2 X.25 packet level protocol .62
12.2.3 Outgoing and incoming virtual calls.62
12.2.4 Identification of X.25 terminals .62
Annex A (normative): TBR Requirements Table (TBR-RT).63
A.1 Guidance for completion of the TBR-RT.63
A.2 General classification of requirements TBR-RT.64
A.3 Layer 1 and overvoltage requirements TBR-RT .66
A.4 Layer 2 TBR-RT .69
A.5 Layer 3 TBR-RT .72
Annex B (normative): Layer 1 tests .74
Annex C (normative): Layer 2 tests .75
C.1 The TTCN Graphical form (TTCN.GR) .75
C.2 The TTCN Machine Processable form (TTCN.MP) .75
Annex D (normative): Layer 3 tests .76
D.1 The TTCN Graphical form (TTCN.GR) .76
D.2 The TTCN Machine Processable form (TTCN.MP) .76
Annex E (normative): Partial Protocol Implementation eXtra Information for Testing (PIXIT)
proforma .77
E.1 Introduction.77
E.2 References.77

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TBR 34: December 1997
E.3 Proforma structure and contents . 77
E.4 Table/item identification. 77
E.5 Guidance on completing the PIXIT. 77
E.6 Environmental test conditions. 77
E.7 Layer 1 PIXIT. 78
E.8 Layer 2 PIXIT. 78
E.9 Layer 3 PIXIT. 80
Annex F (informative): Justifications for the requirements in the TBR. 82
F.1 Principles applicable to the whole TBR . 82
F.1.1 General principles . 82
F.1.2 Requirements vs. definitions . 82
F.1.3 Error conditions . 82
F.1.4 General editorial changes to text from the base standards . 83
F.1.5 Detailed list of changes with respect to the base standards . 83
F.1.6 Untested requirements.84
F.1.7 Primitives. 84
F.1.8 Supplementary services. 84
F.1.9 Differences in clauses 10 and 11 and the TBR-RT between TBR 33 and this TBR. 84
F.1.10 Article 4(e) of the Directive. 85
F.2 Changes with respect to TBR 4. 85
F.2.1 Changes for layer 1. 86
F.2.1.1 Timing considerations. 86
F.2.1.2 CRC-4 monitoring and E-bits. 86
F.2.1.3 Protection requirements. 86
F.2.1.4 Detailed list of changes with respect to TBR 4 . 86
F.2.2 Changes for layer 2 of the D-channel. 87
F.2.3 Changes for layer 3 of the D-channel. 88
Annex G (normative): List of supported telecommunication services. 91
G.1 Basic telecommunication services. 91
G.2 Supplementary services for the access connection . 91
Annex H (informative): Examples for TE access configurations with small sets of requirements. 92
Annex J (informative): Bibliography . 94
History. 95

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TBR 34: December 1997
Foreword
This Technical Basis for Regulation (TBR) has been produced by the Signalling Protocols and Switching
(SPS) Technical Committee of the European Telecommunications Standards Institute (ETSI).
This TBR resulted from a mandate from the Commission of the European Community (CEC) to provide
harmonized standards for the support of the Council Directive 91/263/EEC ("The Terminal Equipment
Directive").
Annex F provides information relating to the articles of the Directive 91/263/EEC.
Annexes A, B, C, D, E and G are normative whereas annexes F, H and J are informative.
Overview of the Abstract Test Suites (ATSs)
This TBR is accompanied by the following ATSs (see annexes C and D):
- T34L2_3 (layer 2);
- T34L3_5 (layer 3).
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TBR 34: December 1997
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TBR 34: December 1997
1 Scope
This Technical Basis for Regulation (TBR) specifies the technical requirements under Articles 4 (c) to 4 (f)
of Council Directive 91/263/EEC ("The Terminal Equipment Directive") for Terminal Equipment (TE) to be
attached to the pan-European Integrated Services Digital Network (ISDN) at an interface at the T
reference point or coincident S and T reference point for a primary rate access. These requirements are
taken from TBR 4 [8], TBR 13 [9], ETS 300 007 [10], ETS 300 046-3 [12], ETS 300 011 [11],
ETS 300 102-1 [13] and ETS 300 125 [14]. This TBR does not contain the essential requirements of
Article 4 (g) for interworking via the public network, and so does not provide any guarantee of correct
terminal-to-terminal operation.
NOTE 1: Although this TBR provides the technical attachment requirements, it does not contain
the full specification of the user side of the ISDN user-network interface. Important
information necessary for correct working can be found only in the base standards
mentioned above.
This TBR specifies these requirements for TE that:
a) access Packet Handler (PH) of an ISDN through either on-demand B- or D-channel mode or
semi-permanent B-channel mode basic services (case B of X.31) and those related supplementary
services that are specified in annex G; and
b) is capable of handling either incoming calls only, outgoing calls only or both incoming and outgoing
calls.
This TBR applies to all TE that is intended for connection to the forms of ISDN access referred to above,
irrespective of whether the TE provides additional interfaces, telecommunications services or functions for
which other TBRs or national approval requirements apply.
This TBR does not specify the requirements for a packet mode TE that uses on-demand B-Channel circuit
mode access (case A of X.31); the requirements for such a TE are contained in TBR 4 [8].
NOTE 2: This TBR is not applicable to TE which may be supplied in some countries for
connection to an ISDN telecommunications service corresponding to, but not
compatible with, the pan-European ISDN primary rate access standards.
2 Normative references
This TBR incorporates by dated or 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 TBR only when incorporated into it by amendment or revision. For undated references the
latest edition of the publication referred to applies.
[1] ITU-T Recommendation G.703 (1991): "Physical/electrical characteristics of
hierarchical digital interfaces".
[2] ITU-T Recommendation G.706 (1991): "Frame alignment and cyclic redundancy
check (CRC) procedures relating to basic frame structures defined
in Recommendation G.704".
[3] ITU-T Recommendation I.411 (1988): "ISDN user-network interfaces -
Reference configurations".
[4] ISO/IEC 9646-1: "Information technology - Open Systems Interconnection -
Conformance testing methodology and framework - Part 1: General Concepts".
[5] ISO/IEC 9646-2: "Information technology - Open Systems Interconnection -
Conformance testing methodology and framework - Part 2: Abstract Test Suite
specification".
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TBR 34: December 1997
[6] ISO/IEC 9646-3: "Information technology - Open Systems Interconnection -
Conformance testing methodology and framework - Part 3: The Tree and
Tabular Combined Notation (TTCN)".
[7] TBR 2 (1997): "Attachment requirements for Data Terminal Equipment (DTE) to
connect to Packet Switched Public Data Networks (PSPDNs) for CCITT
Recommendation X.25 interfaces at data signalling rates up to 1 920 kbit/s
utilizing interfaces derived from CCITT Recommendations X.21 and X.21bis".
[8] TBR 4 (1995) and A1: "Integrated Services Digital Network (ISDN); Attachment
requirements for terminal equipment to connect to an ISDN using ISDN primary
rate access".
[9] TBR 13 (1996): "Business TeleCommunications (BTC); 2 048 kbit/s digital
structured leased lines (D2048S); Attachment requirements for terminal
equipment interface".
[10] ETS 300 007 (1991): "Integrated Services Digital Network (ISDN); Support of
packet-mode terminal equipment by an ISDN".
[11] ETS 300 011 (1992): "Integrated Services Digital Network (ISDN); Primary rate
user-network interface; Layer 1 specification and test principles".
[12] ETS 300 046-3 (1992): "Integrated Services Digital Network (ISDN); Primary
rate access - safety and protection; Part 3: Interface I - protection".
a
[13] ETS 300 102-1 (1990): "Integrated Services Digital Network (ISDN); User-
network interface layer 3; Specifications for basic call control".
[14] ETS 300 125 (1991): "Integrated Services Digital Network (ISDN); User-network
interface data link layer specification, Application of CCITT Recommendations
Q.920/I.440 and Q.921/I.441".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of this TBR, the following definitions, together with those given in ITU-T
Recommendation I.411 [3] and TBR 4 [8] apply:
access connection: Used instead of call in situations where interchanging with virtual call should be
prevented (the term access being derived from "packet handler access").
call: Call establishment according to ETS 300 102-1 [13] procedures.
conditional notification class: Class of methods to inform a user of an incoming virtual call, where the
network can use ETS 300 102-1 [13] procedures to activate or provide a channel for the delivery of the
virtual call (applicable to the use of the D-channel or of a B-channel).
notification class: Class of methods to inform a user of an incoming virtual call and provide a channel for
that call.
no notification class: Class of methods to inform a user of an incoming virtual call, where the network
requires that an access connection (use of B-channel) or a data link with SAPI=16 (use of D-channel) is
currently established between the TE and the PH (if none of the conditions is fulfilled, the network will not
inform the user of the incoming call).
on-demand B-channel: Case where a B-channel is established as a result of D-channel signalling
procedures (as opposite to semi-permanent B-channel).
on-demand layer 1: Case where the physical layer is activated/deactivated as a result from requests
from layer 2.
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TBR 34: December 1997
semi-permanent: Case where a B-channel is semi-permanently established at installation or subscription
time and is kept activated semi-permanently.
semi-permanent layer 1: Case where the physical layer is activated (established) at installation or
subscription time and is kept activated semi-permanently.
semi-permanent layer 2: Case where the data link is kept established independently of an active call or
virtual call.
virtual call: Call according to the procedures of X.25 layer 3.
3.2 Abbreviations
For the purposes of this TBR, the following abbreviations apply:
ADPCM Adaptive Differential Pulse Code Modulation
AFI Authority and Format Identifier
AMI Alternate Mark Inversion
ATS Abstract Test Suite
BCD Binary Coded Decimal
C/R Command/Response field bit
CRC Cyclic Redundancy Check
DISC DISConnect
DM Disconnected Mode
DSP Domain Specific Part
DSS1 Digital Subscriber Signalling System No. one
DTE Data Terminating Equipment
EA Address field Extension bit
EMC Electro-Magnetic Compatibility
FCS Frame Check Sequence
HDLC High level Data Link Control
I Interface point a
a
I Interface point b
b
IDI Initial Domain Identifier
ISDN Integrated Services Digital Network
IUT Implementation Under Test
LAN Local Area Network
LAPB Link Access Procedure - Balanced
LAPD Link Access Procedure on the D-channel
MFAS Multi-Frame Alignment Signal
NSAP Network Service Access Point
NT Network Termination
PCO Point of Control and Observation
PH Packet Handler
PICS Protocol Implementation Conformance Statement
PIXIT Protocol Implementation eXtra Information for Testing
PLL Pre-allocated Logical Link (former: Permanent Logical Link)
NOTE: The terms PLL and Long-term-PLL have been defined in ETS 300 049 second edition.
The option of accessing the PH over the D-channel using PLL or Long-term-PLL is
covered by this TBR, although the terms themselves are not further used.
ppm parts per million
PSPDN Public Switched Packet Data Network
REJ REJect
Ri Reference number
RNR Receive Not Ready
RR Receive Ready
Rx Receive
...