ETSI ETS 300 125 ed.1 (1991-09)
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
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
T/S 46-20 [CC]
Digitalno omrežje z integriranimi storitvami (ISDN) - Specifikacija podatkovne povezovalne plasti vmesnika uporabnik-omrežje - Aplikacija priporočil CCIT Q.920/I.440 in Q.921/I.441
General Information
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Integrated Services Digital Network (ISDN); User-network interface data link layer specification; Application of CCITT Recommendations Q.920/I.440 and Q.921/I.44135.100.20Podatkovni povezovalni slojData link layer33.080Digitalno omrežje z integriranimi storitvami (ISDN)Integrated Services Digital Network (ISDN)ICS:Ta slovenski standard je istoveten z:ETS 300 125 Edition 1PSIST ETS 300 125:1996en01-oktober-1996PSIST ETS 300 125:1996SLOVENSKI
STANDARD
PSIST ETS 300 125:1996
EUROPEANETS 300 125TELECOMMUNICATIONSeptember 1991STANDARDSource: ETSI TC-SPSReference: T/S 46-20 [CC]ICS:33.080Key words:ISDN, data link layer, Q.920, Q.921Integrated Services Digital Network (ISDN);User-network interface data link layer specification;Application of CCITT Recommendations Q.920/I.440and Q.921/I.441ETSIEuropean Telecommunications Standards InstituteETSI SecretariatPostal address: F-06921 Sophia Antipolis CEDEX - FRANCEOffice address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCEX.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.frTel.: +33 92 94 42 00 - Fax: +33 93 65 47 16Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and theforegoing restriction extend to reproduction in all media.© European Telecommunications Standards Institute 1991. All rights reserved.PSIST ETS 300 125:1996
Page 2ETS 300 125: September 1991Whilst 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 Standards Approval Dept." at the address shown on the title page.PSIST ETS 300 125:1996
Page 3ETS 300 125: September 1991ContentsForeword.9Scope.9PART 1: Application of CCITT Recomendation Q.920(I.440).11ISDN user-network interface data link layer - General aspects.111General.112Concepts and terminology.113Overview description of LAPD functions and procedures.153.1General.153.2Unacknowledged operation.173.3Acknowledged operation.173.4Establishment of information transfer modes.173.4.1Data link connection identification.173.4.2Data link states.183.4.3TEI administration.193.4.4Establishment of multiple frame operation.204Service characteristics.204.1General.204.2Services provided to layer 3.204.2.1Unacknowledged information transfer service.204.2.2Acknowledged information transfer service.204.3Services provided to layer management.214.4Administrative services.224.5Model of the data link service:.224.5.1General.224.5.2Data link layer representation as seen by layer 3.234.5.2.1Data Link Connection Endpoint States.234.5.2.2Broadcast data link layer connection services.234.5.2.3Point-to-point data link connection endpoint services.234.5.2.4Sequences of primitives at one point-to-point data linkconnection endpoint.234.6Services required from the physical layer.245Data link layer - Management structure.245.1Data link procedure.265.2Multiplex procedure.265.3Structure of the data link procedure.265.4Additions for European networks.275.4.1General.275.4.2Enhanced functional block diagram and block interaction diagram.27References (used in Q.920).31PART 2: Application of CCITT Recomendation Q.921(I.441).32ISDN user-network interface - Data link layer specification.321General.322Frame structure for peer-to-peer communication.32PSIST ETS 300 125:1996
Page 4ETS 300 125: September 19912.1General.322.2Flag sequence.322.3Address field.322.4Control field.332.5Information field.332.6Transparency.332.7FCS field.332.8Format convention.342.8.1Numbering Convention.342.8.2Order of bit transmission.342.8.3Field mapping convention.342.9Invalid frames.352.10Frame abort.353Elements of procedures and formats of fields for data link layer peer-to-peer communication.353.1General.353.2Address field format.353.3Address field variables.363.3.1Address field extension bit (EA).363.3.2Command/Response field bit (C/R).363.3.3Service Access Point Identifier (SAPI).363.3.4Terminal Endpoint Identifier (TEI).373.3.4.1TEI for broadcast data link connection.373.3.4.2TEI for point-to-point data link connection.373.4Control field formats.373.4.1Information transfer (I) format.383.4.2Supervisory (S) format.383.4.3Unnumbered (U) format.383.5Control field parameters and associated state variables.383.5.1Poll/Final bit.383.5.2Multiple frame operation - variables and sequence numbers.393.5.2.1Modulus.393.5.2.2Send state variable V(S).393.5.2.3Acknowledge state variable V(A).393.5.2.4Send sequence number N(S).393.5.2.5Receive state variable V(R).393.5.2.6Receive sequence number N(R).393.5.3Unacknowledged operation - variables and parameters.393.6Frame types.393.6.1Commands and responses.393.6.2Information (I) command.403.6.3Set asynchronous balanced mode extended (SABME) command.403.6.4DISConnect (DISC) command.413.6.5Unnumbered Information (UI) command.413.6.6Receive Ready (RR) command/response.413.6.7REJect (REJ) command/response.413.6.8Receive Not Ready (RNR) command/response.413.6.9Unnumbered Acknowledgement (UA) response.423.6.10Disconnected Mode (DM) response.423.6.11Frame reject (FRMR) response.423.6.12Exchange identification (XID) command/response.434Elements for layer-to-layer communication.444.1General.444.1.1Generic names.444.1.1.1DL-ESTABLISH.444.1.1.2DL-RELEASE.444.1.1.3DL-DATA.444.1.1.4DL-UNIT DATA.444.1.1.5MDL-ASSIGN.444.1.1.6MDL-REMOVE.454.1.1.7MDL-ERROR.454.1.1.8MDL-UNIT DATA.45PSIST ETS 300 125:1996
Page 5ETS 300 125: September 19914.1.1.9MDL-XID.454.1.1.10PH-DATA.454.1.1.11PH-ACTIVATE.454.1.1.12PH-DEACTIVATE.454.1.1.13MPH-ACTIVATE (See Appendix III).454.1.1.14MPH-DEACTIVATE (See Appendix III).454.1.1.15MPH-INFORMATION.454.1.2Primitive types.454.1.2.1REQUEST.464.1.2.2INDICATION.464.1.2.3RESPONSE.464.1.2.4CONFIRM.464.1.3Parameter definition.464.1.3.1Priority indicator.464.1.3.2Message unit.464.2Primitive procedures.474.2.1General.474.2.2Layer 3 - data link layer interactions.475Definition of the peer-to-peer procedures of the data link layer.495.1Procedure for the use of the P/F bit.505.1.1Unacknowledged information transfer.505.1.2Acknowledged multiple frame information transfer.505.2Procedures for unacknowledged information transfer.505.2.1General.505.2.2Transmision of unacknowledged information.515.2.3Receipt of unacknowledged information.515.3Terminal Endpoint Identifier (TEI) management procedures.515.3.1General.515.3.2TEI assignment procedure.525.3.2.1Expiry of timer T202.545.3.3TEI check procedure.545.3.3.1Use of the TEI check procedure.545.3.3.2Operation of the TEI check procedure.555.3.4TEI removal procedure.565.3.4.1Action taken by the data link layer entity receiving theMDL-REMOVE-REQUEST primitive.565.3.4.2Conditions for TEI removal.565.3.5TEI identity verify procedure.575.3.5.1General.575.3.5.2Operation of the TEI identity verify procedure.575.3.5.3Expiry of timer T202.585.3.6Formats and codes.585.3.6.1General.585.3.6.2Layer management entity identifier.595.3.6.3Reference number (Ri).595.3.6.4Message type.595.3.6.5Action indicator (Ai).595.4Automatic negotiation of data link layer parameters.605.5Procedures for establishment and release of multiple frame operation.605.5.1Establishment of multiple frame operation.605.5.1.1General.605.5.1.2Establishment procedures.605.5.1.3Procedure on expiry of timer T200.615.5.2Information transfer.615.5.3Termination of multiple frame operation.615.5.3.1General.615.5.3.2Release procedure.615.5.3.3Procedure on expiry of timer T200.625.5.4TEI-assigned state.625.5.5Collision of unnumbered commands and responses.625.5.5.1Identical transmitted and received commands.625.5.5.2Different transmitted and received commands.63PSIST ETS 300 125:1996
Page 6ETS 300 125: September 19915.5.6Unsolicited DM response and SABME or DISC command.635.6Procedures for information transfer in multiple frame operation.635.6.1Transmitting I frames.635.6.2Receiving I frames.635.6.2.1P bit set to 1.635.6.2.2P bit set to 0.645.6.3Sending and receiving acknowledgements.645.6.3.1Sending acknowledgements.645.6.3.2Receiving acknowledgements.645.6.4Receiving REJ frames.645.6.5Receiving RNR frames.655.6.6Data link layer own receiver busy condition.675.6.7Waiting acknowledgement.675.7Re-establishment of multiple frame operation.685.7.1Criteria for re-establishment.685.7.2Procedures.685.8Exception condition reporting and recovery.695.8.1N(S) sequence error.695.8.2N(R) sequence error.695.8.3Timer recovery condition.695.8.4Invalid frame condition.705.8.5Frame rejection condition.705.8.6Receipt of an FRMR response frame.705.8.7Unsolicited response frames.705.8.8Multiple-assignment of TEI value.715.9List of system parameters.715.9.1Timer T200.715.9.2Maximum number of retransmissions (N200).715.9.3Maximum number of octets in an information field (N201).725.9.4Maximum number of transmission of the TEI identity request message(N202).725.9.5Maximum number of outstanding I frames (k).725.9.6Timer T201.725.9.7Timer T202.725.9.8Timer T203.725.10Data link layer monitor function.735.10.1General.735.10.2Data link layer supervision in the multiple-frame-established state.735.10.3Connection verification procedures.735.10.3.1Start timer T203.735.10.3.2Stop Timer T203.745.10.3.3Expiry of timer T203.74Annex A (to Recommendation Q.921) Provision of point-to-point signalling connections.75Annex B (to Recommendation Q.921) SDL for point-to-point procedures.76B.1General.76B.2An overview of the states of the point-to-point data link layer entity.76B.3Cover notes.79B.4The use of queues.80B.5SDL representation.81Annex C (to Recommendation Q.921) An SDL representation of the broadcast procedures of the datalink layer.109Annex D (to Recommendation Q.921) State transition table of the point-to-point procedures of the datalink layer.110PSIST ETS 300 125:1996
Page 7ETS 300 125: September 1991D.1State transition table.110D.2Key to the state transition table.111D.2.1Definition of a cell of the state transition table.111D.2.2Key to the contents of a cell.111Appendix I (to Recommendation Q.921) Retransmission of REJ response frames.143I.1Introduction.143I.2Procedure.143Appendix II (to Recommendation Q.921) Occurence of MDL-ERROR-INDICATION within the basicstates and actions to be taken by the management entity.144II.1 Introduction.144II.2 Layout of table II-1/Q.921.144II.3 Preferred management actions.144Appendix III (to Recommendation Q.921) Optional basic access deactivation procedures.146III.1 Introduction.146III.2 Description of the conceptual model.146III.3 Deactivation procedure with MPH-DEACTIVATE-INDICATION.147III.4 Deactivation procedure without MPH-DEACTIVATE-INDICATION.149Appendix IV (to Recommendation Q.921) Automatic negotiation of data link layer parameters.150IV.1 General.150IV.2 Parameter initialisation.150IV.3 Internal parameter initialisation.150IV.4 Automatic notification of data link layer parameter values.150History.153PSIST ETS 300 125:1996
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Page 9ETS 300 125: September 1991ForewordThis European Telecommunication Standard (ETS) has been produced by the Signalling, Protocols andSwitching (SPS) Technical Committee of the European Telecommunications Standard Institute (ETSI),and was adopted, having passed through the ETSI standards approval procedure.This ETS is a revised version of CEPT Recommendation T/CS 46-20 (Edition of December 15, 1987;Revision 1988). It is based on the CCITT Recommendations Q.920/I.440 and Q.921/I.441 versionsaccording to the BLUE BOOK, Vol. VI, Fascicle VI.10. It is intended to reduce the number of options whichexist within CCITT Recommendations Q.920/I.440 and Q.921/I.441 and to provide the additionalspecification text necessary for clarification as well as to ensure harmonisation of the ISDN user-networkinterface within Europe.All procedures at the ISDN user-network interface apply to both, the S and T reference points.CCITT Recommendations Q.920/I.440 and Q.921/I.441 apply with the modifications specified below. Themodifications are presented based on the CCITT Recommendations according to the BLUE BOOK, Vol.VI, Fascicle VI.10.The following editorial conventions have been applied:-the layout of this standard is aligned with CCITT Recommendations Q.920/I.440 and Q.921/I.441according to the BLUE BOOK, Vol. VI, Fascicle VI.10, exept modifications which require additionalsections or figures and tables;-modifications made for the sake of consistency and clarification are indicated by a revision bar inthe left margin;-modifications to select options for networks are indicated by a revision bar in the left margin orencircled by asterisks;-editorial modifications are indicated by a revision bar in the right margin.In addition this standard is based on the following considerations:(a)CCITT Recommendations Q.920/I.440 and Q.921/I.441 define the ISDN user-network interfacedata link layer;(b)there are a number of options and points requiring further specification in RecommendationsQ.920/I.440 and Q.921/I.441;(c)the harmonisation of the ISDN user-network interface is an important requirement for Europeannetwork operators;(d)European network operators who wish to provide ISDN services should apply the CCITTRecommendations Q.920/I.440 and Q.921/I.441 in accordance with the specification defined below.NOTE:Some references in this standard reflect the fact that the text was initially produced inCEPT working groups. For practical reasons it has not always been possible to re-editthe text appropriately at this stage.ScopeThis standard specifies the user-network interface data link layer of the pan European Integrated ServicesDigital Network (ISDN) as provided by European public telecommunication operators at the T referencepoint or coincident S and T reference point (as defined in CCITT Recommendation I.411 [13] by means ofDigital Subscriber Signalling System No. 1 (DSS1).PSIST ETS 300 125:1996
Page 10ETS 300 125: September 1991Blank pagePSIST ETS 300 125:1996
Page 11ETS 300 125: September 1991PART 1: Application of CCITT Recomendation Q.920(I.440)ISDN user-network interface data link layer - General aspects1GeneralThis Recommendation describes in general terms the Link Access Procedure on the D-channel, LAPD.The application of this protocol to other channel types is for further study. Details are provided inRecommendation Q.921(I.441) [1].The purpose of LAPD is to convey information between layer 3 entities across the ISDN user-networkinterface using the D-channel.The definition of LAPD takes into consideration the principles and terminology of:-Recommendations X.200 [2] and X.210 [3] - the reference model and layer service conventions forOpen Systems Interconnection (OSI);-Recommendation X.25 [4] - LAPB user-network interface for packet mode terminals; and-ISO 3309 [5] and ISO 4335 [6] - High-level Data Link Control (HDLC) standards for frame structureand elements of procedures.LAPD is a protocol that operates at the data link layer of the OSI architecture. The relationship betweenthe data link layer and other protocol layers is defined in Recommendation I.320 [7].NOTE 1:The physical layer is currently defined in Recommendations I.430 [8] and I.431 [9], andprETS 300 012 [14] and prETS 300 011 [15], respectively, and layer 3 is defined inRecommendations Q.930 (I.450) [10], Q.931 (I.451) [11] and ETS 300 102-1 [16], ETS300 102-2 [17], and X.25 [4]. References should be made to these Recommendationsfor the complete definition of the protocols and procedures across the ISDN user-network interface.NOTE 2:The term "data link layer" is used in the main text of this Recommendation. However,mainly in figures and tables, the terms "layer 2" and "L2" are used as abbreviations.Furthermore, in accordance with Recommendations Q.930 (I.450) [10] and Q.931(I.451) [11], and ETS 300 102-1 [16] and ETS 300 102-2 [17], the term "layer 3" isused to indicate the layer above the data link layer.LAPD is independent of transmission bit rate. It requires a duplex, bit transparent D-channel.The characteristics of the D-channel are defined in Recommendation I.412 [12].§ 2 below describes basic concepts used in this Recommendation andRecommendation Q.921.§ 3 gives an overview description of LAPD functions and procedures.§ 4 summarises the services that the data link layer provides to layer 3 and theservices that the data link layer requires from the physical layer.§ 5 provides an overview of the data link layer structure.2Concepts and terminologyThe basic structuring technique in the OSI reference model is layering. According to this technique,communication among application processes is viewed as being logically partitioned into an ordered set oflayers represented in a vertical sequence as shown in figure 1/Q.920.A data link layer Service Access Point (SAP) is the point at which the data link layer provides services tolayer 3. Associatied with each data link layer SAP is one or more data link connection endpoint(s). SeePSIST ETS 300 125:1996
Page 12ETS 300 125: September 1991figure 2/Q.920. A data link connection endpoint is identified by a data link connection endpoint identifier asseen from layer 3 and by a Data Link Connection Identifier (DLCI) as seen from data link layer.Entities exist in each layer. Entities in the same layer, but in different systems which must exchangeinformation to achieve a common objective are called "peer entities". Entities in adjacent layers interactthrough their common boundary. The services provided by the data link layer are the combination of theservices and functions provided by both the data link layer and the physical layer.System managementLM Layer management (see figure 10/Q.920)Highest layer(N+1) layer(N) layer(N-1) layerLowest layerLMLMLMLMLMFigure 1/Q.920: LayeringLayer 3Data link layerLayer 3entityData link layerentityData link layer serviceaccess pointData link connectionendpoint*Figure 2/Q.920: Entities, service access points and endpointsCooperation between data link layer entities is governed by a peer-to-peer protocol specific to the layer. Inorder for information to be exchanged between two or more layer 3 entities, an association must beestablished between the layer 3 entities in the data link layer using a data link layer protocol. Thisassociation is called a data link connection. Data link connections are provided by the data link layerbetween two or more SAPs (see figure 3/Q,920).PSIST ETS 300 125:1996
Page 13ETS 300 125: September 1991**Layer 3entityLayer 3entityData link layerservice accesspointData linkconnectionendpointData link connectionFigure 3/Q.920: Peer-to-peer relationshipData link layer message units are conveyed between data link layer entities by means of a physicalconnection.Layer 3 requests services from the data link layer via service primitives. The same applies for theinteraction between the data link layer and the physical layer. The primitives represent, in an abstract way,the logical exchange of information and control between the data link layer and adjacent layers. They donot specify or constrain implementation.The primitives that are exchanged between the data link layer and adjacent layers are of the following fourtypes (see also figure 4/Q.920):a)REQUEST;b)INDICATION;c)RESPONSE; andd)CONFIRM.Layer 3CONFIRMREQUESTINDICATIONRESPONSESAPData link layerData link layer
peer - to - peer
protocol
**NOTE 1:The same principle applies for data link layer - physical layer interactions.Figure 4/Q.920: Primitive action sequence:PSIST ETS 300 125:1996
Page 14ETS 300 125: September 1991The REQUEST primitive type is used when a higher layer is requesting a service from the next lowerlayer.The INDICATION primitive type is used by a layer providing a service to notify the next higher layer of anyspecific activity which is service related. The INDICATION primitive may be the result of an activity of thelower layer related to the primitive type REQUEST at the peer entity.The RESPONSE primitive type is used by a layer to acknowledge receipt, from a lower layer, of theprimitive type INDICATION.The CONFIRM primitive type is used by the layer providing the requested service to confirm that theactivity has been completed.Layer-to-layer interactions are specified in Recommendation Q.921.Information is transferred, in various types of message units, between peer entities and between entities inadjacent layers that are attached to a specific SAP. The message units are of two types:-message units of a peer-to-peer protocol; and-message units that contain layer-to-layer information concerning status and specialised servicerequests.The message units of the layer 3 peer-to-peer protocol are carried by the data link connection. Themessage units containing layer-to-layer information concerning status and specialised service requestsare never conveyed over a data link connection or a physical connection.This Recommendation specifies (see also figure 5/Q.920):a)the peer-to-peer protocol for the transfer of information and control between any pair of data linklayer service access points; andb)the interactions between the data link layer and layer 3, and between the data link layer and thephysical layer.*Layer 3
entity
Layer 3
entityData link
layer entity
Data linklayer entityPhysical
layer entity
Physicallayer entity
Data link layer
service access point
Data link layer
Peer-to-peer protocol
Physical layer
service access point
Physical connection
Data link layer /layer 3
primatives
Data link layer /
physical layer
primatives
***Figure 5/Q.929: Data link layer reference modelPSIST ETS 300 125:1996
Page 15ETS 300 125: September 19913Overview description of LAPD functions and procedures3.1GeneralThe purpose of LAPD is to convey information between layer 3 entities across the ISDN user-networkinterface using the D-channel. Specifically LAPD will support:-multiple terminal installations at the user-network interface;-multiple layer 3 entities.All data link layer messages are transmitted in frames which are delimited by flags. (A flag is a unique bitpattern.) The frame structure is defined in Recommendation Q.921.LAPD includes functions for:a)the provision of one or more data link connections on a D-channel. Discrimination between the datalink connnections is by means of a data link connection identifier (DLCI) contained in each frame;b)frame delimiting, alignment and transparency, allowing recognition of a sequence of bits transmittedover a D-channel as a frame;c)sequence control, to maintain the sequential order of frames across a data link connection;d)detection of transmission, format and operational errors on a data link connection;e)recovery from detected transmission, format, and operational errors;f)notification to the management entitiy of unrecoverable errors; andg)flow control.Data link layer functions provide the means for information transfer between multiple combinations of datalink connection endpoints. The information transfer may be via point-to-point data link connections or viabroadcast data link connections. In the case of point-to-point information transfer, a frame is directed to asingle endpoint, while in the case of broadcast information transfer, a frame is directed to one or moreendpoints.Figure 6/Q.920 shows three examples of point-to-point information transfer. Figure 7/Q.920 shows anexample of broadcast information transfer.PSIST ETS 300 125:1996
Page 16ETS 300 125: September 1991*********** ***********User sideNetwork sideTETE(1)ET/NT2
DatalinklayerTEET/NT2Layer 3
TE (2)ET/NT2SAPx(a)Layer 3
DatalinklayerTE(1)TE(2)ET/NT2SAPx(b)TE(1)TE(2)ET/NT2TE(1)TE(2)ET/NT2SAPxSAPxLayer3
Datalinklayer(c)Physical connection
Data link connection
ET
Exchange termination
TE
Terminal equipment
NT
Network termination
SAP
Service access point
Figure 6/Q.920: Point-to-point data link connectionsPSIST ETS 300 125:1996
Page 17ETS 300 125: September 1991********User side
Network side
TE(1)
TE(2)ET/NT2
Layer 3 Data
link
layer
B BBTE(1)TE(2)ET/NT2 SAPxSAPxSAPxPhysical connection
Data link connection
B = Broadcast
Figure 7/Q.920: Broadcast data link connectionTwo types of operation of the data link layer are defined for layer 3 information transfer: unacknowledgedand acknowledged. They may coexist on a single D-channel.3.2Unacknowledged operationWith this type of operation layer 3 information is transmitted in Unnumbered Information (UI) frames.At the data link layer the UI frames are not acknowledged. Even if transmission and format errors aredetected, no error recovery mechanism is defined. Flow control mechanisms are not defined.Unacknowledged operation is applicable for point-to-point and broadcast information transfer; that is, a UIframe may be sent to a specific endpoint or broadcast to multiple endpoints associated with a specificService Access Point Identifier (SAPI).3.3Acknowledged operationWith this type of operation, layer 3 information is transmitted in frames that are acknowledged at the datalink layer.Error recovery procedures based on retransmission of unacknowledged frames are specified. In the caseof errors which cannot be corrected by the data link layer, a report to the management entity is made.Flow control procedures are also defined.Acknowledged operation is applicable for point-to-point information transfer. One form of acknowledgedinformation transfer is defined, multiple frame operation.Layer 3 information is sent in numbered Information (I) frames. A number of I frames may be outstandingat the same time. Multiple frame operation is initiated by a multiple frame establishment procedure using aSet Asynchronous Balanced Mode Extended (SABME) command.3.4Establishment of information transfer modes3.4.1Data link connection identificationA data link connection is identified by a Data Link Connection Identifier (DLCI) carried in the address fieldof each frame.The data link connection identifier is associated with a connection endpoint identifier at the two ends of thedata link connection (see figure 8/Q.920).PSIST ETS 300 125:1996
Page 18ETS 300 125: September 1991The connection endpoint identifier is used to identify message units passed between the data link layerand layer 3. It consists of the SAPI and the Connection Endpoint Suffix (CES).The DLCI consists of two elements: the SAPI and the Terminal Endpoint Identifier (TEI).The SAPI is used to identify the service access point on the network side or the user side of the user-network interface.The TEI is used to identify a specific connection endpoint within a service access point.The TEI is assigned by the network, if the user equipment is of the automatic TEI assignment category, orit is entered into the user equipment, for example, by the user or the manufacturer, if the user equipmentis of the non-automatic TEI assignment category (see § 3.4.3).The DLCI is a pure data link layer concept. It will be internally used by the data link layer entitiy and is notknown by the layer 3 entity or management entity. In these latter entities, the concept of ConnectionEndpoint Identifier (CEI) will be used instead.The CEI is composed of the SAPI information and a reference value named CES. The CES is a valueselected by the layer 3 or management entity to address the data link layer entity. When the relevant TEIis known
...
SLOVENSKI STANDARD
01-GHFHPEHU-1996
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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
Ta slovenski standard je istoveten z: ETS 300 125 Edition 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
35.100.20 Podatkovni povezovalni sloj Data link layer
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
PSIST ETS 300 125:1996
PSIST ETS 300 125:1996
EUROPEAN ETS 300 125
TELECOMMUNICATION September 1991
STANDARD
Source: ETSI TC-SPS Reference: T/S 46-20 [CC]
ICS: 33.080
Key words: ISDN, data link layer, Q.920, Q.921
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
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 92 94 42 00 - Fax: +33 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 1991. All rights reserved.
PSIST ETS 300 125:1996
Page 2
ETS 300 125: September 1991
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 Standards Approval Dept." at the address shown on the title page.
PSIST ETS 300 125:1996
Page 3
ETS 300 125: September 1991
Contents
Foreword .9
Scope .9
PART 1: Application of CCITT Recomendation Q.920(I.440) .11
ISDN user-network interface data link layer - General aspects.11
1 General.11
2 Concepts and terminology.11
3 Overview description of LAPD functions and procedures .15
3.1 General .15
3.2 Unacknowledged operation .17
3.3 Acknowledged operation.17
3.4 Establishment of information transfer modes .17
3.4.1 Data link connection identification.17
3.4.2 Data link states.18
3.4.3 TEI administration .19
3.4.4 Establishment of multiple frame operation.20
4 Service characteristics .20
4.1 General .20
4.2 Services provided to layer 3.20
4.2.1 Unacknowledged information transfer service .20
4.2.2 Acknowledged information transfer service .20
4.3 Services provided to layer management.21
4.4 Administrative services .22
4.5 Model of the data link service: .22
4.5.1 General.22
4.5.2 Data link layer representation as seen by layer 3.23
4.5.2.1 Data Link Connection Endpoint States.23
4.5.2.2 Broadcast data link layer connection services.23
4.5.2.3 Point-to-point data link connection endpoint services .23
4.5.2.4 Sequences of primitives at one point-to-point data link
connection endpoint.23
4.6 Services required from the physical layer.24
5 Data link layer - Management structure .24
5.1 Data link procedure.26
5.2 Multiplex procedure.26
5.3 Structure of the data link procedure.26
5.4 Additions for European networks.27
5.4.1 General.27
5.4.2 Enhanced functional block diagram and block interaction diagram .27
References (used in Q.920) .31
PART 2: Application of CCITT Recomendation Q.921(I.441) .32
ISDN user-network interface - Data link layer specification .32
1 General.32
2 Frame structure for peer-to-peer communication .32
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2.1 General. 32
2.2 Flag sequence. 32
2.3 Address field. 32
2.4 Control field . 33
2.5 Information field. 33
2.6 Transparency . 33
2.7 FCS field. 33
2.8 Format convention .34
2.8.1 Numbering Convention. 34
2.8.2 Order of bit transmission . 34
2.8.3 Field mapping convention. 34
2.9 Invalid frames. 35
2.10 Frame abort. 35
3 Elements of procedures and formats of fields for data link layer peer-to-peer communication . 35
3.1 General. 35
3.2 Address field format . 35
3.3 Address field variables .36
3.3.1 Address field extension bit (EA). 36
3.3.2 Command/Response field bit (C/R). 36
3.3.3 Service Access Point Identifier (SAPI). 36
3.3.4 Terminal Endpoint Identifier (TEI). 37
3.3.4.1 TEI for broadcast data link connection. 37
3.3.4.2 TEI for point-to-point data link connection . 37
3.4 Control field formats. 37
3.4.1 Information transfer (I) format. 38
3.4.2 Supervisory (S) format. 38
3.4.3 Unnumbered (U) format. 38
3.5 Control field parameters and associated state variables . 38
3.5.1 Poll/Final bit . 38
3.5.2 Multiple frame operation - variables and sequence numbers. 39
3.5.2.1 Modulus. 39
3.5.2.2 Send state variable V(S) . 39
3.5.2.3 Acknowledge state variable V(A) . 39
3.5.2.4 Send sequence number N(S) . 39
3.5.2.5 Receive state variable V(R) . 39
3.5.2.6 Receive sequence number N(R). 39
3.5.3 Unacknowledged operation - variables and parameters . 39
3.6 Frame types . 39
3.6.1 Commands and responses. 39
3.6.2 Information (I) command . 40
3.6.3 Set asynchronous balanced mode extended (SABME) command. 40
3.6.4 DISConnect (DISC) command . 41
3.6.5 Unnumbered Information (UI) command. 41
3.6.6 Receive Ready (RR) command/response . 41
3.6.7 REJect (REJ) command/response . 41
3.6.8 Receive Not Ready (RNR) command/response. 41
3.6.9 Unnumbered Acknowledgement (UA) response . 42
3.6.10 Disconnected Mode (DM) response . 42
3.6.11 Frame reject (FRMR) response. 42
3.6.12 Exchange identification (XID) command/response . 43
4 Elements for layer-to-layer communication .44
4.1 General. 44
4.1.1 Generic names . 44
4.1.1.1 DL-ESTABLISH. 44
4.1.1.2 DL-RELEASE. 44
4.1.1.3 DL-DATA. 44
4.1.1.4 DL-UNIT DATA . 44
4.1.1.5 MDL-ASSIGN. 44
4.1.1.6 MDL-REMOVE. 45
4.1.1.7 MDL-ERROR . 45
4.1.1.8 MDL-UNIT DATA . 45
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4.1.1.9 MDL-XID.45
4.1.1.10 PH-DATA.45
4.1.1.11 PH-ACTIVATE.45
4.1.1.12 PH-DEACTIVATE.45
4.1.1.13 MPH-ACTIVATE (See Appendix III) .45
4.1.1.14 MPH-DEACTIVATE (See Appendix III) .45
4.1.1.15 MPH-INFORMATION .45
4.1.2 Primitive types .45
4.1.2.1 REQUEST .46
4.1.2.2 INDICATION.46
4.1.2.3 RESPONSE.46
4.1.2.4 CONFIRM.46
4.1.3 Parameter definition .46
4.1.3.1 Priority indicator .46
4.1.3.2 Message unit .46
4.2 Primitive procedures .47
4.2.1 General.47
4.2.2 Layer 3 - data link layer interactions.47
5 Definition of the peer-to-peer procedures of the data link layer .49
5.1 Procedure for the use of the P/F bit.50
5.1.1 Unacknowledged information transfer.50
5.1.2 Acknowledged multiple frame information transfer .50
5.2 Procedures for unacknowledged information transfer.50
5.2.1 General.50
5.2.2 Transmision of unacknowledged information.51
5.2.3 Receipt of unacknowledged information .51
5.3 Terminal Endpoint Identifier (TEI) management procedures.51
5.3.1 General.51
5.3.2 TEI assignment procedure .52
5.3.2.1 Expiry of timer T202 .54
5.3.3 TEI check procedure .54
5.3.3.1 Use of the TEI check procedure.54
5.3.3.2 Operation of the TEI check procedure .55
5.3.4 TEI removal procedure.56
5.3.4.1 Action taken by the data link layer entity receiving the
MDL-REMOVE-REQUEST primitive .56
5.3.4.2 Conditions for TEI removal.56
5.3.5 TEI identity verify procedure.57
5.3.5.1 General.57
5.3.5.2 Operation of the TEI identity verify procedure .57
5.3.5.3 Expiry of timer T202 .58
5.3.6 Formats and codes .58
5.3.6.1 General.58
5.3.6.2 Layer management entity identifier.59
5.3.6.3 Reference number (Ri).59
5.3.6.4 Message type .59
5.3.6.5 Action indicator (Ai).59
5.4 Automatic negotiation of data link layer parameters.60
5.5 Procedures for establishment and release of multiple frame operation .60
5.5.1 Establishment of multiple frame operation.60
5.5.1.1 General.60
5.5.1.2 Establishment procedures.60
5.5.1.3 Procedure on expiry of timer T200 .61
5.5.2 Information transfer .61
5.5.3 Termination of multiple frame operation.61
5.5.3.1 General.61
5.5.3.2 Release procedure .61
5.5.3.3 Procedure on expiry of timer T200 .62
5.5.4 TEI-assigned state .62
5.5.5 Collision of unnumbered commands and responses .62
5.5.5.1 Identical transmitted and received commands .62
5.5.5.2 Different transmitted and received commands.63
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5.5.6 Unsolicited DM response and SABME or DISC command . 63
5.6 Procedures for information transfer in multiple frame operation. 63
5.6.1 Transmitting I frames. 63
5.6.2 Receiving I frames. 63
5.6.2.1 P bit set to 1 . 63
5.6.2.2 P bit set to 0 . 64
5.6.3 Sending and receiving acknowledgements . 64
5.6.3.1 Sending acknowledgements . 64
5.6.3.2 Receiving acknowledgements . 64
5.6.4 Receiving REJ frames . 64
5.6.5 Receiving RNR frames . 65
5.6.6 Data link layer own receiver busy condition. 67
5.6.7 Waiting acknowledgement. 67
5.7 Re-establishment of multiple frame operation . 68
5.7.1 Criteria for re-establishment . 68
5.7.2 Procedures . 68
5.8 Exception condition reporting and recovery . 69
5.8.1 N(S) sequence error . 69
5.8.2 N(R) sequence error. 69
5.8.3 Timer recovery condition . 69
5.8.4 Invalid frame condition. 70
5.8.5 Frame rejection condition . 70
5.8.6 Receipt of an FRMR response frame. 70
5.8.7 Unsolicited response frames . 70
5.8.8 Multiple-assignment of TEI value. 71
5.9 List of system parameters. 71
5.9.1 Timer T200 . 71
5.9.2 Maximum number of retransmissions (N200) . 71
5.9.3 Maximum number of octets in an information field (N201). 72
5.9.4 Maximum number of transmission of the TEI identity request message
(N202). 72
5.9.5 Maximum number of outstanding I frames (k). 72
5.9.6 Timer T201 . 72
5.9.7 Timer T202 . 72
5.9.8 Timer T203 . 72
5.10 Data link layer monitor function. 73
5.10.1 General . 73
5.10.2 Data link layer supervision in the multiple-frame-established state. 73
5.10.3 Connection verification procedures . 73
5.10.3.1 Start timer T203 . 73
5.10.3.2 Stop Timer T203 . 74
5.10.3.3 Expiry of timer T203. 74
Annex A (to Recommendation Q.921) Provision of point-to-point signalling connections. 75
Annex B (to Recommendation Q.921) SDL for point-to-point procedures . 76
B.1 General. 76
B.2 An overview of the states of the point-to-point data link layer entity. 76
B.3 Cover notes . 79
B.4 The use of queues. 80
B.5 SDL representation. 81
Annex C (to Recommendation Q.921) An SDL representation of the broadcast procedures of the data
link layer . 109
Annex D (to Recommendation Q.921) State transition table of the point-to-point procedures of the data
link layer . 110
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D.1 State transition table.110
D.2 Key to the state transition table .111
D.2.1 Definition of a cell of the state transition table .111
D.2.2 Key to the contents of a cell.111
Appendix I (to Recommendation Q.921) Retransmission of REJ response frames .143
I.1 Introduction.143
I.2 Procedure.143
Appendix II (to Recommendation Q.921) Occurence of MDL-ERROR-INDICATION within the basic
states and actions to be taken by the management entity.144
II.1 Introduction.144
II.2 Layout of table II-1/Q.921 .144
II.3 Preferred management actions.144
Appendix III (to Recommendation Q.921) Optional basic access deactivation procedures.146
III.1 Introduction.146
III.2 Description of the conceptual model .146
III.3 Deactivation procedure with MPH-DEACTIVATE-INDICATION .147
III.4 Deactivation procedure without MPH-DEACTIVATE-INDICATION .149
Appendix IV (to Recommendation Q.921) Automatic negotiation of data link layer parameters .150
IV.1 General .150
IV.2 Parameter initialisation.150
IV.3 Internal parameter initialisation .150
IV.4 Automatic notification of data link layer parameter values.150
History.153
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Foreword
This European Telecommunication Standard (ETS) has been produced by the Signalling, Protocols and
Switching (SPS) Technical Committee of the European Telecommunications Standard Institute (ETSI),
and was adopted, having passed through the ETSI standards approval procedure.
This ETS is a revised version of CEPT Recommendation T/CS 46-20 (Edition of December 15, 1987;
Revision 1988). It is based on the CCITT Recommendations Q.920/I.440 and Q.921/I.441 versions
according to the BLUE BOOK, Vol. VI, Fascicle VI.10. It is intended to reduce the number of options which
exist within CCITT Recommendations Q.920/I.440 and Q.921/I.441 and to provide the additional
specification text necessary for clarification as well as to ensure harmonisation of the ISDN user-network
interface within Europe.
All procedures at the ISDN user-network interface apply to both, the S and T reference points.
CCITT Recommendations Q.920/I.440 and Q.921/I.441 apply with the modifications specified below. The
modifications are presented based on the CCITT Recommendations according to the BLUE BOOK, Vol.
VI, Fascicle VI.10.
The following editorial conventions have been applied:
- the layout of this standard is aligned with CCITT Recommendations Q.920/I.440 and Q.921/I.441
according to the BLUE BOOK, Vol. VI, Fascicle VI.10, exept modifications which require additional
sections or figures and tables;
- modifications made for the sake of consistency and clarification are indicated by a revision bar in
the left margin;
- modifications to select options for networks are indicated by a revision bar in the left margin or
encircled by asterisks;
- editorial modifications are indicated by a revision bar in the right margin.
In addition this standard is based on the following considerations:
(a) CCITT Recommendations Q.920/I.440 and Q.921/I.441 define the ISDN user-network interface
data link layer;
(b) there are a number of options and points requiring further specification in Recommendations
Q.920/I.440 and Q.921/I.441;
(c) the harmonisation of the ISDN user-network interface is an important requirement for European
network operators;
(d) European network operators who wish to provide ISDN services should apply the CCITT
Recommendations Q.920/I.440 and Q.921/I.441 in accordance with the specification defined below.
NOTE: Some references in this standard reflect the fact that the text was initially produced in
CEPT working groups. For practical reasons it has not always been possible to re-edit
the text appropriately at this stage.
Scope
This standard specifies the user-network interface data link layer of the pan European Integrated Services
Digital Network (ISDN) as provided by European public telecommunication operators at the T reference
point or coincident S and T reference point (as defined in CCITT Recommendation I.411 [13] by means of
Digital Subscriber Signalling System No. 1 (DSS1).
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PART 1: Application of CCITT Recomendation Q.920(I.440)
ISDN user-network interface data link layer - General aspects
1 General
This Recommendation describes in general terms the Link Access Procedure on the D-channel, LAPD.
The application of this protocol to other channel types is for further study. Details are provided in
Recommendation Q.921(I.441) [1].
The purpose of LAPD is to convey information between layer 3 entities across the ISDN user-network
interface using the D-channel.
The definition of LAPD takes into consideration the principles and terminology of:
- Recommendations X.200 [2] and X.210 [3] - the reference model and layer service conventions for
Open Systems Interconnection (OSI);
- Recommendation X.25 [4] - LAPB user-network interface for packet mode terminals; and
- ISO 3309 [5] and ISO 4335 [6] - High-level Data Link Control (HDLC) standards for frame structure
and elements of procedures.
LAPD is a protocol that operates at the data link layer of the OSI architecture. The relationship between
the data link layer and other protocol layers is defined in Recommendation I.320 [7].
NOTE 1: The physical layer is currently defined in Recommendations I.430 [8] and I.431 [9], and
prETS 300 012 [14] and prETS 300 011 [15], respectively, and layer 3 is defined in
Recommendations Q.930 (I.450) [10], Q.931 (I.451) [11] and ETS 300 102-1 [16], ETS
300 102-2 [17], and X.25 [4]. References should be made to these Recommendations
for the complete definition of the protocols and procedures across the ISDN user-
network interface.
NOTE 2: The term "data link layer" is used in the main text of this Recommendation. However,
mainly in figures and tables, the terms "layer 2" and "L2" are used as abbreviations.
Furthermore, in accordance with Recommendations Q.930 (I.450) [10] and Q.931
(I.451) [11], and ETS 300 102-1 [16] and ETS 300 102-2 [17], the term "layer 3" is
used to indicate the layer above the data link layer.
LAPD is independent of transmission bit rate. It requires a duplex, bit transparent D-
channel.
The characteristics of the D-channel are defined in Recommendation I.412 [12].
§ 2 below describes basic concepts used in this Recommendation and
Recommendation Q.921.
§ 3 gives an overview description of LAPD functions and procedures.
§ 4 summarises the services that the data link layer provides to layer 3 and the
services that the data link layer requires from the physical layer.
§ 5 provides an overview of the data link layer structure.
2 Concepts and terminology
The basic structuring technique in the OSI reference model is layering. According to this technique,
communication among application processes is viewed as being logically partitioned into an ordered set of
layers represented in a vertical sequence as shown in figure 1/Q.920.
A data link layer Service Access Point (SAP) is the point at which the data link layer provides services to
layer 3. Associatied with each data link layer SAP is one or more data link connection endpoint(s). See
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ETS 300 125: September 1991
figure 2/Q.920. A data link connection endpoint is identified by a data link connection endpoint identifier as
seen from layer 3 and by a Data Link Connection Identifier (DLCI) as seen from data link layer.
Entities exist in each layer. Entities in the same layer, but in different systems which must exchange
information to achieve a common objective are called "peer entities". Entities in adjacent layers interact
through their common boundary. The services provided by the data link layer are the combination of the
services and functions provided by both the data link layer and the physical layer.
System management
LM Highest layer
LM (N+1) layer
LM (N) layer
LM (N-1) layer
LM Lowest layer
LM Layer management (see figure 10/Q .920)
Figure 1/Q.920: Layering
Layer 3
Layer 3
entity
Data link layer service
access point
*
Data link connection
endpoint
Data link layer
Data link layer
entity
Figure 2/Q.920: Entities, service access points and endpoints
Cooperation between data link layer entities is governed by a peer-to-peer protocol specific to the layer. In
order for information to be exchanged between two or more layer 3 entities, an association must be
established between the layer 3 entities in the data link layer using a data link layer protocol. This
association is called a data link connection. Data link connections are provided by the data link layer
between two or more SAPs (see figure 3/Q,920).
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ETS 300 125: September 1991
Layer 3 Layer 3
entity entity
Data link layer
service access
point
**
Data link
connection
endpoint
Data link connection
Figure 3/Q.920: Peer-to-peer relationship
Data link layer message units are conveyed between data link layer entities by means of a physical
connection.
Layer 3 requests services from the data link layer via service primitives. The same applies for the
interaction between the data link layer and the physical layer. The primitives represent, in an abstract way,
the logical exchange of information and control between the data link layer and adjacent layers. They do
not specify or constrain implementation.
The primitives that are exchanged between the data link layer and adjacent layers are of the following four
types (see also figure 4/Q.920):
a) REQUEST;
b) INDICATION;
c) RESPONSE; and
d) CONFIRM.
Layer 3
CONFIRM INDICATION RESPONSE
REQUEST
**
SAP
Data link layer
Data link layer
p eer - to - peer
protocol
NOTE 1: The same principle applies for data link layer - physical layer interactions.
Figure 4/Q.920: Primitive action sequence:
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ETS 300 125: September 1991
The REQUEST primitive type is used when a higher layer is requesting a service from the next lower
layer.
The INDICATION primitive type is used by a layer providing a service to notify the next higher layer of any
specific activity which is service related. The INDICATION primitive may be the result of an activity of the
lower layer related to the primitive type REQUEST at the peer entity.
The RESPONSE primitive type is used by a layer to acknowledge receipt, from a lower layer, of the
primitive type INDICATION.
The CONFIRM primitive type is used by the layer providing the requested service to confirm that the
activity has been completed.
Layer-to-layer interactions are specified in Recommendation Q.921.
Information is transferred, in various types of message units, between peer entities and between entities in
adjacent layers that are attached to a specific SAP. The message units are of two types:
- message units of a peer-to-peer protocol; and
- message units that contain layer-to-layer information concerning status and specialised service
requests.
The message units of the layer 3 peer-to-peer protocol are carried by the data link connection. The
message units containing layer-to-layer information concerning status and specialised service requests
are never conveyed over a data link connection or a physical connection.
This Recommendation specifies (see also figure 5/Q.920):
a) the peer-to-peer protocol for the transfer of information and control between any pair of data link
layer service access points; and
b) the interactions between the data link layer and layer 3, and between the data link layer and the
physical layer.
Layer 3 Layer 3
entity entity
Data link layer
service access point
Data link layer /
layer 3
*
*
primatives
Data link layer
Peer-to-peer protocol
Data link Data link
layer entity layer entity
Physical layer
Data link layer / service access point
physical layer
* *
primatives
Physical Physical
layer entity layer entity
Physical connection
Figure 5/Q.929: Data link layer reference model
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3 Overview description of LAPD functions and procedures
3.1 General
The purpose of LAPD is to convey information between layer 3 entities across the ISDN user-network
interface using the D-channel. Specifically LAPD will support:
- multiple terminal installations at the user-network interface;
- multiple layer 3 entities.
All data link layer messages are transmitted in frames which are delimited by flags. (A flag is a unique bit
pattern.) The frame structure is defined in Recommendation Q.921.
LAPD includes functions for:
a) the provision of one or more data link connections on a D-channel. Discrimination between the data
link connnections is by means of a data link connection identifier (DLCI) contained in each frame;
b) frame delimiting, alignment and transparency, allowing recognition of a sequence of bits transmitted
over a D-channel as a frame;
c) sequence control, to maintain the sequential order of frames across a data link connection;
d) detection of transmission, format and operational errors on a data link connection;
e) recovery from detected transmission, format, and operational errors;
f) notification to the management entitiy of unrecoverable errors; and
g) flow control.
Data link layer functions provide the means for information transfer between multiple combinations of data
link connection endpoints. The information transfer may be via point-to-point data link connections or via
broadcast data link connections. In the case of point-to-point information transfer, a frame is directed to a
single endpoint, while in the case of broadcast information transfer, a frame is directed to one or more
endpoints.
Figure 6/Q.920 shows three examples of point-to-point information transfer. Figure 7/Q.920 shows an
example of broadcast information transfer.
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ETS 300 125: September 1991
U ser side N etw ork side
**
TE ET/NT2
ET/NT2
Layer 3
TE
**
Data
link SAPx
(a)
layer
*
TE(1)
*
ET/NT2
*
TE (2)
*
Layer 3
TE(1) TE(2) ET/NT2
** * *
Data
link
SAPx
layer
(b)
*
TE(1)
*
ET/NT2
*
TE(2)
*
Layer3
TE(1) TE(2) ET/NT2
*** *
Data
SAPx
link SAPx
layer
(c)
Physical connection Data link connection
**
ET Exchange term ination TE Term inal equipm ent
NT Network term ination
SAP Service access point
Figure 6/Q.920: Point-to-point data link connections
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ETS 300 125: September 1991
User side Network side
*
TE(1)
*
ET/NT2
TE(2)
*
B B B
Layer 3 TE(1) TE(2)ET/NT2
* * *
Data
link SAPx SAPx SAPx
layer
Physical connection B = Broadcast
Data link connection
* *
Figure 7/Q.920: Broadcast data link connection
Two types of operation of the data link layer are defined for layer 3 information transfer: unacknowledged
and acknowledged. They may coexist on a single D-channel.
3.2 Unacknowledged operation
With this type of operation layer 3 information is transmitted in Unnumbered Information (UI) frames.
At the data link layer the UI frames are not acknowledged. Even if transmission and format errors are
detected, no error recovery mechanism is defined. Flow control mechanisms are not defined.
Unacknowledged operation is applicable for point-to-point and broadcast information transfer; that is, a UI
frame may be sent to a specific endpoint or broadcast to multiple
...
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