SIST EN 12795:2003
(Main)Road transport and traffic telematics - Dedicated Short Range Communication (DSRC) - DSRC data link layer: medium access and logical link control
Road transport and traffic telematics - Dedicated Short Range Communication (DSRC) - DSRC data link layer: medium access and logical link control
This European Standard:
- defines the Data Link Layer of DSRC;
- is positioned with respect to other related standards by the layers defined in OSI Basic Reference Model [EN ISO/IEC 7498-1] as adopted for DSRC;
- supports broadcast and half-duplex transmission modes;
- supports a variety of fixed equipment configurations. It supports configurations where one fixed equipment communicates with one mobile equipment unit, as well as configurations where one fixed equipment can communicate with several mobile equipment units;
- takes into account that the mobile equipment communicates with the fixed equipment while passing through a limited communication zone;
- defines neither any specific configuration nor the layout of the communication zone;
- does not define to what extent different instances of fixed equipment, operating in the vicinity of each other, need to be synchronised with each other;
- defines parameters to be used in negotiation procedures taking place between fixed equipment and mobile equipment.
By defining two distinct sublayers, namely the medium access control sublayer and the logical link control sublayer, this standard defines:
a) medium access control procedures for the shared physical medium;
b) addressing rules and conventions;
c) data flow control procedures;
d) acknowledgement procedures;
e) error control procedures;
f) services provided to application layer.
The MAC sublayer is specific to the DSRC. The LLC services offered are unacknowledged and acknowledged connectionless services based on [ISO/IEC 8802-2].
Télématique de la circulation et du Transport routier - Communication a courte portée - Couche de liaison de contrôle d'acces au média et de contrôle logique de liaison
Cestna transportna in prometna telematika – Posebna komunikacija kratkega dosega (DSRC) – Podatkovna povezovalna plast DSRC: srednji dostop in krmiljenje logične povezave
General Information
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.NUDWNHJDTélématique de la circulation et du Transport routier - Communication a courte portée - Couche de liaison de contrôle d'acces au média et de contrôle logique de liaisonRoad transport and traffic telematics - Dedicated Short Range Communication (DSRC) - DSRC data link layer: medium access and logical link control35.240.60Uporabniške rešitve IT v transportu in trgoviniIT applications in transport and trade35.100.20Podatkovni povezovalni slojData link layerICS:Ta slovenski standard je istoveten z:EN 12795:2003SIST EN 12795:2003en01-oktober-2003SIST EN 12795:2003SLOVENSKI
STANDARD
SIST EN 12795:2003
EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 12795March 2003ICS 35.100.20; 35.240.60Supersedes ENV 12795:1997English versionRoad transport and traffic telematics - Dedicated Short RangeCommunication (DSRC) - DSRC data link layer: medium accessand logical link controlTélématique de la circulation et du Transport routier -Communication à courte portée - Couche de liaison decontrôle d'accès au média et de contrôle logique de liaisonThis European Standard was approved by CEN on 4 December 2002.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and UnitedKingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2003 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 12795:2003 ESIST EN 12795:2003
EN 12795:2003 (E)2ContentspageForeword.31Scope.52Normative references.73Terms and definitions.74Abbreviations and variables.85Frame format.106Address establishment.137Medium Access Control (MAC) sublayer.158Logical Link Control sublayer.26Annex A
(normative)
Data link layer parameters.38Annex B
(informative)
Data link layer overhead.39Annex C
(informative)
Evolution of the MAC sequence bit.40Annex D
(informative)
Address establishment.42Annex E
(informative)
A-deviations.47SIST EN 12795:2003
EN 12795:2003 (E)3ForewordThis document (EN 12795:2003) has been prepared by Technical Committee CEN TC 278 “Road Transport andTraffic Telematics”, the secretariat of which is held by NEN.This European Standard shall be given the status of a national standard, either by publication of an identical text orby endorsement, at the latest by September 2003, and conflicting national standards shall be withdrawn at thelatest by September 2003.This document supersedes ENV 12795:1997.The development of this standard was carried out under European Commission Mandate M/018.This European Standard forms part of a series of European Standards defining the framework of a Dedicated ShortRange Communication (DSRC) link in the Road Transport and Traffic Telematics (RTTT) environment.The communication requirements of many RTTT applications can be fulfilled by DSRC. The DSRC standardsenable compliant communication systems to serve multiple RTTT applications in parallel.The small service areas and severe real-time constraints require a specific protocol architecture leading to thereduced protocol stack shown in Figure A, consisting of the Application Layer, the Data Link Layer, and thePhysical Layer. Such architecture is very common for real-time environments.This European Standard gives the architecture and services offered by the DSRC Data Link Layer.Figure A — DSRC protocol stackThe following set of European Standards for the DSRC link is issued by CEN:— EN 12253 "DSRC Physical Layer using Microwave at 5.8 GHz”;— EN 12795 “DSRC Data Link Layer: MAC and LLC” (this European Standard);— EN 12834 "DSRC Application Layer" ;— EN 13372 "DSRC Profiles for RTTT Applications".Annex A is normative. Annexes B, C, D and E are informative.SIST EN 12795:2003
EN 12795:2003 (E)4According to the CEN/CENELEC Internal Regulations, the national standards organizations of the followingcountries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal,Slovakia, Spain, Sweden, Switzerland and the United Kingdom.SIST EN 12795:2003
EN 12795:2003 (E)51 ScopeThis European Standard:— defines the Data Link Layer of DSRC;— is positioned with respect to other related standards by the layers defined in OSI Basic Reference Model[EN ISO/IEC 7498-1] as adopted for DSRC;— supports broadcast and half-duplex transmission modes;— supports a variety of fixed equipment configurations. It supports configurations where one fixed equipmentcommunicates with one mobile equipment unit, as well as configurations where one fixed equipment cancommunicate with several mobile equipment units;— takes into account that the mobile equipment communicates with the fixed equipment while passing through alimited communication zone;— defines neither any specific configuration nor the layout of the communication zone;— does not define to what extent different instances of fixed equipment, operating in the vicinity of each other,need to be synchronised with each other;— defines parameters to be used in negotiation procedures taking place between fixed equipment and mobileequipment.By defining two distinct sublayers, namely the medium access control sublayer and the logical link control sublayer,this standard defines:a) medium access control procedures for the shared physical medium;b) addressing rules and conventions;c) data flow control procedures;d) acknowledgement procedures;e) error control procedures;f) services provided to application layer.The MAC sublayer is specific to the DSRC. The LLC services offered are unacknowledged and acknowledgedconnectionless services based on [ISO/IEC 8802-2].SIST EN 12795:2003
EN 12795:2003 (E)6Figure 1 — Architecture and data flow of the DSRC stackFigure 1 illustrates the global data flow between the elements of the DSRC stack, (Physical, Data Link andApplication Layers) and the application.NOTEFor definitions of the terms used in Figure 1 see [EN ISO/IEC 7498-1].SIST EN 12795:2003
EN 12795:2003 (E)72 Normative referencesThis European Standard incorporates by dated or undated reference, provisions from other publications. Thesenormative references are cited at the appropriate places in the text, and the publications are listed hereafter. Fordated references, subsequent amendments to or revisions of any of these publications apply to this EuropeanStandard only when incorporated in it by amendment or revision. For undated references the latest edition of thepublication referred to applies (including amendments).EN ISO/IEC 7498-1:1995, Information technology – Open Systems Interconnection – Basic ReferenceModel: The Basic Model (ISO/IEC 7498-1:1994).ISO/IEC 8802-2, Information technology – Telecommunications and information exchange between systems– Local and metropolitan area networks – Specific requirements – Part 2: Logical link control.ISO/IEC 3309, Information technology – Telecommunications and information exchange between systems –High-level data link control (HDLC) procedures – Frame structure.3 Terms and definitionsFor the purposes of this European Standard, the following terms and definitions apply.3.1beacon service tabledata structure transmitted by the fixed equipment indicating available services3.2downlinkcommunication channel on which the fixed equipment transmits its information3.3fixed equipmentfixed communication facility with one or more downlink channels and, optionally, one or more uplink channelsNOTENormally the fixed equipment is installed at a fixed location, but it may be installed on a mobile platform.3.4link identifierunique address used for addressing the mobile equipment3.5mobile equipmentmobile communication facility capable of receiving information from the fixed equipment on the downlink and,optionally, also capable of transmitting information to the fixed equipment on the uplinkNOTEThe mobile equipment normally corresponds to the vehicle’s communication unit.3.6profileunique set of parameter values controlling the behaviour of the DSRCSIST EN 12795:2003
EN 12795:2003 (E)83.7service access pointinterface point between data link layer and application layer, that has a unique Link Identifier and that allows layersto communicate[EN ISO/IEC 7498-1:1995]3.8uplinkcommunication channel on which mobile equipment transmits its information3.9windowperiod of time during which the physical medium is allocated either to the fixed equipment or to the mobileequipment4 Abbreviations and variablesFor the purposes of this European Standard, the following abbreviations and variables apply.4.1 Abbreviations4.1.1ACKACKnowledge4.1.2CanACKnowledged command / response4.1.3BSTBeacon Service Table4.1.4C/RCommand/Response4.1.5FFinal4.1.6FCSFrame Check Sequence4.1.7FEFixed Equipment4.1.8HDLCHigh-level Data Link Control4.1.9LIDLink IdentifierSIST EN 12795:2003
EN 12795:2003 (E)94.1.10LPDULink layer Protocol Data Unit4.1.11LLCLogic Link Control4.1.12LSBLeast Significant Bit4.1.13LSDULink layer Service Data Unit4.1.14L1Layer 1 of DSRC (Physical Layer)4.1.15L2Layer 2 of DSRC (Data Link Layer)4.1.16L7Application Layer Core of DSRC4.1.17MModifier function bit4.1.18MACMedium Access Control4.1.19MEMobile Equipment4.1.20MSBMost Significant Bit4.1.21OBUOn-Board Unit, an alternative descriptor to Mobile Equipment4.1.22OSIOpen Systems Interconnection4.1.23PPoll4.1.24PDUProtocol Data UnitSIST EN 12795:2003
EN 12795:2003 (E)104.1.25P/FPoll/Final4.1.26RResponse4.1.27RRResponse Request4.1.28RSURoad Side Unit, an alternative descriptor to Fixed Equipment4.1.29SAPService Access Point4.1.30UIUnnumbered Information4.2 Variables4.2.1V(RI)receive state variable (LLC)4.2.2V(SI)transmit state variable (LLC)5 Frame formatAll DSRC transmissions are in frames, and each frame conforms to the structure shown in Figure 2.FlagLink AddressFieldMAC ControlFieldLPDUFrame CheckSequenceFlagFigure 2 — Frame structureFrames containing no LPDU form a special case, see Figure 3.FlagLink AddressFieldMAC ControlFieldFrame CheckSequenceFlagFigure 3 — Frame structure, no LPDUSIST EN 12795:2003
EN 12795:2003 (E)11NOTEThe physical bit stream can also comprise a preamble and / or a trailing bits, see Figure 4.PreambleLayer 2 frame (including flags and zero bit insertion between flags)Optional trailingbitsFigure 4 — Physical layer bit stream5.1 FlagsAll frames shall start and end with a flag. A flag is a zero bit followed by six one bits followed by a zero bit (01111110). When in receiving state, all stations shall continuously check on a bit-by-bit basis for this sequence. Atransmitter shall send only complete eight bit flags.The flag which ends a frame shall not be used as the start flag for the next frame.In order to achieve transparency the flag is prevented from accidentally occurring in the link address field, MACcontrol field, LPDU and frame check sequence via a zero bit insertion procedure described in 5.7.5.2 Link address fieldThe link address field carries the Link Identifier (LID). The link address field shall contain either a private LID(contained in 4 octets), a multicast LID (contained in one octet) or a broadcast LID (contained in one octet). TheLSB of each octet in the link address field is an extension bit.5.2.1 Private LIDThe private LID is a number in the range of 0 to 268435455. Thus the private LID consists of 28 bits. The privateLID is encoded into the link address field as shown in Figure 5.XXXXXXX0XXXXXXX0XXXXXXX0XXXXXXX176543210MSBLSBFigure 5 — Private link address field formatThe LSB of the first three octets are set to 0 indicating that a further octet of the link address field follows. The LSBof the fourth octet is set to 1 indicating that this is the last octet of the link address field.5.2.2 Broadcast LIDThe broadcast LID equals 127. Thus the broadcast LID consists of 7 bits. The broadcast LID is encoded into thelink address field as in Figure 6.SIST EN 12795:2003
EN 12795:2003 (E)121111111176543210MSBLSBFigure 6 — Broadcast link address field formatThe LSB of the link address field in Figure 6 is set to 1 indicating that it consists of one octet only.5.2.3 Multicast LIDThe multicast LID is a number in the range of 0 to 126. Thus the multicast LID consists of 7 bits. The multicast LID0 is reserved for test purposes and multicast LID in the range 120 to 126 are reserved for private use. The multicastLID is encoded into the link address field as in Figure 7.XXXXXXX176543210MSBLSBX - other than all 1sFigure 7 — Multicast link address field formatThe LSB of the link address field in Figure 7 is set to 1 indicating that it consists of one octet only.5.3 MAC Control control fieldThe MAC control field shall have the encoding as described in 7.3.2.5.4 LPDU formatThe LPDU shall have the encoding as described in 8.3.5.5 Frame Check SequenceAll frames shall include a 16-bit Frame Check Sequence (FCS) just prior to the end flag for error detectionpurposes. The contents of the link address field, MAC control field and LPDU shall be included in the calculation ofthe FCS.The FCS shall be compliant with 16-bit frame checking sequence as defined in [ISO/IEC 3309] (3.6.2). Thegenerator polynomial shall be X16 + X12 + X5 + 1, and the initial value used shall be FFFF16. The ones complementof the resulting remainder shall be transmitted as the 16-bit FCS.5.6 Bit orderFlag, Link address, MAC control field, and LPDU shall be transmitted with the LSB first in each octet.The FCS shall be transmitted with the coefficient of the highest term first.SIST EN 12795:2003
EN 12795:2003 (E)135.7 TransparencyThe occurrence of the flag within a frame other than the start and end flags shall be prevented by a zero bitinsertion procedure as follows:The transmitter shall insert a 0 bit following five contiguous 1 bits anywhere between the start flag and the end flagof the frame. The insertion of the 0 bit thus applies to the contents of the Link address, the MAC control field, theLPDU and the FCS.The receiver shall continuously monitor the received bit stream; after receiving five contiguous 1 bits, the receivershall inspect the following bit. If it is a 0, the five 1 bits are passed as data and the 0 is deleted. If the sixth bit is a 1,the receiver shall inspect the seventh bit. If this bit is a 0, a valid flag has been received; if it is a 1, an abort hasbeen received, and the receiving station shall ignore that frame.6 Address establishmentEach fixed equipment shall contain one broadcast SAP as well as one SAP for each mobile private SAP currentlyknown by the fixed equipment to be in the communications zone.Each mobile equipment shall contain one broadcast SAP, and if required for uplink transmissions, one private SAP.In addition to that, it may contain one or more multicast SAPs.The broadcast SAP establishment is defined in 6.1, the mobile private SAP establishment in 6.2 and the fixedprivate SAP establishment in 6.3.Figure 8 — Link addressing overviewNOTEMobile equipment arriving in the communication zone is, in many cases, not fully powered, but is in a sleep mode.However, in the following sub-clauses, the mobile equipment is described as if it were always fully powered.6.1 Broadcast SAP establishmentThere shall be a broadcast SAP in every fixed and mobile equipment.The broadcast SAP shall always be active.SIST EN 12795:2003
EN 12795:2003 (E)146.2 Mobile private SAP establishmentThe mobile private SAP is created by the data link layer on request from the application layer.The private LID associated with this SAP is generated at the application layer.The mobile data link entity shall use its private LID in all uplink transmissions.6.3 Fixed private SAP establishmentWhen a fixed equipment data link entity receives a frame containing a private LID not known to it, a correspondingSAP shall be created.NOTEDeletion of fixed private SAPs is under the responsibility of upper layers or applications.SIST EN 12795:2003
EN 12795:2003 (E)157 Medium Access Control (MAC) sublayer7.1 OverviewThe MAC sublayer is responsible for controlling the use of the physical medium by the MAC sublayer entity residingin the fixed equipment and the MAC sublayer entity residing in the mobile equipment.The mobile MAC sublayer offers the M-MA-DATA primitives to the mobile LLC sublayer. The fixed MAC sublayeroffers the F-MA-DATA primitives to the fixed LLC sublayer.The medium access control is characterised by:— half duplex mode;— asynchronous time division multiple access (TDMA).The medium access control is unbalanced, in that the fixed equipment is always in control of the physical medium,granting access to the physical medium to either:— the fixed MAC (downlink window); or— one mobile MAC exclusively (private uplink window); or— any mobile MAC, according to certain rules (public uplink window).The mobile MAC can also request access to the medium.7.2 MAC Service service primitivesThe MAC sublayer offers the following primitives to the LLC sublayer:— MA-DATA.request;— MA-DATA.indication.These are shown in Figure 9.Figure 9 — MAC services and primitivesSIST EN 12795:2003
EN 12795:2003 (E)167.2.1 Fixed MAC service primitives7.2.1.1 F-MA-DATA.requestThe primitive shall be passed from the LLC sublayer to the MAC sublayer to request that an LPDU is transmitted toa mobile SAP in the first available downlink window.The primitive shall provide the following parameters:F-MA-DATA.request(LID, LPDU, RR)The LID shall be the LID of the mobile SAP for which the frame is intended.It may be a private LID, the broadcast LID or a multicast LID.The LPDU may be null (in this case no LPDU shall be included in the frame transmitted).The response request (RR) shall indicate whether or not the fixed equipment shall allocate an uplink window inimmediate connection to the downlink frame transmitted.7.2.1.2 F-MA-DATA.indicationThe primitive shall be passed from the MAC sublayer to the LLC sublayer to indicate the successful reception of avalid frame from a mobile SAP.The primitive shall provide the following parametersF-MA-DATA.indication (LID, LPDU)The LID shall be the content of the link address field of the frame received.The LPDU shall not be null.7.2.2 Mobile MAC service primitives7.2.2.1 M-MA-DATA.requestThe primitive shall be passed from the LLC sublayer to the MAC sublayer to request that an LPDU is transmitted tothe fixed SAP in an uplink window.The primitive shall provide the following parametersM-MA-DATA.request(LID, LPDU)The LID shall be the private LID of the mobile SAP.The LPDU may be null, in which case no LPDU shall be included in the frame transmitted.NOTEThe uplink window can be public or private, as described in 7.3.4.SIST EN 12795:2003
EN 12795:2003 (E)177.2.2.2 M-MA-DATA.indicationThe primitive shall be passed from the MAC sublayer to the LLC sublayer to indicate the successful reception of avalid frame from a fixed SAP.The primitive shall provide the following parametersM-MA-DATA.indication(LID, LPDU)The LID shall be the content of the link address field of the frame received.The LPDU shall not be null.7.3 Window management7.3.1 OverviewUplink windows allocated by the fixed equipment are indicated by the MAC control field of the downlink frame andfollow immediately after the downlink window containing the frame.The distinction between allocation of public and private uplink windows is made by the fixed equipment by meansof the LID of the frame allocating the uplink window. A public uplink window is allocated if the link address fieldcontains a broadcast LID while a private uplink window is allocated if the link address field contains a private LID.Figure 10 gives an overview of window management.downlink windowno allocation in downlink framebroadcast or private LIDno uplink window allocateddownlink windowprivate uplink windowallocation in downlink frameprivate LIDprivate uplink window allocateddownlink windowpublic uplink windowpublic uplink windowpublic uplink windowbroadcast LIDallocation in downlink frameconsecutive public uplink windowsallocatedFigure 10 — Window management overview7.3.2 MAC control fieldThe MAC control field is used to:— indicate whether the frame contains an LPDU;— indicate the transmission direction;— allocate public and private windows;— request for private windows;— specify type of LPDU.SIST EN 12795:2003
EN 12795:2003 (E)18The MAC control field has the length of one octet. The content of the MAC control field is different on the downlinkand on the uplink. Bits that are not specified are reserved for future use.7.3.2.1 MAC control field of the downlinkThe MAC control field of the downlink shall be used by frames transmitted by the fixed equipment. The format isdescribed in Figure 11.MSBLSBFigure 11 — MAC control field of the downlinkLD(0)AC/RSXXX76543210LThe LPDU Existence bit shall be used to indicate whether the frame contains an LPDU.0 = frame contains no LPDU1 = frame contains LPDUNOTE: Minimum LPDU consists of a LLC control field only.DThe Direction Identifier bit shall be used to identify the link direction0 = downlink direction.AThe Medium Allocation bit shall be used to allocate medium (see 7.3.4).0 = no uplink window is allocated by the fixed equipment1 = uplink window is allocated by the fixed equipmentNOTE
The allocation may be a private uplink window or N5 public uplink windowsC/RThe LLC Command/Response bit shall be used to identify the LPDU as command orresponse. The LLC Command/Response bit shall be ignored if the LPDU Existence bit isset to 0. (For the use of the C/R bit see 8.3.1)0 = command LPDU1 = response LPDUSThe MAC Sequence bit shall be used to distinguish between first allocation of a privateuplink window and reallocation of a private uplink window. (see 7.4.2)XReserved, shall be set to 0.SIST EN 12795:2003
EN 12795:2003 (E)197.3.2.2 MAC control field of the uplinkThe MAC control field of the uplink shall be used by frames transmitted by the mobile equipment. The format isdescribed in Figure 12.MSBLSBFigure 12 — MAC control field of the uplinkLD(1)RC/RXXXX76543210LThe LPDU Existence bit shall be used to indicate whether the frame contains an LPDU.0 = frame contains no LPDU1 = frame contains LPDUDThe Direction Identifier bit shall be used to identify the link direction1 = uplink directionRThe Medium Request bit shall be used to request uplink window (see 7.4.3).0 = no window is requested by mobile equipment1 = window is requested by the mobile equipmentC/RThe LLC Command/Response bit shall be used to identify the LPDU as command orresponse. The LLC Command/Response bit shall be ignored if the LPDU existence bit isset to 0. (For the use of the C/R bit see 8.3.1)0 = command LPDU1 = response LPDUXReserved, shall be set to 0.SIST EN 12795:2003
EN 12795:2003 (E)207.3.3 Downlink windowsThe fixed equipment allocates a downlink window simply by transmitting a frame.A downlink window starts at the start of the first bit of the preamble and ends at the end of the last bit of the endflag of the downlink layer 2 frame transmitted.NOTETrailing bits defined at the physical layer are not part of the window.A downlink window shall not start before T1 after the end of the previous window if the previous window is an uplinkwindow.A downlink window shall not start before T2 after the end of the previous window if the previous window is adownlink window.A layer 2 frame transmitted in a downlink window shall consist of not more than N2 octets.See Figure 13 and Figure 14 for the timing of the downlink window.Figure 13 — Timing of downlink window after uplink windowFigure 14 — Timing of downlink window after downlink windowNOTEFrom N2 the maximum duration in time of the downlink window can be calculated, taking bit rate and zero bit insertioninto account.max. length of downlink windowuplink windowT1downlink windowPreamblestart flag.end flaglayer 2 framemax. length of downlink windowDownlink windowT2downlink windowPreamblestart flag.end flaglayer 2 frameSIST EN 12795:2003
EN 12795:2003 (E)217.3.4 Uplink windowsThe fixed equipment allocates one or more uplink windows immediately following a downlink frame by setting the Abit of the MAC control field of the downlink frame to 1.There are two kinds of uplink windows, private uplinkwindows and public uplink windows.A private uplink window may only be used by one mobile equipment, while public uplink windows may be used byany mobile equipment according to certain rules described in 7.3.4.3.If the LID of the allocating downlink frame is a private LID, the allocated uplink window is a private uplink window. Ifthe LID of the allocating downlink frame is a broadcast LID, the allocated uplink windows are public uplink windows.Each allocating downlink frame may thus allocate either:a) one private uplink window; orb) N5 consecutive public uplink windows.7.3.4.1 Private uplink windowsA private uplink window may only be used by the mobile equipment having a private LID equal to the LID of theframe allocating the window.A private uplink window starts T3 after the end of the downlink window containing the frame allocating the uplinkwindow.A private uplink window ends:a) T4a after the start of the window, if no mobile equipment has started transmitting before that time; orb) at the end of the last bit of the end flag of the uplink layer 2 frame transmitted; andc) no later than the maximum time of private uplink windows, defined by N3.Figure 15 shows the timing for the private uplink window.Figure 15 — Private uplink window timingT4adownlink windowT3¬¾
private uplink window
¾®preamblestart flag.end flaglayer 2 framemax. length of private uplink windowSIST EN 12795:2003
EN 12795:2003 (E)227.3.4.2 Public uplink windowsEach downlink frame can simultaneously allocate N5 public uplink windows.A public uplink window may be used by any mobile equipment according to certain rules given in 7.3.4.3.A public uplink window starts:a) T3 after the end of the downlink window containing the frame allocating the window if the public uplink window isthe first window after the downlink window; orb) immediately after the end of the previous window if that window is a public uplink window.A public uplink window ends T5 after the start of that window.The transmission of the first bit of the preamble of the layer 2 frame in a public uplink window shall start before T4bafter the start of that window.A layer 2 frame transmitted in a public uplink window shall consist of not more than N4 octets.See Figure 16 for the timing of the public uplink window.Figure 16 — Public uplink window timing7.3.4.3 Public uplink window selectionThe public uplink window selection mechanism is a (pseudo-)random mechanism. The probability of selecting oneout of N5 public uplink windows shall be 1/N5. If a pseudo-random mechanism is used, an appropriate seeding ofthe algorithm has to be taken into account.T4bT4bDownlink windowT3¬¾public uplink window¾®¬¾public uplink window¾®Pre-amblestartflag.endflaglayer 2 frameT3T5T5SIST EN 12795:2003
EN 12795:2003 (E)237.4 MAC elements of procedure7.4.1 Private medium response flagIn each mobile equipment there is a private medium response flag. It shall be set to zero each time a private uplinkwindow is requested and incremented for each public uplink window allocation received.After a private upl
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