BS EN 13757-3-2013 pdf download.Communication systems for and remote reading of meters Part 3 : Dedicated application layer.
The access number is a part of the fixed header. It is used by the extended link layer, the transport layer or the apphcation layer protocol. It is coded as a 1-byte unsigned binary number. It has originally been added to support the wired M-Bus of EN 13757-2 and signals to the user that its meter has been read out by the pull-type readout of EN 13757-2. This function supports the detection of unwanted frequent readouts. For the typical push type frequent transmit modes of radio devices based on the EN 13757-4, this usage is no longer sensible. Acceptable maximum readout or transmit frequency is either an issue of legal requirements or of a contract between the meter reading organisation and the consumer. For devices based on the EN 13757-4, the access number shall be applied for clear indication of a new datagram for both pushed data as well as for requested data by strict rules of increments. For the partnergenerated datagrams, relaxed generation rules simplify the implementation. The detection rules are identical between meter and partner; i.e. a datagram with an identical access number is considered a repetition of an identical previously already received datagram and shall be ignored. All datagrams with a different access number than the last received datagram are considered as a new data datagram. The term new data” means that the meter application provides newer measurement values than in the last (old) data datagram.
NOTE 1 This link layer oriented function is not sufficient to uniquely identify messages from an application. If an application requires such a unique identificabon of a message. a data record with a VIF of FDh,08h (unique message identification) and a data length of at least 4 bytes is added to each massage.
NOTE 2 If both extended link Layer and transportation layer exist, then the access number of the extended link layer will be applied for the timing of synchronous transmission, whereas the access number in the transport layer will be used to indicate new data.
5.9.2 Generation of access number for meter initiated datagrams
If a meter generates a datagram (SND-NR. SND-IR, ACC-DMD or ACC-NR). the access number is incremented (modulo 256) by one before or after each new transmission of the meter.
If the meter supports the feature of synchronised transmission for battery operated receiver (refer to EN 13757-4) then the access number shall be incremented by one and only one with every synchronous transmission. For every asynchronous transmission between two synchronous datagrams, the meter shall use the access number from the last synchronous transmission. To avoid the detection of zero consumption, it is recommended to add a time stamp or an incremental counter (VIFE “unique message identification”) to the message content if one or several asynchronous transmissions are sent in the interval between two synchronous transmissions. If such asynchronous transmission contains new data, the extended link layer may be applied to separate the indication of synchronous transmission from the indication of new data.
After the power up of the meter, its value of the access number shall be set by a randomised initial value from 0 to 255. The access number of the meter shall not be resettable.
When the datagram is a response to a datagram of the communication partner (i.e. ACK, RSP-IJD or other) then the access number repeats the received access number of the communication partner. Otherwise, the access number of the meter is used.
NOTE Meter inihated datagrams are not allowed by the wired M-Bus standard (refer to EN1 3757-2).
5.9.3 Generation of access number for partner generated datagrams
If the communication partner generates a wireless datagram (SND-UD, SND-NKE. REQ-UD1, REQU02), each such datagram shall use a different value of the access number within 300 s or within a frequent access cycle as defined in EN 13757-4. A simple increment is recommended to fulfil this requirement. If the partner transmits an answer to the meter (i.e. ACK or CNF-IR with Cl-field), the transmitted access number of the partner repeats the received access number of the meter. After the power up of the communication partner, its value is undefined, but an initial value of zero is recommended. For the usage of the FCB, see E.7.
For the wired communication according to EN13757-2, the meter shall increment its own access number before or after each response (RSP_UD) to the communication partner. If the communication partner (M-Bus-master) transmits an access number (e.g. SND-UD), it may be ignored.
NOTE The communication partner has to provide an access number in case of encrypted commands.BS EN 13757-3-2013 pdf download.