11.3 Non standard L3 messages
24.0073GPPGeneral AspectsMobile radio interface signalling layer 3Release 17TS
In some protocols, the structure of part or all of the messages might not always follow the standard L3 message structure. As a design rule, this should be consistent for a given protocol, direction and lower layer SAP.
A possibility is to describe the message with the compact notation described in Annex B.
A few consistent structures are found in the present protocol specifications, and are described hereafter.
Other structures can be described directly in the protocol specifications.
11.3.1 Case A: BCCH and AGCH/PCH messages
In these cases, the SAP capability is for fixed length messages. The messages are structured as standard L3 messages plus one octet in front, the L2 pseudo length octet, and a rest octet part at the end.
11.3.1.1 L2 Pseudo Length octet
This octet, the L2 pseudo length indicator octet, indicates the length in octets of the subsequent octet string that can be analysed as a standard L3 message.
The octet is structured as follows:
Bits 3 to 8 encodes in binary the L2 pseudo length, i.e., the length of the part to be analysed as a standard L3 message;
Bit 2 is set to "0";
Bit 1 is set to "1".
A receiver expecting a message so structured and receiving a message with bit 1 of octet 1 (i.e., the 8th bit of the message) set to "1" and bit 2 of octet 1 (i.e., the 7th bit of the message) different from "0", shall abandon the analysis of the message.
A receiver expecting a message so structured and receiving a message on AGCH/PCH:
– with an L2 pseudo length indicator encoding 0 or 1 shall skip the indicated number of octets and not try to analyse the standard L3 message part;
– with a L2 pseudo length indicator bigger than what is compatible with the SAP capability shall abandon the analysis of the message.
11.3.1.2 Rest Octets
The part after the part structured as a standard L3 message, and up to the end of the message as constrained by lower layers, is presented as a non standard IE of variable length (sometime indicated as of type 5), the "rest octets" IE.
The rest octets element may be described by table description, or, preferably, using the compact notation described in Annex B of the present document.
11.3.1.3 Description of a modified standard L3 message
The description can be provided in the same way as a standard L3 message, with in the case of a tabular description one non standard IE at the beginning (of type L2 pseudo length), and one non standard IE at the end.
11.3.2 Case B: SACCH / SDCCH / FACCH messages sent in unacknowledged mode
The messages are structured either as standard L3 messages, or in the so-called short header format. The value of the 8th bit (bit 1 of octet 1) of the link layer PDU distinguishes the two cases. In the case of the short header, the L3 message is the same bit string as the link layer PDU, and has a fixed length. The following description includes the 2-bit link layer header.
11.3.2.1 The first octet
Bits 1 and 2 are the link layer header. Bit 2 of octet 1 is set to "0", and bit 1 is reserved for the link layer.
A protocol discriminator is the first part of the message (starting bit 8 of octet 1). The protocol discriminator field may have different lengths. The following protocol discriminator is defined:
– 0 RR.
All additional PD defined for this structure shall start by 1. The reception of a message with bit 8 of octet 1 set to 1 when expecting a message structured as defined by this clause shall be diagnosed as an unknown PD, and the message ignored.
As a design rule, a message type field should follow the PD, and of a length such that the PD and the message type fit in the 6 first bits of the message.
11.3.2.2 The rest of the message
The rest of the structure is not more constrained.
The preferred description method is the one described in Annex B.
11.3.3 Design guidelines for non standard parts
The guidelines in this clause apply to non standard parts, such as rest octets, short header broadcast message or fully non standard L3 messages.
11.3.3.1 General
The structure should be as far as possible be such that the analysis can be conducted from beginning to end. In other terms, the conditions determining the syntactic analysis of a part (e.g., tags, lengths) should appear before that part.
The part should be structured as a succession of information elements, each carrying an elementary semantic information. An information element should be composed of (possibly) a tag, than (possibly) a length indicator, then a value part.
Tags can be of fixed or variable length, their extent being analysable from beginning to end. A typical tagging is the one bit tagging, which should preferably used as follows: value "0" indicates that the IE is no more than the tag bit, and "1" indicates that the IE continues at least with the next bit.
Variable length tagging should be used to distinguish between several possible formats of the element. Tag lengths are then chosen according to packing efficiency criteria.
The T field of standard IEs can be presented as a variable tagging with only two lengths: 4 and 8 bits.
The length indicator can be of fixed or variable length, their extent being analysable from beginning to end. It should preferably be presented as encoding the length in bits of the value part.
The L field of standard IEs can be presented as a fixed length (one octet) length indicator which can encode only lengths multiple of 8 bits.
The value part can be described as further structured, in a similar way. This can be used to help the reading, and to cover some presence dependence.