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The CSN.1 [10] notation method for describing, encoding, and decoding of air interface Layer 2 signaling messages are used by the GPRS RLC/MAC layer. It was described in Section 4.1.1 that the air interface Layer 3 messages are encoded and decoded on the octet (8 bits) level in TLV format. However, using the CSN.1 notation, only the value, and not type or length, of the IEs of a signaling message are encoded and decoded on a bit level. This results in a compact bits stream for transmission over the GPRS air interface. A CSN.1 encoded signaling message contains a bits string having an ordered sequence of symbols, i.e. 0 and 1. The resulting bits string may not be multiple of 8, i.e. an octet. An example CSN.1 description of a message with two IEs of length 3 bits each is shown below:

<Message>:: = 11 { 1 < IE1 : bit(3)> | 0 <IE2: bit(3)> }11

In the above CSN.1 description of the sample message, the vertical bar “|” represents a choice. If the third bit of the message is 1, then IE1 shall be encoded; if the third bit is 0, then IE2 shall be encoded. Both the IEs are illustrated with length = 3 bits. This is further followed by 11 bits, giving an overall bits stream of 8 bits.

The CSN.1 method is based on the Backus‐Naur Form (BNF) that contains various rules to be followed while describing a message. For more information on such rules of encoding and decoding of RLC/MAC layer signaling message using the CSN.1 method and its other aspects, refer to Annex‐B, TS 24.007 [44]. Example messages and their descriptions/structures using CSN.1 notation can be found in 3GPP TS 44.060 [131].

Reusable software modules/functions/procedures are required to be developed for CSN.1 encoding, at the sender end, and decoding, at the receiving end, of GPRS RLC/MAC signaling message. More about the implementation of encoding and decoding in the code is described in later sections.