G.3 Application of truncated concatenation including ‘padding bits’

04.603GPPGeneral Packet Radio Service (GPRS)Mobile Station (MS) - Base Station System (BSS) interfaceRadio Link Control / Medium Access Control (RLC/MAC) protocolRelease 1999TS

The truncated concatenation may include ‘padding bits’ at the end of a message. In that case, the resulting concatenation shall fit exactly with the received message length, otherwise the message is syntactically incorrect.

The construction is useful, e.g., when a message ends with a sequence of optional components, where the transmitter may need to truncate tailing bits ‘0’, indicating optional components not included in the message.

< Packet ZZZ message content > ::=

{ { 0 | 1 < Optional component 1 > }

{ 0 | 1 < Optional component 2 > }

{ 0 | 1 < Optional component N > }

< padding bits > } // ;

If the optional components from k to N are not needed in the message, the transmitter may use the full message length for the components up to optional component k – 1. The receiver accepts this message and assumes that the choice bits for optional components from k to N are all set to zero (i.e., these components are not present).

However, if the receiver detects a syntactical error within one optional component which is indicated as present in the message, that results in a truncated concatenation which does not fit with the received message length. In this case, the receiver shall not accept the message as being syntactically correct.

An error label may be provided within a truncated concatenation to allow the receiver to accept part of a concatenation in case of a syntactical error within it. This is useful for recurring components at the end of a message.

< Packet TTT message content > ::=

{ { 1 { < Recurring component > ! < Ignore : bit (*) = < no string > > } } ** 0

< padding bits > } // ;

If one of the recurring components is syntactically incorrect, the error branch is called. The error branch expands to the end of the message. The tail bit ‘0’, terminating the recursion, and the ‘spare padding’ are truncated. The receiver accepts any syntactically correct instance of the recurring component preceding the syntactically incorrect one in the message.