G.2 Application of the ‘Message escape’ error label

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

The ‘Message escape’ branch protects the comprehension of the description following bit ‘0’, as shown in the example below.

< Packet YYY message content > ::= — Protocol version 1

< FIELD_1 : bit (3) >

{ 0 < FIELD_2 : bit (16) >

< padding bits >

! < Message escape : 1 bit (*) = <no string> > } ;

The comprehension of ‘FIELD_2’ is required. If the receiver detects bit ‘1’, the ‘Message escape’ branch is called and the remaining part of the message can be ignored.

The ‘Message escape’ branch may be used to introduce an new alternative coding of the message in a later version of the protocol.

< Packet YYY message content > ::= — Protocol version 2

< FIELD_1 : bit (3) >

{ 0 < FIELD_2 : bit (16) >

< padding bits >

| 1 — New code option, replacing old ‘Message escape’:

{ 00 < FIELD_3 : bit (12) >

< padding bits >

! < Message escape : { 01 | 10 | 11 } bit (*) = <no string> > } } ;

An alternative coding, including ‘FIELD_3’, is introduced following ‘bit 1’ in the former ‘Message escape’ branch. A new ‘Message escape’ is defined, this time using to control bits to allow future modification.

A receiver implemented according to the original syntax will not accept the new coding. The original ‘Message escape’ branch will be called and the remaining part of the message, including ‘FIELD_3’ is ignored. The content of ‘FIELD_1’ (e.g. information to identify the receiver) is accepted and can be used to determine appropriate condition handling.