## A.2 False Z sequence detection

08.203GPPRate Adaptation on the Base Station System - Mobile Service Switching Centre (BSS-MSC) InterfaceTS

The Framing Pattern Substitution algorithm presented in subclause 10.2 ensures sure that all the Z sequences found in the original sub frame are removed, but it shall be checked that the transformations performed do not introduce new unwanted Z sequences.

The goal of this subclause is to show that the transformed sub frame does not contain new Z sequences introduced by the algorithm itself.

The coding of the ZSP is the key point to avoid such an emulation. The different cases are considered below.

1 : Sequence ZSP

The worst case is when the address is equal to 1 :

 1 C A0 A1 A2 A3 A4 1 1 0 0 0 0 0 1 1

There is a maximum of 5 zeroes.

2 : Sequence Di / ZSP.

By definition, a data block always ends up with a one (except the last one of the message) and the ZSP always starts with a 1.

3 : Sequence ZSP / Di

ZSP always ends up with a 1 and Di has a maximum of 7 zeroes : it is not possible to find 16 zeroes in a row.

4 : Sequence Di / Dj

Di is not the last data block of the message.

As already mentioned, Di ends up with a one (except the last one) : this is the same case as 3.

5 : Sequence Zi / D or D / Zi

This case only occurs when there is no substitution. In this case, the Zi bit close to the D field is always a one: this does not change the number of zeroes in sequence.

6 : Sequence last Di / new framing pattern

The last D sequence can end up with up to 7 zeroes, followed by the 16 zeroes of the next frame.

There is anyhow no ambiguity, when considering that the framing pattern is made up of 16 zeroes followed by a one.

7 : Sequence last Di / Z bit of the next sub frame

The last D sequence can end up with up to 7 zeroes, followed in the worst case by Z=0 and then a ZSP. As a ZSP starts with a one, this makes a maximum of 8 zeroes in a row.

8 : Sequence ZSP / ZSP (not shown on the figure)

This case arrives when the original message has at least 16 zeroes in a row.

As the ZSP element always starts and ends up with a one, this always induces two consecutive ones.

Annex B (informative):
Change History

 Change history Date TSG # TSG Doc. CR Rev Subject/Comment Old New s27 A005 Synchronisation 5.3.0 7.0.0 s29 A006 Introduction of EDGE channel codings into the specifications 7.0.0 8.0.0 s30 A007 Asymmetric channel coding 8.0.0 8.1.0 09-2000 TSG#09 NP-000551 A008 1 32 kbit/s UDI/RDI multimedia in GSM 8.1.0 8.2.0 12-2000 TSG#10 NP-000604 A009 Removal of 1200/75 bit/s data rate and clean-up 8.2.0 8.3.0 03-2001 TSG#11 NP-010040 A013 Correction of downgrading procedure for HSCSD 8.3.0 8.4.0