A.2 False Z sequence detection
08.203GPPRate Adaptation on the Base Station System  Mobile Service Switching Centre (BSSMSC) 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 

092000 
TSG#09 
NP000551 
A008 
1 
32 kbit/s UDI/RDI multimedia in GSM 
8.1.0 
8.2.0 
122000 
TSG#10 
NP000604 
A009 
Removal of 1200/75 bit/s data rate and cleanup 
8.2.0 
8.3.0 

032001 
TSG#11 
NP010040 
A013 
Correction of downgrading procedure for HSCSD 
8.3.0 
8.4.0 