FRS Signalling Processor
CMX882
1.7.2 Data Interleaving
The built in FFSK packeting includes the option of interleaving the data in each block (Type 5). This,
together with Forward Error Correction (FEC), reduces the effects of one of the commonest sources of
data errors which is burst noise. Interleaving does not add any bits to the message, the packet is
assembled in 'rows' and then transmitting in 'columns'.
Data (8 bits)
FEC (4 bits)
0
1
2
3
4
5
6
7
8
9
10
22
34
46
11
23
35
47
12
24
36
13
25
37
14
26
38
15
27
39
16
28
40
17
29
41
18
30
42
19
31
43
20
32
44
21
33
45
In the above example the packet is assembled as 4 rows with 12 bits of information per row. When this
packet is transmitted interleaved the bits are sent over the communication channel in the following order:
0, 12, 24, 36, 1, 13, 25, 37, 2, 14, ... , 33, 45, 10, 22, 34, 46, 11, 23, 35, 47.
In the receiving modem the packet is re-assembled (de-interleaved) before error correction. The 882's
built in packet receive modem is able to recognise (by using the frame head bytes) when the data has
been interleaved by the transmitter and will decode the data using the correct method.
1.7.3 Scrambler Seed Transfer
The CMX882 data scrambler provides some security for information transfer by allowing the use of
different seed patterns. This requires that the transmitting and receiving devices have identical
scrambler seeds. Should two radios wish to transfer data using the scrambler the seed must be
transferred between them before data transfer takes place. This is best done 'off air' by prior
arrangement. Obviously sending the seed over air by voice or by using FFSK (with either no scrambling
or the standard scrambler) will enable an eavesdropper to grab the seed as it is transferred and then
decode any following messages using that seed.
The following method allows the transfer of a seed between two unrelated radios (from A to B) without the
actual 16 bit seed pattern appearing over the air:
Example (4 bit), X=3, Y=9
Radio
Calculates
Seed + X
Rx + Y
Sends over air
Seed + X
Seed + X + Y
Seed + Y
Calculates
8 + 3
Sends over air
A
B
A
B
11
4
11 + 9
4 - 3
Rx - X
1
Rx - Y
-
1 - 9 = 8
The values used for the Seed, X and Y can be any number up to 65535 and all 3 numbers should be
different for each seed transfer.
This and other simple methods allow an eavesdropper to use basic mathematical techniques (11-4+1 in
the above example) to use the over air information to derive the original seed (they must however have
detected all three over air messages to do this). To reduce this possibility the host can employ more
sophisticated techniques for seed transfer (e.g. functions using large prime numbers) as this will greatly
increase the mathematical complexity required to find the original seed. However such methods are;
beyond the scope of this document, may not be permitted in certain territories and probably not
appropriate for the level of security provided by the inbuilt scrambler.
2004 CML Microsystems Plc
58
D/882/7
CMLMICRO [ CML MICROCIRCUITS ]
