Mocomtech CIM-550 - Manual (Page 117)

91
Chapter 7.
Forward Error Correction Options
Viterbi 91
Sequential 92
TURBO Product Codec 93
Uncoded Operation 96
7.1 I
NTRODUCTION
As standard, the CiM-550 Modem is equipped with two Forward Error Correction
Decoders, Viterbi and Sequential. The constraint lengths and encoding polynomials are
compatible with the vast majority of existing modems from other manufacturers, and
Comtech EF Data has performed compatibility testing to ensure inter-operability.
As an option, the CiM-550 may be fitted with two additional Forward Error Correction
(FEC) devices. The first is a concatenated Reed-Solomon Codec. This is always used in
conjunction with, rather than instead of, the main FEC method (Viterbi or Sequential).
The second option is Comtech EF Data’s Turbo Product Codec, which is an entirely
stand-alone method of FEC. These options are small daughter cards, and may be fitted at
the factory at the time of order, or may be added by the user at a later date.
7.2 V
ITERBI
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The CiM-550 complies with the Intelsat
IESS 308/309 standards for Viterbi decoding with a constraint length of seven. This is a
de facto standard, even in a closed network environment, which means almost guaranteed
inter-operability with other manufacturer’s equipment. It provides very useful levels of
coding gain, and its short decoding delay and error-burst characteristics make it
particularly suitable for low data rate coded voice applications. It has a short constraint
length, fixed at 7, for all code rates. (The constraint length is defined as the number of
output symbols from the encoder which are affected by a single input bit.)