Viterbi; Chapter 7. Forward Error Correction Options - Comtech EF Data CDM-550T Manual

7.1 Introduction
As standard, the CDM-550T 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 CDM-550T 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). It
comprises a small daughter card, and may be fitted at the factory at the time of order, or
may be added by the user at a later date. The second option is Comtech EF Data's Turbo
Product Codec, which is an entirely stand-alone method of FEC. This too is a small
daughter card.
7.2

Viterbi

The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The CDM-550T 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
Rev. 1.3
7
FORWARD ERROR
CORRECTION OPTIONS
Chapter 7.
7–1