Comtech EF Data RCS10 - Manual (Page 233)

RCS10 Redundant Communication System Principles of Operation
TM058 - Rev. 2.3 Page 5-17
The T1-ESF mode of operation supports a T1 interface with 24 frames per multiframe. The CRC-6
checksum is automatically checked by the Drop function and generated by the Insert function and
placed in the appropriate F-bit positions in the terrestrial multiframe. The user may independently
program n timeslots to Drop, and n timeslots to Insert, where n = 1, 2, 4, 6, 8, 12, 16, or 24. In the
DMD10, Robbed Bit Signaling (RBS) is handled without any need for operator intervention and is
transparent to the user.
5.10.7 SLC-96
The T1 SLC-96 mode supports a T1 Interface with 12 Frames per Multiframe (as per T1-D4) except
that the signaling frames (F
s
bits) are sent twice in succession, and then during the subsequent four
signaling frames the F
s
bits are replaced with data link information bits. The data frame is
composed of six signaling frames with a length of 9 ms. The user may independently program n
timeslots to Drop, and n timeslots to Insert, where n = 1, 2, 4, 6, 8, 12, 16, or 24. In the DMD10,
Robbed Bit Signaling (RBS) is handled without any need for operator intervention and is transparent
to the user.
5.11 Multidestinational Systems
Because the Drop and Insert functions are completely independent, the DMD10 easily supports
multidestinational communications. Figure 5-8 illustrates a multidestinational system with one Hub
site and three remote sites. At the Hub site, 30 channels are being transmitted to all 3 remote
sites and a fractional set of channels is being received from each remote site. At the other end of
the link, each remote site is transmitting a fractional E1 to the Hub site as well as receiving all 30
channels from the Hub site, identifying those channels intended for it, and inserting them into the
terrestrial data stream.
5.12 Reed-Solomon Codec (Refer to Figures 5-9, 5-10 and Table 5-1)
Utilizing a Reed-Solomon (RS) outer codec concatenated with a convolutional inner codec is an
effective way to produce very low error rates even for poor signal-to-noise ratios while requiring only
a small increase in transmission bandwidth. Typically, concatenating an RS codec requires an
increase in transmission bandwidth of only 9-12% while producing a greater than 2 dB improvement
in Eb/No. RS is a block codec where K data bytes are fed into the encoder which adds 2t=(N-K)
check bytes to produce an N byte RS block. The RS decoder can then correct up to “t” erred bytes
in the block.
5.12.1 Operation in the DMD10
When the Reed-Solomon Codec is enabled, data is fed to the RS encoding section of the DMD10
where it is scrambled, formed into blocks, RS encoded, and interleaved. Unique words are added so
that the blocks can be reformed in the receiving modem (Refer to Figure 5-10). Data is then sent to
the modulator where it is convolutionally encoded, modulated and transmitted to the satellite.
When the signal is received and demodulated by the receiving modem, it is fed to a Viterbi decoder
for the first layer of error correction. After error correction by the Viterbi decoder, the unique words
are located and the data is deinterleaved and reformed into blocks. The RS decoder then corrects
the leftover errors in each block. The data is then descrambled and output from the RS section.
5.12.2 Reed-Solomon Code Rate
The RS code rate is defined by (N,K) where N is the total RS block size in bytes - data + check
bytes - and K is the number of data bytes input into the RS encoder. The transmission rate
expansion required by the RS codec is then defined by N/K. The DMD10 automatically sets the
correct RS code rate for IDR/IBS open network operation in accordance with the data shown in
Table 5-1. In Closed Net mode, the DMD10 allows any N or K setting up to N=255, and K=235 to
allow tailoring of the code rate to meet system requirements.