Comtech EF Data SLM-5650C CyberLynx, Installation And Operation Manual

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SLM-5650C / SLM-5650C-ODU Satellite Modem
Revision 1
Appendix C C–3 MN-SLM-5650C
C.3 Sources of Overhead
C.3.1 Framing Overhead
Several modes of operation add framing overhead to the satellite channel.
C.3.1.1 AUPC Framing
AUPC overhead channel
is referred to as
“Operand 3” in Section C.2:
•
The AUPC communications channel also supports modem-to-modem receiver signal
Eb/No status used to automatically adjust the Tx uplink power when (AUPC Power
Control) is enabled. If the AUPC framing mode is enabled, 16 framed output bits are
created for every 15 input bits of the transport stream regardless if the features are being
used. The AUPC overhead factor is a function of the programmed Data Rate.
C.3.1.2 TRANSEC Framing
When the TRANSEC encryption is enabled, the transport Frame size is user selectable as:
( N * AES block )
The parameter “ N ” is programmable from 1 to 16; each AES block is 16 bytes in length; each
frame has a 3-byte header. This leads to the
F
T
factor, identified as “Operand 5” in Section C.2.
Larger values of “ N ” create lower overhead, at the expense of slightly longer TRANSEC frame
acquisition times.
C.3.2 Total Framing Overhead
There may be times where the framing overhead is of interest, expressed as a percentage of
framing overhead bits within the framed transport stream. Identified as “
Operand 4 ” in Section
H.2, this value can be expressed as:
Framing Overhead[%]=[1-FTFC]*100
As per “Equation 1” (see Section C.2), since the factors FT and FC are less than unity, the
Framing Overhead (when framing is present) increases the symbol rate. Framing overhead has
no effect on the programmed modem bit rate, which is equivalent to the input bit rate.
C.3.3 IP Traffic Encapsulation Overhead
With Ethernet traffic, a framed transport overhead is necessary to encapsulate the packets
into the transport stream. With the modem operating in (Static) Bridge Mode, Ethernet
packets received from the interface are encapsulated using a form of HDLC. A start flag
byte (0x7E) is added to each packet and a 2-byte CRC is appended to the end. In addition,
in order to enable the receiver to recognize the start flag within the data stream, a “0” is
inserted wherever five “1”s occur in a row in the transport data stream. Assuming random
bits, the probability of bit stuffing using this technique is 1/62.