Comtech EF Data SLM-5650B, Installation And Operation Manual

Page: 409 / 412

SLM-5650B Satellite Modem
Revision 2
Appendix E E–5 MN-SLM-5650B
E.3.1.3
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 E.2. Larger values of “
N
” create lower overhead, at the expense of slightly longer
TRANSEC frame acquisition times.
E.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 5 ” in Section
H.2, this value can be expressed as:
Framing Overhead[%]=[1-FTFC]*100
As per “Equation 1” (see Section E.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 of a serial
data stream input to the modem.
E.3.3 IP Traffic Encapsulation Overhead
When the Ethernet traffic interface is used, a framed transport overhead is necessary to
encapsulate the packets into the transport stream. When the SLM-5650B operates in
(Static) Bridge Mode, Ethernet packets received from the interface are encapsulated as-is.
When the SLM-5650B operates in Router Modes, the Ethernet MAC header and CRC are
removed from the input Ethernet packets, and IP packets are encapsulated into the HDLC
frame.
In either (Static) Bridge Mode or Router Mode, a form of the High-Level Data Link Control
(HDLC) is used to encapsulate the Ethernet or IP packets. 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.
See RFC 4814 “Hash and Stuffing: Overlooked Factors in Network Device
Benchmarking” for a derivation of this probability
.
Considering both sources of HDLC Overhead means that, for a packet length “
L
”, the ratio of input
bits to encapsulated output bits is expressed as “Equation 2”:
F
H
=L/[L+3+L/62] = L/[(63/62)L+3]
Unlike framing overhead, HDLC Overhead reduces the effective throughput of packet bits relative
to the programmed modem data rate. Packet Throughput, in packet bits/second, is expressed as
“Equation 3”:
Packet Throughput=(Programmed Modem Data Rate)*FH