Comtech EF Data SLM-5650A Installation And Operation Manual page 513

I.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 I.2,
this value can be expressed as:
Framing Overhead[%]=[1-F T F C ]*100
As per "Equation 1" (see Section I.2), since the factors
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.
I.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-5650A operates in
(Static) Bridge Mode, Ethernet packets received from the interface are encapsulated as-is.
When the SLM-5650A operates in Router Modes, the Ethernet MAC header and CRC are
removed from the input Ethernet packets, and IP packets are encapsulated into the High-
Level Data Link Control (HDLC) frame.
In either (Static) Bridge Mode or Router Mode, a form of the 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 =L/[L+3+L/62] = L/[(63/62)L+3]
H
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)*F
HDLC Overhead – referred to as "Equation 4" – is expressed as a percent overhead of the
modem date rate:
HDLC OH[%]=(1-F
The HDLC Overhead ranges from approximately 6% for L=64 to approximately 1.8% for L=1518.
Appendix J
)*100=([(1/62)L+3]/[(63/62)L+3])*100
H
I–5
SLM-5650A Satellite Modem
F F
and are less than unity, the
T C
H
Revision 10
MN-SLM-5650A