Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix I I–14 MN-CDM-425
I.5 Ethernet Overhead over WAN Interface
I.5.1 Managed Switch Mode (without IP Packet Processor)
When the optional IP Packet Processor is either not installed or installed but disabled:
After Ethernet packets are sent from the internal switch to the modem WAN interface, the link
overhead per packet is as follows:
1 Byte (HDLC Start Flag) + 2 Bytes HDLC C 14 Bytes
Ethernet Ethernet P 2 Bytes HDLC CRC
Additionally, on average, an additional 3% of overhead is added to account for HDLC bit stuffing.
I.5.2 Router Mode or Managed Switch Mode (with IP Packet Processor)
When the optional IP Packet Processor is installed and enabled:
The CDM-425 Advanced Satellite Modem uses Comtech EF Data’s patented Streamline
Encapsulation (SLE) Framing instead of HDLC Framing.
Note that in SLE Framing, no bit stuffing overhead is involved; hence, WAN overhead is
deterministic. After Ethernet packets are sent from the internal switch to the IP Packet
Processor interface, per packet overhead on the WAN Interface is as follows:
1 Byte SLE start Flags + Control Byte(s)+ Ethernet Payload (IP +
UDP + payload; IP + TCP + payload; etc.) + 2 bytes CRC
For
Control Byte(s)
, note the following:
•
If payload compression or header compression or both are enabled,
Control
Byte(s)
= 2 bytes;
•
If Segmentation and Reassembly (SAR) is enabled,
Control Byte(s)
= 2 bytes;
•
Otherwise,
Control Byte(s)
= 1 byte.