Ethernet Overhead Over Wan Interface; Managed Switch Mode (Without Ip Packet Processor; Router Mode Or Managed Switch Mode (With Ip Packet Processor - Comtech EF Data CDM-425 Installation And Operation Manual

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 Control + 14 Bytes
Ethernet Header + Ethernet Payload + 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.
Appendix I
CDM-425 Advanced Satellite Modem
I–14
Revision 0
MN-CDM-425