Comtech EF Data CDM-625 - Manual (Page 391)

CDM-625 Advanced Satellite Modem Revision 15
Ethernet Network Configuration MN-CDM625
16–13
16.5 Ethernet Overhead over WAN Interface
16.5.1 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.
16.5.2 Router Mode or Managed Switch Mode (with IP Packet Processor)
When the optional IP Packet Processor is installed and enabled : The CDM-625 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.