Comtech EF Data CDM-625A, Инструкция по установке и эксплуатации

Страница: 661 / 756

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix K Revision 4
K–13
K.5 Ethernet Overhead over WAN Interface
K.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.
K.5.2 Router Mode or Managed Switch Mode (with IP Packet Processor)
When the optional IP Packet Processor is installed and enabled : The CDM-625A 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.