Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 242)

Network MN/22156, rev 12
4-24 VMS User Guide
Series800 Satellite Framing
Generic Stream Encapsulation (GSE) is a data link layer protocol that provides
the means to carry packet oriented protocols, such as IP, on top of uni-direc-
tional physical layers like DVB-S2. Packets transmitted by the CDM-800 use an
Enhanced GSE framing.
The CDM-840 modems transmit with VersaFEC physical layer and frame the
packets using StreamLine Encapsulation (SLE).
Header Compression
When compression is enabled, some of the initial traffic sent between two
devices will not be received over the satellite until a full header is transmitted
(based on the Refresh rate). If a ping is sent over the satellite, it will time out
until the full header packet is sent. The header compression refresh rate can be
reduced to minimize the amount of traffic lost when traffic is first sent between
two devices. Separate refresh rates for UDP flows, RTP flows, and all other
flows can be specified.
The default refresh values (50 packets) reflect the recommended settings for a
typical modem/router used in a VMS network. However, the refresh rates can
be decreased for poor satellite link conditions, or they can be increased to
reduce overhead even further. The valid range is 1 to 600 packet(s).
Payload Compression
Only traffic past the headers is effected by payload compression. Payload is
considered everything inside the Streamline Encapsulation satellite frame.
Therefore, IP headers could be compressed as well. Payload compression is an
optional feature of the modem and has the following functions:
• All modems used in a VMS network operate in router mode requiring that
payload compression be set on a per route basis.
• The compression algorithm is applied to all data (SLE header excluded).
• Compression statistics are fed back to QoS in order to maximize WAN
utilization while optimizing priority, jitter and latency.
• The modem runs 1024 simultaneous compression sessions to maximize
compression across multiple distinct traffic flows.
• Compression algorithm is not applied to RTP streams because this traffic
is already compressed and would only increase the satellite bandwidth if
compressed again.
Receive payload compression is auto-sensed by a bit carried in packet headers
and the modem unit will perform decompression. This function is always avail-
able if the payload compression FAST code option has been purchased.