Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 244)

Network MN/22156, rev 12
4-26 VMS User Guide
• DiffServ– QoS rules based on Differentiated Services settings.
This mode is described in the following section.
• Max/Priority – QoS rules based on maximum bandwidth and priority.
This mode is described under the Rules section, in the subsections
“Priority” on page 4-33 and “Minimum & Maximum Bandwidth” on
page 4-34.
• Min/Max – QoS rules based on minimum and maximum bandwidth.
This mode is described under the Rules section, in the subsection
“Minimum & Maximum Bandwidth” on page 4-34.
Packet Segmentation and Reassembly (SAR) is an option that can be enabled
for QoS on the CDM-840 Remote unit. This feature reduces the packet size,
providing two advantages:
• packets are routed more quickly through the network
• compliance with specified packet size restrictions for a given path
SAR is an adaptive process; it will trigger only if the packet latency exceeds the
threshold value (default is 20 msec). At lower data rates, SAR improves the
jitter and latency performance for high priority packets. Latency value is calcu-
lated based on the satellite transmission bandwidth. The minimum segment size
is limited to 480 bytes—excluding satellite HDLC header information—in
order to avoid satellite overhead and consumption of CPU cycles.
DiffServ QoS Mode
Selecting the DiffServ Mode radio button makes the target unit fully compliant
with the Differentiated Services QoS standards.
Ideally, each node in the path should be DiffServ compatible, but it is not neces-
sary. DiffServ Code Points (DSCP) can be set by traffic source or by edge rout-
ers. Comtech CDM-IP modems do not set the DiffServ Code Points; rather, they
prioritize traffic.
Some implementations of DiffServ will prioritize traffic by Class Selector
assignment. This is defined in the DSCP within the IP header. The first 3 bits of
the DSCP define the Class Selector Precedence (or Priority), as shown in
table 4-2.
Table 4-2 DiffServ Code Points (DSCP)
Class Selector DSCP
Modem/Router
Priority
Default (CS0) 000 000 8
Precedence 1 001 000 7