Mocomtech CDM-570 - Manual (Page 285)

CDM-570/570L Satellite Modem with Optional IP Module Revision 4
IP Module Ethernet Interface Overview MN/CDM570L.IOM
Weighted Random Early Detection: The MIN-MAX and MAX-Priority QoS modes
allow the user to enable or disable the WRED option. In Diffserv mode, WRED
applicable to Asure forwarding only, however user can change the WRED option.
WRED allows for more graceful dropping of packets, as QoS queues get full. Without
WRED, typically packets are dropped based upon a simple tail drop algorithm, which
was applied to packets as they were being added to the QoS queues. This can result in
large numbers of contiguous packets being dropped which causes many protocols such as
RTP and TCP to ungracefully degrade performance in an over-consumed or bursty
scenario. WRED applies a randomization which means that the percentage change to
drop packets
System Latency: System latency is used to define the maximum duration that a packet
will stay in a QoS queue. This mechanism serves to ensure that old packets are “aged”
out of the system rather than waste satellite bandwidth on invalid packets.
15.4.4.1 Maximum Bandwidth/Priority QoS Mode
QoS Rules can be assigned to up to 32 different types of flows to be defined by the user.
Flows can be defined by any combination of Protocol (FTP, UDP, RTP, etc.),
Source/Destination IP (specific or range), and/or Layer 3 Source/Destination Port.
Priority - A Priority level from 1 to 8 is assigned for each flow. The IP Module classifies
each packet that is to be forwarded over the satellite. The packet will then have a Priority
assigned according to the defined QoS Rules. Any packet that does not meet a QoS Rule
is assigned to the Default Rule and will be assigned a Priority of 9. Priority 1 packets will
be forwarded immediately, Priority 2 packets will be forwarded as soon as there are no
Priority 1 packets in the Queue, and so on. Any latency critical traffic, such as VoIP/RTP
should always be assigned Priority 1.
Maximum Bandwidth - This can also be assigned to a flow to restrict the Maximum
Bandwidth that any particular flow will utilize, or the default of no bandwidth restriction
can be selected.
Filtering - QoS also allows specific flows to be designated as “filtered,” so the IP
Module will discard traffic that the user does not want to forward over a satellite link.
QoS Rule Hierarchy - It is quite possible to have traffic that meets the definitions of
several QoS Rules. All traffic will be classified into the first QoS Rule that is a match, or
fall into the Default Rule. The most specific QoS Rule will always be first. For example,
a QoS Rule that identified a Source and Destination IP Address would be assigned ahead
of a rule that just defined RTP protocol. QoS Rules that have the same amount of
variables defined are sorted as follows:
15–9