Quality Of Service; Maximum Bandwidth/Priority Qos Mode - Comtech EF Data CDM-570 Installation And Operation Manual

CDM-570/570L Satellite Modem with Optional IP Module
Ethernet IP Module Interface
13.4.5

Quality of Service

Quality of Service (QoS) is an optional feature of the IP Module. Select one of three modes of
QoS operation:
•
Mode 1 – QoS Rules based on Maximum Bandwidth and Priority
•
Mode 2 – QoS Rules based on Minimum and Maximum Bandwidth
•
Mode 3 – DiffServ.
QoS Segmentation and Reassembly (SAR): Packet Segmentation and Reassembly (SAR) is
enabled automatically while QoS is enabled. However, SAR is an adaptive process; it will trigger
only if the packet latency exceeds the threshold value (default to 20 msec). Latency value is
calculated based on the satellite transmission bandwidth. There is no minimum segment size.
However if the last segment is less than 16 bytes, then it will be appended to the previous
segment excluding satellite HDLC header in order to avoid satellite overhead and consumption of
CPU cycles.
Weighted Random Early Detection (WRED): The MIN-MAX and MAX-Priority QoS modes
allow enabling or disabling of the WRED option. In Diffserv mode, WRED is applicable to Asure
forwarding only; however, the WRED option can be changed. WRED allows for more graceful
dropping of packets as QoS queues get full. Typically, without WRED, packets are dropped
based upon a simple tail drop algorithm that is applied to packets as they are 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 increases as the queue becomes full, and minimizes the chances of global
synchronization. Thus, WRED allows the transmission line to be used fully at all times.
System Latency: System latency is used to define the maximum duration that a packet will stay
in a QoS queue. Rather than waste satellite bandwidth on invalid packets, this mechanism serves
to ensure that old packets are "aged" out of the system.

13.4.5.1 Maximum Bandwidth/Priority QoS Mode

QoS Rules can be assigned to up to 32 different types of flows to be user-defined. 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
then has a Priority assigned according to the defined QoS Rules
•
Any latency critical traffic such as VoIP/RTP should always be assigned Priority 1
•
Priority 1 packets are forwarded immediately; Priority 2 packets are forwarded as soon as
there are no Priority 1 packets in the Queue; and so on
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Revision 12
MN/CDM570L.IOM