Mitel MiVOICE BUSINESS, Engineering Manualline

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Engineering Guidelines
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The 3300 ICP controller and IP phones use, by default, a value that is compatible with the
Type-of-Service format for priority and TOS. This complies with RFC 791, but also by choice
of value, RFC 1122 and RFC 1349. However, updates in the definition of DiffServ mean that
voice is better suited to a slightly different class of service. This is the Expedited Forwarding
class and uses a DSCP value of 46, rather than the older TOS value of 44.
Figure 32: Type-of-Service field using precedence
Figure 33: Differentiated Services Code Point in the Type-of-Service field (newer definition)
The Layer 3 precedence field (TOS), and DSCP, are similar in operation to the Layer 2 IEEE
802.1p field. In fact, many network devices offer the capability of mapping between the different
schemes. Once a TOS value, or DSCP, is chosen, it generally never changes. The voice
application sets the appropriate values before data is sent.
For networks based around legacy TOS and Precedence routers, the Mitel voice applications
should use the TOS value of 0xB0, or 176 decimal, or 0xB8 (184 decimal), for the
Type-of-Service field, providing a precedence of five with minimum delay (the D-bit is set). This
is equivalent to a DSCP value of 44, or 46 respectively.
For newer installations based on DSCP routers, a programmable DSCP value of 46 is
recommended, in order to utilize the highest priority queue, Expedited Forwarding (EF).
For DiffServ routers, the fixed value equates to a value of 0x2C, or 44 decimal. This is the
default value. However, it is recommended for DiffServ (DSCP) based routers that the value of
46 be used to utilize the highest priority queue, Expedited Forwarding (EF).
The only requirement is that the router support priority queuing mechanisms, such as Weighted
Fair Queuing, Class-Based Weighted Fair Queuing (also known as Low Latency Queue, LLQ)
or similar. For DSCP configurations ensure that the Expedited Forwarding queue is enabled.
Generally, routers that support DSCP will group different classes or groups of numbers into
particular queues. Check the settings on these and include, where possible, DSCP value 44
into the Expedited Forwarding class with DSCP value 46. Note also that a number of access
Layer 2 switches can overwrite the DSCP value based on the incoming Layer 2 priority
information. Ensure that such ports use a consistent DSCP value, in this case 46.
With a Layer 3 device, such as a router, the packet-per-second (PPS) throughput is also
important. An IP phone with a packet rate of 20 ms means that the phone sends 50 packets
per second and also receives 50 packets per second. Be aware of how vendors specify the
PPS rating. For example, with two phones connected to a router, each port sends and receives
50 PPS—that is, 100 PPS per port, requiring that 200 PPS be handled. However, between the