Nortel Networks NB5PLUS4/W manual TOS-to-Priority Mapping

The weighted values used for the WRR scheduler will be calculated based on the percentage weights the user inputs in the Web UI as stated above in 1.) Queue Priorities.

Example: User selects a Medium Queue Weight = 60 %, and Low Queue Weight = 40%. Then the O(1) scheduling array will look like {L, M, M, L, M, M, L, M, M, L} where L and M represents a scheduling cycle for the respective Low and Medium queues.

Low Latency Queue (Fragmentation and Interleaving) for Voice Traffic

With Voice traffic shared over same PVC with Data traffic, the simple packet classifi- cation and prioritization will not suffice to achieve the low latency required by voice. In this case, a voice call triggers dynamic flushing of existing data packets from device queues (including DSL device driver) for Head of Line Blocking removal, and IP MTU resizing based on uplink bandwidth for fragmentation and packet interleav- ing of voice and data. Below is an example of MTU calculations:

	Total delay PSTN delay			Maximum Data Fragment size
	end-to-end budget			based on upstream bandwidth (bytes)
VIF	(ms)	(ms)	100kbps	150kbps	200kbps	250kbps
10ms	200	100	207	363	519	675
20ms	200	100	82	175	269	363
30ms	200	100	x	x	19	50

For Voice traffic priority an extra EF queue was added to PRIOWRR. This extra queue should not be exposed via WebUI config for data usage. Its use is triggered internally by the voice app using the socket options system calls. Voice packets are using this EF queue. Signalling for Voice uses the next EF queue that’s also exposed on the web config. This means that voice signalling can be mixed with data if user configures data for High Priority.

TOS-to-Priority Mapping

High Priority Marking for Expedited Forwarding Queue: DSCP Mark: xx1000

Medium Priority Marking:	DSCP Mark: xx0100
Low Priority Marking for Best Effort:	DSCP Mark: xx0000

The four TOS bits (the ‘TOS field’) are defined as:

Binary		Meaning
1000		Minimize delay (md)
0100		Maximize throughput (mt)
0010		Maximize reliability (mr)
0001		Minimize monetary cost (mmc)
0000		Normal Service
TOS	Bits	Means	Linux Priority	Queue Priority	Band
0x0	0	Normal Service	0	Best Effort	2
0x2	1	Minimize Monetary Cost	1	Filler	2

YML754 Rev1	NB5Plus4/W User Guide
www.netcomm.com.au	47