Requested revision
Standard:802.1Q-2011Clause:I.5
Clause title:Supporting the credit-based shaper algorithm
Rationale for revision
Table 8-4 does not show "a set of recommended priority to traffic class mappings where the credit-based shaper algorithm (8.6.8.1) is supported by one or two of the available traffic classes". As correctly stated in 8.6.6 "Table 8-4 shows the recommended mapping for the number of classes implemented, in implementations that do not support the credit-based shaper transmission selection algorithm (8.6.8.2)". The mappings which support the credit-based shaper are shown in 34.5. "the recommended mappings shown are intended for use where priority 5 is used to support SR class A and priority 4 is used to support SR class B" The recommended priorities are 3 (SR class A) and 2 (SR class B). The reference to table G-3 is wrong. (Such a table does not exist.) In the tables I-4 and I-5 the acronyms VO ("voice") and VI ("video") are used for the traffic types of the two SR classes. In table I-2 the traffic type "voice" is associated with priority 5 and "video" is associated with priority 4. This does not correspond with the recommended priorities for the SR classes. Table I-4 "Credit-based shaper support of one SR class" shows the traffic types in the case that only SR class A is supported. "SR class B only" seems to be the more common case. (According to IEEE 802.1BA "All Bridges shall support SR class B".) Table I-4 should show the "SR class B only" case instead (in order to be consistent with table 34-2).
Proposed text
Change "Table 8-4 defines a set of recommended priority to traffic class mappings where the credit-based shaper algorithm (8.6.8.1) is supported by one or two of the available traffic classes; the recommended mappings shown are intended for use where priority 5 is used to support SR class A and priority 4 is used to support SR class B." to "The tables 34-1 and 34-2 define a set of recommended priority to traffic class mappings where the credit-based shaper algorithm (8.6.8.1) is supported by one or two of the available traffic classes; the recommended mappings shown are intended for use where priority 3 is used to support SR class A and priority 2 is used to support SR class B." Change "table G-3" to "table I-3" Change the traffic type - priority mapping concept so that it fits to AVB. Change table I-4 to "SR class B only".
Impact on existing networks
None, assuming every implementation is based on the normative text. A bridge or an end station using the wrong priority would not be able to send a stream over a network or to an end station which uses the correct one. Using the shaper with the wrong priority - traffic class mapping would lead to a higher and completely unpredictable latency for AVB streams.
Originator
Name:Christian BoigerEmail:christian.boiger@fh-deggendorf.de
Affiliation:Hochschule Deggendorf - University, Hirschmann
Submitted:2011-09-14