Some more details inside regarding wireless QoS. jamal wrote: > On Mon, 2007-11-06 at 17:30 +0300, Cohen, Guy wrote: > > > > > For WiFi devices the HW often implements the scheduling, especially when > > QoS (WMM/11e/11n) is implemented. There are few traffic queues defined > > by the specs and the selection of the next queue to transmit a packet > > from, is determined in real time, just when there is a tx opportunity. > > This cannot be predicted in advance since it depends on the medium usage > > of other stations. > > WMM is a strict prio mechanism. > The parametrization very much favors the high prio packets when the > tx opportunity to send shows up.
Sorry, but this not as simple as you describe it. WMM is much more complicated. WMM defines the HW queues as virtually multiple clients that compete on the medium access individually. Each implements a contention-based medium access. The Access Point publishes to the clients the medium access parameters (e.g. back off parameters) that are different for each access category (virtual client). There is _not_ a strict priority assigned to each access category. The behavior of each access category totally depends on the medium usage of other clients and is totally different for each access category. This cannot be predicated at the host SW. > > Hence, to make it possible for wireless devices to use the qdisc > > mechanism properly, the HW queues should _ALL_ be non-empty at all > > times, whenever data is available in the upper layers. > > agreed. > > > Or in other > > words, the upper layers should not block a specific queue because of the > > usage of any other queue. > > This is where we are going to disagree. > There is no way the stack will send the driver packets which are low > prio if there are some which are high prio. There is therefore, on > contention between low and high prio, no way for low prio packets to > obstruct the high prio packets; And this is not the right behavior for a WLAN stack. QoS in WLAN doesn't favor strictly one access category over another, but defines some softer and smarter prioritization. This is implemented in the HW/Firmware. I just think that providing a per-queue controls (start/stop) will allow WLAN drivers/Firmware/HW to do that while still using qdisc (and it will work properly even when one queue is full and others are empty). > however, it is feasible that high prio > packets will obstruct low prio packets (which is fine). No this is _not_ fine. Just to emphasize again, WMM doesn't define priority in the way it is implemented in airplane boarding (Pilots first, Business passengers next, couch passengers at the end), but more like _distributed_ weights prioritization (between all the multiple queues of all the clients on the channel). As a side note, in one of the WFA WMM certification tests, the AP changes the medium access parameters of the access categories in a way that favors a lower access category. This is something very soft that cannot be reflected in any intuitive way in the host SW. > cheers, > jamal - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
