Bugzilla – Bug 1142
Wrong 802.11p Slot time
Last modified: 2011-05-13 11:40:47 EDT
By looking at the source code of the WifiMac class (src/devices/wifi/wifi-mac.cc) I have found a wrong slot time value for 802.11p CCH and SCH, in particular, at lines 340 and 351, you find: SetSlot(Microseconds(16)); instead of 13 micros. 802.11p uses an OFDM PHY layer in the 5.9 GHz band, BW 10MHz and table 17-15 at page 626 of the 802.11-2007 standard says that the slot time for OFDM with 10MHz BW must use a 13 micros slot time, which is indeed how it is done in WifiMac::Configure80211_10MHz. In my opinion, WifiMac::Configure80211p_CCH and WifiMac::Configure80211p_SCH should simply invoke WifiMac::Configure80211_10MHz. NB: this error is present also in the ns-3.10 release. Best Michele Segata
I looked into http://dx.doi.org/10.1109/IEEESTD.2010.5514475 (IEEE 802.11p) and it does not mention anything. The same for http://dx.doi.org/10.1109/IEEESTD.2006.254109 (IEEE 1609.4-2006, which says "The specific designations of these channels and the specification of the PHY are defined in IEEE Std 802.11, as amended by IEEE P802.11p."). So I would refer to 802.11-2007.
I did the same check and I came to the same conclusion, i.e., the slot time is defined by 802.11-2007 section 17.3.8.6 Slot time: "The slot time for the OFDM PHY shall be 9 s for 20 MHz channel spacing, shall be 13 µs for 10 MHz channel spacing, and shall be 21 µs for 5 MHz channel spacing. Where dot11RegulatoryClassesRequired is true, the value of the slot time shall be increased by the value of 3 µs × coverage class. The default value of coverage class shall be zero." I have been wondering whether 802.11p or 1609.4 specify any non-zero coverage class (this could justify a 16us slot) but after a quick search I would say that this is not the case.
Created attachment 1112 [details] quick fix I think the Configure80211p_CCH and Configure80211p_SCH methods will eventually go away when bug 945 is resolved, but I would leave them around for the moment being. That said, here's a proposed patch. Ruben, Michele, do you think this is satisfactory?
(In reply to comment #3) > Created attachment 1112 [details] > quick fix > > I think the Configure80211p_CCH and Configure80211p_SCH methods will eventually > go away when bug 945 is resolved, but I would leave them around for the moment > being. > > That said, here's a proposed patch. Ruben, Michele, do you think this is > satisfactory? Yes, this is exactly what I meant :)
(In reply to comment #3) > Created attachment 1112 [details] > quick fix > > I think the Configure80211p_CCH and Configure80211p_SCH methods will eventually > go away when bug 945 is resolved, but I would leave them around for the moment > being. > > That said, here's a proposed patch. Ruben, Michele, do you think this is > satisfactory? When a patch removes code, I always like it :-) +1
changeset: 7162:c29fbeddbc04 tag: tip user: Nicola Baldo <nbaldo@cttc.es> date: Fri May 13 17:40:37 2011 +0200 summary: Bug 1142 - Wrong 802.11p Slot time