802.11p [FreeBSD WIKI]

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This is a draft summary of the IEEE 802.11p amendment, which is referred to 11p in this text. The amendment is now part of the IEEE 802.11 2012 standard.

Key points seems to be: how to calculate timing and estimate error. Also one can use external time source like GPS. Another thing seems to be 5.9GHz, each channel is 10 Mhz bandwidth. There seems to be some changes to power transmit spectrum mask and receiver performance for improved immunity to out-of-channel interfering emissions. Also new transmit power levels.

11p is only part of a group of standards related to all layers of protocol for DSRC based operations (Dedicated short-range communications). The upper layers are defined in the IEEE 1609 family of standards.

List of notes about the 11p amendment. The goal of this list is to give a good overview of what needs to be changed in net80211 to support 11p:

* Wilcard BSS (WAVE BSS beacon)

* 11p allows STA transmission of data outside BSS context. That is allowed only when not associated with IBSS or BSS (even MBSS).

* 11p does not utilize authentication, association or data confidetiality servies.

* 11p is well suited for use in rapidly varying communication enviroments for very short-duration in the order of tens or hundreds of milliseconds.

* Communication for 11p may take place at predefined frequency bands (regulator designation) or other out-of-band communication.

* 11p is single radio multichannel defined to work between 5.860 - 5.920 GHz. The inerval is divided in 7 channels with 10 Mhz non-interleaving channels. The channels are 172, 174, 176, 178, 180, 182, 184

* The 7 channels are divided into 1 channel for broadcasting and control and 6 channels for ongoing data transactions.

* A strict requirement is for the MAC to reset in 2 TU.

* 11p defines a new frame of type management called Timing Advertisement.

* A frame with toDS=0 and fromDS=0 can also mean outside the context of a BSS.

* For %%QoS%% traffic streams are not used and the TID always corresponds to a TC (XXX:not sure what TC is).

* For 11p there is a new set of EDCA parameter set for %%QoS%% frames (Tabel 7-37a in amendment).

* 11p time advertisement is used by a receiving STA to align it's own estimate of the time standard based on that of another STA.

* TXOP limits shall be zero for each AC.

* 11p adds a new field to to MLME-VSPICIFC: RCPI which is the measured value of received channel power on the received Vendor Specific Action frame.

* 11p adds MLME-GETTSFTIME to request the current value of the TFS timer that the STA maintains.

* From 11p amendment:  "NOTE—The TSF timer value can be used, along with other information, by the SME to compute an offset between an external time standard such as a version of Universal Coordinated Time (UTC) from a Global Positioning System (GPS) unit and the TSF timer.".

* 11p STA are not a member of a BSS and therefore not required to to synchronize to a common clock or use the mechanism.

* 11p STA maintains a TSF timer for other purposes than synchronization.

* 11p may send management of subtype Action and if it maintains a TSF timer it may send of subtype Timing Advertisement.

* 11p may send control frames except those of subtype PS-Poll, CF-End, and CF-End + CF-
Ack.

* 11p STA may send data frames of subtype Data, Null, %%QoS%% Data, and %%QoS%% Null.

* 11p shall set BSSID to wildcard in all management and data frames.

* 11p shall se information from the CF Parameter Set element of all received Beacon frames, without regard for the BSSID, to update its NAV.

* From amendment: "When a STA transmits the Timing Advertisement frame, the Timestamp shall be set to the value of the STA’s TSF timer at the time that the data symbol containing the first bit of the Timestamp is transmitted to the PHY plus the transmitting STA’s delays through its local PHY from the MAC-PHY interface to its interface with the WM [e.g., antenna, light emitting diode (LED) emission surface].
A STA can advertise a time standard by transmitting a Timing Advertisement frame, which includes the Time Advertisement information element. As defined in 7.3.2.61 the Time Advertisement information element contains two estimates. The Time Value field contains an estimate of the difference between a time standard and the timestamp included in the same frame. The Time Error field contains an estimate of the standard deviation of the error in the estimate in the Time Value field. The time standard can be derived from an external time source. A STA with an external time source can implement an estimator in a variety of ways, which are beyond the scope of this standard."

References:

* 11p amendment

* "An Efficient %%QoS%% MAC for IEEE802.11p Over Cognitive Multichannel Vehicular Networks" thesis by Hikmat El Ajaltouni.