WLAN and CSMA/CA Questions.

Based on the Lammle book, it seems that only 802.11b uses CSMA/CA. And that 802.11g only use CSMA/CA when a client uses a 802.11b NIC that forces the 802.11g AP to use DSSS.

Does 802.11b, 802.11a, 802.11g all use CSMA/CA?

But on the Wendell Odom book, it seems that all of them use CSMA/CA.

Lammle says that CSMA/CA is also called RTS/CTS but on the page on wikipedia. CSMA/CA is a different protocol to RTS/CTS.

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Comments

alliasneo

as I understand it I thought all wireless standards used the CSMA/CA technology. Basically taking the CSMA/CD from the Ethernet standard and using Collision Avoidance instead. How else would they know when to transmit? But I could be wrong.

SharkDiver

Seems right that they are all CSMA/CA.

Can someone find the white paper for that? I looked and couldn't find it.

onesaint

From the IEEE 802.11 standards, the definition for an HCF and CSMA/CA can be found which appears to be applicable to all 802.11 standards (although not explicitly stated). From the standards definition - 2007:

3.26 coordination function: The logical function that determines when a station (STA) operating within a
basic service set (BSS) is permitted to transmit protocol data units (PDUs) via the wireless medium (WM).
The coordination function within a BSS may have one hybrid coordination function (HCF), or it may have
one HCF and one point coordination function (PCF) and will have one distributed coordination function
(DCF). A quality of service (QoS) BSS will have one DCF and one HCF...

9.1.1 DCF
The fundamental access method of the IEEE 802.11 MAC is a DCF known as carrier sense multiple access
with collision avoidance (CSMA/CA). The DCF shall be implemented in all STAs, for use within both IBSS
and infrastructure network configurations.

For a STA to transmit, it shall sense the medium to determine if another STA is transmitting. If the medium is
not determined to be busy (see 9.2.1), the transmission may proceed. The CSMA/CA distributed algorithm
mandates that a gap of a minimum specified duration exist between contiguous frame sequences. A
transmitting STA shall ensure that the medium is idle for this required duration before attempting to transmit. If
the medium is determined to be busy, the STA shall defer until the end of the current transmission. After
deferral, or prior to attempting to transmit again immediately after a successful transmission, the STA shall
select a random backoff interval and shall decrement the backoff interval counter while the medium is idle. A
transmission is successful either when an ACK frame is received from the STA addressed by the RA field of
the transmitted frame or when a frame with a group address in the RA field is transmitted completely. A
refinement of the method may be used under various circumstances to further minimize collisions—here the
transmitting and receiving STA exchange short control frames (RTS and CTS frames) after determining that
the medium is idle and after any deferrals or backoffs, prior to data transmission. The details of CSMA/CA,
deferrals, and backoffs are described in 9.2. RTS/CTS exchanges are also presented in 9.2.

But to further explain is this great white paper done on the "Impact of Legacy Devices on
802.11n Networks" from the folks at (Airmagnet) NLE, which states,

...it is useful to think of STAs in terms of the PHY
(physical) layer that is in use by them, as opposed to their 802.11 amendment
designator(s). 802.11 has defined seven PHYs as of 802.11n. Ignoring the IR
(Infrared) and FHSS (Frequency Hopping Spread Spectrum) PHYs, they are:

Clause 15 (DSSS PHY for 2.4 GHz, defined in the original 802.11 specification)
Clause 17 (OFDM PHY for 5 GHz, defined in the 802.11a amendment)
Clause 18 (HR/DSSS PHY for 2.4 GHz, defined in the 802.11b amendment)
Clause 19 (ERP PHY for 2.4 GHz, defined in the 802.11g amendment)
Clause 20 (HT PHY for 2.4 and 5 GHz, defined in the 802.11n amendment)

Each of these PHYs define the operating band(s), radio preamble / PLCP format(s),
modulation types and operating rules that a STA may use. It is important to
understand PHY compatibilities, particularly the radio preambles, since they are used
to sense the medium’s condition as busy or not. The point here is that devices with
incompatible PHYs can not sense each other as occupying the medium – which would
clearly lead to a break-down in the CSMA/CA system. 802.11 copes with this by
mandating that later-amendment PHYs (operating in the same band as previously
defined PHYs) must also support those previously defined PHYs.

Great question. It was fascinating to research.