WLAN Technologies – Purpose of CSMA and OFDM

ieee 802.11lanwireless

Do wireless networks employ both Carrier Sense Multiple Access and Orthogonal Frequency Division Multiplexing? If so, what are their corresponding roles? I'm not a professional and I am confused as to which of them makes P2MP communication possible. I always thought WLANs use CSMA to allow different nodes to communicate to the same access point on the same channel, but here I am reading about how every 802.11 standard uses OFDM as its access technology (except 802.11b which uses DSSS instead), allowing multiple devices to communicate with the same AP at the same time.

Is this the right place for PHY-related questions?

EDIT: changed TDM to CSMA to be more accurate.

Best Answer

Finally found out that in Wifi, CSMA is the one used to allow Multiple Access. To be exact, IEEE 802.11 releases use CSMA/CA (Collision Avoidance) instead of CSMA/CD (Collision Detection) which is used in wired networks.

OFDM, on the other hand, is just an advanced digital modulation scheme which divides the data stream being exchanged between the node and the AP to multiple individual streams of lower rate and transmits them on different, closely-spaced carrier frequencies simultaneously (multiplexing). Making the signal more robust against error--thus allowing higher data rates--is what OFDM does in a nutshell.

Related Solutions

Wireless – configuring IBSS WLANs

On Linux for all cards that support it (used to be all, many these days don't) you simply run:

iwconfig wlan0 mode Ad-Hoc
iwconfig wlan0 essid Foo
iwconfig wlan0 channel 6

You can do WEP, and apparently even TKIP-based WPA.

Once configured you can treat it like a classic ethernet bus (Think Coax or 10baseT hubs), with one big caveat, the "hidden node problem", in ad-hoc there's no repeaters so if two nodes can't make direct radio contact with each other they can't talk (at that layer, you can of course bridge or route at a higher level).

Wireless – what to know about DSSS vs OFDM

From what I've gathered, the primary differentiation between DSSS and OFDM comes in a congested or constrained RF environment. Because DSSS is transmitting as many bits as possible all at once, it runs the risk of having some of the transmission disrupted in transit should the RF become less than ideal. In contrast, OFDM sacrifices a bit of performance to transmit the data in those multiple carrier "packets" that allow for reassembly or retransmission in the event of a failure.

If I use an extra-large semi truck to haul freight, it works well so long as the highway doesn't have any obstructions (like narrow lanes or short overpasses). If I break my freight up into a greater number of smaller trucks, I can ensure that some of the freight arrives even if conditions (like a traffic jam) prevent the entire shipment from arriving.

Best Answer

Related Solutions

Wireless – configuring IBSS WLANs

Wireless – what to know about DSSS vs OFDM

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