How Bandwidth allocation works in 802.11 Network using DCF for e.g

Question

I'm studying how 802.11 dcf mode with CSMA/CA works but I got to a confusing point...

since DCF only one user can transmit successfully to an AP (receiver) (considering ideal case when exposed terminals and they don't transmit together at the same time)

so we are allocating the full bandwidth available in the receiver to that transmitting user ? so here how bandwidth allocation works if I have another user for example contending for the channel.

since there is only one user getting access to transmit what's the logic behind bandwidth allocation ?

thanks in advance. Kind Regards.

Answer 1

well, bandwidth doesn't get allocated so much as used, and DCF is used to determine whether a host can get dedicated access to a shared medium (in this case the channel) long enough to send its packet. Often we use the term 'bandwidth' incorrectly. Bandwidth is really a constant based on the network media. What you are really interested in is Transfer rate, which can loosely be defined as the amount of time the channel is yours to use exclusively times the actual width of the band the channel occupies.

Remember, at any given Nanosecond, exactly 0-1 host can be sending. There can never be two hosts transmitting at exactly the same nano, or a collision will occur, and hosts need to back off.

Just like with 802.3 (CSMA/CD) networks, when the medium is in use, its full bandwidth is being used (it is not possible to use less than 100%), and the different packets are being Time-sliced so that they can get access to the medium after waiting in line a second, so that other terminals can get their turn too.

From that perspective, the transfer rate you are referring to is the aggregation of the actual bandwidth X the amount of time its used. using a 10Mib pipe/channel for 1 second yields a " bandwidth" of 10Mib/s. The real question is, does it take 5 seconds in wall time to get 1 second of access to the wire? if that is the case, the channel has used 100% of its capacity for 5 seconds (10Mbps), but your terminal got only one second of the time [1/5 * (10Mbps)].

In Summation, these algorithms are NOT about allocating bandwidth. they are exclusively concerned about whether the channel is free so your terminal can use it this nano, without a collision.

Hope that helps clarify

Answer 2

On DCF each equipment tests the channel and if it's free, it simply transmits using all the bandwith available. The other equipments when sense that the channel is busy wait a random time before trying again.

To make it clear, I will use an example:

Imagine you are part of a group of people trying to give an speech on a dark room. You don't know how many people is in the room and don't know exactly when you have to start your speech.

When you hear a long silence you try to take that turn to give your speech and all the others don't have any choice than hear you because you are using all the "bandwith".

However, it can happen that you start at the same time that another one, so only one continues its speech (two speeches at the same time would be total confusion) and the other one waits for another long silence.