In 802.11 (CSMA/CA), does a higher data rate imply more number of collisions?

Question

In a wireless environment working with the 802.11 protocol suite, where the MAC protocol is based on CSMA-CA (collision avoidance), a higher data rate would mean that data is sent more quickly, which in turn implies that more number of packets are sent successively.

Wouldn't a greater number of packets being sent mean a greater chance of collision? Note that even if powerful error correcting codes or higher modulation schemes are used, collision can't be prevented, as that entirely depends on how many packets are being transmitted 'over-the-air' at any point of time.

Overall, I'm confused how high data rates would work well in a wireless environment, if it resulted in more number of collisions (which would be counter-productive).

Answer 1

a higher data rate would mean that data is sent more quickly, which in turn implies that more number of packets are sent successively.

A higher data rate is one factor. The number of packets sent by a node and the time allocated to a node for exclusive use are other factors.

Wouldn't a greater number of packets being sent mean a greater chance of collision?

collisions are not associated with the number of packets. The more nodes wanting to transmit simultaneously will increase collisions. On the other hand if a single node is the only one transmitting lots of packets, there is no change of collisions

collision can't be prevented, as that entirely depends on how many packets are being transmitted 'over-the-air' at any point of time.

At any point in time only one packet is being transmitted otherwise there is a collision.

how high data rates would work well... if it resulted in more collisions

High data rates will not result more collisions. The faster the packets can get from point A to B, the less time for a second node to want to use the networks at the same time. Less time for the second node to wait for a clear channel means there is less time for a third node to come along and want access. Similar to lower speed limits creating more traffic congestion.

Answer 2

The premise that a higher raw data rate implies a higher packet rate is false. The packet rate would only go up if the network were already saturated with traffic, and the application actually transmitted more packets as a result of the increased available bandwidth.

All other parameters being equal (packet size, payload bandwidth, etc.) a higher raw data rate means that the individual packets are shorter in time, resulting in fewer collisions, not more.

Answer 3

In a wireless environment working with the 802.11 protocol suite, where the MAC protocol is based on CSMA-CA (collision avoidance), a higher data rate would mean that data is sent more quickly, which in turn implies that more number of packets are sent successively.

No. 802.11 allows different modulations, with more bits per symbol the better the S(I)NR gets. So, same number of packets is able to transport more data in a good signal scenario than under a bad signal condition.

Wouldn't a greater number of packets being sent mean a greater chance of collision?

Yes/No. Notice the Carrier Sense in CSMA. Admittedly, assuming stations are trying to access the channel more often, the likelihood of one missing the fact that the channel is currently used within a fixed duration becomes higher, but: Wifi cards (and drivers) are clever. They tend to adapt their MAC strategy (and: aggressiveness) to the occupancy of the channel.

as that entirely depends on how many packets are being transmitted 'over-the-air' at any point of time.

No. Not entirely. It also depends on how you sense, and what arbitration scheme is actually in effect.

Overall, I'm confused how high data rates would work well in a wireless environment, if it resulted in more number of collisions (which would be counter-productive).

Even with more collisions, you'd get more packets across – that's like saying "in a city with 100,000 cars, there's gotta be more traffic jam than in a city with 10,000 cars. I don't see how having 100,000 cars helps getting 100,000 people to work". Well, it does.