CSMA/CD vs CSMA/CA

[Abdul salaam]

Hello guys

I read cisco documentary about CSMA/CD it is a mechanism used with hub to detect collision while multiple access but I couldn't find anything about its timer and jam message also is it still mechanism for switch while each port of switch is single collision domain.

CSMA/CA is used in wireless, a cisco instructor said at a time from one channel with multiple host, only one host can transmit the rest should await until turns come, but i see multiple users connected to Wi-Fi simultaneously send and receive data.

Thank you 

[Kasun Bandara]

this will give very good explanation for CSMA/CD. https://www.softwaretestinghelp.com/what-is-csma-cd/

and https://en.wikipedia.org/wiki/Carrier-sense_multiple_access_with_collision_detection

for CSMA/CA, time sharing is not use slots which are like seconds. it working in very small time slots. so users are not feels time slot is swapping and allocating new slots. 

[TotallyTodd · Accepted solution]

Hi, with a hub totally, it uses CSMA/CD which is collision detection that is mainly used after a collision has already occurred. When a collision occurs there becomes a jam signal on the wire. Before resending, the host will wait a random amount of time before resending. With a wireless connection using CSMA/CA , it uses a mechanism to avoid collisions before they occur. Essentially, this mechanism will look on the wire before sending the traffic to make sure that it is clear. It uses signals called a request to send to an access point and the access point will in return send a clear to send signal to signal that the wire is clear to allow traffic. To allow multiple users to use the same wifi, some protocols use MIMO which is multiple input multiple output totally. Although this may seem simultaneous, each packet his handled individually and 802.11 headers allot a certain amount of time for each packet to stay on the network called the duration id. Totally alot that goes into wifi. I can post a flow chart if you would like for each process totally. 

[Abdul salaam]

Yes please post the flow chart

[Joseph W. Doherty]

"CSMA/CD it is a mechanism used with hub to detect collision while multiple access"

Firstly, CSMA/CD predates Ethernet hubs.  Secondly, unlike 10Base2 or 10Base5, with 10BaseT and hubs, the latter's collisions are no longer purely physical.

"also is it still mechanism for switch while each port of switch is single collision domain."

Yes it is, if 10BaseT switch port running in half-duplex mode, basically interacting with Ethernet hosts as if they were connected to a hub, well, physically, at L1.  But some switches can "play tricks" with half-duplex hosts, such as mimicking collisions for flow management (where a switch with a full-duplex host could use pause frames for flow management).

"CSMA/CA is used in wireless, a cisco instructor said at a time from one channel with multiple host, only one host can transmit the rest should await until turns come"

Yea, that what's desired, but that's not exactly how Wi-Fi works.  It's more complex.

"multiple users connected to Wi-Fi simultaneously send and receive data."

I'm sure that seems so.  Just as you'll see multiple hosts on a hub simultaneously send and receive data.  Often there's no simultaneously, just very, very fast sharing.

BTW Ethernet's CSMA/CD also does collision avoidance.  Wi-Fi's collision avoidance, doesn't always work, nor, as mentioned by others, is RTS and CTS always used.  Again, both are more complex.

I just quickly skimmed the Wiki articles on both, and Wiki articles (to me) seem to be fairly accurate without getting deep, deep into those subjects, you might want to look at Wiki's CSMA/CD and/or CSMA/CA write ups.

[addendum - edit]

Oops, just noticed one of @Kasun Bandara references is the same CSMA/CD Wiki article.  So, independently, we both appear to believe it's good.