I have multiple questions.

  1. Where is/were CSMA protocol really implemented ? i.e In our computer ? routers ? switch ? or which component ? is is s/w or h/w based ?

  2. Is it still used ? if not, which one is being currently used ?

  • Did any answer help you? if so, you should accept the answer so that the question does not keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Dec 23, 2021 at 18:53

3 Answers 3


CSMA is used in both Ethernet (CSMA/CD) and Wi-Fi (CSMA/CA). Every device that uses one or the other uses it.

It's almost always implemented in hardware on the network interface.

  • wow! you answered all my questions clearly. Thank You.
    – Xenon OP
    Apr 9, 2021 at 18:33
  • 2
    It used to be used on Ethernet, back when we were using coax (10base2) or non-switched half-duplex twisted pair. With switched Ethernet, the medium is no longer shared, so there no CSMA/CD on use. No carrier sense, no multiple access, no collision detection.
    – jcaron
    Apr 10, 2021 at 0:18

CSMA is for a shared medium. A host senses the carrier to determine if it is clear before sending.

This is required for something like Wi-Fi, where the medium is shared, otherwise all hosts would just send, stepping on each other, resulting in garbled signals.

It is still used for network protocols used on a shared medium.

  • Don't forget Ethernet!
    – Ron Trunk
    Apr 9, 2021 at 17:49
  • I was trying to give an example of a use case.
    – Ron Maupin
    Apr 9, 2021 at 17:51
  • 2
    CSMA/CD is all but obsolete - it's only supported for 10/100 Mbit/s operation. It's been defined for 1000 Mbit/s as well but there are no devices supporting it. Modern Ethernet is fully switched and full duplex at all times.
    – Zac67
    Apr 9, 2021 at 18:36

CSMA/CA is a generic packet-based multiple access method, which the MAC layer of the 802.11 specification decides to employ. It is one of the functions of medium access control (MAC). In communication protocols, MAC layers always encompass a set of packet types and then the structure of those packets. This encompasses the separate packets (in the case of 802.11), or the fields in the core packets, which are used to negotiate multiple access over the channels presented by the physical layer specification. It also encompasses the packet addressing.

The multiple access method simply defines how multiple devices can transmit over a channel at the same time. The channel is defined by the PHY layer of 802.11.

The MAC layer actually uses the DCF medium access control method, which employs CSMA/CA and applies binary exponential backoff and specific timing parameters such as DIFS, as well as RTS/CTS. DCF is packet based and 802.11 control packets implement DCF, such as RTS/CTS and ACK which are sent by the MAC layer state machine to the physical layer circuitry when a packet needs to be sent. The physical layer then sends these packets to the antenna, by first adding a PLCP header and preamble defined by the physical layer specification and then sending them to the antenna according to the multiplexing and modulation scheme (and coding rate) that should be used for the preamble and body according to the physical layer specification – in 802.11a PHY, the preamble and signal field of the PLCP header is sent as OFDM/BPSK and the rest is sent as whatever is defined in the signal field which may be OFDM/QAM for instance (where / is the multiplexing and modulation scheme respectively)

It is the layer 2 hardware that determines high level properties of the wireless channel, such as knowing when there's a collision when physical layer doesn't pass to it a received transmission that happens to be the expected CTS within an expected time period. These are higher level details than the physical specification.

CSMA/CA and CSMA/CD differ in that CA uses an ACK packet because with a radio you can't sense collisions and send data at the same time. CSMA/CA can be optionally enhanced with RTS/CTS.

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