Q

What kind of systems use the CSMA protocol?

Besides the CSMA/CD protocol, there is a CSMA protocol. What kind of systems use the CSMA protocol? Why CSMA not CSMA/CD?
Carrier Sense Multiple Access (CSMA) refers to a family of protocols used by stations contending for access to a shared medium like an Ethernet cable or a radio channel. There are multiple "flavors" of CSMA; each has a different way of dealing with the collisions that can occur when more than one station attempts to transmit on the shared medium at the same time.

CSMA/CD (Collision Detection) is used by Ethernet stations to detect frame collisions on 802.3 wired LANs. Collision detection requires sending stations to listen for collisions and use a backoff algorithm to avoid repeating the collision when re-transmitting.

CSMA/CA (Collision Avoidance) is used by Wi-Fi stations to avoid frame collisions on 802.11 wireless LANs. Collision avoidance requires that stations request to send data before actually sending it. This control frame request/response (RTS/CTS) means higher overhead. But it can be more efficient when the cost of re-transmission is high, such as when longer data frames are sent over wireless.

One reason that CSMA/CA is used in wireless instead of CSMA/CD is the "hidden node" problem. On an Ethernet LAN, every station can "hear" every other station. In a wireless LAN, some stations can "hear" the access point but not other distant stations on the far side of the access point. Two stations that cannot hear each other cannot detect collisions, so CSMA/CD would not work properly. CSMA/CA lets these stations ask for permission first, thereby avoiding collisions.

 

This was first published in September 2003

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