wifi - What is the reason CSMA/CD can't be used on a wireless network?

  • Ian Muscat

    I'm new to wireless networking and wondering why the same Collision Detection mechanisms used for Ethernet can't be applied to WiFi. I think I'm starting to understand, but not sure if I've got it:

    The physical characteristics of WiFi make it impossible and impractical for the CAMA/CD mechanism to be used. This is due to CSMA/CD’s nature of ‘listening’ if the medium is free before transmitting packets. Using CSMA/CD, if a collision is detected on the medium, end-devices would have to wait a random amount of time before they can start the retransmission process. For this reason, CSMA/CD works well for wired networks, however, in wireless networks, there is no way for the sender to detect collisions the same way CSMA/CD does since the sender is only able to transmit and receive packets on the medium but is not able to sense data traversing that medium. Therefore, CSMA/CA is used on wireless networks. CSMA/CA doesn’t detect collisions (unlike CSMA/CA) but rather avoids them through the use of a control message. Should the control message collide with another control message from another node, it means that the medium is not available for transmission and the back-off algorithm needs to be applied before attempting retransmission.

    Am I on the right track or is there something else I should be considering?

  • Answers
  • Chris S

    You've got acronyms mixed up left and right, but the basic gist is right.

    Wireless transceivers can't send and receive on the same channel at the same time, so they can't detect collisions. This is due to the fact that there's an incredible difference between send power (generally around 100mw) and receive sensitivity (commonly around 0.01 to 0.0001mw). The sending would cover up any possible chance of receiving a foreign signal, no chance of "Collision Detection". For this reason Collision Avoidance with Control Messages is necessary.

    On most wired networks the (like Ethernet) the voltage is around 1 to 2.5v; both sending and receiving are roughly the same voltage. So if you're sending a 2.5v signal, and someone else collides with a -2.5v signal, the "Detection" parts will see a signal somewhere around 0v and know a collision occurred.

  • Related Question

    802.11 - What is the maximum length of a wifi access point's SSID?
  • leeand00

    I was wondering if anybody knew what the maximum string length of a browser's SSID is or where I could go to look for that sort of information. (From a spec of some sort)

  • Related Answers
  • splattne

    According to the documentation of the standard, the length of an SSID should be a maximum of 32 characters (32 octets, normally ASCII letters and digits, though the standard itself doesn't exclude values).

    Some access point/router firmware versions use null-terminated strings and accept only 31 characters.

    Here is the paragraph defining from the IEEE standard document :

    PDF excerpt

    Download link: PDF. Telecommunications and information exchange between systems — Local and metropolitan area networks — Specific requirementsPart 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications)*

  • koenigdmj

    This page says 32 characters.

  • MDMarra

    An SSID for a wireless network has a maximum length of 32 characters

  • Marcin

    Changing the length of SSID is irrelevant to the security of the wifi network. It can be sniffed from air easily, kismet will show it to you regardless if you're 'hiding' SSID or not.

  • Area 51

    it is 32 characters so if your 64 bit WEP key is 32 characters long you can use it also as your SSID. it is not safe to have both the SSID and the WEP KEY the same.