Networking / Beginners

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CSMA/CA

Because only a single device can use any network at a time, network nodes must have a way to access the network media without stepping on each other's data packets. Wired Ethernet networks use carrier sense multiple access/collision detection (CSMA/CD), but Wi-Fi networks use carrier sense multiple access/collision avoidance (CSMA/CA). Let's compare both methods.

TIP Wired Ethernet networks use carrier sense multiple access/ collision detection (CSMA/CD). Wi-Fi networks use carrier sense multiple access/collision avoidance (CSMA/CA).

How do multiple devices share the network media, such as a cable? This is fairly simple: each device listens in on the network media by measuring the level of voltage currently on the wire. If the level is below the threshold, the device knows that it's clear to send data. If the voltage level rises above a preset threshold, the device knows that the line is busy and it must wait before sending data. Typically, the waiting period is the length of the current frame plus a short, predefined silence period called an interframe space (IFS). So far, so good-but what happens when two devices both detect that the wire is free and try to send data simultaneously? As you probably guessed, packets transmitted on the network from two different devices at the same time will corrupt each other, thereby canceling each other out. This is called a collision. Collisions are a fact of networking life. So, how do network nodes deal with collisions? They either react to collisions after they happen, or they take steps to avoid collisions in the first place.

CSMA/CD is the reactive method. With CSMA/CD, each sending node detects the collision and responds by generating a random timeout period for itself, during which it doesn't try to send any more data on the network-this is called a backoff. Once the backoff period expires (remember that we're only talking about milliseconds here), the node goes through the whole process again. This approach may not be very elegant, but it gets the job done.

CSMA/CD won't work for wireless networking as, wireless devices simply can't detect collisions; therefore, wireless networks need another way of dealing with them. The CSMA/CA access method, as the name implies, proactively takes steps to avoid collisions. The 802.11 standard defines two methods of collision avoidance: Distributed Coordination Function (DCF) and Point Coordination Function (PCF). Currently, only DCF is implemented.

TIP Current CSMA/CA devices use the Distributed Coordination Function (DCF) method for collision avoidance.

DCF specifies much stricter rules for sending data onto the network media. For instance, if a wireless network node detects that the network is busy, DCF defines a backoff period on top of the normal IFS wait period before a node can try to access the network again. DCF also requires that receiving nodes send an acknowledgement (ACK) for every packet that they process. The ACK also includes a value that tells other wireless nodes to wait a certain duration before trying to access the network media. This period is calculated to be the time that the data packet takes to reach its destination based on the packet's length and data rate. If the sending node doesn't receive an ACK, it retransmits the same data packet until it gets a confirmation that the packet reached its destination.

802.11 was the very oldest wireless standard. Over time, more detailed additions to 802.11 came along that improved speed and took advantage of other frequency bands.

Standard	Frequency	Spectrum	Speed	Range	Compatibility
802.11	2.4 GHz	DSSS	11 Mbps	~300'	802.11