Code Library

This section takes a closer look at a specific technology centered around the emerging IEEE 802.11 standard, also known as Wi-Fi . Wi-Fi is technically a trademark, owned by a trade group called the Wi-Fi alliance, that certifies product compliance with 802.11. Like its Ethernet and token ring siblings, 802.11 is designed for use in a limited geographical area (homes, office buildings, campuses), and its primary challenge is to mediate access to a shared communication medium — in this case, signals propagating through space. 802.11 supports additional features (e.g., time-bounded services, power management, and security mechanisms), but we focus our discussion on its base functionality.


Physical Properties
802 .11 runs over six different physical layer protocols (so far). Five are based on spread spectrum radio and one on diffused infrared (and is of historical interest only at this point). The fastest runs at a maximum of 54 Mbps.

The original 802.11 standard defined two radio-based physical layer standards, one using frequency hopping (over 79 1-MHz-wide frequency bandwidths) and the other using direct sequence (with an 11-bit chipping sequence). Both provide up to 2 Mbps. Then physical layer standard 802.11b was added. Using a variant of direct sequence, 802.11b provides up to 11 Mbps. These three standards run in the license-exempt 2.4 GHz frequency band of the electromagnetic spectrum. Then came 802.11a, which delivers up to 54 Mbps using a variant of frequency-division multiplexing (FDM) called orthogonal frequency-division multiplexing (OFDM) . 802.11a runs in the license-exempt 5-GHz band. On one hand, this band is less used, so there is less interference. On the other hand, there is more absorption of the signal and it is limited to almost the line of sight. The most recent standard is 802.11g, which is backward compatible with 802.11b (and returns to the 2.4-GHz band). 802.11g uses OFDM and delivers up to 54 Mbps. It is common for commercial products to support all three of 802.11a, 802.11b, and 802.11g, which not only ensures compatibility with any device that supports any one of the standards, but also makes it possible for two such products to choose the highest bandwidth option for a particular environment.


Collision Avoidance
At first glance, it might seem that a wireless protocol would follow the same algorithm as the Ethernet — wait until the link becomes idle before transmitting and back off should a collision occur — and to a first approximation, this is what 802.11 does. The additional complication for wireless is that, while a node on an Ethernet receives every other node’s transmissions, a node on an 802.11 network may be too far from certain other nodes to receive their transmissions (and vice versa).

A related problem, called the exposed node problem, occurs under the circumstances, where each of the four nodes is able to send and receive signals that reach just the nodes to its immediate left and right. For example, B can exchange frames with A and C but it cannot reach D, while C can reach B and D but not A. Suppose B is sending signal to A. Node C is aware of this communication because it hears B’s transmission. It would be a mistake, however, for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission. For example, suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B. (It would interfere with A sending to B, but B is transmitting in our example.) 802.11 addresses these two problems with an algorithm called multiple access with collision avoidance (MACA).

The idea is for the sender and receiver to exchange control frames with each other before the sender actually transmits any data. This exchange informs all nearby nodes that a transmission is about to begin. Specifically, the sender transmits a request-to-send (RTS) frame to the receiver; the RTS frame includes a field that indicates how long the sender wants to hold the medium (i.e., it specifies the length of the data frame to be transmitted). The receiver then replies with a clear-to-send (CTS) frame; this frame echoes this length field back to the sender. Any node that sees the CTS frame knows that it is close to the receiver, and therefore cannot transmit for the period of time it takes to send a frame of the specified length. Any node that sees the RTS frame but not the CTS frame is not close enough to the receiver to interfere with it, and so is free to transmit.

There are two more details to complete the picture. Firstly, the receiver sends an acknowledgment (ACK) to the sender after successfully receiving a frame. All nodes must wait for this ACK before trying to transmit. Secondly, should two or more nodes detect an idle link and try to transmit an RTS frame at the same time, their RTS frames will collide with each other. 802.11 does not support collision detection, but instead the senders realize the collision has happened when they do not receive the CTS frame after a period of time, in which case they each wait a random amount of time before trying again. The amount of time a given node delays is defined by the same exponential back off algorithm used on the Ethernet.

Source of Information : Elsevier Wireless Networking Complete 2010