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  Wired on Wireless: A New Class of 802.11 Devices Go the Distance March 22, 1999 | ||
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The Essentials of 802.11 Like all network specifications, the 802.11 spec defines the physical layer (PHY) and MAC (Media Access Control) layers. However, unlike most other IEEE specs, 802.11 includes three alternative PHY standards: diffuse infrared operating at 1 Mbps in the 850-nanometer spectrum; frequency-hopping spread spectrum operating at 1 Mbps or 2 Mbps in the 2.4-GHz to 2.4835-GHz spectrum; and direct-sequence spread spectrum operating at 1 Mbps or 2 Mbps in the 2.4-GHz to 2.4835-GHz spectrum. A single 802.11 MAC standard is based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). While the frame structure is similar to 802.3 Ethernet, the 802.11 specification includes provisions designed to minimize collisions. Because two mobile units may both be in range of a common access point, but not in range of each other, it is not always possible for stations to sense the medium's true availability. The 802.11 standard deals with this "hidden-node" problem in two ways. First, a positive acknowledgement (ACK) ensures that frames reach their destination. Second, the spec enables a request to send (RTS), clear-to-send (CTS) scheme that regulates data transmissions. Because this scheme adds overhead, many vendors implement definable, frame-sized thresholds in their RTS/CTS implementations. In this way, requests are only made when attempting to transmit large frames. To promote fair access to the medium, the CSMA/CA scheme implements a minimum time gap between frames from a given user. The 802.11 standard fosters two basic modes of operation: Ad hoc mode enables peer-to-peer transmission between mobile units. Infrastructure mode--in which mobile units communicate via an access point that serves as a bridge to a wired network infrastructure--is the more common wireless LAN application. Because radios require a significant amount of power, the 802.11 standard includes power management provisions via a power-save mode that you can invoke when a mobile unit is not transmitting. When operating in power-save mode, the PC Card periodically wakes up to see if there are any transmissions from an AP destined for that mobile unit. If there are, the PC Card shifts from power-save mode to receive mode. Although this mode is not an optional component of the 802.11 standard, relatively few vendors have implemented power savings. Most plan to add this capability by midyear. The 802.11 standard addresses the issue of security in two different ways. First, before communication can take place, all mobile units must associate themselves with an AP. In most cases, this requires that the mobile unit and AP are configured to use a common network name, called an Extended Service Set ID (ESSID). Protection is quite limited, however, and some products let you associate with any AP, while others let you browse for ESSIDs and dynamically attach to the network of your choice. Many APs lets you restrict access by MAC address, but this is limited somewhat by most PC Cards' ability to change their MAC address.
Since wireless communication uses a more open medium for communication in comparison to wired LANs, the 802.11 designers also included a shared-key encryption mechanism, called WEP (wired equivalent privacy), in the specification. Unfortunately, many vendors have not yet implemented this feature in their products, though most should have this feature in place some time later this year. As wireless "sniffers" become available, more robust security will clearly be necessary. |
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