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This chapter is from the book

This chapter is from the book

Introduction to the IEEE 802.11 Standard

The initial 802.11 PAR states that "...the scope of the proposed [wireless LAN] standard is to develop a specification for wireless connectivity for fixed, portable, and moving stations within a local area." The PAR further says that "...the purpose of the standard is to provide wireless connectivity to automatic machinery and equipment or stations that require rapid deployment, which may be portable, handheld, or which may be mounted on moving vehicles within a local area."

The resulting standard, which is officially called "IEEE Standard for Wireless LAN Medium Access (MAC) and Physical Layer (PHY) Specifications," defines over-the-air protocols necessary to support networking in a local area. As with other IEEE 802–based standards (such as 802.3 and 802.5), the primary service of the 802.11 standard is to deliver MSDUs (MAC Service Data Units) between peer LLCs. Typically, a radio card and access point provide functions of the 802.11 standard.

NOTE

To order a copy of the IEEE 802.11 standard, contact the IEEE 802 Document Order Service at 800-678-4333. You can also order the standard via IEEE's Web site at http://www.ieee.org.

The 802.11 standard provides MAC and PHY (Physical Layer) functionality for wireless connectivity of fixed, portable, and moving stations moving at pedestrian and vehicular speeds within a local area. Specific features of the 802.11 standard include the following:

  • Support of asynchronous and time-bounded delivery service.

  • Continuity of service within extended areas via a distribution system, such as ethernet.

  • Accommodation of transmission rates of 1Mbps and 2Mbps (802.11a and 802.11b extensions offer higher data rates than the base standard).

  • Support of most market applications.

  • Multicast (including broadcast) services.

  • Network management services.

  • Registration and authentication services.

Target environments for use of the standard include the following:

  • Inside buildings, such as offices, banks, shops, malls, hospitals, manufacturing plants, and residences

  • Outdoor areas, such as parking lots, campuses, building complexes, and outdoor plants

The 802.11 standard takes into account the following significant differences between wireless and wired LANs:

  • Power management Because most wireless LAN NICs are available in PCMCIA Type II format, obviously you can outfit portable and mobile handheld computing equipment with wireless LAN connectivity. The problem, though, is that these devices must rely on batteries to power the electronics within them. The addition of a wireless LAN NIC to a portable computer can drain batteries quickly.

    The 802.11 working group struggled with finding solutions to conserve battery power; however, they found techniques enabling wireless NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. The MAC layer implements power management functions by putting the radio to sleep (lowering the power drain) when no transmission activity occurs for some specific or user-definable time period. The problem, though, is that a sleeping station can miss critical data transmissions. The 802.11 standard solves this problem by incorporating buffers to queue messages. The standard calls for sleeping stations to awaken periodically and retrieve any applicable messages.

  • Bandwidth The ISM spread spectrum bands do not offer a great deal of bandwidth, keeping data rates lower than desired for some applications. The 802.11 working group, however, dealt with methods to compress data, making the best use of available bandwidth.

  • Security Wireless LANs transmit signals over much larger areas than do those using wired media, such as twisted-pair, coaxial cable, and optical fiber. In terms of privacy, therefore, a wireless LAN has a much larger area to protect. To employ security, the 802.11 group coordinated its work with the IEEE 802.10 standards committee responsible for developing security mechanisms for all 802-series LANs.

  • Addressing The topology of a wireless network is dynamic; therefore, the destination address does not always correspond to the destination's location. This raises a problem when routing packets through the network to the intended destination. Thus, you may need to use a TCP/IP-based protocol such as MobileIP to accommodate mobile stations.

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