How WLAN Systems Are Different

Wireless networks have fundamental characteristics that make them significantly different from traditional wired LANs. Some countries impose specific requirements for radio equipment in addition to those specified in the 802.11 standard.

Destination Address Does Not Equal Destination Location

In wired LANs, an address is equivalent to a physical location. This is implicitly assumed in the design of wired LANs. In IEEE 802.11, the addressable unit is a station (STA). The STA is a message destination, but not generally a fixed location.

The Media Impact of the Design

The physical layers used in IEEE 802.11 are fundamentally different from wired media. Thus, IEEE 802.11 PHYs

• Use a medium that has neither absolute nor readily observable boundaries outside of which stations with conformant PHY transceivers are known to be unable to receive network frames.

• Are unprotected from outside signals.

• Communicate over a medium significantly less reliable than wired PHYs.

• Have dynamic topologies.

• Lack full connectivity, and therefore the assumption normally made that every STA can hear every other STA is invalid (that is, STAs may be hidden from each other).

• Have time-varying and asymmetric propagation properties.

Because of limitations on wireless PHY ranges, WLANs intended to cover reasonable geographic distances may be built from basic coverage building blocks.

The Impact of Handling Mobile Stations

One of the requirements of IEEE 802.11 is to handle mobile as well as portable stations. A portable station is one that is moved from location to location, but that is only used while at a fixed location. Mobile stations actually access the LAN while in motion.

For technical reasons, it is not sufficient to handle only portable stations. Propagation effects blur the distinction between portable and mobile stations; stationary stations often appear to be mobile due to propagation effects. Another aspect of mobile stations is that they may often be battery powered. Hence, power management is an important consideration. For example, it cannot be presumed that a station's receiver will always be powered on.

Interaction with Other IEEE 802 Layers

IEEE 802.11 is required to appear to higher layers (Logical Link Control [LLC]) as a current style IEEE 802 LAN. This requires that the IEEE 802.11 network handle station mobility within the MAC sublayer. To meet reliability assumptions (that LLC makes about lower layers), it is necessary for IEEE 802.11 to incorporate functionality that is untraditional for MAC sublayers.