802.11 WirelessLAN  (80211.pdf)

Figure 1: A access point usually defines a BSS. A BSS can exist without an access point such as in an IBSS network. For such a network to work, there should be no hidden terminals. This requires that all mobiles are within range of each other.	Figure 2: Example of an ESS. An ESS consists of more than one BSS. Each BSS has an access point. BSS's can overlap or be overlaid. BSS-transitions are guarnteed by the specifications but ESS transitions are not.

Network Topology

The 802.11 spec does not really define a network topology. In fact, it probably shouldn't since it exists in the link-layer and physical layers of the OSI stack. Networking is not really in the scope of the wireless lan. How the 802.11 is configured is up to the system planner. The services that 802.11 is guaranteed to provide are

• Station Services (the services that mobiles provide)
  • Authentication (means a secure  login)
  • Deauthentication (means to secure logout)
  • Privacy (prevent snooping by others)
  • MSDU delivery (MAC Service Data Units, deliver packets between the link layers)
• Distribution system services (the services that  base stations or  access points provide)
  • Association (means a login)
  • Diassociation (means a logout)
  • Distribution (somehow MAC frames are distributed but 802.11 does not specify how)
  • Integration (means that the wireless lan can work with other [wired] networking technology)
  • Reassociation (to have an change in Association - means need to support base station transition and update access point - mobile mappings)

There are two popular styles for using 802.11 wireless lans. One is called the "ad-hoc" or Independent Basic Service Set (IBSS) configuration and the other is labeled the Extended Service Set (ESS) configuration. In the IBSS configuration, there is no backbone infrastructure and there must be at least two wireless stations. An IBSS is one BSS (Figure 1) or Basic Service Set. Mobiles can talk to each other without the use of a master. However, if a cell contains many mobiles then the network planner has the option to setup a master for better link utilization. The ESS configuration (Figure 2)   consists of more than one BSS which may be connected to another type of distribution service, such as Ethernet. BSS's can overlap, therefore, a mobile may fall within multiple BSS's. The 802.11 specification guarantees that a mobile can transition to other BSS's but the whether this is a hard or soft handoff is unknown. One thing to keep in mind is that most mobiles in this network are stationary and the mobility of any mobile is very slow. As a result, this type of network topology doesn't necessarily need to provide support for continuity of connections since the user will likely use the mobile when it is stationary.

One interesting aspect of the 802.11 is that it supports different types of physical layers. The three types are FHSS, DSSS, and IR. Which one the network planner decides to use depends on the environment selection criteria listed below [2].

Acronyms/Terms

BSS - Basic Service Set

ESS - Extended Service Set

IBSS - Independent

FHSS - Frequency Hoppng Spread Spectrum

DSSS - Direct Sequence Spread Spectrum

IR - Infrared

Access Point - Essentially the base station.

For more information

• [1] Julio L. da Silva Jr. 's  802.11 Overview
  • (pdf) (html)
• [2] Jim Geier, Wireless LANs: Implementing Interoperable Networks, MacMillan Technical Publishing, USA 1999
• [3] IEEE 802.11 Website: WLANA page
  • http://www.wlana.com/intro/standard/index.html

last modified 11/29/01 05:49:19 PM dpatel@noyce.eecs.berkeley.edu