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The First Wireless Network Protocol: 802.11
Approval of the IEEE 802.11 standard for wireless local area networking (WLAN) and rapid progress made toward higher data rates have put the promise of truly mobile computing within reach. While wired LANs have been a mainstream technology for at least fifteen years, WLANs are uncharted territory for most networking professionals. Some obvious questions come to mind when considering wireless networking:
+ How can WLANs be integrated with wired network infrastructure?
+ What is the underlying radio technology?
+ How is multiple access handled?
+ What about network security?
IEEE 802.11 is limited in scope to the Physical (PHY) layer and Medium Access Control
(MAC) layer (Lough, 3), but it shares MAC characteristics with the IEEE 802.3 Ethernet standard (3Com, 2). The following overview explains major differences between wired and wireless LANs and should answer some of the questions that arise when evaluating WLAN technology.
WLANs can be used either to replace wired LANs, or as an extension of the wired LAN infrastructure. The basic topology of an 802.11 network is shown in Figure 1. A Basic Service Set (BSS) consists of two or more wireless nodes, or stations, which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given cell coverage area. This type of network is often formed on a temporary basis, and is commonly referred to as an ad hoc network, or Independent Basic Service Set (IBSS) (Geier, 3).
Figure 1 (Intel, 1)
In more structured environments, the BSS contains an Access Point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is similar to a basestation used in cellular phone networks. When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP. AP s are not mobile, and form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode.
The Extended Service Set (ESS) consists of a series of overlapping BSSs (each containing an AP) connected together by means of a Distribution System (Geier, 3). Although the Distribution System could be any type of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs and seamless campus-wide coverage is possible.
IEEE 802.11 provides for two variations of the physical layer. These include two RF technologies, namely, Direct Sequence Spread Spectrum (DSSS), and Frequency Hopped Spread Spectrum (FHSS). The DSSS and FHSS physical layer options were designed specifically to conform to FCC regulations for operation in the 2.4 GHz ISM bands, which has worldwide allocation for unlicensed operation (Geier, 3). Both FHSS and DSSS physical layers currently support 1 and 2 Mbps. However, all 11 Mbps radios are DSSS.
The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA) (Lough, 4) similar to AppleTalk. This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station waits until transmission stops, and then enters into a random back off procedure. This prevents multiple stations from seizing the medium immediately after completion of the preceding transmission.
Packet reception in DCF requires acknowledgement. The period between completion of packet transmission and start of the acknowledgement (ACK) frame is one Short Inter Frame Space (SIFS) or 28 microseconds (Brenner, 8). ACK frames have a higher priority than other traffic. Fast acknowledgement is one of the significant features of the 802.11 standard, because it requires ACKs to be handled at the MAC sublayer.
Transmissions other than ACKs must wait at least one DCF inter frame space (DIFS) or 128 microseconds before transmitting data (Brenner, 8). If a transmitter senses a busy medium, it determines a random back-off period by setting an internal timer to an integer number of slot times. Upon expiration of a DIFS, the timer begins to decrement. If the timer reaches zero, the station may begin transmission. However, if the channel is seized by another station before the timer reaches zero, the timer setting is retained at the decremented value for later transmission.
The method described above relies on the assumption that every station can hear all other stations. This is not always the case. Referring to Figure 2, the access point is within range of the Station A, but Station B is out of range. Station B would not be able to detect transmissions from Station A, and the probability of collision is greatly increased. This is known as the Hidden Node (Lough, 4).
Figure 2 (Intel, 3)
To combat this problem, a second carrier sense mechanism is available. Virtual Carrier Sense enables a station to reserve the medium for a specified period of time through the use of RTS/CTS frames. In the case described above, Station A sends an RTS (ready to send) frame to the AP. Station B will not hear the RTS. The RTS frame contains a duration/ID field, which specifies the period of time for which the medium is reserved for a subsequent transmission. The reservation information is stored in the all stations detecting the RTS frame (Brenner, 6).
Upon receipt of the RTS, the AP responds with a CTS (clear to send) frame, which also contains a duration/ID field specifying the period of time for which the medium is reserved. While Station B did not detect the RTS, it will detect the CTS and update itself accordingly. Thus, collision is avoided even though some nodes are hidden from other stations. The RTS/CTS procedure is invoked according to a user specified parameter. It can be used always, never, or for packets that exceed an arbitrarily defined length (Brenner, 6).
As mentioned above, DCF is the basic media access control method for 802.11 and it is mandatory for all stations. The Point Coordination Function (PCF) is an optional extension to DCF. PCF provides a time division duplexing capability to accommodate time bounded, connection-oriented services such as cordless telephony.
The authors of the 802.11 standard allowed for the possibility that the wireless media, distribution system, and wired LAN infrastructure would all use different address spaces. IEEE 802.11 only specifies addressing for over the wireless medium, though it was intended specifically to facilitate integration with IEEE 802.3 wired Ethernet LANs. The IEEE802 48-bit addressing scheme was therefore adopted for 802.11, thereby maintaining address compatibility with the entire family of IEEE 802 standards. In the vast majority of installations, the distribution system is an IEEE 802 wired LAN and all three logical addressing spaces are identical.
IEEE 802.11 provides for security via two methods: authentication and encryption (Brenner, 13). Authentication is the means by which one station is verified to have authorization to communicate with a second station in a given coverage area. In the infrastructure mode, authentication is established between an AP and each station.
Authentication can be either Open System or Shared Key. In an Open System, any station may request authentication. The station receiving the request may grant authentication to any request, or only those from stations on a user-defined list. In a Shared Key system, only stations which possess a secret encrypted key can be authenticated. Shared Key authentication is available only to systems having the optional encryption capability.
Encryption is intended to provide a level of security comparable to that of a wired LAN. The Wired Equivalent Privacy (WEP) feature uses the RC4 PRNG algorithm from RSA Data Security, Inc. (Brenner, 13). The WEP algorithm was selected to meet the following criteria:
+ Reasonably strong
+ Computationally efficient
Brenner, Pablo. A Technical Tutorial on the IEEE 802.11 Protocol. SSS-Online. February 25, 2001. .
Geier, Jim. Overview of the IEEE.11 Standard. Wireless-Nets. February 24, 2001. .
Lough, Blankenship, and Krizman. A Short Tutorial on Wireless LANs and IEEE 802.11. Computer.org. February 24, 2001. .
What is Wireless Networking? Intel. February 24, 2001. .
What s New in Wireless LANs: The IEEE 802.11b Standard. 3Com. February 24, 2001. .
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