IEEE 802.11 Tutorial Part 2

By Jim Zyren and Al Petrick

Return to the first part of the IEEE 802.11 Tutorial Part 1.

Multiple Access

The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA). 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 offprocedure. This prevents multiple stations from seizing the medium immediately after completion of the preceding transmission.

Packet reception in DCF requires acknowledgement as shown in Figure 7. The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS). ACK frames have a higher priority than other traffic. Fast acknowledgement is one of the salient 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) before transmitting data. 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 subsequent transmission.

The method described above relies on the Physical Carrier Sense. The underlying assumption is that every station can "hear" all other stations. This is not always the case. Referring to Figure 8, the AP is within range of the STA-A, but STA-B is out of range. STA-B would not be able to detect transmissions from STA-A, and the probability of collision is greatly increased. This is known as the Hidden Node.

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, STA-A sends an RTS frame to the AP. The RTS will not be heard by STA-B. 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 Network Allocation Vector (NAV) of all stations detecting the RTS frame.

Upon receipt of the RTS, the AP responds with a CTS frame, which also contains a duration/ID field specifying the period of time for which the medium is reserved. While STA-B did not detect the RTS, it will detect the CTS and update its NAV 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 which exceed an arbitrarily defined length.

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, connectionoriented services such as cordless telephony.

Logical Addressing

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. 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.

Security

IEEE 802.11 provides for security via two methods: authentication and encryption. 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 STA may request authentication. The STA 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. The WEP algorithm was selected to meet the following criteria:

• reasonably strong
• self-synchronizing
• computationally efficient
• exportable
• optional

Timing and Power Management

All station clocks within a BSS are synchronized by periodic transmission of time stamped beacons. In the infrastructure mode, the AP serves as the timing master and generates all timing beacons. Synchronization is maintained to within 4 microseconds plus propagation delay.

Timing beacons also play an important role in power management. There are two power saving modes defined: awake and doze. In the awake mode, stations are fully powered and can receive packets at any time. Nodes must inform the AP before entering doze. In this mode, nodes must "wake up" periodically to listen for beacons which indicate that AP has queued messages.

Roaming

Roaming is perhaps the least defined feature among those discussed in this article. The standard does identify the basic message formats to support roaming, but everything else is left up to network vendors. In order to fill the void, the Inter-Access Point Protocol (IAPP) was jointly developed by Aironet, Lucent Technologies, and Digital Ocean. Among other things, IAPP extends multi-vendor interoperability to the roaming function. It addresses roaming within a single ESS and between two or more ESSs.

The Wireless Ethernet Compatibility Alliance

The recently adopted Complimentary Code Keying (CCK) waveform delivers speeds of 5.5 and 11 Mbps in the same occupied bandwidth as current generation 1 and 2 Mbps DSSS radios and will be fully backward compatible. Now that a standard is firmly in place, WLANs will become a part of the enterprise networking landscape within the next twelve months.

The mission of the Wireless Ethernet Compatibility Alliance is to provide certification of compliance with the IEEE 802.11 Standard and to ensure that products from multiple vendor meet strict requirements for interoperability. With cross vendor interoperability assured, WLANs are now able to fulfill the promise of high speed mobile computing.

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