Support for Time-Bounded Data

Time-bounded data such as voice and video issupported in the 802.11 MAC specification
through the Point Coordination Function (PCF).As opposed to the DCF, where control is distributedto all stations, in PCF mode a singleaccess point controls access to the media. If a
BSS is set up with PCF enabled, time isspliced between the system being in PCF mode and in DCF (CSMA/CA) mode. Duringthe periods when the system is in PCF mode, the access point will poll each station for data, and after a given time move on to the next station. No station is allowed to transmit unless it is polled, and stations receive data from the access point only when they are polled. Since PCF gives every station a turn to transmit in a predetermined fashion, a maximum latency is guaranteed. A downside to PCF is that it is not particularly scalable, in that a single point needs to have control of media access and must poll all stations, which can be ineffective in large networks.

Power Management
In addition to controlling media access, the 802.11 HR MAC supports power conservation to extend the battery life of portable devices. The standard supports two power-utilization modes, called Continuous Aware Mode and Power Save Polling Mode. In the former, the radio is always on and drawing power, whereas in the latter, the radio is “dozing” with the access point queuing any data for it. The client radio will wake up periodically in time to receive regular beacon signals from the access point. The beacon includes information regarding which stations have traffic waiting for them, and the client can thus awake upon beacon notification and receive its data,
returning to sleep afterward.

Security
802.11 provides for both MAC layer (OSI Layer 2) access control and encryption mechanisms,
which are known as Wired Equivalent Privacy (WEP), with the objective of providing wireless LANs with security equivalent to their wired counterparts. For the access control, the ESSID (also known as a WLAN Service Area ID) is programmed into each access point and is required knowledge in order for a wireless client to associate with an access point. In addition, there is provision for a table of MAC addresses called an Access Control List to be included in the access point, restricting access to clients whose MAC addresses are on the list.

For data encryption, the standard provides for optional encryption using a 40-bit shared-key RC4 PRNG algorithm from RSA Data Security. All data sent and received while the end station and access point are associated can be encrypted using this key. In addition, when encryption is in use, the access point will issue an encrypted challenge packet to any client attempting to associate with it. The client must use its key to encrypt the correct response in order to authenticate itself and gain network access.

Beyond Layer 2, 802.11 HR WLANs support the same security standards supported by other 802 LANs for access control (such as network operating system logins) and encryption (such as IPSec or application-level encryption). These higher-layer technologies can be used to create end-to-end secure networks encompassing both wired LAN and WLAN components, with the wireless piece of the network gaining unique additional security from the 802.11 feature set.

Considerations for Choosing a Wireless LAN
While the bulk of this paper has described how 802.11b wireless LANs are alike, there are still many ways for wireless LAN vendors to differentiate themselves in the marketplace that will affect a customer’s purchasing decision. We cover some of these areas below.

Ease of Setup
To install a wireless LAN one must install and configure APs and PC Cards. The most 9 important piece of this effort is proper placement of the APs. Access point placement is
what ensures the coverage and performance required by the network design. There are several
features that provide assistance in the installation process:

• Site survey. For complete wireless LANs employing a cellular architecture, proper placement of APs is best determined by performing a site survey, in which the person installing the WLAN can place APs and record signal strength and quality information while moving about the intended coverage area. While most vendors provide a site survey tool, these utilities vary in the
amount and quality of information they provide, as well as in their logging and reporting capabilities.

• Power over Ethernet. Some vendors ship APs that can be powered over the Ethernet cable that connects the access point to the wired network. This is usually implemented by a piece of equipment in the wiring closet that takes in AC power and the data connection from the wired switch, and then outputs DC power over unused wire pairs in the networking cable that runs between the module and the access point. This feature eliminates the need to run an AC power cable out to the access point (usually located on the wall or ceiling), making installation quicker and more affordable.

• Easy-to-use NIC and access point configuration tools. Once the APs are installed,both APs and NICs must be configured for use. As with any technical product, the quality of the user interface determines the amount of time required to configure the network for operation. In addition, some vendors supply tools for bulk configuration of access points on the same network, greatly easing network setup. Finally, having a variety of methods to access the access point is
helpful to ensure simple setup. Configuration options include telnet; Web-based; ormSNMP-based over the Ethernet cable, from a wireless station, or via a serial port built into the access point.

Ease of Management
Since an 802.11 wireless LAN differs from standard 802.3 and 802.5 wired LANs only at OSI Layers 1 and 2, one should expect at least the same level of manageability from these products as one finds for wired networking products. At a minimum, the products should come with SNMP 2 support so that they can be automatically discovered and managed using the same tools employed for wired LAN equipment. And one should assess carefully what can be controlled via the SNMP MIB. Some products measure and control a number of Ethernet and radio variables in the access point, while others provide only a basic Ethernet MIB.

Beyond SNMP, it is useful to be able to configure and probe APs via an easy-to-use interface like a Web browser. Some vendors have built Web servers into their APs for this reason. Finally, the ability to manage, configure, and upgrade APs in groups simplifies WLAN administration.

Range and Throughput
802.11b WLANs communicate using radio waves because these waves penetrate off many indoor structures or can reflect around obstacles. WLAN throughput depends on several factors, including the number of users, microcell range, interference, multipath propagation, standards support, and hardware type. Of course, anything that affects data traffic on the wired portions of the LAN, such as latency and bottlenecks, will also affect the wireless portion.

When it comes to range, more is not always better. For example, if the network requirement is for high performance (5.5 Mbps or 11 Mbps) and complete coverage, long range at lower network speeds (1 Mbps and 2 Mbps) may make it difficult to employ a channel reuse pattern while maintaining high performance.