In the last month's article I talked extensively about Bluetooth, a Personal Area Network (PAN) technology. Now let’s turn our attention to wireless Ethernet data networking.
The first wireless Local Area Networking (LAN) equipment was introduced in the late 1980s after the FCC authorized its operation on a shared basis in the ISM (Industrial, Scientific, and Medical) band of radio frequencies (2.4 GHz). Unfortunately, the lack of industry standards resulted in each vendor producing proprietary products that did not inter-operate. The lack of an industry standard greatly slowed the acceptance of wireless LANs in the marketplace, so in the late 1980s the Institute of Electrical and Electronics Engineers (IEEE) began work on what would become known as IEEE Standard 801.11 for wireless Ethernet; unfortunately, it took many years before 801.11 was finalized. In 1997 vendors began to offer standards compliant equipment (although not well known, standard 802.11 includes support for both infrared and radio frequency networking, but few products have been produced using infrared technology).
It is also important to note that the original 802.11 standard did not include a mechanism for transferring a mobile user’s connection from one access point to another. In other words, wireless did not equate to mobile.
Originally the 802.11 standard supported data rates of 1 and 2 Mbps, but by 1997 many enterprise LANs had already been upgraded from 10 Mbps to 100 Mbps making the 1 or 2 Mbps data rate of wireless equipment acceptable for only the simplest of applications. Fortunately, it took the 802.11 committee less than two years to ratify and issue standard 802.11b, which supports a data rate of 11 Mbps.
The latest 802.11 committee effort is standard 802.11a (note the reverse alphabetical sequence of the standards: 802.11b was issued before 802.11a). This implementation supports data rates of 54 Mbps, but unfortunately uses a different part of the radio frequency spectrum than older equipment. Thus, to take advantage of 802.11a’s higher data rata an organization would have to replace existing 802.11 and 802.11b equipment. Presently the high cost and complexity of fabricating the required radio frequency circuits has caused a delay in the marketing of 802.11a products. In addition, the IEEE 802.11g committee is working on increasing the 2.4 GHz bandwidth from 11 Mbps to about 22 Mbps (www.wirelessethernet.org).
Standard 802.11 defines two operating modes: Independent Basic Service Set (BSS) and Extended Service Set (ESS) networks. BSS, or ad hoc networking, allows wireless networking devices to communicate directly among each other. ESS requires the use of access points to link wireless devices into the organization’s enterprise LAN.
Access Points (AP) are used to interface wireless devices to the building’s data network. Access points are mounted throughout the building (and campus) and require a wired connection to the organization’s Ethernet network and a power source. The current generation of APs operate on low-voltage (typically 48 or 24 volts DC) provided by plug-in power cubes, but IEEE Task Force 802.3af is working on a standard that will allow power to be delivered over the same conductors used for data transmission resulting in simplified installation (for example, all Ethernet devices could be powered from a central power supply in the data center). This will make retrofitting wireless technology into existing facilities fairly easy since the same small-diameter LAN cable can support both power and data transmission functions. In some cases it may be prudent to install empty conduit to accommodate these cables in hard-to-wire areas of a building if it is anticipated that wireless LANs will be needed later (for example, in libraries or atriums).
The location and spacing of access points depends on a number of variables: building construction, floor layout, the number of simultaneous communications links to be supported, and, most importantly, the bandwidth required supporting the applications. Bandwidth is the key issue since access points share the available bandwidth between all users. For example, one user connected to an 802.11b (11 Mbps) AP has full use of that bandwidth (actual useable bandwidth is substantially less than the raw 11 Mbps data rate; probably less than 8 Mbps). However, eleven users sharing a single 802.11b AP will have, on average, only 1 Mbps each of available bandwidth.
Although vendors have developed software programs to estimate the location of access points based on signal attenuation caused by building materials (steel, concrete), the design must be verified in the field. It is not unusual to need more than the originally estimated access points to achieve the required coverage pattern and range.
Since a single access point supports a fixed maximum number of simultaneous links, it is important to understand the environment in which the wireless equipment is used. If the required number of simultaneous communication links exceeds the number that can be handled by one access point, additional access points will be required in the same physical space.
Useable bandwidth is a function of the signal-to-noise ratio of the communications channel. In other words, as the distance between the AP and the device increases, the useable bandwidth decreases. This will be especially important when migrating from 802.11b (11 Mbps) to 802.11a (54 Mbps). To provide adequate bandwidth, APs for 802.11a may have to be spaced as close as 50 feet apart.
Although the installation of APs may be straightforward, the administration of wireless LANs requires as much thought and careful planning as the wired LAN. One key element of wireless LANs is Wired Equivalent Privacy (WEP). WEP has a number of shortcomings and should not be depended upon to provide truly secure wireless access. When WEP is enabled, most APs exhibit a severe decrease in useable bandwidth (as much as 50%). In addition, all APs are shipped with the same default Service Set Identifier (SSID) to allow out-of-the-box interoperability with existing APs. If the default SSID is not changed, it becomes relatively easy for anyone to attach to the network. Remember, wireless networking authenticates the hardware, not the user.
It is possible that wireless Ethernet will become the universal wireless protocol of choice. For example, the IEEE 802.11e committee is working on adding telephony to the standard (www.personaltelco.net). Starbucks has installed MobileStar 802.11b wireless access in its stores and community networking (free) wireless Ethernet networks has started to appear (www.nycwireless.net). Wireless Ethernet is also being installed in major airports all over the country.
Ernest Schirmer is Vice President for Technology Consulting with Syska & Hennessy, Inc. consulting engineers in New York City. For more information, please visit www.syska.com or email [email protected].
