By Jim Geier
In June 1997, the Institute of Electrical and Electronic Engineers (IEEE) finalized the initial standard for wireless LANs, IEEE 802.11. This standard specified a 2.4GHz operating frequency with data rates of 1 and 2Mbps. When deploying a wireless LAN using the initial version of 802.11, you could opt for using frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS). Since the ratification of the initial 802.11 standard, the IEEE 802.11 Working Group (WG) has made several revisions through various task groups.
What do the letters mean?
Task groups within the 802.11 WG enhance portions of the 802.11 standard. A particular letter corresponding to each standard/revision, such as 802.11a, 802.11b, and so on, represents the different task groups. For example, Task Group B (i.e., 802.11b) was responsible for upgrading the initial 802.11 standard to include higher data rate operation using DSSS in the 2.4GHz band.
Let’s take a closer look at each of the 802.11 task groups and how they impact your WLAN deployments.
802.11a — OFDM in the 5GHz Band
802.11a is a Physical Layer (PHY) standard (IEEE Std. 802.11a-1999) that specifies operating in the 5GHz UNII band using orthogonal frequency division multiplexing (OFDM). 802.11a supports data rates ranging from 6 to 54Mbps. 802.11a-based products became available in late 2001.
Because of operation in the 5GHz bands, 802.11a offers much less potential for radio frequency (RF) interference than other PHYs (e.g., 802.11b and 802.11g) that utilize 2.4GHz frequencies. With high data rates and relatively little interference, 802.11a does a great job of supporting multimedia applications and densely populated user environments. This makes 802.11a an excellent long-term solution for satisfying current and future requirements. Strongly consider the deployment of 802.11a unless extenuating circumstances point you toward a different PHY, such as 802.11b.
802.11b — High Rate DSSS in the 2.4GHz band
The task group for 802.11b was responsible for enhancing the initial 802.11 DSSS PHY to include 5.5Mbps and 11Mbps data rates in addition to the 1Mbps and 2Mbps data rates of the initial standard. 802.11 finalized this standard (IEEE Std. 802.11b-1999) in late 1999. To provide the higher data rates, 802.11b uses CCK (Complementary Code Keying), a modulation technique that makes efficient use of the radio spectrum.
Most wireless LAN installations today comply with 802.11b, which is also the basis for Wi-Fi certification from the Wireless Ethernet Compatibility Alliance (WECA). These products have been available for the past two years. In some cases, you should deploy 802.11b networks today to take advantage of the installed base of 802.11b-equipped users. For example, utilize 802.11b as the basis for public wireless LANs to maximize the number of subscribers.
802.11c — Bridge Operation Procedures
802.11c provides required information to ensure proper bridge operations. This project is completed, and related procedures are part of the IEEE 802.11c standard. Product developers utilize this standard when developing access points. There’s really not much in this standard relevant to wireless LAN installers.
802.11d — Global Harmonization
When 802.11 first became available, only a handful of regulatory domains (e.g., U.S., Europe, and Japan) had rules in place for the operation of 802.11 wireless LANs. In order to support a widespread adoption of 802.11, the 802.11d task group has an ongoing charter to define PHY requirements that satisfy regulatory within additional countries. This is especially important for operation in the 5GHz bands because the use of these frequencies differ widely from one country to another. As with 802.11c, the 802.11d standard mostly applies to companies developing 802.11 products.
802.11e – MAC Enhancements for QoS
Without strong quality of service (QoS), the existing version of the 802.11 standard doesn’t optimize the transmission of voice and video. There’s currently no effective mechanism to prioritize traffic within 802.11. As a result, the 802.11e task group is currently refining the 802.11 MAC (Medium Access Layer) to improve QoS for better support of audio and video (such as MPEG-2) applications. The 802.11e group should finalize the standard by the end of 2002, with products probably available by mid-2003.
Because 802.11e falls within the MAC Layer, it will be common to all 802.11 PHYs and be backward compatible with existing 802.11 wireless LANs. As a result, the lack of 802.11e being in place today doesn’t impact your decision on which PHY to use. In addition, you should be able to upgrade your existing 802.11 access points to comply with 802.11e through relatively simple firmware upgrades once they are available.
802.11f – Inter Access Point Protocol
The existing 802.11 standard doesn’t specify the communications between access points in order to support users roaming from one access point to another. The 802.11 WG purposely didn’t define this element in order to provide flexibility in working with different distribution systems (i.e., wired backbones that interconnect access points).
The problem, however, is that access points from different vendors may not interoperate when supporting roaming. 802.11f is currently working on specifying an inter access point protocol that provides the necessary information that access points need to exchange to support the 802.11 distribution system functions (e.g., roaming). The 802.11f group expects to complete the standard by the end of 2002, with products supporting the standard by mid-2003.
In the absence of 802.11f, you should utilize the same vendor for access points to ensure interoperability for roaming users. In some cases a mix of access point vendors will still work, especially if the access points are Wi-Fi-certified. The inclusion of 802.11f in access point design will eventually open up your options and add some interoperability assurance when selecting access point vendors.
802.11g – Higher Rate Extensions in the 2.4GHz Band
The charter of the 802.11g task group is to develop a higher speed extension (up to 54Mbps) to the 802.11b PHY, while operating in the 2.4GHz band. 802.11g will implement all mandatory elements of the IEEE 802.11b PHY standard. For example, an 802.11b user will be able to associate with an 802.11b access point and operate at data rates up to 11Mbps. In early 2002, 802.11g decided to use OFDM instead of DSSS as the basis for providing the higher data rate extensions.
An issue is that the presence of an 802.11b user on an 802.11g network will require the use of RTS / CTS (request-to-send / clear-to-send), which generates substantial overhead and lowers throughput significantly for all 802.11b and 802.11g users. RTS / CTS ensures that the sending station first transmit a RTS frame and receive a CTS frame from the access point before sending data. A mixture of 802.11b and 802.11g requires RTS / CTS to avoid collisions because 802.11b stations can’t hear 802.11g stations using OFDM.
It’s unclear at this date when 802.11g will ratify the standard. In addition, the FCC (Federal Communications Commission) still needs to approve the use of OFDM in the 2.4GHz band, a generally necessary action when messing with the PHY. As a result, it will likely take a relatively long period of time before 802.11g products appear on the market.
There’s been much debate over the use of 802.11g vs. 802.11a for satisfying needs for higher performance WLAN applications. For the foreseeable future, your only selection for data rates beyond 802.11b’s 11Mbps is to use 802.11a. Because of the earlier time to market and superior performance capacity, 802.11a will likely dominate the high performance WLAN market in the near-term and distant future.
802.11h – Spectrum Managed 802.11a
802.11h addresses the requirements of the European regulatory bodies. It provide dynamic channel selection (DCS) and transmit power control (TPC) for devices operating in the 5GHz band (802.11a). In Europe, there’s a strong potential for 802.11a interfering with satellite communications, which have “primary use” designations. Most countries authorize WLANs for “secondary use” only. Through the use of DCS and TPC, 802.11h will avoid interference in a way similar to HiperLAN/2, the European-based competitor to 802.11a. 802.11h hopes to have their standard finalized sometime before the end of 2003.
To implement DCS and TPC, 802.11h is developing associated practices that affect both the MAC and PHY Layers. The inclusion of DCS and TPC will likely enable 802.11h to become the successor to 802.11a. Fortunately, there shouldn’t be any issues of non-interoperability between existing 802.11a and 802.11h users and access points. The good news is that 802.11h is enabling sales of 802.11a networks in Europe, which will eventually result in higher sales volumes and lower prices.
802.11i – MAC Enhancements for Enhanced Security
802.11i is actively defining enhancements to the MAC Layer to counter the issues related to wired equivalent privacy (WEP). The existing 802.11 standard specifies the use of relatively weak, static encryption keys without any form of key distribution management. This makes it possible for hackers to access and decipher WEP-encrypted data on your WLAN. 802.11i will incorporate 802.1x and stronger encryption techniques, such as AES (Advanced Encryption Standard). In a previous tutorial, I discuss more details of how 802.11i is beefing up security.
Don’t expect 802.11i to be available in the near future. The standard will likely not have IEEE ratification before mid-2003. 802.11i updates the MAC Layer, so you should be able to upgrade existing access points with firmware upgrades. The implementation of AES, however, may require new hardware.
For now, you can obtain stronger forms of security that go well beyond WEP by implementing proprietary security mechanisms available from access points vendors. The problem is that you’ll probably need to deploy network cards and access points from the same vendor. As a minimum, utilize WEP.
802.11 Next Generation
In addition to the above task groups, the 802.11 WG is studying new methods to increase performance and make better use of the radio spectrum. For example, the group is considering the use of ultrawideband modulation as a new mechanism for supporting higher speed applications and reducing the potential for RF interference. You won’t see these newer, faster standards for a number of years, though.
Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.
Reprinted from 80211-planet.com.
