Last October I wrote a number of posts discussing the basics of wireless LANs, along with the associated frequency spectrums, security standards, and security vulnerabilities. The wireless networking process, defined through the IEEE 802.11 set of standards, has become common in private homes and has a significant and growing role in corporate and business settings. However, most of the existing standards are very rapidly being seen as inadequate as applications and business plans become more complex and require more bandwidth.
For instance, streaming voice and video, whether it’s a feature-length movie downloaded to your flat screen at home or a videoconferencing session at your office, have become an increasingly less-than-satisfactory proposition with the existing IEEE 802.11a, b, and g standards. One of the significant issues is the delay of packets passing through a congested wireless network. The worse the congestion, the more significant the delay. Many applications, such as Voice-Over IP (VoIP), are very negatively affected by delay in a network and can delivery choppy or garbled audio.
However, the good news is that a new standard, IEEE 802.11n, should eventually provide significantly higher speeds and range. To better appreciate where we are headed with IEEE 802.11n, it is good to relook at where we are now as a starting point.
802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, and 802.11g versions to provide wireless connectivity in the home, office, and many commercial establishments such as coffee shops and books stores.
- 802.11a – The 802.11a standard operates in the 5 GHz band and uses a 52-subcarrier orthogonal frequency-division multiplexing (OFDM) scheme with a maximum raw data rate of 54 Mbit/s. The advantages of using OFDM include reduced multipath effects in reception and increased spectral efficiency. However, in the real world, 802.11a usually yields a more realistic net achievable throughput in the mid-20 Mbit/s.Using the 5 GHz band gives 802.11a a significant advantage over other standards because of low utilization of that band of frequencies. However, this high carrier frequency also brings a slight disadvantage. The effective overall range of an 802.11a signal is slightly less than that of 802.11b/g. And, 802.11a signals cannot penetrate as far as those for 802.11b because they are absorbed more readily by walls and other solid objects in their path. In addition, 802.11a products are very expensive to manufacture because of the difficulty of manufacturing a solid state device that can provide a useful power output in the 5 Ghz range.
- 802.11b – This standard has a maximum raw data rate of 11 Mbit/s and uses the same Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) media access method. Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s using Transmission Control Protocol (TCP) and 7.1 Mbit/s using User Datagram Protocol (UDP).802.11b products appeared on the market in mid-1999 and are a direct extension of the Direct-sequence spread spectrum (DSSS) modulation technique. The dramatic increase in throughput of 802.11b, along with simultaneous substantial price reductions, led to the rapid acceptance of 802.11b as the definitive wireless LAN technology.802.11b devices suffer interference from other products operating in the 2.4 GHz band. Devices operating in the 2.4 GHz range include; microwave ovens, Bluetooth devices, baby monitors, cordless telephones and, very importantly, your children’s’ wireless game controller. Interference issues and user density problems within the 2.4 GHz band have become a major concern and frustration for users.
- 802.11g – This standard is the third modulation standard for Wireless LANs. Like 802.11b, this standard operates in the 2.4 GHz band. However, it operates at a maximum raw data rate of 54 Mbit/s. This equates to about 19 Mbit/s net throughput. This speed is identical to an 802.11a core, except for some additional legacy overhead for backward compatibility. The 802.11g hardware is fully backwards compatible with 802.11b hardware. In an 802.11g network the presence of a legacy 802.11b participant will significantly reduce the speed of the overall 802.11g network.The modulation scheme used in 802.11g is also the orthogonal frequency-division multiplexing (OFDM) process, which is copied from 802.11a. It offers data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. And, it reverts to CCK, like the 802.11b standard for 5.5 and 11 Mbit/s, and DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s. Even though 802.11g operates in the same frequency band as 802.11b, it can achieve higher data rates because of its heritage to 802.11a.
In my next post I will examine the specifics of the new IEEE 802.11n standard and describe how it uses the new Multiple Input Multiple Output (MIMO) process.
Author: David Stahl