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WLAN 802.11a



Wireless LANs (WLANs): Focus on 802.11a

At this point, you might be wondering why I dedicated the first WLAN column to 802.11b rather than 802.11a. Well, it's not that the IEEE reinvented the alphabet. Rather, 802.11b standards were completed first, although 802.11a standards were proposed first.

The order of completion is the reverse of the order of proposal due to issues of complexity. So, 802.11a is the more recent standard, the one implemented in the fewest products, and the one with the greatest uphill battle in terms of market acceptance.

802.11a (Wi-Fi5)

Dubbed Wi-Fi5 (Wireless Fidelity 5 GHz) by the Wireless Ethernet Compatibility Alliance (WECA), 802.11a runs in three distinct 100 MHz bands totalling 300 MHz of bandwidth in the 5 GHz range, which the FCC allocated in support of U-NII (Unlicensed-National Information Infrastructure).

The specific allocated frequency domains and associated maximum power levels are 100 MHz in the 5.15-5.25 MHz at a maximum of 50 mW (milliWatt), 100 MHz in the 5.25-5.35 MHz band at a maximum of 250 mW, and 100 MHz at 5.725-5.825 MHz at a maximum of 1 W.

The low and middle bands are intended for in-building applications, and the high band for outdoor use (e.g., building-to-building). At speeds of up to 54 Mbps, 802.11a really screams.

802.11a uses a signal modulation technique known as Coded Orthogonal Frequency Division Multiplexing (COFDM), rather than a spread-spectrum technology like those that 802.11b employs. COFDM sends a stream of data symbols in a massively parallel fashion over multiple subcarriers, which essentially are small slices of RF (Radio Frequency) spectrum within a designated channel within a designated carrier frequency band.

Across the two lower bands, for example, each of eight non-overlapping RF channels is 20 MHz wide and is subdivided into 52 subcarrier channels, each of which is approximately 300 KHz wide. Of those subcarrier channels, 48 are used for user data transmission, and the remaining 4 are used for error control purposes.

Note: You may remember that 802.11b specifies several different signalling rates (i.e., raw speeds) and modulation techniques. The specifics depend on issues including interference caused by other systems sharing the same RF band, signal attenuation caused by physical matter in the space between transmitter and receiver, and multipath fading.

The phenomenon of multipath fading occurs as portions of the signal bounce off of dense physical matter at slightly different angles, thereby taking slightly different paths to the receiver and arriving at slightly different times.

You also may remember about the increased susceptibility of Higher Frequency RF signals to attenuation. Those issues are of real significance and all of those points especially apply in the case of 802.11a, which is way up the frequency range.

The better the quality of the signal at 5MHz, the more complex the modulation technique that can be supported within each subcarrier channel and the more bits, therefore, can be impressed on the RF signal.

All 802.11a-compliant devices must support signalling speeds of 6, 12 and 24 Mbps. Optional speeds include 9, 18, 36, 48 and 54 Mbps.

The coding techniques and data rates specified in 802.11a include BPSK (Binary Phase Shift Keying) at 125 Kbps per channel for a total of 6 Mbps across all 48 data channels, QPSK (Quadrature Phase Shift Keying) at for 250 Kbps per channel for a total of 12 Mbps, 16QAM (16-level Quadrature Amplitude Modulation) at 500 Kbps per channel for a total of 24 Mbps, and 64QAM (64-level QAM) at 1.125 Mbps per channel for a total of 54 Mbps.

As you might expect, the higher speeds work only under the best of circumstances and only over very short distances, with the exact distance being fairly unpredictable as so many factors can influence it.

These data rates are all raw data rates, referring to gross signalling speeds, rather than throughputs. In order to get a sense for actual throughput, we have to consider things like the symbol rate and the delay spread.

The symbol rate refers to the rate of transmission of a symbol, or set of bits. The delay spread is the variation in timing between receipt of the signals associated with a given symbol, with the delay spread being caused by multipath fading, which I defined earlier.

It follows that the symbol rate must be slowed down enough that each symbol transmission is longer than the delay spread, which is sensitive to the degree to which the transmitter and receiver have clear line-of-sight.

Since the data rates are so high, the modulation techniques so sophisticated and sensitive, and the quality of the airwaves so uncertain and tenuous, 802.11a specifications include Forward Error Correction (FEC). Through the embedding of some redundant data in the payload, the receiving device typically can detect, isolate, diagnose and correct errors in transmission.

While this approach inherently adds some overhead to the transmission and, thereby, (further) reduces throughput, it largely obviates the need for bandwidth-intensive retransmissions. Although it also necessitates the embedding of some additional intelligence in the receiving device, the costs of doing so are fairly modest in contemporary terms. On balance, this approach is much more efficient and cost-effective than are the alternatives.

At the MAC (Media Access Control) Layer, both 802.11a and 802.11b make use of the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol, which we discussed in the lesson on Ethernet Essentials.

As you will recall, CSMA/CA requires that each attached device broadcast a Request To Send (RTS) frame before transmitting. If the RTS frame gets through, the destination device responds with a Clear To Send (CTS) frame. All other devices on the network honor this reservation, and the transmission ensues.


802.11a may be behind its younger brother in market penetration, but it has its own selling points -- it runs at speeds up to 54Mbps.

CSMA/CA is much more reliable, but the RTS/CTS frames consume some bandwidth, so this approach is somewhat overhead-intensive and, therefore, affects total throughput. Also, the additional programmed logic makes CSMA/CA somewhat more expensive.

CSMA/CA is used in wireless Ethernet, since the allotted RF spectrum is so limited between the shared access point and the client workstations that an unacceptable level of collisions would otherwise result. On balance, CSMA/CA is more cost-effective than are the alternatives.

While the 5 GHz spectrum is relatively clear in the U.S., it is not nearly so readily available elsewhere as military and governments use portions of this band overseas. In Japan, only the 5.15-5.25 MHz spectrum is available. In Europe, the 5.725-5.825 MHz spectrum is already allocated for other uses, including HiperLAN, which competes with 802.11a.

So, as susceptible as 802.11a is to distortion and interference in the U.S., it's even more so outside our borders.

Speaking of HiperLAN...

HiperLAN (High performance radio LAN) is a high-speed European LAN standard also running in the 5 GHz range. HiperLAN, which grew out of efforts to develop a wireless version of ATM, was approved by ETSI (European Telecommunications Standards Institute) in February 2000.

HiperLAN1 operates at rates up to 20 Mbps, and HiperLAN2 at rates up to 54 Mbps. Since the 5.725-5.825 MHz spectrum is already allocated for other uses in Europe, and since it's important to protect incumbent applications and systems running over previously allocated shared spectrum, ETSI requires that two additional protocols be used in conjunction with 802.11a.

DFS (Dynamic Frequency Selection) allows the 802.11a system to dynamically shift frequency channels in consideration of other signals. TPC (Transmission Power Control) reduces the power level of the 802.11a signal in the event that a competing signal is detected.

Taken together, these protocols serve to eliminate interference issues with incumbent signals. HiperLAN uses Orthogonal Frequency Division Multiplexing (OFDM) as the signal modulation technique.

What About Security?

Security is a big deal these days, and especially in the case of WLANs. We've all got to remember that any wireless communications technology is inherently insecure. Sure, we can add some pretty impressive encryption mechanisms to improve security, but we must at the same time understand that it is possible to break any of them, given enough time and effort.

802.11b specifications include WEP (Wired Equivalent Privacy), which uses a 40- or 128-bit encryption key to protect data in transit. WEP doesn't provide great protection, however, as it has been shown to be easily compromised.

Any real significant inherent security will have to wait for another standards-based solution. In the meantime, user organizations have to overlay their own higher-strength security mechanisms, generally in the form of a VPN (Virtual Private Network), which involves more complex authentication and encryption mechanisms.

To a or not to a?

802.11a has a bit of an uphill battle to fight, largely because of the early penetration of its little brother, 802.11b. Not only did 802.11b get to market first, but it's also very inexpensive and simple (Heck, even I did it!) and, therefore, very popular.

In favor of 802.11a is its much greater speed rating of up to 54 Mbps, and that counts for a lot in an enterprise application where 100 Mbps is the norm. Security remains a bit of a wild card, however, and security is paramount in an enterprise context.




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