Wireless LAN Standards and Modes
 


WLAN Standards

he Wireless Local Area Networl (WLAN) technology is defined by the IEEE 802.11 family of specifications. There are currently four specifications in the family: 802.11, 802.11a, 802.11b, and 802.11g. All four use the Ethernet protocol and CSMA/CA (carrier sense multiple access with collision avoidance instead of CSMA/CD) for path sharing.

  • 802.11 -- applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
  • 802.11a -- an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing (OFDM) encoding scheme rather than FHSS or DSSS. The 802.11a specification applies to wireless ATM systems and is used in access hubs.
  • 802.11b (also referred to as 802.11 High Rate or Wi-Fi) -- an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
  • 802.11g -- offers wireless transmission over relatively short distances at 20 - 54 Mbps in the 2.4 GHz band. The 802.11g also uses the OFDM encoding scheme.
  • 802.11n - builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output). IEEE 802.11n offers high throughput wireless transmission at 100Mbps – 200 Mbps.

 

Name

Band (GHz)

Maximum Transmission Rate (Mbit/s)

Note

802.11 Legacy

2.4

2

Outdated; virtually no end devices available

802.11a

5

54

Less interference-prone

802.11b

2.4

11

Less common

802.11g

2.4

54

Widespread, backwards-compatible with 11b

802.11n

2.4 and/or 5

300

Common

802.11

The main problem of radio networks acceptance in the market place is that there is not one unique standard like Ethernet with a guaranteed compatibility between all devices, but many proprietary standards pushed by each independent vendor and incompatible between themselves. Because corporate customers require an established unique standard, most of the vendors have joined the IEEE in a effort to create a standard for radio LANs. This is IEEE 802.11 (like Ethernet is IEEE 802.3, Token Ring is IEEE 802.5 and 100vg is IEEE 802.12).

Of course, once in the 802.11 committee, each vendor has pushed its own technologies and specificities in the standard to try to make the standard closer to its product. The result is a standard which took far too much time to complete, which is overcomplicated and bloated with features, and might be obsoleted before products come to market by newer technologies. But it is a standard based on experience, versatile and well designed and including all of the optimisations and clever techniques developed by the different vendors.

The 802.11 standard specifies one MAC protocol and 3 physical layers : Frequency Hopping 1 Mb/s (only), Direct Sequence 1 and 2 Mb/s and diffuse infrared (can we really call it a "standard" when in includes 3 incompatible physical layers ?). Since then, it has been extended to support 2 Mb/s for Frequency Hopping and 5.5 and 11 Mb/s for Direct Sequence (802.11b). The MAC has two main standards of operation, a distributed mode (CSMA/CA), and a coordinated mode (polling mode - not much used in practice). 802.11 of course uses MAC level retransmissions, and also RTS/CTS and fragmentation.

The optional power management features are quite complex. The 802.11 MAC protocol also includes optional authentication and encryption (using the WEP, Wired Equivalent Privacy, which is RC4 40 bits - some vendors do offer 128 bits RC4 as well). On the other hand, 802.11 lacks to defines some area (multirate, roaming, inter AP communication...), that might be covered by future developments of the standard or complementary standards. Some 802.11 products also implement proprietary extensions (bit-rate adaptation, additional modulation schemes, stronger encryption...), those extensions may or may not be added to the standard over time.

When 802.11 was finalised (september 97), most vendors were slow to implement 802.11 products because of the complexity of the standard and the number of mandatory features (and in some cases they also need to provide backward compatibility with their own previous line of products). Some of the optional features (encryption and power saving) did only appear months after the initial release of the product. But things seem to be sorted out and we now have fully featured products on the market. The complexity of the specification, the tightness of the requirements and the level of investment required made 802.11 products expensive compared to the previous generation of wireless LANs, but because of the higher standardisation and higher volumes, prices are now dropping.

Even if vendors eventually have launched 802.11 products, the standard doesn't fully guarantee inter-operability : the products have to use at least the same physical layer, the same bit rate and the same mode of operation (and there is so many other little important details...). The most cooperative vendors have been busy lately sorting out interoperability issues with independent testing labs, but it is still a touchy subject... 

After 7 years of arguing in sub-committees making 802.11, you would think that most people would had enough of it. In fact no, the 802.11 committee is now busy pushing a new standard at 5 GHz, and also higher speed at 2.4 GHz (by tweaking the Direct Sequence physical layer). Both standard makes changes only to the physical layer, so that the 802.11 MAC can be reused totally unmodified, saving costs.

802.11-a (802.11 at 5 GHz) was standardised first (spring 99), based on OFDM, and using the UNII band (so it won't be available in Europe and Japan). The OFDM physical layer is a very close copy of the one used in HiperLan II (so they might be some sort of compatibility ), using 52 subcarriers in a 20 MHz channel, offering 6, 12 and 24 Mb/s and optional 9, 18, 36, 48 and 54 Mb/s bit-rates. No products are yet on the market.

Very soon after, 802.11 did standardise 802.11-b (802.11 HR), based on a modified DS physical layer . The goal was to extend the life of the 2.4 GHz band by overcoming the major drawback : low speed. On top of the original 802.11-DS standard, 802.11-b offer additional 5.5 Mb/s and 11 Mb/s bit rates. It was approved by the FCC and they are now products on the market (which are quite popular).

 

Basically, wireless networks can be classified into three network modes:

Managed Mode (Infrastructure Mode), via Access Point

Managed networks have a managing element: the access point. In this mode (also referred to as infrastructure mode), all connections of the WLAN stations in the network run through the access point, which may also serve as a connection to an ethernet. To make sure only authorized stations can connect, various authentication mechanisms (WPA, etc) are used.

Ad-hoc Mode (Peer-to-Peer Network)

Ad-hoc networks do not have an access point. The stations communicate directly with each other, therefore an ad-hoc network is usually faster than a managed network. However, the transmission range and number of participating stations are greatly limited in ad-hoc networks. They also do not support WPA authentication. If you intend to use WPA security, you should not use Ad-Hoc_Mode.

Master Mode

In master mode your network card is used as the access point. It works only if your WLAN card supports this mode.