IEEE 802.11 DSSS: The Path To High Speed Wireless Data Networking

By Angela Champness

The 1997 completion of the IEEE 802.11 standard for wireless LANs (WLANs) was a first important step in the evolutionary development of wireless networking technologies. The standard was developed to maximize interoperability between differing brands of wireless LANs as well as to introduce a variety of performance improvements and benefits. On September 16, 1999, the 802.11 ratified a revision of the 802.11 standard, called 802.11 High Rate, that provides much higher data rates, while maintaining the 802.11 protocol.

In addition to providing high performance and robust systems, 802.11 also promises multi-vendor interoperability amongst product with the same PHYs. This means that customers are freer to mix and match vendors to meet their requirements for each given application. Furthermore, standardization also delivers lower cost components, which will translate into lower prices for users. Because of this, almost all WLAN vendors have moved to IEEE compliance. With the upcoming announcement of the new high-speed specification, most of these vendors will be announcing High Speed 802.11 compliant products as well.

The standard specifies a choice of different physical (PHY) layers. Vendor's implementations have either used Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS), both Radio Frequency (RF) based.

In 802.11, the DSSS PHY specifies a 2-Mbps-peak data rate with optional fallback to 1-Mbps in very noisy environments. The standard defines the FHSS PHY to operate at 1 Mbps and allows for optional 2-Mbps operation in very clean environments.

Most vendors chose to implement DSSS as the recently ratified 802.11 High Rate (11Mbps) standard is based on DSSS as well. This makes migration from a 2 Mbps 802.11 DSSS system to an 11 Mbps 802.11 HR system very easy as the underlying modulation scheme is very similar. 2 Mbps 802.11 DSSS systems will be able to co-exist with 11 Mbps 802.11 HR systems, enabling a smooth transition to the higher data rate technology. This is similar to migrating from 10 Mbps Ethernet to 100 Mbps Ethernet, enabling a large performance improvement while maintaining the same protocol.

The 802.11 protocol (Media Access Control layer - MAC), is extremely robust and feature rich. It includes Sequence Control and Retry fields supporting a feature called MAC-layer acknowledge that minimizes interference and maximizes usage of the bandwidth available on the wireless channel.

Type/Subtype and Duration fields ensure reliable communications in the presence of hidden stations. WEP (Wired Equivalent Privacy) fields result in data security that is equal to that achievable with standard Ethernet. Sequence Control and More Frag fields support a concept called fragmentation that can allow a WLAN to operate in the presence of interference or signal fading.

The 802.11 MAC can work seamlessly with standard Ethernet, via a bridge or access point (AP), to ensure that wireless and wired nodes on an Enterprise LAN can interoperate with each other. The WLAN standard uses a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) MAC scheme, whereas standard Ethernet uses a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) scheme.

Roaming Provisions

802.11 allows a client to roam among multiple APs that can be operating on the same or separate channels. For example, every 100 msecs, an AP might transmit a beacon signal that includes a time stamp for client synchronization, a traffic indication map, an indication of supported data rates, and other parameters. Roaming clients use the beacon to gauge the strength of their existing connection to an AP. If the connection is judged weak, the roaming station can attempt to associate itself with a new AP.

Power Management

802.11 adds features to the MAC that can maximize battery life in portable clients via power-management schemes. Power management causes problems with WLAN systems because typical power-management schemes place a system in sleep mode (low or no power) when no activity occurs for some specific or user-definable time period. Unfortunately, a sleeping system can miss critical data transmissions.

To support clients that periodically enter sleep mode, the 802.11 specified that APs include buffers to queue messages. Sleeping clients are required to awaken periodically and retrieve any messages. The APs are permitted to dump unread messages after a specified time passes and the messages go unretrieved.

Wired Equivalent Privacy

One final area of differentiation between 802.11 and either wired LANs or existing WLAN implementations centers on data security. The standard defines a mechanism through which the WLANs can achieve Wired Equivalent Privacy (WEP). If WEP is enabled then all data transmitted over the wireless network in encrypted.

Interoperability

One of the biggest gains from the 802.11 standard is the ability for products from different vendors to interoperate with each other. This means that as a user, you can purchase a wireless LAN card from one vendor and a wireless LAN card from another vendor and they can communicate with each other, independent of the brand of access point you as a user chose. This gives the user the choice to choose the system that best meets the needs for each application. In the past there have been claims made by some vendors that interoperability between vendors was not yet achieved. However this is not the case.As a supplement to the 11 Mbps interoperability testing that will be preformed through WECA,, a number of vendors have successfully tested interoperability together at the University of New Hampshire Interoperability Lab. Further details on these results can be found on http://www.iol.unh.edu/consortiums/wireless/.

Follow-on efforts in IEEE 802.11: Faster data rates

As mentioned above, 802.11 has been moving rapidly to provide continuous improvements, as has been seen within the Ethernet 802.3 environment.

The latest step in the evolution was the ratification of the 802.11b or High Rate, providing data rates of 11 Mbps. The standard's 11 Mbps PHY layer uses Complementary Code Keying (CCK) technology,. This latest standard is based on DSSS technology and provides speeds up to 11 Mbps with fallback rates of 5.5 Mbps, 2 Mbps, and 1 Mbps. It uses the same bandwidth as the 2 Mbps DSSS standard and thus interoperates with legacy IEEE DSSS systems. As in the wired world, higher speeds are continuously desired for applications such as streaming video, telephony and multimedia. Moreover, faster peak rates will allow more nodes to effectively connect to a WLAN via a single channel. Now that the standard development has stabilized, we can reasonably expect that many of the industry's leading vendors will be bringing out high speed products in the near future.

Other Industry Activities

WECA (Wireless Ethernet Compatibility Alliance) was recently formed to provide compliance and standards for the wireless LAN industry. They have announced the Wi-Fi™ (wireless fidelity) standard that is an awarded "seal of approval" for those WLAN products that have successfully completed prescribed interoperability testing. The Wi-FiÔ seal will provide customers the assurance that products bearing this logo will work together. The WECA Group includes a growing number of the industry's leading wireless LAN manufacturers.

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