Wireless Local Area Networks: Technology, Standards & Limits

Introduction to Wireless LANs

The advent of networking — and more particularly the Local Area Network (LAN) — has its origins in the growing need for users to access and share data, applications, and services that could not be stored on their own computers alone. Each LAN follows a particular topology that depicts both the physical arrangement of nodes and the logical method by which data is transferred. Initially, LAN implied a wired topology: the interconnection of nodes in a particular location by means of a cable network. Within that environment, however, there also existed a group of mobile users who needed to remain connected regardless of their physical position. Furthermore, with the rise of small mobile and home offices, area limitations became a significant frustration. The solution was the Wireless LAN (WLAN).

A WLAN enables computers on a local area network to access data and other information without using cables, transmitting data through the air, through walls, ceilings, and cement structures. Notebook PCs and laptops can easily be carried from place to place while remaining connected.

A Wireless LAN is a communication system that transmits and receives data using modulated electromagnetic waves as a substitute for wired cables. It consists of two primary components: an access point or base station that is physically connected to the LAN, and a built-in or add-on wireless card attached to the user's PC. The medium connecting the access point and the PC may be infrared, radio frequency (RF), or microwave.

Transmission Media: Infrared, Microwave, and Radio Frequency

Infrared systems use the same signal frequencies employed on fiber-optic links. Because only the amplitude of signals is detected, there is a substantial reduction in interference. Transmission speeds in these systems are higher because they are not constrained by bandwidth limitations. Moreover, since infrared operates in the light spectrum, no license is required. Infrared LANs can be implemented in two ways: directional and omni-directional.

Directional transmission provides a range of up to 2 kilometers and offers the highest bandwidth and throughput. Omni-directional transmission bounces signals in all directions but is limited to a shorter range of 30–60 feet. The main disadvantages of infrared systems lie in the fact that the transmission spectrum is shared with the sun and other sources such as fluorescent lights, so interference from these sources can adversely affect the LAN. An unobstructed line of sight is also essential, since infrared signals cannot penetrate opaque objects — walls, dividers, curtains, and even fog may obstruct the signal.

Microwave systems using narrowband transmission with single-frequency modulation operate mostly in the 5.8 GHz band. Because there is no spread-spectrum overhead, higher throughput is achieved in microwave systems. These systems operate at less than 500 milliwatts of power.

Radio frequency systems operate using spread spectrum in two ways: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both DSSS and FHSS have lower data rates than infrared and microwave due to the additional overhead involved.

In DSSS, the transmitted signal is permitted to spread over an allowed band and is modulated by a random binary string known as a spreading code. Data bits are mapped to patterns of chips, and the number of chips representing a bit constitutes the spreading ratio. Greater spreading ratios provide more interference resistance; conversely, lower spreading ratios increase the bandwidth available to the user. Both transmitter and receiver must be synchronized with the same spreading code. Because the signal spreads over a wider band, recovery is faster in DSSS systems.

FHSS involves splitting the band into several sub-channels. The signal transmits short bursts by hopping from one sub-channel to another for a predetermined period known as the dwell time. This requires synchronization of signals at both transmitter and receiver. Because the frequency is constantly shifted, interference is reduced. For co-location of several FHSS LANs, an orthogonal hopping sequence is used.

IEEE 802.11 Standard and Network Architecture

The wide acceptance of WLANs — driven by their flexibility, scalability, and mobility — made standardization essential to ensure compatibility and reliability among manufacturers and users. In 1997, the Institute of Electrical and Electronics Engineers (IEEE) standardized the WLAN under the designation IEEE 802.11. The WLAN architecture defined by this standard refers to each computer as a station. Stations are categorized as either fixed or mobile. When two stations come together for communication, a Basic Service Set (BSS) is formed. The IEEE 802.11 standard uses the BSS as its fundamental building block, requiring at least two computers.

A BSS is called an Independent Basic Service Set (IBSS) if it is stand-alone and not connected to any base, established according to a peer-to-peer architecture. The Distribution System (DS) enables two or more BSSs to be interconnected with the help of Access Points, which extends network coverage. Access Points bridge the gap between the BSS and the DS, and data moves through them. When large and complex networks are built using BSSs and a DS, an Extended Service Set (ESS) is created. The entire network under the ESS operates as an independent basic service set to the Logical Link Control Layer.

The benefits of IEEE 802.11 are extended through the use of a portal, which logically integrates a wired LAN with an IEEE 802.11 Wireless LAN. The portal also serves as a gateway to the Distribution System, bridging the gap between wired and wireless networks. The IEEE 802.11 standard specifies the services the Distribution System must support, which — in conjunction with the BSS and DS — may be categorized as Station Services and Distribution System Services.

Conclusion

Irrespective of so many concerns, Wireless LANs retain their distinct advantages in terms of flexibility, scalability, and mobility, making them extremely useful for mobile groups of users. Their ability to transmit data through physical structures without cables represents a genuine advance over traditional wired networks, and for environments where cabling is impractical or impossible, the WLAN remains an indispensable solution. As standards continue to evolve, ongoing improvements in speed, security, and signal reliability will further enhance the viability of wireless networking.

References

1. Prem, Edward C. "Wireless Local Area Network." Retrieved from http://www.cis.ohio-state.edu. Accessed 04/01/2004.

2. "Wireless Local Area Networking: An Introduction." Accessed 04/01/2004.

3. "What Wireless Is/Is Not." Retrieved from http://bits.eller.arizona.edu/services/resources/WLAN/whatiswireless.aspx. Accessed 04/01/2004.