Network Magazine India

Do you need dependable and fast access to information? Do you want network performance with the freedom of mobile computing? Do you want to access the network from anywhere in your organization? If yes, then Wireless LANs is for you.

Wireless LANs allow workstations to communicate and access the network using radio propagation as the transmission medium. Says Anand Mehta, Marketing Manager, D-link India Ltd., "A wireless LAN is an easy data communication system that uses electromagnetic waves (radio and infra red) to communicate information from one point to another without relying on a physical connection. It is implemented as an option for a wired LAN within specified premises".

Just as PCs were the initial driving force behind wired Ethernet innovations in the 1980s, the wide acceptance of laptops, palmtops, PDAs, and wireless telephony is creating an environment that is ripe for innovation in the form of wireless Ethernet connectivity. To find where this trend is headed for, we need to look further into wireless LANs.

WLAN Technologies
Three important WLAN technologies exist today. They are:

Direct Sequence Spread Spectrumx
DSSS systems spread the signal energy across a relatively wide band by increasing the occupied bandwidth. A DSSS transmitter converts a bit stream into a symbol stream, in which each symbol represents a certain number of bits, depending on the phase shift keying (PSK) modulation technique.

The symbol information is converted into a complex-valued signal that is fed to the spreader. The spreader multiplies its input signal with a pseudo noise sequence, called a chip sequence. This multiplication creates a signal with a wider bandwidth. The inphase and quadrature components of the spreader output signal are fed to a quadrature modulator.

The transmitter front end provides filtering, conversion to a higher RF, and power amplification. The channel center frequencies are 2.412 Mhz, 2.417 Mhz, 2.422 Mhz, 2.462 Mhz, 2.467 Mhz and 2.472 Mhz.

Frequency Hopping Spread Spectrum
These systems hop from narrow band to narrow band within a wide bandwidth. FHSS wireless LAN stations send one or more data packets at one carrier frequency, then hop to another carrier frequency to send one or more packets, and continue this hop transmit sequence, called slow frequency hopping. The time these FHSS radios dwell on each frequency is fixed. The hopping pattern appears random, but it is actually a periodic sequence tracked by both the sender and receiver. A FHSS transmitter converts the bit stream into a symbol stream in which each symbol represents one or more bits.

Infrared Technology
On the other hand Infrared (IR) systems use very high frequencies that are just below visible light in the electromagnetic spectrum to carry data. Like light, infra red rays cannot penetrate opaque objects. They are either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range of just about three feet and are typically used for PANs but occasionally are used in specific WLAN applications. High performance directed IR is impractical for mobile users and is therefore used only to implement fixed subnetworks. Diffuse (or reflective) IR WLAN systems do not require line-of-sight, but cells are limited to individual rooms.

Yet another technology that will gain widespread usage in the next few years is Bluetooth. It is a wireless personal area networking (WPAN) technology that has garnered significant industry support and will coexist with most wireless LAN solutions. The Bluetooth specification is for a 1 Mbps, small form-factor, low-cost radio solution that can provide links between mobile phones, mobile computers and other portable handheld devices and connectivity to the Internet.

Embedded in a wide range of devices to enable simple, spontaneous wireless connectivity, this technology, is a complement to wireless LANs that are designed to provide continuous connectivity via standard wired LAN features and functionality.

WLAN Standards
The IEEE Standards board approved the 802.11 wireless LAN standard on June 26th, 1997. The standard was published at the end of '97 and IEEE 802.11 compliance became a minimum requirement for any wireless LAN product. IEEE 802.11 brings multi vendor interoperability and lower prices. The main features of IEEE 802.11 standard are:

IEEE 802.11 Standard specifies the frequency band for Wireless LAN applications as 2.4Ghz ISM band.
Latest release of IEEE 802.11 Standard specifies a maximum bandwidth of 11 Mbps with fall back to 5.5 Mbps, 2 Mbps and 1 Mbps throughput. In addition, the full compliance to IEEE Standards assures not only the interoperability with the earlier 2 Mbps Wireless LAN products but also with expected future higher throughput wireless LAN products.

The 802.11 standard DSSS prescribes a single fixed 11-chip spreading code that is used by all stations. The medium access assignment is not made by a unique code, but rather by a listen-before-talk carrier sensing and at a deferred transmission. All the stations participating in a wireless network share the single-channel medium.

How do Wireless LANs Work?
Wireless LANs use electromagnetic airwaves (radio or infrared) used to communicate information from one point to another without relying on any physical connection. Radio waves are often referred to as radio carriers because they simply perform the function of delivering energy to a remote receiver.

The data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at the receiving end. This is generally referred to as modulation of the carrier by the information being transmitted. Once the data is superimposed (modulated) onto the radio carrier, the radio signal occupies more than a single frequency, since the frequency or bit rate of the modulating information adds to the carrier.

Multiple radio carriers can exist in the same space at the same time without interfering with each other, if the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies.

In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure.

A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna attached to the access point) is usually mounted high but may be mounted essentially anywhere that it is practical as long as the desired radio coverage is obtained.
End users access the wireless LAN through wireless-LAN adapters, which are implemented as PC cards in notebooks or palmtop computers, as cards in desktop computers or integrated within hand-held computers.

Wireless LAN adapters provide an interface between the client network operating system (NOS) and the airwaves via an antenna. The nature of the wireless connection is transparent to the NOS.

How secure is a WLAN?
One of the main concerns of users of Wireless LANs is the assumed reduction in privacy and security. Let us see how wireless solutions address these concerns.

Next to all standard LAN Access Control Mechanisms offered by network operating systems, Wireless LANs uses multiple levels of security to prevent unauthorized access to network resources.

The Wireless LAN offers up to five layers of added protection. The following elements add to the security:

Spread Spectrum
Most of the Wireless LAN products use low power Spread Spectrum technology in sending the signal. In doing so, it transmits data by converting the signal from digital to analog and spreading it eleven times over the waveband. This spreading is done using a unique code, which is built into the product.

Physical access to the LAN does not yield intelligible results unless the Wireless LAN product is used to decode the signal.

"Close Wireless System" option
Wireless LAN systems provide the so-called "Close Wireless System" option, which will prevent wireless stations to associate with an Access Point if the SSID ( = Network Name ) differs.

Shared Key Authentication
Shared Key Authentication feature allows Access Points to verify the user as being authorized to associate to the Access Point. It requires WEP (Wired Encryption Privacy) to be present. The actual authentication procedure consists of an exchange of four messages between the station and the Access Point, allowing the Access Point to verify that the station has the proper key.

Hardware Encryption using WEP
As an added option wireless solutions use hardware encryption to provide added privacy to transmitted data. The traffic between the stations will be encrypted in order to prevent eavesdropping. Wireless LANs use encryption based on various algorithms.

Access Control Table
Wireless Solutions have the included capability to restrict access to the infrastructure network to those stations of which the hardware MAC address is included in a pre-loaded filter table. Network administrators who wish to deploy this capability will create a table of MAC addresses of wireless stations that are allowed to have access to the backbone. Stations with MAC addresses that do not appear in this table are not granted access, and the traffic generated by these stations will be filtered out. This mechanism is known as 'Access Control' and the specific table mentioned is called the Access Control Table.

In addition to the above, user defined schemes can be added such as user passwords on network servers. With the provided security provisions in place, Wireless systems will have equal or more privacy than can be expected from existing wired stations.

Applications of Wireless LAN
Wireless LAN Applications can be broadly categorized into three parts:
Campus Wireless LANs

Typical applications
Organizations, medical facilities, retail outlets,
warehouses, factories, research centers, educational institutions among many others are recognizing the value of flexibility and adaptability provided by wireless LANs.

Roaming users empowered with laptops, PDAs, palmtops can have access to the company's intranet and Internet from any place of their presence. A wireless way to keep them in touch with computer services or resources is to create a wireless network with several access points and supply the roaming staff with portable computers equipped with wireless network interface cards.

Streamlining Inventory Management
Managing stock control and inventory is a necessity for any manufacturing organization and for any retail operation of any size. But fluctuations in volume and human error can increase the potential for more mistakes and ultimately, lose revenue.

With wireless LAN handheld scanners, keypads and bar code readers can be linked to databases, printers and scanners throughout the facility. And the process is virtually paperless. Instead of being a separate time consuming task, inventory becomes a part of every retailer's routine, making stock and inventory control more manageable.

Extending LANs to difficult-to-wire-areas
Wireless LANs coexist with wired LANs and help in extending LAN connectivity to areas with EMI/EMC problems, locations where providing wired connections affect aesthetic qualities of the location (showrooms, conference halls), and locations with frequently changing environments.

Wireless LANs can also be used to provide connectivity to vehicle mounted computers. Wireless VoIP handsets can be used by customer service/field staff to enhance the organization's productivity.

Outdoor Point-to-Point and Point-to-Multipoint Links
Wireless links are being used for providing outdoor point-to-point and point-to-multipoint high bandwidth links connecting different branches of an organization spread over a city. These links can be used to set up Wireless Metropolitan Area Networks (MANs) for e-governance or for linking various government organizations, info kiosks, public utility places, and so on.

Wireless point-to-multi-point links are being deployed by ISPs for providing high bandwidth Internet connectivity to their corporate clients. Similarly broadband service providers and ASPs are making use of these links for offering their services.

Many Wireless LAN vendors incorporate special features such as inbuilt router features, additional security measures, provision for connecting outdoor antennas, high message reliability and robust interference management features, remote monitoring features, site survey tools, etc. which are required for point-to-multi-point applications.

Wireless point-to-multipoint links can be used for providing the necessary connectivity for anywhere banking, online reservation form, and geographically dispersed counters within a city. NM

Mahesh Rathod can be reached at rathodmp@hotmail.com

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