by April 12, 2002 0 comments

Wireless LAN technology has reached a level where it can be used for reliable communication over a network. The way it’s set up will depend upon the specific requirements of a company, but the general topologies remain the same. Plus, the technology used for data transmission has also been standardized thanks to the IEEE 802.11b standard, which allows wireless throughput of up to 11 Mbps. Here, we explore the main topologies and techniques used in Wireless LANs.

The topologies
Simply speaking, there are two types of wireless topologies, called Ad hoc and Infrastructure. The former is purely a wireless-only network. This happens when nodes, be they PCs or notebooks, communicate with each other directly over wireless. There may or may not be an access point. Ad hoc wireless setups are usually suitable for temporary arrangements, such as in meetings, etc. 

The other kind of wireless topology, called infrastructure is basically augmenting your existing wired network with a wireless LAN. For this, there would be access points connecting to your network’s backbone. Where you place these access points depends upon your requirements. You could have them placed near a meeting room so that there’s a network connection whenever it’s required without the need to run around looking for a network cable every time there’s a meeting. You could place them such that the access point ranges overlap in such a way so as to provide roaming to mobile users. This way the user will get seamless connectivity as he passes from one access point to another. 

Wireless vs wired
Wireless LANs can be seamlessly integrated into a wired network, and the wireless nodes can use the same protocols and applications as wired nodes. However, there’s a slight difference in the way they work from wired LANs. This difference is in the way they use the bottom two layers, namely the Physical layer and the MAC (Media Access Control) part of the Data Link layer of the OSI model. Functionality for the remaining layers is the same. 

If you’re familiar with how Ethernet works, you’ll know that they use the CSMA/CD technology to transmit data, which happens at the MAC layer. CSMA/CD stands for Collision Sense Multiple Access/Collision Detection. This technology allows a node to detect whether the packet it transmitted had a collision with another packet, and will retransmit if it finds so. In Wireless LANs, however, this is not possible because of difficulties of detecting collisions in Radio Frequency. Therefore, they have to resort to a technique called CSMA/CA, where the CA stands for Collision Avoidance. Here, the nodes look for a free channel, and then transmit the signal. They have to rely on the receiving node to send an acknowledgement whether the data was received. Other features included in the data link layer of wireless nodes is security called WEP (see Wireless Lans: How Secure, How Safe?, page 88), and power management, wherein portable stations like notebooks go to sleep after a time interval specified by the base station or Access Point.

The physical layer in wireless LANs differs from the wired counterparts because there’s no wire to transmit data over. There’s just air, so the physical layer must choose the technique for data transmission. Two transmission technologies are used here, Photonic or Radio Frequency. Techniques used with RF include Spread Spectrum and Low Power Narrowband use RF technology. Photonic technology uses Infrared frequency for transmission, and is not very widely used in wireless LANs these days. 

Data transmission techniques
Wireless LANs either use Radio wave or infrared frequencies. The earlier wireless access points that came into the market used infrared, but today most of them use radio frequency. The advantage of this is that radio waves can penetrate through surfaces like walls and doors. Infrared devices on the other hand are based on the line of sight, and get blocked or reflected on such surfaces. 

Data transmission over wireless using Radio frequency is similar to the techniques used in an ordinary radio, AM and FM. There’s a constant carrier signal that’s generated, which is superimposed by another signal containing the data to be transmitted. The carrier frequency’s amplitude could be modulated by a signal (Direct Sequence), or its Frequency could be modulated (called Frequency Hopping). After modulation, the carrier signal no longer remains a single frequency, or fixed amplitude, depending upon the modulation technique. 

There are two RF techniques used for data transmission in wireless LANs, called Spread Spectrum and Low power Narrowband. Spread Spectrum is further divided into two more techniques, called Frequency Hopping and Direct Sequence.

Spread spectrum
In Frequency Hopping, there’s a narrowband carrier signal that hops from one frequency to another in a pre-defined fashion. Both the transmitter and receiver know this hopping sequence and therefore have to remain in synchronization in order to send and receive accurate data. Since the method hops over multiple frequencies, it forms a single logical channel used and maintained by the transmitter and receiver. To those who are not supposed to receive this signal, it appears to be wideband noise.

In Direct Sequence Spread Spectrum technique, every binary ‘1’ bit that’s transmitted is in the form of a sequence of ones and zeros. Every binary ‘0’ transmitted uses the inverse sequence of this ‘1’ bit. This redundant pattern for every bit that’s transmitted is called a Chip. A wider chip increases the chances of data recovery by the receiver. The ratio of chips to the original bits is called the spreading ratio or gain. To unintended receivers, this appears as low-power wideband noise.

Low-power narrowband
The second transmission method, called low-power narrowband uses a single frequency narrow enough to transmit the data signal. Every wireless node communicates using a different frequency, so that there’s no crosstalk with other nodes. This prevents one node from listening to others. So while receiving, a node will filter out all other frequencies but for the one that’s meant for it. 

The future
There’s ongoing development in the area of wireless LANs. New standards are being developed by the IEEE 802 group to improve the performance and throughput. For instance, the latest draft standard, the 802.11g, is trying to take the throughput in wireless LANs to 20+ Mbps, and that too in the 2.4 GHz band. Given such a development, wireless LANs seem to be a promising technology for the future.

Anil Chopra

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