Wireless Local Area Networks

("http://www.ent.ohiou.edu/~sangal/lan.html")
Rahul Sangal E-mail: sangal@bobcat.ent.ohiou.edu

Table of Contents

1. Introduction

For some time, companies and indivisuals have connected computer with local area networks
( LANs ). This allowed the ability to access and share data, application and other services not resident on any one computer. The LAN users have at disposal much more information, data and applications then they could otherwise store by themselves.

With the increasing number of portable computers and highly mobile users, the need of wireless Local Area Networks is increasing. Wireless LANs are especially needed in envoirnments that make the use of cable difficult or impractical. The main advantages offered by wireless LANs are portability, low installation costs and quick set up time.

2. Physical Media

1. Infrared ( IR ) Transmission : This uses the same technology as in a television remote control unit. IR wireless LAN is not bandwidth limited and is capable of transmission speeds of 50Mb/s and more. IR systems are easy to design and therefore inexpensive. The drawbacks to IR systems are that the transmission spectrum is shared with the sun and other things such as flouresent lights. If there is enough interference from the other souces it can render the LAN useless. IR systems require an unobstructed LOS ( line of sight) as it cannot penetrate opaque objects.

2. Microwave : Microwave systems( MW ) operate at less than 500 milliwatts of power. The biggest advantage to MW systems is higher throuhput achieved. Radio LAN is an example of systems with microwave technology.

3. Radio : Most RF LANs use spread spectrum modulation. "Spread Spectrum is a digital coding technique where the signal is "spread" so that it appears more like noise and are resilient to interference. There is a lot of overhead involved with spread spectrum and so most of them had lower data rates than IR or MW. There are two type of spread spectrum systems :
a) Direct Sequence ( DS ).
b) Frequency Hopping (FH).

3. Medium Access Control


The Data link layer of the OSI model handles a variety of transmission functions, including ensuring that the data is packaged properly before it is sent over cabling and performing flow and error control.
The MAC - Medium Access Control- layer, which resides at the lower portion of the Data Link Layer controls the access to the physical transmission medium. It's this layer that recieved a lot of attention from the 802.11 committee.
The 802.11 standard specifies that Carrier Sense Multiple Access with Collision Avoidance, or CSMA/CD, should be used as the meathod for transmitting information over Wireless LAN. The CSMA part determines whether the transport medium is currently busy with another transmission. But in cases where two or more end stations hear a quiet network and start to blast information at about the same moment, the collision is inevitable. Collision Detection ( as in IEEE 802.3 standard ) does not quite work because that would require wireless radios to send and recieve at the same time - a requirement that would make products more expensive.
Instead 802.11 supprots CSMA / CA along with something called positive acknowledgement . With the collision avoidance meathod, a station that wants to transmit first checks the medium to see if it is free. If it is free, then the station is allowed to send. The station on the recieving end of the transmission then dispatches an acknowledgemet to inform the sending station that a collision did not occur. If the sending station does not recieve an acknowledgement packet it will assume the original packet did not make it through and will resend until an acknowledgement is recieved.
To minimize the posibility of collision due to the stations not being able to hear each other, 802.11 defines a virtual-carrier sense feature. This allows for a station that wants to send something to first send a Request to Send (RTS), which is a short packet that contains the source and destination addresses, as well as the duration of the transmission. Other stations now know that they must wait that long before they can transmit. If the medium is free, the reciever station will then reply with a short packet called Clear to Send (CTS), which will contain the same duration-of-transmission information. It also indicates to its neighbouring stations about the time period for which it will be busy. Since each packet is also acknowledged the entire sequence is called the 4 way handshake.

4. Network Topologies in Wireless LANs


A "network topolgy" is defined by a set of stations that communicate with one another. The topologies for Wireless LANs can be divided into four distinct categories based on the presence and absence of network infrastructure and how the stations that can communicate are chosen:
1. Ad Hoc Networks
Ad Hoc networks are formed when a number of stations are set up to form a network in the absence of a wired infrstucture. Ad Hoc networks can be divided into two broad categories :

a) Without Centralized Control: Here the stations that can communicate send packets directly to each other. Additionaly, most media access protocols support real time services. This type of network is very difficult to set up and mantain in mobile envoirnments.

b) With Centralized Control: There is a centralized control station called a Base Station (BS). All the stations can communicate with the base station. Whether the stations can also communicate with one another is an issue that is yet to be resolved. If they are allowed to communicate direcly, efficiency increses, but the problem arises if one station drifts out of range. The base station has a built in mechanism that recognizes such drifts and relays appropriate messages.

2. Cellular Topology
In this topology, each mobile station has a access to a base station which in turn is attached to the wired netwoks. Most cellular networks used in Personal Communications employ this topology.Note that at any given point of time a mobile station can communicate with only one base station. The base station advertises which mobile stations are associated with it so that the other stations can communicate with it. Here when the mobile station moves from one cell to another the connection is transferred to the corresponding base station.

3. Non-Cellular Topology
This topology is similar to the cellular topology in which mobile stations gain access to the wired network infrastucture through base stations. However, unlike in cellular topologies, mobile stations are NOT communicating with only one base station. This results in increased efficiency of the communication. Communication between mobile stations and wired infrastucture in a non-cellular topology is via any of the base stations within the reach of the mobile station.

4. Personal Area Networks
This topology is mainly used in situations where a small set of computers require access to a set of peripherals. In this topology there are few masters i.e. the computers ans few slaves i.e. the peripherals. Slaves responds to the commands from the master. Since the geographical area covered is small and there are a small number of devices to handle, this topology is very easy to manage. An example of this kind of networking is found in the standards proposed by Infrared Data Association.

5. Summary


Wireless LANs come in many types : infrared, microwave and radio. Radio is further broken down in to direct sequence and frequency hopping spread spectrum. The MAC layer protocol used by Wireless LANs as standardized in 802.11 is CSMA / CA. A number of topologies for wireless LANs have been discussed. Traditional wired LANs will become a thing of past as more and more users become mobile. There is great interest in the research community regarding the interoperability of Wireless LAN with the current Wide Area Networks such as Internet and ATM. Meanwhile there is lot of effort going on to increase the throughput, reliability and security of Wireless LANs.