The advent of networking more particularly the Local Area Network has its origin to the growing necessity of the user to have access and share much more data, application and other services than they could otherwise store by themselves in their own computer. Each LAN has to follow a particular topology which depicts physical arrangement of the nodes and logical method by which the data is transferred. Initially, the LAN implied to wired topology i.e. interconnection of the nodes in a particular location by means of cable network. Within the LAN there also exist a group of mobile users who desires to be connected irrespective of their positions. Moreover with the advent of small mobile/home office, the limitation of area becomes irritating to the users. The solution becomes Wireless LAN. It enables the computers of local area network to access data or other information without using Cables. Without cables it enables the transmission of data through the air, through walls, ceilings and cement structures. The notebook PC or laptops can easily be carried from place to place even being connected.
The Wireless LAN is a communication system that transmits and receives data using modulated electro magnetic waves as a substitute for wired cables. The Wireless LAN consists of two primary components. An access point or base station physically connected to the LAN and a built in or added wireless card attached to the users PC. The medium that connects between the access point and the PC may be Infrared, Radio Frequency (RF) or Microwave. The infrared systems use the same signal frequencies used on fiber optic links. Only the amplitude of the signals is detected by the Infra Red systems, hence there is substantial reduction in interferences. The transmission speed in these systems is more because they are not limited by the bandwidth. Moreover, since it operates in the light spectrum it does not require necessary license. The Infra Red LAN can be implemented in two ways directional and omni-directional.
The directional transmission gives a range up to 2 kilometers and also offers highest bandwidth and throughput. The omni-directional transmission bounces the signals to all the directions but up to a lower range of 30-60 feet. The IR main disadvantages of the IR system lies in the fact that the transmission spectrum is shared with the sun and others such as fluorescent lights and therefore interferences from them adversely affects the LAN. Moreover an unobstructed Line of Sight is essential the IR systems, since the IR signals cannot penetrate opaque objects. Walls dividers, curtains even fogs may obstruct the signals. The Microwave systems using narrow band transmission with single frequency modulation are operating mostly in 5.8 GHz band. Since the overhead is not involved with spread spectrum systems higher throughput is achieved in Microwave systems.
The Microwave systems operate at less than 500 mill watts of power. The Radio Frequency systems operate using spread spectrum in two ways- Direct Sequence Spread Spectrum and Frequency Hopping Spread Spectrum. The DSSS and FHSS systems have lower data rates than IR and MW due to increasing overheads involved. The transmission signal of Direct Sequence Spread Spectrum is permitted to spread over only to an allowed band. The transmitted signal is modulated by a random binary string known as spreading code. Pattern of chips are mapped by the data bits and the number of chips that represent a bit, constitute the spreading ratio.
The interference is resisted with more spreading ratio conversely, the bandwidth available to the user increases with the lower bandwidth. It is required to synchronize both transmitter and receiver with the same spreading code. Since it involves spread of the signals over a wider band the recovery is faster in DSSS systems. The Frequency Hopping Spread Spectrum (FHSS) involves splitting up of the band into several sub-channels. The signal by hopping from one sub-channel to the other transmits short bursts for a pre-determined period known as dwell time. This necessitates synchronization of the signals both at transmitter and receiver. Since the frequency is constantly shifted the interference is less in frequency hopping. For co-location of several FHSS LANs orthogonal hopping sequence is used. [Prem, Wireless Local Area Network]
The wide acceptability of Wireless LAN due to its benefits of flexibility, scalability and mobility necessitated standardization of the devices ensuring compatibility and reliability among all the manufacturers and users. An effort was made by the Institute of Electrical and Electronics Engineers (IEEE) in 1997 to standardize the Wireless LAN under the term IEEE 802.11. The WLAN as per this standard is implemented through the method of Medium Access Layer is implemet The architecture in for IEEE 802.11 standard refers each of the computers as a station. First of all the stations are categorized as fixed and mobile. With two stations coming together for communication the Basic Service Set is formed. The IEEE 802.11 standard uses BSS as the standard building block with at least two computers. The BSS is said to be Independent Basic Service Set (IBSS) if it is stand alone and not connected to any base.
It is established as per the peer-to-peer architecture. The Distribution System enables two or more BSSs to be interconnected with the help of the Access Points, which increases the network coverage. The access points bridges the gap between the BSS and DS and data moves through them. The Extended Services Set (ESS) is created with the creation of large and complex networks using BSS and DS. The entire network under the ESS operates as an independent basic service set to the Logical Link Control Layer. The benefits of the IEEE 802.11 extended with the use of a portal to logically integrate between the wired LAN and IEEE 802.11 Wireless LAN. The portal is also used as a gateway to access to the Distribution System. The Portal is used to bridge the gap between Wired LANs with Wireless LANs. The standard IEEE 802.11 has specified the services that the Distribution System should support which in consonance with the BSS and DS may be Station Services and Distribution System Services.
Five services are prescribed for the Distribution System to perform. They may be grouped as Association, Re-association, Disassociation, Distribution and Integration. Among them the services of Association, Re-association and Disassociation are associated with mobile stations. There is no transition when the BSS is fixed or even moves within its own BSS. It is said to be BSS-transition when the station moves between BSSs but within the ESS and it is ESS-transition when the station moves between BSSs of different ESSs. For using the LAN the station affiliates with the BSS infrastructure associating itself with the Access Point. The Association supports no transition mobility. The BSS transition is supported by Re-association, which allows the stations to switch affiliation from one Access Point to another. Disassociation terminates the affiliation between the station and Access Point.
While in ESS transition the station has to reinitiate connections. Distribution Service enables the receiver get the data from the sender. The data is first sent to the local Access Point and the Distributed system distributes the same to the Access Point with which the recipient is associated. When the output Access Point is a portal it necessitates Integration. Similarly the Services specified under Station Services are Authentication, De-authentication, Privacy and MaC Service Data Unit (MSDU) Delivery. Authentication of the identity of the stations is essential before their association in order to control their access into the network for security reasons. Two types of authentication services Open System Authentication and Shared key Authentication is provided by the standard. With the use Wired Equivalent Privacy (WEP) privacy algorithm the shared secret is implemented.
De-authentication involves termination of a station's authentication and automatic disassociation from the network. Moreover, the Wireless LANs use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) instead of CSMA/CD being used by the Wired LANs as its Media Access Control Protocol. Under CSMA/CA the station before sending messages sends a short message known as Ready to Send Messages and the actual message is sent after hearing from the destination station known as Clear to Send Messages. The message is sent in packets each of which is acknowledged the failure entails retransmission. The standard IEEE 802.11 is followed by the standards IEEE 802.11a and IEEE 802.11b. The reformulated standards defined new Physical Layers for data transfer rates from 5 Mbps, 11 Mbps to 54 Mbps operating in Industrial, Scientific and Medical frequency bands. [Wireless local area networking an introduction]
The Wireless LAN suffers from some disadvantages which some times are worth analyzing before its actual installation. Most Wireless Networks connect into a wired network eventually for accessing internet. The speed is a major constraint in the Wireless network. The IEEE 802.11a has a speed of only 54 Mbits/s in comparison to Optical Fiber Ethernet speed of 1 Gbit/s. Except for areas having difficulties in cabling wireless LANs cannot be a substitute for the Wired Network. Besides security is…