optical and wireless lan s

ABSTRACT:

Optical LAN means data is carried over optical fiber cables.
Fiber optic LAN, the medium for data transmission in optical fiber (transparent glass with coding). The LAN bus is of fiber optic cable technology has been developed on the basis of theoretical work by DR.CHARLES KAO (ITI, USA) during 1966. The first commercial fiber optic cables system was introduced in 1977. The fiber optic technology has progressed rapidly after 1985 applications include control and communication cables.
The wireless LANs have been developed during 1990s.the technology of wireless LANs (hardware and software) has tremendous commercial scope and continuous developments are taking place in hardware and software and new user friendly wireless LANs are expected to be introduced in market to supplement conventional wired LANs. Conventional LANs have hard wired or fiber-optic data bus to which computer terminals, server computer, other hardware like printer, file-server etc. is connected.
There is no cable connection between computer terminals forming the network. Wireless local area networking is a method of connecting computers or a group of computers without using hard copper wires or fiber-optic cables to form a computer communication network. Wireless LANs are the recent addition to the wireless information systems. The various types of wireless LANs (WLANs) include:
 Infra-red WLANs
 Direct beam infrared LANs
 Diffused infra-red WLANs
 Frequency hopping spread spectrum WLANs
 Direct sequence spread spectrum WLANs
 High frequency radio LANs
 Narrow-band microwave WLANs

INTRODUCTION: -

DEFINITION: -
Fiber LAN means local area n/w in which computer parts are connected by means of fiber optic cable.
TYPES: -
Fiber optic LANs are two types.
 Single mode fiber LAN
 Multi mode fiber LAN.
Single mode fibers LAN is used for long distance transmissions. They have extremely small cores and they accept light only along the axis of the fiber. This requires the use of special lasers as a light source, and they need to be precisely connected to the laser, to other fibers in the system, and to the detector.
Multi mode fibers LAN have cores larger than those of single mode fiber’s LAN and they accept light from a variety of angles, multi mode fiber LAN can use more types of light sources and cheaper connectors than can single mode fiber LAN, but they cant be used over long distances.

CONSTRUCTION DETAILS: -

Fiber optic cable consists of a specially developed silica glass cable. The light signals can be transmitted through such cable very efficiently and effect of electrical noise on transmission is completely eliminated. The light signals are generally digitized.
The optical fiber is a dielectric, long, flexible cylinder of small diameter (125um), which guides the light along the direction of its longitudinal axis almost without loss, even when it is bent.

OPERATION: -
In fiber optic LAN electrical signals (E) are converted into optical signals in E/O converts. The electrical signals need level adjustment, adjustment of impedance before E/O conversion. This is done in the electrical interface circuit.

LAN FIBER TOPOLOGY (configuration): -
The most common topologies for fiber LANs are
 Switched star (fabric star)
 Point-to-point data bus
 Ring bus with arbitrated loop
Fiber LANs needed at least one switching node.

Following are the specialties in the fiber LANs: -
 Full duplex links with two fibers per link
 Data rate at 100 to 800Mbs per fiber hence 200Mbs to 1600Mbs for fiber pair link
 Fiber LAN covers distances up to 10km. Repeaters are not necessary up to specified distances.

Type \data rate-> 800 Mbps 400 Mbps 200 Mbps 100 Mbps
Single mode fiber 10 km 10 km 10 km -
Multimode 50um fiber 0.6 km 1 km 3 km 10 km
Multimode 60um fiber 0.20 km 0 km 1.5 km 1.5 km
Coaxial cable for video 0.025 km 0.5 km 0.1 km 0.1 km
Twisted pair shielded - - 0.05 km 0.1 km
Coaxial cable mini-fiber 0.01 km 0.05 km 0.025 km 0.035 km

 Multiplexed line traffic between multiple sources and destinations.
 Standard components and broad availability
 Extension easy
 Compatible with other existing LAN technologies
 Useful with variety of user computers: personal to super computers
 Greater connectivity than the coaxial cable LANs.


FIBER BUS LAN: -
The user computers are connected to the fiber pair cable by paired links.
Each fiber bus carries light signals in one direction only. Thus the light signals do no collide. Any computer sends out the information in form of digital data by acquiring access. The signal has a destination address on the header. The signal travels in the forward direction along the fiber. The destination computer accepts the signal and the information. Other computers ignore the signal.
A fiber star with switched hub technology named as fabric LAN.
The central hub (controller computer) is linked with each computer (porter terminal) by a pair of fiber link. (One for incoming and another for outgoing light signals). Sender sends the light signals to the destination computer via the hub. The signal has address of the destination in the form of the digital header.
The light signals are diverted by the central hub computer (controller) to the destination computer by appropriate switching action. The optical digital signals carrying information flows as follows.
Sender computer terminal hub destination computer terminal
Twin optical fiber links
FIBER RING LAN: -
Two fiber rings are formed links by connecting several pairs of links via repeaters or arbitrators. Repeaters repeat the file signal and passion to the next computer as it is or send it to its connected computer. Alternatively the arbitrator serves the same purpose

HIERARCHICAL NETWORK WITH FIBER OPTIC CABLES: -
Hierarchy means the order or authority.
Hierarchical LAN topology is form of a tree topology. The user computers are grouped in several hierarchical levels in order of authority (1,2,………).
Each level has a root repeater (level 1 root repeater, level 2 root repeater,………).
The root repeaters are connected in nodal tree formation. Each nodal root is connected to its mother root at higher hierarchical level. Each nodal root is further connected two or more branch nodal repeaters and user computers forming the tree trunks and tree branches.

APPLICATIONS:

 Large data transmission capacity per fiber (up to 30000 simultaneous communications per fiber).
 Higher bandwidth of fiber optic channel (e.g. 500MHz).
 The weight/meter of the fiber optical cable is low.
 Lower attenuation of signal (weakening) as there is no I square R loss like in copper cable with fiber optic cable, the repeaters repeat the digital signal are spaced at longer spacing (such as 30km) than those for copper cables (such as 5 km).
 There is no electro-magnetic interface due to radio waves, TV waves and power transmission line switching surges.
 Fiber optic cable can be laid on high voltage power line, fiber optic cable being insulating.
 Light weight and low cross section. Hence devices and installations are compact and low weight.
 Low error rate (10-9) as compared with that in coaxial cables (10-6).
 Rapidly reducing cost.
 Advances in semiconductor device technologies.
 Advances in transmission technologies for short, long, very long distance.
 Better scalability: more terminals can be added. Bus can be extended.

WIRELESS LANS.
INTRODUCTION:-

Wire less Local Area Networks (WLANs) make it possible for us to keep in touch with other people even when you are on move. Being in touch with your Network and entering data, sending faxes and communicating via a modem while in vehicles and airports are the Major applications that drive wireless fidelity (WI-FI). Using the Radio Frequency (RF) Technology, WLANs transmit and receive data over the air, minimizing the need for wired connections, providing users with mobile access to wired LAN’s in its coverage Area. A data signal at the point transmission is combined with a chipping code (also called higher data-bit rate) that divides the data according to a spreading ratio.
Now lets look at the MAC or the media access control sub-layer of the data link n/w layer. If you are familiar with how Ethernet works, you’ll know that it uses the CSMA/CD technology (collision sense multiple access with collision detection) to transmit data, which happens at the MAC layer.

DEFINITION: - WLAN is defined as a flexible data communication system, which can extend (or) Replace a wired Infrastructure. WLANs transmit and receive data over the air-using network Resources, electromagnetic waves as carriers.
The Primary constituents of a WLAN are a wireless station and an access point. A Station (client) is typically a laptop (or) notebook personal computer with a wireless network interface card (NIC). A WLAN client may also be a desktop (or) hand held device such as PDA, a barcode scanner or equipment with a kiosk.
 ACCESS POINT: -
The access point (AP), which acts as a bridge between the wireless and wired networks typically comprises a radio, a wired Network Interface such as 802d.3, and bridging software.
 RADIO CARDS: -
The radio cards generally have a web interface and can be managed through a browser, so essentially no additional software is revised to run this Network.
 ANTENNA: -
The antenna is connected to the access point (AP) is mainted at a certain height for the required coverage.
 WIRELESS BRIDGES: -
These are similar to the wired bridges and are used to connect two WLAN’s Bridges can provide to point wireless connection between two WLAN’s that may be on two different floors.

Intel’s centrino microprocessor or enables you to connect from WI-FI centered wireless LAN network’s and hotspots all over the world, without hunting for a phone jack or even plugging in a wireless card.

HOW TO SET UP A WIRELESS LAN:-
In setting up a WLAN, the first step is to understand the deployment considerations. If the customer intends to deploy it and a PDA, a then ideally the implementer should look at a low option, such as 802.11b
Elevators shafts, large mirrors, metallic dames, heavy load bearing floor and ceiling constructions are all potentially obstructing or interfering items after the site survey Access points are deployed. Each access point is then configured for various networks such as security, quality of service.

HOW WLAN’S WORK: -
Radio waves are often referred to as radio carriers because they simply perform the function of delivering energy to remote receiver. The data being transmitted is superimposed onto the radio carrier, the radio signal occupies more than a single frequency. Technical details such as negotiating an IP address happen automatically trough a built-in service called dynamic host configuration protocol (DHCP). Each access point will have a service set identifier (SSID) that users will need to know to gain access.
End users access the WLAN through wireless LAN adapters, which are implemented as PC cards in notebook computers, or fully integrated devices with in hand-held computers.

MICROCELLS AND ROAMING: -
Wireless communication is limited how far signals carry for given power output. WLANs use micro cells to extend the range of wireless connectivity. They handle low-power signals and users as they roam through a given geographic area.

RANGE/COVERAGE:-
The range in which the access device works is up to 1000 meters and sniggles can usually penetrate cement, wood and other devise substances.

WIRELESS LAN TOPOLOGIES:-
Wireless LAN’s are built using two basic topologies. These topologies are variously termed, termed, including managed and unmanaged, hosted and peer- to-peer, and Infrastructure and adhoc.
An infrastructure topology is one that extends an existing wired LAN to wireless devices by providing a base station (called an access point). In Infrastructure mode, there may be multiple access points for small building.
An adhoc topology is one in which a LAN is created solely by the wireless devices themselves, with no central controller or access point. Each
Device communicates directly with other devices in the network.

BENEFITS OF WLANS:
: -Wireless LAN’s offer the following advantages over traditional LAN’s.
 Mobility
 Quick development
 Reduced cost –of –ownership
 Scalability

ISSUES INVOLVING WLANs: -

 Wireless security: - Security is a critical Issue with wireless LAN’s. Each access point has a service set identifier (SSID) that users will need to know to gain access. The primary security architectures of WI-FI depend on wired equivalency privacy (WEP). WEP uses a security key to identify authorized users and encrypts their network traffic that travels over the airwaves. A more advanced, dynamic WEP generates a new key with each session providing an additional level of security.
The major known security risks with 802.11b are insertion attacks, interception and monitoring wireless traffic, misconfiguration, and jamming and client-to-client attacks.

 Performance Issues:- Today, most wired users connect at 100mbps with a shared media Ethernet hub, with a shared media Ethernet hub. For a good through put with heavy traffic, such as web browsing, medium-sized files being transferred frequently, the safe number would be two clients per access point.
 Roaming challenges:- As a user or station roams from access point to access point, an association must be maintained or between the NIC and an access point for network connectivity to be maintained. If the user crosses a subnet boundary, the IP address originally assigned to the station may no longer be appropriate for the new subnet. If the transition involves crossing of administrative domains, it is possible that the station may no longer be allowed to access the network in the new domain based on their credentials.

 Multi path effects:- A radio signal can take multiple paths from transmitter to a receiver, an attribute termed as multi path.

APPLICATIONS:
 Wireless education
 Wireless business centers
 Small offices
 Hospitals
 Accounting
 Warehouses


REFERENCES: : -


1. Wireless Communications – by Theodore S. Rappaport
(Pearson Education)
2. IT & Wireless Communications – by Prof. S. Rao
(Khanna Publishers)
3. Information Technology magazine, Sept. 2003 Edition
4. PC Quest magazine, Aug. 2003 Edition