The access point (abbreviated AP or WAP (for wireless access point)), is a networking hardware device, such as a wireless router, that transmits and receives data (sometimes referred to as a transceiver) and also can serve as the bridge between the WAP device and a wired LAN (Local Area Network), which facilitates connectivity between nearby wireless clients. A WAP (also known as a hotspot) acts as a central transmitter and receiver of wireless radio signals.
For example, in the enterprise's wireless network, multiple workers can print documents from their PC that physically connected to the wireless network, with the help of a wireless printer that located at a central location in the office. The WAP device acts as a central hub for sending and receiving data via WLAN (Wireless Local Area Networks).
An access point is most commonly used in homes and office networks. A business may provide secure access points to you and co-worker to work anywhere in the office and remain connected to a central network; you are going through an access point, to access the internet without connecting to it using a cable. In most houses, the wireless access point (WAP) is a wireless router, which connected to a DSL or cable modem. The standards and frequencies are prescribed by IEEE, and all WAP devices use IEEE 802.11 standards.
Difference between Wired or Wireless networking
The wired LAN provides reliable service to users, working in a fixed environment. Once installed, the workstations and the servers of a wired LAN are fixed in their native locations. For users who are highly mobile or in a rough terrain, where there is no possibility to install and lay down the cables of a wired LAN, a good solution is to install a wireless LAN. Wireless LANs transmit and receive data over the atmosphere, using radio frequency (RF) or infrared optical technology, there by, eliminating the need for fixed wired connections. Wireless LANs provides dual advantage of connectivity and mobility. Wireless LANs have gained strong popularity in applications like health-care, retail, manufacturing, warehousing, and academic. These applications use hand-held terminals and notebook computers to transmit real-time information to centralized 'hosts' for processing.
Wireless LANs have limitations when compared with wired LANs. Wireless LANs are slower than wired LAN. Also, they have limitations with their range of operation. When a station is moved out of its range, it suffers from noise and error in the received data due to the poor signal strength.
IEEE formed a working group to develop a Medium Access Control (MAC) and Physical Layer (PHY) standard for wireless connectivity for stationary, portable, and mobile computers within a local area. This working group is IEEE 802.11. The recommendations of the 802.11 committee have become the standard for wireless networking.
Need for Wireless access point
Networking and Internet services are essential requirements for today's business computing. An increasing number of LAN users are becoming mobile. These mobile users require connectivity to a network, regardless of where they are because they want simultaneous access to the network. With wireless LANs, users can access shared information without looking for a place to plug in their systems and do not need network managers to set up networks to install cable and other equipment.
The wireless network access modes are as follows:
Infrastructure mode: To solve particular problems of providing services from a fixed network to mobile users, an infrastructure wireless LAN is used. In an infrastructure LAN, there is a central server, operating on a fixed wired network. Portable clients can access the services of the fixed network through access points called Portable Access Units. Figure illustrates the structure of an infrastructure LAN. Typical range of a PAU is between 50 and 100 meters.
Ad-hoc Mode: The most basic configuration of a wireless LAN consists of PCs equipped with wireless adapter cards as shown in Figure. They can set up an independent network whenever they are within range of one another. This is called a peer-to-peer or ad hoc network. Such a configuration is required on demand. Such networks require no administration or pre-configuration. In this case, each client would only have access to the resources of the other clients, and not to a central server. Access points have a finite range, in the order of 500 feet for indoor and 1000 feet for outdoors. In a very large facility such as a university or on a college campus, it will probably be necessary to install more than one access point. Access point positioning is accomplished by means of a site survey. The goal is to plan the coverage area with overlapping coverage cells so that clients might range throughout the area without ever losing network contact. The ability of clients to move seamlessly among a cluster of access points is called roaming. Access points hand the client off from one to another in a way that is invisible to the client, ensuring unbroken connectivity. Conferences arranged in a building, or airports, meetings arranged on corporate offices, may need to setup wireless LAN.
The below are the advantages & advantages of wireless access points respectively.
Wireless LANs offer the following advantages over traditional wired networks. Mobility Users on a wireless LAN systems can access to real-time information from anywhere within their organization. This mobility supports productivity and service opportunities, which are not possible with wired networks.
Fast Installation and Simplicity Installing a wireless LAN system can be fast and easy and can eliminate the need to pull cables through walls, floor, and ceilings.
Installation Flexibility Wireless network is suitable for any kind of geographical conditions. Installation requires to properly setup the transmitter and the receiver antenna (RF) or infrared system. This is much easier than cable installation of a wired LAN. If a company decided to move to a new location, the wireless system is much easier to move.
Reduced Cost The initial investment required for wireless LAN hardware is higher than the cost of wired LAN hardware. However, the overall installation expenses and life cycle costs are significantly lower. Long-term cost benefits are greatest in dynamic environments, requiring frequent moves and changes.
Scalability Wireless LAN systems can be configured in a variety of topologies to cater to the need for specific applications and installations. Configurations can be easily changed. They scale well. New nodes can be added to the existing wireless LAN without much degradation of performance.
More users access: A wireless router can able to support over 50 or even hundreds of users access, and what’s more, it has a stronger ability to send and receive signals, especially in a large area needing wireless coverage. It is best for mobility because there are no wires.
The broader range of transmission: Generally, the signal range of the wireless router is just dozens of meters, and if beyond this range, the signal lost. At this point, an access point can extend signal coverage proportionally, which enables users to communicate freely in the broader wireless network.
Flexible networking: In commercial locations, many wireless devices used with different networking patterns that should be adopted based on the environment and requirements. Multi-AP is interconnection to extend the coverage of wireless networks, so clients to roam seamlessly in the network.
High cost: The setup cost of wireless AP is a little bit expensive because, for enterprises wireless network, more wireless APs needed. Wireless networks high cost but are easy to implant. However the actual challenge comes when we try to secure its signals. Insecure wireless network can be attacked by hackers.
Poor stability: Due to the air as the transmission medium, the network stability is poor in Wireless networks, and it is slower than wired network especially if there are more devices in WLAN, while in the cable network is faster and more stable than a wireless network. The wireless signals blocked by certain obstacles as walls, gates and human beings. The signal strength depends upon the location;
Less Secure: It is less secure as compare to cable network because you are using radio waves for transmission and someone on the network could sniff traffic.
Overlapping channels problems: If you are using more that one AP and you don’t configure them properly, then you may have facing Overlapping channels problems.
Components of a Wireless Lan
Apart from the components needed by the conventional wired LAN, a wireless LAN needs additional components. They are the transmitters and receivers at radio frequency (RF) or infrared (IR). The RF transmitter and receivers need antennas to perform two-way communication. This area requires a wide knowledge about antenna and propagation. Usually a trial installation is carried out before actual implementation. Hubs, bridges, network operating system, servers, and other components are functioning exactly as they were, on a wired LAN.
Mobile clients are portable computing devices that act as clients. The following are some of the mobile systems.
1. Laptop computers: Laptop PCs with two-way communication facility (Transceiver)
2. Palmtops or Personal Digital Assistants (PDA) with communication capability
3. Portable FAX
4. Cellular phones
For network management and efficient communication, a wireless LAN needs additional equipments.
Communication units: These units perform communications within the network and also with other networks.
Data collecting units: These units collect data from other systems.
Security Units: These units take care of the network security.
Transceivers: A transceiver is a half-duplex device. It performs transmission and reception of data within a wireless LAN. It can be able to transmit in one direction at a time.
Portable bridges: Portable Bridge can support internet working functions. Two wireless LANs can communicate with each other using a bridge. It can be a transceiver or a satellite port or other communication unit that provides a bridge service.
Working of Wireless Lans
Wireless LANs use electromagnetic waves (radio or infrared technology) to communicate information from one point to another without relying on any physical connection. Radio waves are often referred 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 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. The access point receives, buffers, and retransmits 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 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 add-on cards in notebook 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.
Wireless LANs may use either radio wave technology (RF) or infrared technology (optical) for transmission of data. Each technology comes with its own set of advantages and limitations. The properties of these two technologies are discussed here.
Radio Wave Technologies
Radio waves propagate freely on air. They are used for many applications. Radio broadcast, television, telephony, and defense applications use radio waves. The band used for a specific application is highly significant and cannot be used for other applications. There are national level and international agreements in the selection of a specific band for an application. Radio wave transmission and reception requires highly sophisticated circuitry. Both transmitter and receiver must work within a short band.
The following are the problems associated with radio frequency transmission.
Path Loss SNR or signal to noise ratio is defined as the ratio of power of the received signal to power of the noise in the received signal. The performance of the communication system is good if this factor is improved. But the design will be more complex if this parameter is to be improved. Either increasing the transmitting power or reducing the distance between the transmitter and receiver can improve SNR.
Adjacent channel interference: Interference is another phenomenon that affects the radio frequency transmission, when the same frequency band is allocated to two adjacent transceivers, resulting in interference. Hence interference occurs when one useful signal is mixed up with another signal. This problem can be avoided by dividing the available band into sub-bands and allotting different bands to adjacent transceivers.
Multipath: Another problem with radio wave transmission is due to Multipath. A receiver at any point can get two types of signal from the transmitter. One is the direct signal and the other is the reflected signal. Every object reflects the radio wave. Hence, the receiver can get multiple reflected signals through various paths. The signal strength is additive at certain points and out of phase at some other points. Hence the receiver can get peak power at some points and minimum power at some other points. This phenomenon is known as frequency selective fading. By employing two antennas at quarter wavelength separation, this problem can be solved.
A narrowband radio system transmits and receives user information on a specific radio frequency. Narrow-band transmission uses single frequency modulation, set up mostly in the 5.8 GHz band. The big advantage to narrowband systems is high throughput because they do not have the overhead involved with broadband systems. Radio LAN is an example of systems with narrowband technology. Undesirable cross talk between communications channels is avoided by carefully coordinating different users on different channel frequencies. In a radio system, privacy and non-interference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the one to which it is tuned. From the customer's view, one drawback with narrowband technology is that the user must-obtain a license for the usage of the specific frequency.
Direct Sequence Spread Spectrum Technology (DSSS)
Most wireless LAN systems use this technology. More bandwidth is consumed compared to narrowband transmission. With direct sequence spread spectrum, the transmission signal is spread over an allowed band (for example 25 MHz). A random binary string called a spreading code is used to modulate the transmitted signal. The data bits are mapped to a pattern of "chips" at the source and mapped back into a bit-stream at the destination. The number of chips that represent a bit is called spreading ratio. The higher the spreading ratio, the more the signal is resistant to interference, at the expense of increased bandwidth. The Federal Communication Committee for International Radio Transmission (FCC) recommends that the spreading ratio must be more than ten. Most products have a spreading ratio of less than 20 and the new IEEE 802.11 standard requires a spreading ratio of eleven. The transmitter and the receiver must be synchronized with the same spreading code. To an unintended receiver, DSSS appears as low-power wide band noise and is rejected (ignored) by most narrowband receivers.
Frequency-Hopping Spread Spectrum Technology (FHSS)
This technique splits the band into many small sub-channels of each 1 MHz band. The signal then hops from sub-channel to sub-channel transmitting short bursts of data on each sub-channel for a set period of time, called dwell time. The hopping sequence must be synchronized at the sender and the receiver otherwise the whole information will be lost. Frequency hopping is less susceptible to interference because the frequency is constantly shifting. This makes frequency-hopping systems extremely difficult to intercept. This feature gives frequency-hopping systems a high degree of security.
To jam a frequency hopping system, the whole band must be jammed. These features are very attractive to agencies involved with law enforcement or the military. To an unintended receiver, FHSS appears to be a short-duration impulse noise.
A technology, little used in commercial wireless LANs, is infrared (IR). Infrared has extremely high frequency, higher than radio wave range. They are in the frequency range of 1014 Hz and higher. Infrared technology is already used in optical fibers, TV remote control, CD players, etc. IR systems are simple in design and therefore inexpensive. They use the same signal frequencies used on fiber optic links. IR systems detect only the amplitude of the signal and so interference is greatly reduced.
Characteristics of Infrared Transmission
Infrared systems need special infrared emitters and infrared detectors. Infrared transmission is performed in two ways. The first method uses the direct modulation and the second uses carrier modulation. The direct modulation scheme is described below as wireless LANs use only direct modu1ation scheme.
Direct modulation, often referred as on-off keying, is widely used in optical fiber systems. A light source, usually an LED is directly switched on by a binary 1 and switched off by a binary 0. The direct modulation system is similar to the one shown in Figure. The source bit stream is encoded, using a standard encoding technique prior to modulation. The encoded data is then modulated, using a modulator. Pulse position modu1ation or similar modulation technique is employed to reduce the power requirements. Modu1ated signal is then fed to the LED device. At the receiving side, an optical band-pass filter is used to select the required band that contains the transmitted signal component. Photo-detector produces electrical signal, which is in the modulated form. A demodulator extracts the encoded data from this and a decoder recovers the data in the original form. Direct modulation is commonly used within a room or a small area where the transmitter and the receiver are in the line of sight.
Infrared links can be used in two different modes. They are direct (point-to-point) mode and diffuse (Omni-directional) modes. In a point-to-point mode, the light emitter is directly pointed to the detector. Hence, low power emitters or less-sensitive photo detectors can be used. This mode of operation is adequate for providing a direct wireless link between two portable devices. Directed systems give a good range of a couple of kilometers and can be used outdoors. It also offers the highest bandwidth and throughput. High performance directed IR is impractical for mobile users and is therefore used only to implement fixed sub-networks.
In the diffuse mode, the infrared light from the source is optically diffused to scatter the light to a wide area. Thus, this mode is suitable for broadcast operation. Omni-directional IR systems provide very limited range and typically reducing the coverage range to 30 - 60 feet, and are occasionally used in specific wireless LAN applications. All the detectors within the room can receive the signal from one transmitter; each with varying phase. The phase variation is due to the variations of path length between the transmitter and receiver. Multiple reflections of light also cause these phase variation. This phenomenon is known as multipath dispersion. This problem will not affect the communication process much in a typical room environment. Signal rate up to 1Mbps can be satisfactorily achievable. Beyond this rate, Inter Symbol Interference causes the major problem.
Protocol for Wireless Lan
The CSMA protocol is very difficult to implement for wireless LAN. Hence special protocols are needed to avoid collision. MACA and MACAW are the two widely used protocols.
During 1990, Kam developed the MACA (Multiple Access with Collision Avoidance) protocol for wireless transmission. The protocol is very simple to implement and works in the following manner. Station X, willing to transmit data to the nearby station Y, sends a short frame called RTS (Request to Send) first. On hearing this short frame, all stations other than the receiving station, avoid transmission, thereby allowing the communication to take place without interference. The receiving station sends a CTS (Clear to Send) frame to the calling station. After receiving the CTS frame, station X begins transmission. When simultaneous transmission of RTS by two stations Wand X to station Y occurs, both frames collide with each other and are lost. When there is no CTS from station Y, both stations wait for a random amount of time (binary exponential back off) and start the whole process again.
Bhargavan et al (1994) investigated the behavior of MACA protocol and refined it with modifications. The first modification was the acknowledgment frame for the successful receipt of each frame. This modification adds carrier sense to stations. The second modification was to apply the binary exponential back off algorithm to source-destination pair. This improves the fairness of the protocol. They have also added to stations, the ability to exchange information, regarding congestion.
Digital Cellular Radio
Yet another type of wireless networking can be done with digital cellular mobile radio. The protocol for this type of LAN is similar to the telephone systems. Connections are maintained for long durations as in telephone network against small durations in computer networks. There are three different approaches used for channel allocation for cellular wireless LANs. The following sections discuss about these approaches.
Global Systems for Mobile Communications (GSM)
The Global System for Mobile Communication was developed by Europe and subsequently followed by many other countries. The frequency band assigned for .GSM is 900 MHz. Every cell in the GSM system has up to a maximum of 124 full-duplex channels per cell. Each channel uses a pair of frequencies, one for the transmission from base station to the mobile station (downlink) and the other from the mobile station to the base station (uplink). Data from eight stations are combined to form a TOM frame. This scheme is similar to slotted ALOHA with TOM and FDM. Hence, no channel access problem arises.
GSM uses a number of control channels for different purposes. A broadcast control channel has a continuous transmission of signal. It is used for the mobile stations to identify their base station. For the purpose of updating, registration (Sign-up) and call setup, a dedicated control channel is used. All mobile stations use another control channel called common control channel. This channel has three sub- channels, namely:
Paging Channel Used by the base station to announce the incoming call
Access Channel Used by the mobile station to request a slot for transmission
Grant Channel Base station announces the grant of the requested slot, using this channel
Cellular Digital Packet Data (CDPD)
The GSM system is highly error prone and expensive to implement. Hence, a layered protocol similar to the 051 was developed. This system uses physical layer, data link layer, etc.
Three types of systems are used. They are mobile hosts (Portable systems), Mobile Database systems (Base stations), and Base Interface Systems (BIS),for providing connectivity to fixed LANs and routers.
With each CDPD cell, there are two channels, one for uplink and the other for downlink. Each cell has a unique downlink channel, through which the incoming data can be collected. All data sent to a cell are broadcast to it. Each mobile station receives the data addressed to it and rejects all other data. All mobile stations willing to transmit data contend for the channel. The mobile host checks the uplink channel condition before doing any thing. It senses the channel condition whether "busy" or "idle". If the channel is idle, it starts transmission immediately. If a busy channel is heard, it skips a random number of slots and tries to get a free slot. If it again senses "busy", it waits for a random time and starts the whole process again. The algorithm is known as Digital Sense Multiple Access.
The main problem with CDPD is that there is no data channel. The network itself is intended for voice and not for data. However, when the voice channel is idle, they are grabbed for data transmission. As and when a voice channel is allocated, the base station asks the mobile station to close the data transmission immediately. Therefore, data transmission is only a secondary phenomenon.
Code Division Multiple Access (CDMA)
It is a totally new channel allocation technique, compared with the earlier methods. All stations are permitted to transmit freely. The bit pattern used by each station is independent of others. Hence, the receiving station can receive the desired information, while rejecting all other signals as noise. The bit time is divided into m number of short time intervals called 'chips'. Each station uses a unique m-bit pattern called a chip sequence. To transmit a 1/1" bit, the chip sequence is sent. For the transmission of an I/O bit the complement of the chip sequence is sent. No other pattern is permitted by the station. For example, if station X is assigned an 8-bit chip sequence 11100010, it sends the same pattern for transmitting a 1-bit, and sends the pattern 00011101 to transmit a 0-bit. This process of sending multiple bits for a single bit increases the bandwidth by a factor m.
Uses of Wireless Lans
Wireless LANs frequently act as a substitute rather than replacement for a wired LAN network. They often provide the final few meters of connectivity between a wired network and the mobile user. The following list describes some of the many applications made possible through the power and flexibility of wireless LANs:
• Doctors and nurses in hospitals can be more productive because wireless hand-held terminals or notebook computers with wireless LAN capability can deliver patient information instantly.
• Consulting or accounting audit teams or small workgroups can increase the productivity with quick wireless network setup.
• Students or research scholars, attending a class inside an institute campus can instantly access the Internet to consult the catalog of the net digital library.
• Network managers in dynamic environments minimize the overhead caused by moves, extensions to networks, and other changes with wireless LANs.
• Training sites at corporations and students at universities use wireless connectivity to ease access to information, information exchanges, and learning.
• Network managers installing networked computers in older buildings find that wireless LANs are a cost-effective network infrastructure solution.
• Travelers and tourists can book their ticket through the net during their travel.
• Warehouse workers use wireless LANs to exchange information with central databases.
• Network managers implement wireless LANs to provide backup for mission-critical applications, running on wired networks.
• Senior executives in meetings make quicker decisions because they have real-time information at their fingertips.
There are several issues in selecting a new wireless LAN system to install. The following are some of the potential issues to think about while evaluating different wireless LANs. They are:
• Range of operation
• Maximum required throughput
• Integrity and reliability
• Compatibility with the existing network
• Inter-operability of wireless devices
• Interference and coexistence licensing issues