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Wireless LANs

History of Wireless LANs:

Wireless services represent a progression of technology, and perhaps a new era of telecommunications, but these services have been used for over a century and remain synonymous with "radio". The humble beginning of wireless services takes us back to the 19th century at time when Guglielmo Marconi, "the father of radio" made his mark in the world of wireless technology. When Marconi started experimenting with radio waves (Hertzian Waves) in 1894 his objective was to produce and detect radio waves over long distances. In 1896, Marconi was successful and obtained a patent and established the Wireless Telegraph and Signal Company Limited, the first radio factory in the world. In 1901, signals were received across the Atlantic and in 1905 the first wireless distress signal was sent using Morse Code. Wireless technology eventually progressed as an invaluable tool used by the U.S. Military. The Military configured wireless signals to transmit data over a medium that had complex encryption, which makes unauthorized access to network traffic almost impossible. This type of technology was first introduced during World War II when the Army began sending battle plans over enemy lines and when Navy ships instructed their fleets from shore to shore.

The first wireless LAN came together in 1971 when networking technologies met radio communications at the University of Hawaii as a research project called ALOHNET. The bi-directional star topology of the system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines. Since then, wireless networking technology has been targeted for connectivity since it has several advantages. The most obvious advantage of wireless networking is mobility that allowed them to roam freely. It has a great deal of flexibility. Like all networks, wireless networks transmit data over a network medium: the medium is a form of electromagnetic radiation. The two media that have seen the widest use in local-area application are infrared and radio waves. However, infrared light has limitations because walls, partitions, and other office construction easily block it. But radio waves can penetrate most office obstructions and offer a wider coverage range. Wireless devices are constrained to operate in a certain frequency band. And each band has an associated bandwidth, which is simply the amount of frequency space in the band. The use of radio spectrum is controlled by regulatory authorities through licensing processes. For example, the Federal Communications Commission (FCC) is controlled the regulation in the U.S. Some of common U.S. frequency bands are UHF ISM, S-Band, C-Band, X-Band, and Ku-Band. There are three bands labeled ISM. Those bands are being use for industrial, scientific, and medical. The ISM bands are generally license-free and it is use low power. However, wireless-network hardware tends to be slower than wired hardware since it has to guard against the loss due to the unreliability of the wireless medium. Radio waves can suffer from a number of propagation problems that may interrupt the radio link, such as multi-path interference and shadows.

Evolution of Wireless LANs:

For wireless LANs (WLANs) technology, 802.11 refers to a family of specifications developed by the Institute of Electrical and Electronics Engineers (IEEE). 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. The IEEE accepted the specification in 1997. There are several specifications in the 802.11 family:

The original version of the standard IEEE 802.11 released in 1997 specifies two raw data rates of 1 and 2 megabits per second (Mbps) to be transmitted via infrared (IR) signals or by either Frequency hopping or Direct-sequence spread spectrum in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations.The original standard also defines Carrier Sense Multiple Access with Collision Avoidance (CSM/CA) as the medium access method. A significant percentage of the available raw channel capacity is sacrificed (via the CSMA/CA mechanisms) in order to improve the reliability of data transmissions under diverse and adverse environmental conditions. A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realize. It is really more of a "beta-specification" than a rigid specification, allowing individual product vendors the flexibility to differentiate their products.

802.11a is one of several specifications in the 802.11 family applicable to wireless local area networks (wireless LANs or WLANs). 802.11a utilizes 300 MHz of bandwidth in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. Addition, Networks using 802.11a operate at radio frequencies range between 5.725 GHz and 5.850 GHz. Therefore, there is less interference with 802.11a than with 802.11b/802.11g: because 802.11a provides more available channels, and because the frequency spectrum employed by 802.11b/802.11g (2.400 GHz to 2.4835 GHz) is shared with various household appliances and medical devices. 802.11a uses Orthogonal Frequency Division Multiplexing (OFDM), an encoding scheme that offers benefits over spread spectrum in channel availability and data rate. Forward Error Correction (FEC) was also used in the 802.11a for guarding against data loss: FEC sends a secondary copy along with the primary information. If part of the primary information is lost, the second copy then exists to help the receiving device recover the lost data. This way, even if part of the signal is lost, the information can be recovered so the data is received as intended, eliminating the need to retransmit. 802.11a wireless networks support a maximum data transfer speeds at 54 Mbps. 802.11a and 802.11b/802.11g are not compatible because they operate on different frequencies. 802.11a have a range that extends to about 75ft.

The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbps and uses the same CSMA/CA media access method defined in the original standard. 802.11b is usually used in a point-to-multipoint configuration, wherein an access point communicates via an omni-directional antenna with one or more clients that are located in a coverage area around the access point. Typical indoor range is 30 meters (100 ft) at 11 Mbps and 90 meters (300 ft) at 1 Mbps.

In June 2003, a fourth modulation standard was ratified 802.11g. This flavor works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbps (like 802.11a). Networks using 802.11g operate at radio frequencies between 2.400 GHz and 2.4835 GHz, the same band as 802.11b. But the 802.11g specification employs Orthogonal Frequency Division Multiplexing (OFDM), the modulation scheme used in 802.11a, to obtain higher data speed. 802.11g is fully backwards compatible with 802.11b and uses the same frequencies. However, the presence of an 802.11b participant significantly reduces the speed of an 802.11g network; For example, attaching even a single wireless 802.11b client to a wireless 802.11g base station will slow all 802.11g connections down to slightly faster than wireless 802.11b. 802.11g have a range that extends to about 100ft.

In January 2004, IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for WLANs. The real data throughput is estimated to reach a theoretical 540 Mbps (which may require an even higher raw data rate at the physical layer), and should be up to 10 times faster than 802.11a or 802.11g, and near 40 times faster than 802.11b. Addition, 802.11n has not been finished yet: while the details have yet to be firmly established, the goal of the project is to support connection speeds of 100Mbps for each user. It is projected that 802.11n will also offer a better operating distance than current networks. There are two competing proposals of the 802.11n standard, expected to be ratified: WWiSE (World-Wide Spectrum Efficiency), backed by companies including Broadcom, and TGn Sync backed by Intel and Philips. Previous competitors TGn Sync, WWiSE and a third group, MITMOT said that “late July 2005, they will merge their respective proposals as a draft which will be sent to the IEEE in September and a final version will be submitted in November. The standardization process is expected to be completed by the second half of 2006.” 802.11n builds upon previous 802.11 standards by adding Multiple-Input Multiple-Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM). MIMO uses multiple transmitter and receiver antennas to allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity. The 802.11n technology will support all major platforms, including consumer electronics, personal computing and handheld platforms, throughout all major enterprise, home and public hotspot environments. 802.11n is backwards compatible with 802.11a, 802.11b, and 802.11g. In other word, networks using 802.11n operate at radio frequencies at 2.4 GHz and 5 GHz. 802.11n have a range that extends to about 400-500ft.

Types of Wireless Networking:

The independent networks happen when station in Basic Service Set (BSS) communicate directly with each other and is within direct communication range. Due to their short duration, small size, and focused purpose, Independent BSSs are sometimes referred as ad hoc BSSs or ad hoc networks. The infrastructure networks are distinguished by the use of an access point. Access points are used for all communications in infrastructure networks, including communication between mobile nodes in the same service area. The communication must take two hops. First, the originating mobile station transfers the frame to the access point. Second, the access point transfers the frame to the destination station. Although the multi-hop transmission takes more transmission capacity than a directed frame from the sender to the receiver, it has two major advantages. First is that there are no restriction on the distance between mobile stations themselves. Second, it saves power because Access points can note when a station enters a power saving mode and buffer frames for it. Furthermore, a mobile station can be associated with only one access point, and the 802.11 standard places no limit on the number of stations than an access point may serve. However, Implementation consideration may limit the number of mobile stations an access point may serve.

In the extended service areas, it is created by linking BSSs together into an Extended Service Set (ESS). Stations within the same ESS may communicate with each other, even though these stations may be in different basic service areas and may even be moving between basic service areas. For station in a ESS to communicate with each other, the wireless medium must act like a single layer 2 connection.

The distribution system provides mobility by connecting access points. When a frame is given to the distribution system, it is delivered to the right access point and relayed by that access point to the intended destination. However in 802.11, the backbone Ethernet is the distribution system medium. But it is not the entire distribution system. And, most access points operate as bridges. Frames may be sent by the bridge to the wireless network; any frames sent by the bridge’s wireless port are transmitted to all associated stations. Then each associated station can transmit frames to the access point. To fully implement the distribution system, access points must inform other access points of associated stations. Many access points use an Inter-Access Point Protocol (IAPP) over the backbone medium. However, there is no standardized method for communication association information to other members of an ESS. The nature of the wireless medium, 802.11 networks have overlapping boundaries. This allows basic service area to overlap resulting in increasing the probability of successful transitions between basic service areas and offers the highest level of network coverage.