Wireless Local Area Networks WLAN Computer Science Essay

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Farah Liana Bt. Zaimudin Abstract- In this paper, the latest advance in 802.11 has been studied. It was observed that IEEE 802 Wireless Standards comprises a family of networking standards that cover the physical layer (PHY) specifications of technologies from Ethernet to wireless. IEEE 802.11 group of standards specify the technologies for wireless LANs. There are new 802.11 standards that increase WLAN capacity, but they do not even come close to arrive at 1000 Mbps.

Index Terms-IEEE 802.11, MIMO, WLAN, 802.11n

INTRODUCTION

The two standards that are available for wireless networking are High Performance Radio Local Area Network (HIPERLAN), a European standard and IEEE 802.11, an American standard. Compared to wired network, wireless network is easier to install, cost-effective way of network and wireless network gives more other benefits [1].

Wireless networks are becoming well known, proficient and realistic. It has the similar function as conventional wired networks, but they can be as simple as connecting two computers without a cable, and as complicated as having a huge number of computers connected to each other and to a central server in a corporate area. A typical wireless network (see Figure 1) is set up using the following minimal components [3].

Figure 1: Wireless Networks Architecture

Source:http://wireless.ictp.it/school_2006/wiki/pmwiki~28.html

How WIRELESS WORK

Wireless networks works with radio frequency (RF) technology which is a frequency between the electromagnetic spectrum affiliated with radio wave propagation. Electromagnetic field created when an RF current is supplied to antenna will be able to spread through space.

The essential part of a wireless network is a component known as access point (AP). Access point is a device that is usually plugged into an Ethernet hub, switch and server. The major function of an access point is to broadcast a wireless signal and serve as link to make a connection between resources such as printer and personal computer to connect to the physical network and make the internet connection. Wireless network adapters are required to connect to an access point and get associated with wireless network.

The type of wireless networking standard it uses and the technological improvement on the access point will implies the speed of wireless network. Access points comes with speeds of 11mbps (802.11b), 22mbps (802.11a) and 54mbps (802.11g) with 802.11b and 802.11g being the most well-known standards [3].Less data will be able to transmit and receive if there are more device connected to the access point. The closer the device to an access point, the stronger the signal and faster connection speed [5].

Wireless Local Area Networks (WLAN)

AND TECHNOLOGY REVIEW

WLANs comprise under the 802.xx category of the IEEE standards [3]. 802.11 specify 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 [4].

The spreading techniques originally used in wireless LAN products can be divided into two families: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The FH approach oppose interference by jumping rapidly from one frequency to another frequency in a randomness way. Both receiving system and the sender has the same random sequence, and jumps at the same time. The approach that resist interference by combining in a series of random bits with the real data is called the direct sequence spread spectrum approach. The receiver, using the same random sequence, strips out the excess bits. Direct Sequence modem is more complex than the Frequency Hopping but the Direct Sequence has a simpler (Media Access Control) MAC protocol. Both techniques are different in term of bandwidth sharing also in term of resistance to interferences (it depend on the strength and pattern of the interferer) [12].

DEVELOPMENT OF STANDARD WIRELESS LAN TECHNOLOGY

Figure 2: Wireless Networking Standards White Paper Series that comparing various wireless standards that currently exist in the industry.

Source: http://zone.ni.com/devzone/cda/tut/p/id/7129

The wireless standard varies in terms of some characteristics such as compatibility, transmission speed, range, security, cost, modulation scheme, frequency and public access. Below are some wireless standard that have been using in wireless network.

A.802.11

802.11 is the first WLAN standard created by Institute of Electrical and Electronics Engineers (IEEE) in 1997.It only supported 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) which is too slow for most applications. For this reason, ordinary 802.11 wireless products are no longer produced and been extended into 802.11b [6].

B. 802.11b

IEEE expanded on the original 802.11, establish 802.11b specification (also known as 802.11 High rate) based on modified Direct Sequence (DS) physical layer using Complementary Code Keying (CCK) [6].It supports bandwidth up to 11 Mbps and requires fewer access point than 802.11a for coverage of large area and organizations [7][12].

802.11b uses the same unregulated radio signaling frequency (2.4 GHz) as the original 802.11 standard and is backward compatible with the original 802.11 DS systems. The higher bit rate of 802.11b which is 11 Mbps requires a higher SNR which will reduce the range significantly. Increasing the transmit power of the access point will significantly increase the signal strength and automatically improves the SNR. This will allow the client device to receive signals at higher data rates.

C.802.11a

802.11a is a second extension to the original 802.11.It is ratified at same time with 802.11b but 802.11b became most widely used wireless networking standard.802.11a is better than 802.11b in terms of speed.

802.11a supports bandwidth up to 54 Mbps and signals in a regulated frequency (RF) spectrum around 5 GHz and supports 12 non-overlapping channels based on Orthogonal Frequency Division Multiple (OFDM) to encode more data within each channel; therefore, it can be implemented in large coverage area. The greater number of channel reduces possible conflict among access point and avoid interference [3].Some 802.11a products also offer a proprietary "turbo mode" to reach 108 Mbps.

To cover the same area, 802.11a wireless LAN need more access points than the 802.11b/g network .This is because the RF propagation characteristics at 5 GHz operation show off a higher loss than 2.4 GHz. Thus, 802.11a is suit for environments where high throughput is needed for a substantially small coverage area.

 This higher frequency compared to 802.11b shortens the range of 802.11a networks and it has more difficulty penetrating walls and other obstructions, limiting their reach inside buildings. Since 802.11a and 802.11b utilize different frequencies, the two technologies are incompatible with each other [6].

D. 802.11g

802.11g attempts to combine the best of both 802.11a and 802.11b. 802.11g supports Orthogonal Frequency Division Multiple (OFDM) to reach bandwidth up to 54 Mbps [6].Operate in 2.4GHz band now occupied by 802.11b for greater range with 14 available channels and allow only three non-overlapping channels of communication which means  you can put three Wireless Application Protocol (WAP)s in the same area without interference from one another. With more non-overlapping channels frequencies and floor space can be managed much more effectively, particularly by adding WAPs anywhere without interference. This lets the same AP supply higher bandwidth to new 802.11g stations while simultaneously supporting DSSS CCK for backwards compatibility with older 802.11b stations.  Therefore, they are mostly used in hot spots and residential environments, where fewer access points can meet user's needs.

As 802.11g wireless standard offers the highest data transfer rate or bit rate, the standard is more proven in industrial environment. 802.11g is more complicated to implement when using this standard in a corporate environment because of pre-planning of network infrastructure, but it offers backwards compatible with 802.11b, meaning that 802.11g access points or router will work with 802.11b wireless network adapters and vice versa [3]. 802.11b and 802.11g equipment can share a Wi-Fi LAN. If a proper set up was made, the network will function correctly at reasonable speed. Mixing 802.11b and 802.11g gear can reduce cost on component upgrades in the short term. It may support legacy 802.11b devices and improved security enhancements over 802.11 [7].

E. 802.11n

The newest IEEE standard in the Wi-Fi category is 802.11n. It was designed based on amendment of 802.11g in the amount of bandwidth supported by utilizing multiple wireless signals and antennas called multiple-input multiple-output (MIMO) technology to transmit and receive data and doubling the channel bandwidth from 20 MHz to 40 MHz helps increase the physical (PHY) rate up to 600 Mbps thus multiplying total performance. MIMO can transmit three streams of data and receive two thus exhibit more data transmission during the same time. This will increase the range, or the distance over which data can be transmitted. Each 802.11n channel will use a larger frequency range than these earlier standards, also increasing data throughput.

The 2.4 GHz band can support both 802.11b and 802.11g and the 5 GHz band is used by 802.11a devices. The 802.11n based devices are expected to work in both bands and hence backward compatibility with the respective legacy devices. Backward compatible 2.4 GHz version with 802.11b/g systems means 802.11n access points can interact with 802.11b/g devices and 802.11b/g access points can communicate with 802.11n devices. There is limitation in the data rate because of the slower protocol when mixed-use environments such as 802.11 b/g devices do not perform at 802.11n speed even when used with 802.11n access point but still only get 802.11b/g level performance.802.11n also offers somewhat better range over earlier Wi-Fi standards due to its increased signal intensity and up to 4-5 times faster than 802.11g.802.11n has the best signal range. [6] [9].

Table 1: Comparison table of some wireless standard 802.11

802.11n: THE LATEST ADVANCE IN 802.11

As mentioned before, one technology established in 802.11n wireless systems is using multiple antennas for both receiver and transmitter refer as MIMO technology or so called smart antenna systems.

MIMO uses spectrum effectively without forgo reliability. Every receiver listens for signals from every transmitter, enabling path diversity where multi-path reflections may be recombined to enhance the desired signals.

Figure 3: How a smart antenna (MIMO) system work

Basically, multiple antennas send different flows of individually encoded signals (spatial streams) over the air in parallel to drive more data through a given appropriate channel. At the end of receiver, each antenna sees a different mix of the signal streams and the device "demultiplex" them to be use.

The second technology include in 802.11n is channel bonding. Since 802.11n standards can support both 2.4 GHz and 5GHz band, it can use two different non-overlapping channels at the same period to transmit signals thus increase the amount of data transmitted [13].

Payload optimization or so called packet aggregation is the third technology in 802.11n which means more data or signals can be conclude into each packet transmission [13].

Conclusion

Flexibility and mobility have made wireless local area networks (WLANs) a rapidly emerging field of activity in computer networking. In the meantime, the IEEE 802.11 standard has gained global popularity in wireless networking markets and has also been expected to continue being the adopted standard for supporting WLAN's applications.

Each 802.11 wireless networking standard is well suited for different specific environments and applications. 802.11b is the most widely uses by far but it does not mean that it will always be the most appropriate. The 802.11a standard was created in the same time as 802.11b but gives slow feedback.802.11n is on the horizon, finally achieving the speeds of wired networks. Every 802.11 wireless networking standard give impact in their performance and each technology is advantageous.

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