COE 341: Data and Computer Communications


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COE 341: Data and Computer Communications

  1. 1. COE 341: Data and Computer Communications TERM REPORT Wireless LANs By: Armaghan Tawseef Khan (237871) Fakhruddin Mahmood (246810) Sadiq Jafar Hasan (237867) Instructor: Dr. Radwan Abdel-Aal Introduction
  2. 2. A wireless LAN or WLAN is a wireless local area network that enables client computers and the server to communicate with one another without direct cable connections. By using electromagnetic waves, WLANs transmit and receive data over the air, and thus minimize the need for wired connections. There are several different technologies by which these WLANs can be implemented depending on the requirements of the users. Furthermore, several WLAN standards have been developed in order to ensure interoperability between products from different vendors. I) How They Work In a typical WLAN configuration, the access point (a device that translates between the wired LAN and the wireless LAN) is connected to the wired network from a fixed location using standard Ethernet cable. The access point receives, buffers and transmits 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 many hundred feet. End users access the wireless LAN through the wireless-LAN adapters which are like PC cards, PCI or ISA boards with radio transceivers to communicate with the Access point, and these adapters provide an interface between the client network operating system (NOS) and the airwaves. Also, there might be Extension Points which are devices similar to the access point, but not connected to the wired LAN. These extension points serve to extend the range of the wireless network by relaying signals from client computers to the Access point. II) Wireless LAN Technology A) Infrared LANs Infrared is common place in most homes, where it is used for a variety of remote control devices. Recently attention has turned to the use of infrared technology to construct wireless LANs. However, an individual cell of an IR LAN is limited to a single room, because infrared light does not penetrate opaque walls. There are 3 types of transmission techniques: 1) Directed beam 2
  3. 3. A directed beam can be used to create point to point links. In this mode, the range depends on the emitted power and on the degree of focusing. A focused IR data link can have a range of kilometers. Such ranges are needed for constructing outdoor wireless LANs. One indoor use of point to point IR links is to set up a token ring LAN. A set of IR transceivers can be positioned so that data could circulate around them in ring configuration. 2) Omni directional It involves a single base station that is within the line of sight of all other stations on the LAN. This station is usually mounted on a ceiling. The base station acts as multiport repeater. The ceiling transmitter broadcasts an omnidirectional signal that can be received by all of the other IR transceivers in the area. These other transceivers transmit a directional beam aimed at the ceiling base unit. 3) Diffused In a diffused configuration, all of the IR transmitters are focused and aimed at a point on a diffusely reflecting ceiling. IR radiation striking the ceiling is reradiated omnidirectionally and picked up by all of the receivers in the area. Advantages • Virtually unlimited bandwidth. • Unregulated spectrum. • IR light is diffusely reflected by light-colored objects. Can use this property to provide coverage in the presence of obstacles. • Does not penetrate walls or other opaque objects. This makes it easy to secure and allows separate networks to operate without interference. Disadvantages • There is interference in the form of background radiation from sunlight and indoor lighting. • Power needs to be restricted for eye safety. B) Spread Spectrum LANs Spread spectrum is a type of modulation that spreads data transmission across the available frequency band, in excess of the minimum bandwidth required to send the information. Spreading the data across the frequency spectrum makes the signal resistant to noise, interference and eavesdropping. Spread spectrum 3
  4. 4. modulation schemes are commonly used with personal communication devices such as digital cellular phones, as well as with wireless local area networks. Two of the most popular spread spectrum techniques are: 1) Direct Sequence Spread Spectrum (DSSS) This is probably the most widely recognized form of spread spectrum. A Direct Sequence Spread Spectrum transmitter converts an incoming data (bit) stream into a symbol stream where each symbol represents a group of one or more bits. Using a phase-varying modulation technique such as quadrature phase-shift keying (QPSK), the DSSS transmitter modulates or multiplies each symbol with a noise like code called pseudorandom noise (pn) sequence. This is called a "chip" sequence. The multiplication operation in a DSSS transmitter artificially increases the used bandwidth based the length of the chip sequence. 2) Frequency Hopping Spread Spectrum (FHSS) As the name implies the FHSS hops from narrow band to narrow band within a wide band. More specifically, FHSS radios send one or more data packets and continue this hop-transmit sequence. The hopping pattern or sequence appears random but is actually a periodic sequence tracked by sender and receiver. The FHSS systems can be susceptible to noise during any one hop but typically can achieve better transmission during other hops within the wideband. Why do we use spread spectrum? Spread spectrum has many different unique properties that cannot be found in any other modulation technique. Below outlines the advantages and disadvantages for a typical spread spectrum system. Advantages: • Has the ability to eliminate or alleviate the effect of multipath interference; • Can share the same frequency band (overlay) with other users; • Provides privacy due to unknown random codes; • Involves low power spectral density since signal is spread over a large frequency band. Disadvantages: 4
  5. 5. • Bandwidth inefficient • Implementation is somewhat complex. C) Narrowband Microwave LANs These LANs operate at microwave frequencies but do not use spread spectrum. The term narrowband microwave refers to the use of a microwave radio frequency band for signal transmission, with a relatively narrow bandwidth-just wide enough to accommodate the signal. There are two types of narrowband microwave LANs: 1) Licensed Narrowband RF Within the United States, licensing is controlled by the FCC. Each geographic area has a radius of 28 km and can contain five licenses, with each license covering two frequencies. One advantage of the licensed narrowband LAN is that it guarantees interference free communication. Unlike unlicensed spectrum, such as ISM, licensed spectrum gives the license holder a legal right to an interference free data communications channel. Users of an ISM-band LAN are at risk of interference disrupting their communications, for which they may not have a legal remedy. 2) Unlicensed Narrowband RF In 1995, Radio LAN became the first vendor to introduce a narrowband wireless LAN using the unlicensed ISM spectrum. This spectrum can be used for narrowband transmission at low power (0.5 watts or less). The Radio LAN product operates at 10Mbps in the 5.8-GHz band. The product has a range of 50m in a semi open office and 100m in an open office. III) STANDARDS A) IEEE 802.11 When wireless networks initially started appearing in the market, there was no unique standard to ensure interoperability between different products. In order to combat this, the IEEE 802.11 committee was formed in 1990. The purpose of the IEEE 802.11 standard (also known as the WiFi standard) was to define specifications that allow wireless LAN devices from different vendors to operate 5
  6. 6. together. It does this by defining a single Medium Access Control protocol and three different physical layer technologies (as depicted in fig.1): • Frequency Hopping Spread Spectrum (FHSS) operating in the 2.4 GHz band at 1 and 2 Mbps • Direct Sequence Spread Spectrum (DSSS) operating in the 2.4 GHz band at 1 and 2 Mbps • Infrared operating at 1 and 2 Mbps Figure 1: 802.11 protocol Further amendments were made to the protocol later on. These include the 802.11a, 802.11b and 802.11g amendments. 1) 802.11a The 802.11a amendment to the original protocol was ratified in 1999. It uses a technology called Orthogonal Frequency Division Multiplexing (OFDM) rather than Spread Spectrum and operates in the 5 GHz band. Using this technique, data rates of up to 54 Mbps are possible. Unfortunately it is not compatible with 802.11b devices unless the device implements both standards. The advantage of 802.11a is that instead of operating in the crowded 2.4 GHz band (which is also used by cordless telephones, microwave ovens, Bluetooth devices and other devices), it operates in the 5 GHz band which is relatively free of interference. However, the higher frequency has less penetrating power thus reducing the indoor range from 50m to 30m. 2) 802.11b The 802.11b amendment was ratified in 1999 and is an extension of the original standard, improving the DSSS technology. It uses a technique called Complementary Code Keying (CCK) which increases the data rate to 11 Mbps. Because it is similar to the original, it was easy for manufacturers to adapt to 802.11b and due to the higher data rate it became the most widespread technology for wireless LANs. 6
  7. 7. 3) 802.11g In 2003 a new amendment called 802.11g was ratified. Like 802.11b, it operates in the 2.4 GHz band using DSSS but allows data rates of up to 54 Mbps. This is done by using Orthogonal Frequency Division Multiplexing (OFDM) similar to that used by 802.11a. An advantage is that 802.11g is compatible with 802.11b hardware. Vendors were quick to incorporate the new standard in their products and currently most devices in the market can operate using 802.11a, b and g. As mentioned before, the drawback of using 802.11b and 802.11g is that they operate in the crowded 2.4 GHz band and thus are susceptible to interference. 4) 802.11n In 2004, the IEEE announced plans for a new amendment to the 802.11 standard: 802.11n. This amendment will incorporate Multiple-Input Multiple-Output (MIMO) technology into 802.11 to allow a proposed data rate of 540 Mbps. The amendment is expected to appear some time after July 2007. Architecture The IEEE 802.11’s architecture is made up of cells, each one known as a Basic Service Set (BSS). A BSS is made up of clients known as stations in close vicinity of each other and sharing the same medium. The BSS can be isolated or it can be connected to a Distribution System (DS) via an Access Point (AP). A distribution system is like a backbone which connects several BSSs and it can be either a wired network, a wireless network or simply a switch. Several BSSs that are connected through a DS form an Extended Service Set (ESS). Furthermore, an ESS can be connected to another LAN using a Portal which is usually a bridge or router. This is shown in figure 2. 7
  8. 8. Figure 2 : IEEE 802.11 Architecture B) Other Standards: Some other standards also exist but they are not as popular as the IEEE 802.11. HiperLAN is a standard that was developed by a team of researchers at the European Telecommunications Standards Institute (ETSI) but did not gain popularity. HiperLAN II was also developed by ETSI and is used in some parts of Europe. There are also standards like OpenAir, HomeRF and SWAP which have been developed by private corporations. IV) Benefits and Drawbacks of WLANs: A) Benefits of WLANs: 1) Long-Term Cost Savings: The cost of installing and maintaining a wireless LAN is generally lower than it for wired LAN. This is because that a WLAN eliminates the direct costs of cabling and the labor associated with installing and repairing it. Also, because WLANs 8
  9. 9. simplify moves, adds, and changes, they reduce the indirect costs of user downtime and administrative overhead. Moreover, companies usually reorganize, and this results in movement of people, new floor plans, office partitions, etc. This reorganization often requires recabling the network, incurring both labor and material costs. In some cases, the recabling costs of organizational changes are substantial, especially with large networks. 2) Reduced Installation Time Wireless LANs eliminate the time needed with wired LANs for cable installation which might include digging in the streets and on the walls. So, WLAN makes the network available for use much sooner. Moreover, it can reach places that cannot be reached by wires. Thus, many countries lacking a network infrastructure have turned to wireless networking as a method of providing connectivity among computers without the expense and time associated with installing physical media. 3) Mobility: Wireless LAN systems can provide LAN users with access to network information anywhere in their organization. Thus, users can physically move while using an appliance, such as a handheld PC or data collector. In fact, many jobs require workers to be mobile such as healthcare workers, policemen, and emergency care specialists. So, this freedom of movement results in significant advantages due to gains in efficiency. 4) Scalability: Wireless LANs can be designed to be extremely simple or quite complex. These systems grow easily with the need by adding more access points. It is a good solution if you need to connect several buildings without installing a wired connection. 5) Easy access to the Internet in public places: Wireless infrastructure providers are enabling wireless connectivity in public areas around the world. Thus, conference centers and hotels can provide wireless access to the Internet for their visitors. So, for example, when a traveling worker reaches his destination, he can do his job easily using this WLAN. 9
  10. 10. B) Drawbacks of WLANs: 1) Interference: As mentioned before, the frequency at which 802.11b&g operate at is the 2.4 GHz radio band, which is also used by cordless phones and most microwave ovens. Thus, when WLAN is used near other devices sharing the same frequency band, Interference might occur. 2) Speed: Most commonly used wireless LANs have speeds much less than those offered by wired LANs. So, larger offices with high network traffic and demands for speed, should take this into consideration. 3) Health: Because wireless LANs use microwaves similar to those in phones, they have the same health concerns as these. However, the transmission power of a typical wireless access point is less than 100 mW while it might be up to 2 watts in mobile phones. Moreover, WLAN is typically used at much greater distance from the body. 4) Security At a wired network, the access to the network can be restricted by physical means. Usually, the geographical range of a wireless network covers greater than the area it should cover. As a result, any neighbor may be able to gain an unauthorized access to internal network resources as well as to the Internet, possibly sending spam or doing illegal actions using the owner's IP address. Conclusion Wireless LANs allow computers to communicate without the need for wires. This can be achieved through several technologies as we have seen. Standards have also been developed to allow interoperability between products from different vendors. Due to their many advantages, wireless LANs are quickly gaining popularity and are being developed further to provide improved performance. 10