Multi-Hop Wireless Networks - Coursework Example

Methods of tackling and mitigating RF multipath that impact to WLAN throughput. WLAN is the current technology of communication that is being adapted almost by all sectors such as schools, business, large organization among others. The frequencies that transmit data from one source to destination are exposed to Radio Frequency (RF) interface by nature from different sources. This frequency band is unlicensed hence the growing number of users which makes it overcrowded (Akella, 2005). The range of devices such as cordless phones, Bluetooth headsets, ‘WirelessUSB’ peripherals which operate in 2.4GHz band are on the 802.11b/g networks (http://tinyurl.com). Multipath propagation is where the signals from the source to destination take different channels or path(Geier, 2002). The signals travel through a medium with many obstacles such as the wall, seats and other items which may cause signal to bounce in various directions. A signal transmitted from the source and bounces in many direction reaches the destination at different times. Some signal may be distorted along the way or lost signal strengths. The received signal causes misrepresentation of symbols in 802.11 signal where signals overlap due to delays from the multipath. The receiver demodulates the signal received which is a wrong representation from the original signal. Hence wrong information is decoded. Inter-symbol interference is where the symbol representation overlaps. Sometimes earlier arriving signals and delayed signals may have the same strength but out of phase hence cancel each other and there is nothing received. The destination will detect the errors in the received data by checking the check sum which will not add up. This causes the destination not to send an acknowledgement message to the source. The source will resend the packet if time for receiving acknowledgement elapses. This retransmission reduces the users’ throughput. Antennas have been used to boast the signals after loosing energy along the way as they pass through obstacles. But it is hard to overcome some challenges of multipath problems such as cancelling out of the signals of the signals at the destination. The rake receiver has been used to overcome this problem where it holds every delayed multipath carrier. The rake receivers become ineffective if the multipath signals are completely distorted or strongly cancelled each other rendering the rake receiver with on carrier to lock on to. Military GPS receivers are be mitigation for multipath problem where the beams are directed to a satellite. This receivers uses adaptive beamforming and nulling antennas to overcome multipath problem and improve the throughput of WLAN by ensuring that there will be retransmission. Purposes of POE in WLAN design Power over Ethernet (PoE) This is a technology for Local area network (LAN) wired Ethernet which reduces the number of cables needed while installing LAN where the power which is required by the device to operate is supplied by data cables instead of power cables or cords. It is important to determine if a given switch can supply power to an access power via PoE by considering the amount of power required by the access point. How PoE works Current and data signals are transmitted in same cable where the electrical current are introduced into the data cable via component called Injector. In the cable the current is transmitted separately from data signals to avoid interfere between them. A picker is installed between two transmission media that are not PoE compatible to ensure that the current is separated from the data signal as it enters in a media that does not support PoE. The current tapped from the data cable is then jacked into power jack. If the devices are compatible then there is no need of a picker or tab component. More advanced devices for PoE are being implemented that ensures that the devices are not damaged by electrical faults such as excessive power supply or short circuited issues by turning off the power supply. Cisco Pre-Standard Power this is example of PoE devices designed specifically to power access point and Cisco IP phones. It has low power require of 6-7 watts. Purpose of PoE in WLAN design PoE devices are installed in order to achieve the following benefits in a WLAN design 1. Minimize number of power cable connection needed to install and connect to very device. Given that the power needed by the access point and other devices will be supplied via data cable the power cords will not be needed. 2. Having minimal cables in the WLAN network reduces the cost, the cost of purchasing the power cords and the cost of installing and maintaining the cords. 3. Flexibility in installation, it will be flexible to install this type of network whereby it will be easy even to shift the access point and other devices. There will be no issues of moving or re-installing the power cables. Spread spectrum technology The first generation mobile system commonly referred to as first generation mobile communication systems used analog technology. Second generation mobile communication systems such as global system for mobile (GSM), Code Division Multiple Access (CDMA) use digital technology which enhanced quality, security and efficiency. The third known as 3G has provided quality global roaming capability for voice, data, internet and multimedia application. Spread spectrum, was developed by the military and has been popular because of its tolerance to frequency interference and co-existence capabilities. Frequency hopping and direct sequence are the two forms of spread spectrum. It is the modulation that spread data transmission to cover the available frequency band. Spread spectrum makes the data signals resistance to noise, eavesdropping and interference. Spread spectrum is a technology that is used with communication devices such as digital cellular phones and WLAN. Frequency-hopping spread spectrum technology (FHSS) It uses narrowband carrier which changes the frequency pattern in a form know to transmitter and receiver. Narrowband is a radio transmission system that allows to transmit and receive information on a given specific frequency. This technology operates on 2.402- 2.480 GHZ range with a fast frequency change of 2.5 hops per second where each hop covers at least 6MHZ. This technology splits the band into 79 channels which are non-overlapping with 1MHZ per channel as illustrated in the graph below. Direct Sequence Spread Spectrum (DSSS) A redundant bit pattern is generated and added to every bit transmitted. The redundant bit pattern is known as chip. It operates on 2.4GHz band. This ensures that there is no loss of data during transmission. The data transmitted may have been damaged but because of the extra bit pattern it can be recovered without retransmission. This spread spectrum technology ensures that there is high throughput in WLAN transmission. Orthogonal Frequency Division Multiplexing (OFDM) It is the 802.11a standard which operates on 5.0 GHZ. It has 200 channels of 5MHz apart which can support 64 users per access point. Infrared Technology This is a technology that operates in very high frequencies, these frequencies in the electromagnetic spectrum below visible light. It is does not penetrate opaque objects. This technology is implemented in WLAN for fixed sub networks because it cannot be applied to mobile devices. Question 4 This solution is for designing a WLAN for 20 people in an office. The task ranges from understanding the type of technology to use, the layout, the type of antenna and security plan with the implemented WLAN, the employees will be able to use laptops to access email, Internet, small accounting applications, printing service and other office requited applications and services for day-to-day activity in the office. WLAN design is a design of internet connection without cabling system structure. The challenge in designing WLAN Understanding how 802.11 radio work one must understand various vendor implementation and how to secure WLAN in the office, Ensuring that external interferences and effect of building structure can be managed WLAN TECHNOLOGY The technology that it uses is radio frequency commonly abbreviated as RF. The WLAN operates 802.11b and2.4GHz. WLAN devices are both receivers and transmitters referred to as transceiver. There are various modulation schemes that generate variety of 2.4GHz radio signal. The transmission of the signals is in the range of 100 meters. The signal transmitted can be affected by the type of materials used in construction of the building, the equipments within the offices for instance an office with many reflective surfaces will have low throughput because the waves will bounce more on the surface before reaching the destination. Three non-overlapping channels such as 1,6 and 11 will be used in design for reuse this will minimize the level of interference. The access point should be installed to ensure that there are no dead spots whereby the cells must overlap each other, interference of similar channels such as channel 1 of one access point with channel 1 of another access point should be minimized by having few access point in one room. Designing a Cell Plan Given that this is an office WLAN design of about 20 employees, it can be estimated that 10 employees are within a radii of 100 foot. This estimation is important because it enables the designer to know the needed access point which will in turn help in estimating other resources needed. There should be site survey which ensures that access point have strong signal levels to enhance throughput. The Ethernet connection devices that one should consider in this structure should support ‘Power over Ethernet’ to cut down the cost of providing AC power supply of 110V for each point. Devices such as personal device application will be needed in the site survey. It is important to point out that this survey could be provided by the vendor. The cell design can reduce the wave concentration in the offices by configuring access points and the network interface cards to reduce the output power of radio frequency. Types of antennas Antennas are important in WLAN set up because it radiates RF signals in a pattern that is predictable hence signal gain. Antennas basically increase the strength of the signal by spreading them in a given pattern that reduces errors in data received. Omni-directional will be the best solution in the implementation of WLAN in the office. The beams are transmitted in 360 degrees hence they should not be put in corners but at the centre where the beam will transmit in all directions. Patch antenna provides signal beam in 80 degrees while ceiling mount is important to overcome the challenges of the ceiling board in signal transmission. Bandwidth requirements To determine the bandwidth that the WLAN will require there are factors that should be considered. These include; The number of users and their access arrangements: Small cells are required if the users are at the same point such as conference room while large cells are appropriate if there are few users at the same point meaning that few users will want to use the same channel. WLAN for office in this case will require small cells because in an office arrangement many users will access the same radio channel. Therefore the number of concurrent users on the internet at a given point is very important. The tasks that people connected to the internet will do and the relative bandwidth of each task is also another important factor. The main aim and tasks of this WLAN solution is to enable employees to access email, Internet, small accounting applications, printing service and other office requited applications and services for day-to-day activity. E-mails and internet do not require a lot of bandwidth. While accounting application and printing will require tremendous bandwidth, it is estimated that one 802.11b channel having a throughput of 6mbps can support 30 to 50 users (Molta, 2002). This will be appropriate in over office WLAN setup. Security plan Security plan should be developed to ensure that only designated users access WLAN. There will be authentication, accounting by ensuring that everyone has an account and their activities monitored, access control and privacy. There will be an open WLAN and another one that will have high security standards to ensure that accounting information is not access via WLAN by unauthorized users. Resources such as printers and other office equipments connected on the network will be protected by authenticating the users. Standard devices from vendors such as Cisco that have proved to be secure and have security measures even to the open network will be installed to ensure security. There will be also other products to manage and enforce security products such as NetMotion and Ecutel will be installed. WLAN to LAN connection This is a technical challenge of ensuring that the access point is connected to the LAN infrastructure. Wireless Virtual local area network (VLAN) will be established and configured to ensure that only those who are supposed to use WLAN access it. This will set a boundary between the LAN infrastructure and the WLAN. Maintenance This will be an ongoing activity to ensure the WLAN is secure from the hackers and other insecurities. This will be done using management software such as Agere which will help in maintenance. Monitoring and troubleshooting tools such as Wildpackets will be purchased to ensure that the network works continuously. To deal with Radio frequency problems spectrum analyzers will be purchased to troubleshoot the problems. Reference: A. Akella, G. Judd, S. Seshan, and P. Steenkiste(2005), Self management in chaotic wireless deployments. In MobiCom Bluetooth coexistence with 802.11, retrieved from ,http://tinyurl.com/2grndv M. Kodialam and T. Nandagopal. (2003), Characterizing achievable rates in multi-hop wireless networks: the joint routing and scheduling problem, In MobiCom T. Karhima, A. Silvennoinen, M. Hall, and S.-G. Haggman ( 2004), IEEE 802.11b/g wlan tolerance to jamming. In MILCOM Read More