This chapter is from the book Because wireless signals travel through the atmosphere, they are susceptible to different types of interference than standard wired networks. Interference weakens wireless signals and therefore is an important consideration when working with wireless networking. Show
Interference TypesWireless interference is an important consideration when you’re planning a wireless network. Interference is unfortunately inevitable, but the trick is to minimize the levels of interference. Wireless LAN communications typically are based on radio frequency signals that require a clear and unobstructed transmission path. The following are some factors that cause interference:
Many wireless implementations are found in the office or at home. Even when outside interference such as weather is not a problem, every office has plenty of wireless obstacles. Table 7.4 highlights a few examples to be aware of when implementing a wireless network indoors. Table 7.4. Wireless Obstacles Found Indoors
Spread-Spectrum TechnologySpread spectrum refers to the manner in which data signals travel through a radio frequency. With spread spectrum, data does not travel straight through a single RF band; this type of transmission is known as narrowband transmission. Spread spectrum, on the other hand, requires that data signals either alternate between carrier frequencies or constantly change their data pattern. Although the shortest distance between two points is a straight line (narrowband), spread spectrum is designed to trade bandwidth efficiency for reliability, integrity, and security. Spread-spectrum signal strategies use more bandwidth than in the case of narrowband transmission, but the trade-off is a data signal that is clearer and easier to detect. The two types of spread-spectrum radio are frequency hopping and direct sequence. Frequency-Hopping Spread-Spectrum (FHSS) TechnologyFHSS requires the use of narrowband signals that change frequencies in a predictable pattern. The term frequency hopping refers to data signals hopping between narrow channels. For example, consider the 2.4GHz frequency band used by 802.11b/g. This range is divided into 70 narrow channels of 1MHz each. Somewhere between 20 and several hundred milliseconds, the signal hops to a new channel following a predetermined cyclical pattern. Because data signals using FHSS switch between RF bands, they have a strong resistance to interference and environmental factors. The FHSS signal strategy makes it well suited for installations designed to cover a large geographic area and where using directional antennas to minimize the influence of environmental factors is not possible. FHSS is not the preferred spread-spectrum technology for today’s wireless standards. However, FHSS is used for some lesser-used standards and for cellular deployments for fixed broadband wireless access (BWA), where the use of DSSS (discussed next) is virtually impossible because of its limitations. Direct-Sequence Spread-Spectrum (DSSS) TechnologyWith DSSS transmissions, the signal is spread over a full transmission frequency spectrum. For every bit of data that is sent, a redundant bit pattern is also sent. This 32-bit pattern is called a chip. These redundant bits of data provide both security and delivery assurance. The reason transmissions are so safe and reliable is simply because the system sends so many redundant copies of the data, and only a single copy is required to have complete transmission of the data or information. DSSS can minimize the effects of interference and background noise. As for a comparison between the two, DSSS has the advantage of providing better security and signal delivery than FHSS, but it is a sensitive technology, affected by many environmental factors. Orthogonal Frequency Division MultiplexingOrthogonal Frequency Division Multiplexing (OFDM) is a transmission technique that transfers large amounts of data over 52 separate, evenly spaced frequencies. OFDM splits the radio signal into these separate frequencies and simultaneously transmits them to the receiver. Splitting the signal and transferring over different frequencies reduces the amount of crosstalk interference. OFDM is associated with 802.11a, 802.11g amendments, and 802.11n wireless standards. Beacon Management FrameWithin wireless networking is a frame type known as the beacon management frame (beacon). Beacons are an important part of the wireless network because it is their job to advertise the presence of the access point so that systems can locate it. Wireless clients automatically detect the beacons and attempt to establish a wireless connection to the access point. The beacon frame is sent by the access point in an infrastructure network design. Client stations send beacons only if connected in an ad hoc network design. The beacon frame has several parts, all of which the client system uses to learn about the AP before attempting to join the network:
These beacons are transmitted from the AP about every 10 seconds. The beacon frames add overhead to the network. Therefore, some APs let you reduce the number of beacons that are sent. With home networks, constant beacon information is unnecessary. Passive and Active ScanningBefore a client system can attempt to connect to an access point, it must be able to locate it. The two methods of AP discovery are as follows:
When designing a wireless network, there are quite a few factors that one must consider. For example, it’s extremely important to estimate the user density in order to lay out the proper number of access points (APs). Another important consideration is what materials go into the building: the walls, the floors, the doors. Learn more about wireless surveys. It’s understandable that each different type of material affects wireless signal differently. Here’s a breakdown of the five different phenomena that can impact a Wi-Fi signal: 1. ReflectionA wireless signal is just radio waves. Just like light, it can bounce off of certain surfaces. Metal, for one, is a highly reflective material. This is a common occurrence for offices since they are generally in complex and intricately designed structures. If a large amount of reflection occurs, signals can be weakened and also cause interference at the receiver. 2. RefractionRefraction is the bending of a wave when it enters a medium where the speed is different. For example, glass or water can refract waves. This can play into consideration when you’re carefully placing APs. Different media have different refractive indexes. It’s important to track possible refraction when designing your wireless network because if a signal changes direction in traveling from sender to receiver, this can cause lower data rates, high retries and lead to an overall lessening of capacity. 3. DiffractionThis is when waves encounter an obstacle and travel around it — the wave’s direction and intensity both change. In fact, diffraction can even be more pronounced or introduce a shadow zone depending on the size and shape of the obstacle. Hills are known to cause diffraction to wireless signals. 4. ScatteringWhile this phenomena is similar to refraction, it’s more unpredictable. Dust, humidity, unevenness and other qualities in a material can cause a signal to scatter in all directions. This can have a significant impact on signal integrity and strength. Chain-link fences and even smog are notorious for scattering RF signals. 5. AbsorptionThis is one of the most common reactions we see wireless signal have to materials. Basically, a material is converting the signal’s energy into heat. This occurs largely due to the molecules in the medium being unable to move fast enough to “keep up” with the RF waves that are trying to pass through it. Different materials naturally have different absorption rates. Wood and concrete, for example, can make a huge impact on signal strength because of how much they absorb the radio waves. It’s important to note that any given material can exert a combination of these impacts signal. Glass can both refract and absorb. That is in part why it is so essential to meticulously plan your wireless network, taking careful consideration of the different materials that your signal will encounter. For more information on the areas on which wireless network engineers must pay close attention, see 7 Gotchas of Wi-Fi. Are you interested in having a wireless survey done for your facility? Or are you looking to upgrade to a smarter Wi-Fi network? Email us or call us at 502-240-0404 to get started! |