Scalable voip mobility intedration and deployment- P14 pptx

130 Chapter 5 www.newnespress.com The formula for the channels to frequencies is 2.407GHz + 0.5GHz * channel for the 2.4GHz band, and the simpler to remember 5GHz + 0.5GHz * channel for the 5GHz band. The only channels that are in the 2.4GHz band are channels 1–14. Everything else is in the 5GHz band. Therefore, channel 36 is 5.18GHz, and channel 100 is 5.50GHz. The total number of channels is large, but many factors reduce the number that can be practically used. First to note is that the 2.4GHz band, where 802.11b and 802.11g run, only has three nonoverlapping channels (four in Japan) to choose from. Unfortunately, the eleven channel numbers available in the United States gives the false impression of 11 independent channels, and to this day there exist some Wi-Fi deployments that mistakenly use all 11 channels, causing an RF nightmare. To avoid overlapping channels, adjacent channel selections need to be four channel numbers apart. Therefore, channels 1 and 5 do not overlap. In the 2.4GHz band, custom usually spreads the channels out even a bit further, and using only channels 1, 6, and 11 is recommended. The authors of the standard recognized the problem the overlap causes, and, for the 5GHz band, disallowed overlap by preventing devices from using the intermediate channels. Therefore, no channels in the 5GHz band overlap, when it comes to 20MHz channels. (40MHz channels are an exception, and will be discussed in the section on 802.11n, Section 5.5.3.) The unlicensed spectrum was originally designed for, and still is allocated to, other uses besides Wi-Fi. The 2.4GHz band was created to allow, in part, for microwave ovens to emit radio noise as they operate, as it is impractical to completely block their radio emissions. Because that noise prevented being able to provide the protections from interference that licensed bands have, the regulatory agencies allowed inventors to experiment providing other services in this band. And so began 802.11. The 5GHz band is, in theory, more set aside from radiation. Except for the top 5.8GHz ISM band, the 5GHz range was designed for communications devices. However, interference still exists. One primary source, and the one important from a regulatory point of view, is radar. Radars operate in the same 5GHz band. Because the radars are given priority, Wi-Fi devices in much of the 5GHz band are required to either be used indoors only, or to detect when a radar is present and shut down or change channels. This last ability is known as dynamic frequency selection (DFS). This is not a feature or benefit, per se, but a requirement from the various governments. DFS complicates the handoff process significantly, and will be further explored in Section 6.2.2. 5.3.2 Radio Basics Radio technology can seem rather complicated. Wi-Fi transmitters emit power at a given strength, but receivers have to be able to correctly decode those signals at power levels up to a billion times weaker, just for normal wireless networks to work. This incredible swing in power levels requires quite a bit of smarts in both the definitions of the signals and in the Introduction to Wi-Fi 131 www.newnespress.com designs of the radios themselves. It is impossible to describe every important concept in the world of radios, but we will cover those concepts that have the most impact for voice mobility. 5.3.2.1 Power and Multipath Power, or signal strength, is the amount of energy the radio pushes into the air over time. Power is measured in watts. However, for wireless, a watt is a large value, and the receiver usually gets signals many millions of times weaker, once the radio waves reach it. Therefore, Wi-Fi devices tend to refer to milliwatts, or mW. To cover the wide range, the logarithmic decibel is also used. Therefore, power can also be measured in dBm, or decibels of milliwatts. (More formally, it can be called dBmW; however, the W for watt is usually left off.) 0 dBm equals 1 mW. The conversion formula is P dBm = 10 × log 10 P mW , where P dBm is the power in dBm and P mW is the power in milliwatts. It is handy to remember that a 3dB change in power is roughly equal to a two-fold change in power. Note that differences between two dBm values are cited in dB, not in dBm. Therefore, the difference between an 18dBm and a 21dBm transmission power is referred to as 3dB. The reason is because the difference of any logarithmic value is really a ratio of two nonlogarithmic values, and thus 21dBm −18dBm is really stating that the ratio of the two power levels is roughly 2, or that 21dBm is 2 times stronger than 18dBm. (The units of milliwatts cancel in the ratio.) The wide difference in power levels stems from the inverse square law. In free space, space with nothing to interfere or interact with the radio waves, the power of radio signals falls off with the square of the distance, as the power is spread evenly across space. (The reason why spreading power evenly causes the exponent to be 2 is that the area of a sphere grows as the square of the distance. Picture the same amount of radiation smeared over the surface of a sphere growing from the antenna.) This means that short amounts of distance can cause the signal to fall off rapidly. As it turns out, when thinking about wireless, the inverse square law should be thought of with a few modifications. First, radio waves do not pass through all materials equally. The denser the material, the more the radio waves are weakened. This process is called attenuation, and how much an object attenuates is measured in the difference in power level the object causes over free space path loss. A vacuum, and for the most part, dry air, does not attenuate radio waves, and thus can be thought of as having 0dB of attenuation. A wooded wall might attenuate radio waves a bit, and so might have 3dB of attenuation. A cement wall, being thicker, might have 10dB of attenuation. The denser, thicker, or more conductive (metallic) the material is, the higher the attenuation Figure 5.6 shows an example of signal falloff. Certain objects are especially bad for Wi-Fi. Wire mesh, such as chicken wire lathing used in certain plaster walls, metal paneling and ductwork, and water-containing materials tend to cause high amounts of attenuation, as they absorb the radio waves better than dry and less dense materials. This shows up in deployments where old buildings, plumbing, elevator cores, and stairwells tend to disproportionately dampen the power of radio signals. 132 Chapter 5 www.newnespress.com Second, radio signals do not travel as far at higher frequencies than at lower ones. The higher the frequency, the more quickly signals fall off, and the more strongly they interact with and are stopped by objects. This can be understood rather easily. AM channels on radios tend to travel for hundreds of miles—long after you have left the city. That is because they are very low-frequency—in the kilohertz range. AM radio waves pass through walls, buildings, and mountains with relative ease. On the other hand, visible light only goes through glass. That is because light is a very high-frequency radio signal—400 terahertz or higher. In Wi-Fi, this is most noticeable in the difference between 2.4GHz (802.11bgn, referring to how 802.11n radios in the 2.4GHz band must support 802.11g and 802.11b as well) and 5GHz (802.11an), where the 5GHz radio waves may be stopped many feet before the 2.4GHz radio waves are, depending on the environment. Third, radio waves reflect, and these reflections get mixed into the radio waves that are being transmitted, amplifying or weakening the signal. This effect, known as multipath, is caused by the constructive or destructive interference of radio waves, most often caused by reflections from multiple surfaces (see Figure 5.7). The name “multipath” comes from how the signal at any point in space can be thought of as the sum of radio waves bouncing as light rays along multiple paths. Multipath is a real problem with Wi-Fi, and is difficult to account for. That is because multipath doesn’t just increase or decrease the strength of the signal. Radio waves travel at a fixed speed, the speed of light, and therefore those components of a signal that are reflected from objects further away are also slightly more delayed. The speed of light is quick—light travels at around 30 cm per nanosecond—but nanoseconds are in the range of times that Wi-Fi signals change to carry information. Therefore, the effect of multipath is add delayed echoes of the signal to the original signal, increasing the distortion of the signal as well. In environments where multipath does not Distance (feet) Power (dBm) 0 dBm Wall (5dB attenuation) Metal (10dB attenuation) 5 ft 10 ft 15 ft 20 ft 25 ft 30 ft 35 ft 40 ft 45 ft 50 ft 55 ft 60 ft -10 dBm -20 dBm -30 dBm -40 dBm -50 dBm -60 dBm -70 dBm -80 dBm -90 dBm -100 dBm Figure 5.6: How Signals Weaken: Distance and Attenuation Introduction to Wi-Fi 133 www.newnespress.com Strongest Signal Crest of wave Trough of wave No Signal Figure 5.7: Two Transmitters and How the Signals Combine or Cancel substantially distort the signals, the greater effect is that attenuation, and is actually modeled by altering the inverse square law’s square parameter. The path loss exponent (PLE) is usually adjusted upwards, from the value 2, to accommodate the effects of the environment in weakening the signal over distance. Office spaces often have path loss exponents around 2.3 to 2.5. Some environments, on the other hand, such as long metallic hallways or airplanes, have path loss exponents less than 2. No, the laws of physics are not being violated—even the inverse square law is still absolutely correct with a PLE of 2 and only 2—but rather, the reflections from the long hallway make the environment serve as a waveguide, and those reflections constructively interfere, or amplify the signal, more than free space would. (The overall energy radiated out remains the same, just concentrated in the areas with PLE less than 2 and necessarily lessened in others.) All this means that the strength of Wi-Fi signals depends heavily on the environment. The 802.11 standard has a diagram that is telling. Figure 5.8, present in the IEEE 802.11 standard, shows the signal strengths throughout a one-room office. The darker the color, the weaker the signal, and the lighter the color, the stronger the signal. It is clear to see that the reflections cause intense ripple patterns across the room. These ripples are spaced a wavelength apart, and the difference from peak to trough is 50dB, or over one hundred thousand times the power. 134 Chapter 5 www.newnespress.com The problem with the ripples is that they are nearly impossible to predict. This has an impact on the performance of the wireless network. 5.3.2.2 Noise and Interference Radio noise occurs in Wi-Fi networks from three main sources. Each source contributes to the noise floor, which represents the background signal power level that will always be present. Signals that can be received must be a certain power level above the noise floor, in order for the signal to be distinct from the din. All of the major theory behind radio reception and information transfer relate the capacity to send information, and the amount of information that can be sent, to the ratio of the signal power level to this noise floor. The first source of noise is the most basic and least interesting to voice mobility networks once it is understood. Thermal noise is the noise produced in every RF frequency because of the temperature of the world and all of the things in it. Thermal noise is a basic property of the fact that radio waves are light, and light is radiated because of the temperature of objects. The phenomenon that causes a piece of metal to become red when heated also causes the thermal background radiation within the Wi-Fi bands. The noise is not only coming from the nonradiating components in the room. The radios themselves—the antennas, the circuit paths, and the amplifiers and active components—all themselves are injecting thermal noise in the radio circuits. This is most important for the receiver’s circuits, as this noise is what can drown out the weak signal coming in. In Wi-Fi networks, the noise floor tends to get no lower than around −100dBm, measured by Wi-Fi devices. This sort of noise is expected to be random and predictable, but predictable in how it is random. Once the thermal noise level is learned, it should not change by more than a couple of dB as time goes on, and that variation is as much the result of temperature variations as the Wi-Fi equipment heats up. Figure 5.8: Signal Combination in a Room Introduction to Wi-Fi 135 www.newnespress.com The second source of noise comes from non-Wi-Fi devices. Microwave ovens, cordless phones, Bluetooth devices, and industrial machinery all inject noise into the Wi-Fi bands. Devices are allowed, by regulatory rules, to inject this noise into the ISM bands that Wi-Fi uses, as long as the power level does not exceed certain thresholds. But intentionally generated communications signals tend to be of greater concern, as coexistence between different technologies in the unlicensed spectrum is mandatory, and Wi-Fi is not the only technology. The two ISM bands, the 2.4GHz and the 5.8GHz, are where these devices should be expected. The 5.8GHz band can especially have surprising sources of noise, because there are outdoor applications for the 5.8GHz band, using high-power but low- beam-width transmitters. A neighbor could easily set up such a system and interfere with a Wi-Fi network already in place. The third, and most important source of noise is self noise, or noise generated by Wi-Fi from its operation. This noise always comes from neighboring access points’ devices, which generate enough power that their signal energy reaches some of all of the devices in the given cell. In the best case, those interfering signals may reach the devices as weak, undecipherable power, contributing to an effective rise in the noise floor, but not necessarily resulting in a direct change into the operation of the Wi-Fi protocol. This sort of noise is always present in Wi-Fi networks that see more than a trivial amount of use. The more densely deployed the network is, as a mixture of access points and the clients that use them, the higher the noise floor will rise because of the contribution of this power. The noise floor can rise as high as −80dBm in many networks when being used, and even higher when the network comes under stress. In networks where the density is reasonably elevated, the effect of the other devices is stronger than noise, as it directly affects the Wi-Fi protocol by causing the devices to detect the distant transmissions and defer transmission. Even when this does not happen, the bursty on-and-off nature of Wi-Fi can mean that transmissions in progress can experience bit errors as the interference disrupts the radios themselves, without being detected as noise or energy outright. 5.3.3 RF Planning RF planning is designed to address the two problems of multicellular networks. The first problem is to ensure that the coverage levels within the network are high enough that the expected data rates, based on the minimum required signal to noise ration, can be achieved at every useful square foot of the building or campus environment. The second problem is to avoid the intercell interference which results from multiple devices transmitting on the air without mitigation. Proper RF planning is an expensive, time-consuming process. The basics of RF planning are for the installers to predict what the signal propagation properties will be in the expected environment. This sort of activity always requires using sophisticated RF prediction tools. RF prediction tools operate by requiring the operator to designate the locations and RF 136 Chapter 5 www.newnespress.com properties—attenuation, mostly—of each physical element in the building, the furniture, the walls, the floors, and the heavy machinery. Clearly a laborious process, the operator must copy in the location of these elements one at a time. Some tools are intelligent enough to take CAD drawings or floor-plan maps and estimate where the walls are, but an operator is required to verify that the guesses are not far from reality. RF planning tools then use RF calculations, based on electromagnetic principles, to determine how much the signal is diminished or attenuated by the environment. The planning tools need to know the transmit power capabilities and antenna gains of all of the access points that will be deployed in the network. RF planning can be used this way to assist in determining where access points ought to be located, to maximize coverage given the particular SNR requirements. Because RF planning uses exact equations to predict the effects of the environment, it can be only as good as the information it is given. Operators must enter the exact RF and physical properties of the building to have a high likelihood of getting an accurate answer. For this reason, RF planning suffers from the garbage-in-garbage-out problem. If the operator has uncertainty about the makeup of the materials in the building, then the results of the RF plan share the same uncertainty. Furthermore, RF planning cannot predict the effects of multipath. Multipath is more crucial than ever in wireless networking, because the latest Wi-Fi radios take advantage of that multipath to provide services and increase the data rate. Not being able to predict multipath places a burden on RF planning exercises, and requires RF planners to look for the worst- case scenarios. Using RF planning tools to determine what power levels or channel settings each access point takes, then, is not likely to be a successful proposition as the network usage increases. Unfortunately, Wi-Fi self noise is a problem that does not show itself until the network is being heavily used, at which point it shows with vigor. Until then, as the network is just getting going, self noise will not be present at high levels and will not occupy 100% of the airtime. Thus, network administrators will see early successes with almost any positioning of Wi-Fi equipment, and can gain a false sense of security. (It is important to note that this is a property of trying to predict how RF propagates. Tools or infrastructure that constantly monitor and self-tune suffer the same problems, but with the added wrinkle that the self- tuning is disruptive, and yet will be triggered when the noise increases and the network needs to be disrupted the least.) The one place where RF planning shows strength is in determining a rough approximation of the number and position of access points that are needed to cover a building. This does not require the sort of accuracy as complete RF plan, and tends to work well because of the fact that Wi-Fi networks are planned for a much higher minimum SNR than is necessary to cover the building. That higher SNR is required, however, to establish a solid data rate, and Introduction to Wi-Fi 137 www.newnespress.com so what appears to be padding or overprovisioning from a coverage point of view can be lost capacity from a data rate point of view. Nonetheless, determining the rough number of access points needed for large deployments is a task that can do with some automation, and RF planning tools used only to plan for coverage (and not for interference), can be reasonably effective—even more so if the infrastructure that is deployed is able to tolerate the co-channel interference that is generated. 5.4 Wi-Fi’s Approach to Wireless The designers of 802.11 took into account the RF properties to create a technology that could transmit in the face of the obstacles in RF environments. Over time, they developed multiple differing, but related, radio technologies that built upon each one’s previous 802.11 radio types and modern RF design principles to continue to improve the speed, range, and resiliency of the transmissions. Let’s look at the principles behind 802.11 transmissions at the physical layer. 5.4.1 Data Rates A data rate, in 802.11, is the rate of transmission, in megabits per second (Mbps) of the 802.11 header and body. The 802.11 MAC header, the body, and the checksum (but not the physical layer header) are transmitted at the same data rate within each frame. A data rate represents a particular encoding scheme, or way of sending bits over the air. Each data rate can be thought of as coming from its own modem, designed just for that data rate. An 802.11 radio, then, can be thought of has having a number of different modems to chose from, one for each data rate. (In practice, modern radios use digital signal processing to do the modulation and demodulation, and therefore the choice of a modem is just the choice of an algorithm in microcode on the radio or software used to design the radio itself.) Each data rate has its own tradeoff. The lowest data rates are very slow, but are designed with the highest robustness in mind, thus allowing the signal to be correctly received even if the channel is noisy or if the signal is weak or distorted. These data rates are very inefficient, in both time and spectrum. Packets sent at the lowest data rates can cause network disruption, as they occupy the air for many milliseconds at a time. Although one millisecond sounds like a short amount of time, if each packet were, say, ten milliseconds long, then the highest throughput an access point could get would be less than 1.2Mbps for 1500-byte packets. The higher data rates trade robustness for speed, allowing them to achieve hundreds of megabits per second. The description of the 802.11 radio types will walk through the principles involved in packing more data in. Occasionally, someone may mention that this 138 Chapter 5 www.newnespress.com effect is related to Shannon’s Law. Shannon’s Law states that the maximum amount of information that can be transmitted in a channel increases logarithmically with the signal-to- noise ratio. The stronger the signal is than the noise floor, the faster the radio can transmit bits. Lower data rates do not take advantage of high SNRs as well as higher data rates do. As data rates go higher, the radios become increasingly optimistic about the channel conditions, trying to pack more bits by making use of the higher fidelity that is possible. That higher fidelity is held to a smaller distance from the radio, and so higher data rates travel less far. (But note that 802.11 uses a concept to ensure that every device within the longest range knows of a transmission, no matter what the data rate is.) Think of it as saying that the amount of available “space” in a channel is determined by the SNR. More SNR means that more bits can be packed, by reducing the “space” between bits. Of course, the smaller the “space” between bits, the harder it becomes to tell the bits apart. Data Rates and Throughput Data rates in 802.11 refer to how fast the bits of the frame are transmitted over the air. Numbers as high as 300Mbps exist for the latest 11n devices; Section 5.5 explains each radio type. However, there is a significant gap between the data rate and the highest possible throughput that an application can see. The main reason for this are that there is a tremendous amount of overhead in 802.11. Because each frame is preceded by a low-data rate header (the preamble), as well as mandatory random waiting times (the backoff), much of the airtime is spent in negotiating which device can transmit. This limits one-way traffic—such as UDP streams—to a significantly lower throughput than the data rate the frames are going at. The peak throughput varies significantly, depending on the vendors and products involved, but good rules of thumb are: • 802.11b: 11Mbps data rate → around 8Mbps UDP throughput • 802.11a/g: 54Mbps data rate → around 35Mbps UDP throughput • 802.11n: 300Mbps data rate → around 250Mbps UDP throughput Furthermore, 802.11 is a half-duplex network, meaning that upstream and downstream traffic compete for the same airtime. Thus, TCP traffic, which must have one upstream packet (also called acknowledgments) for every two downstream data packets, has an even lower throughput, such as: • 802.11b: 11Mbps data rate → around 6Mbps TCP throughput • 802.11a/g: 54Mbps data rate → around 28Mbps TCP throughput • 802.11n: 300Mbps data rate → around 190Mbps TCP throughput Introduction to Wi-Fi 139 www.newnespress.com 5.4.2 Preambles Because 802.11 allows transmitters to choose from among multiple data rates, a receiver has to have a way of knowing what the data rate a given frame is being transmitted at. This information is conveyed within the preamble (see Figure 5.9). The preamble is sent in the first few microseconds of transmission for 802.11, and announces to all receivers that a valid 802.11 transmission is under way. The preamble depends on the radio type, but generally follows the principle of having a fixed, well-known pattern, followed by frame-specific information, then followed by the actual frame. The fixed pattern at the beginning lets the receiver train its radio to the incoming transmission. Without it, the radio might not be able to be trained to the signal until it is too late, thus missing the beginning of the frame. The training is required to allow the receiver to know where the divisions between bits are, as well as to adjust its filters to get the best version of the signal, with minimum distortion. The frame-specific information that is included with the preamble (or literally, the Physical Layer Convergence Procedure (PLCP) following the preamble, although the distinction is unnecessary for our purposes) names two very important properties of the frame: the data rate the frame will be sent at, and how long the frame will be. Preamble (lowest data rate) Training Header Body FCS Data Rate Length Time Frame (data rate for this frame) Figure 5.9: 802.11 Preambles Illustrated All preambles are sent at the lowest rate the radio type supports. This ensures that no matter what the data rate of the packet, every radio that would be interfered with by the transmission will know a transmission is coming and how long the transmission will last. It also tells the receiver what data rate it should be looking for when the actual frame begins. All devices within range of the transmitter will hear the preamble, the length field, and the data rate. This range is fixed—because the preamble is sent at the lowest data rate in every case, the range is fixed to be that of the lowest data rate. Note that there is no way to change the data rate at which the preamble is sent. The standard intentionally defines it to be a fixed value—1Mbps for 802.11b, and 6Mbps for everything else. When a radio hears a preamble with a given data rate mentioned, it will attempt to enable its modem to listen for that data rate only, until the length of the frame, as mentioned in the preamble, has concluded. If the receiver is in range of the transmitter, the modem will be able . 2.4GHz band, and the simpler to remember 5GHz + 0.5GHz * channel for the 5GHz band. The only channels that are in the 2.4GHz band are channels 1–14. Everything else is in the 5GHz band. Therefore,. channels 1 and 5 do not overlap. In the 2.4GHz band, custom usually spreads the channels out even a bit further, and using only channels 1, 6, and 11 is recommended. The authors of the standard. unlicensed spectrum is mandatory, and Wi-Fi is not the only technology. The two ISM bands, the 2.4GHz and the 5.8GHz, are where these devices should be expected. The 5.8GHz band can especially have