Data Rates | Wi-Fi's Approach to Wireless

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 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.

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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. 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: 11 Mbps 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

RF Planning | RF Primer

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 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 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.

Channels | RF Primer

One Wi-Fi radio does not occupy the entire unlicensed spectrum, unlike frequency-hopping technologies such as Bluetooth. 802.11 divides up the spectrum into a number of different channels. Channels are named with whole numbers, assigned by a formula to specific center frequencies for the channels. The idea behind small number of discreet channels is to carve up the spectrum, helping pack in as many devices as possible and avoiding requiring clients to have to tune in across a wide range of frequencies, the way that analog car radios must.

The channel numbers are somewhat arbitrary, and are arranged to let you know what band they occupy. Different 802.11 radio types allow for different channel selections.

The two key properties that define how the 802.11 radio uses the spectrum are its center frequency and bandwidth. The center frequency is the one the radio uses to determine where to look for the transmissions. This concept is similar to car radios: FM channel 97.3 means that the radio tunes its center frequency to 97.3MHz. Unfortunately, Wi-Fi channels do not convert as neatly to their center frequencies. Because of this, many people and tools will either interchangeably use the center frequency or the channel number to describe the channel. Wi-Fi uses center frequencies that are always in the gigahertz range. The bandwidth tells which other frequencies are occupied by a transmission. 802.11 radios used for mobility primarily have 20MHz bandwidth, except for 802.11n radios, which can also use 40MHz bandwidths. The channel and bandwidth together show which part of the spectrum the radio occupies. Although the different 802.11 radio types may fill the carved-out part of the spectrum differently, the amount that is carved out is roughly the same for the same bandwidth. Figure 1 sketches the general concept.

 
Figure 1: Shape of 802.11 Frequency Occupation

Table 1 lists the channels and what radio types can use them.

Table1: 802.11 Channels 

Channel	Frequency	US Band	11b, 11g	11a	11n	Notes
1	2.412GHz	ISM 2.4	✓		✓	Nonoverlapping	High power: 1 W maximum.
2	2.417GHz		✓		✓
3	2.422GHz		✓		✓
4	2.427GHz		✓		✓
5	2.432GHz		✓		✓
6	2.437GHz		✓		✓	Nonoverlapping
7	2.442GHz		✓		✓
8	2.447GHz		✓		✓
9	2.452GHz		✓		✓
10	2.457GHz		✓		✓
11	2.462GHz		✓		✓	Nonoverlapping
12	2.467GHz		✓		✓	Europe, Japan, Australia. No U.S. or Canada
13	2.472GHz		✓		✓
14	2.484GHz		11b only			Japan only. Channel 14 does not follow the channel to frequency formula.
36	5.18GHz	U-NII 2 Lower		✓	✓	Indoor use only. Low power: 40 mW maximum
40	5.20GHz			✓	✓
44	5.22GHz			✓	✓
48	5.24GHz			✓	✓
52	5.26GHz	U-NII 2 Upper		✓	✓	Non-DFS for equipment before July 2007	Radar detection and dynamic frequency selection (DFS) required
56	5.28GHz			✓	✓
60	5.30GHz			✓	✓
64	5.32GHz			✓	✓
100	5.50GHz	U-NII 2 Extended		✓	✓
104	5.52GHz			✓	✓
108	5.54GHz			✓	✓
112	5.56GHz			✓	✓
116	5.58GHz			✓	✓
120	5.60GHz			✓	✓	U.S., Europe, and Japan. No Canada, because of weather radar.
124	5.62GHz			✓	✓
128	5.64GHz			✓	✓
132	5.66GHz			✓	✓
136	5.68GHz			✓	✓
140	5.70GHz			✓	✓
149	5.745GHz	U-NII 3		✓	✓	U.S, Canada and Europe. No Japan	High power
153	5.765GHz			✓	✓
157	5.785GHz			✓	✓
161	5.805GHz			✓	✓
165	5.825GHz	ISM 5.8		✓	✓	U.S., Canada and Europe. No Japan.	High power

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.

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 5 GHz 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 asdynamic 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

Over-the-Air: Virtualized

The virtualized architecture builds upon the layering architecture, but introduces the notion of complete wireless network virtualization. Wireless LAN (WLAN) virtualization involves creating a unique virtual wireless network (a BSSID) for every mobile device. This allows the network to be partitioned for each client, providing each client with its own set of 802.11 autonegotiated features and parameters.

It's important to note that the per-device containment provided by virtualization differs from the per-device rules and access control enforcement provided by the other architectures. Containment addresses the over-the-air behavior of the client directly, using the standard to enforce the segmentation and the tight resource bounds. The client's cooperation is not needed or expected. Access control, on the other hand, is fundamentally a cooperative scheme, and clients can choose not to participate in the optional protocols required to make bidirectional access control work. Even downstream policy enforcement cannot stop a client from transmitting what it wants to upstream.

However, virtualized Wi-Fi partitions are able to maintain the per-device containment, by transferring control of the network resources from the client to the network, and then using Wi-Fi mechanisms from the network side to ensure that client behavior is limited to the resources that the client is allocated.

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Architectures and 802.11 Functions

In 802.11, the concept of an "access point" is defined to carry one BSSID and one SSID over the air to a set of clients. The access point definition includes every function necessary to make the access point a bridge to wireline Ethernet, including encryption, decryption, connection management, medium access control, and timing functions.

However, this concept is only a concept, and the architectures in the market today differ by how they divide the functions of the 802.11 access point across the actual equipment deployed in the network. In general, every architecture ensures that multiple 802.11 access point concepts can be created and operated in each physical access point, thus allowing for multiple BSSIDs—and more importantly, multiple SSIDs—per access point. This starts by having multiple radios within an access point, but is most useful by allowing multiple SSIDs per radio.

This is the point of departure for the architectures. Controller-based architectures move parts of the 802.11 access point out of each physical access point and operates them, instead, in the controller, thus sharing those parts across all of the access points. This does not violate the standard, however, because the standard was designed to allow for all kinds of mappings of logical 802.11 entities to physical devices.

Ultimately, the best way to choose which 802.11 functions should be centralized—and thus, which type of architecture to invest in when creating a voice mobility network—is to choose based on how well the features meet your needs, and not on architectural principles alone.

Over-the-Air: Dynamic or Adaptive Microcell

Dynamic microcell over-the-air architectures take a different approach than static architectures. The goal of dynamic architectures is to use what is known as radio resource management (RRM; some vendors use similar terms) to adaptively configure the channels, power levels, and other settings of the access points.

The reason for transitioning from a stable network to one that is constantly in flux is to attempt to avoid some of the problems inherent in larger 802.11 networks, mentioned in the following sections. The key observation is that radio resources exist and need to be monitored somehow. Broadly, radio resources can be thought of as wireless network capacity, and they are reduced by interference, density, and mobility of wireless clients. The following sections, especially "RF Primer" and "Radio Basics," will shed light on the specifics of what impacts these radio resources.

Dynamic architectures attempt to handle the problem by constantly measuring the various fluctuations in load, density, and neighboring traffic, and then making minute-by-minute adjustments in response. The main tools in the dynamic architecture's arsenal are, as before, choosing channel settings and transmit power levels.

Dynamic architectures end up creating an alternating assignment of channels, in which every access point attempts to chose a different channel from its neighbors and a power level low enough to avoid providing too much duplicated coverage.

The advantages of dynamic radio resource management is that the network is able to avoid situations where static networks completely fail—for example, dynamic networks can continue to operate (albeit with reduced capacity) when a microwave oven is turned on, whereas static networks may succumb completely in the area around the interference. The main disadvantage, however, is that the network and its associated coverage patterns are unpredictably changing, often by the minute. This leads to a necessary tradeoff between the disease and the cure. Thus, dynamic systems provide the expert administrator with the ability to go in and turn down the aggressiveness of the adaptation, providing a choice between a more static network or more dynamic network, allowing the administrator to choose which benefits and downsides are best suited for the given deployment. You will find that many voice mobility networks have disabled many of the adaptive features of their networks to ensure a more consistent coverage.

Additionally, the smaller and changing cell sizes, along with the wide array of channels that end up being used, leads to issues with handoff that directly affect voice mobility. To help mitigate these problems, network assistance protocols can be used to increase the amount of information that clients, who decide when to hand off and where to hand off to, have at their disposal.

Over-the-Air: Static Microcell

Static microcell over-the-air architectures usually require the administrator or a planning tool to generate the radio frequency (RF) parameters—channel selection and transmit power, in this case—for the access points. The most basic implementations just require the user to select a channel and power level. Of course, the system may have some defaults, and may even attempt to make some initial scanning to chose "better" channels. Nevertheless, once a choice is made, the choice does not change unless the administrator selects a new value or uploads a new RF plan.

This does introduce the concept of RF planning, which will be addressed in the section on RF (Section 5.3). The key to the static (and the subsequent dynamic) microcell architectures is the dedication of the available Wi-Fi channels to avoiding neighboring access point interference, thus resulting in an alternating pattern of channel assignments, where the closest neighbors always have different channels. For static systems, the installer is required to know how to do this by sight, or by using the RF planning tools. Furthermore, because these architectures also require reducing power levels significantly to avoid interference from second-order (further away) neighbors, and lower power levels translates into less range and smaller cell sizes, these architectures are also known as microcell.

Standalone access points are the most obvious candidates for static over-the-air architectures, because there is no system changing channels or power levels on the network. However, all of the wireline architectures can be made to behave statically, though how to do so may not be obvious and setting the network in that mode may not be recommended.

The advantage of the static architecture is that the RF plan is consistent, thus allowing for a more predictable coverage. The disadvantage is that the network does not react to changes in its environment, such as persistent noise or neighboring network interference.