Clear Channel Assessment and Details on Carrier Sense

Now that we've covered the preamble, you can begin to understand what the term carrier sense would mean in wireless.

The term clear channel assessment (CCA) represents how a radio determines if the air is clear or occupied. Informally, this is referred to as carrier sense. As mentioned previously, transmitters are required to listen before they transmit, to determine whether someone else is also speaking, and thus to help avoid collisions.

When listening, the receiver has a number of tools to help discover if a transmission is under way. The most basic concept is that of energy detection. A radio can figure out whether there is energy in the channel by using a power meter. This power meter is usually the one responsible for determining the power level, often stated as the Receive Signal Strength Indication (RSSI) of a real signal. When applied to an unoccupied channel, the power meter will detect the noise floor, often around −95dBm, depending on the environment. However, when a transmission is starting, the power meter will detect the signal being sent, and the power level measured will jump—let's say, to −70dBm for this example. That difference of 25dB can be used by the radio to clue in that it should attempt to turn on its modem and seek out the preamble. This allows the radio to have its modem off until real signals come by.

Energy detection can be used as a form of carrier sense to trigger the CCA. When done that way, non-802.11 noise that crosses a certain threshold, determined by the radio, will show up as an occupied channel for as long as the noise is present. This allows the radio to avoid transmitting into a channel at the same time as interference is present. In the 2.4GHz band, microwave ovens can often trigger the energy detection thresholds on radios, causing the radios to stop transmitting at that time.

On the other hand, energy detection for CCA has its limitations. If the noise coming in is something that would not interfere with the transmission, but does trip the energy detection threshold, then airtime is being wasted. Therefore, the carrier acquisition portion of CCA comes into play. Radios know to look for specific bit patterns in a transmission, such as the preamble. When they detect these bit patterns, they can assert CCA as well. Or, more importantly, when they detect some energy in the channel but cannot detect these bit patterns, they can conclude that there is no legitimate 802.11 signal and suppress CCA.

Preambles | Wi-Fi's Approach to Wireless

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 1).

 
Figure 1: 802.11 Preambles Illustrated

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.

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 to properly detect the frame. If, however, the receiver is out of range, the receiver will hear garbage. The garbage will not pass the checksum (also garbage), and so will be discarded.

To prevent radios from interpreting noise as a preamble, and locking to the wrong data rate for a possibly very long length, the frame-specific information has its own checksum bit or bits, depending on the radio type. Only on rare occasions will the checksum bit fail and cause a false reception; thus, there is no concern for real deployments.

In summary, a receiver then works by first setting its radio to the lowest common denominator: the lowest data rate for the radio. If the fixed sequence of a preamble comes in, followed by the data rate and length, then the radio moves its modem up to the data rate of the frame and tries to gather the number of bits it calculates will be sent, from the length given. Once the amount of time necessary for the length of the frame has concluded, the radio then resets back to the lowest data rate and starts attempting to receive again.

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

Wireless Data & Broadcast Television | Services

Wireless Data

There are three basic services offered by wireless data systems: circuit switched data, packet switched data and messaging.

Circuit switched data is a bearer service as it only transports the user’s data between points. When sending data through a circuit switched connection, the user regularly pays a standard per-minute charge for the amount of time that the connection is maintained regardless of how much data is sent through the channel.

It usually takes approximately 10 to 20 seconds to establish a circuit switched connection on a wireless network. This is due to the processing of dialed digits through the telephone network and the amount of time the modem requires to establish which communication language will be used (called training time). The user ordinarily pays for this setup time even if they only have a very small amount of information to send (such as an email message). Once a connection is established on a circuit switched connection, data transfer rates generally range from 9600 bps up to 28,800 bps.

Packet switched data is also a bearer type of service as it only transports the users data between points. When sending packets of data through the network, the user normally pays only for the amount of data or number of packets that they send.

Unlike circuit switched data, the connection time for packets is ordinarily under 1 second (some systems may be below 150 msec) and the user does not pay for this setup time. The typical price for packet data transmission ranges from approximately 4 cents to $1 per kilobyte. A one-time activation fee is as a rule required along with a minimum monthly fee. The usage amount is normally applied to the monthly fee.

Several wireless data service providers in the United States now offer service based on application and number of units. This results in different price plans that can vary from $15-25 per month per unit with some systems offering a flat fee for a fixed or unlimited amount data transmission. The trend is to move away from the per packet charge.

Figure 1 shows a typical wireless packet data rate plan. This table shows that there is usually a monthly recurring fee that provides the user with a monthly amount of data. If the customer uses the maximum data allocation, an additional fee per kilobyte or megabye of data is charge.

Figure 1: Wireless Data Cost

Wireless messaging is a teleservice as it processes the user data. Wireless messaging services include store, forward, and Internet connectivity. Typically wireless messaging is combined (bundled) with wireless data service (such as packet data).

Broadcast Television

Broadcast television revenues are primarily obtained from the providing of advertising. Figure 2 shows the recent growth of the broadcast television advertising industry. This diagram shows that total advertising revenue is increasing.

Figure 2: Television Advertising Revenue.
Source: Television Advertising Bureau

Wireless Data (Wireless Networks)

Wireless Data
Wireless data systems transfer of digital signals between two data devices via a wireless communication path. Most wireless data services are dedicated to specific types of applications. Vertical wireless data applications (vertical) are very specific solutions, and have continued to win over mass market “horizontal” offerings. Vertical solutions include applications such as utility meter reading or mobile dispatch. Horizontal solutions have mass-market appeal such as wireless e-mail.

The growth of the Internet has also enabled low-cost, standardized access to wireless data networks that is accelerating the growth of the wireless data marketplace. In 2001, almost all the new mobile telephones had wireless Internet access capability.

Figure 1 shows a basic wireless data system. In this example, many types of wireless data devices communicate through a public wireless data system. In the core of the system, there is a switching system. The switching system commonly routes the data between the wireless device and a computer system (such as a company computer). In this diagram, there are more receivers than transmitters. This is required to allow low-power mobile data transmitters to reach the system. Base station transmitters can provide up to 500 Watts effective radiated power (ERP) while portable mobile data devices can usually provide less than 1 Watt of transmitted power.


Figure 1: Wireless Data System
Wireless data get the attention it deserves when a mass-market wireless data application (often called the “Killer App”) is embraced by the public. Here are a few successful vertical wireless data applications:

Wireless data for the electric power, waste water, and natural gas industries. New competition in the utility industry demands the benefits of a wireless data solution for timely customer-focused improvements.

Wireless data for field service personnel. Field service organizations use wireless data to close the gap on a geographic distance to improve customer service, technician productivity, and increased revenues.

Companies with mobile sales forces have increased their productivity and efficiency of personnel by filling out much of their paperwork “on-line”. Sales force access to corporate databases has proven paramount in the new paradigm of doing business the 21st century style.

Wireless Networks

Wireless networks are primarily designed to transfer voice and or data from one point to one or more other points, (multipoint). Many networks make use of some wireless technologies as a transport medium even though we do not consider them to be wireless networks. Examples of wireless networks include cellular, personal communication service, (PCS), paging, wireless data, satellite, and broadcast radio and television.

Cellular and PCS
Cellular and PCS systems are comprised of a set of radio towers that are strategically distributed over a geographical area in order to provide a continuous service coverage area. A mobile switching center (MSC) provides the switching and control functions necessary to connect calls from the public switched telephone network (PSTN) to the individual mobile telephones. The MSC also manages the radio resources within the entire network.

Several service providers (carriers) within a particular geographical area can provide wireless services simultaneously as long as they use a different set of radio frequencies that do not interfere with each other. Specific sets of radio frequencies are allocated for use in cellular communications systems. Typically a government agency is responsible for assigning these frequencies and licensing them to specific service providers.

Paging
Paging is a wireless system that is capable of delivering message to one or more people whose exact whereabouts are unknown by the sender of the message. Users typically carry a small paging receiver that displays a numeric or alphanumeric message displayed on an electronic readout. It also could be sent and received as voice message or other data. Pager systems may be classified based upon their capabilities as numeric, alphanumeric, tone, or voice.

Paging networks are comprised of terrestrial based antennas all interconnected by means of some other type of physical network. Satellite transmission systems are often used to distribute the messages to multiple towers for retransmission. The use of satellite interconnection systems permits regional, nationwide, or global paging service. Some paging systems are capable of providing two-way communications.

Each pager in the network has a unique identifier (address) that is stored to the pager’s memory. This address is also stored in the paging network. When an incoming telephone call accesses the pager customer’s account, the network transmits the address of the pager on its radio tower. In addition to sending a specific paging address, the system may send other digits, text, or even voice information that follows the addressing message.

Wireless Data
Wireless data networks transfer data between network access points (wireless data devices) through radio transmission and are primarily designed to transfer data from one point to one or more points (multipoint). These networks may be composed of a variety of communication systems including: mobile data terminals, radio access nodes, packet switching networks. Wireless data networks provide a variety of services including: equipment status monitoring, dispatch services, vehicle and goods tracking, credit card validation, and include wireless Internet access.

Broadcast
Broadcasting is a process that sends voice, data, or video signals simultaneously to group of people or companies in a specific geographic area or who are connected to the broadcast network system (e.g., satellite or cable television system). It is typically associated with radio or television radio transmission systems that send the same radio signal to many receivers in a geographic area. Broadcasting can also be applied to wired distribution or point-to-point networks where all users that are connected to the network can receive the same information signal.

Figure below shows the different types of wireless networks. This diagram shows a private land mobile radio system, television broadcast system, paging system, mobile telephone system, and satellite communication system. Although all wireless networks can transmit information from one point to another, different types of networks better suited to provide specific types of services (e.g., paging compared to television broadcasting).


Wireless Networks