This chapter is from the book
This chapter is from the book
This chapter is from the book
Physical Layer Wireless-Interface Features
Wireless features operate at the physical layer; therefore, your wireless-interface feature evaluation covers a broad range of features. The following sections cover these feature categories:
NLOS—Non line-of-sight and near line-of-sight equipment capabilities
Wireless frequency bands—Propagation characteristics and equipment availability for each of the license-free bands
Modulation types—Various types of modulation used by license-free equipment
Bandwidth and throughput—Tradeoffs between data rate, data throughput, and link distance
Noise and interference-reduction features—Receiver and antenna features that improve signal reception abilities
Security—Physical layer wireless security features
Miscellaneous wireless features—Transmit and receive features that can play a significant role in the performance of your wireless network operation
Non Line-of-Sight Features
In the broadband wireless industry, there is no agreement about the exact meaning of the term NLOS. Here are two common ways that NLOS is used:
Near line-of-sight—When equipment vendors state that their equipment has near-LOS capabilities, they are claiming that it operates satisfactorily even when there is a partially obstructed line-of-sight path, as long as there are not too many obstacles to the line-of-sight path. For example, perhaps a few trees are intruding into the Fresnel zone.
Non line-of sight—When equipment vendors state that their equipment has non-LOS capabilities, they are claiming that it operates satisfactorily even when there is an obstructed line-of-sight path. For example, perhaps buildings, trees, and hills are completely blocking the path.
Because there is no standard definition of NLOS, the process of evaluating NLOS performance claims is a challenging one. Almost all vendors of NLOS equipment (either accidentally or intentionally) exclude information about the range of their NLOS equipment. The impression is left with the customer (you) that the NLOS equipment has the same communications range as LOS wireless equipment. This is never the case; the range of NLOS equipment is always substantially less than the range of equipment that is operating over a true, unobstructed LOS path.
Now, you will learn about features that actually improve performance in NLOS environments. Two significant challenges that an NLOS environment presents for wireless equipment are as follows:
Multipath—Any equipment feature that improves performance in a multipath environment also improves performance in an NLOS environment. These features are as follows:
Diversity antennas
Circularly polarized antennas
Smart antennas that constantly adjust their beamwidth to receive and transmit energy directly to and from each individual end user antenna
Adaptive equalization
Multicarrier modulation, such as OFDM
Whenever possible, always try to design your wireless WANs to use LOS paths. You will achieve more reliable coverage at longer distances.
Attenuation—Attenuation losses in a non-LOS environment are the reason that the communications range in an NLOS environment is less than in a LOS environment. The following equipment and network features reduce attention and improve NLOS performance:
- Receiver sensitivity
- 900 MHz frequency band
- Mesh networks
Wireless Frequency Bands
The following sections describe the license-free frequency bands, including the propagation characteristics and the power levels of each band.
900 MHz
900 MHz is the lowest-frequency industrial, scientific, and medical (ISM) band. The total width of the band is 26 MHz. Signals in this band have a wavelength of approximately 12 inches (30 cm). These signals have the capability to pass through some obstructions without being completely lost. For example, they can pass through light trees and diffract over one low hill and still be strong enough to be received several miles away. 900 MHz is the best band to use when there are just a few obstacles to the LOS path. Table 6-1 shows 900-MHz power levels.
Table 6-1 Power Levels for the 900-MHz Band
|
Band |
Maximum Transmitter Power |
Maximum Antenna Gain |
EIRP (Equivalent Isotropic Radiated Power) |
|
902 to 928 MHz |
+30 dBm (1 Watt) |
+6 dBi |
+36 dBi (4 Watts, relative to an isotropic antenna) |
2.4 GHz
2.4 GHz is the middle ISM band. The total width of the band is 83 MHz. Signals in this band have a wavelength of approximately 4.8 inches (12 cm). These signals have little capability to pass through obstructions without being lost. Passing through one wall can result in 10 to 12 dB of attenuation. Attenuation from trees varies depending on the presence of leaves and whether the leaves are wet or dry but, on average, the attenuation from trees is approximately .5 dB per meter. One 30-ft (10-meter) diameter tree (the tree canopy/leaves are 30 feet across, not the tree trunk) results in about 5 dB of attenuation; 6 dB of attenuation reduces the length of a wireless link to 1/2 of its previous length. You can see that passing a 2.4-GHz signal through a few trees can easily reduce the usable length of the wireless path to a few hundred feet. Table 6-2 shows 2.4-GHz power levels.
Table 6-2 Power Levels for the 2.4 GHz Band
| Band |
Maximum Transmitter Power |
Maximum Antenna Gain |
Maximum EIRP |
| 2403 to 2483 MHz (Point-to-Multipoint) |
+30 dBm (1 Watt) |
+6 dBi |
+36 dBi (4 Watts). |
| 2403 to 2483 MHz (Point-to-Point only) |
+30 dBm (1 Watt) |
(3-to-1 Rule) For every 3 dBi (above +6 dBi) of antenna gain, reduce the transmitter power by 1 dB. (For example, for a +9 dBi antenna, reduce transmitter power to +29 dBm.) |
Depends on antenna size. With a +24 dBi antenna and +24 dBm of transmitter power, +48 dBi (64 Watts) is possible in a Point-to-Point (only) link. |
| 2403 to 2483 MHz Wideband frequency hopping spread spectrum using from 15 to 74 hopping frequencies |
+21 dBm (125 mW) |
+6 dBi |
+27 dBi (500 mW). |
3.5 GHz
The 3.5-GHz band is not available for use in the United States; however, some frequency subbands between 3.3 and 4.0 GHz are available for use (usually on a licensed basis) in a number of other countries. This band is mentioned here because equipment for this band is, in many cases, similar to equipment for the 2.4-GHz band. Signals in this band have a wavelength of approximately 9 cm (3.4 in). The propagation characteristics are somewhat similar to the 2.4-GHz band, although attenuation from trees and other obstructions is higher.
5 GHz
There are four license-free subbands at 5 GHz, although two of these bands overlap each other. There is one ISM band from 5725 to 5850 MHz (5.725 to 5.850 GHz), and there are three Unlicensed National Information Infrastructure (U-NII) bands: 5150 to 5250 MHz, 5250 to 5350 MHz, and 5725 to 5825 MHz. The ISM band is 125 MHz wide, and each U-NII band is 100 MHz wide. Signals in the 5-GHz subbands have a wavelength of approximately 2 inches (5 cm). Each 5 GHz subband is wider than the entire 2.4-GHz band; therefore, it is possible to build 5-GHz wireless equipment that provides more bandwidth and more throughput than equipment for any other license-free band. The attenuation from trees at 5 GHz is about 1.2 dB per meter; therefore, each 30-ft (10-meter) diameter tree (crown) that blocks an LOS path reduces the length of a wireless link by approximately 75 percent. Table 6-3 shows 5-GHz power levels.
Table 6-3 Power Levels for the 5-GHz Band
|
Band |
Maximum Transmitter Power |
Maximum Antenna Gain |
EIRP |
|
ISM 5725 to 5850 MHz |
+30 dBm (1 Watt) |
+6 dBi |
+36 dBi (4 Watts). Note that point-to-point systems can use an antenna with more than +6 dBi gain with no transmitter power reduction. |
|
U-NII 5150 to 5250 MHz |
+17 dBm (50 mW) |
+6 dBi |
+23 dBi (500 mW; indoor use only per FCC regulations.) |
|
U-NII 5250 to 5350 MHz |
+24 dBm (250 mW) |
+6 dBi |
+30 dBi. (1 Watt). |
|
U-NII 5725 to 5825 MHz |
+30 dBm (1 Watt) |
+6 dBi |
+36 dBi (4 Watts) Note that point-to-point systems can use an antenna with up to +23 dBi gain with no transmitter power reduction. |
60 GHz
The 59 to 64-GHz ISM band was approved for use in the United States in 1999. The total width of this band is almost 5 GHz. Signals in this band have a wavelength of about 2/10 of an inch (1/2 cm). Signals at this frequency are attenuated by the presence of oxygen in the air; therefore, the maximum wireless link distance is approximately half a mile (800 m), assuming that a LOS path is available. Obstructions completely block the signal. The advantage of this band is that equipment is available that provides point-to-point raw data rates up to 622 Mbps. In addition, the oxygen absorption means that the likelihood of interference from other networks is low.
Modulation Types
This section covers the following information:
A quick review of the modulation process
A direct sequence spread spectrum (DSSS) description
A frequency hopping spread spectrum (FHSS) description
An orthogonal frequency division multiplexing (OFDM) description
A brief mention of other spread spectrum and non-spread types of modulation
Understanding the Modulation Process
Chapter 1, "An Introduction to Broadband License-Free Wireless Wide-Area Networking," defined modulation as the process of adding intelligence to the signal. The modulation process creates a change in some combination of the amplitude, the frequency, or the phase of a signal. Many types of modulation exist, including amplitude modulation used by commercial AM broadcast stations and frequency modulation (FM) used by police departments and fire departments, for example.
Spread spectrum modulation was originally designed for use by the military to camouflage the existence of and the content of military communications. Descriptions of the two types of spread spectrum modulation follow.
Direct Sequence Spread Spectrum (DSSS) Modulation
DSSS modulation simultaneously widens (spreads) a data signal out and reduces the amplitude (technically, it reduces the power density) of the signal. The resulting modulated signal resembles a low-level noise signal that is widely dispersed around a single frequency. The modulated DSSS signal is wider than the bandwidth of the original data. For example, an 11-Mbps raw data rate signal becomes a 22-MHz-wide DSSS signal. In the 2.4-GHz frequency band, there is enough room for three nonoverlapping 11 Mbps-wide signal channels. Each time a DSSS signal is transmitted, the wireless energy is centered around only one frequency; therefore, DSSS modulation is a single-carrier modulation scheme.
Frequency Hopping Spread Spectrum Modulation
Frequency Hopping Spread Spectrum (FHSS) modulation does not spread its signal energy out, but it rapidly shifts the energy from frequency to frequency. A narrowband FHSS signal is transmitted first on one narrow (1-MHz) channel and then quickly shifted to another channel, then another, and another, and so on. The rapid frequency hopping gives this type of modulation its name. The two following types of FHSS are now allowed in the 2.4-GHz band:
Narrowband frequency hopping—Narrowband FHSS signals are 1 MHz wide. They hop using a total of 79 different frequencies. The signal can hop between these frequencies in 78 unique hopping patterns or hopping sequences. 802.11 standards define narrowband frequency hopping.
NOTE
Hopping sequences are sometimes different in different countries. Check with your national telecommunications authority for the regulations in your country.
Wideband frequency hopping—Wideband FHSS signals can be up to 5 MHz wide and can hop using a total of less than 75 different frequencies. Typical wideband FHSS systems use far less than 75 frequencies; one such system uses 43 frequencies with a signal that is 1.7 MHz wide. Wideband frequency hopping systems are relatively new and are just beginning to be deployed in outdoor wireless WANs.
FHSS equipment changes its center frequency each time it hops; however, each time it transmits, the wireless energy is still centered on only one frequency. FHSS is, therefore, a single-carrier modulation scheme.
Orthogonal Frequency Division Multiplexing Modulation
Orthogonal Frequency Division Multiplexing (OFDM) modulation transmits bursts that use more than one carrier frequency simultaneously. Compared to a DSSS signal, an OFDM signal has the following characteristics:
Occupies the same amount of bandwidth
Uses 52 carriers instead of one carrier
Carries more information with each transmitted burst
Is more resistant to multipath fading
802.11a equipment uses OFDM modulation and operates on the 5-GHz band; 802.11g uses OFDM on the 2.4-GHz band. OFDM is a multicarrier modulation scheme because it transmits using more than one carrier simultaneously.
Other Spread Spectrum Modulation Types
Other types of spread spectrum modulation are now legal to use. These versions are proprietary to particular manufacturers and do not interoperate with 802.11b, 802.11a, or 802.11g systems.
One example of a proprietary spread spectrum modulation type is multicarrier DSSS. Rather than using just one DSSS carrier, multicarrier DSSS uses several simultaneous carrier frequencies to transport data. This multicarrier approach is a hybrid combination of single-carrier and multicarrier modulation.
Other Nonspread Spectrum Modulation Types
In the ISM bands, the FCC originally required that spread spectrum modulation be used. Recent rule changes now allow additional modulation types. In the U-NII bands, nonspread spectrum modulation types are permitted, and equipment manufacturers use proprietary digital modulation schemes to offer a variety of high-bandwidth point-to-point and point-to-multipoint systems.
Bandwidth and Throughput
It is important for you to understand wireless throughput so that you can meet (or exceed) the expectations of your wireless end users. This section describes the following:
The difference between the wireless data rate and the wireless throughput
The tradeoff between throughput and distance
Examples of low, medium, and high throughput equipment
Comparison Between Data Rate and Throughput (Including Simplex Versus Duplex Throughput)
There is a common misunderstanding regarding the bandwidth, the data rate, and the throughput of a wireless device:
Bandwidth refers to the raw data rate of the device.
Throughput refers to the actual amount of end user data that the device can transfer in a given time interval.
The result of this misunderstanding is that wireless network users are frequently disappointed in the wireless throughput (data transfer speeds) that they experience.
Understandably, wireless equipment manufacturers want their equipment to look as attractive as possible to potential buyers. For this reason, they usually use the raw data rate in their sales and advertising material. An 802.11b AP, for example, provides a raw data rate of 11 Mbps.
Wireless users have a different expectation; they are interested in how fast a web page or a file downloads. They are interested in the capability of the wireless device to deliver their data. When the wireless users' 802.11b AP delivers just 5.5 Mbps of data throughput, they feel that there must be a problem with the equipment.
Most frequently, the real data throughput potential of a half-duplex wireless network is approximately 50 percent of the raw data rate. An 802.11b AP operating at the maximum 11-Mbps raw data rate has a maximum throughput potential of about 5.5 Mbps. This difference between raw data rate and actual throughput has several causes, including these:
The framing and signaling overhead
The half-duplex turnaround time between transmit and receive
The lower efficiency inherent in the transmission of small packets
Collisions between wireless users and interference from other networks can reduce the throughput below 50 percent. Chapter 8, "Solving Noise and Interference Problems," discusses this issue in more detail.
Remember that your end users rely on you to set their throughput expectations realistically. When they measure their throughput and discover that it meets or slightly exceeds the throughput that you told them to expect, they will judge your wireless network performance to be good.
Tradeoff Between Data Rate and Distance
As you evaluate wireless equipment, you will invariably compare different equipment brands based on how long of a link they can support. Link distance is important; however, during your comparison, it is important that you compare apples to apples. When you compare two brands of wireless equipment side by side, you must compare their link distances at the same data rate. Other factors being equal, the higher the throughput (or the higher the raw data rate), the shorter the communications range. Table 6-4 lists the typical outdoor link distances from an 802.11b AP (using a standard low-gain omnidirectional antenna).
Table 6-4 Examples of 802.11b Data Rates Versus Distances
|
Data Rate |
Distance in Ft. (m) |
|
11 Mbps |
500 (152) |
|
5.5 Mbps |
885 (270) |
|
2 Mbps |
1300 (396) |
|
1 Mbps |
1500 (457) |
As the data rate increases, the maximum AP link distance decreases. AP data rates automatically fall back to the next lower level when the AP detects the signal quality decreasing as the link distance increases.
Sub-1 Mbps Data Rates
Two types of wireless systems operate at sub-1 Mbps data rates:
Low-speed (such as 4800 bps to 128 kbps) wireless modems that provide a point-to-point wireless extension for an RS-232 serial data system.
128 kbps to 1 Mbps point-to-point or point-to-multipoint wireless Ethernet bridges or AP systems. These systems are useful for Internet access.
1-Mbps to 11-Mbps Data Rates
Most point-to-multipoint wireless WAN systems are in this category. This category also includes both 802.11 (2 Mbps) and 802.11b (11 Mbps) systems.
12-Mbps to 60-Mbps Data Rates
This category includes both high-bandwidth point-to-point backbone equipment and point-to-multipoint equipment. Here are some examples:
Point-to-point equipment is available with bandwidths of 12 Mbps, 20 Mbps, 24 Mbps, and 45 Mbps.
Point-to-multipoint equipment is available with shared, aggregate bandwidths of 20 Mbps, 40 Mbps, 54 Mbps, and 60 Mbps.
802.11a WLAN equipment is becoming available. However, as this book was being written, this equipment was not yet appropriate for use in outdoor wireless networks. The power level was too low, and there is no connector to attach an outdoor antenna.
Over 60-Mbps Data Rates
The higher in frequency wireless equipment operates, the more bandwidth that is available. Products that provide more than 60 Mbps of bandwidth operate almost exclusively in the 5.3- and 5.8-GHz U-NII bands, although a few short-range products operate in the 60-GHz band. Products that operate in these bands have aggregate bandwidths of 90 Mbps, 100 Mbps, 155 Mbps (OC-3), 200 Mbps, 480 Mbps, 622 Mbps (OC-12), and 872 Mbps. All these are full-duplex products.
Noise and Interference Reduction Features
Noise is defined as anything and everything other than the desired signal. Interference reduces the throughput of a wireless network. Interference has many sources, so it is important that you consider and utilize all possible noise-reduction features.
NOTE
Chapter 8 is devoted completely to the topic of understanding and minimizing the effects of noise and interference. Refer to Chapter 8 for additional information as you read about the following interference-reduction features.
In the outdoor wireless environment, many potential interference sources exist. You can use a few equipment characteristics to provide some help in minimizing the effects of interference.
The following interference-reduction features operate at the physical layer to help reduce the effects that interference can have on both AP and CPE throughput.
Receiver Selectivity
Selectivity is the capability of a receiver to reject signals that are not exactly on the desired receiving frequency. No receiver is perfectly selective; no receiver has the capability to completely reject all off-frequency signals; therefore, all receivers are susceptible to being overloaded by nearby, strong off-frequency signals. These off-frequency signals can be within the license-free band (in-band interference), or they can be outside the band (out-of-band interference).
Overloading causes a receiver to become desensitized (to experience a reduced sensitivity) to the desired signals. The symptom of a desensitized receiver is a reduction in the receiving distance. Some receivers allow you to configure a higher receive threshold level. This feature enables you to intentionally reduce the sensitivity and therefore reduce the intensity of the overloading. This feature is similar to a "squelch" control on an FM two-way radio.
It can be difficult for you to compare receiver selectivity and to predict overload resistance because most manufacturers do not publish overload specifications. Keep the following general guidelines in mind as you evaluate wireless equipment:
Wireless equipment that is designed for outdoor WAN use should be less susceptible to being overloaded when compared to indoor wireless LAN equipment.
Wireless equipment that is designed for indoor LAN use is likely to be more susceptible to being overloaded when used outdoors.
All wireless equipment might need to have an external bandpass filter added when there are one or more strong, nearby transmitters such as FM, AM, or television broadcast transmitters.
Multipath Resistance
Multipath fading is a fact of life at microwave frequencies. Multipath is caused when signal reflections cause several signals (echoes) to be received almost simultaneously. Equipment features that minimize the effects of multipath include the following:
Antenna diversity—Antenna diversity helps minimize multipath by using two separate antennas. The antennas are separated from each other, and when the signal fades, one antenna receives a stronger signal than the other antenna. The receiver automatically selects the strongest antenna signal on each incoming packet, so fading is reduced.
Circular antenna polarization—Circularly polarized antennas discriminate against multipath interference. Equipment that offers the option of using a circularly polarized antenna provides more protection against multipath compared to equipment without a circularly polarized antenna.
OFDM—Equipment that uses orthogonal frequency division multiplexing modulation provides more immunity to multipath interference compared to non-OFDM equipment.
Multipath interference is worse in a physical environment where you find many obstacles that reflect wireless signals. The center of a city with many tall, flat, reflective metal building surfaces is a high multipath environment. If you plan to deploy wireless service in a high-multipath environment, use as many multipath-reduction features and techniques as possible.
Miscellaneous Interference Reduction Techniques
There are many sources of noise and interference besides multipath. The following miscellaneous features help reduce the distance-robbing effects of noise and interference:
Selectable antenna polarization—Interference from other wireless systems is usually either vertically polarized or horizontally polarized. Equipment that allows you to select either vertical or horizontal polarization allows you to minimize interference from other systems by selecting polarization opposite to other, interfering networks.
Smart antenna technology—Smart antennas enable the antenna pattern beamwidth to be automatically adjusted under software control. In this way, the antenna pattern can be automatically steered to minimize or avoid interference. Smart antenna technology is a relatively new technology that is just beginning to appear in license-free wireless WAN equipment.
Smart radio features—Smart radio features include the radio's capability to automatically scan the available frequencies and to choose the frequency with the least amount of interference. Automatic power adjustment is another smart radio feature. The wireless equipment measures the strength and the quality of the received signal and adjusts the transmit power level up or down to maintain the desired link quality. Using only the amount of power needed minimizes the interference to other wireless systems.
Physical Layer Wireless Security Features
There are a number of physical layer wireless security features as well as many higher-layer security features. The following sections describe the main physical layer security features.
Antenna Pattern/Signal Strength
Although not immediately obvious, antenna directivity provides a certain measure of security. Unauthorized wireless users must physically position themselves in an area where a usable signal exists. This is another reason to carefully consider where you radiate your signal. Rather than broadcasting it everywhere, use directional antennas to radiate only into the areas where your end users are located.
Modulation Type
Like antenna directivity, modulation type is a not-so-obvious security feature. If a wireless network uses DSSS, a hacker must use the same DSSS modulation type. Likewise, if a network uses FHSS, a hacker must use FHSS. If a network uses another proprietary modulation type, an unauthorized user must use the same proprietary modulation type. Therefore, proprietary modulation types provide a higher level of physical layer security than 802.11b, for example.
Network ID (SSID, ESSID)
Several different logical networks can exist in the same physical space. Wireless packets contain a service set identifier (SSID), extended service set identifier (ESSID), or network ID to specify the logical network that a wireless station belongs to. The ESSID is a basic network security feature. If a wireless station does not possess the correct ESSID (or network ID), it cannot connect to a wireless network.
Miscellaneous Wireless Features
This section describes miscellaneous transmit and receive features. Although these features cannot be neatly classified into a specific section, their presence or absence can play a significant role in the performance of your wireless network operation; evaluate them carefully.
Miscellaneous Transmit Features
The following miscellaneous transmitter features can affect the design and performance of your wireless WAN:
Transmitter output power—Most license-free wireless equipment is limited by Federal Communications Commission (FCC) regulations to one watt (+30 dBm) of transmitter output power. Available transmitter output power levels typically vary from 1 watt (1W) down to 200 mW, 100mW, 50 mW, and 30 mW.
TIP
The role of transmitter power in the successful operation of a wireless network is often misunderstood. Many people believe that more power is always better; however, this is not true in many cases. Your best approach is to transmit with only the amount of power that you need to cover your desired service area. Transmitting with too much power results in a transmitting range that is larger than your receiving range. This causes unnecessary interference to other networks. The owner of the other networks might then feel the need to retaliate with excessive transmitter power, which can lead to a cycle of escalation in which everyone loses.
Configurable transmitter power control—A few models of wireless equipment allow you to configure the transmitter output power; however, for most wireless equipment, the power output is not configurable. Only one or two equipment models exist where the AP automatically configures the transmitter power of the end user nodes. The purpose of automatic power control is to use only the power needed for a reliable link. Avoiding the use of excessive power minimizes interference between the end user nodes.
Miscellaneous Receive Features
The following receive features affect the performance of your wireless WAN in many ways:
Receiver threshold—A receiver starts working (receiving and decoding an incoming signal) when the signal reaches the receiver threshold level. Signals below the threshold are either not received or are received with numerous errors. Signals above the threshold are received with a low error rate. The low error rate allows the wireless link to deliver maximum throughput. If you are comparing two different receiver thresholds, the receiver with the lower threshold receives over a longer distance. For example, a receiver with a –85 dBm threshold is better than a receiver with a –80 dBm threshold.
NOTE
When comparing receiver thresholds, compare the threshold values at the same data rate. Comparisons at different data rates are invalid because as the data rate goes up, a receiver's threshold goes up. Stated another way, as the data rate goes up, the receiver becomes less sensitive.
Noise figure—Receivers create noise in their circuitry. Noise figure refers to internal noise or the relative lack of internal noise created by the receiver. The lower the internal noise, the better a weak signal is received. A 3-dB noise figure is better than a 6-dB noise figure, for example.
Miscellaneous Transmit/Receive Features
The following features, when present, apply on both transmit and receive:
AP and bridge—Some wireless APs can be used either as an AP (connecting to many end users) or as a bridge. An AP with bridging capability provides you with more network flexibility than an AP without the capability to work as a bridge.
AP and repeater—Most APs can serve both as an AP and as a repeater at the same time.
Number of wireless ports—Most wireless equipment has one wireless port. Some equipment has more than one wireless port. Multiport equipment can operate simultaneously on more than one frequency or more than one band. One example is an AP that has one 2.4-GHz and one 5-GHz wireless port.
External antenna connector—Wireless WAN equipment must always be connected to an antenna that has LOS paths to the end users. Except in the case of CPE that has the radio integrated with the antenna, this means that the wireless equipment must have a connector for an external antenna. Equipment that is designed to be used indoors often lacks a connector for an external antenna.
Split (indoor/outdoor) hardware architecture—Indoor/outdoor architecture splits the wireless hardware. The microwave part of the equipment is placed outdoors, near the antenna. The low-frequency part of the equipment is placed indoors. The two halves of the radio are connected with either coax or fiber. With a split architecture, coax cable losses between the microwave section and the antenna are almost eliminated, consequently improving the wireless performance.
Integrated antenna/radio—With increasing frequency, wireless equipment (especially 802.11b) equipment is becoming available with the radio physically located inside the antenna. Integrated equipment has the same advantage as split-architecture equipment—eliminating transmission line losses to improve wireless performance. The connection from the antenna/radio to the end user network is made with Ethernet cable. Power-over-Ethernet (PoE) to the antenna and radio is provided using the nondata conductors in the Ethernet cable.
Multifrequency management commonality—A few equipment vendors now offer a wireless equipment family that operates on different frequency bands but can be managed from a common management platform. This equipment provides management economies for those wireless ISPs that need to deploy wireless systems on different bands.
Antenna alignment aids—Some equipment, especially split architecture or integrated antenna and radio equipment, provides visual or aural antenna alignment aids. These aids, typically a series of LEDs or an audible tone, help the installer align the antenna for the highest signal level without leaving the antenna location.
Availability of FCC-certified antenna systems—Most equipment vendors provide at least one antenna system that is FCC-certified for use with the equipment. Some vendors provide a number of certified antenna systems. The more vendor-certified antenna systems are available, the more flexibility you have to use an antenna system that provides the service-area coverage that you need.