ACMA Capítulo 1

WLAN Organizations

There are several organizations that develop WLAN standards (Figure 1-1). These regulatory bodies restrict how RF technology is deployed. Organizations involved in these regulations include the following:

• Internet Engineering Task Force (IETF)—International standards.

• Federal Communications Commission (FCC)—Local regulatory domain for RF spectrum (United States).

• The Institute of Electrical and Electronic Engineers (IEEE)—802.11 standards.

• The Wi-Fi Alliance—WLAN standards.

The IETF is an international standards organization for the Internet.

The FCC places certain restrictions on power and channel usage in the US.

In other words, the FCC controls which frequencies in the spectrum we can use and at what power settings we can use them. The IEEE committee standardizes how data is transmitted over those frequencies.

RF bands and channels

2.4 GHz ISM band and channels

The 2.4 GHz ISM (Industrial, Scientific and Medical) Band is used by 802.11, 802.11b, 802.11g, and 802.11n. However, the channels are only 5 MHz apart, so adjacent channels overlap. When two physically nearby APs use overlapping channels, they can hear each other’s RF signals. This results in interference, which can severely degrade performance. This specific type of interference is often called “co-channel interference.”

So while there are 11 channels available in the US (13 in Europe, 14 in Japan), only channels 1, 6, and 11 are actually used.

5 GHz U-NII bands and channels

802.11a, 802.11n, and 802.11ac use the 5 GHz Unlicensed National Information Infrastructure (U-NII) bands, as defined by the FCC. Like the 2.4 GHz band, actual channel availability can vary by country.

•U-NII 1, also known as Lower U-NII

•U-NII 2, also known as Middle U-NII

•U-NII 2E, also known as U-NII 2 Extended

•U-NII 3, also known as Upper U-NII

Originally, U-NII bands 1–3 were defined for use by 802.11a. Each of these three bands was broken into four useable channels.

Soon thereafter, the U-NII 2E band was made available, adding an additional 11 channels in the 5 GHz range. (Again, depending on the country). Originally, the Lower U-NII could be used for indoor use only, the Middle U-NII for indoor or outdoor use, and the Upper U-NII for outdoor use only. This meant that you were likely to have 8 U-NII channels that were useable for indoor WLANs. Only channels that are directly next to each other have any risk of interfering. They are usually at low enough power that there is minimal or no interference. Still, it is typically considered a best practice. So, unlike the 2.4 GHz spectrum, all 5 GHz channels are usable, without risk of co-channel interference. However, many client devices lack support for certain 5 GHz channels. This is especially true of the U-NII 2e channels. The U-NII 2 extended bands can be problematic for actual real-world use, as many NICs either do not support them, or only scan for them after other channels are scanned. However, even without the use of the U-NII 2E channels, there are still many more nonoverlapping channels in 5 GHz than 2.4 GHz.

Channel bonding

One of the performance enhancements available in 802.11n and 802.11ac is channel bonding. Channel bonding technically does not bond two channels together It is actually using the frequency ranges of those two channels and treating them as a single, much wider channel. 802.11n is also referred to as High Throughput or HT. HT20 indicates 802.11n is deployed without channel bonding, using a standard 20 MHz wide channel. HT40 indicates 802.11n is deployed with channel bonding, using a 40 MHz wide bonded channel.

5 GHz Channel Bonding

As previously described, each 5 GHz channel is 20 MHz wide. The 802.11a standard can leverage each separate channel for communications. With bandwidth limited to a 20 MHz-wide spectrum, (and due to limitations in encoding methods) 802.11a was limited to a maximum data rate of 54 Mbps.The newer 802.11n standard supports both the 2.4 GHz and 5 GHz spectrum. The 802.11n standard introduced a new concept called “Channel Bonding.” This allows you to “bond” two adjacent, 20 MHz-wide channels into a single 40+ MHz-wide path. Users now have over twice the bandwidth. Along with superior encoding methods, maximum theoretical data rates reach 600 Mbps.The 802.11ac standard further capitalized on this channel-bonding capability, enabling you to bond up to FOUR channels (802.11ac Wave 1) or even EIGHT channels (802.11ac Wave 2), for very high potential bandwidths. However, there is a point of diminishing returns—if you convert twelve 20 MHz channels into three 80 MHz channels, you increase co-channel interference. For this and other reasons, most engineers agree that either 20 MHz or 40 MHz channel width is currently optimal, overall, for a corporate WLAN deployment.

The following list shows channels defined for 5 GHz bands (US regulations) and lists the number of 20, 40, 80, and 160 MHz channels available:

US U-NII I and U-NII II bands:

U-NII I: 5150-5250 MHz (indoors only)

U-NII 2: 5250-5350 MHz

• 8x 20 MHz channels

• 4x 40 MHz channels

• 2x 80 MHz channels

• 1x 160 MHz channel

• U-NII II requires DFS (& TPC if over 500mW/27dBm EIRP.

US intermediate band (U-NII 2 extended):

U-NII 2E 5450-5725 MHz

• 12x 20 MHz channels

• 6x 40 MHz channels

• 3x 80 MHz channels

• 1x 160 MHz channel

• Requires DFS (& TPC if over 500mW /27dBm EIRP)

• 5600-5650 MHz is used by weather radars and is temporarily not available in the US.

US U-NII 3/ISM band

U-NII 3 5725-5825 MHz

• 5x 20 MHz channels

• 2x 40 MHz channels

• 1x 80 MHz channel

• Slightly different rules apply for channel 165 in ISM

802.11 standards and amendments

Compare 802.11a/b/g/n/ac data standards

The 802.11 standard was originally ratified in 1997. It was capable of transmitting at 1 and 2 Mbps using either Frequency Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS). The 802.11 standard used the 2.4 GHz spectrum, as recently discussed. This band has been referred to as the Industrial Scientific and Medical (ISM) band.

The 802.11b and 802.11a amendments were both ratified at the same time in 1999. 802.11b products shipped immediately, but 802.11a products did not ship until almost a year later. 802.11b provided an upgrade to 802.11. 802.11b uses DSSS to support transmission speeds of up to 11 Mbps. It provided backward compatibility with 802.11 DSSS and was the first WLAN technology to gain widespread consumer acceptance. This was mainly due to decreased equipment prices. Like 802.11 and 802.11b, 802.11g operates in the 2.4 GHz spectrum, and so is downward compatible with 802.11b. However, instead of DSSS, 802.11g uses a modulation technique called Orthogonal Frequency Division Multiplexing (OFDM). This is the same modulation technique used by 802.11a, and so both 802.11g and 802.11a support the same theoretical data rates, up to 54 Mbps. The 802.11n amendment increased the data rate to approximately 300 Mbps and theoretically 600 Mbps maximum. 802.11n added the term High Throughput (HT), indicating speeds up to 300 Mbps for WLANs.

The latest amendment to the 802.11 standard is 802.11ac, which only operates in the 5 GHz range. 802.11ac features theoretical data rates up to 6.93 Gbps. Speeds from 150 Mbps and up are also referred to as Very High Throughput or VHT.

802.11h—DFS and TPC

The two enabling features of 802.11h are called Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC). With DFS, Access Points constantly listen for RF energy that is likely sourced by some radars or similar systems. If this is detected, the AP must move to a new channel. DFS finds an appropriate, noninterfering channel, and informs all clients that it is moving to this channel. The AP automatically restarts on the new channel, and the clients automatically reassociate to the AP.

TPC offers another level of protection from interference. When devices communicate, they negotiate toward the minimum power level that still allows reliable communications. The objective is to maintain effective data transfer for the WLAN, while minimizing the potential to interfere with other devices.

802.11e—Wireless Quality of Service

The IEEE’s 802.11e standard defines a Quality of Service (QoS) mechanism to define appropriate levels of service for specific applications. Wi-Fi Multimedia (WMM) is a subset of the 802.11e standard, which has become more commonly supported on end systems. As typically deployed, specific application traffic is placed into one of four queues.

This traffic is categorized and organized into queues. There is a special queue reserved for Voice traffic, and another queue for Video traffic. Other business-critical application traffic is placed in the Best Effort queue. Finally, there is a queue labeled Background that may be used for “scavenger-class” traffic—for applications that are not business critical.

802.11i Security

802.11i is an IEEE security standard that supports authentication, encryption, and hashing.

Authentication ensures only valid users can connect. This can be accomplished using a passphrase, which is a preshared ASCII text string. Another option is to assign unique usernames, passwords, and/or certificates to each user. These credentials are authenticated against a back-end RADIUS server.

Encryption prevents attackers from seeing packet data. This is based on the Advanced Encryption Standard (AES) algorithm and uses CCMP (see note).

Hashing algorithms offer protection against man-in-the middle attacks, or anti-replay. This detects if an attacker captures a packet, modifies the data, and then resends it. Hashing is also based on the AES algorithm.

The IEEE’s 802.11i standard is implemented by the Wi-Fi alliance as Wi-Fi Protected Access version 2 (WPAv2). In other words, WPAv2 uses the same AES-based protocols as 802.11i, for both encryption and hashing.The Wi-Fi alliance’s WPAv1 standard uses the same RC4 encryption algorithm used by the outdated and insecure Wired-Equivalent Privacy (WEP). However, WPAv1 vastly improves upon the security by adding an encryption “wrapper” called the Temporal Key Integrity Protocol (TKIP). Unlike WEP, WPAv1 also provides hashing, based on the “Michael” algorithm in which a Message Integrity Code (MIC) is created for each frame. WPAv1’s algorithms can also be used to support legacy devices in an 802.11i network.

Notice that WPA-Personal and WPA-Enterprise use the same encryption protocol and algorithm for security, but use different authentication. WPA-Personal uses a simple Preshared Key (PSK) method of authentication. This is a much simpler authentication mechanism than WPA-Enterprise, which uses EAP/RADIUS authentication to a RADIUS server.

Other 802.11 Standards

802.11k (Radio measurement standard) improves a WLAN Client’s search for nearby APs, which could be potential roaming targets. It does this by creating an optimized list of channels. When the signal strength of the current AP weakens, your device scans for target APs from this list.

802.11v (Improved transition between access points) enables smooth client transition between access points, using a technique called Basic Service Set (BSS) transition-management. The network’s control layer provides endpoints with the load information for nearby access points. Supporting clients take this information into account when deciding among possible roam targets. Support for this protocol continues to increase, but some clients may not yet support it.

802.11r (Roaming standard) facilitates client roaming between APs on the same network. 802.11r uses a feature called Fast Basic Service Set Transition (FT) to authenticate more quickly. FT works with both PSK and 802.1X/EAP authentication methods.

802.11ax (Higher throughput data standard) is a new WLAN data standard due to be ratified in 2019. It is designed to improve overall spectral efficiency. It is predicted to support data rates of up to approximately 10 Gigabits/second.

802.11 Frame Types

The following describes the three 802.11 frame types and the most important subtypes associated with each frame type.

Management Frames are used to establish, control, and maintain client connections.

APs broadcast periodic Beacon frames—“Here I am,” along with basic information about the WLAN. This facilitates an endpoint’s passive discovery of APs and WLANs.

Clients send probe request frames to facilitate active discovery of APs and WLANs.

APs send a Probe Reply in response to endpoint Probe Request. This contains essentially the same information as a Beacon frame. Clients use the information in Beacon and Probe frames to build a list of available networks.

Authentication and deauthentication frames act as a handshake mechanism for the initial client connection request.

Association frames are used to complete a client’s 802.11 connection. Of course, disassociation frames are used to disconnect clients.

Control Frames are used to control access to the channel and to acknowledge receipt of frames.

• A device transmits a Request-To-Send (RTS) frame to gain exclusive rights to an AP’s channel. The AP confirms this request with a Clear-to-Send (CTS) frame.

• Each frame received by a device must be acknowledged, and this is done with an ACK frame. A Block ACK frame is used to acknowledge successful receipt of a series of frames. This is more efficient that acknowledging each individual frame with an ACK.

Data frames are used to transmit actual data over the WLAN, and on to some destination, such as a web site or corporate application. The interaction of Data and ACK frames is already shown in the previous figure.

• Data frames send data, as described above

• QoS data frames transmit data using a QoS method based on 802.11e, as recently discussed.

RF Basics.

Wireless network design involves a tradeoff between maximizing AP coverage area, versus ensuring good performance and capacity, given multiple users per AP. The first model for designing is based upon purely providing RF coverage in a given area. Meanwhile, the second approach takes into account the number of users and their application bandwidth and speed requirements. Both approaches are concerned with providing 100% coverage for all areas. However, the capacity-based design adds additional requirements.

Coverage

One approach to WLAN design is to maximize the square footage that each AP can cover. This results in relatively few APs spread farther apart. To ensure coverage, each AP operates at high power settings. The motivation is to save money by using fewer APs.

Capacity

Another approach is to design the WLAN with an eye on expected number of users and bandwidth requirements for their applications. Using this approach, APs are more closely spaced, and so can operate at a lower power setting to achieve proper coverage. The motivation is to ensure that all users connect at high data rates, so they will experience superior throughput. Also, since each AP covers a smaller area, there will be fewer users per AP.

In a coverage design, you might have, say 40 users per AP. Using a Capacity design for the same area you may only have 15 or 20 users per AP. This adds yet another performance improvement.

Another advantage to this design is resiliency. Should an AP fail, surrounding APs can increase their power to “fill in the gap.” Also, if new walls or other obstacles are erected, this design can more easily adapt.

Which is best?

Larger cells might be appropriate for a low-density deployment, such as a big open area with only a few users that use something like a simple handheld scanner, which is low bandwidth. The coverage design saves money, since you purchase fewer APs. However, typical office spaces would be better served with smaller cells that included fewer users in each cell. Yes, this means you need to purchase more APs to cover the same area. However, if you try to save money by purchasing few APs, your users will likely be plagued by poor performance, less reliable service, and poor roaming characteristics.

Channel availability

One challenge to designing and troubleshooting wireless networks is the ever-changing nature of RF environments. RF Interference can come from other 802.11 devices, in the form of co-channel interference. You will also encounter non-802.11 RF transmitters, such as microwave ovens and Bluetooth devices.

RF Problems

A common WLAN problem is slow throughput. For example, a WLAN client may be connected to an AP at a 54 Mbps data rate. However, during file transfer, actual throughput may be closer to 5 Mbps. A common cause of this is a noisy RF environment. This raises the “noise floor”—the general “background noise” present in any RF environment. This elevated noise floor can drown out desired transmissions and cause APs or clients to retransmit data more often.

The term “co-channel interference” is used to describe the condition when your own APs interfere with other AP signals on the same (or overlapping) channel. Meanwhile, non-802.11 sources of interference is often referred to as “noise.” Co-channel interference can be mitigated by following best practices for network design and deployment and by using automated channel management. Noise can often be mitigated by finding and eliminating or reducing the source. If it is not feasible to eliminate the source, you can mitigate the interference by situating APs strategically away from the source.

RF WLAN interferers

Two elements of interfering devices to be aware of are “duty cycle” and “decibels” (dB). Duty cycle describes how often an interferer is active, over a given time period. Decibels provide a scale to measure the strength of the signal.

Suppose you are standing near a busy highway and a loud truck passes by. The loudness of the truck noise is measured in dB. RF Power or signal strength measurements are also based on dB. Now, you speak to your friend for two minutes, and a very slow-moving truck crawls along next to you, for the entire two minutes. That is a duty cycle of 100%. Think about an actual WLAN deployment. There are often many Bluetooth devices, which operate in the 2.4 GHz range. However, Bluetooth typically has a very low duty cycle, and so this may not be as much of a concern as first imagined. Although this is not always the case, Bluetooth devices typically have little real-world impact on WLANs. Meanwhile, there may only be a few microwave ovens in your facility, but while they are in use, the interference profile has a 100% duty cycle. This can be an egregious source of interferes. Non-802.11 based security cameras are also of high duty cycle and often have a big impact on the WLAN—especially when operating at high dB levels.

Aruba spectrum analysis

An Aruba spectrum analyzer will help you to visualize the RF environment. Any Aruba AP can be configured to act as a spectrum analyzer.

The channel utilization chart

The channel utilization chart reveals both 802.11 and non-802.11 RF energy. The height of each column indicates the strength of the signal.

The Active Devices chart

The Active Devices chart (Figure 1-18) shows APs or other RF transmitters.

The Swept Spectrogram chart

The Swept Spectrogram chart is sometimes referred to as a waterfall chart it shows RF energy, color coded by signal strength. Darker shades (that will appear as red or orange in the actual chart) indicate hotter or stronger RF energy. Lighter shades (such as blue or purple) indicate cooler, weaker RF energy.

Real-time Fast-Fourier Transform (FFT) chart

The FFT chart draws a “max hold line.” This is the highest dB value received over a period of time. Looking at this chart, you can see the strength of the signal over each channel. With experience, you may also be able to interpret what type of device is transmitting, by analyzing the overall shape of the waveform. This type of experience is far less necessary, since the Aruba Spectrum analyzer will automatically detect several types of devices/categories:

Antenna technology

The antenna can perform two functions. Some devices, like an AM/FM radio only receive RF signals. Other devices only transmit RF signals. Wireless APs and endpoints both transmit and receive, and so the antenna serves both functions. When the device transmits data, the antenna receives an oscillating carrier signal from the transmitter and radiates or directs the RF waves outward from the antenna. When the device receives data, the antenna receives the RF signal and directs an oscillating carrier to the receiver.Two common antenna types are omnidirectional and directional:

• Omnidirectional—“Omni” means “all” or “in all ways or places,” and so an omnidirectional antenna radiates energy in all directions. Often referred to as simply an “omni” antenna, they radiate energy in a kind of oval or a “squashed sphere” shape.

• Directional—A directional antenna focuses more energy in a single direction, resulting in less energy in all other directions. Such an antenna may also be referred to as a “sectional” or “sector” antenna, which again describes its radiation pattern.

Reading radiation patterns horizontal plane (azimuth)

Most vendors provide antenna radiation patterns—how the antenna shapes the RF signal that emanates from an AP shows an overhead view of an antenna radiation pattern, as if you were hovering over the AP, looking down upon it. This is called the horizontal coverage, or H-plane. It can also be referred to as the azimuth.

Reading Radiation Patterns Vertical Plane (elevation)

Together, the azimuth and elevation charts give you a three-dimensional idea of the intended coverage area. Remember, physical obstructions and RF interference may change the actual radiation pattern in any given space. Of course, this affects where clients can receive good RF signals from the AP. Antennas do not draw power from the AP or any other source, and so do not “add power” to the signal. While the antenna does not increase the overall power of the signal, it can shape how that signal radiates into the environment. The more rounded shape represents the E-plane of a low-gain, Omni antenna. Remember, omni means that the antenna radiates energy in all directions—360 degrees around the antenna. Low-gain means that the antenna does not focus much of the energy in any given direction. In other words, it transmits about the same amount of energy in all directions. This low-gain omni is analogous to a fairly round, slightly flattened ball or balloon.

Now, imagine you hold the balloon, with one hand on top, the other on bottom, and squash it by gently pressing your hands closer together. What happens to the balloon? The horizontal dimension increases, at the expense of the vertical dimension

AP/Antenna mounting options

Antennas, or APs with integrated antennas can be mounted in one of three ways.

Ceiling mount—This is a very common mounting method for the typical indoor, medium-density deployment, such as a typical carpeted office space or hospital. The APs are mounted flat, or parallel to the ceiling. You can mount them below the ceiling for easier, quicker installation, and easier maintenance (you can easily see AP status lights). However, some people might object to APs being visible to passersby, and so are willing to spend a bit more time and money to hide the APs above the ceiling.

Side mount—APs are mounted to walls, beams, columns, or other structural supports that exist in the space to be covered. This is a far less common method of mounting APs—humans absorb radio energy, and so variations in user density can have large effects on coverage. Still, when other methods are less suitable for some reason, this can be a viable alternative.

Floor mount—For this option, APs are mounted in, under, or just above the floor of the coverage area. This is not a very common method—occasionally used in very high-density deployments, such as a sports arena. Since the floor, carpet, chairs, and humans absorb a lot of energy, each AP’s coverage area is a very small “pico cell.”

Single Input Single Output (SISO)

Legacy WLANs (based on 802.11a/b/g) use Single-Input Single-Output (SISO) radio technologies, where only one antenna transmits or receives at a time. One device transmits a signal over one antenna. Other devices receive this signal on both antennas and send it to the radio for processing. The radio chooses the signal with the best reception and discards the other signal; so effectively only one stream of data is used for each transmission. This concept is known as antenna diversity. This “diversity” is used to mitigate an RF problem called “multipath distortion.”

Likewise, antenna diversity uses the antenna that received the clearest signal to transmit back.

Multiple Input Multiple Output (MIMO)

802.11n and 802.11ac use MIMO antenna technologies to transmit and receive multiple data streams via multiple antennas, at the same time. These so-called “spatial streams” provide significantly faster throughput, as compared to legacy SISO technology. When you purchase an 802.11n or 802.11ac-based device, the number of antennas it can use to transmit and receive are specified, using an “N by M” matrix, where N is the number of transmit (Tx) antennas and M is the number of receive (Rx) antennas. For example, some devices may be sold as a “2 x 2” MIMO—it can use two transmit and two receive antennas. The maximum “N by M” matrix is 4 x 4 for 802.11n and 8 x 8 for 802.11ac.

Another feature introduced in 802.11ac Phase 2 is called Multi User-MIMO (MU-MIMO).

As the name implies, MU-MIMO allows different users’ data to traverse diverse spatial streams, bouncing along through the air across up to eight paths simultaneously. Thus, several Wi-Fi clients will be able to share this larger pool of streams and antennas.

RF transmit power

RF Power

Most vendors specify an AP’s RF power at its antenna connector. This RF power can be specified in one of two different units of measure—Milliwatts or dBm (decibels relative to one Milliwatts).

0 dBm is equal to 1 Milliwatts. As dBm numbers become negative, Milliwatts become fractional numbers. mW represents the data linearly, dBm represents the data logarithmically. RF power dictates the size of the APs coverage area, or how far the signal will travel. This affects the quality of the signal received by endpoints, and therefore data rate.

dBm versus Milliwatts

The power output of a transmitter, and the strength of a received signal can be measured in Milliwatts, or in dBm. They are simply two different scales that can be used to measure the same data. This is similar to how a weight can be measured in either kilograms or pounds. RF power does not work in a linear fashion. It is logarithmic. or example, if you stand some distance “X” away from an AP, your signal strength will be some value, Y. If you stand twice as far away, your signal will not be half as strong, it will be one-fourth as strong.

Milliwatts can be used to measure RF power. However, it is a linear scale, being used to measure a logarithmic phenomenon. At a close distance, the client’s signal strength might be .00001 mw. At some farther distance, the signal strength could be .0000001 mw. The numbers can get cumbersome.

Meanwhile, dBm is a logarithmic scale, used to measure a logarithmic phenomenon. dBm may seem a bit foreign at first, but it is easier to use. his means that the endpoint’s antenna receives only a small fraction of the total energy radiated by the antenna. A user standing very close to the antenna may receive a strong signal, say –40 dBm… or .0000989 mW. This is a very good, strong signal. Somebody farther away may only receive a single strength of –80 dBm, or .0000000099 mW—a very, very weak, unusable signal.

dBm and mW relationships

If you understand certain relationships between dBm and mW, you will be a more effective WLAN engineer. The two main ways these measurement scales can be perceived involves the “rule of 10s” and the “rule of 3s.” A 3 dBm increase is equal to double the power. A 10 dBm increase is equal to 10 times the power.This rule is also inversely true for a 3 dBm decrease or a 10 dBm decrease. A 3 dBm decrease is equal to half of the power. A 10 dBm decrease is equal to 1/10th the power.

A common target for WLAN quality is around –65 dBm to –67 dBm. When doing a site survey, we set a test AP to 50% power, at a particular location. Using site survey software, we walk around taking measurements to draw the –67 dBm outline for that AP, onto a floorplan. We then place the next AP so its –67 dBm ring overlaps the previous by 15%–20%.

Another key value is between –90 dBm and –100 dBm. This is considered a typical “noise floor” and represents the typical background noise in most environments.

Signal to Noise Ratio (SNR)

The Signal to Noise ratio (SNR) is a measurement of the power of the WLAN signal compared to the background noise of the RF environment. SNR is a very important value and is used to evaluate the quality of the signal

Each country’s authority sets their own legal limits on maximum EIRP, above which, you are breaking the law. We will rarely even care about this concept in most indoor installations, since we are typically doing indoor installations using Aruba APs with integrated antennas. Manufacturers ensure that these products are legal at maximum power. EIRP is mostly relevant when doing outdoor deployments, especially long-distance “bridge shots” using high-gain antennas. (In other words, using the RF system as a WAN, instead of a LAN.)

Calculating EIRP

Say you have set an AP’s transmit power to 20 dBm (100 mW) (Figure 1-33). You have connected around 50 ft–75 ft of cable to the AP, with associated connectors. The resistance of the cable and connectors creates 3 dBm of loss, so we are down to 17 dBm by the time the signal reaches the antenna. We get this number by subtracting the 3 dBm of the cable loss from the 20 dBm of the transmitter.

This antenna has 10 dBi of gain, added to the 17 dBm and arriving at the antenna gives us an EIRP of 27 dBm. This is well within the FCC’s 36 dBm maximum limit for EIRP. Other country’s limits may vary.

WLAN mobility

A Basic Service Set (BSS) is defined as an AP WLAN, and all associated clients in that AP WLAN coverage area. Each BSS is identified by its BSSID, which is based on the AP radio MAC address. A single physical AP radio, which has configured to support two logical WLANs. The physical AP radio’s MAC address is aa:aa:aa:aa:aa:a0, as set from the manufacturer. Suppose that you first define an SSID named “guest.” This SSID will be assigned a unique BSSID, based on the radio’s physical MAC address. As shown, the BSSID to be used for the guest SSID ends in “:a1.” Next, you defined a second SSID, to be supported on the same AP radio. This SSID might be assigned a BSSID ending “:a3.” In this way, a single physical radio can support multiple WLANs and therefore, multiple BSSs.

All WLAN clients associated to the AP radio’s guest SSID are considered part of the same BSS. Therefore, when these stations transmit 802.11 frames, their own MAC address is placed in the “Transmitter Address” frame field, and the connected guest BSSID is used as the “Receiver Address.” Others may connect to the Employee WLAN on the same physical radio. This is a unique BSS, with a unique BSSID. They will transmit their frames to receiver address aa:aa:aa:aa:aa:a3.

An Extended Service Set (ESS) is defined as all clients associated with the same logical network name, often configured across multiple APs. This logical network name is technically called the ESSID, but the de facto term used is SSID. The SSID name is case sensitive and identifies the WLAN to the client. APs each transmit their own unique BSSID and perhaps a common set of SSIDs. These are sent over the air in beacon and in probe frames. When you define an (E)SSID, each AP assigns this (E)SSID a unique a 48-bit MAC address. This MAC address is derived from the AP radio’s physical MAC address and is referred to as the BSSID.

the BSSID for the previously discussed scenario, and another physical AP has now been added. Of course, this AP radio has a unique MAC address assigned from the manufacturer—in this case, bb:bb:bb:bb:bb:b0. This radio has been configured to support (E)SSID guest as well, and so has a BSSID ending in “:b1.” Both APs support the “guest” SSID. They do so by making their unique BSSID known. Thus, the Basic Service Sets of each AP is Extended, across two APs, in the form of the common ESSID named “guest.”

Of course, both AP radios may also support the previously discussed Employee SSID. This is not shown in the figure, to make the figure easier to interpret. n an HPE Aruba WLAN, the 802.11 authentication type must be the same for all users connecting to the same SSID. However, their access to resources may be different, through the use of Role Based Access Control (RBAC) and Firewall Policies. Roles and Firewall Policies will be covered in more detail in a later module. Perhaps you need a “managers” SSID, which allows unlimited access to corporate resources, or maybe you need to differentiate between employees and contractors. This does not require a different SSID unless the employees and the contractors will use a different method of authenticating such as 802.1X or Captive Portal. This is a major differentiator between HPE Aruba WLANs and other enterprise WLAN vendors.

Occasionally, you might even want to separate clients by radio type, with different user groups on the 2.4 GHz and 5 GHz radios. This would also require a different SSID.

Wireless device mobility

Wireless devices may be either fixed or in motion while they access the WLAN. A wireless printer does not move while accessing the network. An example of a WLAN device in motion that is considered a highly mobile device (HMD) would be an iPad running a YouTube video while the user is walking down a hallway. An example of a WLAN device in motion that is considered a somewhat mobile device (SMD) would be a laptop.

Roaming

When 802.11 clients roam from one AP to another, they change their point of attachment to the network (new AP/ BSSID) while remaining in the same logical WLAN. To facilitate this roaming, the controller maintains client authentication, state, and firewall session information. This ensures that roaming is seamless to the users and the applications they use.

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