This chapter covers the following subjects:
Wireless LAN Topologies: This section describes autonomous, cloud-based, centralized, embedded, and Mobility Express wireless architectures.
Pairing Lightweight APs and WLCs: This section explains the process that lightweight APs must go through to discover and bind to a wireless LAN controller.
Leveraging Antennas for Wireless Coverage: This section provides an overview of various antenna types and explains how each one alters the RF coverage over an area.
Chapter 17, “Wireless Signals and Modulation,” described the mechanics of using wireless signals to send data over the air—work that is performed by a wireless AP or client device. This chapter takes a broader perspective and looks beyond a single AP to discuss the topologies that can be built with many APs.
The chapter also discusses the types of antennas you can connect to an AP to provide wireless coverage for various areas and purposes. Finally, this chapter discusses how lightweight APs discover and join with wireless LAN controllers in an enterprise network.
The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 18-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those headings so you can assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A, “Answers to the ‘Do I Know This Already?’ Quiz Questions.”
Table 18-1 “Do I Know This Already?” Section-to-Question Mapping
Foundation Topics Section
Wireless LAN Topologies
Pairing Lightweight APs and WLCs
Leveraging Antennas for Wireless Coverage
1. Suppose that a lightweight AP in default local mode is used to support wireless clients. Which one of the following paths would traffic usually take when passing from one wireless client to another?
Through the AP only
Through the AP and its controller
Through the controller only
None of these answers (Traffic must go directly over the air.)
2. A centralized wireless network is built with 1 WLC and 32 lightweight APs. Which one of the following best describes the resulting architecture?
A direct Layer 2 path from the WLC to each of the 32 APs, all using the same IP subnet
A direct Layer 3 path from the WLC to each of the 32 APs, all using the same IP subnet
32 CAPWAP tunnels daisy-chained between the APs, one CAPWAP tunnel to the WLC
32 CAPWAP tunnels—1 tunnel from the WLC to each AP, with no IP subnet restrictions
3. Which of the following unique features is true in an embedded wireless network architecture?
An access layer switch can also function as an AP.
All WLCs are converged into one device.
Large groups of APs connect to a single access layer switch.
An access layer switch can also function as a WLC.
4. Which one of the following comes first in a lightweight AP’s state machine after it boots?
Building a CAPWAP tunnel
Downloading a configuration
Joining a WLC
5. If a lightweight AP needs to download a new software image, how does it get the image?
From a TFTP server
From an FTP server
From a WLC
You must preconfigure it.
6. Which of the following is not a valid way that an AP can learn of WLCs that it might join?
List from a previously joined controller
Over-the-air neighbor message from another AP
7. If an AP tries every available method to discover a controller but fails to do so, what happens next?
It broadcasts on every possible subnet.
It tries to contact the default controller at 10.0.0.1.
It reboots or starts discovering again.
It uses IP redirect on the local router.
8. Which of the following is the most deterministic strategy you can use to push a specific AP to join a specific controller?
Let the AP select the least-loaded controller
Use DHCP option 43
Specify the master controller
Specify the primary controller
9. Which of the following antennas would probably have the greatest gain?
10. An omnidirectional antenna usually has which of the following characteristics? (Choose two.)
Answers to the “Do I Know This Already?” quiz:
10 A, E
Cisco APs can operate in one of two modes—autonomous or lightweight—depending on the code image that is installed. As the names imply, autonomous APs are self-sufficient and standalone, while lightweight APs require something bigger to complete their purpose. The following sections review each mode and analyze its purpose and the data paths that result. The lightweight mode is interesting because it can support several different network topologies, depending on where the companion wireless LAN controllers (WLCs) are located.
Autonomous APs are self-contained, each offering one or more fully functional, standalone basic service sets (BSSs). They are also a natural extension of a switched network, connecting wireless service set identifiers (SSIDs) to wired virtual LANs (VLANs) at the access layer. Figure 18-1 shows the basic architecture; even though only four APs are shown across the bottom, a typical enterprise network could consist of hundreds or thousands of APs.
Notice that the autonomous APs present two wireless LANs with SSIDs wlan100 and wlan200 to the wireless users. The APs also forward traffic between the wireless LANs and two wired VLANs 100 and 200. That means the wired VLANs must be trunked from the distribution layer, where routing occurs for each subnet, all the way down to the access layer switches where the VLANs touch each AP. The extent of a VLAN is shown in Figure 18-1 as a shaded area around the affected links.
An autonomous AP must also be configured with a management IP address (10.10.10.10 in Figure 18-1) to enable remote management. After all, you will want to configure SSIDs, VLANs, and many RF parameters like the channel and transmit power to be used. The management address is not normally part of any of the data VLANs, so a dedicated management VLAN (in this case, VLAN 10) must be added to the trunk links to reach the AP. Each AP must be configured and maintained individually unless you leverage a management platform such as Cisco Prime Infrastructure.
Because the data and management VLANs may need to reach every autonomous AP, the network configuration and efficiency can become cumbersome as the network scales. For example, you will likely want to offer the same SSID on many APs so that wireless clients can associate with that SSID in most any location or while roaming between any two APs. You might also want to extend the corresponding VLAN (and IP subnet) to each and every AP so that clients do not have to request a new IP address for each new association.
This might seem straightforward until you have to add a new VLAN and configure every switch and AP in your network to carry and support it. Even worse, suppose your network has redundant links between the layers of switches. Spanning Tree Protocol (STP) running on each switch becomes a vital ingredient to prevent bridging loops from forming and corrupting the network. For these reasons, client roaming across autonomous APs is typically limited to the Layer 2 domain, or the extent of a single VLAN. As the wireless network expands, the infrastructure becomes more difficult to configure correctly and becomes less efficient.
A topology using autonomous APs does have one nice feature: a short and simple path for data to travel between the wireless and wired networks. Consider the two wireless users shown in Figure 18-2, which are associated to the same autonomous AP. One can reach the other through the AP, without having to pass up into the wired network. That should come as no great surprise if you remember that wireless users in a BSS must pass through an AP first. As the following sections reveal, this is not always the case with lightweight AP topologies.
As a quick review, recall that Cisco APs can be configured to operate in either autonomous or lightweight AP mode. In lightweight mode, an AP loses its self-sufficiency to provide a working BSS for wireless users. Instead, it has to join a WLC to become fully functional. This cooperation is known as a split-MAC architecture, where the AP handles most of the real-time 802.11 processes and the WLC performs the management functions.
An AP and a WLC are joined by a logical pair of CAPWAP tunnels that extend through the wired network infrastructure. Control and data traffic are transported across the tunnels. Many APs can join the same WLC, each with its own pair of CAPWAP tunnels. A wireless network can scale in this fashion, provided the WLC can support the maximum number of APs in use. Beyond that, additional WLCs would be needed.
Several topologies can be built from a WLC and a collection of APs. These differ according to where the WLC is located within the network. For example, a WLC can be placed in a central location, usually in a data center or near the network core, so that you can maximize the number of APs joined to it. This is known as a centralized or unified wireless LAN topology, as shown in Figure 18-3. This tends to follow the concept that most of the resources users need to reach are located in a central location, such as a data center or the Internet. Traffic to and from wireless users travels from the APs over CAPWAP tunnels that reach into the center of the network. A centralized WLC also provides a convenient place to enforce security policies that affect all wireless users.
Figure 18-3 shows four APs joined to a single WLC, but your network might have more APs—many, many more. A large enterprise network might have thousands of APs in its access layer. Scalability then becomes an important factor in the centralized design. Each Cisco WLC model supports a maximum number of APs. If you have more APs than the maximum, you need to add more WLCs to the design, each located centrally. A Cisco unified WLC meant for a large enterprise can support up to 6000 APs.
Notice that the network infrastructure in Figure 18-3 has the same hierarchy as the autonomous topology in Figure 18-1. The only differences are that the APs are running in lightweight mode, and there is a WLC present high in the topology. Figure 18-3 shows one of the CAPWAP tunnels connecting one AP to the WLC, although each AP would also have its own tunnels to the controller. The Layer 3 boundary for each data VLAN is handled at or near the WLC, so the VLANs need only exist at that location, indicated by the shaded link. Each AP still has its own unique management IP address, but it connects to an access layer switch via an access link rather than a trunk link. Even if multiple VLANs and WLANs are involved, they are carried over the same CAPWAP tunnel to and from the AP. Therefore, the AP needs only a single IP address to terminate the tunnel.
The centralized architecture also affects wireless user mobility. For example, as a wireless user moves through the coverage areas of the four APs in Figure 18-3, he might associate with many different APs in the access layer. Because all of the APs are joined to a single WLC, that WLC can easily maintain the user’s connectivity to all other areas of the network as he moves around.
Locating the WLC centrally also affects the path that wireless data must take. Recall that two wireless users associated with an autonomous AP can reach each other through the AP. In contrast, the path between two wireless users in a centralized network is shown in Figure 18-4. The traffic from one client must pass through the AP, where it is encapsulated in the CAPWAP tunnel, and then travel high up into the network to reach the WLC, where it is unencapsulated and examined. The process then reverses, and the traffic goes back down through the tunnel to reach the AP and back out into the air to the other client.
Now imagine that a WLC can be located further down in the network hierarchy. In Figure 18-5, the WLC is co-located with an access layer switch. This can be desirable when the switch platform can also support the WLC function. This is known as an embedded wireless network topology because the WLC is embedded in the switch hardware.
The access layer turns out to be a convenient location for the WLCs. After all, wireless users ultimately connect to a WLC, which serves as a virtual access layer. Why not move the wireless access layer to coincide with the wired access layer? With all types of user access merged into one layer, it becomes much easier to do things like apply common access and security policies that affect all users.
Notice that each AP connects to an access switch for network connectivity as well as split-MAC functionality, so the CAPWAP tunnel becomes really short; it exists only over the length of the cable connecting the AP!
The embedded topology can be cost-effective because the same switching platform is used for both wired and wireless purposes. Ideally, each access layer switch would have its own embedded WLC, distributing the WLCs across the network. A Cisco embedded WLC typically supports up to 200 APs.
It might seem odd that the number of supported APs is rather low when the physical port density of a switch can be rather large. If you think of this from a wireless perspective, it makes more sense. Each AP is connected to the access switch by a twisted-pair cable that is limited to a length of 100 meters. Therefore, all of the APs must be located within a 100 meter radius of the access switch. There are not too many APs to physically fit into that area.
The embedded model can also solve some connectivity problems at branch sites by bringing a fully functional WLC onsite, within an access layer switch. With a local WLC, the APs can continue to operate without a dependency upon a WLC at the main site through a WAN connection.
If the CAPWAP tunnel is relatively short in an embedded topology, that must mean wireless devices can reach each other more efficiently. Indeed, as Figure 18-6 shows, the traffic path from one user to another must pass through an AP, the access switch (and WLC), and back down through the AP. In contrast, traffic from a wireless user to a central resource such as a data center or the Internet travels through the CAPWAP tunnel, is unencapsulated at the access layer switch (and WLC), and travels normally up through the rest of the network layers.
As you might have guessed, it is also possible to move the WLC even below the access layer and into an AP. Figure 18-7 illustrates the Mobility Express topology, where a fully functional Cisco AP also runs software that acts as a WLC. This can be useful in small scale environments, such as small, midsize, or multi-site branch locations, where you might not want to invest in dedicated WLCs at all. The AP that hosts the WLC forms a CAPWAP tunnel with the WLC, as do any other APs at the same location. A Mobility Express WLC can support up to 100 APs.
A Cisco lightweight wireless AP needs to be paired with a WLC to function. Each AP must discover and bind itself with a controller before wireless clients can be supported.
Cisco lightweight APs are designed to be “touch free,” so that you can simply unbox a new one and connect it to the wired network, without any need to configure it first. Naturally, you have to configure the switch port, where the AP connects, with the correct access VLAN, access mode, and inline power settings. From that point on, the AP can power up and use a variety of methods to find a viable WLC to join.
From the time it powers up until it offers a fully functional basic service set (BSS), a lightweight AP operates in a variety of states. Each of the possible states is well defined as part of the Control and Provisioning of Wireless Access Points (CAPWAP) specification, but they are simplified here for clarity. The AP enters the states in a specific order; the sequence of states is called a state machine. You should become familiar with the AP state machine so that you can understand how an AP forms a working relationship with a WLC. If an AP cannot form that relationship for some reason, your knowledge of the state machine can help you troubleshoot the problem.
The sequence of the most common states, as shown in Figure 18-8, is as follows:
AP boots: Once an AP receives power, it boots on a small IOS image so that it can work through the remaining states and communicate over its network connection. The AP must also receive an IP address from either a Dynamic Host Configuration Protocol (DHCP) server or a static configuration so that it can communicate over the network.
WLC discovery: The AP goes through a series of steps to find one or more controllers that it might join. The steps are explained further in the next section.
CAPWAP tunnel: The AP attempts to build a CAPWAP tunnel with one or more controllers. The tunnel will provide a secure Datagram Transport Layer Security (DTLS) channel for subsequent AP-WLC control messages. The AP and WLC authenticate each other through an exchange of digital certificates.
WLC join: The AP selects a WLC from a list of candidates and then sends a CAPWAP Join Request message to it. The WLC replies with a CAPWAP Join Response message. The next section explains how an AP selects a WLC to join.
Download image: The WLC informs the AP of its software release. If the AP’s own software is a different release, the AP downloads a matching image from the controller, reboots to apply the new image, and then returns to step 1. If the two are running identical releases, no download is needed.
Download config: The AP pulls configuration parameters down from the WLC and can update existing values with those sent from the controller. Settings include RF, service set identifier (SSID), security, and quality of service (QoS) parameters.
Run state: Once the AP is fully initialized, the WLC places it in the “run” state. The AP and WLC then begin providing a BSS and begin accepting wireless clients.
Reset: If an AP is reset by the WLC, it tears down existing client associations and any CAPWAP tunnels to WLCs. The AP then reboots and starts through the entire state machine again.
Be aware that you cannot control which software image release a lightweight AP runs. Rather, the WLC that the AP joins determines the release, based on its own software version. Downloading a new image can take a considerable amount of time, especially if there are a large number of APs waiting for the same download from one WLC. That might not matter when a newly installed AP is booting and downloading code because it does not yet have any wireless clients to support. However, if an existing, live AP happens to reboot or join a different controller, clients can be left hanging with no AP while the image downloads. Some careful planning with your controllers and their software releases will pay off later in terms of minimized downtime. Consider the following scenarios when an AP might need to download a different release:
The AP joins a WLC but has a version mismatch.
A code upgrade is performed on the WLC itself, requiring all associated APs to upgrade, too.
The WLC fails, causing all associated APs to be dropped and to join elsewhere.
If there is a chance that an AP could rehome from one WLC to another, you should make sure that both controllers are running the same code release. Otherwise, the AP move should happen under controlled circumstances, such as during a maintenance window. Fortunately, if you have downloaded a new code release to a controller but not yet rebooted it to run the new code, you can predownload the new release to the controller’s APs. The APs will download the new image but will keep running the previous release. When it comes time to reboot the controller on the new image, the APs will already have the new image staged without having to take time to download it. The APs can reboot on their new image and join the controller after it has booted and become stable.
An AP must be very diligent to discover any controllers that it can join—all without any preconfiguration on your part. To accomplish this feat, several methods of discovery are used. The goal of discovery is just to build a list of live candidate controllers that are available, using the following methods:
Prior knowledge of WLCs
DHCP and DNS information to suggest some controllers
Broadcast on the local subnet to solicit controllers
To discover a WLC, an AP sends a unicast CAPWAP Discovery Request to a controller’s IP address over UDP port 5246 or a broadcast to the local subnet. If the controller exists and is working, it returns a CAPWAP Discovery Response to the AP. The sequence of discovery steps used is as follows:
Step 1. The AP broadcasts a CAPWAP Discovery Request on its local wired subnet. Any WLCs that also exist on the subnet answer with a CAPWAP Discovery Response.
Step 2. An AP can be “primed” with up to three controllers—a primary, a secondary, and a tertiary. These are stored in nonvolatile memory so that the AP can remember them after a reboot or power failure. Otherwise, if an AP has previously joined with a controller, it should have stored up to 8 out of a list of 32 WLC addresses that it received from the last controller it joined. The AP attempts to contact as many controllers as possible to build a list of candidates.
Step 3. The DHCP server that supplies the AP with an IP address can also send DHCP option 43 to suggest a list of WLC addresses.
Step 4. The AP attempts to resolve the name CISCO-CAPWAP-CONTROLLER.localdomain with a DNS request (where localdomain is the domain name learned from DHCP). If the name resolves to an IP address, the controller attempts to contact a WLC at that address.
Step 5. If none of the steps has been successful, the AP resets itself and starts the discovery process all over again.
When an AP has finished the discovery process, it should have built a list of live candidate controllers. Now it must begin a separate process to select one WLC and attempt to join it. Joining a WLC involves sending it a CAPWAP Join Request and waiting for it to return a CAPWAP Join Response. From that point on, the AP and WLC build a DTLS tunnel to secure their CAPWAP control messages.
The WLC selection process consists of the following three steps:
Step 1. If the AP has previously joined a controller and has been configured or “primed” with a primary, secondary, and tertiary controller, it tries to join those controllers in succession.
Step 2. If the AP does not know of any candidate controller, it tries to discover one. If a controller has been configured as a master controller, it responds to the AP’s request.
Step 3. The AP attempts to join the least-loaded WLC, in an effort to load balance APs across a set of controllers. During the discovery phase, each controller reports its load—the ratio of the number of currently joined APs to the total AP capacity. The least-loaded WLC is the one with the lowest ratio.
If an AP discovers a controller but gets rejected when it tries to join it, what might be the reason? Every controller has a set maximum number of APs that it can support. This is defined by platform or by license. If the controller already has the maximum number of APs joined to it, it rejects any additional APs.
To provide some flexibility in supporting APs on an oversubscribed controller, where more APs are trying to join than a license allows, you can configure the APs with a priority value. All APs begin with a default priority of low. You can change the value to low, medium, high, or critical. A controller tries to accommodate as many higher-priority APs as possible. Once a controller is full of APs, it rejects an AP with the lowest priority to make room for a new one that has a higher priority.
Once an AP has discovered, selected, and joined a controller, it must stay joined to that controller to remain functional. Consider that a single controller might support as many as 1000 or even 6000 APs—enough to cover a very large building or an entire enterprise. If something ever causes the controller to fail, a large number of APs will also fail. In the worst case, where a single controller carries the enterprise, the entire wireless network will become unavailable, which might be catastrophic.
Fortunately, a Cisco AP can discover multiple controllers—not just the one that it chooses to join. If the joined controller becomes unavailable, the AP can simply select the next least-loaded controller and request to join it. That sounds simple, but it is not very deterministic. If a controller full of 1000 APs fails, all 1000 APs must detect the failure, discover other candidate controllers, and then select the least-loaded one to join. During that time, wireless clients can be left stranded with no connectivity. You might envision the controller failure as a commercial airline flight that has just been canceled; everyone who purchased a ticket suddenly joins a mad rush to find another flight out.
The most deterministic approach is to leverage the primary, secondary, and tertiary controller fields that every AP stores. If any of these fields are configured with a controller name or address, the AP knows which three controllers to try in sequence before resorting to a more generic search.
Once an AP joins a controller, it sends keepalive (also called heartbeat) messages to the controller over the wired network at regular intervals. By default, keepalives are sent every 30 seconds. The controller is expected to answer each keepalive as evidence that it is still alive and working. If a keepalive is not answered, an AP escalates the test by sending four more keepalives at 3-second intervals. If the controller answers, all is well; if it does not answer, the AP presumes that the controller has failed. The AP then moves quickly to find a successor to join.
Using the default values, an AP can detect a controller failure in as little as 35 seconds. You can adjust the regular keepalive timer between 1 and 30 seconds and the escalated, or “fast,” heartbeat timer between 1 and 10 seconds. By using the minimum values, a failure can be detected after only 6 seconds.
To make the process much more efficient, WLCs also support high availability (HA) with stateful switchover (SSO) redundancy. SSO groups controllers into high availability pairs, where one controller takes on the active role and the other is in a hot standby mode. The APs need to know only the primary controller that is the active unit. Because each active controller has its own standby controller, there really is no need to configure a secondary or tertiary controller on the APs unless you need an additional layer of redundancy.
Each AP learns of the HA pair during a CAPWAP discovery phase and then builds a CAPWAP tunnel to the active controller. The active unit keeps CAPWAP tunnels, AP states, client states, configurations, and image files all in sync with the hot standby unit. The active controller also synchronizes the state of each associated client that is in the RUN state with the hot standby controller. If the active controller fails, the standby will already have the current state information for each AP and client, making the failover process transparent to the end users.
From the WLC, you can configure a lightweight AP to operate in one of the following special-purpose modes:
Local: The default lightweight mode that offers one or more functioning BSSs on a specific channel. During times when it is not transmitting, the AP scans the other channels to measure the level of noise, measure interference, discover rogue devices, and match against intrusion detection system (IDS) events.
Monitor: The AP does not transmit at all, but its receiver is enabled to act as a dedicated sensor. The AP checks for IDS events, detects rogue access points, and determines the position of stations through location-based services.
FlexConnect: An AP at a remote site can locally switch traffic between an SSID and a VLAN if its CAPWAP tunnel to the WLC is down and if it is configured to do so.
Sniffer: An AP dedicates its radios to receiving 802.11 traffic from other sources, much like a sniffer or packet capture device. The captured traffic is then forwarded to a PC running network analyzer software such as LiveAction Omnipeek or Wireshark, where it can be analyzed further.
Rogue detector: An AP dedicates itself to detecting rogue devices by correlating MAC addresses heard on the wired network with those heard over the air. Rogue devices are those that appear on both networks.
Bridge: An AP becomes a dedicated bridge (point-to-point or point-to-multipoint) between two networks. Two APs in bridge mode can be used to link two locations separated by a distance. Multiple APs in bridge mode can form an indoor or outdoor mesh network.
Flex+Bridge: FlexConnect operation is enabled on a mesh AP.
SE-Connect: The AP dedicates its radios to spectrum analysis on all wireless channels. You can remotely connect a PC running software such as MetaGeek Chanalyzer or Cisco Spectrum Expert to the AP to collect and analyze the spectrum analysis data to discover sources of interference.
The world of wireless LANs would be rather simple—too simple, in fact—if all antennas were created equal. To provide good wireless LAN coverage in a building, in an outdoor area, or between two locations, you might be faced with a number of variables. For example, an office space might be arranged as a group of open cubicles or as a strip of closed offices down a long hallway. You might have to cover a large open lobby, a large open classroom, a section of a crowded sports arena, an oblong portion of a hospital roof where helicopters land, a large expanse of an outdoor park, city streets where public safety vehicles travel, and so on.
In other words, one type of antenna cannot fit every application. Instead, antennas come in many sizes and shapes, each with its own gain value and intended purpose. The following sections describe antenna characteristics in more detail.
Recall from Chapter 17 that antenna gain is normally a comparison of one antenna against an isotropic antenna and is measured in dBi (decibel-isotropic). An isotropic antenna does not actually exist because it is ideal, perfect, and impossible to construct. It is also the simplest, most basic antenna possible, which makes it a good starting place for antenna theory.
An isotropic antenna is shaped like a tiny round point. When an alternating current is applied, an RF signal is produced, and the electromagnetic waves are radiated equally in all directions. The energy produced by the antenna takes the form of an ever-expanding sphere. If you were to move all around an isotropic antenna at a fixed distance, you would find that the signal strength is the same.
To describe the antenna’s performance, you might draw a sphere with a diameter that is proportional to the signal strength, as shown in Figure 18-9. Most likely, you would draw the sphere on a logarithmic scale so that very large and very small numbers could be shown on the same linear plot. A plot that shows the relative signal strength around an antenna is known as the radiation pattern.
It is rather difficult to show a three-dimensional plot or shape in a two-dimensional document—especially if the shape is complex or unusual. After all, most physical antennas are not ideal, so their radiation pattern is not a simple sphere. Instead, you could slice through the three-dimensional plot with two orthogonal planes and show the two outlines that are formed from the plot. In Figure 18-9, the sphere is cut by two planes. The XY plane, which lies flat along the horizon, is known as the H plane, or the horizontal (azimuth) plane, and it usually shows a top-down view of the radiation pattern through the center of the antenna. The XZ plane, which lies vertically along the elevation of the sphere, is known as the E plane, or elevation plane, and shows a side view of the same radiation pattern.
The outline of each plot can be recorded on a polar plot, as shown by the heavy dark lines in Figure 18-10. It might be hard to see the plots of an isometric antenna because they are perfect circles that correspond with the outline of each circle shown.
A polar plot contains concentric circles that represent relative changes in the signal strength, as measured at a constant distance from the antenna. The outermost circle usually represents the strongest signal strength, and the inner circles represent weaker signal strength. Although the circles are labeled with numbers like 0, −5, −10, −15, and so on, they do not necessarily represent any absolute dB values. Instead, they are measurements that are relative to the maximum value at the outside circle. If the maximum is shown at the outer ring, everything else will be less than the maximum and will lie further inward.
The circles are also divided into sectors so that a full sweep of 360 degrees can be plotted. This allows measurements to be taken at every angle around the antenna in the plane shown.
Antenna pattern plots can be a bit confusing to interpret. The E and H polar plots of the radiation pattern are presented here because most antenna manufacturers include them in their product literature. The antenna is always placed at the center of the polar plots, but you will not always be able to figure out how the antenna is oriented with respect to the E and H planes. Cisco usually includes a small picture of the antenna at the center of the plots as a handy reference.
As you decide to place APs in their actual locations, you might have to look at various antenna patterns and try to figure out whether the antenna is a good match for the environment you are trying to cover with an RF signal. You will need a good bit of imagination to merge the two plots into a 3D picture in your mind. As various antennas are described in this chapter, the plots, planes, and a 3D rendering are presented to help you get a feel for the thinking process.
Antennas are passive devices; they do not amplify a transmitter’s signal with any circuitry or external power. Instead, they amplify or add gain to the signal by shaping the RF energy as it is propagated into free space. In other words, the gain of an antenna is a measure of how effectively it can focus RF energy in a certain direction.
Because an isotropic antenna radiates RF energy in all directions equally, it cannot focus the energy in any certain direction. Recall from Chapter 17 that the gain of an antenna in dBi is measured relative to an isotropic antenna. When an isotropic antenna is compared with itself, the result is a gain of 10log10(1), or 0 dBi.
Think of a zero gain antenna producing a perfect sphere. If the sphere is made of rubber, you could press on it in various locations and change its shape. As the sphere is deformed, it expands in other directions. Figure 18-11 shows some simple examples, along with some examples of gain values. As you work through this chapter and examine antennas on your own, notice that the gain is lower for omnidirectional antennas, which are made to cover a widespread area, and higher for directional antennas, which are built to cover more focused areas.
The antenna gain can be an indicator of how focused an antenna’s pattern might be, but it is really more suited for link budget calculations. Instead, many manufacturers list the beamwidth of an antenna as a measure of the antenna’s focus. Beamwidth is normally listed in degrees for both the H and E planes.
The beamwidth is determined by finding the strongest point on the plot, which is usually somewhere on the outer circle. Next, the plot is followed in either direction until the value decreases by 3 dB, indicating the point where the signal is one-half the strongest power. A line is drawn from the center of the plot to intersect each 3 dB point, and then the angle between the two lines is measured. Figure 18-12 shows a simple example. The H plane has a beamwidth of 30 degrees, and the E plane has a beamwidth of 55 degrees.
When an alternating current is applied to an antenna, an electromagnetic wave is produced. In Chapter 17, you learned that the wave has two components: an electrical field wave and a magnetic field wave. The electrical portion of the wave will always leave the antenna in a certain orientation. For example, a simple length of wire that is pointing vertically will produce a wave that oscillates up and down in a vertical direction as it travels through free space. This is true of most Cisco antennas when they are mounted according to Cisco recommendations. Other types of antennas might be designed to produce waves that oscillate back and forth horizontally. Still others might produce waves that actually twist in a three-dimensional spiral motion through space.
The electrical field wave’s orientation, with respect to the horizon, is called the antenna polarization. Antennas that produce vertical oscillation are vertically polarized; those that produce horizontal oscillation are horizontally polarized. (Keep in mind that there is always a magnetic field wave, too, which is oriented at 90 degrees from the electrical field wave.) By itself, the antenna polarization is not of critical importance. However, the antenna polarization at the transmitter must be matched to the polarization at the receiver. If the polarization is mismatched, the received signal can be severely degraded.
Figure 18-13 illustrates antenna polarization. The transmitter and receiver along the top both use vertical polarization, so the received signal is optimized. The pair along the bottom is mismatched, causing the signal to be poorly received.
There are two basic types of antennas, omnidirectional and directional, which are discussed in the following sections. An omnidirectional antenna is usually made in the shape of a thin cylinder. It tends to propagate a signal equally in all directions away from the cylinder but not along the cylinder’s length. The result is a donut-shaped pattern that extends further in the H plane than in the E plane. This type of antenna is well suited for broad coverage of a large room or floor area, with the antenna located in the center. Because an omnidirectional antenna distributes the RF energy throughout a broad area, it has a relatively low gain.
A common type of omnidirectional antenna is the dipole, shown in the left portion of Figure 18-14. Some dipole models are articulated such that they can be folded up or down, depending on the mounting orientation, whereas others are rigid and fixed. As its name implies, the dipole has two separate wires that radiate an RF signal when an alternating current is applied across them, as shown in the right portion of Figure 18-14. Dipoles usually have a gain of around +2 to +5 dBi.
The E and H plane radiation patterns for a typical dipole antenna are shown in Figure 18-15. In the E plane, think of the dipole lying on its side in the center of the plot; the H plane is looking down on the top of the dipole. Figure 18-16 takes the patterns a step further, showing how the two planes are superimposed and merged to reveal the three-dimensional radiation pattern.
To reduce the size of an omnidirectional antenna, many Cisco wireless access points (APs) have integrated antennas that are hidden inside the device’s smooth case. For example, the AP shown in Figure 18-17 has six tiny antennas hidden inside it.
Integrated omnidirectional antennas typically have a gain of 2 dBi in the 2.4 GHz band and 5 dBi in the 5 GHz band. The E and H plane radiation patterns are shown in Figure 18-18. When the two planes are merged, the three-dimensional pattern still rather resembles a sphere.
Directional antennas have a higher gain than omnidirectional antennas because they focus the RF energy in one general direction. Typical applications include elongated indoor areas, such as the rooms along a long hallway or the aisles in a warehouse. They can also be used to cover outdoor areas out away from a building or long distances between buildings. If they are mounted against a ceiling, pointing downward, they can cover a small floor area to reduce an AP’s cell size.
Patch antennas have a flat rectangular shape, as shown in Figure 18-19, so that they can be mounted on a wall or ceiling.
Patch antennas produce a broad egg-shaped pattern that extends out away from the flat patch surface. The E and H radiation pattern plots are shown in Figure 18-20. When the planes are merged, as shown in Figure 18-21, you can see the somewhat broad directional pattern that results. Patch antennas have a typical gain of about 6 to 8 dBi in the 2.4 GHz band and 7 to 10 dBi at 5 GHz.
Figure 18-22 shows the Yagi–Uda antenna, named after its inventors, and more commonly known as the Yagi. Although its outer case is shaped like a thick cylinder, the antenna is actually made up of several parallel elements of increasing length.
Figure 18-23 shows the E and H radiation pattern plots. A Yagi produces a more focused egg-shaped pattern that extends out along the antenna’s length, as shown in Figure 18-24. Yagi antennas have a gain of about 10 to 14 dBi.
In a line-of-sight wireless path, an RF signal must be propagated a long distance using a narrow beam. Highly directional antennas are tailored for that use but focus the RF energy along one narrow elliptical pattern. Because the target is only one receiver location, the antenna does not have to cover any area outside of the line of sight.
Dish antennas, such as the one shown in Figure 18-25, use a parabolic dish to focus received signals onto an antenna mounted at the center. The parabolic shape is important because any waves arriving from the line of sight will be reflected onto the center antenna element that faces the dish. Transmitted waves are just the reverse: They are aimed at the dish and reflected such that they are propagated away from the dish along the line of sight.
Figure 18-26 shows the radiation patterns in the E and H planes, which are merged into three dimensions in Figure 18-27. Notice that the antenna’s coverage pattern is long and narrow, extending out away from the dish. The focused pattern gives the antenna a gain of between 20 and 30 dBi—the highest gain of all the wireless LAN antennas.
Exam Preparation Tasks
As mentioned in the section “How to Use This Book” in the Introduction, you have a couple of choices for exam preparation: the exercises here, Chapter 30, “Final Preparation,” and the exam simulation questions in the Pearson Test Prep Software Online.
Review the most important topics in this chapter, noted with the Key Topic icon in the outer margin of the page. Table 18-2 lists these key topics and the page number on which each is found.
Table 18-2 Key Topics for Chapter 18
Key Topic Element
Wireless Network Topology Using Autonomous APs
WLC Location in a Centralized Wireless Network Topology
WLC Location in an Embedded Wireless Network Topology
WLC Location in a Mobility Express Wireless Network Topology
AP controller discovery states
AP controller discovery steps
Cisco lightweight AP modes
Plotting the Radiation Pattern of an Isotropic Antenna
There are no memory tables in this chapter.
Define the following key terms from this chapter and check your answers in the Glossary: