Access Points

As previously mentioned, there are two types of access points:

• Autonomous APs

• Lightweight APs

Some APs are built into modules and deployed in ISR routers at branch sites; other APs are deployed as just standalone devices. Cisco APs are known to offer the best range and throughput in the industry, as well as a number of security features that you do not find with other vendors.

Cisco APs offer multiple configuration options. Some of them support external antennas, some support internal antennas, and some are to be deployed outdoors. Still others are de- signed to be deployed indoors. Some APs are designed to be implemented for wide-area networking and bridging purposes and, while operating as a bridge, may also allow client connections. The point is that Cisco APs can serve a number of purposes.

The benefit of the CUWN APs is that they are zero-touch management, assuming that Layer 2 connectivity is already in place. As soon as they are plugged in and powered on, you don’t have to do anything else at the AP level. The models that you need to be familiar with for the CCNA Wireless exam include the 1130AG, 1240AG, 1250AG, 1300, and 1400 series wireless bridges.

The 1130, 1240, and 1250 can be both autonomous and lightweight APs. Whereas the 1300 and 1400 series are designed to operate as bridges, the 1300 series can also sup- port wireless clients. In turn, the 1400 series supports bridging only. Another model is the outdoor mesh 1500 series, which supports only LWAPP, so that would be designed for a lightweight scenario only.

Cisco is known for being ahead of the curve. That’s where the special functionality of the 1250AG comes in. The 1250AG is one of the first access points to support the 802.11n draft version 2.0 standard and is the basis for all 802.11 Wi-Fi interoperability testing. For a client vendor to get the v2.0 stamp of approval, it must be validated against the 1250, and the 1250 is the only AP used during this validation.

The 1130AG

The 1130AG, shown in Figure 10-3, is a dual-band 802.11 a/b or g AP that has integrated antennas.

The 1130AG can operate as a standalone device or in lightweight AP mode. It also can op- erate as a Hybrid Remote Edge AP (H-REAP) device. An H-REAP device operates on the far side of a WAN, and its controller is back at the core site.

The 1130AG is 802.11i/WPA2-compliant, and it has 32 MB of RAM and 16 MB of flash memory. The 1130 AP typically is deployed in office or hospital environments. Naturally, the internal antennas do not offer the same coverage and distance as APs that are designed for external antennas. Consider the 1130s. They have 3 dB gain and 4.5 dB gain for the 2.4- and 5-GHz frequencies, respectively. If you were to compare the 1131 to the 1242 with the 2.2 dipole antennas, you would see a larger coverage area than with the 1242.


The 1240AG


The 1240AG series AP, shown in Figure 10-4, is also a dual-band 802.11 a/b or g device, similar to the 1130AG; however, it supports only external antennas.

Those external antennas would connect using the RP-TNC connectors. The 1240AG can operate as an autonomous AP and in lightweight AP mode. Like the 1130AG, it also can operate in H-REAP mode. It too is 802.11i/WPA2-compliant.


The 1250 Series AP

Shown in Figure 10-5, the 1250 series AP is one of the first enterprise APs to support the 802.11n draft version 2.0.

Because it supports the 802.11n draft standard, you can get data rates of about 300 Mbps on each radio and the 2-by-3 multiple input and multiple output technology. The 2-by-3 is discussed in Chapter 6, “Overview of the 802.11 WLAN Protocols.” Also, because the 1250 is modular, it can easily be upgraded in the field. It operates in controller-based and standalone mode and is also 802.11i/WPA2-compliant.

The 1250 is designed for a more rugged type of indoor environment. You might see this at more hazardous locations such as packaging plants, or in situations where you might need


to place an antenna in a hazardous location and the AP elsewhere. You might see this type of AP in factories and hospitals. It has 64 MB of DRAM and 32 MB of flash memory. It has 2.4-GHz and 5-GHz radios.

Cisco Wireless Networks Architecture

The Need for Centralized Control

There is certainly a need for centralized control in wireless deployments today. Initial wireless deployments were based on standalone access points called autonomous access or fat APs. An autonomous AP is one that does not rely on a central control device. Al- though this is a great start, the problem lies in scalability. Eventually, you will have prob- lems keeping your configurations consistent, monitoring the state of each AP, and actually taking action when a change occurs. You end up with holes in your coverage area, and there is no real dynamic method to recover from that. There is certainly a need for central- ized control, and the Cisco Unified Wireless Network (CUWN) is based on centralized control.

Eventually you will want or need to convert those standalone APs, if possible, to light- weight APs. A lightweight AP is managed with a controller.

Traditionally after a site survey, you would deploy your wireless network based on the in- formation you gathered. As time passes, the environment you did the original site survey in will change. These changes, although sometimes subtle, will affect the wireless cover- age. The CUWN addresses these issues.


The Cisco Solution

The CUWN solution is based on a centralized control model. Figure 10-1 illustrates the numerous components of the CUWN.


An AP operating in lightweight mode gets its configuration from the controller. This means that you will perform most of your configuration directly on the controller. It dy- namically updates the AP as the environment changes. This also allows all the APs to share a common configuration, increasing the uniformity of your wireless network and eliminat- ing inconsistencies in your AP configurations.


As you can see, five functional areas exist:

• Wireless clients

• Access points

• Network unification

• Network management

• Network services

Supporting Multiple Networks

Previous chapters discussed that an AP can actually advertise multiple SSIDs, which lets the AP offer guest access as well as corporate user access and maybe even access for wire- less IP phones. Each Wireless LAN Controller actually can support 512 different VLAN instances. Remember that on the connection between the AP and the Wireless LAN Con- troller, all your wireless client data is passed via the LWAPP tunnel as it travels toward the wired domain.

To review, recall that an SSID exists only in the wireless space. An SSID is then tied to a VLAN within the controller. Each lightweight AP can support 512 different VLANs, but you don’t very often see that many on one AP.

On the other hand, your Wireless LAN Controller can have up to 16 wireless LANs (WLAN) tied to each AP. Each WLAN is assigned a wireless LAN identification (WLANID) by the controller. This is a number between 1 and 16, and you don’t get to choose which one to use.

So, now you have a WLAN that brings together the concept of an SSID on the wireless space and a VLAN on the wired space. By having separate WLANs, you can assign differ- ent quality-of-service (QoS) policies to the type of traffic encountered on each of them. An example of this would be to have a WLAN for IP Phones and a different WLAN for regular network users.

Each AP supports up to 16 SSIDs; generally, one SSID is mapped to one VLAN. With that said, even though a Wireless LAN Controller can support up to 512 VLANs per AP, you see a maximum of only 16 VLANs in most situations.


The CUWN Architecture

The Cisco Unified Wireless Network defines a total of five functional areas or intercon- nected elements, as shown in Figure 10-2.

The five elements or components all work together. It’s no longer about point products, where you can buy a standalone AP and deploy it and then later get management software to handle it. Today it is all about everything working together to create a smarter, more functional net- work. To illustrate how it all comes together, consider a Cisco wireless network. This type of network includes the following wireless clients (the first component of the CUWN):

• Cisco Aironet client devices

• Cisco-compatible client devices (not necessarily Cisco products, but still compatible)

• Cisco Secure Services Client (SSC)

The client devices get a user connected.

The second component, the access point, is dynamically managed by your controllers, and they use LWAPP to communicate. The AP bridges the client device to the wired net- work. A number of APs that could be discussed here are as follows:

• The 1130AG

• The 1240AG

• The 1250AG

• The 1300 series bridge

• The 1400 series bridge

• The 1500 series outdoor mesh

Delivering Packets from the Wireless to Wired Network

The Association Process

To begin, you need a network. This chapter uses the common logical topology seen in Figure 9-1. As you can see, multiple wireless clients are in range of an AP that is advertis- ing multiple service set identifiers (SSID). One SSID puts users on a network that is of- fered to guest users called Guest. The other SSID is called UserNet and is designed for authenticated users of the corporate network. Naturally, more security is going to be applied to users of UserNet, such as authentication and encryption, as opposed to the net- work Guest. The Guest network places users on the 172.30.1.0/24 subnet. The UserNet places users on the 10.99.99.0/24 network. Although these two networks are on different subnets and users associate with different SSIDs, recall that an AP can advertise multiple SSIDs but actually uses the same wireless radio. In the wireless space, the SSID and IP subnet keep the networks logically separated.


Getting back to the association process, a client scans the channels hoping to hear a beacon from an AP or actively sends a probe request. If a probe response is received or a bea- con is heard, the client can attempt to associate with the SSID received in that probe response or beacon.

The next step is to authenticate and associate with the AP. When the client chooses an SSID, it sends an authentication request. The AP should reply with an authentication response. After this occurs and a “Success” message is received, an association request is sent, including the data rates and capabilities of the client, followed by an association re- sponse from the AP. The association response from the AP includes the data rates that the AP is capable of, other capabilities, and an identification number for the association.

Next, the client must determine the speed. It does this by determining the Received Signal Strength Indicator (RSSI) and signal-to-noise ratio (SNR), and it chooses the best speed to send at based on these determinations. All management frames are sent at the lowest rate, whereas the data headers can be sent faster than management frames, and the actual data frames at the fastest possible rate. Just as the client determines its rates to send, the AP, in turn, does the same. Now that the client is associated, it can attempt to send data to other devices on the network.


Sending to a Host on Another Subnet

When a client is associated with an AP, the general idea is to send data to other devices. To illustrate this, first try to send data between Client A in Figure 9-2, which is on the User- Net network, and Client B, which is on the Guest network. Although a typical network would not allow guest users to send traffic to internal WLAN users for security purposes, this will provide an example of how the connection works.

The two clients are clearly on two different subnets, so the rules of how IP works are still in play. The clients cannot send traffic directly to each other. Based on normal IP rules, they would first determine that the other is not on the same subnet and then decide to use a default gateway to relay the information. If a client has never communicated with the de- fault gateway, it uses Address Resolution Protocol (ARP) to resolve its MAC address. The process would appear as follows:

Step 1. Client A wants to send traffic to Client B.
Step 2. Client A determines that the IP address of Client B is not on the same subnet.
Step 3. Client A decides to send the traffic to the default gateway of 10.99.99.5.
Step 4. Client A looks in its ARP table for a mapping to the gateway, but it is not there.
Step 5. Client A creates an ARP request and sends to the AP, as seen in Figure 9-3.


When the ARP request is sent to the AP, it is an interesting process and actually works a little bit differently than on a wired network. Remember that on a wired network, the header has only two MAC addresses: the source address and the destination address. An 802.11 frame can have four addresses: the source address (SA), destination address (DA), transmitter address (TA), and receiving address (RA). In this situation, the SA is the MAC of the client sending the ARP request, the DA is broadcast (for the ARP), and the RA is the AP. No TA is present in this example.

Figure 9-4 shows the ARP request.


The AP receives the ARP and sees its MAC address. It verifies the frame check sequence (FCS) in the frame and waits the short interframe space (SIFS) time. When the SIFS time expires, it sends an ACK back to the wireless client that sent the ARP request. This ACK is not an ARP response; rather, it is an ACK for the wireless frame transmission.

The AP then forwards the frame to the WLC using the Lightweight Access Point Protocol (LWAPP), as illustrated in Figure 9-5.


The LWAPP frame that travels from the AP to the WLC is traveling on a wired network. This brings forth the question, “What happened to the 802.11 frame format?” LWAPP simply encapsulates the frame inside a 6-byte header. The new 6-byte header has the AP IP and MAC address as the source and the WLC IP and MAC address as the destination. Encapsulated inside of that header is the original 802.11 frame with the three MAC ad- dresses, including the broadcast MAC address for the ARP process. When the WLC re- ceives the LWAPP frame, it opens the frame revealing the ARP request and rewrites the ARP request in an 802.3 frame that can be sent across the wired network. The first ad- dress from the 802.11 frame is dropped, the second address is placed as the source address in the new 802.3 frame, and the third address, the broadcast address, is placed as the desti- nation address. The WLC then forwards the ARP request, in 802.3 format, across the wired network, as seen in Figure 9-6. Here you can see how the frame appears between the wireless Client A and the AP, how the AP encapsulates the frame and sends it to the WLC, and how the WLC rewrites the frame and sends it to the wired network.


As switches receive the ARP request, they read the destination MAC address, which is a broadcast, and flood the frame out all ports except the one it came in on. The exception to this rule is if VLANs are in use, in which case the frame would be flooded to all ports that are members of the same VLAN. Assuming that VLANs are not in use, the frame, as stated, is flooded out all ports except the one it came in on.

At some point, the frame will be received by a Layer 3 device, hopefully the default gate- way. In Figure 9-7, the router has received the ARP request and will respond to it with its MAC address.


That ARP response is sent back as a unicast message, so the switches in the path are going to forward it directly to the port that leads back to the wireless client, rather than flooding the frame out all ports. Eventually the frame is received by the WLC, and it must be re- built as an 802.11 frame. When the WLC rewrites the frame, it places the DA as address 1, the SA as address 3, and the TA as address 2, which is the SSID of the AP. Figure 9-8 illus- trates this process.

As illustrated in Figure 9-9, the newly formed 802.11 frame is placed inside an LWAPP header where the AP IP and MAC is the destination and the WLC IP and MAC is the source. The LWAPP frame is forwarded to the AP.

Next, the AP must remove the LWAPP header, exposing the 802.11 frame. The 802.11 frame is buffered, and the process of sending a frame on the wireless network begins. The AP starts a backoff timer and begins counting down. If a wireless frame is heard during the countdown, the reservation in the heard frame is added to the countdown and the AP continues. Eventually, the timer expires, and the frame can be sent an 802.11 frame.

WiMax

Worldwide Interoperability for Microwave Access (WiMax) is defined by the WiMax fo- rum and standardized by the IEEE 802.16 suite. The most current standard is 802.16e.

According to the WiMax Forum:

“WiMAX is a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to wired broadband like cable and DSL. WiMAX provides fixed, nomadic, portable and, soon, mobile wireless broadband connectivity without the need for direct line-of-sight with a base station. In a typical cell radius deployment of three to ten kilometers, WiMAX Forum Certified systems can be ex- pected to deliver capacity of up to 40 Mbps per channel, for fixed and portable ac- cess applications.

“This is enough bandwidth to simultaneously support hundreds of businesses with T- 1 speed connectivity and thousands of residences with DSL speed connectivity. Mo- bile network deployments are expected to provide up to 15 Mbps of capacity within a typical cell radius deployment of up to three kilometers. It is expected that WiMAX technology will be incorporated in notebook computers and PDAs by 2007, allowing for urban areas and cities to become ‘metro zones’ for portable outdoor broadband wireless access.”

You must understand a few aspects of WiMax; the first is the concept of being fixed line of sight (LOS) or non-LOS (mobile). In non-LOS, mobile doesn’t mean mobile in the sense that most of us think. WiMax mobility is more like the ability to travel and then set up shop temporarily. When you are done, you pack up and move on. A few service providers use this technology to provide end-user access as an alternative to DSL or cable modem. Your signal range in this Non-LOS scenario is about 3 to 4 miles, and data rates are adver- tised at around 30 Mbps, but you can expect less—closer to 15 Mbps.

Other service providers are targeting business customers in a fixed LOS WiMax deploy- ment in which the topology most closely resembles that of a traditional T1, being a point- to-point type of topology and providing backhaul or backbone services. This fixed LOS advertises 30 to 70 Mbps throughput, but you can expect around 40 Mbps.

As the IEEE standardizes WiMax technology, it has progressed from the original 802.16 to 802.16a, c, d, and finally 802.16e.

As mentioned, the WiMax defines last-mile access. Figure 8-6 shows a sample topology in which subscribers have a point-to-point connection back to a service provider and from there have access to the public Internet.


WiMax operates on the 10- to 66-GHz frequency band, so it doesn’t interfere with 802.11 LANs. So why is it discussed in this section? The school of thought here is that, with some planning, a device acting as a gateway can be deployed offering 802.11 LAN access with 802.16 last-mile access or upstream access to a service provider, thus removing the need for wires. The question of how feasible this is lies in the hands of the vendors devel- oping the products and the standards committees ensuring interoperability. Some vendors, however, have tested this technology in lab environments with much success.


Other Types of Interference

Other types of interference can occur in the same frequency ranges. These devices might not be the most obvious, but they should be considered. They can include the following:

• Microwaves (operate at 1 to 40 GHz)

• Wireless X11 cameras (operate at 2.4 GHz)

• Radar systems (operate at 2 to 4 GHz for moderate-range surveillance, terminal traffic control, and long-range weather and at 4 to 8 GHz for long-range tracking and air- borne weather systems)

• Motion sensors (operate at 2.4 GHz)

• Fluorescent lighting (operates at 20000 Hz or higher)

• Game controllers and adapters (usually operate at 2.5 GHz)

When dealing with wireless deployments, you can use tools to determine signal strength and coverage, but just knowing about these additional sources of interference will save you some time in determining where to place APs and clients.

Additional Wireless Technologies

Cordless Phones

Cordless phones have been around as long as I can remember—or at least since I was in junior high. Cordless phones sometimes operate in the wireless spectrum as WLANs, which can cause interference issues. Visit an electronics store, and you’ll find some phones that operate at 2.4 GHz and others that operate at 5.8 GHz. This should be a consideration when you purchase cordless phones. If you have 802.11a deployed, a 2.4-GHz phone should suffice. If you have 802.11b/g, you should avoid a phone that operates in the 2.4- GHz range and go with a 5.8-GHz phone. With that said, let’s look at cordless phone technology in more detail.


To begin with,cordless phones canuse Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA). The Multiple Access technology is used to allow more than one handset to access the frequency band at the same time, as shown in Figure 8-1. As you can see, a cordless phone communicates with the base station. Multiple cordless phones can use the same base station at the same time by using TDMA or FDMA.


It’s common for cordless phones to use the Digital Enhanced Cordless Telecommunica- tions (DECT) standard. DECT is an ETSI standard for digital portable phones and is found in cordless technology that is deployed in homes and businesses. Currently, the DECT standard is a good alternative for avoiding interference issues with any 802.11 technolo- gies. The original DECT frequency band was 1880 to 1900 MHz. It’s used in all European countries. It is also used in most of Asia, Australia, and South America.

In 2005, the FCC changed channelization and licensing costs in the 1920 to 1930 MHz, or 1.9 GHz, band. This band is known as Unlicensed Personal Communications Services (UPCS). This change by the FCC allowed the use of DECT devices in the U.S. with few changes. The modified DECT devices are called DECT 6.0. This allows a distinction to be made between DECT devices used overseas and other cordless devices that operate at 900 MHz, 2.4 GHz, and 5.8 GHz.


Bluetooth

Bluetooth is a personal-area technology that was named after a king of Denmark, Harald “Bluetooth” Gormson. It is said that the use of his name is based on his role in unifying Denmark and Norway. Bluetooth technology was intended to unify the telecom and com- puting industries. Today, Bluetooth can be found integrated into cell phones, PDAs, lap- tops, desktops, printers, headsets, cameras, and video game consoles. Bluetooth has low power consumption, making it a good choice for mobile, battery-powered devices.

The Bluetooth Special Interest Group (SIG) was formed in 1998, and the name “Bluetooth” was officially adopted. In 1999, Bluetooth 1.0 and 1.0b were released, although they were pretty much unusable. Bluetooth 1.1 followed and was much more functional. Eventually, based on Bluetooth 1.1, the 802.15.1 specification was approved by the IEEE to conform with Bluetooth technology.

Bluetooth 1.2 was then adopted in 2003 with faster connections and discovery of devices as well as the use of adaptive Frequency Hopping Spread Spectrum technology. In 2004, Bluetooth 2.0 + Enhanced Data Rate (EDR), supporting speeds up to 2 Mbps, was adopted by the Bluetooth SIG. The IEEE followed with 802.15.1-2005, which is the speci- fication that relates to Bluetooth 1.2. After the 802.15-2005 standard, the IEEE severed ties to the Bluetooth SIG because the Bluetooth SIG wanted to pursue functionality with other standards.

Bluetooth technology might interfere with 802.11 LANs, because it operates in the 2.4- GHz range. However, because it is designed for a proximity of about 35 feet, has low transmit power, and uses Frequency Hopping Spread Spectrum, it is unlikely that Blue- tooth will interfere.

Bluetooth is considered a piconet; it allows eight devices (one master and seven slaves) to be paired, as shown in Figure 8-2. Although the figure is a little extreme, it shows you just how many devices can be paired with a laptop or desktop. You can download photos you’ve taken, while listening to music with your headphones, synchronizing your cell phone’s contacts and PDA calendar with Outlook, and using your mouse to print that new white paper on Cisco.com, all while playing a video game. Imagine the wire mess you would have without Bluetooth.


ZigBee

Many people have never heard of ZigBee, but it’s a technology that is well-designed and very useful. ZigBee was developed by the ZigBee Alliance. It consists of small, low-power digital radios based on the IEEE 802.15.4 standard for wireless personal-area networks (WPAN), such as wireless headphones connecting to cell phones via short-range radio. If you look at the ZigBee Alliance home page at http://www.zigbee.org, you’ll likely notice that ZigBee relates much of its use to control and monitoring. In fact, ZigBee is often used for monitoring, building automation, control devices, personal healthcare devices, and computer peripherals.

A Wireless Connection

Using Figures 7-11 through 7-18, you can step through a simple discovery and association process.

1. The AP sends beacons every 2 seconds, as shown in Figure 7-11.


2. Client A is passively scanning and hears the beacon. This enables the client to deter- mine whether it can connect. You can see this in Figure 7-12.

3. A new client (Client B) arrives. Client B is already configured to look for the AP, so in- stead of passive scanning, it sends a probe request for the specific AP (see Figure 7-13).


4. The AP sends a probe response, seen in Figure 7-14, which is similar to a beacon. This lets Client B determine if it can connect.


5. From this point on, the process would be the same for Client A and Client B. In Figure 7-15, Client B sends an authentication request.

6. Also seen in Figure 7-15, the AP returns an authentication response to the client.

7. The client then sends an association request, as seen in Figure 7-16.

8. Now the AP sends an association response, also seen in Figure 7-16.

9. When the client wants to send, it uses an RTS, assuming this is a mixed b/g cell. The RTS includes the duration, as you can see in Figure 7-17.

10. Also seen in Figure 7-17, the AP returns a CTS.

11. The client sends the data (see Figure 7-17).

12. The AP sends an ACK after each frame is received (Figure 7-17).

13. In Figure 7-18, the client sends a disassociation message.

14. The AP replies with a disassociation response (Figure 7-18).

15. The client returns and sends a reassociation message (Figure 7-18).

16. The AP responds with a reassociation response (Figure 7-18).


Again, this process has other variations, but this should give you a pretty good under- standing of how to manage a connection.

Wireless Frame Transmission

When people talk about wireless networks, they often say that they are just like wired 802.3 LANs. This is actually incorrect, aside from the fact that they use MAC addresses. Wireless LANs use the 802.11 frame structure, and you can encounter multiple types of frames. To get a better understanding, you can begin by learning the three types of wireless frames. Once you are familiar with the three types of wireless frames, you can further your knowledge by taking a deeper look at interframe spacing (IFS) and why it is necessary.


Wireless Frame Types

Wireless LANs come in three frame types:

• Management frames: Used for joining and leaving a wireless cell. Management frame types include association request, association response, and reassociation request, just to name a few. (See Table 7-2 for a complete list.)

• Control frames: Used to acknowledge when data frames are received.

• Data frames: Frames that contain data.

Now that you have an idea of what frames are used, it is helpful to see how these frames are sent. For this, you need to understand a few more terms that might be new to you. Because all the terms meld together to some degree, they are explained in context throughout the next section.


Sending a Frame

Recall that wireless networks are half-duplex networks. If more than one device were to send at the same time, a collision would result. If a collision occurs, the data from both senders would be unreadable and would need to be resent. This is a waste of time and resources. To overcome this issue, wireless networks use multiple steps to access the network. Wireless LANs use carrier sense multiple access collision avoidance (CSMA/CA), which is similar to the way 802.3 LANs work. The carrier sense part means that a station has to determine if anyone else is sending. This is done with clear channel assessment (CCA), and what it means is that you listen. You can, however, run into an issue where two devices cannot hear each other. This is called the hidden node problem. This issue is overcome using virtual carrier sense (VCS). The medium is not considered available until both the physical and virtual carrier report that it is clear.

Each station must also observe IFS. IFS is a period that a station has to wait before it can send. Not only does IFS ensure that the medium is clear, but it ensures that frames are not sent so close together that they are misinterpreted. The types of IFS periods are as follows:

• Short interframe space (SIFS): For higher priority and used for ACKs, among other things

• Point-coordination interframe space (PIFS): Used when an AP is going to control the network

• Distributed-coordination interframe space (DIFS): Used for data frames and is the normal spacing between frames

Each of these has a specific purpose as defined by the IEEE.

SIFS is used when you must send a frame quickly. For example, when a data frame is sent and must be acknowledged (ACK), the ACK should be sent before another station sends other data. Data frames use DIFS. The time value of DIFS is longer than SIFS, so the SIFS would preempt DIFS because it has a higher priority.

Figure 7-1 illustrates the transmission of a frame. In the figure, Station A wants to send a frame. As the process goes, both the physical and virtual carrier need to be free. This means the client has to listen. To listen, the client chooses a random number and begins a countdown process, called a backoff timer. The speed at which the countdown occurs is called a slottime and is different for 802.11a, b, and g.


It works like this:

1. Station A selects the random timer value of 29.

2. Station A starts counting at 29, 28, 27, 26, and so on. While Station A is counting down, it is also listening for whether anyone else is sending a frame.

3. When the timer is at 18, Station B sends a frame, having a duration value in the header of 45.

4. The duration of 45 that is in the header of the frame sent by Station B is called a network allocation vector (NAV) and is a reservation of the medium that includes the amount of time to send its frame, wait for the SIFS, and then receive an ACK from the AP.

5. Station A adds 45 to the 18 that is left and continues counting down, 63, 62, 61, and so on. The total time that Station A waits before sending is called the contention window.

6. After the timer on Station A reaches 0, it can send its frame as illustrated in Figure 7- 2. At this point, the medium should be clear.

If Station A sends but fails, it resets the backoff timer to a new random number and counts down again. The backoff timer gets larger as the frames fail in transmission. For example, the initial timer can be any number between 0 and 31. After the first failure, it jumps to any number between 0 and 127. It doubles for the next failure, then again, then again.


This entire process is known as the distributed coordination function (DCF). This simply means that each station is responsible for coordinating the sending of its data. The alternative to DCF is point coordination function (PCF), which means the AP is responsible for coordination of data transmission.


If the frame is successful, an ACK must be sent. The ACK uses the SIFS timer value to make sure it is sent quickly. Some amount of silence between frames is natural. The SIFS is the shortest period of silence. The NAV reserves this time. A normal silence time is the DIFS. Again, the ACK uses SIFS because you want it to be sent immediately. The station that sends the ACK waits for the SIFS and then ACKs with the duration of 0. This is how the end of the transmission is indicated.


Frame Types

For the most part, all frames are going to have the same type of header. The difference is in the body of the frame. The body is more specific and indicates what the frame is all about. Table 7-2 shows some frame types.

802.11 WLAN Protocols

wireless space consists of numerous protocols. Specifically in the WLAN area, the Institute of Engineers Electrical and Electronic Engineers (IEEE) has created several protocols within the 802.11 category to facilitate the networking process. These protocols define the data rates, the modulation techniques, and more. An understanding of these protocols is essential for any administrator of wireless networks.

The IEEE helps to standardize wireless protocols. Those that you must be familiar with for the CCNA Wireless Exam are the 802.11 a/b/g and n protocols. These four IEEE standards define the wireless family that is used in almost all wireless LANS today. The standardization of wireless networking started with the original 802.11 protocol in 1997, and each protocol thereafter has simply added to the benefit of wireless technologies. This chapter looks at the 802.11 protocol families, their history, and how they operate. The 802.11 protocols encompass the 2.4-GHz and 5-GHz range.


The Original 802.11 Protocol

The original 802.11 protocol was where wireless LANs find there beginnings. It is rare to find this original protocol in new hardware today, probably because it only operates at 1 and 2 Mbps. The 802.11 standard describes frequency-hopping spread spectrum (FHSS), which operates only at 1 and 2 Mbps. The standard also describes direct sequence spread spectrum (DSSS), which operates only at 1 and 2 Mbps. If a client operates at any other data rate, it is considered non-802.11 compliant, even if it can use the 1- and 2-Mbps rates.


The original 802.11 protocol falls within the industry, scientific, and medical (ISM) bands and operates only in the 2.4-GHz range. The 2.4-GHz range has up to 14 channels depending on the country you are in. In the United States, the FCC allows channels 1 through 11 to be used. This gives you 3 nonoverlapping channels: 1, 6, and 11. This is important because you do not want to have APs and clients operating on the same channel placed near each other for interference reasons.


The 802.11b Protocol

802.11b is a supplement to the 802.11 protocol. To get an better feel for how the 802.11 protocols progressed, understand that technology moves faster than the standards do. 802.11 was quickly outgrown because wired networks offered 10 Mbps versus the 1 and 2 Mbps of 802.11. Vendors developed methods of achieving higher data rates. The danger in vendor-designed protocols, of course, is interoperability. The job of the IEEE was simply to define a standard that all vendors could follow based on the proprietary implementations that they were using.

802.11b offers higher data rates—up to 11 Mbps—with backward compatibility at 1 and 2 Mbps. At 1 and 2 Mbps, the same coding and modulation as 802.11 is used. When operating at the new speeds—5.5 Mbps and 11 Mbps—a different modulation and coding is used. 802.11 uses Barker 11 coding, as covered in Chapter 1, “Introduction to Wireless Networking Concepts,” and 802.11b uses complementary code keying (CCK) for coding. For modulation, 802.11 uses differential binary phase-shift keying (DBPSK), whereas 802.11b uses differential quadrature phase-shift keying (DQPSK). The result is more data sent in the same period.

802.11b was ratified in September 1999. The United States has 11 channels, the same as 802.11. In Europe, the ETSI defines 13 channels, and Japan has 14. 802.11b allows dynamic rate shifting (DRS) to enable clients to shift rates to lower rates as they travel farther away from an AP and higher rates as they get closer to an AP. Today, 802.11b is the most popular and most widely deployed wireless standard. Table 6-3 gives some basic information on the 802.11b standard.


The 802.11g Protocol

The IEEE ratified 802.11g in June 2003. In addition to the four data rates of 802.11b, it added eight more. The maximum data rate of 54 Mbps places 802.11g in the same speed range as 802.11a; however, it remains in the 2.4-Ghz frequency range. On the lower end, 802.11g is still compatible with 802.11b, using the same modulation and coding as 802.11b for the 1-, 2-, 5.5-, and 11-Mbps rates. To achieve the higher data rates, 802.11g uses orthogonal frequency division multiplexing (OFDM) for modulation. OFDM is the same modulation that 802.11a uses.

There are still only three nonoverlapping channels. With OFDM, you must be careful about power outputs; the power needs to be reduced to handle the peaks in the modulation technique and still fall within governmental regulations. Table 6-4 shows some details about 802.11g.


The 802.11a Protocol

802.11a was ratified in 1999 and operates in the 5-GHz frequency range. This makes it incompatible with 802.11, 802.11b, and 802.11g, while avoiding interference from these devices in addition to microwaves, Bluetooth devices, and cordless phones. 802.11a had late-market adoption, so it is not as widely deployed as the 802.11b and g protocols.

Another difference is that 802.11a supports anywhere from 12 to 23 nonoverlapping channels as opposed to the 3 nonoverlapping channels in 802.11b/g. Because OFDM is used, subchannels can overlap. 802.11a requires that the data rates of 6, 12, and 24 Mbps be supported but allows for data rates up to 54 Mbps.

Table 6-5 shows some details on the 802.11a standard.


The rules under ETSI specifications are a little different. ETSI allows 19 channels and requires that dynamic frequency control (DFC) and transmit power control (TPC) be used.

What makes 802.11a unique is the way the 5-GHz frequency band is divided into multiple parts. These parts, the Unlicensed National Information Infrastructure (UNII), were designed for different uses. UNII-1 was designed for indoor use with a permanent antenna. UNII-2 was designed for indoor or outdoor use with an external antenna, and UNII-3 was designed for outdoor bridges and external antennas.

The FCC revised the use of the frequency in 2004 by adding channels and requiring compliance of DFC and TPC to avoid radar. The revision also allows all three parts of the UNII to be used indoors. This is not the case with ETSI, however, because it does not allow unlicensed use of UNII-3.


In the 802.11a spectrum, the higher-band channels are 30 MHz apart. This includes UNII- 2 and above. The lower bands are 20 MHz apart.


The 802.11n Protocol

802.11n is currently a draft standard. Again, technology has progressed more rapidly than the standards, because vendors are already shipping 802.11n APs and clients. What makes 802.11n special is that in a pure 802.11n environment, you can get speeds up to 300 Mbps, but most documentation says it will provide 100 Mbps. This is probably because the expectation is that other 802.11 clients will be present. 802.11n is, in fact, backward compatible with 802.11b/g and a.

The backward compatibility and speed capability of 802.11n come from its use of multiple antennas and a technology called Multiple-Input, Multiple-Output (MIMO). MIMO, pronounced Mee-Moh, uses different antennas to send and receive, thus increasing throughput and accomplishing more of a full duplex operation.

MIMO comes in three types:

• Precoding

• Spatial multiplexing

• Diversity coding

Precoding is a function that takes advantage of multiple antennas and the multipath issue that was discussed in Chapter 3, “WLAN RF Principles.” 802.11n uses transmit beamforming (TxBF), which is a technique that is used when more than one transmit antenna exists where the signal is coordinated and sent from each antenna so that the signal at the receiver is dramatically improved, even if it is far from the sender. This technique is something that you would use when the receiver has only a single antenna and is not moving. If the receiver is moving, then the reflection characteristics change, and the beamforming can no longer be coordinated. This coordination is called channel state information (CSI).

Spatial multiplexing takes a signal, splits it into several lower rate streams, and then sends each one out of different antennas. Each one of the lower rate streams are sent on the same frequency. The number of streams is limited to the lowest number of antennas on either the transmitter or the receiver. If an AP has four antennas and a client has two, you are limited to two.

Currently, the Wi-Fi Alliance is certifying 802.11n devices even though they are still in draft status. The Wi-FI Alliance is doing this using the interim IEEE 802.11n draft 2.0.

Common Antenna Types

The two main types of antennas are directional and omnidirectional. In this section you will learn the difference between the two types and look at some of the antennas that Cisco offers. Both send the same amount of energy; the difference is in how the beam is focused. To understand this, imagine that you have a flashlight. By twisting the head of the light, you can make the beam focus in a specific area. When the beam has a wider fo- cus, it doesn’t appear to be as bright. While you twist the head of the light, you never change its output. The batteries are the same. The power is the same. The light is the same. You simply focus it in different ways. The same goes for wireless antennas. When you look at a directional antenna, it appears to be a stronger signal in one direction, but it’s still emitting the same amount of energy. To increase power in a particular direction, you add gain.

The angles of coverage are fixed with each antenna. When you buy high-gain antennas, it is usually to focus a beam.


Omnidirectional Antennas

There are two ways to determine the coverage area of an antenna. The first is to place the AP in a location and walk around with a client recording the signal-to-noise ratio (SNR) and Received Signal Strength Indicator (RSSI). This could take a really long time. The sec- ond method is a little easier. In fact, the manufacturer does it for you. Figures 5-3 and 5-4 show different views of the wireless signal. Figure 5-3 shows how the wireless signal might propagate if you were standing above it and looking down on the antenna.

This is called the horizontal plane (H-plane) or azimuth. When you look at an omnidi-rectional antenna from the top (H-plane), you should see that it propagates evenly in a 360-degree pattern.

The vertical pattern does not propagate evenly, though. Figure 5-4 shows the elevation plane (E-plane). This is how the signal might propagate in a vertical pattern, or from top to bottom. As you can see, it’s not a perfect 360 degrees. This is actually by design. It’s what is known as the “one floor” concept. The idea is that the signal propagates wider from side to side than it does from top to bottom so that it can offer coverage to the floor it is placed on rather than to the floor above or below the AP.


Another way to look at this is to imagine an AP, as shown in Figure 5-5. If you draw in the H-plane and E-plane, you can relate the signal to each plane.

Now that you have a better understanding of how to determine the propagation patterns of an antenna, let’s look at some antennas.


2.2-dBi Dipole

The 2.2-dBi dipole,orrubberduck,showninFigure5-6,ismostoftenseenindoorsbe- cause it is a very weak antenna. In fact, it’s actually designed for a client or AP that doesn’t cover a large area. Its radiation pattern resembles a doughnut, because vertically it doesn’t propagate much. Instead, it’s designed to propagate on the H-plane. The term dipole may be new to you. The dipole antenna was developed by Heinrich Rudolph Hertz and is con- sidered the simplest type of antenna. Dipoles have a doughnut-shaped radiation pattern. Many times, an antenna is compared to an isotropic radiator. An isotropic radiator assumes that the signal is propagated evenly in all directions. This would be a perfect 360- degree sphere in all directions, on the H and E planes. The 2.2-dBi dipole antenna doesn’t work this way; rather, it has a doughnut shape.


IR-ANT1728

The AIR-ANT1728, shown in Figure 5-7, is a ceiling-mounted omnidirectional antenna op- erating at 5.2 dBi.

You would use this when a 2.14-dBi dipole doesn’t provide adequate coverage for an area. This antenna has more gain, thus increasing the H-plane, as shown in Figure 5-8.

The easiest way to express the effect of adding gain—in this case, 5.2 dBi versus 2.2 dBi— is to imagine squeezing a balloon from the top and the bottom, as shown in Figure 5-9.

The squeezing represents the addition of gain. The H-plane widens and the E-plane short- ens, as shown in Figure 5-10.

Table 5-2 details the statistics of the AIR-ANT1728.

Antenna Communications

Principles of Antennas

If someone asked you what the most important part of a wireless network is, what would you say? I’d have to say the antenna. Why? Without it, you have a nice little AP that can offer network services for anyone within about 3 feet. But that’s not what you want. You want to make sure that your space is properly covered. You need antennas to do this. In fact, you need the right antennas to do this. In this section you will learn about the factors involved in dealing with antennas, which include polarity and diversity.


Polarization

The goal of an antenna is to emit electromagnetic waves. The electro portion of the term electromagnetic describes the wave and that it can move in different ways. The way that it moves is its polarization. There are three types of polarization:

• Vertical
• Horizontal
• Circular

As shown in Figure 5-1, vertical polarization means that the wave moves up and down in a linear way. Horizontal polarization means that the wave moves left and right in a linear way.

The third type of polarization, circular polarization, indicates that the wave circles as it moves forward, as illustrated in Figure 5-2.

The electric field is generated by stationary charges, or current. There is also a magnetic field hence the term electromagnetic. The magnetic field is found perpendicular (at a 90- degree angle) to the electric field. This magnetic field is generated at the same time as the electric field; however, the magnetic field is generated by moving charges. Cisco antennas are always vertically polarized in wireless networks. This makes the electric field vertical. Why is this important? The importance is that the antenna is designed to propagate signals in a certain direction. Here is where installation errors can hurt you. For example, if you have a long tube-like antenna, it would face up/down. If you placed it flat instead, the signal would propagate in a different direction and would end up degraded.

Although this is not a huge factor in indoor deployments, it can be in outdoor deployments. Usually other factors degrade your wireless signal propagation on indoor deployments.


Diversity

By now you should understand what the multipath issue is. Traffic takes different paths because of the obstacles in the wireless path. One way to deal with multipath issues is to use two antennas on one AP. Diversity is the use of two antennas for each radio to increase the odds that you receive a better signal on either of the antennas.



Here is how it works: The two antennas are placed one wavelength apart. When the AP hears a preamble of a frame, it switches between the two antennas and uses an algorithm to determine which antenna has the better signal. After an antenna is chosen, it is used for the rest of that frame. You can switch antennas and listen to the preamble because it has no real data. As soon as the real data gets there, it uses only one of the antennas.

Most of the time this happens with a single radio in the AP and two antennas connected to it. This is important because the two antennas cover the same area. You wouldn’t try to cover two different areas with the same radio. Additionally, the antennas need to be the same. If you used a weaker antenna on one side versus the other, the coverage area would not be the same.

Vendor-Specific Topology Extensions

The vendor-specific topology extensions are an enablement of additional network functionality by way of vendor-defined protocols, devices, and topologies. In this section you will learn how workgroup bridges, wireless repeaters, outdoor wireless bridges, and wire- less mesh networks through the use of wireless controllers can enhance the functionality and capability of your wireless deployment.

Workgroup Bridges
You will most likely have times when you have an isolated network that needs access to the rest of the network for access to the server farm and the Internet. You might not be able to run an Ethernet cable to the isolated network, or you might not own the property so you can’t drill holes in the walls, and so on. In this scenario, you would use a WGB topology such as the one shown in Figure 4-6.

Notice that the WGB is used to bridge a wired network to an AP that connects to a distri- bution system.

Cisco offers two types of workgroup bridges:

• Autonomous Workgroup Bridge (aWGB): The aWGB was originally just called a workgroup bridge, but Cisco later changed the name when it introduced the Univer- sal WGB. The aWGB is supported in IOS AP version 12.4(3G)JA and later. The aWGB connects only to upstream Cisco APs, and the AP sees multiple Ethernet clients.
  

• Universal Workgroup Bridge (uWGB): The uWGB is supported on IOS AP version 12.4(11)XJ and later. It allows bridging to upstream non-Cisco APs and appears as a single client.

Repeaters
Recall that in an Extended Service Set (ESS), multiple APs connect clients. This is all well and good until you have clients roaming about who get into areas where coverage is neces- sary but not possible. The solution of a WGB doesn’t work, because a WGB connects users who are wired. An example is a worker at a warehouse who carries a barcode scan- ner or even a wireless Cisco IP Phone. There are scenarios where you can’t run a cable into a location to install an AP. This is where you want to use a wireless repeater. A wireless repeater is simply an AP that doesn’t connect to a wired network for its connectivity to the distribution network. Instead, it overlaps with an AP that does physically connect to the distribution network. The overlap needs to be 50 percent for optimal performance. Figure 4-7 shows an example. A repeater is allowing a client to connect to the network when in fact the client would normally be out of the service area of the AP.

You can get APs that act as a repeater as well, which is how the Cisco solution works. The catch is that you need a Cisco AP as the upstream “root” device, and only one SSID is supported in repeater mode. Additionally, the overall throughput is cut in half for each re- peater hop.


Outdoor Wireless Bridges

When you have two or more LANs within a few miles of each other and you want to link them, you can use a wireless bridge. Because you are “bridging,” the technology works at Layer 2. This means that the LANs do not route traffic and do not have a routing table.

You can connect one LAN directly to another in a point-to-point configuration, as shown in Figure 4-8, or you can connect many LANs through a central hub, as shown in Figure 4-9.


Each end of a point-to-multipoint topology would have to communicate through the hub if it wanted to communicate with the others. Cisco offers the Cisco Aironet 1300 series wireless bridge and the Cisco Aironet 1400 series wireless bridge. When using a 1400 se- ries, you can bridge only networks, but if you use a 1300 series, you can allow clients to connect as well as bridge networks. The 1300 series operates in the 2.4-GHz range, and the 1400 series operates in the 5-GHz range.


Outdoor Mesh Networks

As you can see, bridges are a good way to connect remote sites. However, suppose that you are operating in a point-to-multipoint topology, and the central site experiences con- gestion. Who suffers? Just the central site? Just the remote site? No; the answer is every- one. When two remote sites communicate through a central site, the central site makes all the difference.

Assume that the central site goes down, as shown in Figure 4-10.


Now the remote sites can’t communicate with each other or the central site. This can be a major issue to contend with. The solution is to deploy a mesh network such as the one illustrated in Figure 4-11.

The mesh solution is appropriate when connectivity is important, because multiple paths can be used. The IEEE is currently working on a mesh standard (802.11s). However, the solution discussed here is a Cisco solution in which a wireless controller, also shown in Figure 4-11, is involved.

When you have a mesh network, some nodes (another term for APs in a mesh network) are connected to a wired network. Some nodes simply act as repeaters. A mesh node re- peats data to nearby nodes. More than one path is available, so a special algorithm is used to determine the best path. The alternative paths can be used when there is congestion or when a wireless mesh node goes down.

Original 802.11 Topologies

Although the previous sections discussed network topologies that you might encounter, it was a very general discussion. You also need to understand the original topologies, defined by the 802.11 committees, including the following:

• Ad hoc mode
• Infrastructure mode

The following sections give more details on these topologies.

Overview of Ad Hoc Networks

When two computers want to communicate directly with one another, they do so in the form of an ad hoc network. Ad hoc networks don’t require a central device to allow them to communicate. Rather, one device sets a group name and radio parameters, and the other uses it to connect. This is called a Basic Service Set (BSS), which defines the area in which a device is reachable. Because the two machines don’t need a central device to speak to each other, it is called an Independent Basic Service Set (IBSS). This type of ad hoc network exists as soonas two devices see each other. Figure4-2 shows an ad hoc network.

Each computer has only one radio. Because there is only one radio, the throughput is lower and acts as a half-duplex device, because you can’t send and receive at the same time.

You don’t have much control in these networks, so you’re stuck when it comes to methods such as authentication. In addition, you need to address who starts the conversation and who decides on the order of communication, to name just a couple issues.

Network Infrastructure Mode

In wireless networks, an access point acts as a connection point for clients. An AP is actually a cross between a hub and a bridge. Here’s why:

• There is one radio, which cannot send and receive at the same time. This is where the AP is likened to a hub. It’s a half-duplex operation.
• APs have some intelligence that is similar to that of a bridge. That is how an AP can see a frame and decide to forward it based on MAC addresses.

What is different on an AP versus a bridge is that wireless frames are more complex. Standard Ethernet frames have a source MAC address and a destination MAC address. Wireless frames can have three or four MAC addresses. Two of them are the source and destination MAC addresses, and one is the AP’s MAC address that is tied to a workgroup.


The fourth that could be present is a NEXT_HOP address in the event that you are using a workgroup bridge (WGB).

An AP is actually just one type of wireless station. This terminology could cause some confusion between an AP and a client on a network, so to differentiate between them, a client is called a station (STA), and an AP is called an infrastructure device.

So what does a typical wireless topology look like? Of course, wireless clients are associated with an AP. In the wireless space, the coverage area of the AP is called a Basic Service Area (BSA), which is also sometimes known as a wireless cell. They mean the same thing. When only one AP exists, this coverage area is called a BSA, as shown in Figure 4-3. That AP then usually has an Ethernet connection to an 802.3 LAN, depending on the function of the AP.

Assuming that the AP has an Ethernet connection, it bridges the 802.11 wireless traffic from the wireless clients to the 802.3 wired network on the Ethernet side.

The wired network attached to the AP’s Ethernet port is a path to a wireless LAN controller (or controller for short). The client traffic is passed through the controller and then is forwarded to the wired network, called the distribution system. The distribution system is how a client accesses the Internet, file servers, printers, and anything else available on the wired network.
When more than one AP is connected to a common distribution system, as shown in Figure 4-4, the coverage area is called an Extended Service Area (ESA).


Why would you want more than one AP connected to the same LAN? There are a few reasons:

• To provide adequate coverage in a larger area.
• To allow clients to move from one AP to the other and still be on the same LAN.
• To provide more saturation of APs, resulting in more bandwidth per user.

This process of a client moving from one AP to another is called roaming. For roaming to work, the APs must overlap. You might wonder why they need to overlap, because interference in a wireless network is a common issue. The reason for the overlap is so that a client can see both APs and associate to the one with the stronger signal. As soon as the signal from the associated AP hits the threshold built into the client, the client looks for another AP with a better signal.


Service Set Identifiers

Think about how you connect to a wireless network. On your laptop, you might see a popup that says “Wireless networks are available” or something to that effect. When you look at the available networks, you see names. On older Cisco autonomous APs, the network was called “Tsunami.” On a store-bought Linksys, the network is actually called “linksys.” So the client sees a name that represents a network.

On the AP, the network is associated with a MAC address. This network or workgroup that your clients connect to is called a Service Set Identifier (SSID). So on an AP, the SSID is a combination of MAC address and network name. This MAC address can be that of the wireless radio or another MAC address generated on the AP. When an AP offers service for only one network, it is called a Basic Service Set Identifier (BSSID). APs offer the ability to use more than one SSID. This would let you offer a Guest Network and a Corporate Network and still use the same AP. When the AP has more than one network, it is called a Multiple Basic Service Set Identifier (MBSSID). You can think of it as a virtual AP. It offers service for multiple networks, but it’s the same hardware. Because it’s the same hardware and the same frequency range, users on one network share with users on another and can collide if they send at the same time.

Now let’s return to the roaming discussion. To get roaming to work, the BSA of each AP must overlap. The APs also need to be configured for the same SSID. This enables the client to see that the same network is offered by different MAC addresses, as illustrated in Figure 4-5.


When a client roams and moves from one AP to the other, the SSID remains the same, but the MAC address changes to the new AP with a better signal.

Another issue to consider when roaming is the possibility of interference between the two overlapping APs. Even though they offer the same SSID, they need to be on different channels, or frequency ranges, that do not overlap. This prevents co-channel interference, which should be avoided. The 2.4 spectrum allows only three nonoverlapping channels. You must consider this fact when placing APs.