Basic Service Set

The solution is to make every wireless service area a closed group of mobile devices that forms around a fixed device; before a device can participate, it must advertise its capabilities and then be granted permission to join. The 802.11 standard calls this a basic service set (BSS). At the heart of every BSS is a wireless access point (AP), as shown in Figure 26-4. The AP operates in infrastructure mode, which means it offers the services that are necessary to form the infrastructure of a wireless network. The AP also establishes its BSS over a single wireless channel. The AP and the members of the BSS must all use the same channel to communicate properly.

Because the operation of a BSS hinges on the AP, the BSS is bounded by the area where the AP’s signal is usable. This is known as the basic service area (BSA) or cell. In Figure 26-4, the cell is shown as a simple shaded circular area that centers around the AP itself. Cells can have other shapes too, depending on the antenna that is connected to the AP and on the physical surroundings that might affect the AP’s signals.

The AP serves as a single point of contact for every device that wants to use the BSS. It advertises the existence of the BSS so that devices can find it and try to join. To do that, the AP uses a unique BSS identifier (BSSID) that is based on the AP’s own radio MAC address.

In addition, the AP advertises the wireless network with a Service Set Identifier (SSID), which is a text string containing a logical name. Think of the BSSID as a machine-readable name tag that uniquely identifies the BSS ambassador (the AP), and the SSID as a nonunique, human-readable name tag that identifies the wireless service.

Membership with the BSS is called an association. A wireless device must send an association request to the AP and the AP must either grant or deny the request. Once associated, a device becomes a client, or an 802.11 station (STA), of the BSS. What then? As long as a wireless client remains associated with a BSS, most communications to and from the client must pass through the AP, as indicated in Figure 26-5. By using the BSSID as a source or destination address, data frames can be relayed to or from the AP.

You might be wondering why all client traffic has to traverse the AP at all. Why can two clients not simply transmit data frames directly to each other and bypass the middleman? If clients are allowed to communicate directly, then the whole idea of organizing and managing a BSS is moot. By sending data through the AP first, the BSS remains stable and under control.

Distribution System

Notice that a BSS involves a single AP and no explicit connection into a regular Ethernet network. In that setting, the AP and its associated clients make up a standalone network. But the AP’s role at the center of the BSS does not just stop with managing the BSS; sooner or later, wireless clients will need to communicate with other devices that are not members of the BSS. Fortunately, an AP can also uplink into an Ethernet network because it has both wireless and wired capabilities. The 802.11 standard refers to the upstream wired Ethernet as the distribution system (DS) for the wireless BSS, as shown in Figure 26-6.

You can think of an AP as a translational bridge, where frames from two dissimilar media (wireless and wired) are translated and then bridged at Layer 2. In simple terms, the AP is in charge of mapping a virtual local-area network (VLAN) to an SSID. In Figure 26-6, the AP maps VLAN 10 to the wireless LAN using SSID “MyNetwork.” Clients associated with the “MyNetwork” SSID will appear to be connected to VLAN 10.

This concept can be extended so that multiple VLANs are mapped to multiple SSIDs. To do this, the AP must be connected to the switch by a trunk link that carries the VLANs. In Figure 26-7, VLANs 10, 20, and 30 are trunked to the AP over the DS. The AP uses the 802.1Q tag to map the VLAN numbers to the appropriate SSIDs. For example, VLAN 10 is mapped to SSID “MyNetwork,” VLAN 20 is mapped to SSID “YourNetwork,” and VLAN 30 to SSID “Guest.”

In effect, when an AP uses multiple SSIDs, it is trunking VLANs over the air, and over the same channel, to wireless clients. The clients must use the appropriate SSID that has been mapped to the respective VLAN when the AP was configured. The AP then appears as multiple logical APs—one per BSS—with a unique BSSID for each. With Cisco APs, this is usually accomplished by incrementing the last digit of the radio’s MAC address for each SSID.

Even though an AP can advertise and support multiple logical wireless networks, each of the SSIDs covers the same geographic area. The reason is that the AP uses the same transmitter, receiver, antennas, and channel for every SSID that it supports. Beware of one misconception though: multiple SSIDs can give an illusion of scale. Even though wireless clients can be distributed across many SSIDs, all of those clients must share the same AP’s hardware and must contend for airtime on the same channel.

Extended Service Set

Normally, one AP cannot cover the entire area where clients might be located. For example, you might need wireless coverage throughout an entire floor of a business, hotel, hospital, or other large building. To cover more area than a single AP’s cell can cover, you simply need to add more APs and spread them out geographically.

When APs are placed at different geographic locations, they can all be interconnected by a switched infrastructure. The 802.11 standard calls this an extended service set (ESS), as shown in Figure 26-8.

The idea is to make multiple APs cooperate so that the wireless service is consistent and seamless from the client’s perspective. Ideally, any SSIDs that are defined on one AP should be defined on all the APs in an ESS; otherwise, it would be very cumbersome and inconvenient for a client to be reconfigured each time it moves into a different AP’s cell.

Notice that each cell in Figure 26-8 has a unique BSSID, but both cells share one common SSID. Regardless of a client’s location within the ESS, the SSID will remain the same but the client can always distinguish one AP from another.

In an ESS, a wireless client can associate with one AP while it is physically located near that AP. If the client later moves to a different location, it can associate with a different nearby AP automatically. Passing from one AP to another is called roaming. Keep in mind that each AP offers its own BSS on its own channel, to prevent interference between the APs. As a client device roams from one AP to another, it must scan the available channels to find a new AP (and BSS) to roam toward. In effect, the client is roaming from BSS to BSS, and from channel to channel.

Independent Basic Service Set

Usually a wireless network leverages APs for organization, control, and scalability. Sometimes that is not possible or convenient in an impromptu situation. For example, two people who want to exchange electronic documents at a meeting might not be able to find a BSS available or might want to avoid having to authenticate to a production network. In addition, many personal printers have the capability to print documents wirelessly, without relying on a regular BSS or AP.

The 802.11 standard allows two or more wireless clients to communicate directly with each other, with no other means of network connectivity. This is known as an ad hoc wireless network, or an independent basic service set (IBSS), as shown in Figure 26-9. For this to work, one of the devices must take the lead and begin advertising a network name and the necessary radio parameters, much like an AP would do. Any other device can then join as needed. IBSSs are meant to be organized in an impromptu, distributed fashion; therefore, they do not scale well beyond eight to ten devices.

Repeater

Normally, each AP in a wireless network has a wired connection back to the DS or switched infrastructure. To extend wireless coverage beyond a normal AP’s cell footprint, additional APs and their wired connections can be added. In some scenarios, it is not possible to run a wired connection to a new AP because the cable distance is too great to support Ethernet communication.

In that case, you can add an additional AP that is configured for repeater mode. A wireless repeater takes the signal it receives and repeats or retransmits it in a new cell area around the repeater. The idea is to move the repeater out away from the AP so that it is still within range of both the AP and the distant client, as shown in Figure 26-10.

If the repeater has a single transmitter and receiver, it must operate on the same channel that the AP is using. That can create the possibility that the AP’s signal will be received and retransmitted by the repeater, only to be received again by the AP—halving the effective throughput because the channel will be kept busy twice as long as before. As a remedy, some repeaters can use two transmitters and receivers to keep the original and repeated signals isolated on different channels. One transmitter and receiver pair is dedicated to signals in the AP’s cell, while the other pair is dedicated to signals in the repeater’s own cell.

Workgroup Bridge

Suppose you have a device that supports a wired Ethernet link but is not capable of having a wireless connection. For example, some mobile medical devices might be designed with only a wired connection. While it is possible to plug the device into an Ethernet connection when needed, a wireless connection would be much more practical. You can use a workgroup bridge (WGB) to connect the device’s wired network adapter to a wireless network.

Rather than providing a BSS for wireless service, a WGB becomes a wireless client of a BSS. In effect, the WGB acts as an external wireless network adapter for a device that has none. In Figure 26-11, an AP provides a BSS; Client A is a regular wireless client, while Client B is associated with the AP through a WGB.

You might encounter two types of workgroup bridges:

• Universal workgroup bridge (uWGB): A single wired device can be bridged to a wireless network.

• Workgroup bridge (WGB): A Cisco-proprietary implementation that allows multiple wired devices to be bridged to a wireless network.

Outdoor Bridge

An AP can be configured to act as a bridge to form a single wireless link from one LAN to another over a long distance. Outdoor bridged links are commonly used for connectivity between buildings or between cities.

If the LANs at two locations need to be bridged, a point-to-point bridged link can be used. One AP configured in bridge mode is needed on each end of the wireless link. Special purpose antennas are normally used with the bridges to focus their signals in one direction—toward the antenna of the AP at the far end of the link. This maximizes the link distance, as shown in Figure 26-12.

Sometimes the LANs at multiple sites need to be bridged together. A point-to-multipoint bridged link allows a central site to be bridged to several other sites. The central site bridge is connected to an omnidirectional antenna, such that its signal is transmitted equally in all directions so that it can reach the other sites simultaneously. The bridges at each of the other sites can be connected to a directional antenna aimed at the central site. Figure 26-13 shows the point-to-multipoint scenario.

Mesh Network

To provide wireless coverage over a very large area, it is not always practical to run Ethernet cabling to every AP that would be needed. Instead, you could use multiple APs configured in mesh mode. In a mesh topology, wireless traffic is bridged from AP to AP, in a daisy-chain fashion, using another wireless channel.

Mesh APs can leverage dual radios—one using a channel in one range of frequencies and one a different range. Each mesh AP usually maintains a BSS on one channel, with which wireless clients can associate. Client traffic is then usually bridged from AP to AP over other channels as a backhaul network. At the edge of the mesh network, the backhaul traffic is bridged to the wired LAN infrastructure. Figure 26-14 shows a typical mesh network. With Cisco APs, you can build a mesh network indoors or outdoors. The mesh network runs its own dynamic routing protocol to work out the best path for backhaul traffic to take across the mesh APs.

Wireless Bands and Channels

Because a range of frequencies might be used for the same purpose, it is customary to refer to the range as a band of frequencies. For example, the range from 530 kHz to around 1710 kHz is used by AM radio stations; therefore, it is commonly called the AM band or the AM broadcast band.

One of the two main frequency ranges used for wireless LAN communication lies between 2.400 and 2.4835 GHz. This is usually called the 2.4-GHz band, even though it does not encompass the entire range between 2.4 and 2.5 GHz. It is much more convenient to refer to the band name instead of the specific range of frequencies included.

The other wireless LAN range is usually called the 5-GHz band because it lies between 5.150 and 5.825 GHz. The 5-GHz band actually contains the following four separate and distinct bands:

5.150 to 5.250 GHz

5.250 to 5.350 GHz

5.470 to 5.725 GHz

5.725 to 5.825 GHz

It is interesting that the 5-GHz band can contain several smaller bands. Remember that the term band is simply a relative term that is used for convenience. Do not worry about memorizing the band names or exact frequency ranges; just be aware of the two main bands at 2.4 and 5 GHz.

A frequency band contains a continuous range of frequencies. If two devices require a single frequency for a wireless link between them, which frequency can they use? Beyond that, how many unique frequencies can be used within a band? To keep everything orderly and compatible, bands are usually divided into a number of distinct channels. Each channel is known by a channel number and is assigned to a specific frequency. As long as the channels are defined by a national or international standards body, they can be used consistently in all locations. Figures 26-21 and 26-22 show the channel layout for the 2.4 and 5 GHz bands, respectively.

You might assume that an AP can use any channel number without affecting any APs that use other channel numbers. In the 5-GHz band, this is the case because each channel is allocated a frequency range that does not encroach on or overlap the frequencies allocated for any other channel. In other words, the 5-GHz band consists of nonoverlapping channels.

The same is not true of the 2.4-GHz band. Each of its channels is much too wide to avoid overlapping the next lower or upper channel number. In fact, each channel covers the frequency range that is allocated to more than four consecutive channels! Notice the width of the channel spacing in Figure 26-21 as compared to the width of one of the shaded signals centered on channels 1, 6, and 11. The only way to avoid any overlap between adjacent channels is to configure APs to use only channels 1, 6, and 11. Even though there are 14 channels available to use, you should always strive for nonoverlapping channels in your network.

APs and Wireless Standards

It might be obvious that wireless devices and APs should all be capable of operating on the same band. For example, a 5-GHz wireless phone can communicate only with an AP that offers Wi-Fi service on 5-GHz channels. In addition, the devices and APs must also share a compatibility with the parts of the 802.11 standard they support.

As the IEEE 802.11 Wi-Fi standard evolves and develops, new amendments with new functionality get proposed. These amendments are known by “802.11” followed by a one- or two-letter suffix until they are accepted and rolled up into the next generation of the complete 802.11 standard. Even then, it is common to see the amendment suffixes still used to distinguish specific functions.

You should be aware of several amendments that define important characteristics such as data rates, methods used to transmit and receive data, and so on. For the CCNA 200-301 exam, you should know which band each of the amendments listed in Table 26-3 uses. The ENCOR 300-401 exam goes further into the data rates and modulation and coding schemes used by each.

The 802.11 amendments are not mutually exclusive. Wireless client devices and APs can be compatible with one or more amendments; however, a client and an AP can communicate only if they both support and agree to use the same amendment. When you look at the specifications for a wireless device, you may find supported amendments listed in a single string, separated by slashes. For example, a device that supports 802.11b/g will support both 802.11b and 802.11g. One that supports b/g/a/n/ac will support 802.11b, 802.11g, 802.11n, and 802.11ac. You should become familiar with Table 26-3 so that you can know which bands a device can use based on its 802.11 amendment support.

If a device can operate on both bands, how does it decide which band to use? APs can usually operate on both bands simultaneously to support any clients that might be present on each band. However, wireless clients typically associate with an AP on one band at a time, while scanning for potential APs on both bands. The band used to connect to an AP is chosen according to the operating system, wireless adapter driver, and other internal configuration. A wireless client can initiate an association with an AP on one band and then switch to the other band if the signal conditions are better there.

In open space, RF signals propagate or reach further on the 2.4-GHz band than on the 5-GHz band. They also tend to penetrate indoor walls and objects easier at 2.4 GHz than 5 GHz. However, the 2.4-GHz band is commonly more crowded with wireless devices. Remember that only three nonoverlapping channels are available, so the chances of other neighboring APs using the same channels is greater. In contrast, the 5-GHz band has many more channels available to use, making channels less crowded and experiencing less interference.