Wireless Technology Overview
In its most simplistic form, a WLAN is an LAN that uses radio frequency (RF) to communicate instead of using a wire. As shown in Figure 5-1, wireless clients connect to wireless access points (WAPs).
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Figure 5-1 Wired and Wireless Networks
Because WLANs use RF, the throughput (speed) is inversely proportional to the distance between the transmitter and the receiver.[2] Therefore, everything being equal (notwithstanding interferences), the closer a wireless client is to a transmitter, the greater is the throughput, as shown in Figure 5-2.
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Figure 5-2 Throughput (Coverage) Is Related to the Distance from the RF Transmitter
However, wireless communication brings a trade-off between flexibility and mobility versus battery life and usable bandwidth.
Wireless Standards
WLAN standards that are currently supported by major vendors were developed by the working group 11 of the Institute of Electrical and Electronics Engineers (IEEE) 802 committee. The most common standards are shown in Table 5-1.
Table 5-1. Wireless Standards
|
Standard |
Maximum Throughput (Mbps) |
Frequency (GHz) |
Compatibility |
Ratified |
|
802.11b |
11 |
2.4 |
— |
1999 |
|
802.11a |
54 |
5 |
— |
1999; Product availability 2001 |
|
802.11g |
54 |
2.4 |
Backward-compatible with 802.11b |
2003 |
The 802.11a standard operates in the unlicensed 5-GHz band, which makes the transmission vulnerable to interference from microwave ovens and cordless phones. The strength of 802.11b and 802.11g signals, which operate in the 2.4-GHz band, is affected negatively by water, metal, and thick walls.
The 802.11b and 802.11g standards divide the 2.4 GHz into 14 overlapping individual channels. Channels 1, 6, and 11 do not overlap and therefore can be used to set up multiple networks. The 802.11a standard is an amendment to the original standard. The advantage of using 802.11a is that it suffers less from interference, but its use is restricted to almost line of sight, thus requiring the installation of more access points than 802.11b to cover the same area.
The medium access method of the 802.11 standards, called the Distribution Coordination Method, is similar to the carrier sense multiple access collision detect (CSMA/CD) mechanism of Ethernet.
The following types of frames are transferred over the airwaves:
- Data frame— Network traffic.
- Control frame— Frame controlling access to the medium, similar to a modem's analog connection control mechanism, with its Request To Send (RTS), Clear To Send (CTS), and acknowledgment (ACK) signals.
- Manager frame— Frames similar to data frames, pertaining to the control of the current wireless transmission.
Wireless Components
The main components of wireless networks are as follows:
- Wireless access points
- Wireless client devices
Wireless Access Points
WAPs provide connectivity between wireless client devices and the wired network, as shown earlier in Figure 5-1.
Integrated Access Point
The WAP does not need to be a stand-alone device. Cisco offers integrated access point functionality[3] for some small- to medium-business (SMB) routers, as shown in Figure 5-3. By installing a high-speed wireless interface card (HWIC) in Cisco 1800, 2800, or 3800 routers, customers can run concurrent routing, switching, and security services and include IEEE 802.11 wireless LAN functionality in a single platform.
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Figure 5-3 Integrating Routing and Wireless Functionality
Wireless Client Devices
A wireless client device is equipped with a wireless interface card (WIC), which the device uses to communicate over RF with WAPs. Wireless clients can be the following items, among other things:
- User workstations and laptops
- PDAs
- Wireless IP phones
User Workstations and Laptops: Ad-Hoc Network
In addition to connecting to a WLAN access point, two wireless end stations can form an exclusive, point-to-point, wireless network without the intervention of an access point. This type of independent network is known as an ad-hoc network.
PDAs
Wireless PDAs—PDAs that connect directly on the corporate network—play a significant role in an organization where time is extremely sensitive. An example of where 802.11b-compatible devices (wireless PDAs) are put to benefit is triage nurses who are faster at inputting their assessment and sharing their findings on the spot rather than walking back to the nurses' station to do so.
Wireless IP Phones
Absolute campus mobility is probably best demonstrated by Cisco wireless IP phones.[4] These 802.11b phones have built-in security, QoS, and management features. Wireless IP phones leverage existing IP telephony deployments, as shown in Figure 5-4.
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Figure 5-4 Deploying Wireless IP Phones