Devices on a wireless LAN (WLAN) transmit and receive data using radio or infrared signals, sent through an access point (AP). WLANs function similarly to Ethernet LANs with the access point providing connectivity to the rest of the network as would a hub or switch. WLANs use an Institute of Electrical and Electronics Engineers (IEEE) standard that defines the physical and data link specifications, including the use of Media Access Control (MAC) addresses. The same protocols (such as IP) and applications (such as IPSec) can run over both wired and wireless LANs.
WLANs are local to a building or a campus, use customer-owned equipment, and are not usually required to have radio frequency (RF) licenses.
Service Set Identifiers (SSID) correspond to a VLAN and can be used to segment users. SSIDs can be broadcast by the access point, or statically configured on the client, but the client must have the same SSID as the AP to register with it. SSIDs are case sensitive. Clients associate with access points as follows:
Step 1. The client sends a probe request.
Step 2. The AP sends a probe response.
Step 3. The client initiates an association to an AP. Authentication and any other security information is sent to the AP.
Step 4. The AP accepts the association.
Step 5. The AP adds the client’s MAC address to its association table.
Characteristics of Wireless LANs
The following lists some characteristics of wireless LANs, and the data transmitted over wireless networks.
- WLANs use Carrier Sense Multi-Access/Collision Avoidance (CSMA/CA). Wireless data is half-duplex. CSMA/CA uses Request to Send (RTS) and Clear to Send (CTS) messages to avoid collisions.
- WLANs use a different frame type than Ethernet.
- Radio waves have unique potential issues. They are susceptible to interference, multipath distortion, and noise. Their coverage area can be blocked by building features, such as elevators. The signal might reach outside the building and lead to privacy issues.
- WLAN hosts have no physical network connection. They are often mobile and often battery-powered. The wireless network design must accommodate this.
- WLANs must adhere to each country’s RF standards.
Clients can roam between APs that are configured with the same SSIDs/VLANs. Layer 2 roaming is done between APs on the same subnet; Layer 3 roaming is done between APs on different subnets.
WLAN Topologies
Use of the Cisco Aironet line of wireless products falls into three categories:
- Client access, which allows mobile users to access the wired LAN resources
- Wireless connections between buildings
- Wireless mesh
Wireless connections can be made in ad-hoc mode or infrastructure mode. Ad-hoc mode (or Independent Basic Service Set [IBSS]) is simply a group of computers talking wirelessly to each other with no access point (AP). It is limited in range and functionality. Infrastructure mode’s BSS uses one AP to connect clients. The range of the AP’s signal, called its microcell, must encompass all clients. The Extended Service Set (ESS) uses multiple APs with overlapping microcells to cover all clients. Microcells should overlap by 10–15 percent for data, and 15–20 percent for voice traffic. Each AP should use a different
channel.
Wireless repeaters extend an AP’s range. They use the same channel as their AP, they must be configured with the AP’s SSID, and they should have 50 percent signal overlap.
Workgroup bridges connect to devices without a wireless network interface card (NIC) to allow them access to the wireless network. Wireless mesh networks can span large distances because only the edge APs connect to the wired network. The intermediate APs connect wirelessly to multiple other APs and act as repeaters for them. Each AP has multiple paths through the wireless network. The Adaptive Wireless Path (AWP) protocol runs between APs to determine the best path to the wired network. APs choose backup paths if the best path fails.
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