There are several main components of a WLAN. I would like to briefly touch upon these before I go any further.
First of all, there must be Mobile Units (MUs). These are also called stations, and can include any device that can associate with an Access Point and gain access to the wireless network. This can include laptops, PDAs, telephones, desktop PCs, and any other device that has a wireless network adapter. A WLAN MU must have a WLAN NIC installed to send and receive WLAN traffic. WLAN MUs must also include one ore more client software models that provide some extra functionality within the network.
Access Points, or APs, allow the MU to connect to the wired network by providing a point of access. Some APs are centrally managed, while others are managed individually. In a traditional WLAN implementation, each AP maintains its own intelligence. Independent servers located on a switched network provide security, such as traffic shaping and filtering, as well as SNMP management for the AP.
Access Ports are the next component I would like to mention. The wireless switch collects management intelligence from individual access points. By using access points to gather data tat is then moved to the switch, access points become smart antennas, or Access Ports, that are centrally managed from the switch. Because Access Ports receive their intelligence from the switch, upgrades are as simple as upgrading the software on the switch.
WLAN switches provide management, additional security, and campus area mobility for the WLAN.
Gateway and VPN Termination Points provide additional security for WLAN MUs to connect to the network. Some WLAN switches have VPN functionality built right in, so two separate boxes are not required.
Other servers can be required for a WLAN implementation. These are often shared with the wired network, and include DHCP servers, DNS Servers, and Remote Authentication Dial-In User Servers (RADIUS).
There are three main configurations for a WLAN: The Basic Service Set (BSS), the Extended Service Set (ESS), and the Independent Basic Service Set (IBSS). The BSS requires one access point for all MUs on the network. An ESS has multiple access points that can be accessed by multiple MUs on the network. The IBSS does not have an access point; instead devices connect directly to each other. This is often referred to as an Ad-Hoc Wireless connection.
Using Radio Frequencies, MUs connect to Access Points (or to each other in an ad-hoc network). Access points send out a beacon at frequent and periodic intervals. Mobile Units will connect to APs by scanning the airwaves to find available APs. Once the MU finds an AP that it wants to connect to it will begin the process of authentication and association.
There are two types of communications that are used by MUs to connect to APs. The first is narrow band communications. Narrow Band Communications use only enough of the RF to send information. It must be a high powered signal to be heard over other signals in the area. The higher power can lead to RF noise. It can also be easily jammed, and is susceptible to interference from other devices. Spread Spectrum is a more widely used technology. It uses a broader frequency range to send information at a lower power density. It is less susceptible to interference and is less likely to be intercepted than narrow band. Spread Spectrum can be further divided into two technologies: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). FHSS uses a pattern of frequency hopping to send data. This pattern is repeated until all frequencies in the band have been used and the hop sequence is complete, at which point the sequence is restarted. There are four components to a FHSS system that allow it to communicate: Channels, Hop Sequence, Dwell Time, and Hop Time. FHSS is not very widely used by devices, and is not commonly used in WLAN deployments. Some technologies, such as Bluetooth, do use FHSS. DSSS is the most common method of deploying a WLAN. It has a lower overall cost, more available hardware, and a better price-to-throughput ratio. DSSS sending stations multiply the information bit with a continuous string of pseudo-noise signals (Chips, discussed previously). The process begins when the carrier signal is modulated with a code sequence. The code sequence contains the chipping code, which determines how much spreading (how much of the frequency range is used) will occur. It also has information that determines the data rate of transmission.
In the United States, the FCC has allocated two frequency ranges for wireless devices. The first is ISM, which operates in the 2.4 GHz – 2.4835 GHz range. The second range is the UNII range, operating from 5 GHz – 6 GHz. In Europe and the rest of the world there are different regulations regarding these frequency ranges, but they are very close. The FCC also regulates how much power output devices can have to 100 mW, but has no further restrictions on FHSS and DSSS Power Density.
Post a Comment