3.3.1 This page will introduce the regulations and standards that apply to wireless technology. These standards ensure that deployed networks are interoperable and in compliance.
Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC).
A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS). DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps. The next standard approved was 802.11b, which increased transmission capabilities to 11 Mbps. Even though DSSS WLANs were able to interoperate with the Frequency Hopping Spread Spectrum (FHSS) WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task was simply to create a standard that matched the manufacturer’s solution.
802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps. All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates for DSSS only. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points.
802.11b devices achieve the higher data throughput rate by using a different coding technique from 802.11, allowing for a greater amount of data to be transferred in the same time frame. The majority of 802.11b devices still fail to match the 11 Mbps bandwidth and generally function in the 2 to 4 Mbps range.
802.11a covers WLAN devices operating in the 5 GHZ transmission band. Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ. 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps. In production networks, a more standard rating is 20-26 Mbps.
802.11g provides the same bandwidth as 802.11a but with backwards compatibility for 802.11b devices using Orthogonal Frequency Division Multiplexing (OFDM) modulation technology. Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate.
The next page explains the devices and topologies used in wireless networks.
3.3.2 This page describes the devices and related topologies for a wireless network.
A wireless network may consist of as few as two devices. - The nodes could simply be desktop workstations or notebook computers. Equipped with wireless NICs, an ‘ad hoc’ network could be established which compares to a peer-to-peer wired network. Both devices act as servers and clients in this environment. Although it does provide connectivity, security is at a minimum along with throughput. Another problem with this type of network is compatibility. Many times NICs from different manufacturers are not compatible.
To solve the problem of compatibility, an access point (AP) is commonly installed to act as a central hub for the WLAN infrastructure mode. The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell. Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell could greatly vary. Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet). To service larger areas, multiple access points may be installed with a degree of overlap. The overlap permits "roaming" between cells. This is very similar to the services provided by cellular phone companies. Overlap, on multiple AP networks, is critical to allow for movement of devices within the WLAN. Although not addressed in the IEEE standards, a 20-30% overlap is desirable. This rate of overlap will permit roaming between cells, allowing for the disconnect and reconnect activity to occur seamlessly without service interruption.
When a client is activated within the WLAN, it will start "listening" for a compatible device with which to "associate". This is referred to as "scanning" and may be active or passive.
Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response. The authentication and association steps are completed.
Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes.
The first Interactive Media Activity shows the levels of the OSI reference model and the related networking devices.
The second Interactive Media Activity shows the addition of a wireless hub to a wired network.
The next page explains how wireless LANs communicate.
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