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Factors Affecting Wireless Signals

Because wireless signals travel through the atmosphere, they are susceptible to different types of interference than standard wired networks. Interference weakens wireless signals and therefore is an important consideration when working with wireless networking.

Interference Types

Wireless interference is an important consideration when you’re planning a wireless network. Interference is unfortunately inevitable, but the trick is to minimize the levels of interference. Wireless LAN communications typically are based on radio frequency signals that require a clear and unobstructed transmission path.

The following are some factors that cause interference:

  • Physical objects: Trees, masonry, buildings, and other physical structures are some of the most common sources of interference. The density of the materials used in a building’s construction determines the number of walls the RF signal can pass through and still maintain adequate coverage. Concrete and steel walls are particularly difficult for a signal to pass through. These structures will weaken or at times completely prevent wireless signals.
  • Radio frequency interference: Wireless technologies such as 802.11b/g use an RF range of 2.4GHz, and so do many other devices, such as cordless phones, microwaves, and so on. Devices that share the channel can cause noise and weaken the signals.
  • Electrical interference: Electrical interference comes from devices such as computers, refrigerators, fans, lighting fixtures, or any other motorized devices. The impact that electrical interference has on the signal depends on the proximity of the electrical device to the wireless access point. Advances in wireless technologies and in electrical devices have reduced the impact that these types of devices have on wireless transmissions.
  • Environmental factors: Weather conditions can have a huge impact on wireless signal integrity. Lightning, for example, can cause electrical interference, and fog can weaken signals as they pass through.

Many wireless implementations are found in the office or at home. Even when outside interference such as weather is not a problem, every office has plenty of wireless obstacles. Table 7.4 highlights a few examples to be aware of when implementing a wireless network indoors.

Table 7.4. Wireless Obstacles Found Indoors

Obstruction

Obstacle Severity

Sample Use

Wood/wood paneling

Low

Inside a wall or hollow door

Drywall

Low

Inside walls

Furniture

Low

Couches or office partitions

Clear glass

Low

Windows

Tinted glass

Medium

Windows

People

Medium

High-volume traffic areas that have considerable pedestrian traffic

Ceramic tile

Medium

Walls

Concrete blocks

Medium/high

Outer wall construction

Mirrors

High

Mirror or reflective glass

Metals

High

Metal office partitions, doors, metal office furniture

Water

High

Aquariums, rain, fountains

Spread-Spectrum Technology

Spread spectrum refers to the manner in which data signals travel through a radio frequency. With spread spectrum, data does not travel straight through a single RF band; this type of transmission is known as narrowband transmission. Spread spectrum, on the other hand, requires that data signals either alternate between carrier frequencies or constantly change their data pattern. Although the shortest distance between two points is a straight line (narrowband), spread spectrum is designed to trade bandwidth efficiency for reliability, integrity, and security. Spread-spectrum signal strategies use more bandwidth than in the case of narrowband transmission, but the trade-off is a data signal that is clearer and easier to detect. The two types of spread-spectrum radio are frequency hopping and direct sequence.

Frequency-Hopping Spread-Spectrum (FHSS) Technology

FHSS requires the use of narrowband signals that change frequencies in a predictable pattern. The term frequency hopping refers to data signals hopping between narrow channels. For example, consider the 2.4GHz frequency band used by 802.11b/g. This range is divided into 70 narrow channels of 1MHz each. Somewhere between 20 and several hundred milliseconds, the signal hops to a new channel following a predetermined cyclical pattern.

Because data signals using FHSS switch between RF bands, they have a strong resistance to interference and environmental factors. The FHSS signal strategy makes it well suited for installations designed to cover a large geographic area and where using directional antennas to minimize the influence of environmental factors is not possible.

FHSS is not the preferred spread-spectrum technology for today’s wireless standards. However, FHSS is used for some lesser-used standards and for cellular deployments for fixed broadband wireless access (BWA), where the use of DSSS (discussed next) is virtually impossible because of its limitations.

Direct-Sequence Spread-Spectrum (DSSS) Technology

With DSSS transmissions, the signal is spread over a full transmission frequency spectrum. For every bit of data that is sent, a redundant bit pattern is also sent. This 32-bit pattern is called a chip. These redundant bits of data provide both security and delivery assurance. The reason transmissions are so safe and reliable is simply because the system sends so many redundant copies of the data, and only a single copy is required to have complete transmission of the data or information. DSSS can minimize the effects of interference and background noise.

As for a comparison between the two, DSSS has the advantage of providing better security and signal delivery than FHSS, but it is a sensitive technology, affected by many environmental factors.

Orthogonal Frequency Division Multiplexing

Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique that transfers large amounts of data over 52 separate, evenly spaced frequencies. OFDM splits the radio signal into these separate frequencies and simultaneously transmits them to the receiver. Splitting the signal and transferring over different frequencies reduces the amount of crosstalk interference. OFDM is associated with 802.11a, 802.11g amendments, and 802.11n wireless standards.

Beacon Management Frame

Within wireless networking is a frame type known as the beacon management frame (beacon). Beacons are an important part of the wireless network because it is their job to advertise the presence of the access point so that systems can locate it. Wireless clients automatically detect the beacons and attempt to establish a wireless connection to the access point.

The beacon frame is sent by the access point in an infrastructure network design. Client stations send beacons only if connected in an ad hoc network design. The beacon frame has several parts, all of which the client system uses to learn about the AP before attempting to join the network:

  • Channel information: Includes which channel the AP uses.
  • Supported data rates: Includes the data transfer rates identified by the AP configuration.
  • Service Set Identifier (SSID): This beacon includes the name of the wireless network.
  • Time stamp: Includes synchronization information. The client system uses the time stamp to synchronize its clock with the AP.

These beacons are transmitted from the AP about every 10 seconds. The beacon frames add overhead to the network. Therefore, some APs let you reduce the number of beacons that are sent. With home networks, constant beacon information is unnecessary.

Passive and Active Scanning

Before a client system can attempt to connect to an access point, it must be able to locate it. The two methods of AP discovery are as follows:

  • Passive scanning: The client system listens for the beacon frames to discover the AP. After it is detected, the beacon frame provides the information necessary for the system to access the AP.
  • Active scanning: The client station transmits another type of management frame known as a probe request. The probe request goes out from the client system, looking for a specific SSID or any SSID within its area. After the probe request is sent, all APs in the area with the same SSID reply with another frame, the probe response. The information contained in the probe response is the same information included with the beacon frame. This information enables the client to access the system.
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