WIRELESS COMMUNICATION IN A WIRELESS NETWORK
By RIAZ FAROOQ
Dec 27, 2004 - Jan 02, 2005
Wireless networks, like their wired counterparts, rely on the manipulation of electrical charge to enable communication between devices. Changes or oscillations in signal strength from 0 to some maximum value (amplitude) and the rate of those oscillations (frequency) are used singularly or in combination with each other to encode and decode information.
When two devices understand the method(s) used to encode and decode information contained in the changes to the electrical properties of the communications medium, they can communicate with each other. A network adaptor is able to decode the changes in the electric current it senses on the wire and convert them to meaningful information (bits) that it can subsequently send to higher levels for processing. Likewise, a network adaptor can encode information (bits) by manipulating the properties of the electric current for transmission on the communications medium (the cable, in the case of wired networks).
The obvious and primary difference between wired and wireless networks is that wireless networks use a special type of electric current, commonly known as Radio Frequency (RF), which is created by applying alternating current (AC) to an antenna to produce an electromagnetic field (EM). The resulting RF field is used by devices for broadcast and reception. In the case of wireless networks, the medium for communications is the EM field, the region of space that is influenced by the electromagnetic radiation (unlike audio waves, radio waves do not require a medium such as air or water to propagate). As with wired networks, amplitude decreases with distance, resulting in the degradation of signal strength and the ability to communicate. However, the EM field is also dispersed according to the properties of the transmitting antenna, and not tightly bounded as is the case with communication on a wire. The area over which the radio waves propagate from an electromagnetic source is known as the Fresnel Zone.
Like the waves created by throwing a rock into a pool of water, radio waves are affected by the presence of obstructions and may be reflected, refracted, diffracted, or scattered, depending on the properties of the obstruction and its interaction with the radio waves. Reflected radio waves can be a source of interference on wireless networks. The interference created by bounced radio waves is called multipath interference.
When radio waves are reflected, additional wave fronts are created. These different wave fronts may arrive at the receiver at different times and be in phase or out of phase with the main signal. When the peak of a wave is added to another wave (in phase), the wave is amplified. When the peak of a wave meets a trough (out of phase), the wave is effectively cancelled. Multipath interference can be the source of hard-to-troubleshoot problems. In planning for a wireless network, administrators should consider the presence of common sources of multipath interference. These include metal doors, metal roofs, water, metal vertical blinds, and any other source that is highly reflective to radio waves. Antennas may help to compensate for the effects of multipath interference, but these have to be carefully chosen. In fact, many wireless access points have two antennas for precisely this purpose. But, a single omni-directional antenna may be of no use at all for this kind of interference.
Another source of signal loss is the presence of obstacles. While radio waves can travel through physical objects, they will be degraded according to the properties of the object they travel through. A window, for example, is fairly transparent to radio waves, but may reduce the effective range of a wireless network by 50-70%, depending on the presence and nature of coatings on the glass. A solid core wall can reduce the effective range of a wireless network by up to 90% or greater.
EM fields are also prone to interference and signal degradation by the presence of other EM fields. In particular, 802.11 wireless networks are prone to interference produced by cordless phones, microwave ovens, and a wide range of devices that use the same unlicensed Industrial, Scientific and Medical (ISM) or Unlicensed National Information Infrastructure (UNII) bands. To mitigate the effects of interference from these devices and other sources of electromagnetic interference, RF-based wireless networks employ Spread Spectrum technologies. Spread spectrum provides a way to "share" bandwidth with other devices that may be operating in the same frequency range. Rather than operating on a single, dedicated frequency such as is the case with radio and television broadcasts, wireless networks use a "spectrum" of frequencies for communication.
First conceived of by Hedy Lamarr and George Antheil (a Hollywood actress and composer respectively) in 1940 as a method to secure military communications from jamming and eavesdropping during WWII, spread spectrum defines methods for wireless devices to use a number of narrowband frequencies over a range of frequencies simultaneously for communication. The narrow-band frequencies used between devices change according to a random-appearing but defined pattern, allowing the use of individual frequencies to contain parts of the transmission. Someone listening to a transmission using spread spectrum would hear only noise, unless their device understood in advance what frequencies were used for the transmission and could synchronize with them.
Two methods to synchronize wireless devices are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). As the name implies, FHSS works by quickly moving from one frequency to another according to a pseudo-random pattern. The frequency range used by the frequency hop is relatively large (83.5 MHz), providing excellent protection from interference. The amount of time spent on any given frequency is known as dwell time; the amount of time it takes to move from one frequency to another is known as hop time. FHSS devices will begin their transmission on one frequency and move to other frequencies according to the pre-defined pseudo-random sequence and then repeat the sequence after reaching the final frequency in the pattern. Hop time is usually very short (200-300 Is) and not significant relative to the dwell time (100-200 ms). However, Bluetooth devices use very short dwell times, and the hop times in this case can be significant, resulting in lower throughput. In general, the longer the dwell time, the greater the throughput and the more susceptible the transmission may be to narrowband interference.
The frequency hopping sequence creates the channel, allowing multiple channels to coexist in the same frequency range without interfering with one another. As many as 79 FCC-compliant FHSS devices using the 2.4 GHz ISM band may be co-located with each other. However, the expense of implementing such a large number of systems limits the practical number of co-located devices to well below this number. Wireless networks that use FHSS include HomeRF and Bluetooth, which both operate in the unlicensed 2.4GHz ISM band. FHSS is less subject to EM interference than DSSS, but usually operates at lower rates of data transmission (usually 1.6Mbps, but can be as high as 10 Mbps) than networks that use DSSS.
DSSS works somewhat differently. With DSSS, the data is divided and simultaneously transmitted on as many frequencies as possible within a particular frequency band (the channel). DSSS adds redundant bits of data known as chips to the data to represent binary 0s or 1s. The ratio of chips to data is known as the spreading ratio: the higher the ratio, the more immune to interference the signal is because if part of the transmission is corrupted, the data can still be recovered from the remaining part of the chipping code. This method provides greater rates of transmission than FHSS, which uses a limited number of frequencies, but fewer channels in a given frequency range. And, it also protects against data loss through the redundant, simultaneous transmission of data. However, because DSSS floods the channel it is using, it is also more vulnerable to interference from EM devices operating in the same range. In the 2.4-2.4835 GHz frequency range employed by 802.11b, DSSS transmissions can be broadcast in any one of 14 22 MHz-wide channels. The number of center-channel frequencies used by 802.11 DSSS devices depends on the country. For example, North America allows 11 channels operating in the 2.4-2.4835 GHz range, Europe 13, and Japan 1. Because each channel is 22 MHz wide, channels may overlap with each other. With the 11 available channels available in North America, only a maximum of 3 channels (1, 6, and 11) may be used concurrently without the use of overlapping frequencies.
When comparing FHSS and DSSS technologies, it should be noted that FHSS networks are not inherently more secure than DSSS networks, contrary to popular belief. Even if the relatively few manufacturers of FHSS devices were not to publish the hopping sequence used by their devices, a sophisticated hacker armed with a spectrum analyzer and a computer could easily determine this information and eavesdrop on the communications.
Wireless networks operate at the Physical and Data Link Layers of the OSI model. The PHY layer is concerned with the physical connections between devices, such as the medium and how bits (0s and 1s) are encoded and decoded. Both FHSS and DSSS, for example, are implemented at the PHY layer. The Data Link Layer is divided into two sub layers, the Media Access Control (MAC) and Logical Link Control (LLC) layers. The MAC layer is responsible for such things as the framing of data, error control, synchronization, and collision detection and avoidance. The Ethernet 802.3 standard, which defines the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method for protecting against data loss as result of data collisions on the cable, is defined at this layer.
The Wireless Application Protocol (WAP) is an open specification designed to enable mobile wireless users to easily access and interact with information and services instantly. WAP is designed for handheld digital wireless devices such as mobile phones, pagers, two-way radios, smartphones and other communicators. It works over most wireless networks and can be built on many operating systems including PalmOS, Windows CE, JavaOS, and others. The WAP operational model is built on the World Wide Web (WWW) programming model with a few enhancements. This model is shown in Figure 1.
WAP 2.0 ARCHITECTURE PROGRAMMING MODEL
WAP browsers in the wireless client are analogous to the standard WWW browsers on computers. WAP URIs are the same as those defined for traditional networks and are also used to identify local resources in the WAP enabled client. The WAP specification added two significant enhancements to the above programming model — push and telephony support (Wireless Telephony Application — WTA). WAP also provides for the use of proxy servers as well as supporting servers providing such functions as PKI support, user profile support, and provisioning support.