Wireless antennas act as both transmitters and receivers. There are two broad classes of antennas on the market, ommi directional (Omni, or point-to-multipoint) and directional (Yagi or point-to-point). As general rule, Yagi antennas have greater range than Omni antennas of equivalent gain because Yagis focus all their power in a single direction whereas Omnis must disperse the same power in all directions at once. The drawback of using a directional antenna, though, is that more care must be taken to align communication points, generally making Yagi a good choice only for oint-to-point bridging of access points. Most WAPs use Omnis because clients and other APs could be in any direction at any given moment. A non-networking example of an Omni antenna is the FM antenna on your automobile. The orientation of your car does not affect the reception of the signal. The television aerials that some of us are old enough to remember rotating into a specific direction for a certain channel (how many of you labeled your set-top antenna dial for the actual TV stations you could receive?) are examples of Yagi antennas.
Omnis and Yagis are both rated according to their signal gain with respect to an actual or the oretical laboratory reference antenna. These ratings are relative indicators of the corresponding production antenna’s range. Range is also affected by the bit rae of the underlying technology, with higher bit rates extending shorter distances. Remember, a Yagi will always have a longer ranger than an equivalently rated Omni, but the straight-line Yagi will be limited in coverage area.
Manufacturers rate these antennas in units of decibel isotropic (dBi) or decibel dipole (dBd), based on the type of reference antenna (isotropic or dipole) of equivalent frequency operation used to rate the production antenna. A positive value for either unit of measure represent a gain in signal strength with respect to the reference antenna. Webstar’s defines isotropic as “exhibiting properties (as velocity of light transmission with the same values when measured along axes in all directions. “Isotropic antennas are not able to be produced in reality, but their properties can be engineered from antenna theory for reference purposes.
As a practical example, consider Cisco System’s series of Aironet Access Point (indoor) and Bridge (outdoor) antennas. Table 6.1 illustrates the effect gain ratings and attempted bit rates have on range limitations.
Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM), not to be confused with automated teller machines, first emerged in the early 1990s. ATM was designed to be a high-speed communications protocol that does not depend on any specific LAN topology. It uses a high-speed cell-switching technology that can handle data as well as real-time voice and video. The ATM protocol breaks up transmitted data into 53-byte cells. A cell is analogous to a packet or frame, except that an ATM cell is always fixed in length, whereas a frames’ length can very.
ATM is designed to switch these small cells through an ATM network very quickly. It does this b setting up a virtual connection between the source and destination nodes; the cell may go through multiple switching points before ultimately arriving at their final destination. The cells may also arrive out of order, so that receivings system may have to reassemble and correctly order the arriving cells. ATM, like Frame Relay, is a connection-oriented service in contrast to most Data Link protocols, which are best-effort deliver services and do not require virtual circuits to be established before transmitting user data.
Data rates are scalable and start as low as 1.5 Mbps, with speeds of 25Mbps, 51Mbps, 100Mbps, 155Mbps, and higher. The common speed of ATM networks today are 51.84 Mbps and 155.52 Mbps. Both of these speeds can be used over either copper or fiber-optic cabling. An ATM with a speed of 622.08 Mbps is also becoming common but is currently used exclusively over fiber-optic cable. ATM supports very high speeds because it is designed to be implemented by hardware rather than software; faster processing speeds are therefore possible, Fiber-based service-provider ATM networks are running today at data rates of 10GBps are becoming more and more common.
In the U.S., the standard for synchronous data transmission on optical media is Synchronous Optical Network (SONET); the international equivalent of SONET defines a base data rate of 51.84Mbps; multiples of this rate are known as optical carrier (OC) levels, such as OC-3, OC-12, and so on.