Copyright Goodheart-Willcox Co., Inc. 455 Chapter 14 Wireless Network Security channels is that many devices operate on this frequency, including cordless tele- phones, microwave ovens, Bluetooth devices, and even some car alarms. These devices can affect wireless transmissions and availability. The 5-GHz band has more channels than the 2.4-GHz band, but the 5-GHz band includes 23 channels that do not overlap. This reduces the possibility of using an overlapping channel. It also reduces interference from nearby channels. However, the 5-GHz channel is used in the United States by the military and many weather apps therefore, the FCC requires any device using the 5.250 to 5.350, 5.470, and 5.725 frequencies to have a feature called DFS. Dynamic frequency selection (DFS) is a channel-allocation mandate that detects interference with military and weather radar and automatically shifts the frequency being used by the wireless device. This is needed to ensure critical radar applications are not disrupted. Antennae Types and Placement Access points have one or more antennae built into the device. An antenna may be omnidirectional or unidirectional, as shown in Figure 14-6. Omnidirectional means it will send and receive in all directions. Unidirectional means it will only send and receive from one specific direction. Since a unidirectional antenna only transmits in one direction, the power of the transmission will be greater than if the same device transmitted in all directions. There is also less interference at the access point since the receiver must listen and respond to signals from only one direction. While there are strong benefits to unidirectional antennae, in many situations they are not flexi- ble enough for establishing communication between a transmitter and receiver. For example, if there are receivers on opposite sides of a transmitter, a single unidirec- tional antenna will not provide coverage for both. Placement of access points is an important consideration to ensure proper cov- erage. An AP using omnidirectional signaling should be placed in the center of the coverage area. With unidirectional antenna placement, ensure you are not too far from the host devices so the signal can provide adequate coverage. If you are trying to avoid individuals from connecting or attempting to connect in a specific physical area, you may need to move the AP or reduce the power on the AP so it cannot receive a connection in that specific location. Antenna type is often based on if the signal is point-to-point or point-to-​ multipoint. Point-to-point is a telecommunication method that describes a connec- tion between two distinct points. An example of a point-to-point communication is a broadcast link between two businesses to send and receive data over long distances. Point-to-multipoint is a transmission method where one device or network is config- ured to transmit to multiple devices. A common example is an outdoor wireless 3.4 3.5 Unidirectional Antenna Omnidirectional Antenna Goodheart-Willcox Publisher Figure 14-6 An omnidirectional antenna broadcasts in all directions, while a unidirectional antenna broadcasts in only one specific direction.