Title: 3.3 Components of a Wireless Transmission System
Course: Information and Communication Technology: Module B > Section: Section 3 - Wireless Technology Basics

3.3.1 Components of a Wireless Transmission System

A typical wireless communication system consists of:

- a transmitter or source
- one or more receiver(s)—also known as destinations
- antennas and medium
- protocols for communication

The transmitter initiates the signal. It takes data and generates a radio frequency (RF) signal. The transmitter generates an AC (alternating current) signal that corresponds to the frequency of the selected wireless standard. For example, 802.11n may transmit in the 2.4 GHz range (an AC signal that oscillates at 2.4 billion times per second.). The AC signal radiates RF waves away from the antenna. In the United States, the maximum power level is regulated by the FCC. The higher the power level, the farther the signal will travel but the greater the likelihood becomes that the signal will interfere with another nearby network. The U.S. allows a maximum power level of 100 milliwatts (mW). Calculating the power level will be discussed later.

The receiver's antenna collects the RF signal receives and passes it to the receiver. It takes the modulated signal received from the antenna, converts it into 1s and 0s (binary), and passes the data to the computer to be processed. The received signal is usually much weaker than what was transmitted and could be altered or corrupted. This may be caused by free space loss (line of sight), attenuation (distance), interference from other signals or obstacles, or multipath (what the signal bounced off during transmission).

The Intentional Radiator (IR) is a device that intentionally generates and emits radio frequency energy by radiation or induction. It consists of all components from transmitter to antenna, excluding the antenna: the transmitter, all cables, all connectors, grounding, lightning arrestor, amplifier, and attenuator. The FCC limits the power that can be generated by an IR. An IR's power is measured at the connector providing input to the antenna. Typically, this signal is measured in milliwatts (mW) or decibels relative to 1 milliwatt (dBm).

The Equivalent Isotropically Radiated Power (EIRP) is the highest RF signal that can be transmitted from a particular antenna. It will be higher for a signal that has been focused or directed using a higher gain antenna. The FCC also limits the amount of EIRP from an antenna.

3.3.2 Components of a Wireless Transmission System

The Antenna transmits and receives the signal. Different types of antennas can be used depending on the wireless environment. An antenna that radiates its signal equally in all directions is known as an isotropic radiator. Such an antenna can only be approximated in practice.

Most common wireless devices used within a single floor of a building use an antenna whose horizontal radiation characteristics are omnidirectional.

There are also other options:

- Horizontally-Omnidirectional antennas work with signals in any direction in the horizontal plane. Wi-Fi routers and mobile adapters frequently use this type of antenna. They typically use a short vertical dipole antenna, similar to those used on walkie-talkie radios. They have a gain relative to an isotropic radiator of between 2 and 9 dBi. Horizontally-omnidirectional antenna must radiate power over 360 degrees. This means that gain measured from any one direction is lower than alternative directional antennas that focus energy in single direction. However, there are some varieties that offer increased gain.

- Directional or Unidirectional antennas provide a more focused signal. Signals are typically transmitted in an oval-shaped pattern with a beam width of only a few degrees. With higher gain, directional antennas can also be used outdoors to extend point-to-point links over a longer transmission distance or to form a point-to-multipoint network. Directional antennas are frequently used in situations where 360-degree coverage is unnecessary, such as extending a Wi-Fi network into hard-to-reach corners of a building. Directional antennas are parabolic, dish, Yagi, and panel type antennas. These are typically used outdoors and when long distances are needed.

3.3.3 Components of a Wireless Transmission System

The FCC regulates the radio frequencies used for wireless transmissions. Channels are created by dividing up the available RF spectrum. Each channel can carry a separate transmission or conversation. The FCC has designated the 2.4 GHz (2.412-2.484) and 5 GHz (5.180-5.825) ranges.

Globally, the 2.4 GHz range is divided into 14 channels, but the United States only uses 1-11 (2.412-2.462) each spaced 5 MHz apart. Channels may be bonded together into 20 MHz to 40 MHz widths to increase bandwidth. The 5 GHz range is divided into seven clear channels plus a few with restrictions.

Wireless technology uses an access method called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) which creates a reservation on the channel for use by a specific conversation. While a reservation is in place, no other device may transmit on the channel.  Therefore, collisions are avoided.

When a Client or device needs to send packets, it connects to an access point by sending a “Request to Send” packet to that access point. If the channel is available, the AP will respond to the device with a “Clear to Send” message indicating to the device that it is clear for the device to transmite on the channel. A “Clear to Send” message is broadcast to all devices within the BSS so all devices know that the requested channel is now in use.

Once the conversation or transmission is complete, the device that requested the channel sends another message to the AP known as an Acknowledgement. The Acknowledgement indicates to the AP that the channel can be released. This message is also broadcast to all devices so they know that the channel is available.