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OSI LAYERS

AL ICT 2019 TAMIL

GCE AL ICT PAPER 2019

PROPERTIES OF SIGNALS

Wavelength, frequency, amplitude and phase

Wavelength

• An RF signal is an alternating current (AC) that continuously changes between a positive and negative voltage.
• An oscillation, or cycle, of this alternating current is defined as a single change from up to down to up, or as a change from positive to negative to positive.
• A wavelength is the distance between the two successive crests (peaks) or two successive troughs (valleys) of a wave pattern.

As RF signals travel through space and matter, they lose signal strength (attenuate).

It is often thought that a higher frequency electromagnetic signal with a smaller wavelength will attenuate faster than a lower frequency signal with a larger wavelength. In reality, the frequency and wavelength properties of an RF signal do not cause attenuation. Distance is the main cause of attenuation. All antennas have an effective area for receiving power known as the aperture. The amount of RF energy that can be captured by the aperture of an antenna is smaller with higher frequency antennas.

Although wavelength and frequency do not cause attenuation, the perception is that higher frequency signals with smaller wavelengths attenuate faster than signals with a larger wavelength. Theoretically, in a vacuum, electromagnetic signals will travel forever. However, as a signal travels through our atmosphere, the signal will attenuate to amplitudes below the receive sensitivity threshold of a receiving radio.
Essentially, the signal will arrive at the receiver, but it will be too weak to be detected.

The perception is that the higher frequency signal with smaller wavelength will not travel as far as the lower frequency signal with larger wavelength.
The reality is that the amount of energy that can be captured by the aperture of a high frequency antenna is smaller than the amount of RF energy that can be captured by a low frequency antenna.

Higher frequency signals will generally attenuate faster than lower frequency signals as they pass through various physical mediums such as brick walls.

1) The coverage distance is dependent on the attenuation through the air (referred to as free space path loss, discussed later in this chapter).

2) The higher the frequency, typically the less the signal will penetrate through obstructions.

For example, a 2.4 GHz signal will pass through walls, windows, and doors with greater amplitude than a 5 GHz signal. Think of how much farther you can receive an AM station’s signal (lower frequency) versus an FM station’s signal (higher frequency).

Frequency

RF signal cycles in an alternating current in the form of an electromagnetic wave. Frequency is the number of times a specified event occurs within a specified time interval.

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A standard measurement of frequency is hertz (Hz).

An event that occurs once in 1 second has a frequency of 1 Hz.
An event that occurs 325 times in 1 second is measured as 325 Hz.

The frequency at which electromagnetic waves cycle is also measured in hertz. Thus, the number of times an RF signal cycles in 1 second is the frequency of that signal, as pictured in Figure 2.7.

Amplitude

An important property of an RF signal is the amplitude, which can be characterized simply as the signal’s strength, or power.

Amplitude is represented by the positive crests and negative troughs of the sine wave. In Figure 2.8, you can see that λ represents wavelength and a represents amplitude.

The first signal’s crests and troughs have more magnitude; thus the signal has more amplitude.
The second signal’s crests and troughs have decreased magnitude, and therefore the signal has less amplitude.

The radios used in most indoor 802.11 access points have a transmit power range between 1 mW and 100 mW.
Wi-Fi radios can receive signals with amplitudes as low as billionths of a milliwatt.

Phase

Phase is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency.

The phase involves the relationship between the position of the amplitude crests and troughs of two waveforms.

Phase can be measured in distance, time, or degrees.

If the peaks of two signals with the same frequency are in exact alignment at the same time, they are said to be in phase.

If the peaks of two signals with the same frequency are not in exact alignment at the same time, they are said to be out of phase.

It is important to understand is the effect that phase has on amplitude when a radio receives multiple signals.

Signals that have 0 (zero) degree phase separation actually combine their amplitude, which results in a received signal of much greater signal strength, potentially as much as twice the amplitude.

If two RF signals are 180 degrees out of phase (the peak of one signal is in exact alignment with the trough of the second signal), they cancel each other out and the effective received signal strength is null.

Phase separation has a cumulative effect. Depending on the amount of phase separation of two signals, the received signal strength may be either increased or diminished.
The phase difference between two signals is very important to understanding the effects of an RF phenomenon known as multi path.

NETWORKING (COMMUNICATION AND SIGNALS)

What is Data Communications?

Data communication is a process of transferring data electronically from one place to another. Data can be transferred by using different medium. The basic components of data communications are as follows:

1. Message
2. Sender
3. Receiver
4. Medium/ communication channel
5. Encoder and decoder

Message

The message is the data or information to be communicated. It may consist of text, number, pictures, sound, video or any a combination of these.

Sender

Sender is a device that sends message. The message can consist of text, numbers, pictures etc. it is also called source or transmitter. Normally, computer is used as sender in information communication systems.

Receiver

Receiver is a device that receives message. It is also called sink. The receiver can be computer, printer or another computer related device. The receiver must be capable of accepting the message.

Medium

Medium is the physical path that connects sender and receiver. It is used to transmit data. The medium can be a copper wire, a fiber optic cable, microwaves etc. it is also called communication channel.

Encoder and decoder

The encoder is a device that converts digital signals in a form that can pass through a transmission medium. The decoder is a device that converts the encoded signals into digital form. The receiver can understand the digital form of message. Sender and receiver cannot communicate successfully without encoder and decoder.

What is signal? 

Signal is an electromagnetic or light wave that represents data. Signals are used to transfer data from one device to another through a communication medium.

Types of Signals

Different forms of communication signals are as follows:

1. Digital signals
2. Analog signals

Digital Signals

Digital signal is a sequence of voltage represented in binary form. The digital signals are in the form of electrical pulses of ON and OFF. These signals are in discrete form. Digital signals are faster and efficient. They provide low error rates. They also provide high transmission speed and high-quality voice transmission.

All data communication between the computers is in digital form. Computers understand and work only in digital form. The following figure represents a high voltage as a 1 and a low voltage as a 0.

Data Communications

Analog Signals

Analog signal is a continuous electrical signal in the form of wave. The wave is known as carrier wave. Telephone line is most commonly used media for analog signals.

Characteristics of Analog Signals

Two characteristics of an analog wave are as follows:

1. Frequency: the number of times a wave repeats during a specific time interval is known as frequency.
2. Amplitude: the height of wave within a given period of time is known as amplitude.

What is Data Communications and Signal