Analog and Digital Data
Analog data means information that is changing continuously. An analog data must be converted into anal signal before transmitted. Analog clock is best example of analog data. Its hours minutes and seconds are changing continuously to provide continuous information. In the same way when a person speaks an analog wave is generated in the air. This can be recorded by a microphone and can be converted to an analog .An analog signal can have many levels of intensity over a period of time. Mainly there are two types of analog signals
Periodic Signals
A periodic signal follows a pattern and completes that pattern within a measurable time often called a period periodic signal repeats that pattern over one after the other. Once a full pattern is completed then it called a cycle. Sine wave is the best way to represent periodic analog signals. Sine wave consists of three parameters the frequency, the phase and the peak amplitude.
Non periodic Signals
A signal that changes without any specific pattern or cycle that repeats is called non-periodic signals. In data communications, we mostly use periodic analog signals and non-periodic form as digital signals.
Period and Frequency
• Period means the amount of time in which a signal completes its one cycle.
• Frequency means the number of periods in one second.
• If a signal does not change, it means its frequency is zero.
• If a signal changes continuously, it means its frequency is unlimited.
Composite Signals
A signal that is made of many sine waves is called composite signal. All sine waves are different from each other on the basis of frequency amplitude and time.
Bandwidth
• The difference between the highest and the lowest frequencies present in that signal is called the bandwidth of composite signal.
3.3 DIGITAL SIGNALS
• A digital signal means discrete values that are continuous changing. A digital signal can have more than two levels.
The bit rate is the number of bits sent in one second expressed in bits per second (bps).
• Bit Length Bit length =propagation speed x bit duration
• Digital Signal as a Composite Analog Signal Based on Fourier analysis, a digital signal is a composite analog signal. The bandwidth is infinite.
How to Digital Signals
There are two ways to transmit digital signals.
Baseband transmission
Broadband transmission
Baseband Transmission
Baseband transmission means we don’t need to convert digital signal to an analog signal. This type of direct transmission is called baseband transmission.
Broadband Transmission
Broadband transmission means data must be converted into analog before transmitted. It is used for large data transmission. For example we use broadband devices to access internet as it requires high data transmission speed. The process of digital to analog conversion is called modulation.
TRANSMISSION IMPAIRMENT
Transmission impairment means that the signal at the ending of the medium is not the same as the signal at the start of the medium. Attenuation distortion and noise are three main factors of impairment.
Attenuation
Loss of energy causes attenuation. For example when a signal, composite or simple travels through a channel, it loses some of its energy due to the resistance of the channel or medium.
The decibel (dB) measures the strength of two signals or one signal between two different points
Distortion
Due to distortion the form or shape of signal is changed. Distortion can occur only in a composite signal that is made with different frequencies
Noise
Noise can result in poor signal quality. Noise occurs in both digital and analog signals, and can affect data contained in signals, including text, programs, audio, and video. Several types of noise include thermal noise, cross talking, and impulse noise, that may corrupt the signal
3.6 PERFORMANCE OF NETWORK
Performance is an important issue that network has to face. Some factors that can reduce network performance are as follows.
Bandwidth
One characteristic that measures network performance is bandwidth. The term bandwidth can also refer to the number of bits per second that a channel, a link, or even a network can transmit.
Throughput
• The throughput is a measure of how fast we can actually send data through a network.
Bandwidth in bits per second and throughput seem the same, they are different. A link may have a bandwidth of B bps, but we can only send T bps through this link with T always less than B.
Latency (Delay)
• Over all delay is called latency
• Latency =propagation time + transmission time + queuing time + processing delay
Queuing Time
Device to hold the message before it can be processed. If there are many messages, each message will have to wait. Bandwidth and delay product defines the number of bits that can load on the link.
Jitter
Another performance issue that is related to delay is jitter. Jitter is delay when real time applications data packets arrive out of sequence like audio video massages.
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