**** NOTES SERIES: COMPUTER NETWORKS ****

Hello UGC NET Aspirants!!

Here I am providing important terms and definitions useful for questions related to Data Communication & Computer Networks. Here is the first post and will be followed by many posts in coming days.
Best of Luck!!

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1. Network Reliability: Measured by the frequency of failures.

2. Dedicated Link: Dedicated means the traffic carries traffic only between the two connected devices.

3. The key elements of a protocol are: syntax, semantics and timing. Syntax means structure (format) of data, e.g., first 8 bits of data contain address of the sender. Semantic means the meaning of each data section, e.g., an address identify the route to be taken. Timing means when and how fast data can be sent.

4. Standards can be De facto or De jure. De facto means standards originally defined by the manufacturer and De jure means legalised by a governing body.

5. Periodic vs Nonperiodic signal: a periodic signal completes a pattern within a measurable time frame (period) and repeats the pattern over subsequent identical periods. There is no such property of a  nonperiodic signal.

6. A sine wave is a simple, periodic, analog signal. Every sine wave has 3 properties: 1. Amplitude 2. Frequency 3. Phase

The peak amplitude of a signal is the absolute value of its highest intensity.
Frequency means no. of periods in 1 second (The period is the duration of one cycle in a
repeating event, while the frequency is the number of cycles per unit time. Period is the inverse
of frequency.
Phase is the position of a waveform at time 0. It is measured in degrees or radians.

7. Wavelength is the distance a signal can travel in one period. It is measured in micrometers (microns).

8. The  bit length is the distance one bit occupies on a transmission medium.

9. Baseband transmission means sending a digital signal over a channel without changing it to analog. It requires a low-pass channel - a channel with a bandwidth that starts from zero.

10. Broadband transmission means changing the digital signal to analog for transmission. Here we can use band-pass channel - a channel with bandwidth NOT starting from zero.

11. A low-pass channel can be considered a band-pass channel with lower frequency starting at zero.

12. transmission impairments:

Attenuation: Loss of signal energy
Decibel: relative strength of a signal at two points. It is negative is signal is attenuated and
positive is signal is amplified.
Distortion: Signal changing its shape or form.
Jitter: different packets of data face different delays and application is time-sensitive.
Noise: Its types are-thermal, induced, cross-talk and impulse
Thermal noise is due to motion of electrons in a wire. Induced noise comes from appliances.
Cross-talk is due to mingling of two wires. Impulse is voltage spike from power lines.

13. Increasing the levels of a signal reduces the reliability of a transmission system.

14. Latency of a signal is the time it takes for the signal to completely arrive at the destination. It has 4 parts: propagation time (PT), transmission time (TT), queuing time (QT) and processing delay (PD).
                       Latency=PT+TT+QT+PD
                       PT=distance/propagation speed
                       TT=message size/bandwidth
QT is the time required by each intermediate or end device to hold the message before it can be processed.

15. For a noiseless channel, the Nyquist's bit rate formula (bit rate=2 x bandwidth x log2 x L)  is used for max bit rate of the channel. For a noisy channel, Shannon's capacity theorem (bandwidth x log2 (1+SNR)) finds max. data rate.

16. There are 4 levels of addresses under TCP/IP Protocols: 1. Link (physical) address 2. Logical address 3. Port address 4. Specific address. Link address is handled at layer 1 & 2 of OSI Model, logical address at Network layer, port address at Transport layer and specific address by the particular process running at Application layer.