Chapter II Physical layer of computer network notes

This chapter highlights
The task of the physical layer.
Several common channel multiplexing techniques.
Several common broadband access technology, mainly ADSL and FTTx.

Basic conceptual role of the physical layer

As far as possible to shield the difference between the transmission media and communication means , so that the upper layer of the data link layers Do not feel these differences, only need to consider how to implement this layer of protocol and services.

Main tasks

I. Certain features related to the interface of the transmitting media

1. Mechanical characteristics : Specify the characteristics of the interface connector
2. Electrical characteristics : Specify the voltage range on each line of the interface cable
3. Function characteristic : Indicate the meaning of the voltage of a certain level on a line
4. Process characteristics : Indicates the order of occurrence of various possible events for different functions

Second, complete the transfer mode conversion (parallel and serial conversion)

Data in the computer many uses parallel transmission , but the data in the transmission media is serial , that is, bit-by-byte transmission in chronological order.

The basic knowledge of data communication system model P39

组成: source system, transmission system, purpose system
路径: end point, demodulator, transmission network, source point, modulator
信号: Digital signal-- analog signal--digital signal

A few basic concepts about channels

Communication interaction Mode

单工通信Data transfer in one direction only

半双工通信Only one-direction transmission at a time

全双工通信Both sides can transmit data at the same time

Note Adaptation Channel

Baseband signals often contain more low- frequency components and DC components , many channels do not transmit this signal, it is necessary to modulate this signal.

modulation
1. Baseband modulation (coding): Only the baseband signal waveform transformation , so that it can be adapted to the channel, modulation is still the baseband signal.
Coding: Non-zero, zero, Manchester, Manchester (P41)

2. Band-pass modulation: using carrier modulation, the frequency of baseband signal is moved to a higher band, modulated to become analog signal .
Basic Band-Pass modulation method: AM, FM (FM), Phase modulation (PM) (P41)
Complex modulation: quadrature amplitude modulated QAM (quadrature amplitude modulation)

Limit capacity of the channel

factors restricting the transmission rate of the code element on the channel
code element: The number of bits carried.
(1) frequency range through which the channel can pass
Example: too high frequency signal, resulting in transmission in the channel, the distortion, resulting in inter-code disturbance.
(2) Snr: The average power of the signal and the average power of the noise ratio, recorded as S/N, Unit decibel (db).

Shannon Formula : The channel limit transmission rate c,w is the channel bandwidth (HZ), S is the signal power, n is the noise power

Conclusion: The higher the channel bandwidth or signal-to-noise ratio, the higher the limit transmission rate of the channel.

Increase the channel transfer rate: let each code element carry more bits, the new code element modulation, at the same transmission rate to transmit the code element, the transmission of more information .

Transport media-guided transport media under the physical layer

Electromagnetic waves are guided by solid media (copper or fiber) transmission;

Twisted pair Wire

The twisted pair can be used to transmit analog signals or to transmit digital signals. The bandwidth obtained depends on the thickness of the wire (i.e. diameter) and the distance between the transmission distances. In many cases, the twisted-pair transmission can reach a bandwidth of several Mbps at a distance of thousands of meters. Because the twisted pair has sufficient transmission performance and a relatively low cost, it is widely used and may continue for many years to come.

Coaxial cable

The coaxial cable has better shielding characteristics and greater bandwidth than unshielded twisted pair, so it can transmit a considerable distance at a very high rate. There are two types of coaxial cables that are widely used. One is a 50 euro cable, which is used for digital transmission from the outset: the other is a 75 euro cable, typically used for analog transmission and cable TV transmissions. Mainly used in the residential area of the limited television network.

Optical cable

Optical fiber transmits optical signals with a refractive angle greater than the angle of incidence, so when the incident angle is sufficient the General Assembly appears full reflection.
Optical fiber is mainly used for long-distance network backbone transmission, high-speed local area network (although so far, copper cores are still trying to catch up with optical fiber) and high-speed Internet access, such as fiber to the user. The optical fiber transmission system consists of three key components, the z light source, the transmission medium and the detector.

Multimode fiber and single-mode fiber

Non-guided transmission media (P47~P50) channel multiplexing Technology

Frequency Division Multiplexing

Frequency Division multiplexing FDM, leveraging the advantages of the pass-band transmission, allows multiple users to share a single channel. It divides the spectrum into several bands, each of which has a frequency band to send its own signals.

Time Division Multiplexing

Time Division Multiplexing TDM, divided into equal length TDM frames, each user periodically occupy the same bandwidth, when the user has no data transmission, it occupies a time slot is not released, other users can not take advantage of this resource

Statistical Time Division Multiplexing

Statistical Time Division Multiplexing (STDM) is an improved division multiplexing, which can significantly improve the utilization rate of the channel by using a multi-level concentrator.

Wavelength Division Multiplexing

Frequency division multiplexing of light.

Code Division Multiplexing

Also called Code Division Multiple Access CDMA, each bit time is divided into m short interval, called Chip (CHIP)

Topic

Books 6, 7, 8, 9, 11, 12, 16, 18

Chapter II Physical layer of computer network notes