The physical layer considers how to transmit data bit streams on the transmission media connecting various computers, rather than specific transmission media.

Function: To shield the differences of different transmission media and communication means as much as possible.

Protocols for the physical layer are also often referred to as physical layer procedures.

Second, the basic knowledge of data communication

1. Data, signal, symbol

Data is the entity that conveys information

A signal is an electrical or electromagnetic manifestation of data

symbol

2. Source, channel, and sink

The source is the source that generates and sends data

Sink is the end point of receiving data

A channel is generally used to represent a medium that transmits information in a certain direction

3, rate, baud, bandwidth

The rate indicates the amount of data transferred per unit time.

Bandwidth: Maximum Data Rate

4. Nyquist Theorem

5 Shannon's theorem

7 Shannon and Nye selection

Three, the characteristics of several transmission media

1. Guided transmission media

Electromagnetic waves are directed to propagate along a solid medium (copper wire or optical fiber)

1, twisted pair

Regardless of the type of twisted pair, the attenuation increases with the increase of frequency

The highest rate of twisted pair also has a lot to do with the coding method of digital signal

2 coaxial cables

3. Optical cable

2, non-guided transmission media

1. Radio microwave communication

2. Satellite communication

Fourth, channel multiplexing technology

1. Frequency division multiplexing, time division multiplexing and statistical time division multiplexing

Multiplexing:

1. Frequency division multiplexing

2. Time division multiplexing

2 WDM

3 code division multiplexing

Every user can use the same frequency band to communicate at the same time.

Each user uses a different pattern that has been specially selected so that there is no interference.

When the CDM (Code Division Multiplexing) channel is shared by multiple users with different addresses, it is called CDMA (Code Division Multiple Access).

5. Key exercises after class

Data: Entities that deliver information

Signal: The electrical or electromagnetic manifestation of data

Analog data: Analog signals that carry information

Analog Signals: Continuously Changing Data

Baseband signal: the signal from the source

Band-pass signal: After the baseband signal is modulated by the carrier, the signal frequency is moved to a higher frequency band for transmission in the channel

Numeric data: data whose value is a discontinuous value

Digital signal: a signal with a limited number of discrete values

Symbol: When using a waveform in the time domain to represent a digital signal, the basic waveform representing different discrete values

Simplex communication: that is, there is only one direction of communication, but no interaction in the opposite direction

Half-duplex communication: that is, both parties in the communication can send information, but they cannot send information at the same time, and of course they cannot receive information at the same time. This kind of communication is that one party sends and the other party accepts, and then reverses after a period of time

Full-duplex communication; both communicating parties can send and receive information at the same time

Serial transmission: use a data line to transmit data, one bit at a time, and multiple bits need to be transmitted one by one

Parallel transfer: transfer multiple bits at once using multiple data lines

C=RLog2(16)=20000b/s×4=80000b/s

The highest symbol rate R, symbol amplitude level n. C=Rlog2(n)

C=Wlog2(1+S/N)
64000b/s=3000Hz×log2(1+S/N)
S/N=64.2dB
This shows that this is a channel with very high SNR requirements.

FDM (frequency division multiplexing) frequency division multiplexing

FDMA (frequency division multiple access)

TDM (time division multiplexing) time division multiplexing

TDMA (time division multiple access) time division multiple access

STDM (statistic time division multiplexing) statistical time division multiplexing

WDM (wavelength division multiplexing) wavelength division multiplexing

DWDM (dense wavelength division multiplexing) dense wavelength division multiplexing

CDMA (code division multiple access) code division multiple access

SONET (synchronous optical network) synchronous optical network

SDH (synchronous digital hierarchy) synchronous digital series

STM-1 (Synchronous Transfer Module) level 1 synchronous transfer module

OC-48 (Optical Carrier) level 48 optical carrier

Computer Network Chapter 2 Physical Layer Final Review

First, the basic concept of the physical layer