Chapter II (physical layer)

2.1、 The basic concept of the physical layer

1、 The main purpose of the physical layer ： Transparently transmit the bitstream on the transmission medium ; Be careful ： The physical layer does not refer to a specific transmission medium
2、 The main tasks of the physical layer ： Determine some characteristics related to the interface of the transmission medium ：

① Mechanical characteristics ： Indicate the shape of the connector used for the interface 、 Size 、 Number and arrangement of pins, etc ;
② Electrical characteristics ： Indicate the voltage range of each line on the interface cable ;
③ Functional features ： Indicate the meaning of the voltage at a certain level on a line ;
④ Process characteristics ： Indicate the sequence of various possible events for different functions ;

Data is usually transmitted in parallel in computers , On the communication line , Generally, it is serial transmission , Mainly for economic reasons ;

2.2 Basic knowledge of data communication

2.2.1 Model of data communication
（1） The communication system can generally be divided into three parts ：

①. Source system ： Including source and transmitter ;
②. Transmission system ： It may be a simple transmission line or a complex network system ;
③. Purpose system ： Receiver and sink ;

（2） Several related terms ：

①.  data （data： Carrier of operation information ;
②.  The signal （signal： Electrical or electromagnetic representation of data ;
③.  analog signal （analogous signal： Fluctuations continue to change , Contains an infinite number of values ;
④.  digital signal （digital signal： discrete , Jump , Contains a finite number of values ; Advanced equipment , cheap 
⑤.  Code element （code： In use time domain （ Or time domain for short ） When the waveform of the signal represents a digital signal , Basic waveforms representing different discrete values .

2.2.2 Some basic concepts about channel

1、 channel ： A medium that transmits information in one direction .
Different communication circuits , Usually a communication circuit consists of a transmission channel and a reception channel .
2、 Simplex communication ： There can only be one direction of communication without interaction in the opposite direction , Like radio .
3、 Half duplex communication ： Both sides of the communication can send messages , But it can't be sent by both sides at the same time , Can not receive at the same time .
4、 Duplex communication ： Both sides of communication can send and receive information at the same time .
5、 baseband signal （ I.e. basic frequency band signal ）： Signal from source . Like the computer output on behalf of a variety of text or image file data signals are baseband signals .
6、 Baseband transmission ： The baseband signal is transmitted directly in the channel .
7、 Band pass signal ： A signal modulated by a carrier wave （ That is to say, it can pass through the channel only in a frequency range ）.
8、 Band transmission ： The baseband signal is modulated into an analog signal and then transmitted .

2.3 Channel multiplexing

2.3.1、 Why to use channel multiplexing technology ？

1、 The cost of communication line erection in communication engineering is relatively high , The existing capacity of the line needs to be fully utilized .
2、 The communication capacity of the transmission medium will exceed the transmission traffic of a single channel .

2.3.2、 The essence of multiplexing technology ： The sender uses a multiplexer to combine multiple user data and share a physical line for transmission to the receiver , The receiving end then uses a demultiplexer to separate the data into individual data , Distribute to multiple users of the receiver .
（ This chapter is relatively simple ）