Overview of understanding the physical layer of transmission media

Q1： What is the role of the physical layer ？

The difference between shielding transmission media and communication means , Provide services for the data link layer

Q2： What is a data communication system ？

The data communication system includes three parts , Namely ： Source system → Transmission system → Purpose system

Some basic concepts

• data (data)—— The entity carrying the message .
• The signal (signal)—— The electrical or electromagnetic representation of data .
• “ Simulated ”(analogous)—— The value of the parameter representing the message is continuous .
• “ Digital ”(digital)—— The value of the parameter representing the message is discrete .
• 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 .

Q3： The concept of channel

The way the channel interacts with information ：

• One-way communication （ Simplex communication ）—— There can only be one direction of communication without interaction in the opposite direction .
• Two way alternating communication （ Half duplex communication ）—— Both sides of the communication can send messages , But it can't be sent by both sides at the same time ( Of course, we can't receive it at the same time ).
• Two way simultaneous communication （ Full duplex communication ）—— Both sides of communication can send and receive information at the same time .

baseband signal —— Signal from source , It contains more low-frequency components , In order to enable the channel to transmit low-frequency components and DC components , Modulation required , Modulation can be divided into the following two categories ：

• Baseband modulation （ Transform only the waveform ）, Also known as encoding ;
• Bandpass modulation （ Use carrier modulation ）：① amplitude modulation ;② frequency modulation ;③ Phase modulation ;
  

The limit capacity of the channel :
Shannon's theorem ：
Limit information transmission rate C = W log2(1+S/N) b/s ; lower than C Error free transmission can be realized

Methods of improving information transmission rate ：
Let each symbol carry more bits of information —— That is, a symbol is 3 individual bit Words , Compared with the original situation , At the same time, the amount of information transmitted increases 3 times , Because the transmission rate has nothing to do with the amount of data .

Q4： Transmission media under the physical layer

1. Leading to transmission media

1.1 Twisted pair
It is the oldest but most commonly used transmission medium . Put two insulated copper wires together , Then twisted in a regular way to form a twisted pair . The stranding can reduce the electromagnetic interference to the adjacent wires .

The most common use of twisted pair is the ubiquitous telephone system . Almost all telephones are connected to telephone exchanges with twisted pair cables . This section of twisted pair from subscriber telephone to switch is called subscriber line or subscriber loop . A certain number of such twisted pairs are usually bundled into cables , Wrap it with a sheath .

1.2 Coaxial cable ：
Copper core wire made of inner conductor （ Single strand is new wire or stranded wire ）, Insulating layer , Mesh braided outer conductor shield （ It can also be a single share ） And protective plastic outer layer . Because of the shielding layer of the outer conductor , Coaxial cable has good Anti interference characteristics , By It is widely used to transmit high-speed data .

1.3 optical cable ：
It is the transmission medium of optical fiber communication . There is a light source at the transmitting end , Light emitting diodes or semiconductor lasers can be used , They can produce light pulses under the action of electric pulses . At the receiving end, a photodiode is used to make a photodetector , When the light pulse is detected, the electric pulse can be restored .

2. Undirected transmission media

2.1 Short wave communication ： High frequency communication , Mainly by the reflection of the ionosphere . However, the fading phenomenon caused by ionospheric instability and the multipath effect caused by ionospheric reflection , The communication quality of short wave channel is poor .

2.2 Microwave communication
Ground microwave relay communication
Satellite communications