Computer Network-Physical Layer (2)-Transmission Method
Serial transmission and parallel transmission
Serial transmission: means that data is sent bit by bit in sequence, so only one data transmission line is needed between the sending end and the receiving end.
Parallel transmission: refers to sending n bits instead of one bit at a time, so n transmission lines are needed between the sending end and the receiving end.
The advantage of parallel transmission is that the speed is n times that of serial transmission, but the cost is high
Usually the computer's transmission method on the network is serial transmission, and parallel transmission is used inside the computer.
Synchronous transmission and asynchronous transmission
synchronous transmission
Data is transmitted in the form of a stable bit stream, with no gaps between bytes. The receiving end detects the middle moment of each bit signal to determine whether bit 0 or bit 1 is received. Due to the clock frequency (processing) of different devices There are certain differences in the frequency of pulses that the clock generator of the device can produce, and it is impossible to achieve exactly the same result. In the process of transmitting a large amount of data, the cumulative error in the judgment time will cause the receiving end to misjudge the bit information. Therefore, methods need to be adopted to synchronize the clocks of the receiving and transmitting devices.
- External synchronization: Add an independent clock signal line between the sending and receiving parties, and the receiving end receives data according to the rhythm of the clock synchronization signal
- Internal synchronization: The sending end encodes the clock synchronization signal into the sending data and transmits it together (for example, traditional Ethernet uses Manchester encoding)
asynchronous transfer
Bytes are used as independent transmission units. The intervals between bytes are not fixed. The receiving end only synchronizes the bits in the byte at the beginning of each byte. Therefore, it is usually necessary to synchronize the bits before and after each byte. Add the start bit and end bit respectively
- Asynchronous means that the time interval between bytes is not fixed
- Each bit in the byte still needs to be synchronized (the duration of each bit is the same)
Simplex, half-duplex and full-duplex
Simplex communication: There is only one data transmission direction between the communicating parties. For example, radio broadcasting uses this communication method.
Half-duplex communication: Also known as two-way alternating communication, the two communicating parties can transmit data to each other, but not at the same time, such as walkie-talkies
Full-duplex communication: also known as two-way simultaneous communication, both parties can send and receive information at the same time, such as telephone calls
- Simplex communication requires only one channel, while duplex or half-duplex communication requires two channels (one in each direction)