A data communication base
The basic concept of the data communication
1, the message: humans can perceive described. For example: text and image the eye can see; ears hear sounds; smell the smell.
2. Information: the expression of uncertainty on the status or existence of things. Meaningful content contained in the message, the message is the information carrier
3, the communication: point information reproduced exactly or approximately another point.
4, the communication system: various techniques to achieve an overall, apparatus and method of communication functions.
5, the signal: in a communication system, the information propagating in the transmission channel carrier
6, data (selected) state on the nature and objective things like the relationship between symbols and their combinations described, usually numbers, text, images, etc., may be another abstract symbols
7, a channel (selection): a medium for transmitting signals; narrow channel: refer only to the signal transmission medium. Generalized channel: is not only a transmission medium, but also some of the conversion means in a communication system.
The channel has three states:
(1) the transmission status : a communication station using the channel, other stations prohibited;
(2) a race condition : all communication stations attempt to use the right of the right channel;
(3) idle : no communication stations use the channel.
Constituting the data communication system
- 1, the source: a device to convert the message signal.
- 2, the transmitting apparatus: the source signal generated by means of the appropriate conversion. Including coding and modulation.
- 3, the channel: a medium for transmitting signals.
- 4, the receiving device: the transmitting device to complete inverse transform, i.e., demodulated and decoded.
- 5, the sink: the end signal, converts the signal into a message for the identification of people.
- 6, noise sources: natural and inherent to the communication device, influence of interference on the communication signal and various signals
Analog communications and digital communications
- Analog signals: completely dependent variable signal changes with a continuous change of the message signal. Arguments: can continuously be discrete. The dependent variable: some continuous.
- Digital Signal: Arguments: discrete. The dependent variable: discrete
Data Communication:
- 1, data transmission direction: one-way communication, two-way alternate communication (half-duplex) and the simultaneous two-directional communication (full duplex)
- 2, the transmission data sequence space-time: serial communication and parallel communication
- 3, the data synchronization: asynchronous communication: transmitting character, no clock synchronization is established, is simple and suitable for low speed network; synchronous communication: transmission data block, both the clock synchronization is established, implementation complexity, suitable for high-speed network.
Data communication system according to the function
- 1, the channel utilization;
- 2, the interface and generating a signal;
- 3, synchronization;
- 4, error detection and correction;
- 5, the addressing and routing;
- 6, network management;
- 7, security guarantees;
Second, the physical medium
================ guide type transfer medium ====================
Transmission media guide type: a wired channel; to wire as the transmission medium, the signal transmitted along the wire, the signal energy concentrated in the vicinity of the wire, and therefore high transfer efficiency, but flexible enough to deploy; Category: open wire, twisted pair, coaxial cable, fiber
Open wire: advantages: lower transmission loss; disadvantages: weather and susceptible to external electromagnetic interference. Sensitive to external noise. Limited bandwidth.
UTP: The two mutually insulated copper wires twisted together side by side; 1, Category: unshielded twisted pair (the STP): Good performance. High prices, complex installation process. 2, unshielded twisted pair (UTP): more commonly used
Coaxial: has good shielding effect on the disturbance, has good resistance to electromagnetic interference. Currently used for cable television networks.
Fiber: The basic principle is to use a total reflection phenomenon of light.
advantage:
- Optical fiber communication capacity is very large, up to 100 Gbit / s.
- Transmission loss, long distance relay, in particular long-distance transmission economy.
- Good anti-lightning and electromagnetic interference.
- No crosstalk, confidentiality, nor eavesdropping or interception of data.
- Small size and light weight.
=========== non-guided transmission medium ===============
Non-guided transmission media: radio channel
Propagation of electromagnetic waves in free space
Third, the channel and the channel capacity
Channel communication device connected to a communication system transmitter and the receiver side, for signal transmission from the sender to the receiver;
Channel is divided into broad and narrow channel. Narrow channels: the signal transmission medium; generalized channel: converting means comprises a number of signal transmission medium and the communication system.
========== generalized channel classification and composition ================
Channel coding: digital signal transmission from the output of the encoder to the decoder unit through the input terminal
Modulated channel: transmitting from the output of the modulator to the input terminal through the demodulator portion.
================ channel transmission characteristics ==============
Effect of different types of channel signals of quite different:
- Random parameter channel (with channel parameters): signal distorted by the channel is time-varying.
- Constant channel parameters (constant channel parameters): signal generating time and regardless of the distortion by the channel
With reference channel transmission characteristics: the majority of the radio channel (depending on the wave and sky-wave propagation radio channel)
- 1, transmission signals vary randomly decay time
- 2, the signal propagation delay time vary randomly
- 3, the presence of multipath propagation phenomena
Constant reference channel transmission characteristics: a wired channel, and various portions of a radio channel (line of sight microwave link propagation)
- 1, a fixed attenuation of the signal amplitude
- 2, the fixed delay to produce the output signal
============== channel capacity ==============
Channel capacity: error-free transmission channel information to the maximum average information rate. Ability to measure the transmission channel
1, bandwidth of the channel is described refers to the maximum effective bandwidth with a symbol rate (or symbol rate) of the channel signal, the unit is Baud
2, the transmission rate of the channel is a transmission unit time symbols (or symbol), or the ability to use signaling rate (or speed information) is described information, the unit is bit / s
Fourth, the baseband transmission
Analog sources: telephone
Digital Source: PC
The baseband signal: the original signal has not been emitted from the source is modulated.
Baseband transmission: the baseband signal is directly transmitted in the channel.
Digital baseband transmission: the transmission channel digital baseband signal.
In the digital baseband transmission system, signal former role is the raw digital baseband signal into a digital baseband signal for the transmission characteristics of the channel.
Transmitting the encoded digital baseband
Map the data pulse signal code:
- 1, unipolar NRZ
- 2, bipolar NRZ
- 3, unipolar RZ
- 4, a bipolar RZ
- 5, the differential code (relative code)
The basic code digital baseband signal is converted into digital transmission suitable for transmission baseband transmission code: the AMI code; biphase code; Miller code; the CMI code; nBmB code; nBmT code;
V. band transmission
- A baseband signal having a low-pass characteristic, can be used in low-pass characteristics of the channel (cable channel) transmission line in Zhong. Wireless channel, a band-pass characteristic. Thus the use of only the modulated baseband signal does not correspond to the transmission channel characteristics of a carrier signal matches
- Digital modulation: using a digital base band signal wherein the control parameter of the carrier signal, so that digital information is changed to reflect the baseband signal parameter of the carrier signal, and further the digital baseband signal into a digital passband signal process.
- Digital demodulation: receiving data at the end of the process needs to be modulated to a digital baseband signal is unloaded carrier signals, is reduced to a digital baseband signal.
- Band digital transmission system: modulating implemented, the transmission may not demodulate the transmission system.
Amplitude: distance from the highest point to the x-axis, or the lowest point of the distance to the x-axis
frequency:
Phase: wave translated in the x-axis
1, the basic method of digital modulation
One (or some) changes in the parameters by the digital baseband signal control carrier signal. (0 or 1 by using the control carrier wave amplitude, frequency or phase.)
Amplitude shift keying (ASK): amplitude modulating the carrier
Frequency Shift Keying (FSK): modulating the carrier frequency
Phase shift keying (PSK): the phase modulated carrier
2, three kinds of transmission band modulation
- A binary digital modulation: a digital base band transmission of a communication system Zhuo formula
- Second, and more digital modulation system Zhong
- Third, the quadrature amplitude modulation
============= ========== binary digital modulation
1, the binary amplitude shift keying (2ASK): using a binary baseband signal amplitude variation control carrier signal.
The baseband coding information is 0: for the period of the modulated carrier signal amplitude is zero.
The baseband coding information is 1: is a period of the modulated carrier signal amplitude A
Binary frequency shift keying (2FSK): using a binary baseband signal frequency variation control carrier signal.
The baseband coding information is 0: for the period of the modulated carrier signal of a frequency f1.
The baseband coding information is 1: is a period of the modulated carrier signal of frequency f2
3, binary phase shift keying (2PSK): using binary phase change baseband signal control carrier signal.
The baseband coding information is 0: after a period of a phase modulated carrier signal φ1 is.
The baseband coding information is 1: after a period of a phase modulated carrier signal φ2 is.
Differential binary phase shift keying (2DPSK): using a relative phase change between adjacent two symbols represent digital information carrier the digital baseband signal.
Binary digital modulation properties:
- Bandwidth efficiency: FSK minimum.
- Bit Error Rate: Phase Shift Keying lowest error rate; maximum amplitude shift keying error rate.
- The sensitivity of channel characteristics: amplitude shift keying most sensitive
================ ary digital modulation ==============
=============== quadrature amplitude modulation ==================
Quadrature amplitude modulation QAM (United amplitude phase keying (APK))
The basic idea is: a two-dimensional modulation, the amplitude and phase of the carrier signal while Zhong joint line modulation technique.
Advantages: High bandwidth efficiency; strong resistance to noise; simple modulation and demodulation system;
Sixth, the physical layer interface protocol
The main task of the physical layer
- 1, over a transmission medium to achieve no transmission structure of the bit stream.
- 2, with the relevant characteristics of the interface between data terminal equipment (DTE) and data communications equipment (DCE) is. DTE: Data Terminal Equipment DCE: Data Communication Equipment
Physical layer interface characteristics: mechanical properties, electrical properties, functional properties, protocol properties
- 1, the mechanical properties of: a communication physical hardware connections between specified ports mechanical characteristics. For example: the size of conventional power kneaded head.
- 2, the electrical characteristics: the predetermined electrical connections and associated circuits connected to the physical properties, hunt. For example: the level of the signal.
- 3, features: each signal line use and the like of specified physical interface. For example: data signal lines, control signal lines
- 4, protocol characteristics: a communication protocol, the whole process of using the interface specified transmission bit stream. For example: Event execution order.
Bandwidth efficiency: FSK minimum. Bit Error Rate: Phase Shift Keying lowest error rate; maximum amplitude shift keying error rate. The sensitivity of channel characteristics: amplitude shift keying most sensitive