Physical layer at the bottom of OSI reference model, its main function is to achieve a transparent bit stream transmission, the data link layer provides data transfer services. Take it directly to the actual physical medium for data transmission (ie communication channels). Physical layer transmission in bits (bit), that is, a binary bit ("0" or "1"). It must be emphasized that the physical layer refers to a specific physical device is not, nor is physical media signal transmission, but rather refers to a layer on top of physical media (ie, data link layer) to provide a raw bit stream of the physical transmission connection.
3.1 physical layer interface and protocol
3.1.1 Physical Layer Interface
Physical layer protocol is sometimes also referred to as interfaces. And physical layer protocol provides for the establishment, maintenance, and disconnect the physical channel-related features that include mechanical, electrical, functional, and procedures of the four aspects.
ISO physical layer of the OSI model's defined as: entities in the physical channel through the middle of a reasonable system for the physical bit transmission connection required for activation, maintenance and removal of providing mechanical, electrical, functional, and procedures of means.
DTE (Data Terminal Equipment) refers to the data terminal equipment, all belonging to the user workstation networking equipment or collectively, they are the communication source or the destination, such as computers, terminals and so on.
DCE (Data Circuit-Terminating Equipment or Data Communications Equipment), refers to a data circuit terminating equipment or data communications equipment, is to provide network connection point referred to the network device, such as automatic call answering equipment and modems.
3.1.2 Physical layer functions and services
1. Mechanical properties
2. Electrical Characteristics
DTE and DCE interface of the wire (also known as circuit) of the electrical connections are non-equilibrium approach, using differential receiver and balanced way to approach non-equilibrium three.
3. Signal features
Functional properties of the physical layer provides the interface signal sources, function and relationship between other signals. . Interface signal line according to the functions can be divided into data signals, control signal lines, timing signal line and ground lines, and four categories.
4. Order characteristics
Characteristics of the physical layer provides a point of order to exchange data with the exchange control circuit steps, these steps of the music, which makes the control transfer bit stream to be completed.
3.1.3 Examples of physical layer protocol
1.EIA RS-232C interface standard
EIA RS-232C by the American Electronics Industry Association EIA (Electionic Industry Association) in 1969 issued a currently the most widely used serial physical interface standard. RS (Recommended Standard) means "recommended standard", 232 is the identification number, and the suffix "C" indicates that the recommended standard has been modified frequently.
2.EIA RS-449 and RS-422 and RS-423 interface standard
3.100 series and 200 series interface standards
4.X.21 recommendations and X.21 bis
3.2 Transmission Media
Transmission medium is a communication network between the sender and receiver of physical access, computer network used in the transmission medium can be divided into two major categories of wired and wireless. Twisted pair, coaxial cable and fiber optic cable are three common transmission media; radio communications, microwave communications, infrared communications, and laser communication of information carriers belong to the wireless transmission medium.
3.2.1 Twisted Pair
Twisted pair is divided into two types: unshielded and shielded. Unshielded twisted pair (UTP, Unshielded Twist Pair). Unshielded twisted pair (STP, Shielded Twist Pair)
3.2.2 Coaxial cable
Coaxial cable into the baseband coaxial cable (Impedance 50Ω) and broadband coaxial cable (impedance of 75Ω). Baseband coaxial cable is divided into two kinds of Thick and thin cable are used for direct transmission of digital signals; broadband coaxial cable for frequency division multiplexed analog signal transmission, can also be used without using frequency division multiplexing high-speed digital signal multiplexing and analog signal transmission. CCTV is used in CATV cable broadband coaxial cable.
3.2.3 Optical Fiber
Short fiber is optical fiber, which can be conducted by the tidal wave of additional protective layers of quartz fibers.
Light-emitting diode light source can be LED (Light Emitting Diode) and injection laser diodes ILD (Injection Laser Diode).
At the receiving end is converted to electrical energy used to light the detector is a photodiode, the two solid-state devices currently used are PIN and APD detectors detector.
3.2.4 Wireless Transmission Media
Wireless transmission medium transmitted through space, do not set up or buried cable or fiber shop, the technology currently used in accordance with the frequency from low to high order are: radio waves, microwaves, infrared and visible light.
The first generation was analog cellular mobile communication, which refers to the transmission of voice mail users to simulate the way voice occurs. Second-generation digital cellular mobile communication mode. Involves digitally digital voice signals and digital information processing, transmission problems.
Solve the multiple access method is called multiple access technology. In the cellular mobile communication systems, multiple access methods include three kinds: frequency division multiple access FDMA, Time Division Multiple Access TDMA and Code Division Multiple Access CDMA.
Microwave communication, infrared communication, laser communication are needed between the sender and receiver there is a line of sight (Line of Sight) path, it is referred to them as the media attention.
3.2.5 the choice of transmission media
Choice of transmission medium depends on the following factors: network topology, the actual needs of the communications capacity, reliability, affordable price range.
3.3 Data Communication
3.3.1 Communication Channel
1. Data transfer rate
The so-called data transfer rate refers to the binary information transmitted per second digit in bits / second (Bits Per Second), recorded as bps or b / s.
R = 1 / T * log2N
Where T is a digital pulse width (full width code case) or repeated cycles (NRZ case), in seconds. A digital pulse is also called a symbol, N as a symbol said to the effective number of discrete values, also known as the number of modulation levels.
If a symbol desirable types of discrete values N, the symbol can carry log2N bit binary information.
Signal transmission rate, also known as the symbol rate, modulation rate, or baud rate, unit Potter (Baud). Said signal transmission rate per unit time code transmitted by the channel element number, which is modulated by the signal transmission rate. If the signal code element width of T seconds, the symbol rate is defined as:
B = 1 / T (Baud)
2. Channel capacity
Channel capacity of a channel characterized by the ability to transmit data, the unit also used bits / second (bps). Channel capacity and data transfer rate difference is the former that the maximum channel data rate, transmit data channel capacity is the limit, while the latter represents the actual data transfer rate.
Nyquist (Nyquist) noise-free case is given first symbol rate limits under the B and H of the relationship channel bandwidth:
B = 2 * H (Baud)
Where, H is the width of the channel, also known as frequency range, that is, the channel can be transmitted, the lower the frequency difference, in units of Hz. Characterization of this channel can be introduced transmission capacity of the Nyquist formula:
C = 2 * H * log2N (bps)
Here, N is still a symbol that can carry data to take the number of discrete values, C is the channel the maximum data transfer rate.
Calculation of the Shannon channel capacity formula:
C = H * log2 (1 + S / N) (bps)
Where, S the signal power, N is noise power, S / N compared to noise ratio.
Since the actual use of the channel's signal to noise ratio should be large enough, Guchang expressed as 10log10 (S / N), in decibels (dB) is measured in units.
3. Error rate
Error rate is a measure of data communications system in the normal operation of the transmission reliability under the index, which is defined as the binary data bit transmission error probability. Let the total number of transfer N-bit binary data, in which the median error Ne, the error rate is expressed as:
Pe = Ne / N
Computer network and generally require less than 10-9 bit error rate, an average of 109 per data transmission allows only the wrong one.
Communication There are two basic methods, namely, serial and parallel. Typically, the parallel for short distance communications, serial for long-distance communication.
(1) parallel communication. In parallel data transmission in a number of data bits.
(2) serial communications. Serial data transmission, the data in a communication, a transmission line, and also can transmit several parallel transmission of data compared to the speed serial data transfer much more slowly than parallel transmission.
(3) directional serial communication structure. Directional serial data communication structure there are three, namely, simplex, half duplex and full duplex.
Simplex data transmission only support data transmission in one direction; half-duplex data transmission allows data transmission in both directions, which is actually a half-duplex communication can be switched direction simplex; full-duplex data communications allow data transmission in both directions, so full duplex communication is two way communication with a single work, it requires both sending and receiving devices send and receive individual capacity.
3.3.2 Analog and digital data communications data communications
(1) data. Data can be defined as a meaningful entity, which involves a form of things. Simulated data and the data can be divided into two major categories of digital data. Analog data is continuously changing in a range of values; digital data are discrete values.
(2) signal. Signal is an electronic or electromagnetic encoding of data. The corresponding analog data and digital data, the signal can be divided into analog signals and digital signals.
(3) information. Information is the content and interpretation of data.
(4) source. Communication source that generated in the process and send the message device or computer.
(5) the destination. Letters places that receive and process the communication process device or computer information.
(6) channel. Channel is between the source and destination of communication lines.
Modem MODEM (Modulater Demodulater) modulated digital data into analog signals, so that in such an analog signal suitable for transmission media.
For sound data, the complete analog data and digital signal conversion function facilities codec CODEC (Coder Decoder).
Analog transmission systems use amplifiers to boost the signal energy, but also will increase the noise component, so that cause signal distortion.
Attenuation of digital transmission will also endanger the integrity of the data, digital signal only in a limited transmission distance, in order to obtain greater transmission distance, you can use the repeater. Repeater receives a digital signal attenuation, the digital signal back to "0", "1" standard level, and then re-transmission of this new signal, so that effectively overcome the attenuation.
Simultaneous transmission of a multi-channel signal that multiplexing (Multiplexing).
Frequency Division Multiplexing FDM (Frequency Division Multiplexing) and time division multiplexing TDM (Time Division Multiplexing) are the two most common multiplexing.
For Fibre Channel, or use a variation of frequency division multiplexing is called wave division multiplexing WDM (Wavelength Division Multiplexing).
1. Frequency Division Multiplexing FDM
Physical channel in the available bandwidth over a single bandwidth required for the original signal case, the total bandwidth of the physical channel is divided into a number of individual signal bandwidth and transmit the same (or slightly wider) and self-confidence, the way each sub-channel signal transmission This is the frequency division multiplexing.
2. Time Division Multiplexing
If the media can achieve the bit transfer rate of more than the required data transfer data transfer rate, you can use time division multiplexing TDM technology, but also about a physical channel on a time divided into several time slots allocated to more than one turn signal use.
3. Wave division multiplexing