What Is the Physical Layer?

The physical layer (or physical layer) is the lowest layer in the OSI model of computer networks. Physical layer provision: To create, maintain, and dismantle the physical links needed to transmit data, and to provide mechanical, electronic, functional, and standardized characteristics. Simply put, the physical layer ensures that the original data can be transmitted on various physical media. Both LAN and WAN belong to Layers 1 and 2.

The physical layer is the first layer of OSI. Although it is at the lowest level, it is the foundation of the entire open system. The physical layer provides transmission media and interconnected devices for data communication between devices, and provides a reliable environment for data transmission. If you want to remember this first layer with as few words as possible, it is "signals and media."

OSI uses a variety of ready-made protocols, including RS-232, RS-449, X.21, V.35, ISDN, and FDDI, IEEE802.3, IEEE802.4, and IEEE802.5 physical layer protocols. ^[1]

Chinese name: Physical layer
Foreign name: Physical Layer

Layers: OSI first layer
Features: Transparent transmission bit stream
Unit: Bit

Main functions of the physical layer

Physical layer

The main problems to be solved by the physical layer:

(1) The physical layer should shield the physical equipment and transmission media as much as possible. The difference in communication methods makes the data link layer not aware of these differences, and only considers the completion of the protocols and services of this layer.

(2) Give its service users (data link layer) the ability to transmit and receive bitstreams (generally, serially transmitted bitstreams) on a physical transmission medium. To this end, the physical layer should resolve Build, sustain, and release issues. (3) Uniquely identify the data circuit between two adjacent systems. ^[2]

The main function of the physical layer is to provide a data path for data-end equipment and data transmission.

1. Provide a data transmission path for data-end equipment. The data path can be a physical medium or multiple physical media connected. A complete data transmission, including activating the physical connection, transmitting data, and terminating the physical connection. The so-called activation means that no matter how much physical media is involved, it must be connected between the two data terminal devices communicating to form a path.

2. To transmit data, the physical layer must form an entity suitable for data transmission needs and serve data transmission. One is to ensure that data can pass through it properly, and the other is to provide sufficient bandwidth (bandwidth refers to the number of bits (BIT) that can be passed per second) to reduce congestion on the channel. The data transmission method can meet the needs of point-to-point, point-to-multipoint, serial or parallel, half-duplex or full-duplex, synchronous or asynchronous transmission.

3. Complete some management work at the physical layer. ^[3]

Physical layer components

The physical layer media includes overhead lines, balanced cables, optical fibers, and wireless channels. Communication interconnection equipment refers to the interconnection equipment between DTE and DCE

Physical layer Equipment. DTE is data terminal equipment, also called physical equipment, such as computers, terminals, etc. are included. DCE is a data communication device or a circuit connection device, such as a modem. Data transmission usually takes the path of DTE-DCE and then DCE-DTE. Interconnected devices refer to devices that connect DTE and DCE, such as various plugs and sockets. Various thick and thin coaxial cables, T-connectors, plugs, receivers, transmitters, repeaters, etc. in the LAN are all media and connectors at the physical layer. ^[4]

Important content of the physical layer

Characteristics of the interface of the physical layer

(1) Mechanical characteristics

Indicate the shape and size of the connectors used for the interface, the number and arrangement of the leads, the fixing and locking devices, etc.

(2) Electrical characteristics

Indicate the range of voltages that appear on each line of the interface cable.

(3) Functional characteristics

Indicate what a certain level of voltage on a line means.

(4) The procedure characteristics indicate the order of occurrence of various possible events for different functions.

The main features of the physical layer:

(1) Before OSI, many physical rules or protocols have been formulated, and in the field of data communication, these physical rules have been adopted by many commercialized devices. In addition, the physical layer protocol covers a wide range, so far Instead of formulating a new set of physical layer protocols according to the OSI abstract model, the existing physical procedures are used, and the physical layer is determined to describe the mechanical, electrical, functional, and procedural characteristics of the interface with the transmission medium.

(2) Because there are many physical connection methods and many types of transmission media, the specific physical protocol is quite complicated. ^[2]

Signal transmission cannot be separated from the transmission medium, and there must be interfaces at both ends of the transmission medium for sending and receiving signals. Therefore, since the physical layer is mainly concerned with how to transmit signals, the main task of the physical layer is to specify the characteristics of various transmission media and interfaces related to the transmitted signals.

1. Mechanical properties

Also called physical characteristics, indicates the mechanical characteristics of the hardware connection interface between the communication entities, such as the shape and size of the connectors used for the interface, the number and arrangement of leads, and fixing and locking devices. This is very similar to the usual power plugs of various specifications, and their sizes have strict regulations.

The geometric dimensions of DCE interfaces standardized by ISO and the number and arrangement of jack cores.

DTE (Data Terminal Equipment, data terminal equipment, equipment used to send and receive data, such as the user's computer) connector commonly used pin form, its geometric size and. Data circuit-terminating equipment (DCE) is used to connect DTE and data communication network equipment, such as Modem modem connectors. The number of pin cores and arrangement are mirror-symmetrical to the DCE connector.

2. Electrical characteristics

It stipulates the physical connection, the electrical connection of the wires and the characteristics of the related circuits. Generally, it includes the description of the characteristics of the receiver and the transmitter circuit, the identification of the signal, the description of the maximum transmission rate, the rules related to the interconnection cable, the transmitter Electrical parameters such as the output impedance of the receiver and the input impedance of the receiver.

3 Functional characteristics

Indicate the purpose (usage) of each signal line of the physical interface, including: the method of specifying the function of the interface line, and the functional classification of the interface signal line-data signal line, control signal line, timing signal line and ground line.

4 Regulation characteristics

Indicate the entire process of transmitting the bit stream using the interface and the legal sequence of events for transmission, including the order of event execution and the data transmission method, that is, when the physical connection is established, maintained, and information is exchanged, the DTE / DCE parties Sequence of actions on a circuit.

The above four characteristics realize the requirements of the physical layer for signals, interfaces, and transmission media when transmitting data.

Important physical layer standards

Physical layer Some standards and protocols of the physical layer were formulated and applied long before the OSI / TC97 / C16 sub-technical committee was established.

OSI has also developed some standards and adopted some existing results. Some important standards are listed below for the reader's reference.

ISO2110: It is called "data communication-25-pin DTE / DCE interface connector and pin assignment". It is basically compatible with "RS-232-C" of EIA (American Electronics Industry Association).

ISO2593: It is called "data communication-34-pin DTE / DCE-interface connector and pin assignment".

ISO4902: It is called "data communication-37-pin DTE / DEC-interface connector and pin assignment". Compatible with EIARS-449.

CCITT V. 24: Called "Interface Circuit Definition Table between Data Terminal Equipment (DTE) and Data Circuit Termination Equipment". Its work

Compatible with EIARS-232-C and RS-449 on 100 serial lines.

Physical layer characteristics

The four characteristics of the physical interface reflected in the physical interface protocol are mechanical characteristics, electrical characteristics, functional characteristics, and regulatory characteristics. :

Physical layer (1) Mechanical characteristics, indicating the shape and size of the connectors used for the interface, the number and arrangement of leads, fixing and locking devices, etc. This is very similar to the size of power plugs of various specifications that are usually common.

(2) Electrical characteristics, indicating the range of voltages appearing on each line of the interface cable.

The electrical characteristics of the physical layer specify the signal voltage level, impedance matching, transmission rate, and distance limitations when transmitting a binary bit stream over a physical connection. Earlier electrical characteristics standards defined the electrical characteristics at the physical connection boundary points, while newer electrical characteristics standards defined the electrical characteristics of the transmitter and receiver, and also provided the relevant provisions for interconnecting cables. In comparison, newer standards are more conducive to the integration of transmit and receive lines. The electrical characteristics of the physical layer interface are mainly divided into three categories: unbalanced, new unbalanced and new balanced.

Unbalanced signal transmitters and receivers work in unbalanced mode. Each signal is transmitted by a wire, and all signals share a ground wire. The signal level is + 5V ~ + 15V, which means binary "0", and -5V ~ -15V, which means binary "1". The signal transmission rate is limited to 20Kbps, and the length of the wire is limited to 15M. Because the signal line is a single line, the interference between the lines is large, and the external interference during transmission is also large.

Physical layer In the new unbalanced standard, the transmitter works in an unbalanced manner. The receiver works in a balanced manner (ie differential receiver). Each signal is transmitted with a wire. All signals share two ground wires, that is, one ground wire in each direction. The signal level uses + 4v ~ + 6v to represent binary "0", and -4V to -6V to represent binary "1". When the transmission distance reaches 1000M, the signal transmission rate is below 3kbps. As the transmission rate increases, the transmission distance will be shortened. The transmission rate can reach 300kbps in the short range within 10M. Because the receiver uses differential reception, and uses signal ground independently in each direction, interference between lines and outside interference is reduced.

The new balanced standard stipulates that the transmitter and receiver work in a differential manner, and each signal is transmitted using two wires. The entire interface can work normally without sharing a signal. The level of the signal is determined by the difference between the signals on the two wires Means. Relative to a certain wire, the difference between + 4V ~ + 6V means binary "0", and the difference between -4V ~ -6V means binary "1". When the transmission distance reaches 1000M, the signal transmission rate is below 100kbps; when the short-distance transmission is within 10m, the rate can reach 10Mbps. Because each signal uses two-line transmission, inter-line interference and external interference are greatly weakened, and it has a high ability to resist common mode interference.

(3) Functional characteristics, which specify the source, role, and relationship between other signals. That is, the function assignment and exact definition of each signal line on the physical interface. The physical interface signal line is generally divided into data line, control line, timing line and ground line.

The functional characteristics of the DTE / DCE standard interface are mainly to make precise function definitions for the signal lines of each interface and determine the operation relationship between each other. The definition of each interface signal line usually adopts two methods: one method is one line and one meaning method, that is, each

Common connection mechanical characteristics Signal line is defined as a function, CCITT V24, EIA RS-232-C, EIA RS-449, etc. all adopt this method; the other method is one-line polysemy, which means that each signal line is defined as multiple functions This method helps to reduce the number of interface signal lines, which is CCITT X. 21 adopted.

Interface signal lines are generally divided into ground wire, data wire, control wire, and timing wire according to their functions. The naming of each signal line usually uses numbers, letters, or English abbreviations. For example, EIA RS-232-C uses letters, EIA RS-449 uses English abbreviations, and CCITT V. 24 are named after numbers. In CCITT V. In the 24 recommendations, the name of the DTE / DCE interface signal line starts with 1, so it is usually called the 100 series interface line, and the name of the DTE / ACE interface signal line starts with 2, so it is called the 200 series interface Signal line.

(4) The protocol characteristics define a set of operation processes for binary bit stream transmission on the signal line, including the working order and timing of each signal line, so that the bit stream transmission can be completed.

The protocol characteristics of the DTE / DCE standard interface specify the relationship between the signal lines of the DTE / DCE interface, the sequence of operations, and maintenance and test operations. The characteristics of the procedures reflect the various possible events that may occur between the communicating parties in the data communication process. Because the sequence of these possible events is not the same, and there are multiple combinations, the characteristics of the procedures are often complicated. A better way to characterize procedures is to use state transition diagrams. Because the state transition diagram reflects the transition process of the system state, the system state transition is determined by the current state and the events that occur (referring to the control signals that occurred at the time).

Different physical interface standards are different in the above four important characteristics. The more widely used physical networks are EIA-232-E, EIA RS-449, and CCITT's X. 21 suggestions. EIA RS-232C is still the most commonly used computer asynchronous communication interface.

Physical layer interface protocol

Telephone network modems-V. 92
IRDA physical layer
USB physical layer
EIARS-232, EIA-422, EIA-423, RS-449, RS-485
Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX. 100BASE-T, 1000BASE-T, 1000BASE-SX and other types
Varieties of 802. 11 Wi-Fi physical layer
DSL
ISDN
T1 and otherT-carrierlinks, and E1 and otherE-carrierlinks
SONET / SDH
Optical Transport Network (OTN)
GSMUm air interface physical layer
Bluetooth physical layer
ITURecommendations: seeITU-T
IEEE 1394 interface
TransferJet physical layer
Etherloop
ARINC 818 Avionics Digital Video Bus
G. hn / G. 9960 physical layer
CAN bus (controller area network) physical layer

Physical layer communication hardware

Common equipment at the physical layer are: network card optical fiber, CAT-5 line (RJ-45 connector), hub with full wave function, Repeater signal enhancement, serial port, parallel port, etc.

Communication hardware includes communication adapters (also called communication interfaces) and modems (MODEM) and communication lines. In principle, the physical layer only solves the bit stream transmission between DTE and DCE. Although the communication control device, which is the main component of the network node equipment, is inherent in the physical layer, data link layer, and even higher layers, the The upper boundary is not very clear, but it includes functions such as MODEM interface, bit sampling and sending, and bit buffering that belong to the physical layer category. In order to achieve communication between a PC and a modem or other serial device, the electronic circuit must first be used to convert the parallel data in the PC into a bit stream compatible with these devices. In addition to the transmission of the bit stream, technical issues such as how many bits a character consists of and how to extract characters from the bit stream must be solved, which requires the use of communication adaptation. The communication adapter can be considered as a circuit for performing serial, parallel conversion and other related functions of binary data. The communication adapter is divided into TTY (Tele Type Writer), BSC (Birary Synchronous Commuication) and HDLC (High-level Data link Control).

IBM PC Asynchronous Communication Adapter: Asynchronous communication adaptation using TTY protocol adopts RS-232C interface standard. This communication adapter can be used for PC-on-line communication, and it can also be used to connect a variety of external equipment using RS-232C interface. For example, you can connect input devices such as a mouse and digitizer that use the RS-232C interface; you can connect various output devices such as printers, plotters, and CRT displays that use the RS-232C interface. It can be seen that the use of asynchronous communication adapters is very wide. Asynchronous communication procedures treat each character as an independent message. Characters can appear in the bit stream in sequence. The interval between characters is arbitrary (that is, asynchronous timing is used between characters), but each bit in a character is fixed. Clock frequency transmission. Asynchronous timing between characters and synchronous timing between bits in characters are characteristics of asynchronous transmission procedures.

Each character in the asynchronous transmission procedure consists of four parts:
1-bit start bit: It is represented by logic "0", which is called "SPACE" in communication.
5 to 8 data bits: the content to be transmitted.
1 odd / even check bit: used for error detection.
1 to 2 stop bits: It is represented by logic "1" and is used as the space between characters. In this transmission method, each character is separated by a start bit and a stop bit, so it is also called "start-stop" transmission. Each parallel character in the data to be transmitted by the serial port is first converted into a serial bit string, a start bit is added before the string, a check bit and a stop bit are added after the string, and then sent. The receiving end ensures the integrity of the bit string in the received character by detecting the start bit, check bit and stop bit, and finally converts it into a parallel character. The serial asynchronous communication adapter itself is like a microcomputer, and the above functions are all performed transparently without user intervention. Early asynchronous communication adapters were made as separate plug-in boards, which can be directly inserted into the system expansion slot of a PC for use. Later, most asynchronous communication adapters were combined with other adapters (such as adapters for printers, disk drives, etc.). A plug-in board called a multifunction board. There are also some high-end microcomputers. Asynchronous communication adapters have been made on the system motherboard as a conventional part of the microcomputer system.

Physical layer programming method

Asynchronous serial communication programming methods for PCs include DOS, WINDOWS, and BIOS-level PC communication, PC communication for systems based on asynchronous communication, and communication programming methods.

DOS Physical layer DOS communication

PCs usually have two asynchronous serial ports, which are called COM1 and COM2, respectively, and they both meet the RS-232C standard. In the DOS operating system, COM1 and COM2 are managed as I / O devices, and COM1 and COM2 are their logical device names. According to this, DOS can realize asynchronous serial communication by operating on COM1 and COM2. The DOS MODE command can be used to set the parameters of the asynchronous serial port. The DOS COPY command allows the asynchronous serial port to be used as a special "file" for data transmission. The following is an example of using DOS's MODE and COPY commands to transmit characters on a dual-computer keyboard. The format of the MODE command is as follows:

MODE port name: speed, check mode, number of data bits, number of stop bits

The port name is COM1 or COM2; the transmission rate can be selected from 110, 150, 300, 600, 1200, 2400, 4800, or 9600bps; the verification mode is E (even parity), (odd parity) or N (no parity ); The number of data bits is 7 or 8 bits; the number of stop bits is 1 or 2 bits. The parameters set by both communication parties should be the same. For example, if both parties enter the following command: MODE COM1: 1200, E, 7, 1 means that both parties use COM1 as the asynchronous communication port with 1200bps, even calibration, 7 data bits, and 1 stop bit. Communication of the setting parameters. There is a standard console COM in DOS. In fact, COM is the keyboard when input, and COM is the display when output. ^[1]

The PC to be sent executes the following command: COPY CON: COOM1: indicates that the information received from the keyboard is sent through the COM1 serial port.

The PC that is ready to receive executes the following command: COPY COM1: CON: means that it will receive the information from the COM1 serial port and display it on the display.

After the two PCs execute the above commands respectively, the characters entered on the sender's keyboard will be displayed on the receiver's display. The above is the simplest PC communication implemented using the DOS MODE and COPPPY commands. In higher versions of MS-DOS (such as MS-DOS V6.0), a command is also called INTERLNK, which is actually a communication program. Using the INTERLNK command and a cable connecting the serial ports of two PCs, one PC can access data and run programs from the disk drive of the other PC, eliminating the need to use a floppy disk to copy files. The PC used to enter commands is called the Client, and the PC connected to the client is called the Server. The client uses the server's drive and printer, and the server displays the connection status of the two PCs.

When two PCs are connected by INTERLNK, the drive on the server is mapped to the client in the form of an extended drive. If the two PCs originally had three drives A, B, and C, the client will connect to itself except for itself In addition to the three drives, there are three extended drives E, F, and G (server drive image). Clients can use these extended drives just as they use their own drives. When using INTERLNK, there must be at least one free serial port on each PC, and a null modem (Null MODEM) serial cable of line 3 or line 7, the client must have at least 16K free memory, and the server At least 130K of free memory.

CONFIG system configuration file on the client. Add the following command in SYS: devive = c: \ dos \ interlnk. exe / drives: 5

Restart the client and load INTERLNK. This assumes interlnk. The exe is stored in the DOS subdirectory of the client C drive. The / drives: 5 parameter is used to map 5 server drives, which are 3 drives by default. INTERLNK does not need its CONFIG to start on the server. To make any changes to SYS, just type intersvr at the DOS command prompt. At this time, a line of status information appears at the bottom of the screen, showing the connection status of INTERLNK.

PC PC communication at the physical layer

The Microsoft Windows application Terminal allows the user's PC to connect and exchange data with other computers. It can also emulate the type of terminal required by the remote computer with which data will be exchanged. The communication process of reading a file from a remote system with an on-line service is given by a PC using WINDOWS Terminal.

Open the terminal-use the Settings menu to set parameters-consult the file-use the Transfers menu to receive a file-go offline with the remote computer-use the phone menu to suspend the modem-use the File menu Store file-exit terminal

BIOS Physical layer BIOS

Interrupt 14H in the basic input and output system (BIOS) of the PC provides the service function of asynchronous serial port. Through the four functions provided by INT 14H, the serial communication port can be accessed to realize on-machine communication. The serial port function of INT 14H is. ^[4]