The "physical layer" in the tcp/ip protocol suite sends binary streams on the physical medium. The physical layer provides transmission media and interconnection equipment for data communication between devices, and provides a reliable environment for data transmission; the electrical characteristics of the physical layer specify the signal voltage and impedance matching on the line when transmitting binary bit streams on the physical connection. , transmission rate and distance limitations, etc.
The operating environment of this tutorial: Windows 10 system, Dell G3 computer.
The "physical layer" in the tcp/ip protocol suite sends binary streams on the physical medium.
The physical layer (or physical layer, Physical Layer) is the lowest layer in the computer network OSI model. Physical layer specification: Provides mechanical, electronic, functional and normative characteristics for the creation, maintenance, and teardown of physical links required to transmit data. Simply put, the physical layer ensures that raw data can be transmitted over various physical media. Both LAN and WAN belong to layer 1 and 2.
The physical layer is the first layer of OSI. Although it is at the bottom, it is the foundation of the entire open system. The physical layer provides transmission media and interconnection equipment for data communication between devices, and provides a reliable environment for data transmission. If you want to remember this first layer in as few words as possible, it's "Signals and Medium."
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. Different communication methods will make the data The link layer does not feel these differences and only considers completing the protocols and services of this layer.
(2) Give its service users (data link layer) the ability to transmit and receive bit streams (generally serial and sequential transmission bit streams) on a physical transmission medium. For this purpose, physical The layer should address the establishment, maintenance, and release of physical connections. (3) Uniquely identify data circuits between two adjacent systems. [2]
The main function of the physical layer: providing data transmission channels and transmitting data for data-side devices.
1. Provide a path for data transmission for data-end devices. The data path can be a physical media or multiple physical media connected. A complete data transfer includes activating the physical connection, transmitting data, and terminating the physical connection. The so-called activation means that no matter how many physical media are involved, the two data terminal devices in communication must be connected to form a path.
2. To transmit data, the physical layer must form an entity suitable for data transmission needs and serve the data transmission. One is to ensure that data can pass through it correctly, 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 method of transmitting data 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 on the physical layer.
Characteristics of the physical layer interface
The four characteristics of the physical interface reflected in the physical interface protocol are mechanical characteristics, electrical characteristics, functional characteristics and procedural characteristics.
(1) Mechanical characteristics
Indicate the shape and size of the connector used in the interface, the number and arrangement of leads, fixing and locking devices, etc. This is very similar to the common power plug sizes of various specifications that have strict regulations.
(2) Electrical Characteristics
Indicates the range of voltages that appear on each line of the interface cable.
The electrical characteristics of the physical layer stipulate the signal voltage on the line, impedance matching, transmission rate and distance limitations when transmitting a binary bit stream on a physical connection. The early electrical characteristics standards defined the electrical characteristics at the physical connection boundary points, while the newer electrical characteristics standards define the electrical characteristics of both transmitters and receivers, and also provide relevant regulations for interconnection cables. In comparison, newer standards are more conducive to the integration of sending and receiving lines. The electrical characteristics of the physical layer interface are mainly divided into three categories: unbalanced type, new unbalanced type and new balanced type.
Unbalanced signal transmitters and receivers work in an unbalanced manner. Each signal is transmitted with one wire, and all signals share a ground wire. The signal level is 5V~15V, which represents binary "0", and -5V~-15V, which represents binary "1". The signal transmission rate is limited to 20Kbps, and the wire length is limited to 15M. Since the signal line is a single line, the interference between lines is large, and the external interference during the transmission process is also large.
In the new unbalanced standard, the transmitter works in an unbalanced manner. The receiver works in a balanced manner (that is, a differential receiver). Each signal is transmitted using one wire. All signals share two ground wires, one in each direction. The signal level uses 4v~6v to represent binary "0", and -4V~-6V represents binary "1". When the transmission distance reaches 1000M, the signal transmission rate is below 3kbps. As the transmission rate increases, the transmission distance will shorten. In the case of short distance within 10M, the transmission rate can reach 300kbps. Since the receiver adopts differential reception and uses signal ground independently in each direction, inter-line interference and external interference are reduced.
The new balanced standard stipulates that both the transmitter and receiver work in a differential manner. Each signal is transmitted using two wires. The entire interface can work normally without sharing signals. The level of the signal is determined by the signal on the two wires. The difference is represented by . Relative to a certain wire, a difference between 4V and 6V represents a binary "0", and a difference between -4V and -6V represents a binary "1". When the transmission distance reaches 1000M, the signal transmission rate is below 100kbps; when the transmission distance is within 10m, the signal transmission rate can reach 10Mbps. Since each signal is transmitted using two lines, inter-line interference and external interference are greatly weakened, and it has a high ability to resist common mode interference.
(3) Functional characteristics
specifies the source and function of interface signals and the relationship between other signals. That is, the function allocation and exact definition of each signal line on the physical interface. Physical interface signal lines are generally divided into data lines, control lines, timing lines and ground lines.
The functional characteristics of the DTE/DCE standard interface are mainly to define the exact functions of each interface signal line and determine the operational relationship between them. Two methods are usually used to define each interface signal line: one method is the one-line-one-meaning method, that is, each signal line is defined as a function, CCITT V24, EIA RS-232-C, EIA RS-449, etc. Use this method; the other method is the one-line polynomial method, which means that each signal line is defined as multiple functions. This method is conducive to reducing the number of interface signal lines, and it is CCITT X. 21 adopted.
Interface signal lines can generally be divided into ground lines, data lines, control lines, timing lines and other types according to their functions. The naming of each signal line usually uses three forms: numbers, letter combinations or English abbreviations. For example, EIA RS-232-C uses letter combinations, EIA RS-449 uses English abbreviations, and CCITT V. 24 is named after a number. 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) Procedure characteristics
defines a set of operating procedures for binary bit stream transmission on signal lines, including the working sequence and timing of each signal line, so that the bit stream transmission can be completed .
The regulatory characteristics of the DTE/DCE standard interface stipulate the mutual relationships between the signal lines of the DTE/DCE interface, the sequence of actions, and maintenance and testing operations. The procedure characteristics reflect various possible events that may occur between the communicating parties during the data communication process. Because the order of occurrence of these possible events is not the same and there are many combinations, the characteristics of the procedures are often complex. A better way to describe the characteristics of a procedure is to use a state transition diagram. Because the state transition diagram reflects the transition process of the system state, and the system state transition is determined by the current state and the events that occurred (referring to the control signals that occurred at that time).
Different physical interface standards are different in the above four important characteristics. The physical interface standards that are more widely used in actual networks include 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.
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