Good morning, all customers, since today, the shop will launch OSI network seven-layer related packages one after another. The related concepts in the OSI network seven-layer have been roughly introduced, so in the next few days, I will take you to learn more about it. Each of the seven layers of OSI, hoping to have a deeper understanding of the operation of each layer, then next, Xiao Er serves.
In fact, you may not be unfamiliar with the physical layer. You may think that it is the lowest layer of the seven layers of the OSI network. From the literal meaning, it is really okay. Then if I want to ask you about the physical layer. What do you know? You must not just say that the physical layer is the transmission medium of the physical cable used for transmission. I can tell you clearly that this understanding is wrong. The physical layer in the true sense does not refer to the specific transmission medium used to connect the device, but a layer between the data link layer and the transmission medium, which serves as the interface between the data link layer and the transmission medium. . Because of the existence of the physical layer, the data link layer cannot feel the existence of the transmission medium, so everyone mistakenly believes that the physical layer is the transmission medium. I hope this passage of mine can help you correct this mistake.
In the previous article, we talked about functions, services, and protocols, so what do they look like in the physical layer?
1. Functions and services of the physical layer
The function of the physical layer is to transmit data bits, and its service is to establish, maintain and release a physical connection for the upper data link layer, and transparently transmit the bit stream on the physical connection.
2. Physical layer protocol
In the previous content, we have mentioned that a protocol is actually a tool, and the services provided by this layer for the upper layer are implemented according to the protocol. Therefore, the role of the physical layer protocol is to solve how the network device and the physical channel are connected and transmit the bit stream. The current EIA RS-232C of the serial interface between computers and modems is an example of a physical layer protocol.
Expanded content:
The characteristics of the physical layer interface
(1) Mechanical characteristics
Specify the shape and size of the connector used in the interface, the number and arrangement of leads, fixing and locking devices, and so on.
(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 appears on a certain line means.
(4) The characteristics of the procedure indicate the order of occurrence of various possible events for different functions.
The main features of the physical layer:
(1) Because before OSI, many physical regulations or protocols have been formulated, and in the field of data communication, these physical regulations have been adopted by many commercial 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 abstract model of OSI, the existing physical procedures are followed and the physical layer is determined to describe the mechanical, electrical, functional, and procedural characteristics of the interface with the transmission media.
(2) Because there are many ways of physical connection, and there are many types of transmission media, the specific physical protocol is quite complicated. [2]
Signal transmission is inseparable 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 transmission signals.
1. Mechanical properties
Also called physical characteristics, it indicates the mechanical characteristics of the hardware connection interface between communication entities, such as the shape and size of the connector used in 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 dimensions are strictly regulated.
The geometric dimensions of the DCE interface and the number and arrangement of jack cores have been standardized by ISO .
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. DCE (Data Circuit-terminating Equipment, data circuit termination equipment, used to connect DTE and data communication network equipment, such as Modem modem ) connector to match, the number of pins and arrangement are mirrored and symmetrical with the DCE connector.
2. Electrical characteristics
It specifies the physical connection, the electrical connection of wires and the characteristics of related circuits, generally including: description of receiver and transmitter circuit characteristics, signal identification, description of maximum transmission rate, rules related to interconnecting cables, transmitter The output impedance of the receiver, the input impedance of the receiver and other electrical parameters.
3. Features
Indicate the purpose (usage) of each signal line of the physical interface, including: the stipulation method of the interface line function, the functional classification of the interface signal line-data signal line, control signal line, timing signal line and grounding line 4 categories.
4. Procedure characteristics
Specify the entire process of using the interface to transmit bit streams and the legal sequence of events used for transmission, including the execution sequence of the events and the data transmission method, that is, when the physical connection is established, maintained, and exchanged, the DTE/DCE parties are in their respective The sequence of actions on the circuit.
The above four characteristics implement the requirements for signals, interfaces, and transmission media when the physical layer transmits data.