Table of contents
Four important characteristics
Common standards for the physical layer
basic introduction
The physical layer is the lowest layer in the network architecture. It neither refers to the specific physical equipment connected to the computer nor the specific physical medium responsible for signal transmission, but refers to the upper layer (referring to the physical media connecting the open system). Data link layer) provides a physical connection for transporting bit streams .
The main function of the physical layer is to provide its service users (i.e., entities of the data link layer) with the ability to " transparently " transmit bit streams on specific physical media .
The role of the physical layer - to shield the differences in physical equipment, transmission media and communication methods used by computer networks as much as possible, so that the data link layer does not have to consider the specific characteristics of physical equipment and transmission media, but only needs to consider the protocols and protocols that complete this layer. Serve.
Four important characteristics
The physical layer protocol is related to specific physical equipment, transmission media and communication means. The protocols used for the physical layer are also often called protocols . Many protocols of the physical layer were developed before the release of OSI/RM and were not described in OSI terms. The main functions implemented by the physical layer can only be described as four important characteristics related to the transmission medium interface :
- Mechanical characteristics: mainly define the boundary points of physical connections, that is, the physical structure of the connector. Specify the shape and size of the connector used, the number and arrangement of pins or sockets, fixing and locking devices, etc.
- Electrical characteristics: Mainly define the boundary points of physical connections, that is, the physical structure of the connector. Specify the shape and size of the connector used, the number and arrangement of pins or sockets, fixing and locking devices, etc.
- Functional characteristics: It mainly defines the exact functions of each interface signal line and the operational relationship between them.
- Procedure characteristics : Mainly stipulates the mutual relationship between the signal lines of the interface, the sequence of actions, maintenance and test operations, etc. Reflects various possible events that may occur between the communicating parties during the data communication process.
Common standards for the physical layer
THIS IS RS-232
Mechanical Characteristics of the RS-232-C Interface
- Uses 25-pin connector ( compatible with ISO 2110).
- The DTE side adopts a pin type (male plug) structure, and the DCE side adopts a hole type (female socket) structure.
- In actual use, a 9-core connector with fewer core pins can be used.
Electrical Characteristics of RS-232-C Interface
- It adopts single-ended transmitting, single-ended receiving and bipolar power supply. Its logic 1 level is -5V ~ -15V, logic 0 level is +5V ~ + 15V, and the transition zone is -3V ~ + 3V . Noise margin is 2 V.
- The interface level of RS-232-C is not compatible with the TTL and DTL output and input levels ( 1 is 2.4V , 0 is 0.4V ) , and an external transmission line driver / receiver must be added to achieve level conversion. Since the inter-line capacitance of currently used multi-core cables is 150pF /m , and the maximum load capacitance on the signal line should be less than 2500pF , the maximum transmission distance of RS-232-C is 15m . The actual data transfer rate should be selected based on the transmission distance and channel quality
Functional characteristics of RS-232-C interface
- The function definition of the signal line is shown in Table 5-6 . There are 20 signal lines in total, which can be divided into four categories: data lines ( 4 ) , control lines (11 ) , timing lines (3 ) and ground lines (2 ) . The remaining 5 are undefined or proprietary.
- The RS-232-C interface has primary and secondary channels. The auxiliary channel is used to transmit some auxiliary control information between interconnected devices. It is usually rarely used and its rate is lower than the main channel.
Procedural Characteristics of the RS-232-C Interface
- The procedure characteristics describe the sequence and relationship in which each signal line presents an "on" (positive level, logic 0 ) or "off" (negative level, logic 1 ) state under different conditions . For example, if DTE wants to send data to the transmission line, it must ensure that all four control lines CC , CD , CA , and CB are in the " on " state, that is, both the equipment and the line must be ready .
- Since RS-232-C has limitations on many user environments, users are eager to improve the original features, such as increasing the speed, increasing the distance, adding some necessary functions (such as loopback testing), etc. As a result, EIA revised version C to version D in 1987 , revised it to version E in 1991 , and revised it to version F in 1997 . Because there are not many revisions in each version, many manufacturers still use the original name RS-232-C.
THIS IS RS-449
EIA RS-449 is a physical layer standard interface proposed to replace EIA RS-232-C . It consists of 3 standards.
- RS-449 specifies the mechanical characteristics, functional characteristics and process characteristics of the interface ( equivalent to V.35).
- RS-423-A specifies the electrical characteristics when unbalanced transmission is used ( that is, all circuits share a common ground ) .
- RS-422-A specifies the electrical characteristics when balanced transmission is used ( that is, all circuits have no common ground )
Mechanical Characteristics of RS-449 Interface
- Use 37-pin and 9- pin connectors, the latter for secondary channel operation.
Electrical Characteristics of RS-449 Interface
- RS-423-A specifies the characteristics of an unbalanced electrical connection using differential reception. The signal level adopts negative logic of ± 6 V, and the transition area is - 4 ~ + 4 V. When the transmission distance is 100 m , the rate is 10 kb/s ; when the distance is 10 m , the rate is 300 kb/s .
- RS-422-A specifies the use of balanced electrical connection characteristics, the signal level adopts negative logic of ± 6 V , and the transition zone is - 2 ~ +2 V. When the transmission distance is 1000 m , the rate is 100 kb/s ; when the distance is 10 m , the rate can reach 10 Mb/s .
Functional characteristics of RS-449 interface
- Functional definitions were made for 30 signal lines. Compared with RS-232-C , the newly added signal line is mainly to solve the problems of loopback testing and other functions.
- These signal lines include: transmit common return line (SC) , receive common return line (RC) , local loop return (LL) , remote loop return (RL) and test mode (TM) , etc.
Procedural Characteristics of RS-449 Interface
- The protocol characteristics of RS-232-C are inherited.
RJ-45
RJ-45 connector refers to an 8 -core modular jack or socket defined by the IEC (60) 603-7 standardized connector standard , also known as RJ-45 plug . IEC (60)603-7 is the reference standard for connection hardware of the ISO/IEC 11801 international general integrated cabling standard .
The RJ-45 plug is a plastic connector that can only be inserted in a fixed direction and automatically prevents it from falling off. It is commonly known as " crystal head ".
Two ways to sort network cables on RJ-45 plugs
- T568A line sequence is used when network equipment requires cross - connection . The so-called " crossover " means that one end of the network cable is connected according to the T568A line sequence , and the other end is connected according to the T568B line sequence . A cross-connect is used to connect two network devices , such as two computers , a hub, or a switch .
- T568B line sequence is used for direct connection of network equipment . Both ends of the network cable use T568B wire sequence . A pass-through connection is suitable for connections from a wall outlet in the user's workspace of a wiring system to a computer , as well as between two different network devices .
The purpose of each network cable on RJ-45
| Line order |
function |
Line order |
function |
| 1 |
Send + |
5 |
Need not |
| 2 |
send- |
6 |
take over- |
| 3 |
receive + |
7 |
Need not |
| 4 |
Need not |
8 |
Need not |
Notice:
- The line sequence cannot be changed at will . If the line sequence is messed up at will , and 1 and 3 are used for transmission , and 2 and 4 are used for reception , then the anti-interference ability of these lines will be reduced , and the normal operation of the network cannot be guaranteed .
- Cores 4 and 5 can be used for voice services.
digital transmission system
In the early telephone network, the subscriber line from the local office to the user's telephone set used the cheapest twisted pair cable, while the long-distance trunk line used the analog transmission method of frequency division multiplexing FDM. Compared with analog communication, digital communication has obvious advantages in terms of transmission quality and economy.
At present, most long-distance trunk lines adopt the digital transmission method of time division multiplexing TDM. The PCM transmission system was originally designed to transmit multiple telephone calls on trunk lines between telephone exchanges.
Due to historical reasons, the old digital transmission system has many shortcomings, the most important of which are two aspects:
- Rate standards are not unified : If the digital transmission rate of high-order groups is not standardized, international high-speed data transmission based on optical fiber will be difficult to achieve.
- Communication is not synchronous transmission: For a long time in the past, in order to save money, digital networks in various countries mainly used quasi-synchronous methods. When the data transmission rate is very high, the clock synchronization of both the sender and the receiver becomes a big problem.
PCM has two mutually incompatible international standards
The T1 system used in North America has a total of 24 channels . The sampling pulse of each voice channel is encoded with 7 bits , and the signaling symbol is encoded with 1 bit , so one voice channel occupies 8 bits . Frame synchronization is to add 1 bit after 24- way encoding , so that there are 193 bits in each frame . Because the sampling frequency is 8kHz , the data rate of the T1 primary group is 1.544Mb/s .
In the E1 system used in Europe , each time division multiplexing frame ( its length T = 125 m s ) is divided into 32 equal time slots , which are numbered CH0 ~ CH31 . CH0 is used for frame synchronization , and CH16 is used for transmitting signaling . The remaining 30 time slots are used as voice channels for users . Each time slot carries 8 bits , so there are 256 bits in total for the entire 32 time slots . The sampling frequency is also set to 8kHz , so the data rate of the E1 primary group is 2.048Mb/s .
Our country adopts the E1 standard . Japan's primary group uses T1 , but a separate set of standards for higher-order groups is established .
When higher data rates are required, multiplexing can be used.
In order to achieve effective and reliable operation of the data communication system, the system must have a synchronization system with good performance.
Before the synchronized number series was proposed. For a long time in the past, in order to save money, digital networks in various countries mainly adopted the quasi-synchronous PDH method. PDH uses a pulse filling method to compensate for timing errors caused by frequency inaccuracies . When the transmission rate is low, the slight difference in the sending and receiving clocks will not have a serious impact. However, when the data transmission rate continues to increase, the synchronization of the sending and receiving clocks becomes very important and becomes an urgent problem to be solved.
Synchronous Optical Network SONET
In 1988, the United States first proposed a physical layer standard for optical fiber transmission, named SONET (Synchronous Optical Network). The standard stipulates that clocks at all levels of the entire synchronization network come from a very accurate master clock, such as a cesium atomic clock, with an accuracy better than ± 1 × 10 -11 . It also specifies speed, fiber optic interface, operation and maintenance.
SONET defines the line hierarchical structure of synchronous transmission for the optical fiber transmission system. The transmission rate of the first-level synchronous transmission signal STS-1 (Synchronous Transport Signal) is 51.84 Mb/s. For optical signals, it is called the first level optical carrier OC-1 (Optical Carrier).
Synchronous Digital Series SDH
ITU-T developed the international standard synchronous digital series SDH in 1988 based on the American standard SONET .
Generally, SDH and SONET can be considered synonymous. The main differences are: The basic rate of SDH is 155.52 Mb/s, which is called the first- level synchronous transmission module STM-1 , which is equivalent to the OC-3 rate in the SONET system .
SONET/SDH regulations
- The wavelengths of standard optical signals are 1310 nm and 1550 nm .
- The physical layer uses frame-structured transmission technology for broadband interfaces. For example, the frame structure of SDH is a block frame, and its basic signal is STM-1, and N STM-1 can be multiplexed to form STM-N. SDH simplifies multiplexing and demultiplexing technology and can be directly connected to low-speed branches when needed. SDH also adopts a self-healing hybrid ring network structure and is combined with the digital handover system DACS to improve the flexibility and reliability of the communication network.
The significance of SONET/SDH standards
- Unifying different digital transmission systems at the STM-1 level.
- For the first time, a global standard in digital transmission system has been truly realized.
- It has become recognized as a new generation of ideal transmission network system.
- The SDH standard is also suitable for microwave and satellite transmission systems.
Broadband access technology
As the application of the Internet becomes more and more widespread, more and more users use the Internet. Of course, users require higher Internet speeds, the better.
There is no unified definition of “broadband” . There are usually three opinions: ① The rate of access to the Internet is much greater than 56kb/s ; ② The rate of access to the Internet is greater than 1Mb/s ; ③ The FCC of the United States believes that as long as the sum of the two-way rates exceeds 200kb/s .
It is generally believed that it refers to network infrastructure and services with a transmission rate exceeding 1 Mb/s, non-dial-up access, and 24 -hour connection.
In terms of broadband access media, broadband access is divided into two categories: broadband wired access and broadband wireless access.
Broadband wired access technologies include
- Ethernet access technology based on Category 5/6 lines
- Copper-based xDSL technology
- Access technology based on hybrid optical fiber/coaxial cable
- Optical fiber access technology
Ethernet access technology based on Category 5/6 lines
- Ethernet has become a commonly used networking technology. However, traditional Ethernet technology does not belong to the category of access networks, but to the field of user premises networks.
- The broadband access network based on Ethernet technology consists of office-side equipment and user-side equipment. Office-side equipment is generally located in a community or in a commercial building. User-side equipment is generally located on residential floors. The office-side equipment provides an interface with the IP backbone network, and the user-side equipment provides a 10/100 BASE-T interface connected to the user's computer. The office-side equipment has the function of aggregating user-side equipment network management information and billing
- Category 5 line (CAT5) , bandwidth 100MHz , rate 100MB/s , mainly used for 100BASE-T . Category 5e cable (CAT5e) , with a bandwidth of 155M , is mainly used for Gigabit Ethernet. Category 6 cable (CAT6) , with a bandwidth of 250M , is used to build 10 Gigabit Ethernet and is the future development trend.
- High-speed Ethernet access technology based on Category 5/6 lines is particularly suitable for China's national conditions and suitable for the development of FTTZ , and then connects to users with Fast Ethernet (the user-end rate is 10 Mb/s or 100 Mb/s ) .
Copper-based xDSL technology
xDSL technology is a technology that uses digital technology to transform existing analog telephone subscriber lines so that it can carry broadband services. xDSL is the collective name for various digital subscriber line access technologies. Among them, DSL is the abbreviation of digital subscriber line, and the letter x is used as a prefix to indicate different broadband access solutions on digital subscriber lines.
Based on hybrid optical fiber/coaxial cable access technology
- Fiber-coaxial hybrid HFC access technology is a highly distributed access network technology that comprehensively applies analog and digital transmission technology, coaxial cable and optical cable technology, and radio frequency technology. It is the product of the combination of telecommunications network and cable TV network, and is the first broadband access technology to mature and enter the market.
- In addition to receiving CATV television signals, the HFC network also provides telephone, data and other broadband interactive services.
- The traditional cable TV network CATV is a coaxial cable network with a tree topology. It uses frequency division multiplexing of analog technology to transmit TV programs in one direction. The HFC network needs to transform the CATV network.
Based on HFC access technology
- The advantage of HFC network is that it has a wide frequency band. And be able to take advantage of cable television networks that already have considerable coverage. Transform the existing 450 MHz one-way transmission cable TV network into a 750 MHz two-way transmission HFC network.
- The disadvantages of the HFC network are: ① It adopts a tree structure and has poor security and confidentiality; ② It adopts two-way transmission, and the frequency interference problem cannot be ignored; ③ The uplink channel of the HFC network is shared by all users, and channel access conflicts are prone to occur.
Optical fiber access technology
- Optical fiber access network refers to an access network in which the transmission medium in the access network is optical fiber. Optical fiber access networks can be divided into two categories: active optical network AON and passive optical network PON . Because PON has high reliability, easy maintenance, and low investment, the PON structure is often used.
- There are currently two popular PON structures: one is the Ethernet passive optical network EPON , and the standard is 802.3ah . EPON uses the Ethernet protocol at the link layer and uses the PON topology to achieve Ethernet access. The advantages are good compatibility with existing Ethernet, low cost, strong scalability, and easy management . The other is Gigabit Passive Optical Network GPON , and the standard is ITU-T G.984 . It adopts a universal packaging method, can carry a variety of services, and can provide service quality assurance. It is a broadband optical fiber access technology with great potential.