Introduction to basic knowledge of communication technology

Introduction to basic knowledge of communication technology

The telecommunication network (telecommunicaTIon network) is a communication system that interconnects multiple telecommunication systems that constitute multiple users to communicate with each other. It is an important infrastructure for humans to achieve long-distance communication. It uses cable, wireless, optical fiber or other electromagnetic systems to transmit, Receive logos, text, images, sounds or other signals. The telecommunications network is composed of three elements: terminal equipment, transmission link and switching equipment, and it should be supplemented by a signaling system, communication protocol and corresponding operation support system during operation. At present, the communication systems of countries around the world are developing towards digital telecommunication networks, which will gradually replace the transmission and exchange of analog communications, and develop towards intelligent and integrated directions. The telecommunication network cannot be updated at the same time, therefore, the development of the telecommunication network is a gradual process.

The telecommunications network can be divided according to different classification systems as follows:
According to the types of telecommunications services, it is divided into: telephone network, telegraph network, user telegraph network, data communication network, fax communication network, image communication network, cable TV network, etc.

According to the service area, it is divided into: local telecommunication network, rural telecommunication network, long-distance telecommunication network, mobile communication network, international telecommunication network, etc.
According to the types of transmission media, it is divided into: overhead open wire network, cable communication network, optical cable communication network, satellite communication network, user optical fiber network, low-orbit satellite mobile communication network, etc.

According to the switching mode, it is divided into: circuit switching network, message switching network, packet switching network, broadband switching network, etc. According to the structure, it is divided into: mesh network, star network, ring network, grid network, bus network, etc.

According to the form of information signal, it is divided into: analog communication network, digital communication network, digital and analog hybrid network, etc.

According to the information transmission method, it is divided into: synchronous transfer mode (STM) integrated service digital network (ISDN) and remote transfer mode (ATM) broadband integrated service digital network (B-ISDN).

What is an intelligent network?
The idea of ​​the Intelligent Network (IntelligenTIzed Network) originated in the United States. In the early 1980s, AT & T used a centralized database method to provide 800 (called pay) business and telephone billing card business, which is the prototype of the intelligent network. Later, ITU-T (InternaTIonal TelecommunicaTIons Union) officially named the term intelligent network in 1992. The intelligent network is an additional network structure set up to provide new telecommunication services (or value-added services) quickly, conveniently, and economically on the basis of the existing switching and transmission basic network structure. The outstanding advantage of intelligent network to provide new services is that it can be fast, economical and convenient. Because the intelligent network technology has standard model constraints, the implementation of the system can be independent of the new services to be generated, and there are standard communication protocols to support the interconnection of products, thereby creating the basic conditions for quickly providing new services.

The intelligent network is based on computers and databases. In theory, the new services that the intelligent network can provide are unlimited. However, to start a new business, we must consider factors such as actual needs and economic benefits. There are now dozens of smart new services in the world. However, the types provided by different regions are different. For example, China currently has three major categories: international, national, and provincial, and the services provided are also different.

The common smart new services that have been provided in the world are as follows:
Called centralized payment service: Americans call this kind of call "toll-free", in fact, the caller does not pay, but the called user pays. When using this service, users need to dial "800" first, so it is also called "800 service".

Public service business: Users dial a phone number with a specific number prefix to get certain information or services that can be consulted. In the United States, users use this service to dial "900" first, so it is also called "900 service".

Optional accounting business: referred to as "ABS business". It can provide multiple billing methods, such as calling payment, called payment, calling and called sharing payment, third-party payment or credit card payment and other forms of billing.

Private virtual network service: Users can flexibly form non-permanent private networks according to their own wishes, called "virtual network".

Wide area centralized small exchange service (WAC service): Users can enjoy all the functions of private exchanges in the city without setting up a private exchange.

General number service: Assign a common phone number to companies with multiple semicolons to handle business.

The main components of the intelligent network are: a service switching point (SSP), which is used to identify users' calls to the intelligent network; a service control point (SCP), which completes the control of the business, usually composed of large and medium-sized computers and large databases; business A management system (SMS) is an operation, maintenance, management and monitoring system in an intelligent network.

In short, the entire telecommunications network is gradually developing in the direction of intelligence, broadband and personalization. With the development of the intelligent network, the interconnection of the intelligent network can be realized, the combination of the intelligent network and the Internet Internet, the combination of the intelligent network and the broadband integrated service digital network B-ISDN, tomorrow's intelligent network will be more intelligent.

What is "One Link"?
The Chinese name of ISDN (Intergrated Service Digital Network) is the integrated service digital network. China Telecom commonly refers to the "narrow-band integrated service digital network" (N-ISDN) as "one line communication". "One Line" uses digital transmission and digital switching technology to integrate various services such as telephone, fax, data, and image into a unified digital network for transmission and processing, providing users with basic rates (2B D, 144kbit / s) and Primary interface (30B D, 2Mkbit / s) two interfaces. The basic rate interface includes two independent B channels (64Kkbit / s) and one D channel (16Kkbit / s). The B channel is generally used to transmit voice, data and images, and the D channel is used to transmit signaling or packet information. "One Line" is a communication network developed based on the integrated telephone digital network. It can provide end-to-end digital connections, can carry voice and non-voice services, and users can access the network through a multi-purpose user network interface. "One Line" can not only provide circuit-switched services, but also provide packet-switched and non-switched dedicated line services. Customers can flexibly choose according to their needs, and can communicate with existing telephone networks and packet networks.

"One Line" can provide users with three major types of services: â‘ , bearer services (regardless of the type of user terminal, such as circuit-switched bearer services and packet-switched bearer services, etc.); â‘¡, user terminal services (such as digital phones, four Fax, data communication, video communication, etc.); â‘¢, rich supplementary services (such as calling / called user number identification display / restriction, call waiting, call forwarding, multi-user number, sub-address, three-party communication, etc.).

ISDN has the following characteristics:

â‘  Integration of communication services: using one subscriber line, you can make calls or send and receive faxes while surfing the Internet. Just like having two telephone lines, you can also implement conference TV functions by configuring appropriate terminal equipment.

â‘¡Realize high-reliability and high-quality communication: the use of "one-line communication", because the information between terminals is completely digitized, noise, crosstalk and signal fading distortion are very small, and the bit error characteristics of data transmission are at least improved over telephone lines 10 times, so the communication quality is very high.

â‘¢You can use the Internet at a rate of 64kb / s or 128kb / s through "One Line Connect".

â‘£ "One Line" can also provide a wealth of additional functions: calling number display, call waiting, call holding, call transfer, multi-user number, sub-address, terminal mobility, etc.

⑤ The price is suitable: Because a single network is used to provide multiple services, ISDN greatly improves the utilization rate of network resources and provides services to users at a low cost. ISDN greatly improves the utilization rate of network resources at a low cost To provide services to users, users do not have to purchase and install different equipment and lines to access different networks, and only need one interface to get various services, which greatly saves investment.

â‘¥ Flexible and convenient to use: Only one network access interface and a unified number can be used to get various services needed from the network.Users can connect multiple different types of terminals on this interface, and there are multiple terminals Simultaneous communication. Unified access allows communication equipment to be easily moved between different locations like household appliances.

⑦ Broad application prospects: It can be used in trade-oriented enterprises (companies), financial and insurance institutions, stock and stock exchanges, hospitals and schools, especially personal computer users. In terms of voice communications, ISDN provides more services than traditional analog telephone networks. Because ISDN provides comprehensive business capabilities, it can be used in more fields. Individual users have more advantages than MODEM when using ISDN to access the Internet, such as reducing costs and providing true 128K connection.

What is xDSL?
xDSL is the abbreviation of Digital Subscriber Line (DSL), which means digital subscriber line. It is a point-to-point transmission technology that uses copper telephone lines as the transmission medium. In xDSL, "x" represents different types of digital subscriber line technologies. DSL technology supports symmetrical and asymmetrical transmission modes on the traditional telephone network (POTS) user loop, which solves the "last mile" transmission bottleneck that often occurs between network service providers and end users. Since telephone subscriber loops have been laid in large numbers, how to make full use of existing copper cable resources and achieve high-speed access through copper twisted pairs have become the research focus of the industry. Therefore, DSL technology has been quickly paid attention to and has been adopted in some countries. And regions have been widely used.

The differences between various digital subscriber line technologies are mainly reflected in the signal transmission rate and distance. The xDSL technology is mainly divided into two categories: symmetric and asymmetric. Symmetrical DSL technology is mainly used to replace traditional T1 / E1 access technology. Compared with traditional T1 / E1 access, DSL technology has the characteristics of low line quality requirements and simple installation and debugging. It is widely used in communications, campus network interconnection and other fields. Through multiplexing technology, it can transmit multiple channels of voice at the same time. , Video and data. Asymmetric DSL technology is very suitable for applications with different requirements for bidirectional bandwidth, such as Web browsing, multimedia on-demand, information release, etc., so it is suitable for Internet access, VOD systems, etc.

Symmetric DSL technology mainly includes:

HDSL (High-bit-rate DSL), SDSL (Single-line DSL), IDSL (ISDN Digital Subscriber Line), etc.

Among them, HDSL is the most mature kind of xDSL technology, and has been widely used. This technology can be transmitted in full-duplex T1 or E1 via existing copper twisted pairs (a version that will appear called HDSL2 will use a single twisted pair to accomplish the same task). Its characteristics are: the use of two pairs of twisted pair transmission, support various rates of N × 64kbps, up to E1 rate. HDSL is an alternative technology of TI / E1, which is mainly used for the connection of digital exchanges, high-bandwidth video conferencing, distance learning, cell phone base station connection, private network establishment, etc. It has the characteristics of low price and easy installation.

SDSL (Single-line DSL) is a single-line version of HDSL, which can provide two-way high-speed variable bit rate connection, the rate range from 160kbps to 2.084Mbps. Its characteristics are: the use of single-pair twisted pair; supports multiple rates to T1 / E1; users can choose the most economical and suitable rate according to data traffic, up to E1 rate, saving a pair of copper wire than using HDSL; at 0.4 The maximum transmission distance of mm twisted pair is more than 3 kilometers.

IDSL (ISDN Digital Subscriber Line) uses an ISDN terminal adapter at the user end and an ISDN-compatible interface card at the other end of the twisted pair. This technology can provide 128Kbps services.

Asymmetric DSL technologies mainly include ADSL (Asymmetric DSL, Asymmetric DSL), RADSL (Rate Adaptive DSL, Rate Adaptive DSL), VDSL (Very High Data Rate DSL, Very High Speed ​​Digital Subscriber Line), etc .:

ADSL: ADSL provides upstream traffic from 32Kbps to 8.192Mbps and downstream traffic from 32kbps to 1.088Mbps for the network. Simultaneously, it can provide voice telephone services on the same line. Its characteristics are: the use of a pair of twisted pair transmission; the up / down rate from 1.5Mbps / 64Kbps to 6Mbps / 640Kbps supports simultaneous transmission of data and voice.

RADSL: This technology allows service providers to adjust the bandwidth of xDSL connections to suit actual needs and solve line length and quality issues. Its characteristics are: the use of a pair of twisted pair transmission; support for synchronous and asynchronous transmission methods; rate adaptation, the downlink rate from 640kbps to 12Mbps, the uplink rate from 128kbps to 1Mbps supports simultaneous transmission of data and voice.

VDSL: In the case where the user loop length is less than 1054 meters (5000 feet), the rate can be provided up to 13Mbps or even higher, this technology can be used as part of the fiber-to-road network structure. This technology can provide extremely high transmission rates over short distances, but there are not many applications yet.

What is ADSL?
Among various digital subscriber lines, asymmetric digital subscriber line (ADSL) technology has the characteristics of asymmetric upstream and downstream rates, and is suitable for a variety of broadband services. The characteristic of this kind of business is that the downlink needs to transmit TV images and requires a very high transmission rate; the uplink mainly transmits control signaling and low-speed signals, etc., and a narrower frequency band can be used. ADSL is also suitable for Internet access. Because it uses the existing copper wire user line resources, it has less investment and quick results, and is especially suitable for small and medium-sized enterprise users.

The technical characteristics of ADSL are as follows:
(1) High-speed transmission. Provide upstream and downstream asymmetric transmission bandwidth, the maximum downlink speed is 8Mbps, and the maximum uplink speed is 1Mbps.
(2) The Internet and phone calls do not interfere with each other. ADSL data signals and telephone audio signals are modulated in their respective frequency bands on the principle of frequency division multiplexing, without interfering with each other. You can make or receive calls while surfing the Internet, avoiding the trouble of not being able to use the phone when dialing to the Internet.
(3) Exclusive bandwidth, safe and reliable. ADSL utilizes a telephone network that penetrates thousands of households to form a star-shaped network topology. The backbone network uses China Telecom's fiber-optic transmission throughout the country. Each node uses ATM broadband switches to process and exchange information. Information transmission is fast, reliable and safe.
(4) Quick and convenient installation. To install ADSL on an existing telephone line, only one ADSL modem needs to be installed on the user side. The most important thing is that you do not need to re-route or change the line for broadband Internet access.
(5) The price is affordable. The ADSL service Internet tariff is composed of: basic monthly fee information fee, no need to pay Internet communication fee (ie telephone fee).
ADSL is an ideal copper wire broadband access technology. The use of such asymmetric digital line equipment can not only ease the congestion of the telephone network based on the full use of existing telephone subscriber lines, but also transfer Internet and other data services from the public switched telephone network to the data communication network, thereby reducing The pressure on the telephone exchange reduces the possibility of congestion in the telephone network.
According to a survey report from the US telecommunications market, Asymmetric Digital Subscriber Line Equipment (ADSL) increased by 153% in 1998 compared with 1997, far exceeding the 18% annual growth rate of Integrated Services Digital Network (ISDN).

What is packet switching?
Packet switching is also called packet switching. The packet switching method is not for circuit connection, but for information distribution. The packet switch divides the data to be transmitted by the user into several data segments according to a certain length. These data segments are called "packets" (or packets). In the transmission process, control information and address identification (ie packet header) must be added before each packet, and then transmitted in a "store-forward" manner in the network. At the destination, the switch removes the packet header, installs the divided data segments in order, and restores the original file to the receiving user. This process is called packet switching. The communication network that performs packet switching is called a packet switching network. This process is similar to our usual mailing letter. People wrap the written credit envelope, and then write the address and name of the recipient on the envelope, which is equivalent to the routing control information in the packet header; after the envelope is ready, it is put into the mail box , Sorted by the post office, sent to different locations, and finally delivered to the recipient; the recipient opens the letter to read, just like unpacking in the group. This entire process is like a packet switching process, except that packet switching is much more technically complicated in order to accurately, reliably, and at high speed. In addition, the address field and control field must be added to indicate the type of information and where to send it, plus an error check bit to check for errors that occur during transmission. The task of packet switching is to read data packets from each input and distribute them to each output according to the address and control fields above them.
Visually speaking, the circuit is a "extensive" and "macro" exchange method, only the circuit and not the information transmitted on the circuit. In contrast, packet switching is more subtle and detailed, and it manages the information that is transmitted.
The characteristics of packet switching are: ① The packet switching mode has a strong error control function, and the information transmission quality is high. ②Strong network reliability. In packet-switched networks, the routing of "packets" in the network is a dynamic routing algorithm, that is, each packet can freely choose the transmission path, and the switch calculates an optimal path. Therefore, when a switch or trunk in the network fails, the packet can automatically avoid the fault location and select another detour route for transmission, which will not cause communication interruption. ③The packet-switched network can store and forward the transmitted data, so that terminals of different rates and different types can communicate with each other. ④Because the storage and forwarding in the network in units of packets is much smaller than the packet exchange delay in the storage and forwarding in units of messages, it can meet the real-time requirements of conversational communication. ⑤ In packet switching, due to the use of "virtual circuit" technology, multiple information channels can be provided on a physical line at the same time, that is, statistical time division multiplexing of the line is realized, and the line utilization rate is high. ⑥The transmission cost of packet switching has nothing to do with the distance. Whether the user uses it in the same city or across provinces, it is calculated at the same unit price. Therefore, the packet network provides users with an affordable means of information transmission.

What is ATM?

ATM (Asynchronous Transfer Mode) is an asynchronous transfer mode and is a standard formulated by the International Telecommunication Union ITU-T. In fact, in the mid-1980s, people started experimenting with rapid packet switching, and established a variety of models with different names. Europe focuses on image communication, and the corresponding technology is called asynchronous time division multiplexing (ATD); The United States focuses on high-speed data communications, and the corresponding technology is called fast packet switching (FPS); after coordinated research, ITU officially named it Asynchronous Transfer Mode (ATM) technology in 1988, and recommended it as the broadband integrated service data network B -ISDN information transmission mode.
ATM is a transmission mode. In this mode, information is organized into cells. Since each cell containing information from a user does not need to appear periodically, this transmission mode is asynchronous.
ATM cells are fixed-length packets with a total of 53 bytes, divided into 2 parts. The first 5 bytes are letterheads, which mainly complete the addressing function. The next 48 bytes are the information segment, used to load information from different users and different services. All digital information such as voice, data, and images must be cut, and the cells encapsulated into a uniform format are transmitted on the network and restored to the required format at the receiving end. Because the ATM technology simplifies the switching process, removes unnecessary data verification, and uses a fixed cell format that is easy to handle, the ATM switching rate is much higher than traditional data networks, such as x.25, DDN, and frame relay. In addition, for such a high-speed data network, the ATM network uses some effective business traffic monitoring mechanisms to monitor user data online in real time, minimizing the possibility of network congestion. Different "privileges" are given to different services, such as voice, which has the highest real-time privileges, and the highest data transmission correctness privileges. The network allocates different network resources to different services, so that different services can achieve "peace" in the network. Coexistence ".
The characteristics of ATM network: flexibility, high speed, multi-service, reliability and security. There are two major categories of ATM applications in the commercial field, namely multimedia and high-speed data. Multimedia applications mainly include conference TV, vocational education and technical training, e-mail, desktop cooperative working groups, home office, telemedicine, remote exploration, etc .; high-speed data applications mainly involve local area network (LAN) interconnection and data network synthesis.

What is IP?
IP is currently a popular technology. IP is the abbreviation of English Internet Protocol, which means "internet interconnection protocol", which is a protocol designed for computer networks to communicate with each other. In the Internet, it is a set of rules that enables all computer networks connected to the Internet to communicate with each other. It stipulates the rules that computers should follow when communicating on the Internet. Any computer system produced by a manufacturer can be interconnected with the Internet as long as it complies with the IP protocol. It is precisely because of the IP protocol that the Internet has rapidly developed into the world's largest and open computer communication network. Therefore, the IP protocol can also be called "Internet Protocol".
How does IP achieve network interconnection? Studying IP technology is inseparable from the specific network environment. INTERNET is a most typical IP network, and it is also a most successful application of IP technology. After decades of development, the scale of the Internet has grown faster than people had expected. It has expanded from 4 nodes originally located in the United States to today's computer network distributed in 175 countries, connecting millions of hosts.
New applications based on INTERNET are also emerging, such as IP telephony, IP fax, video conferencing, e-commerce, etc. These objective facts have aroused great interest from people, especially many telecommunications experts and practitioners. From the current situation, IP technology is also the best solution for integrated services. Therefore, some people predict that an information revolution that combines communication and computer technology is quietly emerging. Today's Internet is a harbinger of this revolution.
What is Internet? Some people say that Internet is a "network of networks." It uses the TCP / IP protocol cluster to enable thousands of users around the world to communicate and share resources. In general, INTERNET has the following characteristics: it is composed of numerous computer networks; it is a worldwide network; it mainly uses TCP / IP protocol; it uses packet switching technology; it is formed by connecting many routers; it is an information resource network .
China officially connected to the Internet in 1994. China's Internet business has developed very rapidly, and China Science and Technology Network (CSTNET), China Public Computer Internet (CHINANET), China Education and Research Computer Network (CETNET), China Golden Bridge Information Network (CHINAGBN), and China Unicom Internet ) Wait for several major Internet networks.
For users, the Internet is a unified network. This is the basic idea of ​​TCP / IP and the essence of its flexibility and versatility. The IP layer protocol plays a central role in the network hierarchy established by TCP / IP: First, it uses connectionless transmission of datagrams, so that upper-layer applications can improve the efficiency of data transmission without caring about the details of low-level data transmission; second, Unify various physical network technologies through IP datagrams and IP addresses, so as to shield low-level technical details and provide consistency. This makes the diversity of the physical network transparent to the upper layer. Therefore, INTERNET can make full use of various communication media to connect computer networks around the world through a unified IP protocol.

There is also a very important content in the IP protocol, which is to assign a unique address to each computer and other devices on the Internet, called "IP address". Conceptually, an address is an identifier of an object in the system. In the physical network, each site has an address that the machine can recognize. This address is called a physical address (also called a hardware address or MAC address). In the Internet, unified through the upper software. (IP layer) Provides a common address format, which is allocated under unified management to ensure that an address corresponds to a host. In this way, the physical address differences of the entire network are shielded by the IP layer, and the addresses used by the IP layer are generally referred to as Internet addresses, or IP addresses. It is included in the header of the IP datagram.

What is an IP phone?
IP telephony is a telephone service that is opened according to the network technology content stipulated in the international Internet protocol. Chinese translation is an Internet phone or Internet phone. In short, it is a real-time voice transmission service through the Internet. It is a brand-new communication technology that uses the Internet as a medium for voice transmission, thereby realizing voice communication. Because of its low communication cost (internet communication costs RMB 6 minutes and 6% per minute, and the international communication cost of ordinary telephones requires more than ten yuan per minute), it is also called cheap phone. Internet telephony, Internet telephony, economic telephony or cheap telephony are all different names for IP telephony. Their essence is basically the same. Nowadays, the most widely used and more scientific name is "IP telephony". The principle is to compress and pack the analog signal of the ordinary telephone, transmit it through the Internet, decompress it after reaching the other party, and restore it to an analog signal, and the other party can answer it with ordinary telephones and other equipment.
The original IP phone was a call between a personal computer and a personal computer. Both parties of the call have a computer and can access the Internet. Using the computers and modems of both parties, install the sound card and related software, plus the microphone and the speaker. The two parties agree to go online at the same time and then make the call. At this stage, only point-to-point calls where both parties know the other party's network address and must be scheduled for Internet access at the same time are quite troublesome in the ordinary business field. Therefore, they cannot be commercialized or enter the public communication field.
At present, many large international telecommunications companies have launched calls between ordinary telephones and ordinary telephones. Ordinary telephone customers dial the local Internet telephone gateway (Gateway) through the local telephone, enter the account number and password, and enter the called party after confirmation Number, so that the local and remote VoIP phones are connected through the Internet network through the gateway, and the remote Internet gateway calls the called user through the local telephone network, thereby completing the telephone communication between ordinary telephone customers. As a gateway for VoIP, there must be a dedicated line connected to the Internet network, that is, a host on the Internet. At present, the gateways of both parties must use the products of the same company.
This way of calling from ordinary telephones to ordinary telephones via the Internet is the IP telephones that people usually speak of, and is also the fastest-growing and most commercially promising telephone.

Can I use the power line to go online and make calls?
"Power line Internet access", that is, PLC technology, English is Power Line Communication, mainly refers to a communication method that uses power lines to transmit data and voice signals. There have been decades of development history through power line communication, which first appeared in the early 1920s. In our country, carrier aircraft made in Japan were operating in the Northeast in the 1940s as a communication means for long-distance scheduling. It uses the power line as a transmission channel, which has unique advantages such as high channel reliability, less investment and quick results, and synchronization with the power grid construction. In recent years, with the rapid development of Internet technology, the value of using 220V low-voltage power lines to transmit high-speed data is becoming more and more important, because it has the remarkable characteristics of no wiring, wide coverage, and convenient connection. It is considered to provide "last One of the most competitive technologies for the "One Kilometer" solution.
Power line users only need to add a special modem. The top of the modem's data line is a plug with the same specifications as the conventional power plug. Users only need to connect the computer's network card to the modem, and then plug the modem into the power supply, the computer can access the Internet, and this method transmits information more than 30 times faster than ISDN.
In addition to the large data throughput, this technology is also convenient. In theory, as long as there is a public power grid, you can surf the Internet, specifically for families, users can access the Internet in any room, even in the kitchen and bathroom, without having to go from a room with a telephone line interface Pull the wire.
The urban distribution network can be transformed into broadband households. Making calls through the existing distribution network is naturally the next target market. The specific plan is to call IP through the phone on the "cat" or the microphone on the multimedia computer. Telephone, local calls, and domestic long-distance calls are all cheap. In addition to surfing the Internet and making phone calls, "wire Internet access" can also help people complete the remote automatic reading of water, electricity, and gas meter data, which saves utility companies a lot of costs and also facilitates users. At the same time, the use of "wire online" permanent online connection to build a fire protection, anti-theft, anti-toxic gas leakage and other security monitoring systems, so that office workers can sit back and relax. The first-aid medical emergency system built makes the family with the elderly, children and patients feel relieved.
PLC technology that uses high-speed data transmission through 220V low-voltage power lines spreading all over the country, urban and rural areas, and direct users. It is considered to be the most competitive solution for providing the "last mile" solution because of its remarkable characteristics of no wiring, wide coverage and convenient connection One of the technologies. At present, high-speed PLC can transmit data up to more than 1M bps, and it is expected that the rate will reach more than 1OM bps in the near future, and can simultaneously transmit data, voice, video and power, which may bring a new trend of "four networks in one".
High-speed PLC technology has many advantages: First, PLC makes full use of the existing low-voltage power distribution network infrastructure, without any wiring, is a "NoNewWires" technology, saving resources; second, PLC can provide users with high-speed Internet access services , Voice services, thereby adding new options for users to surf the Internet and making calls, which is conducive to other telecommunications service providers to improve services and lower prices; in addition, PLC also provides support for home networking, so that people can enjoy the benefits brought by PLC technology Home audio and video networks, multiplayer games and other entertainment. At the same time, PLC technology is a new force in home automation. Smart home appliances are networked through wall outlets throughout the rooms to enjoy the comfort and convenience of digital homes in advance. Using PLC technology to remotely and automatically read out water, electricity, and gas meter data, you can use a bill to solve all the charging items in the user's life, saving a lot of manpower and material resources, and also greatly convenient for users.
Of course, this technology is still facing certain problems in achieving breakthroughs in China, because China's low-voltage power grid is more complicated, each household's electricity consumption difference is relatively large, and the electricity load is also constantly changing. However, although China's power grid construction has a certain gap compared with Europe and the United States, the overall quality of the line is still relatively high. In recent years, the line transformation has greatly improved the line quality. If the technology matures further, Internet access via wires will no longer be an unattainable dream. It is understood that there are currently less than 400 million telephone users in China, but more than 1 billion electricity users. If the interconnection network is constructed with electric wires, it will save a lot of manpower and material resources compared with the current way of laying optical fibers.
At present, PLC technology has formed two development models: one is the home networking model represented by the United States. This mode of PLC only provides home internal networking, which refers to the networking of the home through the ordinary power lines inside the home. Local area network and outdoor access use other traditional communication methods. The international organization supporting this mode is Home-Plug, a forum established to provide open specifications for high-speed home power line communication network products and services.

What is mobile communication?
Mobile communication is communication between mobile bodies, or communication between a mobile body and a fixed body. The moving body may be a person, or it may be a moving object such as a car, train, ship, radio, etc. The mobile communication system consists of two parts:
(1) Space system;
(2) Ground system: â‘  satellite mobile radio station and antenna; â‘¡ gateway station and base station.
Since the birth of the mobile communication system in the 1980s, it will generally go through five generations of development by 2020, and by 2010, it will transition from the third generation to the fourth generation (4G). To 4G, in addition to cellular phone systems, broadband wireless access systems, millimeter-wave LANs, intelligent transmission systems (ITS) and stratospheric platform (HAPS) systems will be put into use. The most obvious trend of future generations of mobile communication systems is the requirement for high data rates, high mobility and seamless roaming. Meeting these requirements will face greater technical challenges. In addition, system performance (such as cell size and transmission rate) will largely depend on the frequency. Considering these technical issues, some systems will focus on providing high data rates, and some systems will focus on enhancing mobility or expanding coverage.
From the user's perspective, the available access technologies include: cellular mobile wireless systems such as 3G; cordless systems such as DECT; short-range communication systems such as Bluetooth and DECT data systems; wireless local area network (WLAN) systems; fixed wireless access Or wireless local loop system; satellite system; broadcasting system, such as DAB and DVB-T; ADSL and Cable Modem.

There are many types of mobile communications. It can be divided into:
(1) Trunked mobile communications, also known as large-area mobile communications. It is characterized by only one base station, an antenna height of tens of meters to more than 100 meters, a coverage radius of 30 kilometers, and a transmitter power of up to 200 watts. The number of users is about tens to hundreds, and it can be a car-mounted station or a handheld station. They can communicate with the base station, can also communicate with other mobile stations and local telephone users through the base station, and the base station is connected with the city station wired network.
(2) Cellular mobile communication, also called cell-based mobile communication.它的特点是把整个大范围的服务区划分成许多小区，每个小区设置一个基站，负责本小区各个移动台的联络与控制，各个基站通过移动交换中心相互联系，并与市话局连接。利用超短波电波传播距离有限的特点，离开一定距离的小区可以重复使用频率，使频率资源可以充分利用。每个小区的用户在1000以上，全部覆盖区最终的容量可达100万用户。
(3)卫星移动通信。利用卫星转发信号也可实现移动通信，对于车载移动通信可采用赤道固定卫星，而对手持终端，采用中低轨道的多颗星座卫星较为有利。
(4)无绳电话。对于室内外慢速移动的手持终端的通信，则采用小功率、通信距离近的、轻便的无绳电话机。它们可以经过通信点与市话用户进行单向或双方向的通信。
使用模拟识别信号的移动通信，称为模拟移动通信。为了解决容量增加，提高通信质量和增加服务功能，目前大都使用数字识别信号，即数字移动通信。在制式上则有时分多址(TDMA)和码分多址(CDMA)两种。前者在全世界有欧洲的GSM系统(全球移动通信系统)、北美的双模制式标准IS一54和日本的JDC标准。对于码分多址，则有美国Qualcomnn公司研制的IS-95标准的系统。总的趋势是数字移动通信将取代模拟移动通信。而移动通信将向个人通信发展。进入21世纪则成为全球信息高速公路的重要组成部分。移动通信将有更为辉煌的未来。

什么是集群移动通信?
集群移动通信是20世纪70年代发展起来的一种较经济、较灵活的移动通信系统，它是传统的专用无线电调度网的高级发展阶段。传统的专用无线电调度系统，整体规划性差，型号、制式混杂，网小台多，覆盖面窄，加以噪音干扰严重，频率资源浪费。因此，一种新的无线电调度技术——集群移动通信便应运而生。国内外通信界普遍认为，20世纪80-90年代是集群移动通信在专用无线电通信中占据比重较大的10年，是与蜂窝移动通信齐头并进的一种先进通信系统。美、英、法、德、日、北欧四国、加拿大及澳大利亚等国家都广泛开发和使用这一系统。以后曾一度衰落。近年来，集群移动通信系统再次兴起。
所谓集群(Trunking)，即使用多个无线信道为众多的用户服务，就是将有线电话中继线的工作方式运用到无线电通信系统中，把有限的信道动态地、自动地、迅速地和最佳地分配给整个系统的所有用户，以便在最大程度上利用整个系统的信道的频率资源。它运用交换技术和计算机技术，为系统的全部用户提供了很强的分组能力。可以说，集群移动通信系统是一种特殊的用户程控交换机。
我国最早引进集群移动通信系统的城市是上海。集群移动通信很适合于各个专业部门，如部队、公安、消防、交通、防汛、电力、铁道、金融等部门作分组调度使用。后来，北京、天津、广东、沈阳等地相继开发了集群移动通信业务。随着地方经济的发展，一些省会、直辖市和有条件的城市也都逐步建立起80OMHz无线集群移动通信系统，并逐步放开800MHz集群移动通信业务，一些地方和非邮电部门已纷纷开发此项业务。
据不完全统计，我国450MHz与800MHz频段的集群移动通信投入使用的有198个系统，绝大部分均为非邮电单位拥有。“九五”期间，各部门计划建立的集群网，仅所需手持台，即将达到20万台。例如：河南省防汛部门等三家投资5-6亿元，在全省建立无线寻呼网和集群通信网。我国的几个大油田和大型企业及农垦部门也打算扩建、兴建其集群通信网。可见，集群移动通信在我国的发展前景十分广阔。目前，我国集群通信系统多为国外原机引进，品牌型号不少，占据前列的有美国摩托罗拉、优利电(UNIDEN)和芬兰诺基亚等公司的产品，还有香港泰和以及欧洲和日本等地产品。
我国自行设计制造的集群移动通信系统近年也有投入运行的。有关方面认为，当前急需优选型号，统一机型，引进样机进行消化分析，与国外公司合作开发软件和移动台，先突破量大面广的移动台和软件技术，再逐步实现基地台、交换控制系统的国产化，提高发展起点，尽快形成规模经济。据悉，有关方面拟组建集团公司以加速我国集群移动通信系统的发展。

什么是移动电话网?
移动电话网就是可以使移动用户之间进行通信的网络。我国自1987年开始开通移动电话业务以来，移动电话迅猛发展，用户增长迅速，到现在我国已经出现了五种移动电话网共存的局面，这五种网各有不同的通话范围和不同的业务功能。用户选择配备移动电话手机时，需要对现有的五种网有所了解。
我国的五种移动电话网又被称为A、B、C、D、G网，其中A网和B网是模拟网，C、D、G网是数字网。
1)A网和B网：模拟移动电话网模拟网是我国早期建设的移动电话网。由于各地分别建设、时间先后不同，又有爱立信和摩托罗拉两大移动电话系统等原因，模拟移动电话网形成了A网和B网系统，A网地区使用A网的手机，B网地区使用B网的手机。A网的地区是北京、天津、上海以及除河北、山东以外的全国各地。可见在大部分地区是共存的，但原来是不能互通的。B网的地区主要是在北京、天津、上海、河北、辽宁、江苏、浙江、四川、黑龙江、山东等地。1996年1月起，我国各省模拟移动电话系统实现了联网，模拟移动电话已有可能在全国30个省(市、自治区)实现自动漫游。但是，如果要从A网区到B网区，需要用户在自己的手机上进行操作，将手机转换为B网，否则不能使用；如果从B网区回到A网区，也必须先在手机上操作，将手机变回才能使用。变换的方法可见说明书。
2)C网：CDMA制式移动电话网
C网是指CDMA(码分多址)制式的移动电话网，CDMA制式是接通率高、噪声小、发射功率小的新型数字网，能实现移动电话的各种智能业务。我国目前在上海、北京、广州、西安等市建设了C网，沿海的10省也在建设，已经建成的城市间已联网，使用CDMA手机可以在上述地区漫游。
3)G网：全球通(GSM)数字移动电话网
20世纪90年代中期，我国开始建设“全球通”(GSM)数字移动电话网，这就是G网。数字网具有许多新的业务功能，特别是具有漫游范围最为广泛的特点，因而被称为“全球通”。G网工作于900兆赫频段，频带比较窄，随着近年来移动电话用户迅猛增长，许多地区的G网已出现因容量不足而达到饱和的状态。为了满足广大用户的需求，近来又建设了“D”网。
4)D网：工作在DCS1800系统的移动电话网它的基本体制和现有的GSM900系统完全一致，但工作于1800兆赫频段，需要用全球通1800的手机。如果使用双频手机，那么在G网中也能漫游、自动切换。现在有许多城市是DCS1800系统和GSM900系统同时覆盖一个地区，就称为全球通双频系统，使全球通移动通信系统的容量成倍增长。

移动电话网络的组成如下：
(1)小区：是实际场地，配有一个基站，包括发射器、接收器和在窝小区范围内通过无线电通道与用户的移动手机进行通信时所用的其他设备；
(2)基站控制器：用于连接和控制每一蜂窝小区内的基站；
(3)移动交换中心：用于控制基站控制器和通话的呼叫；
(4)传输线：用于连接移动交换中心、基站控制器、基站和PSTN。

什么是GSM?
20世纪80年代中期，当模拟蜂窝移动通信系统刚投放市场时，世界上的发达国家就在研制第二代移动通信系统。其中最有代表性和比较成熟的制式有泛欧GSM (Global System for Mobile Communication)，美国的ADC(D-AMPS)和日本的JDC(现在改名为PDC)等数字移动通信系统。在这些数字系统中，GSM的发展最引人注目。1991年GSM系统正式在欧洲问世，网络开通运行。
GSM系列主要有GSM900、DCS1800和PCS1900三部分，三者之间的主要区别是工作频段的差异。
蜂窝移动通信的出现可以说是移动通信的一次革命。其频率复用大大提高了频率利用率并增大系统容量，网络的智能化实现了越区转接和漫游功能，扩大了客户的服务范围，但上述模拟系统有四大缺点：各系统间没有公共接口；很难开展数据承载业务；频谱利用率低无法适应大容量的需求；安全保密性差，易被窃听，易做“假机”。尤其是在欧洲系统间没有公共接口，相互之间不能漫游，对客户造成很大的不便。
GSM数字移动通信系统源于欧洲。早在1982年，欧洲已有几大模拟蜂窝移动系统在运营，例如北欧多国的NMT(北欧移动电话)和英国的TACS(全接入通信系统)，西欧其它各国也提供移动业务。当时这些系统是国内系统，不可能在国外使用。为了方便全欧洲统一使用移动电话，需要一种公共的系统，1982年，北欧国家向CEPT(欧洲邮电行政大会)提交了一份建议书，要求制定900MHz频段的公共欧洲电信业务规范。在这次大会上就成立了一个在欧洲电信标准学会(ETSI)技术委员会下的“移动特别小组(Group Special Mobile)”，简称“GSM”，来制定有关的标准和建议书。
我国自从1992年在嘉兴建立和开通第一个GSM演示系统，并于1993年9月正式开放业务以来，全国各地的移动通信系统中大多采用GSM系统，使得GSM系统成为目前我国最成熟和市场占有量最大得一种数字蜂窝系统。截至2002年11月，中国手机用户2亿，比2001年年底新增5509.2万。

GSM系统包括哪些部分?
GSM是世界上第一个对数字调制、网络层结构和业务作了规定的蜂窝系统。如今GSM移动通信系统已经遍及全世界，即所谓“全球通”，目前我国的移动通信网就是以GSM系统为基础的移动网络系统。
GSM数字公用陆地蜂窝移动通信系统主要由下述功能单元组成：移动台(MS)，它包括移动设备和用户识别模块;基站子系统(BSS)，为一个小区服务的无线收发信设备;移动业务交换中心(MSC)，对于位于它管辖区域中的移动台进行控制和交换的功能实体;拜访位置寄存器(VLR)，它存储与呼叫处理有关的一些数据，例如用户的号码，所处位置区的识别，向用户提供的服务等参数;归属位置寄存器(HLR)，它是管理部门用于移动用户管理的数据库;设备识别寄存器(EIR)，存储有关移动台设备参数的数据库;鉴权中心(AUC)和操作维护中心(OMC)它是操作维护系统中的各功能实体，依据厂家的实现方式可分为无线子系统的操作维护中心(OMC-R)和交换子系统的操作维护中心(OMC-S)。其中基站子系统(BSS)包括基站控制器(BSC)和无线基站(BTS)两部分。各个网元之间的接口均由GSM规范所定义，但各个厂家在部分接口上采用自己的标准，使得设备选型和组网工作具有一定的难度。

什么是WAP?
WAP (Wireless Application Protocol无线应用协议)是在数字移动电话、因特网或其他个人数字助理机(PDA)、计算机应用之间进行通讯的开放全球标准。它是由一系列协议组成，用来标准化无线通信设备，可用于Internet访问，包括收发电子邮件，访问WAP网站上的页面等等。WAP将移动网络和Internet以及公司的局域网紧密地联系起来，提供一种与网络类型、运行商和终端设备都独立的移动增值业务。通过这种技术，无论你在何地、何时只要你需要信息，你就可以打开你的WAP手机，享受无穷无尽的网上信息或者网上资源。如：综合新闻、天气预报、股市动态、商业报道、当前汇率等。电子商务、网上银行也将逐一实现。你还可以随时随地获得体育比赛结果、娱乐圈趣闻以及幽默故事，为生活增添情趣，也可以利用网上预定功能，把生活安排的有条不紊。WAP协议包括以下几层：
(l)Wireless Application Environment (WAE);(2)Wireless Session Layer (WSP);(3)Wireless Transaction Layer (WTP);(4)Wireless Transport Layer Security (WTLS)；(5)Wireless Transport Layer (WDP)。
其中，WAE层含有微型浏览器、WML、WMLSCRIPT的解释器等等功能。WTLS层为无线电子商务及无线加密传输数据时提供安全方面的基本功能。WAP协议的诞生是WAP论坛成员多年努力的结果。它是针对不同的协议层定义了一系列协议，这些协议使得各方面的厂商和公司可以协同工作，开发无线通信网络的应用。目前有超过100个成员加入WAP论坛，包括有终端和基础设备的制造商，有移动通信的网络运营商，有业务提供商，有软件公司，也有网络内容提供商等，共同为移动设备开发服务和应用。但是，目前由于无线网的带宽等因素的限制，WAP手机的在多媒体上的应用，如可视会议、多媒体教学等，还须一段时日。WAP规范还在不断地完善，WAP论坛成员们在加紧开发功能完善的WAP设备，这无疑加快了其在无线因特网综合服务领域的扩展速度。

什么是GPRS?
GPRS (General Packet Radio Service)是通用分组无线业务的简称。GPRS是GSM Phase2.1规范实现的内容之一，能提供比现有GSM网9.6kbit/s更高的数据率。GPRS采用与GSM相同的频段、频带宽度、突发结构、无线调制标准、跳频规则以及相同的TDMA帧结构。因此，在GSM系统的基础上构建GPRS系统时，GPRS中的绝大部分部件都不需要作改动，只需作软件升级，构成GPRS系统的方法是：
1) GSM系统中引入3个主要组件：(1)PRS服务支持结点(SGSN，Serving GPRS Supporting Node)；(2)GPRS网关支持结点(GGSN，Gateway GPRS Support Node)；(3)分组控制单元(PCU)。
2) 对GSM的相关部件进行软件升级：
ETSI指定了GSM900、1800和1900三个工作频段用于GSM，其中GSM900频段还有G1（E-GSM）频段和P频段。相应地，GPRS也工作于这三个频段，包括GSM900的G1频段和P频段，当然，GPRS可以限制每个小区只工作于P频段。现有的GSM移动台(MS)，不能直接在GPRS中使用，需要按GPRS标准进行改造(包括硬件和软件)才可以用于GPRS系统。
GPRS被认为是2G向3G演进的重要一步，不仅被GSM支持，同时也被北美的IS-136支持。GPRS是一组新的GSM承载业务，是以分组模式在PLMN和与外部网络互通的内部网上传输。在有GPRS承载业务支持的标准化网络协议的基础上，GPRS网络管理可以提供(或支持)一系列的交互式电信业务，例如承载业务、用户终端业务、附加业务。

GPRS业务的具体应用：(1)信息业务；(2)交谈；(3)网页浏览；(4)文件共享及协同性工作；(5)分派工作；(6)企业E-mail ；(7)因特网E-mail ；(8)交通工具定位；(9)静态图像；(10)远程局域网接入；(11)文件传送。

GPRS与WAP的关系如何?
WAP是无线Internet的标准，由多家大厂商合作开发，它定义了一个分层的、可扩展的体系结构，为无线Internet提供了全面的解决方案。WAP协议开发的原则之一是要独立于空中接口，所谓独立于空中接口是指WAP应用能够运行于各种无线承载网络之上，如TDMA、CDMA、GSM、GPRS、SMS等。
GRPS对应于GSM中的9.6kbps的数据业务，与之不同的是GPRS是通过分组交换来实现高速率的数据传输，换句话说，GPRS是对GSM数据业务的改进，是在GSM中实现高速数据传输的手段。
可见，WAP是高层应用，而GPRS是底层传输。可以引用著名的邮递信件的例子来说明WAP与GPRS的关系：如果说WAP是平信或贺卡或明信片，那么GPRS和GSM的CSD(9.6kbps电路交换的数据业务)就是运送信件的交通工具，只不过后者是汽车，前者则是飞机罢了。

什么是FDMA?
FDMA是数据通信中的一种技术，即不同的用户分配在时隙相同而频率不同的信道上。按照这种技术，把在频分多路传输系统中集中控制的频段根据要求分配给用户。同固定分配系统相比，频分多址使通道容量可根据要求动态地进行交换。
在FDMA系统中，分配给用户一个信道，即一对频谱，一个频谱用作前向信道即基站向移动台方向的信道，另一个则用作反向信道即移动台向基站方向的信道。这种通信系统的基站必须同时发射和接收多个不同频率的信号，任意两个移动用户之间进行通信都必须经过基站的中转，因而必须同时占用2个信道(2对频谱)才能实现双工通信。
以往的模拟通信系统一律采用FDMA。频分多址(FDMA)是采用调频的多址技术。业务信道在不同的频段分配给不同的用户。如TACS系统、AMPS系统等。频分多址是把通信系统的总频段划分成若干个等间隔的频道(也称信道)分配给不同的用户使用。这些频道互不交叠，其宽度应能传输一路数字话音信息，而在相邻频道之间无明显的串扰。

什么是TDMA？
时分多址TDMA是把时间分割成周期性的帧，每一帧再分割成若干个时隙（无论帧或时隙都是互不重叠的），再根据一定的时隙分配原则，使各个移动台在每帧内只能按指定的时隙向基站发送信号，在满足定时和同步的条件下，基站可以分别在各时隙中接收到各移动台的信号而不混扰。同时，基站发向多个移动台的信号都按顺序安排。在预定的时隙中传输，各移动台只要在指定的时隙内接收，就能在合路的信号中把发给它的信号区分出来。

什么是CDMA?
CDMA是码分多址的英文缩写(Code Division Multiple I Access)，它是在数字技术的分支——扩频通信技术上发展起来的。CDMA是为现代移动通信网所要求的大容量、高质量、综合业务、软切换、国际漫游等要求而设计的一种移动通讯技术。
CDMA技术的原理是基于扩频技术，即将需传送的具有一定信号带宽信息数据，用一个带宽远大于信号带宽的高速伪随机码进行调制，使原数据信号的带宽被扩展，再经载波调制并发送出去。接收端使用完全相同的伪随机码，与接收的带宽信号作相关处理，把宽带信号换成原信息数据的窄带信号即解扩，以实现信息通信。
CDMA移动通信网是由扩频、多址接入、蜂窝组网和频率复用等几种技术结合而成，含有频域、时域和码域三维信号处理的一种协作，因此它具有抗干扰性好，抗多径衰落，保密安全性高，同频率可在多个小区内重复使用，容量和质量之间可做权衡取舍等属性。这些属性使CDMA比其它系统有很大的优势。
(1) 系统容量大理论上，在使用相同频率资源的情况下，CDMA移动网比模拟网容量大20倍，实际使用中比模拟网大10倍，比GSM要大4-5倍。
(2) 系统容量的配置灵活在CDMA系统中，用户数的增加相当于背景噪声的增加，造成话音质量的下降。但对用户数并无限制，操作者可在容量和话音质量之间折衷考虑。另外，多小区之间可根据话务量和干扰情况自动均衡。
这一特点与CDMA的机理有关。CDMA是一个自扰系统，所有移动用户都占用相同带宽和频率，打个比方，将带宽想像成一个大房子，所有的人将进入惟一的大房子。如果他们使用完全不同的语言，他们就可以清楚地听到同伴的声音而只受到一些来自别人谈话的干扰。在这里，屋里的空气可以被想像成宽带的载波，而不同的语言即被当作编码，我们可以不断地增加用户直到整个背景噪音限制住了我们。如果能控制住用户的信号强度，在保持高质量通话的同时，我们就可以容纳更多的用户。
(3) 通话质量更佳
TDMA的信道结构最多只能支持4Kb的语音编码器，它不能支持8Kb以上的语音编码器。而CDMA的结构可以支持13kb的语音编码器。因此可以提供更好的通话质量。CDMA系统的声码器可以动态地调整数据传输速率，并根据适当的门限值选择不同的电平级发射。同时门限值根据背景噪声的改变而变，这样即使在背景噪声较大的情况下，也可以得到较好的通话质量。另外，TDMA采用一种硬移交的方式，用户可以明显地感觉到通话的间断，在用户密集、基站密集的城市中，这种间断就尤为明显，因为在这样的地区每分钟会发生2至4次移交的情形。而CDMA系统“掉话”的现象明显减少，CDMA系统采用软切换技术，“先连接再断开”，这样完全克服了硬切换容易掉话的缺点。
(4) 频率规划简单用户按不同的序列码区分，所以不相同CDMA载波可在相邻的小区内使用，网络规划灵活，扩展简单。
(5)建网成本低
CDMA技术通过在每个蜂窝的每个部分使用相同的频率，简化了整个系统的规划，在不降低话务量的情况下减少所需站点的数量从而降低部署和操作成本。CDMA网络覆盖范围大，系统容量高，所需基站少，降低了建网成本。
CDMA数字移动技术与现在众所周知的GSM数字移动系统不同。模拟技术被称为第一代移动电话技术，GSM是第二代，CDMA是属于移动通讯第二代半技术，比GSM更先进。

CDMA有哪些主要技术?
CDMA是应用在第三代移动通信系统中的基本技术，即RAKE接收机、功率控制、软切换和话音编码技术。
(1)RAKE接收机发射机发出的扩频信号，在传输过程中受到不同建筑物、山岗等各种障碍物的反射和折射，到达接收机时每个波束具有不同的延迟，形成多径信号。如果不同路径信号的延迟超过一定的时延，则在接收端可将不同的波束区别开来。将这些不同波束分别经过不同的延迟线，对齐以及合并在一起，则可达到变害为利，把原来是干扰的信号变成有用信号组合在一起。这就是RAKE接收机的基本原理。
(2)功率控制在CDMA系统中，不同用户发射的信号由于距基站的距离不同，到达时的功率也不同。距离近的信号功率大，距离远的功率小，相互形成干扰，这种现象称为远近效应。CDMA系统要求所有用户到达基站接收机信号的平均功率要相等才能正常解扩，功率控制就能解决这一问题。它调整各个用户发射机的功率，使其到达基站接收的平均功率相等。功率控制的原理有两种类型：开环控制与闭环控制。
(3)软切换移动台如

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