What are the various fields of mobile phone network signals realized through?

GSM900 and DCS 1800 are what we usually call dual-band networks. Both are GSM standards. The two systems have the same function, mainly with different frequencies. GSM900 works at 900MHZ and DCS 1800 works at 1800MHZ. GSM900 was first used in China. With the rapid development of communication network scale and the number of users, the frequency of the original GSM900 network has become increasingly tense. In order to better meet the growing needs of users, China recently introduced DCS 1800, and adopted the networking mode of GSM900 network as the main network, supplemented by DCS 1800 network, forming GSM 900/DCS 1800. Users can switch freely between GSM 900/DCS 1800 and automatically select the best channel to make a call by using dual-band mobile phones. Even during the call, the mobile phone can automatically switch between the two networks without the user's notice, and the mobile phone chooses the best channel, which improves the connection rate. In order to adapt to this trend and further seize the market share, Nokia, Motorola, Ericsson and other world-renowned mobile phone equipment manufacturers compete to develop and launch multi-band mobile phones.

(A) GSM system network structure

The history of GSM can be traced back to 1982. At that time, the four Nordic countries submitted a proposal to CEPT (Conference Europe of Post and Telecommunications), demanding the establishment of a European public telecommunications service specification in the 900MHZ frequency band, so as to establish a unified cellular system in Europe. In the same year, GSM Group dedicated mobile phone was established. During the period from1982 to1985, the focus of discussion was to formulate the standard of analog cellular network or digital cellular network, and it was not until 1986 that the standard of digital cellular network was decided. From 65438 to 0986, eight systems of different companies and schemes were compared in Paris, including field tests. 1May, 987, narrow-band TDMA scheme was selected. At the same time, 18 countries signed a memorandum of understanding and reached an agreement to fulfill the norms. 1988 promulgated the GSM standard, also known as the pan-European digital cellular communication standard. At present, GSM includes GSM900 and DCS 1800. The two systems have the same function, mainly with different frequencies. In the GSM proposal, hardware is not specified, only functions and interfaces are specified in detail, so that different products can communicate with each other. GSM recommends * * * 12 system.

The main component of 1 GSM system

The main components of GSM digital cellular communication system can be divided into mobile station, base station subsystem and network subsystem. Base station subsystem (BS for short) consists of base transceiver station (BTS) and base station controller (BSC). The network subsystem consists of mobile switching center (MSC), operation and maintenance center (OMC), home location register (HLR), visitor location register (VLR), authentication center (AUC) and equipment identification register (EIR).

2.GSM area, number, address and logo

1) regional branch

According to geographical location, GSM system is divided into GSM service area, public land mobile network (PLMN) service area, mobile switching control area (MSC area), location area (LA), base station area and cell.

*GSM service area

It consists of all GSM member countries connected to the network. As long as the mobile users are in the service area, they can get all kinds of services of the system, including international roaming.

*PLMN service area

The public land mobile network (GSM/PLMN) composed of GSM system is at the level of international or domestic tandem office. This area is a PLMN service area, which can be interconnected with Public Switched Telecommunication Network (PSTN), Integrated Services Digital Network (ISDN) and Public Data Network (PDNN). In this regard, there are the same numbering methods and routing plans. A PLMN service area includes multiple MSC service areas and can even be extended to the whole country.

* Master of Science business areas

In this regard, there are different numbering methods and routing plans. The area controlled by the mobile switching center is called MSC service area. The MSC area can be composed of one or more location areas.

* Location area

Each MSC service is divided into several location areas (LA), which consist of several base station areas and are related to one or several base station controllers (BSC). When the mobile station moves in the location area, there is no need to update the location. When paging a mobile user, all base stations in the location area can send paging signals at the same time. In this system, location area distinguishes different location areas of MSC service area by location area identification code (LAI).

* Base station area

Generally speaking, the area of several cells controlled by a base station controller is called the base station area.

* Community

Cells are also called cell regions, and their ideal shape is a regular hexagon. A cell contains a base station, and each base station contains several groups of receivers and transmitters. Its effective coverage depends on the transmission power, antenna height and other factors, generally several kilometers. The base station can be located in the center of a regular hexagon and adopt an omnidirectional antenna, which is called central excitation; It can also be located at the vertices of a regular hexagon (set at intervals) and adopt a directional antenna of 120 degrees or 60 degrees, which is called vertex excitation. If the business volume in the cell increases sharply, the cell can be reduced (divided into four). The new cell is usually called "small cell" and is called cell splitting in cellular networks.

2) Identification number

GSM network is very complex, including switching system, base station subsystem and mobile station. Mobile users can make continuous calls with local telephone network users, integrated services digital network users and other mobile users, so they must have multiple identification numbers.

1 & gt； International mobile subscriber identity (IMSI)

International Mobile Subscriber Identity (IMSI) is used to identify users in GSM/PLMN networks. According to the GSM recommendation, the maximum length of IMSI is 15 decimal digits.

MCC MSIN/NMSI multinational company

3 digits 1 or 2 digits10-1digits.

MCC- mobile phone country code, 3 digits. For example, the MCC in China is 460.

MNC- mobile network number, up to 2 digits. A mobile communication network (PLMN) for identifying a residence.

MSIN- mobile user identity. Which is used for identifying mobile users in a mobile communication network.

NMSI- domestic mobile subscriber identification code. It consists of a mobile network number and a mobile subscriber identification code.

2> temporary user identity (TMSI)

For the sake of safety, we use TMSI instead of IMSI when transmitting user identification codes in the air, because TMSI is only valid locally (that is, in MSC/VLR area), and its composition structure is selected by the management department, but the total length is no more than 4 bytes.

3> International Mobile Equipment Identification (IMEI)

IMEI is a unique number used to identify a mobile device. Used to monitor stolen or invalid mobile devices. The composition of IMEI is shown in the following figure.

IMEI=TAC+FAC+SNR+SP( 15 digits).

TAC FAC SNR SP

6 digits 2 digits 6 digits 1 digits

TAC- Type Approval Code (TAC) Type approval code, designated by European Type Approval Center. The first two digits are the country code. (For example: Nokia's, Ericsson's and Motorola's, the approval codes of various types are different. Just like Ericsson's, GH388 is different from GF388, although there is only one cover; But as long as they are the same model, the first six sizes must be the same. If it is different, it may be a fake! )

FAC- Assembly Code (FAC) The assembly code is coded by the manufacturer, indicating the manufacturer or assembly location. If it is 40, it is Motorola's factory in Britain, 07 is Motorola's factory in Germany, and if it is 67, it is also in the United States. For Nokia, the FAC is 5 1. SNR- Serial Number (SNR) Serial numbers independently and uniquely identify each TAC and FAC mobile device, so the SNR of the same brand and model cannot be the same.

SP- Alternate alternate code, usually 0.

4 > PSTN/ISDN number (MSISDN) of mobile station.

The MSISDN is used for the number dialed to GSM system by public switched telecommunications network (PSTN) or integrated services digital network (ISDN), and its composition is as follows:

MSISDN=CC+NDC+SN (total length does not exceed 15 digits)

CC= country code (such as China 86), NDC= area code, and SN= user number.

5> Mobile Roaming Number (MSRN)

When a mobile station roams to the service area of another mobile switching center, the mobile switching center will assign a temporary roaming number to the mobile station for routing. The roaming number format is the same as the PSTN/ISDN number format of the visited mobile station. When a mobile station leaves the area, both the Visitor Location Register (VLR) and the Home Location Register (HLR) should delete the roaming number so that it can be reassigned to other mobile stations.

The MSRN allocation process is as follows:

Local telephone users send MSISDN numbers to GSMC and HLR through public switched telecommunications network. HLR requests the visited MSC/VLR to assign a temporary roaming number, and then sends the number to HLR. On the one hand, HLR sends relevant parameters of mobile station, such as International Mobile Subscriber Identity (IMSI), to MSC; On the other hand, HLR informs GMSC of the roaming number of the mobile station, and GMSC can choose the route to complete the local telephone user->; GMSC->； The MSC- > mobile station continues the task.

6> Location area identification code

LAI is used to update the location of mobile users. LAI=MCC+MNC+LAC .MCC= mobile country code, which is used to identify the country and is the same as the three digits in the IMSI. MNC= mobile network number, which identifies different GSMPLMN networks and is the same as MNC of IMSI. LAC= location area number, which identifies the location area in GSMPLMN network. The maximum length of LAC is 16 bits, and 65536 different location areas can be defined in a GSMPLMN.

7> unit global identifier (CGI)

CGI is used to identify cells in a location area. It adds a small area identification code (CI) after the location area identification code (LAI).

CGC=MCC+MNC+LAC+CI .

CI= cell identification code, which is used to identify the cell in the location area, and the maximum length is 16 bits.

8> base station identity code (BSIC)

BSIC is used for mobile stations to identify different neighboring base stations, and BSIC adopts 6-bit coding.

(B) GSM system channel classification

Cellular communication system needs to transmit different types of information, including business information and various control information, so it is necessary to arrange corresponding logical channels on physical channels. Some of these logical channels are used in the call connection stage, some are used in the communication process, and some are used in the whole time of system operation.

1, traffic channel (TCH) transmits voice and data.

According to different rates, voice traffic channels can be divided into full-rate voice traffic channels (TCH/FS) and half-rate voice traffic channels (TCH/HS).

Similarly, data traffic channels can also be divided into full-rate data traffic channels (such as TCH/F9.6, TCH/F4.8, TCH/F2.4) and half-rate data traffic channels (such as TCH/H4.8, TCH/H2.4) according to different data rates (where numbers 9.6, 4.8 and 2.4 represent data rates in kb/s).

2. The control channel (CCH) transmits various signaling information.

Control channels are divided into three categories:

1) Broadcast Information (BCH) is a "one-to-multipoint" one-way control channel, which is used by the base station to broadcast public information to all mobile stations. The content of transmission is all kinds of information that the mobile station needs to access the network and establish a call. It is divided into:

A. frequency correction channel (FCCH): sending information for the mobile station to correct its working frequency;

B. Synchronization channel (SCH): sending information for mobile stations to synchronize and identify base stations;

C broadcast control channel (BCCH): transmitting general information for mobile stations to measure signal strength and identify cell marks.

2) CCCH is a "one-to-many" bidirectional control channel, which is used to transmit control signaling and information needed for link connection in the call connection stage. It is divided into:

A. Paging channel (PCH): the base station sends information for paging the mobile station;

B. Random Access Channel (RACH): When the mobile station applies for network access, it sends network access request information to the base station;

C.AGCH: When the call connection starts, the base station sends signaling to the mobile station to allocate a dedicated control channel.

3) The dedicated control channel (DCCH) is a "point-to-point" bidirectional control channel, which is used to transmit necessary control information between the mobile station and the base station during the call connection stage and communication. It is divided into:

A. Independent Dedicated Control Channel (SDCCH): signaling for transmission connection and channel allocation between mobile stations and base stations;

B. Slow Auxiliary Control Channel (SACCH): transmitting some specific information periodically between the mobile station and the base station, such as power adjustment, frame adjustment and measurement data; SACCH is arranged in traffic channels and related control channels to transmit information through multiplexing. Sach/t is used when it is arranged in the traffic channel and Sach/c is used when it is arranged in the control channel. Sach/ is often used with SDCCH.

C. Fast Auxiliary Control Channel (FACCH): transmitting the same information as SDCCH. When used, the service information (4 frames) should be interrupted and the FACCH should be inserted. However, this control channel should only be used when there is no SDCCH allocated. The transmission rate of this control channel is fast, occupying 4 frames at a time, about18.5 ms.

It can be seen that the GSM communication system has established various special control channels in order to transmit the required signaling. This is not only because digital transmission makes it possible to establish multiple logical channels, but also can enhance the control function of the system (for example, using mobile assisted handover technology to improve the speed of cross-border handover mentioned later), and can also ensure the quality of voice communication. In an analog cellular system, in order to transmit control information during a call, the transmission of voice information must be interrupted (100ms), which is called. When the channel is interrupted by 100 milliseconds, the sound will produce an audible tick. If this interruption is too frequent, the speech quality will be obviously reduced. Therefore, the analog cellular system must limit the capacity of transmitting control information during the call. Different from this, GSM cellular system uses dedicated control channel to transmit control information, and voice information is not interrupted during communication except FACCH, thus ensuring the quality of voice transmission. Among them, FACCH also adopts the "interrupt-burst" control mode, but it only takes a short time (18.5ms) and its influence is obviously reduced. GSM cellular system also uses information processing technology to estimate and compensate the voice deleted by inserting FACCH.