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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING How GSM networks work. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering /Mobile telephony GSM (or Global System for Mobile Communications) was developed in 1990. The first GSM operator accepted subscribers in 1991, by the beginning of 1994, networks based on the standard in question already had 1.3 million subscribers, and by the end of 1995 their number had increased to 10 million! Let's start with the most complex and, perhaps, boring - network flowchart. The description will use internationally accepted English abbreviations. The simplest part of the block diagram - a portable phone, consists of two parts: the "handset" itself - ME (Mobile Equipment - a mobile device) and a SIM smart card (Subscriber Identity Module - subscriber identification module), obtained when concluding a contract with an operator. Like any car is equipped with a unique body number, so the cell phone has its own number - IMEI (International Mobile Equipment Identity - international mobile device identifier), which can be transmitted to the network at its request. SIM, in turn, contains the so-called IMSI (International Mobile Subscriber Identity - international subscriber identification number). Thus, IMEI corresponds to a specific phone, and IMSI - to a specific subscriber. The "central nervous system" of the network is NSS (Network and Switching Subsystem - network and switching subsystem), and the component that performs the functions of the "brain" is called MSC (Mobile services Switching Center - switching center). It is the latter that everyone calls the "switch", and also, in case of problems with communication, they blame it for all mortal sins. There may be more than one MSC in the network. MSC handles call routing, generating data for the billing system, manages many procedures - it's easier to say what is not the responsibility of the switchboard than to list all its functions. The next most important network components, also included in the NSS, I would call the HLR (Home Location Register - the register of own subscribers) and the VLR (Visitor Location Register - the register of movements). Pay attention to these parts, in the future we will often refer to them. HLR, roughly speaking, is a database of all subscribers who have entered into a contract with the network in question. It stores information about user numbers (numbers mean, firstly, the IMSI mentioned above, and secondly, the so-called MSISDN-Mobile Subscriber ISDN, i.e. a telephone number in its usual sense), a list of available services and much more the other - further down the text will often describe the parameters that are in the HLR. Unlike HLR, which is one in the system, there can be several VLRs - each of them controls its part of the network. The VLR contains data about subscribers who are on its (and only its!) territory (and not only its subscribers are served, but also roamers registered in the network). As soon as the user leaves the coverage of a VLR, information about him is copied to the new VLR, and deleted from the old one. In fact, there is a lot in common between what is about a subscriber in the VLR and in the HLR - look at the tables for a list of long-term (Table 1) and temporary (Table 2 and 3) subscriber data stored in these registries. Once again about the difference between HLR and VLR: the first contains information about all subscribers of the network, regardless of their location, and the second contains data only about those who are located in the territory subordinate to this VLR. In the HLR, for each subscriber, there is always a link to the VLR that is currently working with him (the subscriber) (in this case, the VLR itself may belong to a foreign network located, for example, at the other end of the Earth). Long-term data stored in HLR and VLR 1. International Subscriber Identity (IMSI)
Temporary data stored in HLR
Temporary data stored in the VLR
NSS contains two more components - AuC (Authentication Center - authorization center) and EIR (Equipment Identity Register - equipment identification register). The first block is used for subscriber authentication procedures, and the second, as the name implies, is responsible for allowing only authorized cell phones to operate on the network. The executive, so to speak, part of the cellular network is BSS (Base Station Subsystem - a subsystem of base stations). If we continue the analogy with the human body, then this subsystem can be called the limbs of the body. BSS consists of several "arms" and "legs" - BSC (Base Station Controller - base station controller), as well as many "fingers" - BTS (Base Transceiver Station - base station). Base stations can be observed everywhere - in cities, fields - in fact, they are simply transceivers containing from one to sixteen emitters. Each BSC controls an entire group of BTSs and is responsible for channel management and distribution, base station power level, and the like. Usually there is not one BSC in the network, but a whole set (there are hundreds and thousands of base stations in general). The operation of the network is managed and coordinated using OSS (Operating and Support Subsystem - a subsystem for management and support). OSS consists of all sorts of services and systems that control operation and traffic. Each time you turn on your phone after selecting a network, the registration procedure starts. Let's consider the most general case - registration not in the home, but in someone else's, so-called guest, network (we will assume that the roaming service is allowed for the subscriber). Let the network be found. When requested by the network, the phone transmits the subscriber's IMSI. IMSI starts with the country code of the owner's "registration", followed by numbers that identify the home network, and only then - the unique number of a particular subscriber. For example, the beginning of IMSI 25099 ... corresponds to the Russian operator Beeline. (250-Russia, 99 - Beeline). Based on the IMSI number, the VLR of the guest network identifies the home network and contacts its HLR. The latter transmits all the necessary information about the subscriber to the VLR that made the request, and places a link to this VLR in itself, so that, if necessary, it knows where to look for the subscriber. The process of determining the authenticity of the subscriber is very interesting. When registering, the home network AuC generates a 128-bit random number - RAND, sent to the phone. Inside the SIM, using the Ki key (identification key - like IMSI, it is contained in the SIM) and the A3 identification algorithm, a 32-bit response is calculated - SRES (Signed RESult) according to the formula SRES = Ki * RAND. Exactly the same calculations are done simultaneously in AuC (according to the user's Ki selected from the HLR). If the SRES calculated in the phone matches the SRES calculated by AuC, then the authorization process is considered successful and the subscriber is assigned a TMSI (Temporary Mobile Subscriber Identity). The TMSI serves solely to improve the security of the subscriber's interaction with the network and may change from time to time (including when changing the VLR). Theoretically, during registration, the IMEI number should also be transmitted, but I have big doubts about the fact that the Minsk operator tracks the IMEI of the phones used by subscribers. Let's consider some "ideal" network, functioning as it was intended by the creators of GSM. So, when the IMEI is received by the network, it is sent to the EIR, where it is compared with the so-called "lists" of numbers. The white list contains the numbers of phones authorized for use, the black list consists of IMEIs, stolen phones or for some other reason not approved for use, and finally the gray list - "handsets" with problems that are allowed by the system, but behind which is constantly monitored. After the procedure of identification and interaction of the guest VLR with the home HLR, a time counter is started, which sets the moment of re-registration in the absence of any communication sessions. Usually, the mandatory registration period is a few hours. Re-registration is necessary in order for the network to receive confirmation that the phone is still in its coverage area. The fact is that in standby mode, the “handset” only monitors the signals transmitted by the network, but does not emit anything itself - the transmission process begins only if a connection is established, as well as during significant movements relative to the network (this will be discussed in detail below) - in such cases, the timer counting down the time until the next re-registration is restarted. Therefore, if the phone "falls out" from the network (for example, the battery was disconnected, or the owner of the device entered the subway without turning off the phone), the system will not know about it. All users are randomly divided into 10 equal access classes (with numbers from 0 to 9). In addition, there are several special classes with numbers from 11 to 15 (various types of emergency and emergency services, network staff). Access class information is stored in the SIM. A special, class 10 access allows you to make emergency calls (to number 112) if the user does not belong to any permitted class, or does not have an IMSI (SIM) at all. In the event of emergencies or network congestion, some classes may be temporarily denied access to the network. As already mentioned, the network consists of many BTS - base stations (one BTS - one "cell", cell). To simplify the functioning of the system and reduce service traffic, BTS are combined into groups - domains called LA (Location Area - location areas). Each LA has its own LAI (Location Area Identity) code. One VLR can control multiple LAs. And it is the LAI that is placed in the VLR to set the location of the mobile subscriber. If necessary, it is in the corresponding LA (and not in a separate cell) that the subscriber will be searched. When a subscriber moves from one cell to another within the same LA, re-registration and changing records in the VLR / HLR is not performed, but as soon as he (the subscriber) enters the territory of another LA, the phone will begin to interact with the network. When LA is changed, the code of the old area is erased from the VLR and replaced by a new LAI, but if the next LA is controlled by another VLR, then the VLR will be changed and the record in the HLR will be updated. Generally speaking, partitioning a network into LA is a rather difficult engineering task, which is solved when building each network individually. Too small LAs will lead to frequent re-registration of phones and, as a result, to an increase in traffic of various kinds of service signals and faster discharge of mobile phone batteries. If the LA is made large, then, if it is necessary to connect with the subscriber, a call signal will have to be sent to all cells included in the LA, which also leads to an unjustified increase in the transmission of service information and overloading the internal channels of the network. Now let's look at a very beautiful algorithm of the so-called handover (this name was given to the change in the channel used during the connection process). During a conversation on a mobile phone, due to a number of reasons (removal of the "handset" from the base station, multipath interference, the subscriber moving into the so-called shadow zone, etc.), the signal strength (and quality) may deteriorate. In this case, there will be a switch to a channel (maybe another BTS) with a better signal quality without interrupting the current connection (I will add - neither the subscriber nor his interlocutor, as a rule, notice the handover that has occurred). Handovers are usually divided into four types: 1. changing channels within the same base station
In general, handover is the task of the MSC. But in the first two cases, called internal handovers, in order to reduce the load on the switch and service lines, the channel change process is controlled by the BSC, and the MSC is only informed about what happened. During a conversation, the mobile phone constantly monitors the signal level from neighboring BTSs (the list of channels (up to 16) that need to be monitored is set by the base station). Based on these measurements, the six best candidates are selected and reported continuously (at least once per second) to the BSC and MSC to organize a possible switch. There are two main handover schemes: "Least switching mode" (Minimum acceptable performance). In this case, when the quality of communication deteriorates, the mobile phone increases the power of its transmitter as long as possible. If, despite the increase in the signal level, the connection does not improve (or the power has reached its maximum), then a handover occurs. "Energy saving mode" (Power budget). At the same time, the power of the mobile phone transmitter remains unchanged, and in case of deterioration in the quality, the communication channel (handover) changes.
Let's now talk about how incoming calls are routed on a mobile phone. As before, we will consider the most general case when the subscriber is in the coverage area of the guest network, registration was successful, and the phone is in standby mode.
The MSC forwards the subscriber's number (MSISDN) to the HLR. The HLR, in turn, makes a request to the VLR of the visited network in which the subscriber is located. VLR allocates one of the available MSRN (Mobile Station Roaming Number - the number of "wandering" mobile station). The MSRN assignment ideology is very similar to the dynamic assignment of IP addresses in dial-up Internet access via a modem. The home network HLR receives the MSRN assigned to the subscriber from the VLR and, after accompanying it with the user's IMSI, transmits it to the home network switch. The final stage of connection establishment is the direction of the call, accompanied by IMSI and MSRN, to the guest network switch, which generates a special signal transmitted over PAGCH (PAGer CHannel - call channel) throughout the LA where the subscriber is located. Outgoing call routing does not represent anything new and interesting from an ideological point of view. Here are just some of the diagnostic signals that indicate the inability to establish a connection and which the user may receive in response to a connection attempt. Basic diagnostic signals for a connection error Subscriber's number is busy - 425±15 Hz - 500ms beep, 500ms pause
Publication: cxem.net
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