In comments on posts about the WiMAX network ( 1 , 2 ) and about GPRS , interest was expressed in cellular networks, so I decided to implement my long-standing idea and describe to the habrasociety how modern cellular networks are arranged.



The picture above shows the general structure of cellular networks. Initially, the network is divided into 2 large subnets - the radio access network (RAN - Radio Access Network) and the switching network or core network (CN - Core Network).

I want to emphasize that I will describe exactly the existing cellular networks for the CIS, because in Europe, America and Asia, the networks are more developed and their structure is somewhat different from our networks, I will write about this later, if there is interest.

First, I would like to tell you in general terms about the network, and then I will tell you in more detail about the functions of each of the network elements.

Radio access network

Our operators' existing radio access networks are a long evolutionary product, therefore they consist of a GSM radio access network (GERAN - GSM EDGE Radio Access Network) and a radio access network to UMTS (UTRAN - UMTS Terrestrial Radio Access Network). In the upper left in the picture you see GERAN, lower left, respectively, UTRAN. The biggest changes in the transition from GSM to UMTS occur just in the radio access network - the operator needs to build a second network and re-cover existing territories.

Radio access network - this is the web that covers the vast territories of cities and open areas, due to it, this enormous destruction is provided by the cellular networks.

Core network

The core network is the core of cellular networks. The reference name is my free translation, in GSM this part of the network is called the switching network, in UMTS - Core Network, which in essence can be translated as the core of the network. To this core, as peripheral devices to the system unit, various radio access networks can be connected. The backbone network does not evolve much in connection with the evolution from GSM to UMTS, this strong evolution occurs a little later - it has already been passed by Western and Asian operators, but it is only beginning here.

The core network in the above picture is divided into 2 parts - the upper right side is responsible for voice connections, or CS connections (Circuit Switch), the lower right side is responsible for packet connections, or PS connections (Packet Switch).

The core network is concentrated in one or several buildings owned by the cellular operator, in large machine rooms - in other words, a huge server room, where there are a large number of equipment cabinets, they are sometimes called refrigerators, because they look very similar :)

Hlr

HLR - Home Location Register.
In fact, this is a large database that stores everything about the subscriber of this network. In large networks, such as those of the Big Three operators, there are several such nodes - they are scattered across regions. Their quantity is measured in units of pieces. In order to understand the rules, there is one such node in St. Petersburg, another in Moscow, one more in the Urals, another in the Caucasus, in Siberia - 3-4 things ... In practice, this may be a distributed database, because the capacity of one HLR may not be enough to store data about all subscribers. Then the operator buys another HLR (physical device) and organizes a distributed database.

What information is stored there? For the most part, this is information about the services connected to the subscriber:
- whether the subscriber can make outgoing calls
- whether the subscriber can send / receive SMS
- whether the conference call service is allowed
- well, all other possible services
Also important information is stored here, such as the identifier of the MSC in the coverage area of ​​which the subscriber is now located. Later we will see why this may be necessary.

MSC / VLR

MSC - Mobile Switching Center, a switching center for mobile subscribers;
VLR - Visitor Location Register, register of the location of guest subscribers.
Logically, these are 2 separate nodes, but in practice, it is implemented in the same device.
The VLR stores a copy of the data that is recorded in the HLR with the only difference that there is no longer any information about the MSC in whose coverage area the subscriber is located. Information is stored here on which BSC this subscriber is in the coverage area. Well, here, of course, only data is stored about those subscribers who are now in the coverage area of ​​the MSC to which this VLR is connected.


MSC is a classic switch (of course, not the classic one that can be seen in museums where grandmothers sat and pushed wiring). Its main functions - for an outgoing call - determine where to switch the call, for an incoming connection - determine which BSC to send the call to. To perform these functions, he turns to the VLR for the information stored there. It is worth noting here that this is plus the diversity of HLR and VLR - MSC will not knock on the HLR every time the subscriber needs something, but will do everything on his own. MSC also collects data for billing, then this data is fed to the corresponding systems.

Auc

AUC - AUthentication Center, Subscriber Authentication Center. This site is responsible for ensuring that the attacker could not gain access to the network from your person. Also this node generates encryption keys, with which your connection to the network is encrypted in the most vulnerable place - on the radio interface.

Gmsc

GMSC - Gateway MSC, Gateway Switch. This host is used only for incoming calls. Operators have a certain numbering capacity; gateway switches of communication networks (cellular, fixed) are associated with this numbering capacity. When you dial a friend’s number, your call reaches the switch (MSC) of your network and it determines where to send this call further based on the correspondence it has between the numbers and gateways of the networks. The call is sent to the GMSC of the mobile operator that your friend is using. Next, the GMSC makes a request to the HLR and finds out in the coverage area of ​​which MSC the called party is currently located. There further and the call is redirected.

SGSN

SGSN - Serving GPRS Support Node serving the GPRS Support Node. This node is responsible for determining how to provide services based on the requested APN (Access Point Name, access points, for example, mms.beeline.ru). Also, traffic counting is performed on this node.

Ggsn

GGSN - Gateway GPRS Support Node, Gateway GPRS Support Node. Well, this is the gateway responsible for the correct delivery of packets to the user.

Bsc

BSC - Base Station Controller, base station controller. The node to which the base stations are connected, then it manages the base stations - assigns to which subscriber where how many resources to allocate, determines how handovers are implemented. When the signal about the incoming connection for the subscriber comes from the MSC, the controller carries out the paging procedure - through all the base stations subordinate to it, sends a call to this subscriber, which must answer through one of the base stations.

TRC

TRC - TRansCoder, transcoder. The device is responsible for transcoding speech from the GSM format to the standard telephony format used in fixed communication networks and vice versa. Thus, it turns out that the speech is transmitted in the format of fixed networks in the GSM network in the area from GMSC to TRC.

Bts

BTS - Base Transceiver Station, the base transceiver station. This is what is directly located close to the user. It is the base stations that form the very web that cellular operators cover, and the territory on which cellular operators provide services depends on their number. In fact, it’s a rather stupid device, it provides users with separate communication channels, converts the signal to a high-frequency one, which will be transmitted on the air, and produces this same high-frequency signal to antennas. But the antennas we can observe every day.

I want to note that antennas are not a base station :) The base station is like a refrigerator - a cabinet with modules, which is in a special place. This is a special place - for example, little blue cars, which are placed under the red-white towers somewhere in the suburbs.

More details can be found in a recently published article about base stations .

Rnc

RNC - Radio Network Controller, radio access network controller. In fact, it plays the same role as BSC in GERAN.

NodeB

NodeB, the base station in UMTS. Analogue of BTS in GSM.

In general, all the vital elements of the GSM / UMTS network are described here. Here I did not mention some more nodes, such as SMS-C (SMS-Center), MMS-C (MMS-Center), WAP-GW (WAP-Gateway).

If the article is of interest, then in the future I can tell you in more detail about the radio access networks GERAN and UTRAN, because for the most part I deal with radial things.

Also, there are already ideas for a number of articles based on questions of interest in the comments on articles on telecommunications, until I reveal the intrigue - ask interesting questions - there will be interesting articles! ;)

UPD: experts in their fields unsubscribed in the comments, which is very interesting to read:
1.A branch about software installed on equipment;
2. A branch about the differences between our (CIS) networks and networks in Europe / USA / Asia ;
3. Comments from the user DeSh with corrections and clarifications: tyts , tyts .
And indeed in the comments, quite a lot of interesting things surfaced in addition to the comments I highlighted.