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



The picture below shows the general structure of cellular networks. Initially, the network is divided into 2 large subnetworks - the radio access network (RAN - Radio Access Network) and the switching network or core network (CN - Core Network).
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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 it sometime later if there is interest.

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

Radio access network

The existing radio access networks of our operators are a product of a long evolution, 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, in the lower left, respectively UTRAN. The greatest 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 already existing territories.

Radio access network - this is the web that covers the vast territories of cities and open areas, due to it that provides a huge hiding, which provide cellular networks.

Core network

The core network is the core of cellular networks. The name of the reference is my free translation, in GSM this part of the network is called a switching network, in UMTS it is called the Core Network, which in fact 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 core network evolves little due to the evolution from GSM to UMTS, this strong evolution occurs a little later - Western and Asian operators have already passed it, but it is just beginning here.

The reference 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 part is responsible for packet connections, or PS connections (Packet Switch).

The core network is concentrated in one or several buildings belonging to a cellular operator, in large machine rooms - in simple terms, the largest server room, where there is a large number of equipment cabinets, they are also sometimes called refrigerators, because they look very similar :)

Hlr

HLR - Home Location Register, 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 number is measured in units of pieces. In order to understand the order - there is one such node in St. Petersburg, another in Moscow, another in the Urals, 3-4 in the Caucasus and Siberia - in practice ... This may be a distributed database, because the capacity of one HLR may not be enough for storing data about all subscribers. Then the operator buys one more 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:
- can the subscriber make outgoing calls
- can the subscriber send / receive SMS
- is the conference call enabled
- well, all other possible services
Also here is stored such important information as the identifier of the MSC, in whose coverage area 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, guest location register.
Logically, these are 2 separate nodes, but in practice, this is implemented in the same device.
VLR stores in itself 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 which the subscriber is located. Information is stored here about which BSC is located in this subscriber. Well, here, of course, the data is stored only on those subscribers who are now in the range of the MSC to which this VLR is connected.


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

AUC

AUC - Authentication Center, the Subscriber Authentication Center. This node is responsible for ensuring that an attacker cannot access the network on your behalf. Also, this node generates encryption keys, with the help of which your connection to the network is encrypted in the most vulnerable place - at the radio interface.

GMSC

GMSC - Gateway MSC, gateway switch. This host is used only for incoming calls. Operators have a certain number capacity, this number capacity is assigned to gateway switches of communication networks (cellular, fixed). When you dial a friend’s number, your call reaches the switch (MSC) of your network and it determines where to send this call based on its correspondences between the numbers and gateways of the networks. The call is sent to the GMSC of the cellular operator, which is used by your friend. Then the GMSC makes a request to the HLR and finds out in the coverage area of ​​which MSC the called subscriber is currently located. There further and the call is redirected.

SGSN

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

GGSN

GGSN - Gateway GPRS Support Node, a 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 which subscriber where to allocate how many resources, determines how handover is performed. When a signal comes in from the MSC for an incoming connection for a subscriber, the controller performs a paging procedure — through all the base stations subordinate to it, it sends a call to this subscriber, which must respond through one of the base stations.

TRC

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

Bts

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

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

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

Rnc

RNC - Radio Network Controller, radio access network controller. Essentially playing the same role as the BSC in GERAN

NodeB

NodeB, base station in UMTS. Analog BTS in GSM.

In general, all vital elements of a 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 arouses interest, then in the future I can tell in more detail about the radio access network GERAN and UTRAN, because for the most part I do radio things.

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

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