MNP: How it works *

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    * Nobody knows how everything works ...


    The passion for introducing the mobile number portability service has calmed down a bit and now it's time to figure out what and how has been done, what is the logic of work and look at some technical aspects. I want to ask all telecom gurus in advance to forgive me for the deliberate simplifications and ignore of irrelevant details.

    One of the main tasks solved by networks supporting MNP is to establish the subscriber’s affiliation with any operator. This is necessary in order to correctly send a voice call or SMS, and for payment systems - to whom to transfer money. To solve this problem, we use the central computer database - the central database of the transferred numbers.

    Everyone remembers that the TsNIIIS FSUE was appointed the operator of the Central Bureau of Strategic Development, the tasks of which included the deployment of the mentioned base, to which all players should be connected. Of course, TsNIISS did not have any of its own developments, but there was an offer to buy a ready-made Numlex solution from the Lithuanian company MediaPhon. Actually, this is what happened. Money for the purchase of 70 million rubles was allocated by the Ministry of Communications, which allowed to launch the Central Pulp and Paper Center in an extremely short time. By the way, on the website of TsNIIS there is an opportunity to check whether the number belongs to the operator: www.zniis.ru/bdpn/check

    TSNIIS constantly provides meager statistics on the number of successful transitions, from which, unfortunately, it is impossible to understand the main thing: who won what share from whom. But you can see the dynamics of demand for services. If this goes on, I think that the predictions that MNP is in demand only for 1-2% will turn out to be erroneous.

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    We will consider MNP in two planes:

    • the transfer procedure itself, during which the subscriber migrates from one operator to another, and the CDCN base is replenished with routing information, the main of which is the routing number RN;
    • work of the operator’s basic services, taking into account new features.

    It would also be interesting to look at payment systems. But, alas, I do not own this issue at all, therefore I am forced to leave it to more competent authors. Connect, tell.

    Looking ahead, I’ll say that the cornerstone of the entire call processing logic is the RN routing number, which has the following format:

    DXXYY , where:

    D is always the symbol “D”
    XX is the region code according to the Constitution . It looks like the traffic police code, but for many regions it does not match. For example, for RN, code 55 refers to the Novgorod region, although the traffic police in Novgorod has code 53.
    YY is the operator code. For MTS-01, for MegaFon - 02, for Beeline - 99, for Tele2 - 20

    PORTATION OF SUBSCRIBER


    In the TsNIIS documents you can find a lot of colorful diagrams of processes and a fairly detailed description of commands, completion codes and all necessary procedures. We will consider a typical transfer scenario, in which the subscriber of the donor operator leaves to the recipient operator. In a word, this is called Portation.

    Figure 1 is a diagram of the porting process.

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    Consider the main stages of the process
    1. The subscriber comes to the office of the new operator (recipient) and writes a statement of the transition.
    2. The recipient operator sends a request to the CDC, where the application is registered and the porting process is started.
    3. The donor operator receives an application and performs a series of checks - the subscriber’s ownership, no debt, no ban on transfer for 6 months. from the date of connection, etc.
    4. If all checks are successful, the donor sends the estimated date of the start of the transfer. Usually this date is selected after 7 full days from the current moment. As practice shows, during this period the donor operator begins to process the subscriber, trying to find out from him the reasons for his departure and offers to think carefully. Not sure about the effectiveness of such a policy; it seems to me that a subscriber, if he has already thought up and reached the office with his legs to write a statement, nothing will stop him.
    5. On the appointed day and hour, the small portion stage begins , when the main participants in the process make changes in their systems and after which the new SIM card of the subscriber will be able to work in the new network. The first to start is a donor who deactivates a subscriber in their systems - HLR and billing.
    6. Then the recipient activates his new subscriber at home. For all these manipulations, according to the regulations, half an hour is allocated. It is during this period that the subscriber remains completely without communication. I must say, usually, all fit in a few minutes. And after that, the subscriber can register in the new network and even use many services.
    7. Further, the donor makes changes to his SFR so that calls from his network to his “former” subscriber are correctly sent to his new home network. This concludes the small porting phase and our subscriber, exhausted but happy, begins to get used to the new operator.
    8. The CDCU uploads incremental data on the new RN of the ported subscriber to its database, and all connected operators collect this information every 2 hours and update their SRF

    Everything! Now all operators know that our client has moved to a new network.

    And here it remains to add that most operations are performed automatically. Exceptions are, of course, interaction with the subscriber and handling various errors and special situations. For example, a subscriber could change his mind about changing the network - for this case, there are their own typical scenarios.

    WORK OF BASIC SERVICES


    Since this whole story is about a phone number, the main interest is voice calls and SMS. As I already wrote, the main task for servicing a call is to establish the location of the called subscriber. In a classical mobile network, its switch, called GMSC, solves it and the STP signaling gateway helps it, which describes the static routes for sending requests to HLRs based on the number of the desired subscriber. If you are not familiar with the principles of mobile networks, I’ll explain that HLR is a database where information is stored not only about the technical parameters of subscribers, but also about the current serving switch of each of them. And to find the called party, the network makes a request to the HLR.

    With the advent of MNP, the static signaling routing based on MSISDN no longer allows us to find the desired HLR, and therefore the operator has a new node called SRF. The main objective of SRF is to store, update and provide information about the affiliation of any number to the region and operator. A sign of ownership is the routing number RN, which, in conjunction with the MSISDN number, is used further for routing MAP and ISUP traffic by GMSC / MSS switches.

    Now we will analyze the scenario of a voice call to a subscriber ported to a new network. For example, the subscriber A from the Tele2 network calls the subscriber B who switched to MTS **. Where he came from now does not matter, since his last statement in the procedures is no longer involved. Operators may have different internal algorithms for working with SRF. But, we look at the scheme in general terms, keeping in mind that this can be only one option.

    ** operators are chosen arbitrarily.


    Figure 2 - Call to a ported subscriber
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    1. The number was dialed by subscriber A, from the T2 home network. The MSC switch serving it transfers the call to the gateway GMSC, the task of which will be the further organization of the call.
    2. The gateway GMSC makes a request to SRF, where there is information about the RN routing number with the region and operator codes (for example, D5501 - MTS Novgorod)
    3. The SRF node returns a response to the SRI in the GMSC, where a bundle from RN + MSISDN appears as a temporary number
    4. GMSC, based on the received RN + MSISDN, routes the call to the MTS operator.
    5. Well, and then - the standard procedure for searching a subscriber within the network - a request to HLR-B, obtaining a temporary number and GT VLR, establishing a voice connection to the final MSC of subscriber B.

    For ease of understanding, the step of checking subscriber B ownership on the side of the receiving operator (MTS) is omitted. It is also performed via SRF and is necessary in case other operators send calls to the wrong address, i.e. in our case, to numbers not belonging to MTS. MTS will send such calls to the place where the called subscriber is now actually registered and will take a pretty penny for it for transit.

    I repeat that the details of the internal solutions of different operators may differ. But, in general, the principle is unchanged in one thing: to find the routing path, the network uses its own SRF node, which “knows” which subscriber is ported and where and which is not.

    To consider in detail a similar scenario for sending SMS, it makes little sense. I can only say that to search for the recipient, the SMSC initiates the standard request SRI_for_SM, which should be in the HLR of the recipient. Along the way, this request is intercepted by SRF, which will help find the real operator-receiver. In response, the SMSC will receive the GT VLR where the receiver is located and the IMSI required to send the message to the serving switch. How exactly this information will be obtained by SRF is a matter of purely routing the signaling network of the operator and the add-on used. functionality on STP nodes.

    PROBLEMS


    At the initial stage of MNP implementation, it often happened that some individual operators did not always have time to make changes to call routing and signaling. Because of this, some ranges of numbers could be worked out according to the previous scheme, without the participation of SRF. Now this is extremely rare.

    Some problems arise due to the untimely updating of local operator databases. The fact is that each major player has several SFR nodes and they are not synchronized directly with the CBDPN, but through the operator’s local BDPN. These processes somewhat reduce the efficiency of the porting procedure. For a subscriber, this may look like a lack of incoming communication from some operators.

    If basic services are already more or less tested and cleared of bugs, then there are tons of exotic, “smart” services built on IN platforms - various types of call forwarding, SMS, short numbers, mobile numbers in the ABC city format, and so on - all this remains to be rechecked. I am not saying that everything is bad here. But, you need to understand that the more crutches and the less functional it is in demand, the more likely it is that some mechanisms forgot to adjust under MNP.

    CONCLUSION


    When in the fall of 2013 all market participants criticized the organization of the MNP project, they were right. On the part of the Ministry of Communications, distinct rules were announced very late. But, the project was implemented thanks to the fact that our country was not a pioneer in this area. There were ready-made solutions on the market - both for the pulp and paper machine, and for each operator and vendor of its equipment. So the abolition of mobile slavery, in general, was calm, without tantrums. And, although the roughness still remained, in general, telecom operators, contrary to the prevailing stereotype, worked very quickly and in concert.

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