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Roaming Gateway




There are several technical solutions for cheap roaming, which are already on the market, but all that solutions leading to loose of your phone number (MSISDN). Main reason for this is that these solutions are eliminating Mobile Operator from the roaming itself.

But what if combine VoIP technologies, Application, SIM card and such technologies like SMS and USSD? Then we can have one nice solution for all Mobile Operators in the World.

Let's assume some Gateway, which understands all SS7 and SIGTRAN commands from the MSC, but instead of connecting E1 trunks it generates VoIP sessions for Internet with addresses similar to webinar sessions or public link to files in the Cloud - i.e. HTTPS://

  • For each call there is own last sequence after «.com/» and this sequence is transferring over SMS.
  • Also, there will be the login and the password for each session, for more security. The login is the MSISDN number for exact session and the password is some 20 characters, transferred over SMS.
  • Finally, SMS can contain the CALLING MSISDN number.

As SMS is limited to 160 characters, then we can have 15 characters for CALLING MSISDN, 20 characters for the password, some 5 more we will leave for future use and then we can nave a sequence of 120 characters for determination of each session for HTTPS address. It's well enough considering 26 small characters+ 26 big characters + 10 digits and 120 combinations of all that stuff.

Then, we need the VoIP Roaming Application on the smartphone which taking some actions, after receiving such SMS from special number of Home Mobile Operator (for example some short number  like 1111). The functionality of this application are the following:

  • It showing that there is incoming call in progress and showing Calling Number received in SMS.

  • If the subscriber has selected to answer and the smartphone has Wi-Fi connection or 3G/4G on the second local SIM card, the application adding to (stored in the application itself) the sequence of 120 characters of the exact session and establish HTTPS session with sending own MSISDN as the Login and the Password received in the SMS. Then incoming call from home country remains in E1 trunks of home country and international connection is done over the Internet for which Mobile Operator doesn't pay.
  • If the subscriber has selected not to answer then application sending back SMS to the special home operator number (i.e. 1111) with the sequence of 120 characters of the exact session and one more special symbol (for example «%»). Then this SMS is transferring in home network to the Roaming Gateway and session is terminated in both - HTTPS session and in E1 trunk, while calling subscriber hears short rings (call is cancelled by called subscriber).
  • If the subscriber wants to answer but the smartphone neither Wi-Fi connection nor 3G/4G on the second local SIM card, the application is sending back SMS to the special home operator number (i.e. 1111) with the sequence of 120 characters of the exact session and one another special symbol (for example «&»).Then this SMS is transferring in home network to the Roaming Gateway, HTTPS session is terminating but in E1 trunk calling subscriber hears an announcement something like this - «Called subscriber is in weak network area. Please try to call 5-10 minutes later» or like this - «Called subscriber using VoIP solution. Please try to call 5-10 minutes later». Called subscriber knows that he/she should get access to internet on his/her smartphone within 5 minutes and calling person most probably will make one more call, as he/she heard an announcement.
  • If incoming call is very important (and right now smartphone is without the Internet connection) or for security reason have to be done over GSM network (even if there is the Internet in smartphone) or for any other reason - the application has a button like «Get the call over GSM». When subscriber have pressed this button, the application is sending back SMS to the special home operator number (i.e. 1111) with the sequence of 120 characters of the exact session and one another special symbol (for example «#»).Then this SMS is transferring in nome network to the Roaming Gateway, HTTPS session is terminating and the Roaming Gateway transferring the call to generic Media Gateway and call goes in generic way over E1 trunks abroad.

So, now we already have the description for the incoming call from homeland.

Let's look at the outgoing call in roaming to homeland.

Subscriber can run generic phone application and make generic call over GSM network. Absolutely same as it's now and for the same high price. This is still valuable for important calls (immediate or secure).

If subscriber wants to make a cheap call - he/she starts the Roaming Application input digits and press «CALL» button. Then application sending SMS to the special home operator  number (i.e. 1111) with the MSISDN number of the called subscriber. Then this SMS is transferring in home network to the Roaming Gateway, which is establish HTTPS and E1 sessions (for MSC it will be emulating generic incoming call). Then the Roaming Gateway generates SMS similar to incoming call (the sequence of 120 characters of the exact session and establish HTTPS session with the Password) but with some special symbol (for example «@»), for the reason that application knows it's for outgoing call and not showing a message about incoming call. The call goes same way too - outgoing call from home country  Roaming Gateway remains in E1 trunks of home country and international connection is done over the Internet for which Mobile Operator doesn't pay.

Point of Concern - is that called number getting a call from Gateway and there can be some generic wireline number of this gateway, instead of MSISDN number of calling subscriber. We all know this situation with pure Internet VoIP services like Skype.

Good news for us - is that calling numbers can be changed in Operator's Network, while routing will remain safe using other low level identifications. This is the benefit to have innovative roaming solution still inside Mobile Operator's network, compare to other roaming solutions, separated from Mobile Operator!


  • How to make that Home MSC knows that incoming calls should not go generic way, but call should be transferred to the Roaming Gateway? The solution is that when the subscriber go abroad he start the Roaming Application and set slider ON. Then application sending an SMS with text «ON» on the number 1111. Then there should be some Database where stated HLR data for this subscriber and MSC knows that subscribers from this database are always should be routed to the Roaming Gateway. Most obvious solution is to implement this in local Number Portability Database - own roaming subscribers are placed here in a separate group with declared route to the Roaming Gateway. As an alternative - in HLR can be added one more parameter about redirecting to the Roaming Gateway. If both - NPB and HLR are hard to upgrade, then there can be separate server which copying HLR data of the necessary subscriber and adding one more parameter. Finally, after return to homeland, the subscriber set slider to «OFF» and the application sends a SMS «OFF» to 1111 and then subscriber have generic routing and normal Circuid Swithed calls.
  • In the incoming call description, when subscriber doesn't wants to reply or have no the Internet connection - can be used USSD instead of SMS, as outgoing SMS in roaming is expensive. In fact, all this system can be created on base of USSD instead of SMS - depending on the exact Operator's preferences and capabilities.
  • Transferring of the incoming call from the Roaming Gateway to the Generic Media Gateway when the subscriber has pressed a button is not so simple and can't be done on a level of gateways only. The reason is that billing system should know about expensive generic call. More simple solution is that the Operator has GMSC and all roaming calls are controlled here somehow. If not in GMSC, then the Roaming Gateway should inform MSC, MSC redirect call to another Gateway itself and then we have no problems with billing.
  • For outgoing call the button CALL can be pressed only if there is available Internet connection, of course. If no Internet connection, then the button has no activity, can be coloured grey and there can be a notification about necessity to have an Internet connection.
  • In general, application has an access to whole generic Address Book (Contacts) and the subscriber can select phone number (no need to remember numbers or copy-paste), same as during incoming call the subscriber see the contact name, not just digits of the number.

Local calls in roaming and calls from/to other countries (not from/to homeland)

We have examined incoming calls from home country and outgoing calls from to home country, where all E1 connections are local in country where Operator's MSC is. But what to do with local calls in roaming country or calls to another countries? Even we still can use Internet connection for our subscriber connection to our MSC, E1 links to other country appearing here. Two times less E1 connections, compare to generic way can decrease price two times, but still be expensive enough.

Most obvious way to solve the problem is that after some time in most countries and in most operators there will be implemented this Roaming Gateway Solution and in roaming agreements will be included usage of local Roaming Gateway for calls. In particular, if subscriber from Russia is in France make a call to French number, then French Roaming Gateway is used, while if subscriber from Russia is in France make a call to Chinese number, then Chinese Roaming Gateway is used - for the purpose that all E1 calls are local to Roaming Gateway and there is no international E1 links. Tricky situation to be covered in roaming agreements is that even the subscriber from Russia is in France, but when make a call to Chinese number, then Chinese Roaming Gateway is controlled by MSC in Russia, not MSC in France. Even if French MSC will be involved, it works only as the Signalling Transfer Point.

So, bad news are that there are lot of administrative tasks. Good news are that all that connections are for signalling only and no need to have international voice channels in E1, thus we have a great economy.


All-IP and VoLTE

It will be quite reasonable that finally solution will be developed as pure VoIP client working in the pier-to-pier mode for voice, with initial connection establishing by Mobile Operators and using operators authentication procedures.

I mean, that subscriber dial MSISDN number, then Roaming Application generates request to IP Gateway (no need SMS or USSD, but still SIM style authentication is processed), then Home MSC dials with called MSC, this second MSC manage local IP connection for the called subscriber, while for calling subscriber it generates HTTPS address. Then this HTTPS address is transferred via IP to calling smartphone, but transferred over IP, not SMS or USSD, however - with operator's authentication. In result we have the situation, when signalling is still transferred among countries, but international voice channel not a care of mobile operator and thus not to be paid.

The difference to VoLTE roaming - we do not use international IP sessions on a side of Mobile Operator. However in VoLTE is using IP everywhere, but VoLTE traffic has higher priority inside LTE traffic. This is most critical for the Radio part, but for wireline Ethernet it has influence too. So, VoLTE traffic is not the same as VoIP traffic in the regular Internet and price for VoLTE is much higher and always will be like this! Therefore, the only way to have cheap roaming even after implementation of VoLTE around the World - is to avoid international VoLTE priority IP connections among Mobile Operators, but have cheap generic Internet connections which are not provided by Mobile Operators, in advance.



Last technical partition is about using SMS and MMS within Roaming Application. Even it looks like simple, but still requires most of the procedures described above for calls.

Incoming SMS - nothing special to do. Incoming SMS are free of charge already for most roaming cases. The only thing to care - is to have one conversation frame (window) with outgoing SMS.

Outgoing SMS are quite expensive and we can do same job as for calls. In the application you write an SMS, then application deals with Roaming Gateway using USSD (or IP in the future) and in HTTPS session of Roaming Gateway we transfer text over IP. There can be special symbol too. For example, application requesting HTTPS session with «*» in the text, then Roaming Gateway sends back an SMS with the sequence of 120 characters of the exact session and establish HTTPS session with the Password) but with special symbol «*», for the reason that application knows it's a session for outgoing SMS, but not incoming call. After Roaming Gateway got text, it interacts with local SMS Centre and SMS goes regular way to destination mobile phone, but SMS itself is not transferred over international E1 channels and thus we have an economy!

The only task left is that there should be one application (window) where whole conversation is presented with sent and received messages. The idea is that outgoing messages are copied locally in SMS client on the phone (however these are sent not using this SMS client). If it's not so easy to do with base SMS client due to limitations of OS, then there can be an alternative solution, that while roaming mode is on, all outgoing SMS are remains in the Roaming Application and incoming SMS from exact number are copied from SMS client to Roaming Application and thus we have normal conversation. In most cases 2nd variant is acceptable, as SMSes have a value for the exact moment and it's not so important to have these SMS in generic SMS client permanently for months and years.

Outgoing MMS are similar to outgoing SMS. The only difference is that we can use another special symbol to make Roaming Gateway logic more simple (and product more cheap). If use «+» instead of «*», for example, then Roaming Gateway knows that routing should be done to MMS Centre and/or another protocol/signalling should be used, compare to SMS.

Incoming MMS may not require efforts, same as incoming SMS. However, it's assumed that GPRS roaming is turned on. It was not a problem on old phones, when Internet was used only for getting MMS, but in smartphones enabling GPRS roaming may cause big amount of downloaded data even if turned on just for couple of minutes. Therefore, Roaming Application may send back USSD request when received SMS part of the MMS and then Roaming Gateway will organize an HTTPS session for transferring content of the MMS in this session over Wi-Fi or local 3G/4G Internet, without necessity to enable GPRS roaming on the own SIM.


Lawful Intercept

Last technical remark is about the Lawful Intercept, which is strong requirement to all Mobile Operators in all Countries.

First of all - Roaming Gateway is still Telecom Gateway with E1 trunks and thus connection to corresponding government equipment are performed same generic way as it's doing with other gateways.

Considering to the world practice of Lawful Intercept and general principle that law is available in exact country for all the people which are in this country, but not law is for people of exact country wherever in the World they are, we can conclude that if subscriber from Russia came to France and here in roaming and calling local French phone number - intercept should be done only in France. This is how exactly the proposed solution works! This is how exactly it's implemented with E1 in many countries, that source country can only know  from signalling - who and when performed a call, but have no access to voice traffic exactly, as it's a traffic in the another Country.

If he/she decided to call Russia (being in roaming in France), then in the proposed system call over E1 channels will be in Russia only. Yet, HTTPS traffic and all SMS and USSD are available for French affairs and if they want - they still can hear the call. The only difference that it will be VoIP call, not Circuit Switched over E1. At present time it's not a problem in most Countries.

If subscriber is from Russia came to France in roaming and call China - also no problem, as we have Roaming Gateway in China and E1 channels are there, France still tracing SMS and USSD and HTTPS traffic goes over French Operator.

So, even this solution is introducing some "WEB-like" VoIP, yet government affairs of roaming country having all necessary keys, passwords and traffic itself and technically they can do Lawful Intercept. May be some formal changes will be necessary to perform in some countries, but these changes are quite reasonable and will be accepted easily.



There is of course the question about security, that call goes via public Wi-Fi and that Roaming Gateway creates sessions in plain Internet, not in limited networks, not in Intranet and without any VPN.

Let me summarize technical stuff:

  • Usage of HTTPS,
  • Random generation of 120 characters links which are valuable couple of minutes (in general),
  • Using different media for HTTPS sessions (public internet of one operator) and initial session information (SMS or USSD of another operator).
  • In result system has not generic 2 level protection (login+password), even not banking level of 3 parameters (login+password+SMS) but unique 4 level protection consisting of (1) exact link (2) available only in exact time (3) logined with MSISDN (4) and one-time password!

In advance, Roaming Gateway can be enhanced with the following functionality - if some exact HTTPS link is accessed but login is not done or wrong, then Roaming Gateway remaining E1 session, but assign for this E1 session new HTTPS session and inform Application on the exact smartphone once again. So, even brut force attacks on the Roaming Gateway do not let in sessions. Can slow down the system, can increase call setup time, but still have no access to the information itself!

Along with technical stuff - we still keep generic roaming over Circuit Switching networks and GSM, while offering this solution as some cheap alternative with several limitations. In other words - we providing some VAS and regulations for that are not so tough, as for major telecom services. If the Operator want's income increase and growth of subscribers' loyalty, then he can manage all these questions, for the sure.

Yet there can be contrary situation - that Mobile Operators want to keep high pricing for roaming. Complex contracts quite allowing to have complex tariffs and make money out of the air, regardless of subscribers' loyalty. Yes. Still can be! For that case it's possible to promote solution (when ready) to government and press and trying to lobby a law that makes it obligatory to have alternative cheap solution for roaming and that subscribers must to have a choice! I do believe that this is the way and can be quite a successful lobbying, as it will be have a great support from millions of regular people.



What we got on the Mobile Operator's side is that it has no international connection for the traffic! There still remain international connections for signalling, but this is much cheaper than traffic. Even locally, instead of two channels in E1 trunks for local call in the proposed solution it's required only one channel in E1 while for another part is generated HTTPS session. Considering economy on one channel in E1 and still international signalling we can assume, that price of such roaming call has to be similar to price for generic local call. This is the main result! Depending on the exact Country price of the local call compare to call in roaming can differ 15 to 50 times and this is awful!

This solution is providing to a Mobile Operator a Competitive Advantage and Subscriber's Loyalty.  In fact - if real cost goes down 15-50 times, then there is no reason to keep same price, but the opportunity to have same margin due to decreased cost. It even can be more margin for the Operator, with price reduction to subscribers in 10 to 45 times!

But most wise specialists already understood that main driver for margin growth will be the fact, that subscribers will start make same amount of calls in roaming, as they doing in homeland and real income will grow up dramatically! There is even no reason to increase margin per minute (i.e. having 10 lower cost and decrease price only 9 times), but provide honest price and honest terms and conditions and start earning on the effect of the volume!


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