Automatic network selection in roaming - random principle?

Does anyone know what criteria the mobile device uses to select the network operator when roaming?

I meant that Swisscom can set some priorities (probably the provider that charges the cheapest prices) and then the signal strength also plays a role, or if there is a provider in one location in whose network the mobile phone may have previously been registered, If the network is not available, another one will be selected automatically if you have set your mobile phone to automatic network selection.

Nevertheless, I find the matter a bit strange and can’t really explain certain behavioral patterns after today’s experience:

Today I visited a place in France that I visited about a month ago.

  • During my last visit, my cell phone selected the Orange F network provider.
  • When I visited the exact same place today, my cell phone now selected the Bouygues network!
  • Interestingly, the cell phone of a fellow traveler (also a Swisscom customer) dialed into the SFR network!

I then used manual network selection to see which networks were available and Orange F, Bouygues, SRF and 2x Free were shown to me.

As an experiment, I then manually selected Orange F and my cell phone immediately registered on this network.

After a while, I thought I’d better switch back to automatic network selection because I was leaving France on the same day and I wanted to make sure that my cell phone would automatically log into the Swisscom network again when I drove back to Switzerland .

However, as soon as I switched from “manual”, where I selected Orange F, to “automatic”, the device logged into the Bouygues network again after a few moments!

It somehow seems to me as if a preferred network is written to your cell phone or SIM every day, because a month ago I had Orange F with automatic network selection and today I had Bouygues. It seems somewhat random, I hardly think that I changed anything in the coverage/antennas at the place I visited within a month, so that the signal situation is no longer the same. In addition, I couldn’t really tell any difference in reception strength between Orange F and Bouygues at this location.

During other, earlier visits to France (but in different places), my cell phone also logged into the SFR network.

Sorry for the long text, but maybe someone here has a little more background information, it would be interesting.

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Many providers have their preferred roaming partners in various countries, where they probably have the best conditions. Accordingly, it is no coincidence that the same provider is often always selected in the relevant country. You can see this clearly if you manually select a different network operator abroad and then set the network selection on the device back to automatic. The device usually switches back to the preferred partner within a few minutes, despite good reception. A different network is usually only selected if you have no or very poor reception and another network is stronger. When and how quickly this happens depends partly on the smartphone in question. I can’t say why different providers were chosen for you or a fellow traveler. I think the smartphone you use certainly also plays a role.

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@Samsi wrote:

A different network is usually only selected if you have no or very poor reception and another network is stronger.


If the mobile phone is booked into a third-party mobile network (roaming), it will (unfortunately) only be transferred to another, third-party mobile network when “no” mobile signal is received from the currently booked third-party mobile network. During a rebooking, all ongoing data connections (mobile data) and all ongoing telephone calls will be interrupted.

“No” mobile signal received means that the mobile signal has a signal strength of less than:

- 2G/GSM: -104 dBm (RSSI)

- 3G/UMTS: -114 dBm (RSCP)

- 4G/LTE: -94 dBm (?)

=> Minimum sensitivity of 4G/LTE according to ETSI TS 136 101 V14.3.0, Chapter 7.3.1 for band 20 (800 MHz) with 10 MHz bandwidth

was received by the mobile phone. Cell phone signal received that is “no” at all is shown as “Limited Service” in the cell phone’s internal network monitor.

The rule of thumb for reliable mobile communications is a transmission power reserve of 25 dB. 25 dB means that the radio signal received by the receiver is received 316 times stronger than would actually be minimally required.

[https://community.sunrise.ch/d/26711-feedback-zur-netzcover-an-eine-specific-ort/8](https://community.sunrise.ch/d/26711-feedback-zur- network coverage-at-a-specific-location/8)

Therefore, for stable, reliable mobile reception, a minimum signal strength of:

- 2G/GSM: -79 dBm (RSSI)

- 3G/UMTS: -89 dBm (RSCP)

- 4G/LTE: -69 dBm (?)

measured in the cell phone for the received cell phone signal. Unfortunately, no rebooking takes place in (silent) roaming if the received mobile signal falls below these limits.

Technical details can be found at:

[https://community.swisscom.ch/t5/Mobile/2G-Abschaltung-2020-neue-weisse-Flecken-kein-Empfang/td-p/617747] (https://community.swisscom.ch/t5/Mobile/2G-Abschaltung-2020-neue-weisse-Flecken-kein-Empfang/td-p/617747)

[https://community.swisscom.ch/t5/Mobile/4G-VoLTE-telefonieren-mit-externer-antenna-oder-analogem-telefon/td-p/638022](https://community.swisscom.ch/ t5/Mobile/4G-VoLTE-telephoning-with-external-antenna-or-analog-phone/td-p/638022)

be read.

Here too, the technical advantages of 3G/UMTS voice telephony compared to 2G/GSM, 4G/LTE and 5G are clearly visible. More details can be found in the multi-page comments at:

[https://community.swisscom.ch/t5/telefonie-Wissensbox/2025-ist-Schluss-3G-macht-Platz-f%C3%BCr-mehr-leistung/ta-p/702909] (https://community.swisscom.ch/t5/telefonie-Wissensbox/2025-ist-Schluss-3G-macht-Platz-f%C3%BCr-mehr-leistung/ta-p/702909)

When logging into a 3G/UMTS mobile network, the value of Ec/Io must also be taken into account. See the chapter “Emergency call via emergency call channel (161.300 MHz)” in the German-language Wikipedia article “Emergency call”:

[https://de.wikipedia.org/wiki/Notruf#Notruf\_%C3%BCber\_Notrufkanal\_(161,300_MHz](https://de.wikipedia.org/wiki/Notruf#Notruf_%C3% BCber_Emergency Channel_(161,300_MHz))

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Unfortunately, I also know the problem when I go to Germany. The provider that offers 5G or 4G locally is not always chosen - roaming costs the same with all of them. Then I have e.g. Vodaphone Edge and can’t surf well.

If I’m at the location for a longer period of time, I often search manually and choose the “strongest” provider - or hire Deutsche Telecom straight away (they’re usually the best - but not always).

There used to be a bug in Switzerland to switch the manually selected network back to automatic and thus Swisscom. That works well today - you just shouldn’t forget it completely (because then you won’t be reachable). When traveling abroad, the automatic system is probably recommended.

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@Koniferski

For 99% of customers, automatic network selection, which is active by default, is always the best and should not be changed unless you have problems or are familiar with the smartphone. Especially in Germany, however, I usually select Telekom manually, as it offers by far the best network coverage thanks to VoLTE roaming.

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However, this summer I had the impression that Telekom band 20 was quite overloaded in the countryside.

I then got along much better with Vodafone.

Dosc

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I would like to see an option so that the iPhone searches again if a connection drops below 4G and then reconnects. So Edge ignored.

In the past, you could still search for your preferred provider (i.e. Deutschland Telekom) or even choose the order. But that wouldn’t really help because no one is the best everywhere.

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@Koniferski wrote:

I would like an option so that the iPhone searches again if a connection drops below 4G and then reconnects. So Edge ignored.


Apple will fulfill this wish with the upcoming iOS 17 in lockdown mode.

[https://www.heise.de/news/iPhone-Lockdown- Modus-blockiert-kuenhafte-2G-Connections-9194247.html](https://www.heise.de/news/iPhone-Lockdown- Modus- blocked-future-2G-connections-9194247.html)

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Too bad, only in lockdown mode. As a mortal you don’t really need it because it’s also limiting.
Also interesting in the article are the security concerns about 2G - these apply to everyone. Maybe you have to wait until the providers only offer 4G and above.

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Swisscom switched off the 2G network in 2021, so it is no longer relevant, at least in this country.

I think that many roaming partners will no longer operate 2G at all today.

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Not only Vodafone, Telekom also has EDGE. In general, only the 3G network was switched off in Germany, but not the 2G network. There is also currently no date as to when 2G will be switched off in Germany. This will certainly happen sooner or later, but it may take some time.

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Apparently the 2G thing varies greatly depending on the country you are traveling to.

Swisscom has published a list showing the possible roaming partners and the supported network standard:

Roaming tariff query

Just wondering how current this list is. But you can see that apparently all providers in Germany (Telekom, Vodafone and Telefonica O2) have switched off the 3G network, but that 2G is still in operation. In Germany, calls can be made via both 2G and 4G and apparently even via 5G. So you will hardly make calls over 2G unless you are in an area where the provider whose network you are logged into only offers 2G (EDGE).

In France, on the other hand, all providers still offer 3G, but all of them still have 2G, with the exception of Free.

However, according to this list, calls cannot be transferred via LTE (i.e. 4G). In this case, the mobile phone chooses 3G for calls or, in the worst case, even 2G. The new 2G lockdown mode could prevent this in the future.

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@MagicMax

This list has existed at Swisscom for ages and is, as far as I know, relatively up to date. Unfortunately, Sunrise and Salt no longer have such a list of roaming partners. The fact that you can also make calls via 4G (VoLTE) with Vodafone and O2 has apparently only recently become the case. At least I only had VoLTE roaming in Germany with Telekom. This didn’t work for the others, so calls were made over 2G.

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@MagicMax wrote:

Swisscom has published a list showing the possible roaming partners and the supported network standard:

Roaming tariff query


I didn’t know the list yet - it’s interesting. 2G or EDGE still seems to be the minimum standard everywhere. I found on the internet: Only the coverage via the older and slower 2G (GSM, GPRS and Edge) is retained, otherwise cell phones without internet functionality would no longer work at all.

This probably means that 2G or EDGE will be retained for a longer period of time. As a result, security on the Internet does not seem to be guaranteed under 2G (which is one reason for blocking it in lockdown mode) and nothing happens when the EDGE network is selected (you can make phone calls and surf very slowly). The search and automatic switching would only have to happen if you are surfing and something from 4G is available. Maybe that’s too complicated.

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The first generation of mobile communications was known in Switzerland as “Natel C”. Natel C was based on NMT technology and therefore on analog radio (1G/NMT).

https://en.wikipedia.org/wiki/Nordic_Mobile_Telephone

https://de.wikipedia.org/wiki/Natel

Second generation mobile communications (2G/GSM) is based on time slot procedures. Second generation mobile communications uses the advantages of digital radio.

[https://de.wikipedia.org/wiki/B%C3%BCndelfunk#Digitaler\_B%C3%BCndelfunk](https://de.wikipedia.org/wiki/B%C3%BCndelfunk#Digitaler_B%C3% BCndelfunk)

Important cornerstones of 2G/GSM are:

- Time-division multiplexing => https://de.wikipedia.org/wiki/Multiplexverfahren#Time-division multiplexing_(TDMA)

- ISDN (B channel) => https://de.wikipedia.org/wiki/Integrated_Services_Digital_Network

- Primary multiplex connection => [https://de.wikipedia.org/wiki/Prim%C3%A4r multiplex connection](https://de.wikipedia.org/wiki/Prim%C3%A4r multiplex connection)

- SDH => https://de.wikipedia.org/wiki/Synchrone_Digitale_Hierarchie

- SS7 => https://de.wikipedia.org/wiki/Signalling_System_7

The basic element of these technologies is the time slot.

https://de.wikipedia.org/wiki/Zeitschlitz

With third-generation mobile communications (3G/UMTS), the shift away from time slot methods towards pure IP networks began.

https://de.wikipedia.org/wiki/Internet_Protocol

The basic element in the IP network is the data packet.

https://de.wikipedia.org/wiki/Datenpaket

Up to and including 3G/UMTS, voice telephony was implemented exclusively using circuit switching (circuit switching=> CS).

https://www.elektronik-kompendium.de/sites/kom/0211191.htm

https://de.wikipedia.org/wiki/Leitungsvmietung

Mobile communications from the fourth generation (4G/LTE, 5G and 6G) ​​are pure IP networks. Mobile communications from the fourth generation (4G/LTE, 5G and 6G) ​​only support voice telephony via packet switching (packet switching => PS).

[https://de.wikipedia.org/wiki/Packet brokerage](https://de.wikipedia.org/wiki/Packet brokerage)

Packet-switched voice telephony in the 4G/LTE mobile network is called VoLTE (Voice over LTE).

https://de.wikipedia.org/wiki/VoLTE

Packet-switched voice telephony in the fifth generation (5G) mobile network is called VoNR.

[https://community.swisscom.ch/t5/Mobile/5G-in-Liechtenstein/m-p/772675#M12457](https://community.swisscom.ch/t5/Mobile/5G-in-Liechtenstein/m-p/ 772675#M12457)

IMS is a very important part of the core network of modern mobile networks that work with packet switching.

[https://community.swisscom.ch/t5/Archiv-telefonie/Handy-Empfang-im-Minergiehaus-besser/m-p/656772#M61190](https://community.swisscom.ch/t5/Archiv-telefonie/ Cell phone reception-poor-in-the-minergie-house/m-p/656772#M61190)

[https://community.swisscom.ch/t5/Archiv-telefonie/Advanced-Calling-VoLTE-und-WiFi-Calling/m-p/658091#M56525] (https://community.swisscom.ch/t5/Archiv-telefonie/Advanced-Calling-VoLTE-und-WiFi-Calling/m-p/658091#M56525)

https://en.wikipedia.org/wiki/IP_Multimedia_Subsystem

The switch from circuit-switched voice telephony (CS) to packet-switched voice telephony (PS) is anything but trivial, especially in the area of ​​roaming, and is therefore subject to many problems and restrictions for all mobile phone providers worldwide.

https://community.swisscom.ch/t5/Mobile/VoLTE-Roaming/m-p/657972

If the mobile phone does not support packet-switched voice telephony or the mobile phone provider has not enabled the use of packet-switched voice telephony, voice telephony will fall back to circuit-switched voice telephony (circuit switch fallback => CSFB). See:

https://community.sunrise.ch/d/23412-telefonat-willkuerlich-ruf-beendet/15

https://community.sunrise.ch/d/24019-auto-telefonieren/4

[https://community.sunrise.ch/d/28020-voip-empfang-mit-5g-besser/29](https://community.sunrise.ch/d/28020-voip-empfang-mit-5g- bad/29)

Anyone traveling to a country in which there is no nationwide public 2G/GSM or 3G/UMTS mobile network, for example the USA, should take satellite communication equipment with them for emergency communication. See the multi-page comments at:

https://community.swisscom.ch/t5/Mobile/VoLTE-Roaming/m-p/657972

https://de.wikipedia.org/wiki/Notruf#Notruf_per_Satellitenkommunikation

[https://community.swisscom.ch/t5/Mobile/Umleitung-von-Anrufen/m-p/763315#M12065](https://community.swisscom.ch/t5/Mobile/Umleitung-von-Anrufen/m-p/ 763315#M12065)

In Germany, the switch from UMTS2100 to UMTS900 was missed. Details:

[https://community.swisscom.ch/t5/telefonie-Wissensbox/2025-ist-Schluss-3G-macht-Platz-f%C3%BCr-mehr-leistung/tac-p/703050/highlight/true#M533 ] (https://community.swisscom.ch/t5/telefonie-Wissensbox/2025-ist-Schluss-3G-macht-Platz-f%C3%BCr-mehr-leistung/tac-p/703050/highlight/true#M533 )

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@Koniferski wrote:

Also interesting in the article are the security concerns about 2G - they apply to everyone. Maybe you have to wait until the providers only offer 4G and above.


As long as the latest mobile communications standard (currently: 5G) provides insufficient protection against IMSI catchers due to inadequate authentication of the mobile network by the mobile phone, I do not consider a single mobile network on this planet to be sufficiently secure.

https://www.dr-datenschutz.de/authentisierung-authentification-und-autorisierung/

[https://www.heise.de/news/IMSI-Catcher-Ueberwachung-auch-in-5G-Netzen-moeglich-9190322.html](https://www.heise.de/news/IMSI-Catcher- Monitoring-also-possible-in-5G-networks-9190322.html)

[https://www.eff.org/files/2019/07/09/whitepaper\_imsicatchers\_eff\_0.pdf](https://www.eff.org/files/2019/07/09/whitepaper_imsicatchers_eff_0. pdf)

[https://www.heise.de/news/IMSI-Catcher-Why-Ueberwacher-es-so-einfach-haben-4646749.html](https://www.heise.de/news/IMSI-Catcher- Why-supervisor-makes-it-so-easy-4646749.html)

As long as standards bodies such as 3GPP and ETSI are receptive to lobbying by secret services, the security of mobile networks will not improve significantly. Today, any WLAN that is properly secured with WPA2-Enterprise, EAP-TLS, AES-128, SHA-256 and IEEE 802.11w (Protected Management Frames => PMF) is more secure than the most modern (5G) mobile network!

https://www.securew2.com/solutions/wpa2-enterprise-and-802-1x-simplified

[https://www.heise.de/ratgeber/WLAN-und-LAN-securen-mit-IEEE-802-1X-und-Radius-979513.html](https://www.heise.de/ratgeber/ Secure WLAN-and-LAN-with-IEEE-802-1X-and-Radius-979513.html)

How secure standards for radio networks from ETSI are can currently be seen in the TETRA trunked radio standard. TETRA is often used across Europe for police radio.

[https://www.heise.de/news/Digitaler-Behoerdenfunk-Massive-Schwachstellen-bei-TETRA-entdeckt-9226620.html](https://www.heise.de/news/Digitaler-Behoerdenfunk-Massive- Vulnerabilities-discovered-in-TETRA-9226620.html)

[https://www.rtl-sdr.com/encryption-on-the-tetra-protocol-has-been-broken/](https://www.rtl-sdr.com/encryption-on-the-tetra -protocol-has-been-broken/)

https://de.wikipedia.org/wiki/Terrestrial_Trunked_Radio

https://de.wikipedia.org/wiki/B%C3%BCndelfunk

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It also annoys me when the optimal network is not selected in terms of reception. Especially if roaming with 5G is possible and the provider that Swisscom always uses always has an overloaded 4G network. You can quickly switch to manual.

I wouldn’t say that the control over the provider specified by Swisscom doesn’t play a role for 99% of customers, but rather that 99% simply don’t care about it because many no longer even know that you could change it manually. But the cursing because of the overloaded network still happens.

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@roman70

And why don’t 99% of customers know it? Because they don’t care at all. The main thing is that the cell phone works abroad and that is what it does. Correct settings provided. I have only very rarely experienced overloaded networks, especially via 5G. The speed is often slower if you surf the Internet as a “foreigner” because you certainly have a lower priority than your own “domestic” customers. The devices change provider if there is no reception and a different network is available. But this takes a few seconds. Especially when you’re traveling, it hardly makes sense for cell phones to automatically always select the strongest network, since one is actually always slightly better than the other. This would also have an impact on the battery life.

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@Samsi

I also didn’t talk about overloaded 5G networks, but rather overloaded 4G networks. In the UK it was annoying that the cell phone always switched to the Swisscom preferred network. And for this one they didn’t have a 5G roaming agreement (although they do have a 5G network). This means you were constantly being redirected from a 5G network with strong reception and very good bandwidth to a network with completely overloaded 4G because it was prioritized that way by Swisscom. And that’s annoying. And you can fix it yourself as an expert. And yes, the customers are not interested in being able to select manually, but they are interested in the network moving smoothly and with these prioritizations it doesn’t always do that and that’s where the swearing I mentioned occurs.

The cell phones don’t always have to choose the strongest network, but they switch very quickly to the one prioritized by Swisscom, which is often counterproductive for the customer. They also remain on this network. This is my experience confirming the original statement of this thread.

Because of these prioritizations, depending on the situation, you have no choice but to manually switch to a “sliding” network (which many people are no longer able to do these days).

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