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As an slight extension of my project, I am trying to see if I can improve connectivity resilence by using a dual-SIM approach. Dual-SIM phones (or Tri-SIM, quad-SIM phones) are quite popular in many developing countries, and AFAIK, most are based on Mediatek chipsets. However, I am wondering if these use a single GSM module (RF baseband + controller), multiplexed for use by multiple SIM's, or they actually have as many GSM modules as the number of SIM cards.

If it is multiple modules, then I think I know already what is needed, but if it is a single module multiplexed with multiple SIMs, then I'd like to understand how this works. If someone has a high-level schematic of how this works, would be great to be able to take a look.

Edit (Feb 1, 2012): Call me crazy, but I went ahead and purchased an el-cheapo dual-SIM phone to pry it open and see inside, although thanks to the very high integration and density I wasn't hopeful of figuring out much. Sure enough, there are obviously 2 SIM's and the traces seem to lead into what is probably the module that is put under a metallic EMI shield box, without any markings. Can't see an obvious way to remove the EMI shield box, as it is not clear how it is stuck to the board. So, cannot tell for sure, but given the size of the shielding probably a single module.

Another reason why multi-SIM's might be using a single GSM RF module is because I found some phones claiming to have 4 SIMs !! Huh.

Edit (Feb 15, 2012): After quite a bit of reading around, I've come to believe that the way to use a single module with 2 (or more) SIM's is to modify the standard single-SIM firmware on the GSM module itself. This might be difficult to achieve, purely via the firmware on the application-processor (or uC in my case). The firmware on the module, exposes few different types of API's s.a. Hayes AT-command set, or native API that uses some kind of message-passing, and only provides somewhat higher-level control than what is required to deal with multiple SIMs. This means that, implementing dual (or more) SIM for my projects, using off-the-shelf GSM modules like the ones from Telit, Siemen/Benq, SimCom etc., isn't going to be easy (or possible). Will keep looking, and if someone has credible knowledge to challenge this finding, I shall be more than happy to learn about it.

Edit (Jul 25, 2012): I have come across 2 different handsets (el-cheapo bottom-bracket shenzhen Android phones), and a key difference in the technical specification of the 2 phones caught my attention. One said that while the handset supported 2 SIM cards, but only one of them could be active at a time, I believe it is the single module, dual-SIM approach. For the other handset, it said that for the 2 SIM cards supported, both could be used simultaneously, s.a. one being used for a phone call, and other being used to maintain a 3G data-connection. I see no way of being able to do it, without using 2 GSM RF modules. Of course, the second handset is more expensive -- by about $25, which can be explained by the presence of a better ARM11 processor (800MHz instead of 650MHz) and the extra GSM module, plug a slightly larger battery (just 200mAh extra). Of course, this hasn't been validated by ripping the phones apart, or any kind of schematic study.

Edit (Feb 12, 2013): Just to confirm, that on el-cheapo Android phones, which claim to be dual-SIM, there is quite certainly only 1 GSM module, because the behavior is that if you have data-session established, all call to the other SIM (incoming calls), fail, with the network operator playing the announcement that the number is not reachable. Effectively, it means that while the 2nd mobile number is "registered" to the network (in GSM sense), it is not reachable, because the handset (with the SIM corresponding to the mobile number), did not respond to network's "paging request". That happened because the only GSM module is already busy serving data-connection to the first SIM.

Edit (Sep 15, 2015): Much water has flown under the bridge since my last post. I have found documentation that indicates that both variants seem to thrive, though the former is lot more popular for low-end devices, due to reasons of overall device cost (including that of the battery), i.e.

  1. single radio, multi-SIM
  2. dual radio (yet to find multi radio > 2, for GSM), multi-SIM

In the telephone handset industry, the former seems to be called 'dual standby' and latter is called 'dual active'.

Some interesting links relevant in this context:

However, more interestingly, here is the application note on using Quectel GSM module supporting dual-SIM.

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    \$\begingroup\$ +1 for the craziness! :D \$\endgroup\$ – Count Zero Feb 14 '12 at 23:52
  • \$\begingroup\$ Wish I could update this one more time! \$\endgroup\$ – AndrejaKo Sep 15 '15 at 17:18
  • \$\begingroup\$ Update what ? Please feel free to edit, if you have more information to add. \$\endgroup\$ – icarus74 Sep 15 '15 at 17:20
  • \$\begingroup\$ A patent to use dual sim combined with dedicated antennas for each sim has just been claimed by Apple. Remarkable however is that apple is the only big player that does not have a dual sim phone on the market yet. \$\endgroup\$ – Menzo Wijmenga Dec 14 '16 at 16:53
  • \$\begingroup\$ @MenzoWijmenga - where can you see that claim? I see a claim for The ornamental design for a security attachment, as shown and described. . Am I looking in the wrong place? \$\endgroup\$ – Арсений Пичугин Jan 5 '17 at 22:24
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No, they don't have multiple GSM modules. That would drive up cost, size, weight. Since the user is only on one call or the other, there isn't a need for more than on GSM module.

EDIT Jan 31 '12. Wikipedia says there are phones that can use both at once. But I still believe there is only on GSM module.

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  • \$\begingroup\$ Thanks @Brian. Do you have anything to add on how they are used with multiple SIM cards ? \$\endgroup\$ – icarus74 Jan 31 '12 at 3:34
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    \$\begingroup\$ In fact, now are spreading the phones like NGM but also bigger brands, with dual-sim also in stand-by mode, so they must be active at the same time; they also consume more, so if they don't have two modules the duty cycle is increased. \$\endgroup\$ – clabacchio Jan 31 '12 at 22:12
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As far as I know from my brother who uses dual sim phone, both SIMs are active at once. It means you can receive calls on both SIMs without any special switching. Also you can receive or send messages on both SIMS. But the phone has only one GSM hardware. Only one antenna, only one transmitter etc. GSM use time divided modulation so you can run two SIMs without having two pieces of everything - that would make the phone a lot heavier than normally. (Nobody would want a lot heavier phone.) Most of the time the transmitter/receiver is turned off to save valuable power.

In the past I also saw a phone which needed to be turned off and on to switch the sims. It automatically switched the SIMs on every turn-off turn-on cycle. That was easy to use but quite inconvenient.

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    \$\begingroup\$ My phone has two radio's, two antenna's, two SIM's, two EMEI's. When you open the back cover you can nicely see both antenna testing connectorlike-thingies. I can disable either radio independently and when I do that, the battery lasts longer. \$\endgroup\$ – jippie Jul 21 '12 at 18:08
  • \$\begingroup\$ @jippie: Very interesting. But is it particularly useful in praxis? \$\endgroup\$ – Al Kepp Jul 21 '12 at 19:08
  • \$\begingroup\$ yup very practical. My colleagues have my busness number, my friends have my private number. \$\endgroup\$ – jippie Jul 21 '12 at 19:22
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    \$\begingroup\$ @jippie: I ask if it is useful in praxis to have two radios and two antennas, when you can have your two SIMs in a simpler mobile phone with shared antenna and shared radio (and less weight). \$\endgroup\$ – Al Kepp Jul 21 '12 at 22:23
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    \$\begingroup\$ AFAIK single radio dual SIM mobile phones can be called at either number too, without two radios. (I don't know exactly how they behave during a call.) \$\endgroup\$ – Al Kepp Jul 22 '12 at 9:18
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If you have one GSM modem, you can, in theory add many SIM cards ad-infinitum. The trick is to use multiplexers to select between the SIM card receptacles, i.e. routing the SIM interface signals to the appropriate SIM card, and to exploit the SIM_PRESENCE signal. This is frequently used in devices such as trackers, that have to cross borders, and it becomes important to avoid situations where roaming is required (data on roaming is extremely expensive)

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  • \$\begingroup\$ @sonic2k thanks for sharing the hint about SIM_PRESENCE, and reading this link, the logic of SIM multiplexing is a bit clear, but not completely. I can understanding that using SIM multiplexing, it is possible to use 2 or more connections, but one at a time. If I am not mistaken, then to switch between 2 connections, one needs to bring down prior GSM connection (registration to network), and reestablish a new GSM connection (registration to network). However... \$\endgroup\$ – jay Jan 5 '14 at 14:24
  • \$\begingroup\$ ... is it possible to use standard GSM modules s.a. the ones from SIMcom, Benq, SierraWireless, Motorola, Siemens etc., such that multiple GSM registrations could be maintained in parallel, such that, incoming/outgoing calls on any of the registered numbers could be done. Of course, once a call is underway using one of the numbers, other numbers cannot be used (s.t. incoming calls are met with busy tone) ? Is this something that the GSM modem firmware needs to support ? Or can this be achieved somehow using the multiplexer circuit and AT-commands ? \$\endgroup\$ – jay Jan 5 '14 at 14:28
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    \$\begingroup\$ Unless the modem is specifically designed to support two concurrent SIM registrations, it is unfortunately not possible to do what you ask. The multiplexer of choice for SIM interfaces is the Fairchild FSA2567 \$\endgroup\$ – s0nic2k Apr 22 '16 at 13:30

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