LTE CS Fallback (CSFB) Issues Around SMS

While doing some research on CS Fallback (CSFB), the method currently favored by 3GPP to bring voice and SMS to LTE, I came across this discussion paper (SP-090429) initiated by Vodafone, China Mobile, Alcatel-Lucent, CATT, Huawei, Starent and ZTE. In the paper, the authors point to a number of issues with CSFB concerning SMS delivery and think about an LTE native implementation of SMS. According to the SA#44 meeting report, it looks like there was a heated discussion over it but no real agreement was reached on how to move forward.

A quick CSFB intro before moving forward: In its core, what CSFB does is to establish a signaling channel between a circuit switched MSC and the LTE core network. This way, mobiles currently attached to the packet switched LTE network can be informed about incoming circuit switched voice calls and SMS messages. In the case of voice calls, the mobile jumps back to 2G or 3G coverage and accepts the call. SMS messages can be directly delivered over the signaling link so no fallback is necessary. In that respect, CS fallback is not quite the right description for that part of the functionality.

The main issue with CSFB apart from having to fall back to a legacy technology for a core application is the increased call establishment time due to the fallback. It has been estimated that even in the best fallback case, this adds at least 1.5 seconds to the process. In many practical cases, it's likely to be longer. As call establishment times in mobile networks are already significantly longer than in fixed line networks, adding yet another source of delay is very undesirable.

In addition to this issue, the document lists a number of other issues, especially around SMS. Here are some examples:

• Availability at launch: There is no guarantee that CSFB will be available at the launch of LTE as at least one circuit switched component, the Mobile Switching Center (MSC) has to be enhanced to support the new signaling interface (called SGs).

• Roaming: It's not guaranteed that CSFB will be available in a foreign LTE network the mobile roams into when the user travels to another country.

• Deferred Delivery: When an SMS can't be delivered immediately, like for example when the user has temporarily no coverage, an SMS waiting flag is set in the network. When the mobile is back in the network, the MSC is aware of the waiting message and delivers it. Unfortunately, this mechanism is not available with CSFB as the Mobility Management Entity (MME) in the LTE network does not inform the MSC that the mobile is back.

• LBS: Location services based on SMS and Cell-ID interrogation are not possible with CSFB as LTE cell-id's can't be used in the circuit switched part of the network.

• Resource Use: For SMS delivery the MME and the MSC are involved in addition to the SMSC. From a resource point of view, to many network nodes are used.

• Specification Issues: It looks like there are some gaps in the current CSFB specification. One of them concerns SMS delivery during an ongoing fallback procedure due to an incoming call just before the SMS is delivered. Another, potentially even more significant issue, is concatenated SMS delivery, which is also still missing in the specification.

• Test Scenarios: And finally, the current test plan for early LTE implementation does not include CSFB SMS test scenarios yet.

Quite a list of issues. While some can probably be fixed in short order, others are likely to be a bit more tricky. In combination with roaming and the EU mandate to inform users of roaming charges, hot-billing info, activation / deactivation of features and many other applications using SMS, it's clear that something has to be done to fix the issues. If native SMS over LTE is the solution, well, we'll see. Given the result of the discussions in the meeting report, this solution doesn't seem very likely to me.