Mobile Terminated Roaming Retry for LTE CSFB

One of the many issues with LTE Circuit Switched Fall Back (CSFB) is what happens when the UE falls back on the target RAT (can be 3G or 2G, operator defined) and the LAC is not the same as the one the UE is registered on through the Combined Attach procedure in LTE. This typically occurs on the borders of Location Areas (LA) and Tracking Areas (TA) or when the target layer is 3G and the UE can only acquire 2G (e.g. indoors).

In these cases, a Mobile Originated CS call would experience a delay as the UE would first have to perform a LAU procedure on the target RAT followed by the subsequent call setup procedure.

A Mobile Terminated CS call on the other hand would fail as the call has already been routed to the "old" MSC (i.e. the one the UE registered on though the Combined Attach procedure) but the UE finds itself in the LAC of a different, "new", MSC.

To prevent this happening two 3GPP defined procedures can be re-used. The first one is called Mobile Terminated Roaming Retry (defined in rel.07) and the second one is called Mobile Terminated Roaming Forwarding (defined in rel.10). Both of these require an upgrade to the CN elements involved in the call setup procedure.

It is interesting to note, that both of these procedures were defined originally to handle the (rare) occasion of a UE being paged in one LA but at the same time moving through a LA border and thus performing a LAU procedure on a different MSC than the one being paged on.

For this post we will have a closer look at the Mobile Terminated Roaming Retry (MTRR) procedure.

The signalling flow is shown above (click to enlarge) and can be broken down into 4 distinct phases.

During phase 1, a MT call comes through the GMSC which initiates the MAP procedures to inform the MSC the UE is registered on (through the Combined Attach procedure). The MSC contacts the MME through the SGs interface which pages the UE. The paging action initiates the CSFB procedure and the UE is directed towards the target RAT.

Under normal circumstances the UE would fall back on the LA it registered on and would send a Paging Response. In this case however the UE falls back on a different LA and thus has to perform a LAU procedure. This is shown in phase 2, as is the subsequent procedure to transfer the UE subscription from the "old" MSC to the "new" one.

This is also where the call setup procedure would fail as the "old" MSC would not have the ability to inform the GSMC that the UE has changed MSCs.

With MTRR however the "old" MSC informs the GMSC through the Resume Call Handling procedure that the call setup procedure should be repeated by contacting the HLR once more. This is shown in phase 3 of the signalling flow.

Finally, phase 4 is a repeat of phase 1, but this time the GMSC contacts the correct "new" MSC and the call setup procedure is successful.

It is also important to note that during the LAU procedure in phase 2, the UE populated an IE indicating that a CS MT call was in progress. This ensures the new "MSC" keeps the signalling link towards the HLR for the subsequent signalling exchange in phase 4.

Obviously this whole procedure would also add a couple of seconds more on the overall call setup time, which due to the general CSFB procedure is already longer that a "native" CS call setup in 2G/3G.

In the next few posts I will look at the second option, MTRF, and also at a completely different approach that uses an inter working function between the MME and MSC in order to avoid any upgrades in the legacy CS network.

RRC state machine in Vodafone Greece

As I travel I find it interesting to look at how operators configure their networks. I travel to Greece quite often and have noticed that for quite some time now, Vodafone Greece do not use CELL_FACH or CELL/URA_PCH on their network.

This is illustrated in the log extract above. A PS Radio Bearer is established with no subsequent data transfer. After 20s of inactivity the RRC Connection is released and the UE returns to IDLE. So essentially the RRC state machine that Vodafone Greece use consists of two states only. CELL_DCH and IDLE. This is quite strange as there are a number of advantages in using CELL_FACH and CELL/URA_PCH. Obviously for small amounts of data (keep alive messages etc) CELL_FACH can be used without consuming dedicated resources. When there is no data transfer, CELL_PCH or URA_PCH can be used where no radio resources are consumed, the UE can go into DRX and at the same time allow for a fast transition (i.e. low latency) back to CELL_DCH if data transfer is required.

I find it hard to think of a reason why someone would configure their network this way, as it seems very signalling intensive and high latency. Unfortunately my test device did not support Fast Dormancy rel8 so I could not see how the RNC would react if the UE indicated it had no further data to transfer.

What is 3G?

Ok, so my posts are usually a bit more detailed than this, but I was looking at the Google Search stats for 2012 and it seems the question "What is 3G" was the 3rd most searched "What is..." type question (1st was love and 2nd was iCloud in case you are wondering). So here is my attempt to explain the mysterious 3G..

3G refers to the 3rd Generation mobile technology standard. But if we are talking about the 3rd Generation, what were the 1st and 2nd?

The 1st Generation was based on analogue technology (like your FM radio) and appeared around the 1980's in a few countries. The mobile phones were enormous, the batteries even bigger (sometimes external to the handset) and they were very expensive. I imagine very people reading this post ever used a 1st Generation analogue mobile phone. 1st Generation networks are a thing of the past now and none exist anymore (at least in the so called "developed" countries).

The 2nd Generation was based on digital technology (like your digital TV) and appeared around the beginning of the 1990's. The most popular 2G standard is called GSM (Groupe Speciale Mobile) and is still incredibly popular today. I imagine the vast majority of people reading this post have used and are still using GSM. 2G technology standards were initially developed to support voice calls. The transfer of data was a bit of an after thought and the GSM standard was further enhanced with GPRS (General Packet Radio Service) and then EDGE (Enhanced Data rates for GSM Evolution).

So now we come back to 3G. The 3rd Generation was also based on digital technology and appeared around the beginning of the 2000's. The most popular standard is called UMTS (Universal Mobile Telecommunications System) and it too has proven incredibly popular. The main benefit of UMTS over GSM is that is supports much higher data rates so the web browsing, downloading, tweeting etc happen much faster. The UMTS standard was further enhanced with HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) to further increase the data rates possible. Of course if all you are interested is making voice calls and sending some text messages, there is not much benefit in 3G as 2G will do that just fine.

So 1G has disappeared and we are left with 2G and 3G. Most phones today support both standards and most operators, support both networks. These can be thought of as layers, like the graphic above. A dual standard phone will have a preference to camp on the 3G layer (point 1 above). When the 3G layer is not available (typically 3G networks use higher frequencies and thus don't propagate as far) the mobile will transition to the 2G layer. This can happen both in idle and during a voice call or data session (point 2 above). Once 3G coverage improves the mobile will re-select back to the 3G layer.

As an end user you can tell which standard/layer you are using by looking at the icon next to the signal strength bars on your phone. Unfortunately the actual icon itself is not standardised so mobile phone manufacturers usually pick from a collection. The possibilities are:

For 2G it is typically "2G", or sometimes just "G" or sometimes "E" (for EDGE), or "O" if you are using an iPhone (Apple, go figure..)

For 3G it is typically "3G", or sometimes "H" (for HSDPA/HSUPA), or "3G+" (again for HSDPA/HSUPA), or even "H+" (for some further enhancements to HSDPA)

A bit confusing, but you get the idea.

We are now at a time where 4G networks have started appearing. These are based on a standard called LTE (Long Term Evolution) and support even higher data rates. This can be thought of as just another layer on the graphic above, and a 4G capable phone will have a preference to camp on the 4G layer and when that is not available on the 3G layer and when that is not available on the 2G layer.

That is my attempt at answering "What is 3G?". Hopefully it makes some sense!