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Sunday, 19 February 2012Sunday, 19 February 2012

LTE CSFB performance in a live networkA lot has been written about LTE and the issues with supporting voice services. Nowthat the dust has settled it is clear that initially CSFB (Circuit Switched Fall Back) will beused and once some maturity has been reached in networks and devices, VoLTE (Voiceover LTE) will be introduced. My personal opinion is that this will be sometime towardsthe end of 2012 (the soonest) but most probably in 2013.

CSFB has been the victim of a lot of bad press and one of the main drawbacks of thesolution is the additional voice setup delay is causes while the UE has to "fallback" toUMTS/GSM (or even CDMA2000 1x) to establish the CS call.

I recently obtained some logs from a network that supports CSFB and I thereforethought it would be interesting to quantify exactly what sort of delays we are takingabout.

First of all though, a very quick overview of CSFB. From a network perspective, a newinterface is introduced between the MME and the MSC Server/VLR. This is is called SGsand is very similar to the "old" Gs interface that was defined in the specs quite a fewyears back to enable an SGSN to communicate with an MSC.

When a UE that supports CSFB performs a registration on the LTE network it will do sowith a combined Tracking Area Update/Location Area Update. The MME will process theTracking Area Update itself and forward the Location Area Update to the MSC Server/VLRvia the SGs interface. The VLR will process this Location Area Update as normal byupdating the HLR.

Now when a MT call arrives for the UE in question, the MSC Server will send the pagingmessage via the SGs to the MME which will forward it to the UE. The UE responds withan Extended Service Request that triggers the MME to instruct the UE to perform a CSFB(which essentially is a blind re-direction) to UMTS/other. Once there the UE has to readthe system information messages and initiate the call as normal.

With that quick introduction, let's have a look at a log extract and especially thetimestamps associated with each part of the procedure.

- at 01:27:40.218 the UE receives a pagingmessage which indicates that it originates fromthe CS core network (IE cn-Domain : cs)

- this triggers the EMM layer to send anExtended Service Request (IE Service TypeValue : (1) mobile terminating CS fallback or1xCS fallback) to the RRC layer which initiatesthe RRC connection establishment procedure

- once the RRC connection is established themandatory security procedures are performedfollowed by a UE capability enquiry and a RRCConnection Reconfiguration that configures themeasurement events and establishes the DRB

- once the above are completed the messagethat triggers the CSFB is sent which is just aRRC Connection Release with re-directioninformation instructing the UE to go UMTS (IERedirectedCarrierInfo : utra-FDD). Timeelapsed up to this point is 01:27:40.421 - 01:27:40.218 = 203ms

- the UE then takes a further 01:27:40:671 -

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Posted by radio access blogger at 20:16

01:27:40.421 = 250ms to detect andsynchronise with the UMTS cell

- the UE then reads the system informationmessages on the UMTS cell which takes01:27:40:984 - 01:27:40:671 = 313ms

- from here on the UE responds to the pagingmessage as it would have if it was on UMTS inthe first place. This means an RRC connectionis established, followed by a paging responseand the typical CS call set-up signalling flow.

The Alerting message is then received, 01:27:43:625 - 01:27:40:218 = 3s and 407msafter the paging message was sent on LTE.

For those people familiar with measuring call setup delays, 3s 407ms is a pretty goodtime and quite a few networks would be hard to achieve this natively in UMTS.

Even though this is just one example of a CSFB MT call, it clearly shows that call set-updelay is not impacted that much by CSFB. The delay purely associated with CSFB isonly (203ms + 250ms) 453ms.

It is also worth noting that in the rel9 version of the specs, the UE can be sent theSystem Information associated with the target cell in the redirection message which willspeed up the process even more.

A final point to make is that SMS is supported over the SGs interface so there is noneed for CSFB to send or receive text messages.

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5 comments:

Federico Descalzo 5 June 2012 20:32

Very interesting! Thanks for posting!

Reply

Anonymous 10 January 2013 16:51

Thanks for posting this!

Reply

Anonymous 7 February 2013 20:03

Good article.

Reply

Anonymous 4 March 2013 18:57

Good summary of the CS fallback feature.Thank you!

Reply

Anonymous 17 March 2013 16:06

good explanation, Thanks

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radio access blogger

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