Ever seen ATTs “Only AT&T's network lets your iPhone
talk and surf at the same time” ad? If not here is the youtube link.
Ever wondered why so much emphasis on network rather than
phone? Well, this is a new feature on network . Not new exactly; this
feature has been there since Release 5. Good to see that it is actually rolled
out to production; otherwise it seemed very stupid to own a smart phone.
Ok! Here is how this is made possible. When a phone is
switched on, it goes and attaches to the network. If phone is data capable, the
phone does PS attach towards SGSN and CS attach towards MSC. Basically there
are two separate radio control channels for data and voice and a phone can be hooked on
to either one of it at a give time (not an expert on radio though). So if you are browsing, and MSC is paging the
phone for a voice call, phone has to disconnect the GPRS session and receive
CS paging over CS signalling channel which does make a smartphone look stupid.
For this sake 3GPP has done enhancements on network side. On the network side they introduced a new interface between
MSC and SGSN, the Gs interface. This interface has been upgraded to SGs and
further to Sv for CS Fallback and SRVCC features respectively. The additional Gs
interface made phone to connect to both CS and PS service with one attach
towards PS, which is referred to as combined attach. When a phone is powered on
it looks for PS network and sends PS Attach Request with CS parameters towards
SGSN. Based on the attach request, SGSN figures out the location area from
routing area, finds out the MSC based on location area and sends location
update towards it for CS attach. So with single attach UE is connected to both
CS and PS services.
All is good. Now assume a case where phone is using PS services
and has an incoming call. MSC sees the incoming voice call and sends a CS Page
request to SGSN, because that is where the location update request came from,
instead of UE directly. Upon receiving the CS page, SGSN sends paging request
towards MS over PS channel with CS indicator. Since the paging request has CS indication the
phone prepares of a CS call. Phone responds to the paging and MSC pushes
the phone call toward RNC via CS voice channels. But the best part is phone can
still have its PS connection continued while still having a CS connection. The
trick is to not make phone listen on both CS and PS channels instead use one signaling
channel on radio side and make SGSN and MSC communicate for CS services. I
tried to capture the same in above call flow. This is a brief overview, there
are many more procedures inside this for feature to work flawlessly. Also the network mapping
between location area and routing area needs to done correctly. It’s all easy for a
green field operator, but networks like ATT does need a lot of time and money.
Hence the ad :-)
For more details, refer to 3GPP TS 23.060
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