Saturday, January 30, 2010

LTE End to End Signaling:- Part 2

I wrote about LTE End to End Signaling some time back. Based on public demand I enhanced the call flows a bit :-). Here are two calls showing Initial Attach and UE initiated detach procedures.

LTE UE Initial Attach


This is a huge call flow and there is a lot of processing involved. I would like give a brief out line of messages.

S1 Setup: This is where eNB is attached to the network. As long the eNB is functioning the S1 setup remains.

RRC Connections: Once UE comes up a RRC connection is established for communication with the network.

NAS: After RRC is established then begins the NAS signaling. UE sends Attach request along with PDN connectivity request to network. Attach is for attaching to the network and other message message is for establishing the bearers.

HSS: This is Home Subscriber System and it understands diameter protocol. Once MME receives Attach Request, it queries HSS for authentication details. HSS sends the authentication vectors to MME in Authentication Info Answer.

Authentication/Security: Networks requests UE for Auth vectors. Once UE provides the same MME compares the same with what HSS has sent. If they match UE is authenticated. Next is security. After the security all the NAS messages are encrypted using the security algorithms that were exchanged.

Bearers: After security network creates the EPS bearers first. (GTP messages). Then the radio bearers are created and RRC connections are modified accordingly. Once the radio bearers are created eNB downlink addresses are sent to SGW in GTP messages.

LTE UE Initiated Detach


UE decides to detach from the network. Its sends a detach request message to network. Network deletes the EPS bearers then the radio bearers are torn down. Finally RRC connection is released.

I hope the messages are correct. Comments are always welcome.

Thursday, January 14, 2010


With LTE gaining more light I guess most of the network manufacturers are ready with basic attach, default and dedicated bearers, X2 and S1 handovers. But LTE as standalone will not make sense as it will deployed in just few places. Rest of the world will be still running on 2G/3G/CDMA networks. So there is a strong need for 3G/CDMA convergence with LTE which means inter RAT handovers. I think Europe will see 3G convergence with LTE while Verizon is working hard to converge their CDMA network into LTE in US. So, may be, this year will be about convergence and figuring out a proper solution for voice over LTE.

My interest here is SGSN. SGSN is a major signaling element in 3G packet core. It runs RANAP/NAS protocols towards RNC and GTPv1 towards GGSN. If 3G is to be converged into LTE then there has to be a software upgrade on SGSN as it needs to speak GTPv2 over S4 interface with LTE MME. This makes SGSN much more important and signaling intensive device. I am sure that both operators and equipment manufacturers are looking closely at the SGSN's now. The question here is how will the operators choose which 3G networks should support LTE and which shouldnt. Imagine this. I have a UE which has APN configured with a LTE PGW. While in roaming I may not find a LTE network so I will have to switch back to available 3G network. Now, that 3G network should be capable enough to support my UE's existence in the network. This means SGSN should go all the way to talk to LTE MME, pull the bearer contexts, map the QoS and serve the UE. This needs quite a lot of processing. Will existing SGSN be capable to do all this? This will probably need much powerful hardware on SGSN. I am very keen to see how things will go in this direction. I am hoping that Mobile World Congress this year will have some answers. Any thoughts?

Friday, January 1, 2010

Thank you all!

Let me start with wishing you all a very happy and peaceful new year.

I started this blog as a new year(2009) resolution to keep track of my work. This day marks first anniversary of the blog. I never thought it would be useful to anybody. The idea was to make a record of what I was learning. What I was writing did make sense to some of you and I am quite happy about it. As of now the blog has around 125 subscribers and close to 50,000 hits. This blog goes into my achievements list.

This year I would like to continue writing. I moved a bit closer to technology which means the posts would be more accurate and simplified. In the process of writing blog I did learn a lot from many of you and also made some very good contacts. I take a moment to thank you all for the support. I also request you to let me know if any changes or enhancements are required to the blog. Any suggestions or comments are greatly welcomed.

Cheers, Santosh