Saturday, December 19, 2009

Happy Holidays

Time to take a break. I will be off for next two weeks. See you all in the new year.

Monday, December 14, 2009

LTE: Mobility Management States

Manish from Layers7 blog is writing a series of articles on LTE mobility management states. There are two posts on his blog that discuss LTE EMM and ESM. Go ahead and check them out!


Sunday, December 13, 2009

LTE: Inter RAT Handover (E-UTRAN to UTRAN)

Below is the case where UE moves from LTE network to 3G network.


Source network is LTE and target network is 3G. Target SGSN communicates with MME over S3 interface. GTPv2 is protocol used here. This means existing 3G network SGSN's need a software upgrade. (GTPv1 --> GTPv2). We also assume that a LTE SGW is serving the 3G network. This means that SGSN again uses S4 interface to communicate with SGW.(GGSN is no longer required?) Below is the call flow and is self explanatory. I am considering that Indirect Data Forwarding tunnel is used. Details are in 3GPP TS 23.401 Chapter


The messages are almost similar to what S1 handover uses. Only change is communication between RNC and SGSN which is RANAP based. Does this mean that no software upgrade is required in RNC?

I am interested in two things here;-

NSAPI: NSAPI is the identifier used to identify a control plane tunnel/PDP context. EBI is used in LTE for the same purpose. So when the UE moves from LTE to 3G, EBI is mapped to NSAPI. However this mapping is not complex as both NSAPI and EBI are 4 bit values. Also they both start with integer value of 5.

QoS:- QoS values are quite different in LTE when compared to 3G. EPS QoS to pre release 8 QoS mapping is explained in 3GPP TS 23.401 Annex 5.   

Tuesday, December 8, 2009

Secondary PDP context and Direct tunneling!

LTE dedicated bearers are something similar to secondary PDP contexts from GPRS world.

Direct tunneling is one of the release 7 features which says the user plane path could be directly from RNC to GGSN. This will avoid user plane processing in SGSN.

My question is how many service provider networks have implemented direct tunneling? Also who all have provided the support for activating secondary PDP context? I believe most of the smart phones do have support to activate secondary PDP contexts, but how many are actually doing it?

Could somebody please enlighten me in this regard? If the information is confidential please feel free to email me. I shall not disclose the information anywhere, it is for my personal study. Thanks!

Wednesday, December 2, 2009

LTE: PMIP & GTP based interface mapping

We all know that S11 interface is GTP based while S5 interface could be GTP or PMIP based. If 3GPP TS 23.401 specifies GTP based interface then 3GPP TS 23.402 gives out PMIP based S5 interface details.

So I had this question for a while:- In PMIP based S5 interface there is only user plane tunnel per UE, while GTP based S1-U has several tunnels which we call as default and dedicated bearers. So how are so many tunnels with different QoS schemes mapped to single tunnel over PMIP S5 interface?

My theory:- For GTP based S5 interface PCRF communicates to PGW but for PMIP based S5 interface SGW interacts with PCRF. The interaction here is for enforcing the qos values on the tunnels. That is PCRF informs SGW what are the QoS values it should enforce in the downlink for user plane. Lets look at below figure.

Consider this. There are 3 bearers established, one default and two dedicated bearers. The uplink/downlink tunnel id's are as shown in figure. On S5 interface uplink/downlink GRE keys are exchanged. PCRF gives out QoS values that SGW should enforce on the GTP tunnels. Dedicated bearer 1 is associated with TFT 1. Say this is for HTTP traffic and TFT consists of Remote Port information, i.e Port 80. Dedicated bearer 2 is associated with TFT-2. TFT-2 indicates FTP traffic, i.e Remote Port Range 20 and 21. Corresponding TEID's are shown. So SGW happily enforces the QoS rules on the GTP tunnels. But how are these tunnels mapped over S5 interface which is PMIP based?
This is what I think. SGW pushes all the information that is coming on GTP tunnels 0x01, 0x02, 0x03 to GRE tunnel 0x11. The uplink should be blind and SGW shouldnt have anything to worry. But downlink is little complex. The data is coming to SGW over 0x0z GRE tunnel. Now SGW has to map this information into one of the three GTP tunnels. This is where it uses the TFT. SGW reads the incoming data and it forwards the data to HTTP tunnel if it sees that data is coming from port 80, else to FTP tunnel if data is coming on port 21 or it will blindly send it over default bearer. (Does deep packet inspection makes this possible?)
So the SGW mapping should be something like this. All the GTP tunnels ID's mapped to single GRE tunnel and TFT information used for segregating data in downlink.
Correct? Any other thoughts/ideas? Please feel free to comment.