Monday, January 5, 2009

LTE handoff

I was reading LTE specs for a while now. I am more interested in core network of LTE. Well, iam an IP guy so my interests are in EPC. The pic below describes the LTE handoff scenario where source eNodeB knows the target eNodeB. I with my friend tried to simulate some LTE request responses. We are more interested in control plane signalling. So we picked up this call flow, removed the air interface portion and wrote a small client server programs for the rest.

This particualr call flow has both X2 and S1 interface control plane signalling. So after reading the X2AP(TS 36423-840) and S1AP(TS 36413-840) protocol specs for hours, i could finally figure out the packet structures. Then, we had to figure out SCTP stack. Since all the controlling is done on SCTP. Some google search and some test programs told me that Linux kernel version 2.6 and above have SCTP stack built in. I got the lksctp stack from source forge, complied the kernal and zoom. God, I love linux. Well you can get SCTP for Mac too and i got it in my mac. Thanks NKE SCTP!

Well we had everything in hand, SCTP sockets up and running, S1 AP and X2AP request response packets and nice ubuntu and mac pc's to write and test scripts. After couple of hours of colourful C, we had our test programs ready. Hurray!! Below is what we implemented

Next aim to implement GTP-U header!! Will take some time and lot of reading. Office is pretty relaxing these days, might help in speeding up things.


amisha said...


I am also trying to do something similar,but I want to simulate LTE handover based on signal level.

Can you please help me.

Santosh said...

Well...not an eNB guy. Sorry!

Nikolay said...

can you share this code, please?

Santosh said...

hey...i lost the code. Anyway the S1AP encoding was not correct!

kamal said...

When handover happens from one node to another, the point of attachment changes. The UE obtains new IP...and SGW acts as mobility anchor and route data to this new IP.

Please correct me?

Santosh said...

UE ip will never change during handovers!

Sergey said...

Hi all
I am implementing X2AP decoder and I should test it. Do you know where I can get X2AP captured traffic?

Santosh said...

No idea Sergey!