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Sun, 04 Aug 2013
Tracing LTE on the phone

As a short additional note to my first experiments running an LTE eNodeB: I wanted to know how this looks on the phone. The protocol I am interested in is RRC (Radio Resource Control) for LTE (specified in TS 36.331). RRC also transports the NAS messages which are exchanged between the eNodeB and the MME.

The USB LTE Dongle I use for my experiments is based on a Qualcomm chipsets. I used Qualcomm based phones for quite a while so I already know some parts of how tracing works on them. I was able to locate the LTE messages in the traces and extend GSMTAP slightly so that it is able to handle LTE RRC messages too.

Here is how it looks in WireShark when a phone registers to the LTE cell:

The red colored lines are uplink messages (from UE to eNodeB), the blue lines are the response from the eNodeB. The trace also contains a few of the System Information messages the phone has to decode to get all the required information to communicate with the eNodeB. Otherwise you can see that the NAS messages exactly match those seen on the S1-AP interface (of course, this is expected).

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