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Mon, 16 Apr 2012
3G User Plane

Continuing with my Node-B experiments I started to implement handling the user plane. I can now do voice calls, video calls and data connections. Just to make it clear, those are the very first steps only without any optimisation for speed and far from being complete or ready for release. I consider it as the essential "building blocks" necessary for the next steps towards a small Open Source RNC.

So how does it work? One way to allocate a channel for user data is to modify the existing signalling channel by sending the RRC "Radio Bearer Setup" message to the phone and the NBAP "Radio Link Reconfiguration Prepare/Commit" messages to the Node-B. Besides being much more complex due to the number of possible parameters and options this is somehow comparable to TS 04.08 "Channel Mode Modify" and A-bis TS 08.58 "Mode Modify" in GSM. If the commands succeed the Node-B will allocate another UDP port for the User Plane data besides the existing UDP port for the Control Plane data (UDP ports because I use "Iub over IP" to talk with the Node-B).

The User Plane data are embedded in FP (Frame protocol, TS 25.427). For voice the data are the AMR class A, B and C bits. To ease testing with a single phone I use a loopback which simply sends the received uplink AMR class bits back to the downlink.

Signalling for a Video Call is very similar to a Voice call. The main difference is in the CC Setup message, the bearer capability indicates UDI (Unrestricted Digital Information) for Video. UDI is basically a 64 kbit/s channel which transports video according to the 3GPP umbrella protocol 3G-324M) for video telephony. Loopback is not as easy as for voice because first the four-way H.245 protocol handshake has to be completed to agree on communication parameters, after that the uplink video data can be resent to the downlink.

Although UMTS is generally much more complex than GSM/GPRS, handling of data connections is easier than for GPRS because there is no need for a PCU (Packet Control Unit) with all its complexity. There is the optional protocol PDCP (Packet Data Convergence Protocol, TS 25.323) on top of RLC which is used for things like header compression. But if you don't use it, you get the raw IP packets embedded inside RLC.

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