HSDPA And Simultaneous Circuit Switched Voice Calls

In theory, 3GPP standards allows simultaneous HSDPA and circuit switched voice calls on a DCH. The standard even describes how the terminal can signal the maximum speed to the network it supports on a DCH while an HSDPA session is ongoing. In practice, however, this does not seem to be implemented yet.

While testing such a scenario with a Motorola V3xx in practice I noticed that an incoming circuit switched voice call makes the radio network interrupt the HSDPA session, put the data connection on a dedicated channel (DCH) and then opens an additional dedicated channel for the voice call. During the voice call data transmission is possible with 64 kbit/s in one network  (Tim, Italy) and 128 kbit/s in another (SFR, Paris). In a third network I tested this scenario the incoming call was not delivered at all. As this network has not officially launched HSDPA yet, I grant them anonymity.

After the voice call is over I would have expected that the networks put the data connection back onto HSDPA. I was quite surprised that this was not the case. Instead, the DCH was upgraded to 384 kbit/s. It would only go back into HSDPA mode once the data transfer is interrupted. While this is not a problem for web surfing, where many interruptions occur, this behavior is very undesired for file downloads. Here, the file download will continue but never be put back on a faster HSDPA connection.

The picture on the left shows the result of the test run. At first the download speed is around 75 kbyte/s. Then a voice call is started. This shows on the graph with the short interruption of the data traffic followed by a download speed of around 7 kbyte/s (64 kbit/s bearer). After the voice call is over the DCH speed increased to around 40 kbyte/s (384 kbits/s). The second part of the graph shows a repetition of the test (to make sure I was not dreaming).

Looks like it is still early days for HSDPA. While switching from HSDPA to DCH during a voice call could be both a network or a mobile implementation limitation, not switching back to HSDPA after the call very much seems like a network limitation to me.

One thought on “HSDPA And Simultaneous Circuit Switched Voice Calls”

  1. Hi Martin, it seems you have discovered first hand something that operators have been frustrated with from the early days of HSDPA. As it turns out in the rush for deployment of HSDPA networks a lot of RAN manufacturers took the easy road and did not implement any (Ericsson) or not full (Nortel, Nokia) HSDPA + voice functionality.. These limitations are being addressed in the second wave of HSDPA roll-outs (a lot of them coinciding with HSUPA introduction) together with code multiplexing, up to 15 HS-PDSCHs channels and other “fine tuning” features.

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