GPRS EXperiments

Hi List! As there isn't so much about GPRS on here, I though I might write something about experiments over the last couple of weeks with data inside and outside of the lab. I've installed up to date versions of osmo-bts and osmo-pcu on sysmobts 2050 hardware and it's working great! Dynamic channels are really nice, with half rate TCH and AMR working perfectly. Thanks for all that work! The question for this experiment was if it was going to be feasible to actually do anything with several hundred data hungry spying devices.... I mean mobile phones on the network. For the traffic control, I setup a local blacklisting or whitelisting dns server. I've tried both. Blacklisting the worst culprits would be nice, but in practice I think I'll have to go for whitelisting only the intended permitted services. I configured pcodns1 in the ggsn to point to this DNS server, AND I setup a port 53 redirect to catch quite a lot of traffic from android that likes to just talk directly to 8.8.8.8 anyway. In the wild, some dns request analysis reveals the worst culprits (this is a very basic analysis) appear to be all the google update stuff, play store etc, facebook  iCloud,  (all to be expected) , and certain CDNs. Some research shows that these CDN companys specialize in delivery of advertising content inside apps such as mobile platform games. Such is the sad state of affairs on today's internet. Fortunately, we have iptables and ip sets and we have AS blocks assigned to certain bandwidth hungry corporations :-) So turns out it seems quite feasible to supply service for text messages with certain popular IM services to many phones. Short voice clips worked quite well in the lab tests,  although support for media such as pictures and videos was not so great. I have yet to successfully send an image (sourced via device camera within the app) over a "secret" chat with Telegram messenger. As this is not a very low level report, rather intended as some light reading :) I also have a question in a similar light vein. I'm still getting to grips with the log messages available in the pcu , the sgsn and not so much the ggsn, and I'm observing and learning the sequence of events, so at some point I should be able to present a better report about this with some relevant traces and better analysis. For now, In the lab tests I am constantly monitoring the RF uplink; I observe that a phone will attach and then go quiet. A foreground running app may report that it is "connecting" or some such, and the little arrows may be flashing to show that apparently we are transmitting data, but there is nothing on the uplink. My guess here is the OS has sent something and the baseband is saying yes yes doing it.. but the baseband at the same time is waiting for something from the network (and not getting it)? This situation can persist for some time.. several minutes. I have observed that if I initiate any data transfer from the network side then the uplink is established. By the same token, If I transfer a file from the network (http download or some such), the same applies. The link stays active and the IM chat session is very responsive alongside the file transfer. Shortly after the file transfer completes, the uplink is quiet again and the latency in the IM session becomes a problem. start an IM chat, send a message.. but then we get to this quiet uplink sitation and the messages stop sending.. so from the user's point of view it's frustrating. the phone looks like it's transmitting.. until there are timeouts and disconnections and the app may show some indication to the user that it is having trouble connecting to the network. However if I run something on the network side like a script that sends one ping to the phone every ten seconds, this keeps the connection 'alive' and the IM session is much more satisfactory for the user. I should note I believe I observe this also on commercial networks  in some places like certain Berlin U-bahn stations where you can still find (only) GPRS data coverage. Also, a more scientific report is needed, but I seem to observe some phones behaving "better" than others, as in being a little more active on the uplink. Maybe it is related to power saving configuration? The not very low level and scientific question here is: Is this kind of thing tunable with gprs parameters? Any tips on which ones to play with? ( Quite happy to wait until I can send a more useful  report too! ) Thanks! Keith.