Pete,
When I used to do cellular coverage studies for mobile data services, I
found a bit more out about the operation of these things. Those that offer
CDMA as the network system have a piece that still depends on the old
DUN (Dial Up Network) dialer, that piece has a timer that will drop the
connection between the subscriber unit and the tower, but the overlaying
connection still remains. When you go to send data over the network there
should be a background piece that will get the link back between the sub-
scriber unit and the tower. The specification allows for up to 45 seconds
for that to happen (at least that was on 1xRT, not sure if the timers are the
same for EVDO). When we did CDMA testing we used two computers, one
to just set there and round trip messages to a server somewhere, gathering
latency and environment data and the second one that would establish a
connection and the let the DUN hang up, and then attempt to recover, it
had 45 seconds to do so, if it failed it got a error code, recorded it and
started over again.
If I recall, there was a way to set those timers so that you did not have it
drop your link too fast, but you should also have something in the back
ground that should scramble to bring the link back up if you have data
to send, and if the tower side has data for you, the link should also come
back up in the background.
The only problem that we found with this scheme was if you were on a very
loaded base station and your link went away, someone else grabed the
resource, when you went to recover, there was no longer any resources (
available links) to use and your system dropped the call with a "circuits
busy" or "no resources available" message. This very issue caused one
local law enforcement group to go with the GSM people bacause they
treat the network like a ethernet card does it's resource, there is not a
virtual circuit (the DUN) setup it just registers on the tower, if the card is
paid up then it is recognized, and life is good. If it has data to send it
attempts to do so, if there is a collision, it backs off and does so again,
you may get into a base that has a lot of activity but sooner or later you
will grab the channel and do your thing.
I have never understood why the CDMA people keep that old VC thing
around they have a much better (in my view) protocol than the GSM
system as they do not do hard handoffs from base to base so you never
see a problem handoff while the card tries to figure out who it is talking
to to pass data, but that stupid DUN thing can sure mess up your day.
Maybe it is because the telcos still have trouble understanding packetized
comms and the VC is easier to handle with it comes to billing, not sure,
but I would look at that piece as that was where the problems were
when we saw this sort of issues.
-- Chuck Hast -- KP4DJT -- To paraphrase my flight instructor; "the only dumb question is the one you DID NOT ask resulting in my going out and having to identify your bits and pieces in the midst of torn and twisted metal." ----------------------------------------------------------------------- This list is provided as an unmoderated internet service by Networked Knowledge Systems (NKS). Views and opinions expressed in messages posted are those of the author and do not necessarily reflect the official policy or position of NKS or any of its employees.
This archive was generated by hypermail 2.1.3 : Fri Aug 01 2014 - 16:52:22 EDT