Data Usage / Billing - Any explanation?

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MainMeat

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Hi (vodacom3G) guys,

Perhaps there are some theories or hard facts about this issue, and if there are any logical explanation it would be much appreciated!

I have come across and issue with the data usage and billing on the VodaCom GPRS network. My testing was done using two telemetry devices reporting with data via the GPRS network to a Central Server Environment where the data is logged. These Telemetry devices can be configured to report data from every 'n' seconds up to once every several hours. For my testing I configured these units to report once every 60 seconds (or once a minute). Two scenarios are described below:

Scenario 1: The units connect to the GPRS network, sends a small packet of data and then 30 seconds later disconnects. When it has reached the state where the units should report again (effectively 30 seconds later), it reconnect and sends the packet of data again. Total bill over a 12 hour test came to R8 (using Prepaid sim cards)

Scenario 2: The units connect to the GPRS network, sends the same data but this time the connection is maintained and the unit will only reconnect if the connection via the GPRS network fails for some reason. It therefor does not do a intended disconnect and reconnect. Total bill over a 12 hour period came to R0.48 (using the same Prepaid sim cards)

The conclusion is that sending the same amount of data over a 12 hour period, but having to reconnect once every minute causes the data usage bill to be 16 times higher than just maintaining the connection? I understand there will be some additional overhead for the connection to be established every time, but can the difference in Data costs be this high?

Questions that come to mind immediately is: What if the GPRS node / network fails and the units reconnect as intended, or the units move from one GSM cell to another and the hand over fails - will I then have to carry the cost of reconnecting every time? What happens if I connect, and disconnect on a frequent bases using the mobile connect data card - am I paying for data transfer I am not aware off? :confused:

Comments appreciated and thanks for a great forum!

(PS. V3G I reported this to the data help desk and it was logged as no explanation could be provided at the time)
 
There is a bit of handshaking involved in the background as you realised. Also, every time you connect you retrieve a new DNS server settings, a new DHCP IP etc

Usually these few kb are not an issue, but your test indicated the overhead of the device was dropping constantly.

Using static settings would save a bit of data traffic, but the actually handshaking of AT commands will still happen.
 
I don't disagree that there is hand shaking in establishing a TCP Socket connection to a server, and clearly that is data used by me, but having this difference in data usage does not make sense at this time.

I also agree and I don't see why I should be charged for connecting to the GPRS service (if this is the case).

Perhaps the Vodacom Technicians on this forum can provide more feedback. My main concern is that where the connection hops from one GSM cell to the next the probability of having to reconnect to the GPRS server is high and the socket might be lost - and every time this is costing me the above mentioned amounts...
 
It is indeed the handshaking overhead that you're seeing and is normal operation for GPRS.

Before you get an IP, you need to connect to the network and get configured onto the network. This layer-2 process (setting up the PDP-context) uses some data.

Then some IP-related activities might also take place, resolving a URL, etc.

When you disconnect, a similar process happens as the PDP-context is torn down.

GPRS was designed to be an always-on connection and this is how most people use it. So normally you won't even notice the few handshaking bytes in the session as the data usage itself will be much greater.

But in applications like telemetry, where the data itself can be very small, this can become noticeable as you've discovered. Many telemetry systems were originally designed for dial-up, only connecting when needed, and when these moved to GPRS, this quickly became apparent for the developers.

The solution is to keep the GPRS connection and not tear it down.

Also watch for using other channels like SMS. On some modems the PDP-context is torn down to send the SMS and is then rebuilt. So even if you keep the IP connection open, there might be extra data being used. This is modem-specific.

So, if you keep the connection open, but still let the units report every 60 seconds, you'll sort this.
 
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Hi V3G -

Thanks for the info. Pitty that the overhead consumes so much data...will try and keep the connection open as long as Vodacom Allows me.

Can you confirm there is data handover between cells should the unit move from one GSM cell to the next, or is the probability high that the socket connection will fail and a reconnect will be required?
 
MainMeat said:
Hi V3G -

Thanks for the info. Pitty that the overhead consumes so much data...will try and keep the connection open as long as Vodacom Allows me.

Can you confirm there is data handover between cells should the unit move from one GSM cell to the next, or is the probability high that the socket connection will fail and a reconnect will be required?
Click to expand...

There is handover, just like with your normal handset. But sometimes it might happen that the new cell is fully utilised and the call might drop. Again just like with your phone, but this is not frequent.

I've not calculated the number of bytes used to connect and disconnect, but it won't be high. What makes it noticeable, in your case, is the high frequency of connects / disconnects + the low payload count.

It's exactly the same in normal IP comms, you have protocols such as ARP, BOOTP, DHCP ICMP and DNS that all consume bandwidth before you can actually work. But because you do it once per session, the impact is small.

What is your application, BTW? Obviously mobile? I kinda act as the telemetry interface for Vodacom, so if you need any help, please shout.
 
vodacom3g said:
What is your application, BTW? Obviously mobile? I kinda act as the telemetry interface for Vodacom, so if you need any help, please shout.
Click to expand...

It's a device providing GPS coordinates and some additional data via GPRS and using a standard TCP Socket to upload the data.

Thanks for info and offer to help. I will certainly keep this offer in mind and will get in contact with you when required!
 
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