UDP - really "connectionless"?

[JacoV]

Hi there,

I ran some simple tests to see if UDP packets do get dropped between a GPRS modem and the Cell tower. I used two GPRS modems and transmitted a file from the one to the other. Both modules were in close proximity, so I take a very rough guess that the data did not traverse on the backhaul (Telkom?) network, and that there were very little routers in between (major cause of packet loss on normal networks, due to buffer overflow during congestion).

So my speculation is that the lower layer protocol(s) (Below IP) used for GPRS must be quite robust and reliable and optimised for the radio link. I would have expected quite a number of packet drops due to the RF nature of the transmission. Does anyone know if there is quite an elaborate FEC scheme used in the lower layers? Does anyone maybe know more than just my wild guessing?

I must admit, the GPRS modems use quaterwave Marconi antennas, and the signal strenght is quite good: at+csq = 31.

Kind regards,
Jaco

 

[ic]

Even if the underlying GPRS network does attempt to reduce packet loss, I really don't recommend transferring data files using UDP. UDP is fine for streaming and other situations where missing packets aren't going to cause corruption in a data stream, but for transferring data files UDP is a definitely not a good idea, rather use a TCP protocol.

 

[vodacom3g]

Jaco, you can read up on the protocols used in the GSM spec. Pretty well published. Google will get you going.

In general, I agree with ic, use UDP where you don't need handshaking at the Transport layer, otherwise use TCP.

And yes, UDP is really connectionless.