Friday, July 06, 2007

Report-By-Exception? Maybe not

One of the PLC vendors I am just starting to deal with was bragging about the power of their report by expection abilities ... how wonderful for lower cellular data costs they claim. Unfortunately they only support TCP/IP - no UDP/IP. I'll cover this application (& name the PLC) more after I have some time to work at it.

However, the big problem with their "report-by-expection" is that the PLC holds the TCP/IP socket open 24/7 and sends a TCP Keepalive every 1 minute. Sending a TCP Keepalive (2 x TCP packet headers with no data) every 60 seconds costs about 3.4MB per month! I have a hard time seeing that as "low cost". They would be better to just poll real data every 4 minutes and eliminate the TCP keepalives!

Hopefully there is a setting in the PLC to change this behavior, which is why I need to look into this more. I just want to point out that just because a system claims to only move data during exceptions ... that does NOT mean it has very low cellular data costs. Don't forget that data costs include things you don't normally see - TCP socket open/close, TCP keepalive, TCP ack, and TCP retransmissions are all things you pay for.