S120 throughput

[Matej]

Hi to all!

I have few questions about throughput when using s120:

1. What is the max number of packets that could be send/received in one connection interval when using S120? (s120 specification, chapter 13 says 1tx/1rx, Hung Bui in post says 2tx/1rx, other definitions i didn't find...)
2. A bit more tricky... In my project, my device connects up to 5 devices, some of them are smart devices. Now I'm wondering what is the best way of setting up connection interval?

Considering that smart devices have different limitations e.g. if I set min connection interval to 20 ms and max to 50ms then some of "smart" devices will set connection interval to 37.5ms. There is no problem until some other device can't communicate at such interval (Is this possible?). Who or what gurantee to me that devices will have a common factor? If not probably there will occure some collisions and connections could get dropped if supervision timeout is not realy long long.

If i refer to equation in s120 specs, chapter 11.4 where is written that connection interval should be >= "Number of devices" * Teeo + scanWindow + Tscan_reserved... During testing with 20ms connection interval some of devices get disconnected. Increasing it up to 50ms helps and connections get stable, but now is throughput 400KBps (1 packet per interval * 20 * 1/0.05) and it's to low... Is there something i can do (maybe more sophisticated way to set connection intervals) or this is the max what can i reach with s120?

Thank you!

BR Matej

[Hung Bui]

@Matej:

1. On S120 we do have 2 buffer for TX but the scheduler limited it to only one packet per connection interval. So in general it's correct to say that it only support 1 packet per conection event both TX and RX. I will update the case. Thanks for reporting.

2. Could you let me know how many devices have you connected ? and the configuration for scan window you have ? It's pretty strange that you got your devices disconnected. The scheduller is designed so that if the connection going to have timeout, it will have the highest priority and will preempt other connection event. Futhermore, if they all 20ms they should not cause packet drop. As shown in Figure 15 in the S120 spec.

I would suggest you to record a sniffer trace to see what happens over the air. I suspect high rate of packet drop due to interference.

[Hung Bui]

@Matej: There could be dropped packet, but that doesn't mean the data will me missing. The dropped packet will be retransmitted (until timeout) and it's transparent to the application. What you may experience is the data throughput may be lower. The connection that may have timeout will have highest priority and it's packet won't be dropped.

Regarding the suggested connection interval, you can have a look at section 11.4 in the S120 specification. I think 20ms connection interval is recommended.