[ntp:questions] NTP and SNTP in the end, give me precision of 1ms?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Sep 14 23:06:43 UTC 2008

Unruh wrote:
> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>> A 1 baud/second link would have a continually varying latency, and, 
>> unless the bits per signalling unit varied to compensate, a continually 
>> varying delay.  I don't think baud/second was the intended unit.  I 
>> suspect he was suggesting a 1 bit per signalling unit, 1 signalling unit 
>> per second, case.
> Nope, 1 bit per sec as an off the top of my head estimate of the rate for
> an ultra longwavelength radio link with a submarine under the ocean.

I don't think you get the point.  A baud is a signalling unit per 
second.  A signalling unit can carry many bits, and in most transmission 
systems does carry more than one bit.  As I said, I think you are 
assuming the degenerate case where one signalling unit carries one bit, 
and the baud rate and bits per second are the same, and equal to 1.

Incidentally, it would make a lot of sense to me if the strategic 
submarine communication systems used multiple bits per signalling unit.

Examples of bits per second and bauds differing are:

56kbps modem (8k baud).

ADSL (each sub-carrier carries a bit, so there is quite a large ratio).

QAM modems, 1200 baud and 9600 bps.

When marketing and popular computing literature refers to the higher 
speed as the baudrate, they are only correct for the baseband signal, 
not for the signal over the wire.  The actual signalling unit rate is 
important for propagation delay, though, especially over synchronous 

Note packet length needn't matter, on a single hop, as you can calculate 
the start of packet time when working out the transmit and receive 
timestamps, even if the reference implementation doesn't do that.  It 
does in a store and forward environment, but note that I believe some 
more sophisticated routers can start forwarding as soon as they have 
enough information to select the outgoing link.

More information about the questions mailing list