What's the significance of a "zstty[01]: ring overrun"?

I just got hammered with a ton of these when I was ftping from
a remote site and from my PBase at the same time.

I see these occasionally, but this was outrageous!

I have a 57600 connection (via PPP) to the PBase (OT 1.1.1) - and
come to think of it, the throughput I'm seeing right now is
not nearly what it was doing an hour ago (5K/sec) - now it is
maybe 2K/sec.  The modem is a V.34+ (or whatever - 33.6).

NetBSD 1.2C (GENERICSBC-26) P450 (aka LCIII) 25 MHz '030 no
FPU - 20 MB real RAM + 20 MB swap.

I haven't had problems with a 57.6 crtscts link to the PBase
up to now, but at the moment, vi is having severe problems
keeping up with my (35 wpm) typing.

Oh - the connection to the PBase is via a printer cable.

J.