Caps (was Re: AT&T UVERSE Native IPv6, a HOWTO)

Jeff Kell jeff-kell at
Mon Dec 9 06:02:54 UTC 2013

On 12/9/2013 12:48 AM, Jay Ashworth wrote:
> A 3270 that took 5 seconds of delay and then *snapped* the entire screen
> up at once was perceived as "faster" than a 9600 tty that painted the same
> entire screen in about a second and a half or so.  Don't remember who it
> was either, but likely Bell Labs.

This is a "screen/block" mode I/O issue versus a character-mode one. 

And the "screen/block" I/O won't start until the whole screen data is
there, so there is an initial delay.  The character-mode variant will
paint portions of the screen as the data arrives.

Similar anomalies exist on input... the screen/block mode is buffered
locally and proceeds normally; while the character mode version has to
transit the WAN link, whatever it may be.

I won't argue that one is better than the other, depending on your link
speed (transmitting a whole screen will incur longer delays than
transmitting individual fields, though admittedly it happens "less"
often).  But the user perception goes a long way...

I have seen advantages to both, having done serial termainal
applications from back to the 1970s, and won't argue one way or the
other.  You choose your poison.  With 3270 you have little choice other
than "full screen" transactions.  For other ASCII terminal interfaces,
you could optimize the individual fields (while paying the "full screen"

There are "user perceived" throughput values, "transaction perceived"
throughput values, and "application perceived" throughput values.  And
very rarely did the three equal out for every application :(


More information about the NANOG mailing list