[ILUG] ISDN

[Niall O Broin]

On Sat, Sep 09, 2000 at 04:42:44PM +0100, Kenn Humborg wrote:

> > ISDN TAs connected to a PC's serial port offend my engineer's 
> > soul. ISDN is
> > a synchronous technology. There are no start and stop bits (so 1000 bits
> > down an ISDN line = 125 bytes of data, whereas 1000 bits of data down an
> > RS-232 cable = 100 bytes of data). So, a PC using a serial TA is busily
> > converting an 8 kbyte/s data stream into asynchronous serial 
> > data, which is
> > being re-converted into synchronous data by the TA. This leads to poorer
> > latency figures with a serial TA than with a PCI/ISA ISDN card. 
> 
> Well, to be fair, this conversion should be happening on 
> a byte-by-byte basis, therefore the additional latency should
> be of the order of a byte time, ie (1/11520) = 87uS.

This only counts the serial transmisssion time and not whatever time is
taken for processing in the TA and the PC. Also, the latency of 1 byte is of
academic interest - the latency of a whole packet is what matters. For
example, a ping with the default packet size of 64 bytes suffers from
additional latency of 64 x 87 x 2 (there and back) =~ 1 ms. That doesn't
sound bad, but there must be other factors involved because I know I've read
on some game site that there's quite a difference in ping time between
internal ISDN cards and external serial TAs - can't find a reference,
unfortunately. Somebody else (Vin ?) who does actually use ISDN for gaming
mentioned this earlier AFAIR.


Regards,


Niall