Subject: Re: Multiple net interfaces
To: Greg S. Burd <>
From: Robert Shady <>
List: current-users
Date: 03/01/1994 22:03:38
> 	Right.  So does the Livingston Portmaster Terminal Server, but I
> 	don't have one of those, hence the question.  Somehow, there must
> 	be a way to connect two SLIP/PPP devices to a dial-up SLIP/PPP
> 	provider and have 2*(modem speed) throughput (or more!).
> This is impossible for several reasons.  First, the nature of IP routing.   
> Second, the speed at which your system can operate.  SLIP and PPP generate a  
> huge amount of over head even with a really good serial card.  Unless you get 
> a serial card which does DMA, VESA, or PCI data transfers, the processing  
> overhead will saturate your 486-DX4 or even your Pentium.  38400 BPS is lot  
> of information to move across an 8, 16, or even 32 bit bus with all the  
> interrupts, etc.  Even if the serial driver is "smart" enough to do larger  
> transfers, the overhead is a problem.  This is why MorningStar Technologies  
> has something called SnapLink(tm).  It is a SCSI serial device designed to  
> handle T1 bandwidth.

Please.. I *KNOW* it is possible to run several serial ports across an ISA
bus without slowing things down one bit.

First, let's take into account a standard ISA-BUS rate of 8Mhz.  At 8Mhz,
the BUS can theoretically transfer data at 128,000,000 bits per second,
or 16,000,000 bytes per second.  Take into consideration the that there
are 18.2 clock interrupts per second, that allows for 879,121 bytes per
second.  If you allow for a 50% CPU overhead, that still comes out to
439,560 bytes per second.  If you are assuming the use of 16550 FIFO's on
your serial cards, filling up ~10 bytes per interrupt, you can effectively
get a maximum of 4,395,604 bytes per second throughput, if you lock each
port at only 38,400 -- You can have up to 114 serial ports running a solid
38,400 bytes per second (theoretically of course, under ideal conditions).
If given worst case scenario, you should be able to handle at LEAST 64 
connections at 38,400 baud with the appropriate hardware.  Now, let's talk
about how many people have their bus speed bumped up to ~12Mhz, such as
myself... ;)