Subject: Re: Multiple net interfaces
To: Greg S. Burd <>
From: Joe Greco <>
List: current-users
Date: 03/01/1994 18:59:17
> Robert Shady Writes:
> 	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.

I had a FreeBSD 386DX/16 (16550) talking SLIP with a FreeBSD 386DX/20
(16550) at 115200bps hardwired, and got 8K/9Kcps doing FTP's to and from the 
DX/16 to a Sun on an Ethernet connected to the DX/20.  Both cards were
STB DSP/550 el-cheapo $29 cards in ISA systems.  (yes, 8000cps, I was rather 
suprised myself)  It dropped to about 5000 or 6000cps when I ran transfers
simultaneously in two directions.

The system loading did not appear to be prohibitive, as the DX/20 is my
"terminal server" and was handling a 14.4Kbps modem link during these tests.

I believe Telebit's NetBlazer (which I believe is based on 386sx technology)
is quite capable of providing the "dual SLIP or PPP" type of link described
above.  (reference:

I don't know the IP details and haven't hacked IP code, so I cannot offer a
qualified opinion on whether or not this is feasible within the 386BSD
paradigm.  However, I _can_ safely state that 3840cps is trivial to transmit
over an ISA bus, and as far as the serial communications goes, it is

... Joe

Joe Greco - Systems Administrator
Solaria Public Access UNIX - Milwaukee, WI			   414/342-4847