Date: Mon, 15 Dec 1997 09:38:49 -0800 From: "Kevin L. Brokaw" <kevinb@relay.csd.SGI.COM> To: freebsd-isdn@FreeBSD.ORG Subject: Re: [Fwd: What kind of serial card should I use with ISDN and FreeBSD] Message-ID: <34956B29.97070B13@csd.sgi.com> References: <3494BEA2.238F2F07@horizon.ppp.ripco.net>
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Jeremy McMillan wrote: > The 16xxx number refers to the type of FIFO buffer chip (UART) that takes data > from your computer's processor and pumps it out a serial port interface. > Higher numbers mean later standards and higher speeds. A 16450 will not accept > any more than 9600bps. A 16550 will take 57600 bps at a time, and later UARTs > will allow higher speeds on up. I would have to disagree with that overall description of the chip model versus interface speeds. An original 8580 UART is pretty much limited to 9600 BPS, but a 16450 UART can theoretically run at a DTE rate of up to 115,200 BPS. The difference between the 16450 and the 16550 is that the 16550 adds a FIFO buffer. Both the 16450 and 16550 are capable of handling the same serial port rates, but with a machine like a 486/33, you are going to drop data on a 16450 port at high rates of speed, since the CPU most probably can't cycle quickly enough to service the serial port interrupt in a timely manner. The FIFO buffer added in the 16550 allows for some latency in handling the interrupt without losing data. I have been using a 16450-based serial board on an AMD 5x86-133 system for some time, at a port rate of 38400 without losing any data. It's simply a matter of how fast the system is, and how loaded it is. It's a crapshoot. > > > The ISDN adapter I'm using is a 3Com ImpactIQ and it says that with > > compression it can get speeds up to 230kbps. I'm getting about 85kbps with > > it now. > > In general, compression is advertised at about a 2:1 ratio, and thus I'm > guessing your TA has 2 "B" (data) channels at 64Kbps each. Your computer can't > even use the bandwidth of one of these channels at 9.6Kbps. Your serial port > is a major bottleneck. Another place to look is to the PPP protocol negotiation taking place. Is compression even being enabled on the link? There are so many different compression algorithm available under PPP, that a non-optimum one may be selected in negotiation, or if the client only supports one (VJ, for example), and the host does not support that particular algorithm, the link may in fact be running with no compression at all. Most modern PPP implementations can achieve optimum compression close to 4:1. Depending upon your system setup (and if you're using a 16450 chip, this is advisable), it may be best to disable all hardware compression in the ISDN device, and rely upon software compression within the PPP protocol. This will take some of the load off the serial port while still maintaining decent throughput. > > > The 3Com ImpactIQ came with advetisements for serial cards with 16750 UARTS > > on them. Do these work with FreeBSD? Does anyone have any opinions on them? > > You need one to get your money's worth out of your ISDN line. Get one, but > shop around. If you had bought an internal TA, it would have had it's own UART > built in. If you bought an ISDN router, you could have just plugged the thing > into your favorite ethernet port and you'd be practically up and running. You > took the hard way, and now you have to worry about adding a serial card with > all of the address line/IRQ/DMA problems associated with it. I hope your 486 > is cooperative, though you will likely get it working without unreasonable > pains. There's no *technical* reason I see why a 16750 card won't work, since you'd simply replace the existing serial port board with it, and presumably use the previous existing IRQ and I/O port address (most serial boards don't use DMA). However, I don't see support in my release (which is still 2.2.2, so I'm not sure about 2.2.5) for a serial rate above 115200. That may be a limiting factor in getting full-bore throughput. I would agree with the comment above that going with an ethernet-based solution like a low-end Ascend Pipeline would probably be a better solution for full rate throughput. -- Kevin Brokaw (kevinb@sgi.com) | Cynic (SIN-ick) n. : Member, Technical Staff - WebForce | Someone who, upon Silicon Graphics Computer Systems | smelling the flowers, Customer & Professional Services | looks for the casket.
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