This next installment I'm going to discuss the various things that can cause you headaches with your ISDN connection and how to identify and address these problems.
One very important piece of info that is useful for diagnosing problems with your ISDN connection is a realization that there is very little difference between ISDN and a regular analog connection. The only real difference is in the connection speed and the lack of sound.
Often times you'll find that when you try to call your local ZipLink ISDN number, or any ISDN data number, you get a "NO CARRIER" signal almost immediately. There are a couple things that can cause this and it is usually easily fixed. In some areas it is necessarily to dial the full 10 digit number you plan to dial preceded by a "1" (e.g. 14082491433). Also, if you have a Centrex line, you will undoubtedly have to precede all that by a "9."
Another thing to check is if you are configured for 64K or 56K from your CO. In some areas you might be configured from the CO at 56K and if your box is configured for 64K you'll get that "NO CARRIER" signal even though you may be able to make a voice call on you ISDN line. The easiest way to check for this is to try it and 64K and, failing that, try it at 56K. You should also note that even if YOU are configured for 64K on both your box and at the CO, the number you are dialing may not be. This can be seen occasionally when a certain area of the phone companies area still has some older equipment or software, or your call gets routed through another provider which is configured for a slower speed.
The next potential problem I'd like to address is that of older UART chips.
"Universal Asynchronous Receiver/Transmitter (UART) An integrated circuit used for serial communications, containing a transmitter (parallel-to-serial converter) and a receiver (serial-to-parallel converter), each clocked separately...... An example of a late 1980s UART was the Intel 8450. In the 1990s, newer UARTs were developed with on-chip buffer space for data. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the Intel 16550 has a 16 byte FIFO."-Free On-line Dictionary of Computing http://wfn-shop.princeton.edu/cgi-bin/foldoc
This isn't entirely accurate. This definition implies that any computer manufactured after 1989 is going to have a 16550 UART chip, which is just not the case. A good rule of thumb is, if your computer is more than a year and a half old (and, really, even if it's brand spankin' new) you'll want to check the UART chip and make sure it's a 16550 or better. If it's a 16450 or 8250, you'll want to upgrade your serial chip. Otherwise you'll be limited to 9600 bps (basically you'd have a $300+ paper weight).
Symptoms include, Comm overflows, high initial speed then dramatic drop offs. Better performance out of your 14.4 modem.
Another issue to look at, in brief, is that of Multiplexing. First note that Multiplexing does not mean BONDING. The two are not the same thing.
"multiplexing(Or "multiple access") Combining several signals for transmission on some shared medium (e.g. a telephone wire). The signals are combined at the transmitter by a multiplexor (a "mux") and split up at the receiver by a demultiplexor. The communications channel may be shared between the independent signals in one of several different ways: time division multiplexing, frequency division multiplexing or code division multiplexing." -Free On-line Dictionary of Computing http://wfn-shop.princeton.edu/cgi-bin/foldoc
"BONDING is a set of protocols developed by U.S. inverse multiplexor that supports communication over a set of separate channels as if their bandwidth were combined into a single coherent channel. For example it supports a single 384 kb/s data stream over 6 64 kb/s channels."
-comp.dcom.isdn faq http://www.cis.ohio-state.edu/hypertext/faq/usenet/isdn-faq/faq.html
You will be multiplexing over isdn using MultilinkPPP as defined in the rfc1618 document: http://www.cis.ohio-state.edu/htbin/rfc/rfc1618.html So you need to set your software and hardware accordingly. With BONDING, there could be but one channel that is transmitting, while with Multiplexing there are multiple channels involved as a part of the definition.
And now for a potentually confusing discussion.
This next section is to deal with the different switches out there in ISDN land. As I'm sure you are all aware, there are basically three different ISDN switches available in the USA. The AT&T 5ESS (which comes in two basic flavors, the point-to-point and the multi-point), the DMS-100 and the NI1 (or National ISDN). There are other possibilities with in this, but these are the three types that I'll be concerned with here.
The AT&T 5ESS is a nice and easy one, especially if it's point-to-point. You can simply leave the SPID entries blank with the point to point. You need not fill in the DN entries either, though you will undoubtedly want to have them there (you won't recieve incoming calls without 'em in most cases). With the multi-point configuration on the AT&T 5ESS switch, you have to have the SPID's entered. In Pac Bell territory, AT&T 5ESS SPID's are usually have a preceeding 01, the seven digit phone number, and then a trailing 0 (e.g. 0155512120 where 5551212 is the DN). Note that a SPID can be whatever the phone company decides and that the conventions here are simply conventions, they are not written in stone.
The DMS-100 switch is a bit more difficult to configure in that several of the ISDN devices have problems with them and DMS-100 users usually consume larger quantities of asprin in the configuration stage of their ISDN experience. The DMS-100 needs both SPID's entered into the equation. In Pac Bell territory the SPID's will undoubtedly look like the following:
4155551212xy
The "xy" is the TID for the b-channels. The user gets to decide what the TID is within a certain convention. Typically it will be either 00, 01 or 02. You have two SPIDS so it will be some combination of that, usually either 00 and 01 or 01 and 02. The phone companies will discourage you using 00, so you can usually get by with 01 and 02. So, if I had two DN's of 2618835 and 2611927 my SPID's for a DMS-100 could be:
408261883501 for the first SPID.
408261192702 for the second SPID.
Finally there is the NI1 or National ISDN. It has, by far, the most intuative SPID and DN convention of all of them in Pac Bell territory. The SPIDS will usually be the 11 digit phone numbers for the seperate bearer channels so that your SPID will be the same as your DN but with a 1 and the area code in front of the number (e.g. 14155551212).
In Nynex land you'll only deal with the National and AT&T switches. Their SPID convention is to use the 11 digit phone number followed by 4 0's. There are other possible confusions in the ISDN world, but this covers the basic issues you will undoubtedly come accross.