I haven’t posted in a long time, partly because work has had me busy, partly because I haven’t been sure what to write. I have multiple drafts sitting unfinished for various reasons. The best way to get unstuck is to tell some old stories, as I like to do, and I was thinking this morning (for whatever reason) of an old friend called ISDN.
The old saying is of course, “with friends like these…” ISDN was, back in the day, an incredibly useful tool, but it was also a nightmare to configure. As a 2004 (year, not number) CCIE, I had to know it quite well, and I even had an ISDN simulator in my apartment. As a TAC agent, I took several ISDN cases. Long before DSL and cable modems, it was critical for early Internet deployments at smaller companies.
ISDN stood for Integrated Services Digital Network. It was a digital dial-up technology, a phone line that wasn’t analog. Phone lines were a common means of connectivity back in the day, but because they were analog, a modem was required to modulate the digital signal, limiting the bandwidth. 56kbps was about the most you could squeeze out of analog. Because ISDN was a direct digital connection, it could run a lot faster. 128 Kbps for the lower-end ISDN line, 1.5 Mbps for the higher-end.
The 128Kbps line was called a Basic Rate Interface, or BRI. We used BRIs a lot for smaller customers who wanted “dedicated” Internet connectivity. (More on that later.) 128K may sound like a snail’s pace now, but it was blazing fast in 1997. The Primary Rate Interface, or PRI, was just a channelized T1. I never saw it used for data, myself. If you wanted T1 speeds, you just got a T1. There was no point wasting overhead for the additional phone-line signaling. I did see them used extensively for voice, however.
ISDN lines had a per minute charge, so how did we use them for dedicated Internet access? Remember that ISDN lines were a dial-up technology. I don’t know about elsewhere, but in the San Francisco Bay Area, we used a service called “Centrex”. Back in those days, if you wanted a business phone system, you had a few choices. You could put in a “key system”, where you had a few phone lines plugged in to a phone system which also provided internal extensions. Nobody had their own phone number–they simply had an extension–and you would call the main number and get routed to the extension either through a human receptionist or an automated system. Larger businesses used Private Branch Exchanges (PBXs), and their users got their own dedicated phone numbers. For businesses that didn’t want to shell out for the expensive phone equipment on premises, our phone company offered “Centrex”. They provided a bunch of phone lines that were switched back at their central office (CO) instead of on prem. Each user had his own 7 digit number, but for internal calling, four-digit dialing was allowed. You could even do four-digit dialing to other sites if they were in the same Centrex group. Because these were business lines, there was a flat rate and no per minute charges.
OK, long-winded explanation, but here’s how it applied to ISDN. We used to put the customer and their Internet Service Provider (ISP) in the same Centrex group. This allowed the customer to four-digit dial (not a big deal) their ISP, but also avoided per minute charges. Then we’d just configure the ISDN router to never drop the circuit, and if it did drop, to always auto-dial back in. Voila, dedicated Internet via dial-up. We did this for several years until DSL came into the picture, making it dramatically easier to connect. (I was the first person I knew to have an ADSL line installed at my apartment. I remember the phone guy, butt set hanging off his toolbelt, sitting at my Mac trying to figure out how to configure TCP/IP. I told him to just leave me the subnet info.)
The other major use case for ISDN was backup for larger circuits. When I worked at the SF Chronicle, we had 20 or so sites on a frame relay network, the de facto standard before MPLS. What to do if an FR circuit went down? Running a second provider and redundant circuits was cost prohibitive. So, we gave all sites ISDN BRI backup lines. The routers were configured to phone home via the BRI if the FR circuit went down. This was harder than it sounds.
There were quite a few ways to make ISDN backup work. For the CCIE exam, you had to master the nuances of all of them. Simply getting the BRI to dial was one problem–getting the routing protocols, broadcast, and multicast to work were additional challenges. My notes from the CCIE exam are a confusing mishmash of different trivia items for getting it to work.
- On the one hand, you could do a floating static route. Let’s say the FR was your main connection and your default. You’d just point another default with a higher admin distance. If the first route dropped and the new one came up, it would trigger a dial event.
- You could do a dialer watch list, where you configured an access-list with certain IP addresses to watch. If they were lost, it would trigger the dial-up.
- You could add the ISDN interface as “backup-interface” under the main serial interface. In this case, it would only trigger if the interface actually went down, but it was a simple method.
- There were more, but if you read this far you probably don’t want to hear them.
Then there were dialer maps and dialer strings under the BRI interface itself. Goodness, no, I don’t even want to think about it.
The good news is, in networking we are always solving the same problems in the same way. Once you learn what tunneling is, you see it used from the earliest days on networking to modern EVPN fabrics. Most “controllers” are just rebranded NMS. And wide-area networking is about optimal path selection with constrained bandwidth, and determining when to select any given link. In many ways, SDWAN is just solving the same sort of problems we were solving with ISDN backup links.
The very idea of dial-up might be unthinkable to newer engineers who’ve never had a land lane. Then again, I’ve heard of some folks using their Starlink in much the same way. (Apparently it’s fast but expensive.)
I’m sure there are a few ISDN circuits still running out there, but I’ll tell you this much: I’m glad I don’t have to work on them!