X.25

~ the goodbye ~

X.25 arrived in my life as a diagram, which is the wrong way to meet a network. by the time i was sitting in a computer engineering department it had become the thing you were taught right before somebody explained why nobody used it anymore. its working life and my time online overlap by about fourteen years, and in every one of them it was underneath me rather than in front of me.

the CCITT finished the standard in 1976 and published it in what everyone called the Orange Book, with a lot of the drafting work coming out of France, where Rémi Després had already built a packet network that worked. the premise was the virtual circuit. you asked the network for a connection to a specific address, the network set up a path across its own switches, and after that you had something that behaved like a private line even though your packets were sharing wire with everybody else's.

every national phone monopoly with any ambition built one. Transpac in France, Datapac in Canada, Telenet and Tymnet in the United States, PSS in Britain. by 1987 Transpac was the largest public packet network anywhere on revenues near four hundred million dollars, and it carried Minitel underneath, which is the only reason millions of French households ever touched a data network at all. this was the internet's job being done by the phone company, with a bill attached.

Turkey got one in 1989. the PTT called it TURPAK and it was the country's first packet switched data network, running X.25 alongside SNA and a few other things nobody misses. i never dialed it because it was not sold to households. it was sold to institutions, to banks and to ministries, and the account came with an address somebody assigned you. by the time i had a modem in 1998 the thing i wanted was the internet, and the internet reached me over a phone call to an ISP, no address required and no permission asked.

~ the chronicle ~

~ what you were actually paying for ~

the design assumption in 1976 was that the lines were bad, so X.25 checked and retransmitted at every single hop instead of trusting the two ends to sort it out. that is exactly why it was reliable and exactly why it was slow. once the lines got good, the entire safety apparatus turned into overhead you carried for nothing. frame relay took the same shape and threw the checking away. IP threw away more than that and put the recovery at the endpoints, where it turned out to belong all along.

the other thing that dated was the meter. X.25 service was billed by connection time and by volume, so an application that chattered cost real money, and the software of that era is full of abbreviations that exist for no reason except that somebody was paying per character. flat-rate transit between networks killed that pricing model faster than it killed the technology, and the technology never recovered from losing it.

~ the turkish tail ~

TURPAK outlived the network it is usually compared to. Transpac closed in june 2012 and the Turkish one kept running for another four and a half years, until the regulator signed off on Türk Telekom's request to end the service at the close of 2016. somewhere in there is the real shape of a protocol death. not an announcement and not a switch being thrown, just a slowly shortening list of customers who have finally been moved onto something else, and then a regulatory filing to make it official.

~ the long tail ~

strictly speaking it is not gone. aviation kept X.25 on ground links long after everyone else quit, because certifying a replacement costs more than running the old thing, and amateur radio still moves packets over AX.25 on the air. neither of those is a network in the sense that Transpac was a network. they are habitats. the standard has outlived every service that made it matter, which is the fate of a protocol that was good enough to be built into things that are expensive to change.

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