Minitel and Videotex
packet only needed 32 bits of information in total: four bits for the packet type, 12 bits for the virtual circuit identifier (although many of these bits weren’t used in practice), six bits for the sequence number, one bit to mark whether more data is present, and one empty bit that is always set to zero (INTERNATIONAL TELECOMMUNICATION UNION, 1996). There are another further 40 bytes used for framing with every packet sent: this link-layer data helps ensure integrity using a Cyclic Redundancy Check (16-bits), a kind of checksum. Compared to TCP, X.25 has quite a few similarities. TCP provides a virtual-circuit-like interface for data exchange – TCP requires a connection to be set up before use and torn down at the end. While TCP lacks the Permanent Virtual Circuit facility available by default in X.25, i t wasn’t particularly useful . All it allowed was establishing a long-lived connection to a host, which offered no particular advantages over a normal connection except simpler pricing. One thing that really sets the Internet Protocol apart from X.25 is the modularity it entails. With X.25, the developer was tied to using either Virtual Calls, which are slow to create, or Fast Select, which generally came with higher prices and was incapable of carrying more than 128 bytes of data per request (each of which had a response). Virtual Calls are the conceptual equivalent of TCP, while Fast Select is closer to UDP. TCP and Virtual Calls both provide strict ordering of data sent across the stream. In fact, X.25 and IP with TCP/UDP are very much alike in all but technical design and centralization. X.25 has a strict distinction between Data Terminal Equipment (consumer products) and Data Circuit-terminating Equipment. This can be seen in the full name of the X.25 protocol, ‘ Interface Between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) [...] ’ – the protocol is designed in such a way that all connections to the network have to be directly between a DTE and a DCE. An analogy in the modern internet would be that all connections to the Internet must go directly to the ISP, with no switch or gateway in between. At the time, this would have made sense, due to the lack of (relatively) cheap hardware – a ‘ dumb ’ terminal cost the French government 1000 francs, or £230 apiece (Hart, 1988), and even a household switch would have needed to be much more powerful than this. The lack of need for switches and the prohibitive prices meant that the protocol developed with this deep distinction, making it less versatile than the modern internet. X.25 defined a huge number of different options available to the network operator, as well as six different protocol versions. The large number of optional facilities led to fragmentation between countries and manufacturers. Each network (usually national) had its own specification and testing documents despite the X.25 protocol being strictly defined, making difficulties for manufacturers and consumers alike. After purchasing a new X.25 modem, you would have to configure it to match the DCE network it was connected to. While Minitel was successful in bringing technology to French daily life, it held back the adoption of the internet. However, it was a flourishing community with strict privacy standards despite state regulation, which in modern times more often has the opposite effect. The demise of the Minitel was driven by the adoption of the internet in the rest of the world. If X.25 had been simpler to implement and more widely compatible, it could have posed a real alternative to TCP/IP. However, the Minitel was always doomed to failure owing to its inability to function without a server – it was always just a terminal, rather than a
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