What is ISDN?
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ISDN is a communication protocol use to establish a networking connection.
It works somewhat like a modem – a modem generates and listens to sound in order to transmit data. ISDN works over a digital phone line and instead of audible sounds it uses electronic pulses.
Other than that it’s pretty much like another modem. It connects to a service provider and links your computer or your network to whatever the service provider accesses.
ISDN was sometimes used for household internet (I did in the 90s) but was/is more common for either direct connections between business locations or for connecting remote users into a private business network over a dedicated phone line.
I’m pretty sure it’s dropping in popularity rapidly – it has a lot of limits – cost and speed being two big ones.
Eli5 explanation: it is two 56k dial up modems bundled together. One of the modems does upload the other download. A traditional modem could only send datat one direction so it needed to wait to stop recieving then pause to make sure before sending. Combining the two more than doubled the true transfer speed and is considered the first high speed modem.
Fun fact, scott Adam’s the creator of Dilbert cartoons was one of the engineers that worked on the isdn lines creation.
ISDN is a phone operator service that requires specialised equipment in the phone exchange to function (well, not that specialised any more, but surely WAS special) that gives you two separate full-duplex 64kbit connection channels and one 16kbit control channel that sends all the relevant data required to connect, uphold and disconnect.
One of the awesome perks of it was that the connection had a guaranteed quality statement (well, my phone operator had that, at least. They refused ISDN for all customers who couldn’t fully benefit from it) which means that the internet speed WAS 64kbit in full duplex, compared to the modem speeds I had with a 56flex, that were 56kbit AT BEST downlink and most of the time 33.6kbit uplink.
It also created the interesting benefit that it could uphold two connections at the same time (which meant either 128kbit internet speed OR 64kbit internet speed AND a phonecall OR two phonecalls simultaneously.) and all the phonecalls where digitally transferred so the sound quality was comparably AWESOME and basically interference free at all times, which wasn’t really a countryside perk I was used to in the past.
I know of a lot of companies who hooked up on a ISDN service because they only had one pair of wires into their office and wanted to add a company fax machine. Or just wanted to be able to accept two phonecalls at the same time.
But to me, the thing was the guaranteed 64kbit connection. And the second-long handshake compared to the boring 35 second handshake the analogue 56flex modem did every time it connected.
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