Legacy Datacom Technology:  X.25, Frame Relay and ATM

Legacy Datacom Technology: X.25, Frame Relay and ATM
This module describes data communication and networking technologies that were formerly mainstream but no longer in wide use, if at all.  

The idea of a virtual circuit, called a Forwarding Equivalence Class in MPLS lingo, goes all the way back to the 1960s and X.25 data packet networks, which all telephone companies built and sold services on.  Envoy-100 email from Bell Canada ran over Datapac; parts of the Internet ran over TymeNet in the very early days, both were commercial X.25 networks.

Access speeds to X.25 were 1.2 kb/s dial-up modem or for the institutional customer, 56 kb/s data circuit.  The latency, i.e. delay caused by the network, was outlandishly long compared to today's networks.

Frame Relay was a speed improvement over X.25, mostly running at 1.5 Mb/s, but did not come with a guarantee of maximum latency and maximum packet loss necessary to be able to guarantee voice quality when communicating phone calls in packets over Frame Relay.

Asynchronous Transfer Mode (ATM) was supposed to be the answer to all of our problems, carrying phone calls, television, data and Internet traffic together with suitable guarantees for each.  But it got so ridiculously complicated and expensive, it got thrown away and replaced with what we have today: IP packets with MPLS to implement virtual circuits.

In this module are the three predecessors to IP/MPLS.

And lest we forget, "Asynchronous Ports" on PCs to connect modems and mice, communicating one byte at a time:  start bits, stop bits and parity checking.

Telecom 101 Module 19
Detailed Outine

19 Legacy Datacom Technology: X.25, Frame Relay and ATM
19.1 “Asynchronous”: Start/Stop/Parity
...... 19.1.1 Asynchronous Communications
...... 19.1.2 Framing: Start and Stop Bits
...... 19.1.3 Parity Checking
19.2 X.25: Packet-Switching using Virtual Circuits
...... 19.2.1 X.25 Network Structure and Operation
...... 19.2.2 Reliable Network Service: Guaranteed Delivery
...... 19.2.3 Connection-Oriented vs. Connectionless Network Service
19.3 Frame Relay
...... 19.3.1 Elimination of a Layer of Software
...... 19.3.2 Unreliable Service
...... 19.3.3 Network Structure and Operation
...... 19.3.4 No Guarantees for Voice
19.4 ATM
...... 19.4.1 Future-Proof Technology (Not)
...... 19.4.2 ATM Cells
...... 19.4.3 Service Classes