Rick Gallahers MPLS Training Guide: Building Multi Protocol Label Switching Networks

Circuit switching by label is not a new practice. A quick review of telephony shows us how signaling was done in the “old days.” In the early days of telephone systems, telephone switchboard had patch cables and jacks; each jack was numbered to identify its location. When a call came in, an operator would plug a patch cord into the properly numbered jack. This is a relatively simple concept.

Recalling those days, we find that, although the process seemed simple enough, it was really hard work (see Figure 2.1). Telephone operators would attend school for weeks and go through an apprenticeship period before qualifying to operate a switchboard, because the rules for connecting, disconnecting, and prioritizing calls were complex and varied from company to company.

Figure 2.1: Label Switching in the Early Days

Here are some rules of switching:

• Never disconnect the red jacks; these are permanent connections.

• Connect only the company executives to the jacks labeled for long distance.

• Never connect an executive to a noisy circuit.

• If there are not enough jacks when an executive needs to make a call, disconnect the lower-priority calls.

• When the secretary for “Mr. Big” calls up at 9:00 a.m. to reserve a circuit for a 10:00 a.m.–noon time slot, make sure that the circuit is ready and that you’ve placed the call by 9:50 a.m.

• In an emergency, all circuits can be controlled by the fire department.

Essentially, one operator had to know permanent circuits (red jacks), switched circuits, prioritization schemes, and reservation protocols. When automatic switching came along, the same data and decision-making processes had to be loaded into a software program.