separate tandem switch.

Connection of trunks to incoming registers and outgoing senders is not through the four-stage voice fabric. Rather it is through a dedicated single-stage crossbar network known as incoming register link (IRL) or outgoing sender link (OSL) respectively. Registers and senders are in groups of ten, assigned one to each level of as many crossbar switches as are appropriate to the traffic they can handle. Different trunks are wired to different IRLs or OSLs depending on what