The Panel District Sender Selector Frame was used during call setup to connect an idle Sender to the District selector requesting. At call start the rotary switch would step around until an idle Sender was found. When this connection was established the caller would hear dialtone. As the caller dialed a number the sender would call in a Decoder to obtain routing information and start setting up the connection while the caller was still dialing. At the completion of call setup the Sender would 'advance' the District selector into 'talk' position. At this point the Sender is out of the call and the caller could hear the distant end.
A CGO District Sender Selector could handle 22 Senders. Where as a BCO Sender Link Frame Selector could handle 100 Senders.
During very high traffic it was possible for all senders in a given group to be busy. This caused the DSS switch to keep stepping round and round. If this continued too long other DSS switches did the same thing sometimes causing the cat to chase its tail and eventually over heating the DSS switch usually resulting in a DSS switch stopping on a busy Sender. This caused more problems. Stuck Senders. Less Senders available and so on... Not a pretty sight.
Bildquelle: Public Domain