Strowger subscriber access design approaches

In a Strowger (step-by-step) exchange, a calling subscriber must first be connected to a free first group selector that will receive the first dialled digit. Two design approaches are used for this subscriber access:

1. Uniselector (line switch / pre-selector) approach

 Sub A ─> [Uniselector A] ─┐
 Sub B ─> [Uniselector B] ─┼─> free 1st group selector
 Sub C ─> [Uniselector C] ─┘    (outlets of uniselectors)

• Every subscriber has his own uniselector (a single-motion rotary switch) at the exchange.
• When the subscriber lifts the handset, his uniselector rotates and hunts over its outlets until it finds a free first selector, then stops and gives dial tone.
• Fast access (no searching for the caller), simple control.
• Costly: one switch per subscriber, though most subscribers are idle most of the time; poor equipment utilisation.

2. Line finder approach

 Sub A ─┐
 Sub B ─┼─ bank of ─> [Line finder 1] ─> 1st selector
 Sub C ─┘  contacts   [Line finder 2] ─> 1st selector
 (100-200 lines)      (few finders shared)

• A small number of line finders (two-motion switches, each permanently paired with a first selector) is shared by a group of 100–200 subscribers.
• When a subscriber goes off-hook, an allotter assigns a free line finder, which searches its bank (vertical then rotary) to find the calling line and connects it to the first selector.
• Fewer switches are needed because the number of finders is set by traffic, not by the number of subscribers – cheaper for low-traffic lines.
• Slightly slower (search time) and needs an allotter/control circuit.