Crosspoint Switches in Early Digital Telephony
During the 1970s, the Bell System revolutionized telecommunications through Stored Program Control (SPC) exchange systems. While data storage had evolved beyond the need for reed relays, these components remained critical for voice path switching. To manage millions of connections, engineers developed sophisticated crosspoint switch arrays that could route voice signals with high precision and efficiency.
The Evolution of Reed Relay Technology
In the 1ESS switch, engineers implemented the Ferreed method. Unlike standard relays that latch electrically, Ferreeds utilized a magnetically remanent alloy, allowing the relay to latch magnetically. This innovation significantly reduced power consumption and enabled both contacts to be utilized for the voice path.
The wiring of these coils mirrored the logic of magnetic-core memory, employing coincident current selection. In this configuration, activating the contacts for a specific crosspoint would automatically release other crosspoints within the same row and column, ensuring a clean and singular connection.
[ไม่มีภาพประกอบ]How the Ferreed Mechanism Works
The operation of the array relied on a complex interaction of P leads (control leads) and magnetic polarity. Each input and output of the array featured a P lead that controlled the crosspoints on that specific level. Two coils on each crosspoint were wired in series to these leads.
The system functioned through magnetic repulsion and attraction:
- Repulsion: When a pulse passed through a level, the windings on the ends of the reeds were magnetized with the same polarity (north-to-north or south-to-south), causing them to repel and open the crosspoint.
- Attraction: The selected crosspoint received current from both its input and output P leads. Because the windings were opposed and of unequal strength, the resulting magnetic force was north-to-south, attracting the ends and closing the contact.
Once the connection was established by the pulser, the P leads remained "dry," and the crosspoint stayed closed until a new connection involving that level was required.
Comparing Reed Systems and Crossbar Switches
The architecture of reed arrays differed significantly from traditional crossbar switches. Because individual reed crosspoints were more expensive than crossbar components—though the control circuitry was cheaper—reed arrays were typically smaller in size but more numerous.
This design necessitated more stages to complete a connection. For example, a telephone call in a 5XB crossbar exchange typically passed through four switches, whereas a call in a 1ESS reed system typically passed through eight.
| Feature | Crossbar Switch (e.g., 5XB) | Reed System (e.g., 1ESS) |
|---|---|---|
| Typical Switch Stages | 4 | 8 |
| Crosspoint Cost | Lower | Higher |
| Control Circuitry Cost | Higher | Lower |
| Latching Method | Electrical/Mechanical | Magnetic (Ferreed/Remreed) |
Advanced Iterations: Remreed and Crossreed
The subsequent 1AESS system introduced the Remreed design. By using remanent magnetic materials, the system further reduced its physical footprint and power requirements. These were packaged in "grid boxes" containing 1,024 two-wire crosspoints, organized as two stages of eight 8×8 switches.
However, reliability became a challenge. The use of silver plating rather than more expensive precious metals made these arrays less reliable than crossbar switches. To maintain service, failed grid boxes were replaced as entire units and then repaired offline.
Similar technologies were adopted by other manufacturers and regions. Stromberg-Carlson developed the ESC system using crossreed relays, and the British TXE family of telephone exchanges also relied extensively on reed relay technology.
[ไม่มีภาพประกอบ]Key Facts
- Ferreed technology used magnetically remanent alloys to reduce power and enable dual-contact voice paths.
- Coincident current selection was used to isolate specific crosspoints by releasing others in the same row and column.
- 1ESS systems required more stages (typically 8) than 5XB crossbar systems (typically 4) due to the size of the reed arrays.
- Remreed grids in 1AESS systems packaged 1,024 crosspoints into single replaceable units.
- Reliability issues in 1AESS were linked to the use of silver plating on contacts.
Frequently Asked Questions
What is a crosspoint switch?
A crosspoint switch is a mechanism used in telephony to connect an input line to an output line, effectively routing a voice signal from one point to another within an exchange.
How did the Ferreed method differ from standard relays?
Unlike standard relays that require continuous electrical power to maintain a latch, Ferreeds used a magnetically remanent alloy to latch magnetically, which lowered power consumption.
Why did 1ESS calls pass through more stages than 5XB calls?
Because reed crosspoints were more expensive than crossbar ones, they were used in smaller, more numerous arrays, requiring more stages (eight versus four) to complete the routing.
What was the primary cause of failure in Remreed arrays?
The use of silver plating on the contacts, rather than precious metals, made the Remreed arrays less reliable than the older crossbar switches.
Which other systems used similar reed relay technology?
The Stromberg-Carlson ESC system used "crossreed" relays, and the British TXE family of exchanges also utilized reed relays extensively.