Telephone Switch Types

What Is a Central Office Switch?

A Central Office (CO) switch is the hardware and software system at the heart of every telephone exchange. When you dial a number, your call is routed through one or more switches before it reaches the destination. The switch reads the dialed digits, determines where the call should go, and sets up a circuit (or, in modern networks, a packet path) between the two endpoints.

Every wireline telephone number in North America is "served" by a particular CO switch located in a specific building — the central office. That building has a CLLI code (Common Language Location Identifier) that uniquely identifies it. The switch at that CO determines what equipment type prefix appears in the CLLI code for that line.

Switches are classified by their position in the network hierarchy. The two primary classes you'll encounter in the PSTN are:

Complete Switch Reference Table

Class 4 vs. Class 5: The Hierarchy Explained

The North American PSTN was originally organized into a strict 5-level hierarchy defined by AT&T's Bell System. By the time digital switches arrived in the late 1970s and 1980s, the hierarchy had simplified to primarily two levels that matter for number lookups and carrier identification:

Class 5 (End Office) switches are where subscriber lines terminate. Every landline and ported wireless number in the US is associated with a specific Class 5 end office — the physical building where that number's dial tone is generated. When you look up a phone number's carrier or location, you're tracing it back to this Class 5 switch and its associated CLLI code.

Class 4 (Toll/Tandem) switches handle routing between end offices. When a call needs to travel between LATAs or over long distances, it leaves the originating Class 5, rides through one or more Class 4 tandem switches, and arrives at the destination Class 5. Class 4 switches are invisible to subscribers but critical to the network's routing architecture.

The Softswitch Revolution (Early 2000s)

Throughout the 1980s and 1990s, the PSTN ran almost entirely on TDM (Time Division Multiplexing) circuit switches. Each call required a dedicated 64 Kbps circuit through every switch along its path. These systems were enormously expensive, required specialized hardware, and were difficult to upgrade.

The arrival of broadband internet and packetized voice (Voice over IP — VoIP) in the late 1990s triggered a fundamental transformation. Engineers realized that call control (deciding where a call goes) could be separated from the actual transmission of voice. This gave rise to the softswitch architecture:

• Softswitch (Call Agent / Media Gateway Controller) — software running on commodity servers, handling signaling via SS7 or SIP
• Media Gateway — hardware converting TDM circuits to IP packets
• Session Border Controller (SBC) — manages SIP sessions between networks

Companies like Metaswitch, BroadSoft, and Genband built entire carrier-grade platforms on this model. By 2010, most new CLEC deployments were softswitch-based. Today, even the largest ILECs (AT&T, Verizon) are migrating remaining TDM infrastructure to IP.

The softswitch shift has significant implications for number lookups: a number that was originally served by a 5ESS may now be handled by a Metaswitch or Genband C20, or the number may have been ported away entirely — making carrier identification more complex than simply reading a CLLI code.

What Switches Are Still Deployed Today?

As of the mid-2020s, the North American PSTN is in the final stages of a decades-long TDM-to-IP migration. Here's the current landscape:

Explore Individual Switches

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