CO Switches and Switching Systems

Figure 1-3 shows a typical CO switch environment. The CO switch terminates the local loop and makes the initial call-routing decision.


The call-routing function forwards the call to one of the following:

• Another end-user telephone, if it is connected to the same CO

• Another CO switch

• A tandem switch (that is, an intermediary switch between the source and destination switch)

The CO switch makes the telephone work with the following components:

• Battery The battery is the source of power to both the circuit and the telephone. It determines the status of the circuit. When the handset is lifted to let current flow, the telephone company provides the source that powers the circuit and the telephone. Because the telephone company powers the telephone from the CO, electrical power outages should not affect the basic telephone, also known as a POTS (plain old telephone service) phone.

• Current detector The current detector monitors the status of a circuit by detecting whether it is open or closed. Table 1-1 describes current flow in a typical telephone.

• Dial-tone generator When the digit register is ready, the dial-tone generator produces a dial tone to acknowledge the request for service.

• Dial register The digit register receives the dialed digits.

• Ring generator When the switch detects a call for a specific subscriber, the ring generator alerts the called party by sending a ring signal to that subscriber.

Some telephones on the market offer additional features that require a supplementary power source that the subscriber supplies; for example, cordless telephones. Some cordless telephones may lose functionality during a power outage.

When configuring a PBX connection to a CO switch, the signaling should match that of the CO switch. This configuration ensures that the switch and the PBX can detect on hook, off hook, and dialed digits coming from either direction.

Switching systems provide three primary functions:

• Call setup, routing, and teardown

• Call supervision

• Customer ID and telephone numbers

CO switches switch calls between locally terminated telephones. If a call recipient is not locally connected, the CO switch decides where to send the call based on its own call routing information, which is stored in a call-routing table. The call then travels over a trunk to another CO or to an intermediate switch that may belong to an inter-exchange carrier (IXC). Although intermediate switches do not provide dial tone, they act as hubs to connect other switches and provide interswitch call routing.

PSTN calls are traditionally circuit-switched, which guarantees end-to-end path and resources. Therefore, as the PSTN sends a call from one switch to another, the same resource is associated with the call until the call is terminated.

CO switches provide local service to residential telephones. The CO switch provides dial tone, indicating that the switch is ready to receive digits. When you dial your phone, the CO switch receives the digits, then routes your call. The call routing may involve more than one switch as the call progresses through the network.