Central office switch

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Central office switch is the generic name for the network element in the PSTN that establishes temporary connections between two subscribers. It is a voice switch that resides in the central office (CO) of an ILEC, CLEC, or IXC. It can also reside in the hub (STC) or headend, (MTC) of an MSO. These switches have evolved significantly since their introduction in the late 1800s. Originally manual plugboards, they became mechanical switches (e.g., the crossbar switch, then electromechanical switches (e.g., the stepper switch, and finally all electronic switches, (e.g. modern digital switches). All of these switches have operated on the circuit-switching model, and the digital switches have historically used time division multiplexing (TDM). Today, the CO switch is evolving into a packet-based voice switch (e.g., the softswitch) using statistical time-division multiplexing (STDM).

TDM-based CO Switch Components

Components of a central office switch
Components of a central office switch

The traditional TDM-based CO switch implemented five basic components, depicted in the graphic to the right. These include:

  • Lines: also known as local loops, these are the interfaces in the switch that connect directly to the subscriber and provide services. Options include POTS and ISDN. The line side may also implement a TDM-based circuit that connects to a remote terminal, a configuration often referred to as a carrier serving area (CSA), a digital loop carrier (DLC), or a subscriber loop carrier (SLC).
  • Trunks: These are the circuits that interconnect one voice switch to another. Typically these circuits are capable of carrying multiple calls. Historically these trunks have been TDM-based using such technologies as T-1, T-3, or SONET between switches within the PSTN, and T-1 or PRI between a CO switch and a PBX.
  • Service circuits: these are circuits within the CO switch that are used to provide dial-tone, receive and interpret dialed digits (i.e., telephone numbers), play service announcements, and so forth.
  • The switching network: also known as the switch fabric, this is the structure within the switch that forms connections between lines and service circuits (to initiate services), lines and other lines (to complete a call between two telephones connected to the same switch), lines and trunks (to complete a call originating on the local switch and terminating on a remote switch, or the reverse), or one trunk to another trunk (to complete a call originating and terminating on switches other than the local one). The switching network in a digital, all electronic voice switch can take many forms, including shared memory or time slot interchangers.
  • The control element: This is the intelligence of the switch and is responsible for controlling and monitoring call setup, calls in progress, and call termination. Today, this control is performed by an embedded computer system running a real-time OS and specialized software. It is this function that connects to, and communicates with, the SS7 network.

Classes of CO Switch

Historically there have been five classes of CO switch. These include:

  • Class 1 switch: these were international toll switches responsible for coordinating calls originating in two different countries. Because most international backbones have become packet-based, this language has become obsolete.
  • Class 2 switch: these were long distance tandem switches that tied together large regions of the IXC backbone. Again, because most of these backbones have become packet-based, this language has become obsolete.
  • Class 3 switch: these were toll switches that connected an IXC network to a LEC network. These are also known as point of presence (POP) switches.
  • Class 4 switch: These are tandem switches within a LEC network. This terminology is still used.
  • Class 5 switch: This is the switch that connects the subscriber to the LEC network. It is, essentially, the edge of the PSTN. This terminology is still in use.


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