Trunk Connections
Before a telephone call terminates at its final destination,
the call is routed through multiple switches. When a switch receives a call, it
determines whether the destination telephone number is within a local switch or
if the call needs to go through another switch to a remote destination. Trunks
interconnect the telephone company and PBX switches, as shown in Figure 2-9.
The primary
function of the trunk is to provide the path between switches. The switch must
route the call to the correct trunk or telephone line. Although many different
subscribers share a trunk, only one subscriber uses it at any given time. As
telephone calls end, they release trunks and make them available to the switch
for subsequent calls. There can be several trunks between two switches.
The following are examples of the more common trunk types:
-
Private trunk lines
(tie-lines) Companies with multiple PBXs often connect them with tie
trunk lines. Generally, tie trunk lines serve as dedicated circuits that connect
PBXs. On a monthly basis, subscribers lease trunks from the telephone company to
avoid the expense of using telephone lines on a per-extension basis. These types
of connections, known as tie-lines, typically use
special interfaces called recEive and transMit, or E&M interfaces.
-
CO trunks A CO trunk serves as
a direct connection between a PBX and the local CO that routes calls; for
example, the connection from a private office network to the public switched
telephone network (PSTN). When users dial 9, they are connecting through their
PBX to the CO trunk to access the PSTN. CO trunks typically use Foreign Exchange
Office interfaces. Certain specialized CO trunks are frequently used on the
telephony network. A direct inward dial trunk,
for example, allows outside callers to reach specific internal destinations
without having to be connected via an operator.
-
Interoffice trunks An
interoffice trunk is a circuit that connects two local telephone company
COs.
-
Foreign exchange (FX) trunks
FX trunks are
interfaces that are connected to switches supporting connections to either
office equipment or station equipment. Office equipment includes other switches
(to extend the connection) and Cisco devices. Station equipment includes
telephones, fax machines, and modems. The two FX trunk interfaces are:
- - Foreign Exchange Office
(FXO) interfaces An FXO interface connects a PBX to another switch or
Cisco device. The purpose of an FXO interface is to extend the telephony
connection to a remote site; for example, if a user on a corporate PBX wanted a
telephone installed at home instead of in the local office where the PBX is
located, an FXO interface would be used. The FXO interface would connect to a
Cisco voice router, which would serve to extend the connection to the user's
home. This connection is an Off-Premises eXtension (OPX).
- - Foreign Exchange Station
(FXS) interfaces An FXS interface connects station equipment: telephones,
fax machines, and modems. A telephone connected directly to a switch or Cisco
device requires an FXS interface. Because a home telephone connects directly to
the telephone company CO switch, an FXS interface is
used.
Note
The service provided by local telephone companies for
residential phones uses a foreign exchange interface, specifically FXS. This
service is provided on two wires. The service is considered a station-side
connection because the interface terminates with a telephone.
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