Trunk Signaling Systems

Cisco IOS PSTN connectivity complies with the relevant standard signaling systems used by the PSTN and other telephony-switching systems. Cisco IOS routers support all the signaling variations in general use in the world today. No matter where your business is located, you should be able to connect easily to the PSTN with the analog or digital signaling options described in this section.

Analog Signaling

Low-density PSTN connectivity typically implies an analog connection. In some geographies Basic Rate Interface (BRI) is used instead, as discussed in the "Digital Signaling" section. Analog signaling is also used for connections to analog stations (such as fax machines and traditional analog phones). Table 6-1 summarizes the analog signaling variations supported by Cisco IOS voice gateways.

Table 6-1. Analog Signaling Support by Cisco IOS

Signaling	Description	Typical Use
FXS	Foreign Exchange Station	Used to connect to analog phone sets or fax machines. Occasionally also used to connect to a PBX or Key System if it offers only FXO interfaces.
FXO	Foreign Exchange Office	Generally used to connect to an analog PSTN line. Also used to connect to a PBX or Key System FXS interface. Can be connected to any interface where a standard analog phone is currently connected.
E&M	Ear and Mouth	Used to connect to an analog PBX.
Analog DID	Analog Direct Inward Dial	Used to connect to an analog PSTN line that has DID service on it.
CAMA	Centralized Automatic Message Accounting	Used to connect to the PSTN for emergency services (911 calls) in North America.

To connect your Cisco CME system to the PSTN for normal analog business line service, you use FXO interfaces. FXO ports, like all the other analog interfaces, carry one call per port, so each RJ-11 port on your Cisco CME router connects to one line from the PSTN and carries a single call at a time. (A second call gets busy tone if it tries to use the same port or line.)

Note

Note that on voice interface cards such as the NM-HDA and EVM-HD-8FXS/DID, which contain a single RJ-21 50-pin connector, the individual analog ports carried in the single cable are broken into separate RJ-11 ports by a break-out box.

The FXS and FXO voice interfaces are asymmetric, but most of the other signaling methods are symmetric. This means that if the PSTN offers an FXS interface (a normal business line), your Cisco CME router connects to that with an FXO interface. On the other hand, perhaps you have a Key System with FXO interfaces. (Maybe it used to connect to the PSTN, and now you want to connect those same ports to your Cisco CME router.) You require FXS interfaces on the router to connect to these ports.

Asymmetric also means that although you can make calls in both directions across FXS and FXO connections, services typically work in only one direction. For example, caller ID is sent on an FXS interface and received on an FXO interface, but not the other way around.

Analog trunks all support a single call per physical connection or port, so you need as many ports connected to the PSTN as you require simultaneous calls from your business to the PSTN.

FXO connections do not provide dialed digits (DNIS), introducing challenges in providing automatic call switching. You'll learn more about this in the later section "PSTN Call Switching." Analog DID is a variation of FXO that provides DNIS on what is, essentially, an FXO interface. Note, though, that these trunks are one-way and can only receive calls from the PSTN (they cannot make calls to the PSTN). If you use analog DID for incoming calls from the PSTN, you still need some FXO trunks as well to be able to make outgoing calls to the PSTN.

Digital Signaling

If you require only a small number of simultaneous calls to the PSTN, you will most likely use analog FXO connections. In geographic locations outside North America, ISDN BRI is a likely alternative option for low-density PSTN connectivity. However, if you have a larger office and require more than approximately 10 to 16 simultaneous calls to the PSTN, a digital T1 or E1 trunk might provide a more cost-effective option. Table 6-2 summarizes the digital signaling variations supported by Cisco IOS routers.

Table 6-2. Digital Signaling Support by Cisco IOS

Signaling	Description	Typical Use
BRI Q.931	Basic Rate Interface	An ISDN connection to the PSTN or a PBX carrying two simultaneous voice calls. It uses the Q.931 ISDN specification. Calls are controlled via a dedicated channel called the D channel. The term 2B+D is often used for BRI describing two voice channels (or bearer [B] channels) and one signaling channel (or data [D] channel).
BRI QSIG	Basic Rate Interface	Used for PBX ISDN connectivity. It uses the Q Signaling (QSIG) variation of the basic ISDN specification.
T1 CAS	T1 Channel Associated Signaling	Used widely in North America to connect to the PSTN or PBXs. Several variations of this signaling exist, including T1 FXS, T1 FXO, and T1 E&M. T1 E&M signaling supports delay dial, wink, and immediate dial.
T1 FGD	Feature Group D	The T1 CAS variations generally cannot convey caller ID. T1 FGD can. It's used to connect to the PSTN where caller ID is required and PRI is not an option. T1 FGD is an asymmetric protocol.
T1 and E1 PRI	Primary Rate Interface	An ISDN connection to the PSTN carrying 23 (T1) or 30 (E1) simultaneous voice calls, giving rise to the terms 23B+D and 30B+D. It uses the Q.931 ISDN specification. Calls are controlled via a dedicated signaling channel (D channel).
T1 PRI NFAS	Non-Facility Associated Signaling	A variation of PRI available only on T1 that uses a single D channel to control multiple spans of T1s with only B channels (voice calls).
T1 and E1 QSIG	Primary Rate Interface	Used for PBX ISDN connectivity. It uses the QSIG variation of the basic ISDN specification.
E1 R2	The Regional System 2 (R2) CAS protocol	Used in South America and Asia for PSTN connectivity. Numerous country-specific variations of the R2 protocol exist.
J1	Japan interface	PBX connectivity in Japan. Japan also uses the T1 standard.

BRI connectivity on the Cisco IOS routers is supported only for switch (PSTN, PBX, or Key System) connectivity, not for ISDN BRI phones.

All ISDN variations listed in Table 6-2 support both DID and caller ID, which is implicitly supported in the ISDN protocol. The CAS protocols (T1 CAS and E1 R2) may or may not support caller ID. Typically T1 CAS does not, but T1 FGD is a variation that does. All digital trunk types support DNIS and DID.

Cisco IOS PSTN Telephony Interfaces