ISDN Voice Configuration Guide, Cisco IOS Release 12.4T
Overview of ISDN Voice Interfaces
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Overview of ISDN Voice Interfaces

Overview of ISDN Voice Interfaces

Last Updated: December 15, 2011

This chapter provides an overview of ISDN Basic Rate Interface (BRI) and Primary Rate Interface (PRI) for support of voice traffic. With those ports so configured, you can do the following:

  • Bypass PSTN tariffed services such as trunking and administration.
  • Connect your PBXs directly to a Cisco router and route PBX station calls automatically to the WAN.
  • Configure a voice interface on a Cisco router to emulate either a terminal-equipment (TE) or network-termination (NT) interface. All types of PBXs can send calls through a router and deliver those calls across the customer network.
  • Configure Layer 2 operation as point-to-point (static terminal endpoint identifier [TEI]) or point-to-multipoint (automatic TEI).

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to An account on is not required.

Prerequisites for Configuring ISDN Voice Interfaces

  • Obtain PRI or BRI service and T1 or E1 service from your service provider, as required. Ensure that the BRI lines are provisioned at the switch to support voice calls.
  • Establish a working IP, Frame Relay, or ATM network. Ensure that at least one network module or WAN interface card is installed in the router to provide connection to the LAN or WAN.
  • Complete your company's dial plan.
  • Establish a working telephony network based on your company's dial plan and configure the network for real-time voice traffic. This chapter describes only a portion of the process; for further information, see the chapter "Cisco Voice Telephony."
  • Cisco 2600 series and Cisco 3600 series routers--Install digital T1 or E1 packet-voice trunk network modules, BRI voice interface cards, and other voice interface cards as required on your network.
  • Cisco 7200 series routers--Install a single-port 30-channel T1/E1 high-density voice port adapter.
  • Cisco MC3810 multiservice concentrators--Install the required digital voice modules (DVMs), BRI voice module (BVM), and multiflex trunk modules.
  • Configure, for all platforms (as required), the following:
    • Voice card and controller settings
    • Serial and LAN interfaces
    • Voice ports
    • Voice dial peers

Restrictions for Configuring ISDN Voice Interfaces

ISDN Voice Interface Limitations

  • Basic-net3 and basic-qsig are the only ISDN switch types currently supported for an NT interface.
  • When the ISDN BRI port on the router is configured as an NT port, you must use a "rolled" cable (one with the transmit and receive leads swapped) to connect to a TE interface.
  • Layer 1 can be configured only as point-to-point (that is, with one TE connected to each NT). Automatic TEI support issues only one TEI.

QSIG Support Limitations

  • Cisco 2600 series routers do not support VoATM.
  • The following restrictions apply to the Cisco MC3810 multiservice concentrator:
    • QSIG data calls are not supported. All calls with bearer capability indicating a nonvoice type (such as for video telephony) are rejected.
    • Cisco MC3810 supports only one T1/E1 interface with direct connectivity to a private integrated services network exchange (PINX).
    • Cisco MC3810 supports a maximum of 24 B channels.
    • When QSIG is configured, serial port 1 does not support speeds higher than 192 kbps. This restriction assumes that the MFT is installed in slot 3 on the Cisco MC3810. If the MFT is not installed, then serial port 1 does not operate.
  • The following restrictions apply to Cisco 7200 series routers:
    • VoATM is not supported.
    • BRI is not supported.

Information About ISDN Voice Interfaces

ISDN Media Types

Cisco routing devices support ISDN BRI and ISDN PRI. Both media types use bearer (B) channels and data (D) channels as follows:

  • ISDN BRI (referred to as "2 B + D") uses the following:
    • Two 64-kbps B channels that carry voice or data for a maximum transmission speed of 128 kbps
    • One 16-kbps D channel that carries signaling traffic--that is, instructions about how to handle each of the B channels.
  • ISDN PRI (referred to as "23 B + D" or "30 B + D") uses the following:
    • 23 B channels (in North America and Japan) or 30 B channels (in the rest of the world) that carry voice or data
    • One 64-kbps D channel that carries signaling traffic

The D channel, in its role as signal carrier for the B channels, directs the central-office switch to send incoming calls to particular timeslots on the Cisco access server or router. It also identifies the call as a circuit-switched digital call or an analog modem call. Circuit-switched digital calls are relayed directly to the ISDN processor in the router; analog modem calls are decoded and then sent to the onboard modems.

Interface Cards and Network Modules

The VIC-2BRI-NT/TE voice interface card for the Cisco 2600 series and Cisco 3600 series routers and the BVM4-NT/TE voice module for the Cisco MC3810 multiservice concentrator enable Cisco IOS software to replicate the PSTN interface to a PBX that is compatible with European Telecommunications Standards Institute (ETSI) NET3 and QSIG switch types.

Before these cards and modules became available, if your PBXs implemented only a BRI TE interface, you had to make substantial hardware and software changes on the PBX to provide an NT interface to the router. provide an NT interface to the router. VIC-2BRI-NT/NE and BVN4-NT/NE allow you to connect ISDN PBXs and key systems to a multiservice network with minimal configuration changes on the PBX.

Typical ISDN Application

A typical application (see the figure below) allows an enterprise customer with a large installed base of legacy telephony equipment to bypass the PSTN.

Figure 1 Typical Application Using BRI-NT/TE Voice Interface Cards or BVM4-NT/TE Voice Modules

QSIG Protocol

This section contains the following information:

QSIG Basics

QSIG is a variant of ISDN Q.921 and Q.931 ISDN D-channel signaling, for use in private integrated-services network-exchange (PINX) devices such as PBXs or key systems. Using QSIG signaling, a router can route incoming voice calls from a PINX across a WAN to a peer router, which can then transport the signaling and voice packets to another PINX.

The QSIG protocol was originally specified by European Computer Manufacturers Association (ECMA), and then adopted by European Telecommunications Standards Institute (ETSI) and the International Organization for Standardization (ISO). It is becoming the standard for PBX interoperability in Europe and North America.

The table below identifies the ECMA standards and the OSI layer of the QSIG protocol stack to which they relate.

Table 1 QSIG Protocol Stack

OSI Layer



7 to 4

Application mechanisms

End-to-end protocols; network transparent


Multiple ECMA standards

Standards for supplementary services and advanced network features


QSIG generic functional procedures


QSIG basic call



Interface-dependent protocols


I.430 / I.431


QSIG enables Cisco networks to emulate the functionality of the PSTN. A Cisco device routes incoming voice calls from a PINX across a WAN to a peer device, which then transports the signaling and voice packets to a second PINX (see the figure below).

Figure 2 QSIG Signaling

The Cisco voice-packet network appears to the QSIG PBXs as a distributed transit PBX that can establish calls to any PBX, non-QSIG PBX, or other telephony endpoint served by a Cisco gateway, including non-QSIG endpoints.

QSIG messages that originate and terminate on QSIG endpoints pass transparently across the network; the PBXs process and provision any supplementary services. When endpoints are a mix of QSIG and non-QSIG, only basic calls that do not require supplementary services are supported.

QSIG signaling provides the following benefits:

  • It provides efficient and cost-effective telephony services on permanent (virtual) circuits or leased lines.
  • It allows enterprise networks that include PBX networks to replace leased voice lines with a Cisco WAN.
  • It eliminates the need to route connections through multiple tandem PBX hops to reach the desired destination, thereby saving bandwidth, PBX hardware, and switching power.
  • It improves voice quality through the single-hop routing provided by voice switching while allowing voice to be compressed more aggressively, resulting in additional bandwidth savings.
  • It supports PBX feature transparency across a WAN, permitting PBX networks to provide advanced features such as calling name and number display, camp-on/callback, network call forwarding, centralized attendant, and centralized message waiting. Usually these capabilities are available on only a single site where users are connected to the same PBX.

QSIG support enables the following:

  • Digit forwarding on POTS dial peers
  • On Cisco 2600 series, QSIG-switched calls over VoFR and VoIP for T1/E1 and BRI voice interface cards
  • On Cisco 3600 series, QSIG-switched calls over VoFR, VoIP, and VoATM for T1/E1 and BRI voice interface cards
  • On Cisco 7200 series, QSIG-switched calls over VoFR and VoIP on T1/E1 voice interface cards
  • On Cisco MC3810, T1 or E1 PRI and BRI QSIG-switched calls over VoFR, VoIP, and VoATM for Cisco MC3810 digital voice modules and BRI voice module.

See the figure below shows an example of how QSIG support can enable toll bypass.

Figure 3 QSIG Toll-Bypass Application

ISDN Switch Types for Use with QSIG

You can configure QSIG at either the global configuration level or the interface configuration level. To do so requires that you know your switch type. Available types are shown in the table below.

Table 2 ISDN Central-Office Switch Types


ISDN Switch Type




Australian TS013 switches



German 1TR6 ISDN switches


Norwegian NET3 ISDN switches (phase 1)


NET3 ISDN switches (United Kingdom and others)


French VN2 ISDN switches


French VN3 ISDN switches



Japanese NTT ISDN switches

New Zealand


New Zealand NET3 switches

North America


Lucent Technologies basic rate switches


NT DMS-100 basic rate switches


National ISDN-1 switches

The table below lists the ISDN service-provider BRI switch types.

Table 3 ISDN Service-Provider BRI Switch Types

ISDN Switch Type



German 1TR6 ISDN switches


Lucent Technologies basic rate switches


NT DMS-100 basic rate switches


NET3 (TBR3) ISDN, Norway NET3, and New Zealand NET3 switches. (This switch type covers the Euro-ISDN E-DSS1 signaling system and is ETSI-compliant.)


National ISDN-1 switches


Norwegian NET3 ISDN switches (phase 1)


New Zealand NET3 switches


PINX (PBX) switches with QSIG signaling in compliance with Q.931


Australian TS013 switches


Japanese NTT ISDN switches


French VN2 ISDN switches


French VN3 ISDN switches

Cisco platforms that support Q.931 offer both user-side and network-side switch types for ISDN call processing, providing the following benefits:

  • User-side PRI enables the Cisco device to provide a standard ISDN PRI user-side interface to the PSTN.
  • Network-side PRI enables the Cisco device to provide a standard ISDN PRI network-side interface via digital T1/E1 packet voice trunk network modules on Cisco 2600 series and Cisco 3600 series routers.

Traceability of Diverted Calls

European Telecommunication Standard ETSI 300 207-1 specifies that calls must be traceable if diverted. This requires that a VoIP call, when diverted, must translate into divertingLegInformation2 instead of Redirection IE. Cisco's ISDN implementation satisfies this requirement.

Additional References

The following sections provide references related to ISDN.


In addition to the references listed below, each chapter provides additional references related to ISDN.

Related Documents

Related Topic

Document Title


  • ATM Software Segmentation and Reassembly (SAR) at

Basic router configuration

  • Cisco 3600 series documentation at

Cisco IOS command references

Cisco IOS configuration fundamentals and examples

From the website, select a technology category and subsequent hierarchy of subcategories, then click Technical Documentation > Configuration Examples.

Cisco IOS Voice Configuration Library, including library preface and glossary

  • Cisco IOS Voice Configuration Library at

Clock sources

ISDN basics

ISDN cause codes

ISDN configuration

ISDN interfaces for voice

ISDN network modules and interface cards

  • Installing and Configuring 1-Port J1 Voice Interface Cards at

MIX module

RADIUS VSA configuration



  • Cisco IOS Security Configuration Guide at

SS7 for voice gateways

Tcl IVR programming


VoATM configuration

VoIP configuration

WAN configuration





CPE Requirements for MCI ISDN Primary Rate Interface, revision 4.3D, February 10, 1998

ETSI 300 207-1

Integrated Services Digital Network (ISDN): Diversion supplementary services; Digital Subscriber Signalling System No. one (DSS1) protocol; Part 1: Protocol specification , December 1994


AT&T Network ISDN Primary Rate Interface and Special Applications Specifications, User-Network Interface, 1999

TTC JJ-20.10 to JJ-20.12




MIBs Link

  • RFC 1407 MIB

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

Technical Assistance



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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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