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Voice Feature Guide 01

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Introduction to IP Z Interface Extension

Introduction to IP Z Interface Extension


The PSTN network can be used to provision the Z interface extension private line service for the headquarters (HQ) and branch offices of businesses. The service enables users at the branch offices to have the same calling experience as users at the HQ through phones. As devices on existing PSTN networks are approaching the end of their life cycles, many carriers are reconstructing the PSTN networks.

Figure 1-120 illustrates the Z interface extension service network before and after the PSTN network reconstruction. After the reconstruction, the Z interface extension service that used to be carried by the PSTN network is carried by an IP network.

The access devices, one located at the HQ and the other at the branch office, are interconnected through the IP network. The voice signals on POTS lines are carried through RTP packets when transmitted over the IP network. The call signaling on POTS lines, such as offhook and onhook signaling, is carried through RFC2833 packets.

Figure 1-120 Z interface extension service network before and after PSTN reconstruction


The Z interface refers to the analog interface between an access device and an exchange.

Z interface extension means the extension of POTS user signals. Analog POTS user signals are converted into digital signals at the Tx end, transmitted over the network, and restored into analog signals at the Rx end. In this feature, the extension of the Z interface is carried over an IP network, so the feature is called IP Z interface extension.


IP Z interface extension is intended for the following purposes:

  • Connect the private line users at the branch offices of an enterprise that has a small volume of analog phone service requirements to the enterprise's local exchange.
  • Extend the calls of analog phones under a PBX to a remote location, thereby saving long-distance call fee.
  • Connect users at a remote location to a local exchange.

Hardware support

Boards on the HQ-side MSAN (such as MSAN_1 in Figure 1-120) that provide the IP Z interface extension service are the FXO board. Boards on the branch office-side MSAN (such as MSAN_2 in Figure 1-120) for the same purpose are the FXS board.


IP Z interface extension is a technology proprietarily owned by Huawei. Therefore, the MSANs on the FXO side and the FXS side must be Huawei MSANs.

Updated: 2019-02-22

Document ID: EDOC1100067358

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