Use of IP Networks with the PSTN
Internet Protocol (IP) by its nature introduces delay into speech signals as each data packet is formulated and
addressed. Telecom Access Standards recommends that suppliers, designers and installers using this technology
for calls to or from the PSTN refer to ITU E Model requirements in the design of their networks. The overall
aim is to minimise delay, distortion and other transmission impairments, particularly for those calls involving
cellular and international networks, which already suffer extensive delay.
The Use of Voice Compression Through the PSTN
Because of the extensive delay already experienced when calling cellular and international networks, some
of which is already caused by their use of voice compression technologies. Telecom Access Standards will
only approve G711 voice technology for use on the PSTN. G711 is an ‘instantaneous speech-encoding
technique’ whereas G729 and all its variants are considered ‘near instantaneous’ introducing additional delay
into the speech signal.
Echo Cancellation
Echo cancellers are not normally required in the Telecom PSTN because geographic delays are acceptable
where CPE return loss is maintained within Telepermit limits. However, those private networks making use
of Voice-over-IP (VoIP) technology are required to provide echo cancellation for all voice calls. The combined
effect of audio/VoIP conversion delay and IP routing delay can cause the echo cancellation time of 64 mS to
be required.
Cisco Wireless IP Phone 8821 and 8821-EX User Guide
103
Product Safety and Security
Compliance Statements for New Zealand
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