Aastra 800 User Manual
Also see for 800: User guideQuick user guideQuick reference guide
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Contents
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Voice over IP (VoIP) Fundamentals625. You can only conduct a certain number of telephone calls simultaneously withinternal SIP telephones. The number licensed can be viewed on the System:Licences page. The number of SIP telephones currently licensed can be deter-mined on the System Info: Telephony: SIP phones page. If you click on Resetlicences, the available licenses will be reassigned with the next incoming oroutgoing calls.7.2 FundamentalsVoIP makes the transmission of voice and telephony signalling via IP (“InternetProtocol”) possible. After a connection is established, the terminal collects voicedata (PCM data), which is then sent to the receiver using an IP packet. PCM datacan also be compressed to save bandwidth.7.2.1 Propagation Delay and BandwidthIP-based data networks are generally not able to guarantee a specific minimumbandwidth and defined propagation delay. A synchronised 64 kbit/s ISDN lineguarantees a fixed data rate as long as the connection exists. In an IP-based datanetwork, the data rate and propagation delay can vary. Short-term bottlenecks orretransmission due to errors may be the cause. A data flow interruption of a fewseconds is barely noticeable when fetching a Web page, but it can be seriouslyinterfere with a telephone call.A modern Intranet normally offers enough performance reserves and reliability tomake good-quality VoIP telephony possible. Specific components can also beoptimised; for example by using a modern switch which evaluates the TOS byte ofIP packets, by replacing unreliable connections, or by using a separated VLAN forVoIP.7.2.2 Latency and Packet LengthFor technical reasons, there is always a delay (“latency”) between the recording ofvoice data via the microphone and playback via the receiver. Voice data isrecorded for a short period so that it can be sent in an IP packet. The IP packet alsohas a signal-propagation delay before the receiver can begin playback. For thesereasons, the extra time required for voice-data encoding and decoding may beneglected. |
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