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When the remote access is locked, the user receives a notification in his mailbox via a<br />

pre-defined voice message: "Remote access to voice mail is currently locked".<br />

This notification is only shown locally to the user via the blinking mailbox indication and can be<br />

consulted only locally. In this way, a potential malicious attack receives no status information.<br />

6.3.5.2.4 Using the Remote Substitution Service<br />

Using the "remote substitution" service, the user inputs the set number and the user's<br />

password (whether the DISA access code is disabled or not).<br />

In case of failure in entering the password, the counter of attempts is increased: the counter is<br />

the same as the one used for remote access to voice mail.<br />

When the maximum number of attempts is reached, the "remote substitution" service is locked.<br />

Note:<br />

6.4 SIP<br />

For the "remote substitution" service, the voice prompt is absent. Pre-defined voice message is sent to<br />

the user's mailbox but the voice prompt is not played.<br />

6.4.1 Overview<br />

6.4.1.1 SIP Protocol<br />

SIP (Session Initiation Protocol) is an IP signaling protocol designed to establish, to maintain<br />

and to end multimedia sessions between different parties. It operates on a client-server mode.<br />

It is based on the exchange of text messages with a syntax similar to that of HyperText<br />

Transport Protocol (HTTP) messages. Elements of the SIP world are identified by SIP Uniform<br />

Resource Locators (URLs) similar to e-mail addresses.<br />

It is important to note that SIP does not provide an integrated communication system. SIP is<br />

only in charge of initiating a dialog between interlocutors and of negotiating communication<br />

parameters, in particular those concerning the media involved (audio, video). Media<br />

characteristics are described by the Session Description Protocol (SDP). SIP uses the other<br />

standard communication protocols on IP: for example, for voice channels on IP, Real-time<br />

Transport Protocol (RTP) and Real-time Transport Control Protocol (RTCP). In turn, RTP uses<br />

G7xx audio codecs for voice coding and compression.<br />

Unlike H.323, the SIP protocol can rely on the IP network transport protocol in datagram mode<br />

User Datagram Protocol (UDP) in addition to the IP network transport protocol in Transmission<br />

Control Protocol (TCP) connected mode: see figure: H.323 and SIP in the OSI Model . UDP<br />

has the advantage of being an unconnected protocol that facilitates swift exchanges. It does<br />

not guarantee datagram reception and transmission sequence preservation. Thus, SIP carries<br />

out these functions, using retransmission, acknowledgement and sequencing mechanisms.<br />

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