Bedienungsanleitung Aeta Scoopy plus S (Seite 126 von 136) (Englisch)

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SCOOPY+ S - User Manual

What happens then on the network: the Scoopy+ sends the request  (INVITE in SIP protocol) to a

proxy server (often the same device is also the registrar). To make things simple, this proxy then relays

and routes the request  to its destination. Resolving the SIP URI to a physical network and address

uses mechanisms similar to those used for resolving URLs. Several proxys may be involved in cascade to

finally reach the desired destination, but this does not have to be known or dealt with by the end

devices. The following is like the initiation of a phone call: the IP codec "rings"; if it accepts the call, this

is notified to the Scoopy+.

At this stage, the proxy (s) provide the Scoopy+ and the IP codec with all the address data they need for

the link, then the actual audio streams can be exchanged  between both units. As a very important

feature, the end devices now can exchange data directly; the proxies do not have to be on the path,

they are only involved in the setting (and later the ending!) of the session. The codecs will automatically

exchange their coding capabilities, and agree on a coding mode, with no further user intervention.

Alternatively, the call can be done from the station to the reporter, in a way very similar to the above. In

contrast with ISDN links, the operators at the station do not even need to know where the reporter is

located! This is because the registrar deals with this issue.

Note that it is also possible to set a link with a SIP-compliant VoIP phone instead of another codec. This

is one of the numerous advantages of using a standard.

8.2.3. Setting a link without a SIP server

Contrary to a commonplace but wrong idea, the SIP protocol does not impose using a SIP server. The SIP

protocol can be used without a SIP server, i.e. it is possible to set "peer to peer" links without involving

one or more SIP registrars or proxies. In such a case the process is more direct, and the codec contacts

the remote destination with no intermediate entity. However, there are some drawbacks with such type

of session:

 Without a registrar, the identification of the destination is its IP address; this is not as "stable"

as a SIP URI, as it may change depending on the location, or time (with dynamic addressing).

 The two codecs must have open access to the Internet, or otherwise the link must be made

possible by unlocking if necessary the appropriate routes and ports (conversely, when using a

proxy, such allowances can be restricted to the server connection, for a better control on the

access security).

 Most often the gateways perform address translation (NAT), which is an a priori issue for UDP

protocols such as implemented for real time audio links. Proxies can help dealing with this

issue, but without such servers it may be rather difficult to work around such obstacle. The

following chapter provides some recommendations about this.

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