Sequential Forking Time - Snom 4S Administrator's Manual

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Sequential Forking Time

When you allow the proxy to register an account more than once,
the proxy starts "sequential forking" if the registration show different
probabilities. That means, the user agents are called one after another,
depending on their probability. This applies only to INVITE requests that
initiate a new call.
This feature is helpful in cases when you first want to alert a
phone which will most likely pick up that call. Only if nobody answeres
that call, you will go to another destination. A good example for this is
ringing the secretary in an office first, and the boss only later if nobody
answeres. When users have the same probability, they are called at the
same time, this is called parallel forking.
When the proxy receives a 3xx response (redirect), it handles
them in a special way. If there are other requests pending, it determines
the destinations that are provided with the 3xx response and puts them on
the list. This is done using the probability of the underlying registrations,
and redirected contacts may have an even lower probability.
If the 3xx response for the only pending request, the proxy
passes the response through to the request client. The client itself then
takes care of the redirection. This is important in cases where a call from
a PSTN gateway comes in and should be redirected to a PSTN number.
The gateway then does not have to pass the call through the proxy, it
can immediately redirect the call on the PSTN level. This does not work
however, if the call is redirected to more than one PSTN number, in which
case the call will have to go through VoIP.
In many environments, users are registered with a high
probability and the mailbox is registered as well, but with a low value.
Even if the phone is switched off, the proxy will redirect the call to the
mailbox after a timeout.
The sequential forking time defines the time in seconds between
the first ringing of the user with the highest probability and a user that
has the probability 0 (even if no such user is registered).
For example, if there are three users for number "abc" with
the probabilities 0.9, 0.5 and 0.1 and the sequential forking time is 30
seconds, the contact with probability will ring immediately, the contact
with probability 0.5 after 30 x (0.9 – 0.5) / 0.9 = 13 s and the contact
76 • Routing
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