The construction of the voicemail URI also shows the method of using an
“opaque” URI [1] to indicate to the voicemail server the intention of the
INVITE. In this example, the user portion of the URI contains the username
alanand also the keyword msg-deposit-external, indicating that this is
a message deposit session. This indicates to the voicemail server to play an
external greeting and record a message. Another URI possibility would be
alan-msg-retrieval, which could indicate message retrieval. In this case,
the voicemail server would authenticate the caller for appropriate credentials,
and then play back messages to the caller. This is shown in Figures 7.1, 7.2, and
7.3 using different design options for comparison.Comparison
The advantages and disadvantages of these three implementations are sum-
marized in Table 7.1. In summary, each implementation shifts the location of
the service logic (such as the ring-no-answer timer and the recursive retries).
The implementation of a particular service will depend on many factors and
may be influenced by economies of scale.
In addition to these three common methods, there is a fourth method of ser-
vice implementation that involves third-party call control. This approach has
been generalized to an architecture of special proxies that modify SIP mes-
sages (headers and message bodies), and generate and respond to requests.
This is covered in detail in Chapter 19, “SIP Component Services.”Table 7.1 Comparison of Service Implementation
SERVICE
IMPLEMENTATION ADVANTAGES DISADVANTAGES
Server Called user agent does User must change proxy logic in
not need to be registered. order to change nature of
Neither user agent requires service. As a result, the service
any provisioning or special logic is not under the direct
logic. control of the user.
Called user agent Service logic is under con- Feature logic must be in called
trol of user in phone user agent. User agent must be
configuration. registered. This means effec-
tively that the called user agent
must be “on” or else the service
will fail. This type of “24x7”
reliability is more difficult to
achieve on a customer’s
premises, as compared to a
service provider’s centralized
location.140 Chapter 7