network
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... unavailable. These services are still best
located in a network device, rather than in an end system.
Traditionally, network-based ...
... network device, rather than in an end system.
Traditionally, network-based services have been created only by
...
... for these new service creators to describe the desired behavior of
network servers.
This document describes an architecture ...
...
This document describes an architecture in which network devices
respond to call signalling events by triggering user-created programs
...
... gatekeeper: An H.323 entity on the network that provides address
translation and controls access to the network ...
... network that provides address
translation and controls access to the network for H.323
terminals ...
...
gateway: A device which translates calls between an H.323 network
and another network, typically the public-switched telephone
network ...
... gateway: A device which translates calls between an H.323 network
and another network, typically the public-switched telephone
network.
...
... H.323 network
and another network, typically the public-switched telephone
network.
RAS: The Registration ...
... Network model ...
... Call Processing Language operates on a generalized model of an
Internet telephony network. While the details of various protocols
differ, on an abstract level all major Internet telephony
...
...
In the Call Processing Language's network model, an Internet
telephony network contains two types of components.
...
... Call Processing Language's network model, an Internet
telephony network contains two types of components.
...
... gateways -- for example, a device which
connects calls between an IP telephony network and the PSTN -- are
also considered to be end systems. Other devices, such as mixers or
...
...
proxy it: forward it on to one or more other network or end
systems, returning one of the responses received.
...
...
When an end system places a call, the call establishment request can
proceed by a variety of routes through components of the network. To
begin with, the originating end system must decide where to send its
requests. There are two possibilities here: the originator may be
...
... which the request should be sent. A request may pass through any
number of signalling servers: from zero (in the case when end systems
communicate directly) to, in principle, every server on the network.
What's more, any end system or signalling server can (in principle)
receive requests from or send them to any other.
...
... contact the destination without having to communicate with any
network servers at all.
We see, then, that in Internet telephony ...
... limitation implies a number of restrictions on how some services can
be implemented. For instance, a network system cannot reliably know
if an end system is currently busy or not; a call may have been
placed to the end system without traversing that network ...
... network system cannot reliably know
if an end system is currently busy or not; a call may have been
placed to the end system without traversing that network system.
Thus, signalling messages must explicitly travel to end systems to
find out their state ...
... Interaction of CPL with network model ...
... call. It does not attempt to coordinate the behavior of multiple
signalling servers, or to describe features on a "Global Functional
Plane" as in the Intelligent Network architecture [6].
...
...
In general, in an Internet telephony network, an address will denote
one of two things: either a user, or a device. A user address ...
...
Users can have CPL scripts on any network server which their call
establishment requests pass through and with which they have a trust
...
... This model does not specify the means by which users locate a CPL-
capable network server. In general, this will be through the same
means by which they locate a local Internet telephony server to
...
... create call processing language scripts, typically on end
devices, and transmit them through the network to signalling servers.
Scripts persist in signalling servers until changed or deleted,
...
... Scripts persist in signalling servers until changed or deleted,
unless they are specifically given an expiration time; a network
system which supports CPL scripting will need stable storage.
...
... CPL also might not necessarily be created on a device near either
the end device or the signalling server in network terms. For
example, a user might decide to forward his or her calls to a remote
location only after arriving at that location.
...
... NFS.
Users can also retrieve their current script from the network to an
end system so it can be edited. The signalling server should also be
able to report errors related to the script to the user, both static
...
... the second script. When the second script sends back a final
response, that response arrives at the first script in the same
manner as if a request arrived over the network. Note that in some
cases, forwarding can be recursive; a CPL server must be careful to
...
... banned address. This type of problem is unsolvable in an
administratively heterogeneous network, even a "lightly"
heterogeneous network such as current telephone ...
... administratively heterogeneous network, even a "lightly"
heterogeneous network such as current telephone systems. CPL does not
...
... As an aside, [8] discusses a fourth type of feature interaction for
traditional telephone networks, signalling ambiguity. This can arise
when several features overload the same operation in the limited
...
... when several features overload the same operation in the limited
signal path from an end station to the network: for example, flashing
the switch-hook can mean both "add a party to a three-way call" and
...
... services in a traditional circuit-
switched telephone network as a series of decisions and actions
arranged in a directed acyclic graph. Many vendors of IN ...
...
In addition to the general reasons why this is desirable, a
network server might conceivably handle very large call
volumes, and we don't want CPL execution to be a major
...
... its execution environment must be designed so that scripts
cannot use unlimited amounts of network resources, server CPU
time, storage, or memory.
...
... underlying
protocol is SIP, H.323, a traditional telephone network, or any
other means of setting up calls. It should also be agnostic to
...
... flow,
their bandwidth usage, their network destination addresses,
etc. The script should be able to make decisions based on
...
... International Telecommunication Union, "General recommendations on telephone switching and signaling -- intelligent network: Introduction to intelligent network capability set 1," Recommendation Q.1211, Telecommunication Standardization Sector of ITU ...
... telephone switching and signaling -- intelligent network: Introduction to intelligent network capability set 1," Recommendation Q.1211, Telecommunication Standardization Sector of ITU, Geneva, Switzerland, Mar. 1993. ...