desk, we can dial the following numbers:
------------------------------------------------------
| 0 | Local operator |
| 00 | Long distance operator |
| xxxx | Local extension number |
| 8xxxxxxx | Local number |
| #xxxxxxx | Shortcut to local number at|
| | other corporate sites |
| *xx | Star services |
| 91xxxxxxxxxx | Long distance number |
| 9011 + up to 15 digits| International number |
------------------------------------------------------
The solution to this problem is to have the Call Agent load the
gateway with a digit map that may correspond to the dial plan. This
digit map is expressed using a syntax derived from the Unix system
command, egrep. For example, the dial plan described above results
in the following digit map:
(0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (see Appendix A) -
support for basic digit map letters is REQUIRED while support for
extension digit map letters is OPTIONAL. A gateway receiving a digit
map with an extension digit map letter not supported SHOULD return
error code 537 (unknown digit map extension).
A digit map, according to this syntax, is defined either by a (case
insensitive) "string" or by a list of strings. Each string in the
list is an alternative numbering scheme, specified either as a set of
digits or timers, or as an expression over which the gateway will
attempt to find a shortest possible match. The following constructs
can be used in each numbering scheme:
* Digit: A digit from "0" to "9".
* Timer: The symbol "T" matching a timer expiry.
* DTMF: A digit, a timer, or one of the symbols "A", "B", "C",
"D", "#", or "*". Extensions may be defined.
* Wildcard: The symbol "x" which matches any digit ("0" to "9").
* Range: One or more DTMF symbols enclosed between square brackets
("[" and "]").
* Subrange: Two digits separated by hyphen ("-") which matches any
digit between and including the two. The subrange
construct can only be used inside a range construct,
i.e., between "[" and "]".
* Position: A period (".") which matches an arbitrary number,
including zero, of occurrences of the preceding
construct.
A gateway that detects events to be matched against a digit map MUST
do the following:
1) Add the event code as a token to the end of an internal state
variable for the endpoint called the "current dial string".
2) Apply the current dial string to the digit map table, attempting a
match to each expression in the digit map.
3) If the result is under-qualified (partially matches at least one
entry in the digit map and doesn't completely match another
entry), do nothing further.
If the result matches an entry, or is over-qualified (i.e., no
further digits could possibly produce a match), send the list of
accumulated events to the Call Agent. A match, in this
specification, can be either a "perfect match," exactly matching one
of the specified alternatives, or an impossible match, which occurs
when the dial string does not match any of the alternatives.
Unexpected timers, for example, can cause "impossible matches". Both
perfect matches and impossible matches trigger notification of the
accumulated digits (which may include other events - see Section
2.3.3).
The following example illustrates the above. Assume we have the
digit map:
(xxxxxxx|x11)
and a current dial string of "41". Given the input "1" the current
dial string becomes "411". We have a partial match with "xxxxxxx",
but a complete match with "x11", and hence we send "411" to the Call
Agent.
The following digit map example is more subtle:
(0[12].|00|1[12].1|2x.#)
Given the input "0", a match will occur immediately since position
(".") allows for zero occurrences of the preceding construct. The
input "00" can thus never be produced in this digit map.
Given the input "1", only a partial match exists. The input "12" is
also only a partial match, however both "11" and "121" are a match.
Given the input "2", a partial match exists. A partial match also
exists for the input "23", "234", "2345", etc. A full match does not
occur here until a "#" is generated, e.g., "2345#". The input "2#"
would also have been a match.
Note that digit maps simply define a way of matching sequences of
event codes against a grammar. Although digit maps as defined here
are for DTMF input, extension packages can also be defined so that
digit maps can be used for other types of input represented by event
codes that adhere to the digit map syntax already defined for these
event codes (e.g., "1" or "T"). Where such usage is envisioned, the
definition of the particular event(s) SHOULD explicitly state that in
the package definition.
Since digit maps are not bounded in size, it is RECOMMENDED that
gateways support digit maps up to at least 2048 bytes per endpoint.
2.1.6 Packages
MGCP is a modular and extensible protocol, however with extensibility
comes the need to manage, identify, and name the individual
extensions. This is achieved by the concept of packages, which are
simply well-defined groupings of extensions. For example, one
package may support a certain group of events and signals, e.g.,
off-hook and ringing, for analog access lines. Another package may
support another group of events and signals for analog access lines
or for another type of endpoint such as video. One or more packages
may be supported by a given endpoint.
MGCP allows the following types of extensions to be defined in a
package:
* BearerInformation
* LocalConnectionOptions
* ExtensionParameters
* ConnectionModes
* Events
* Signals
* Actions
* DigitMapLetters
* ConnectionParameters
* RestartMethods
* ReasonCodes
* Return codes
each of which will be explained in more detail below. The rules for
defining each of these extensions in a package are described in
Section 6, and the encoding and syntax are defined in Section 3 and
Appendix A.
With the exception of DigitMapLetters, a package defines a separate
name space for each type of extension by adding the package name as a
prefix to the extension, i.e.:
package-name/extension
Thus the package-name is followed by a slash ("/") and the name of
the extension.
An endpoint supporting one or more packages may define one of those
packages as the default package for the endpoint. Use of the package
name for events and signals in the default package for an endpoint is
OPTIONAL, however it is RECOMMENDED to always include the package
name. All other extensions, except DigitMapLetter, defined in the
package MUST include the package-name when referring to the
extension.
Package names are case insensitive strings of letters, hyphens and
digits, with the restriction that hyphens shall never be the first or
last character in a name. Examples of package names are "D", "T",
and "XYZ". Package names are not case sensitive - names such as
"XYZ", "xyz", and "xYz" are equal.
Package definitions will be provided in other documents and with
package names and extensions names registered with IANA. For more
details, refer to section 6.
Implementers can gain experience by using experimental packages. The
name of an experimental package MUST start with the two characters
"x-"; the IANA SHALL NOT register package names that start with these
characters, or the characters "x+", which are reserved. A gateway
that receives a command referring to an unsupported package MUST
return an error (error code 518 - unsupported package, is
RECOMMENDED).
2.1.7 Events and Signals
The concept of events and signals is central to MGCP. A Call Agent
may ask to be notified about certain events occurring in an endpoint
(e.g., off-hook events) by including the name of the event in a
RequestedEvents parameter (in a NotificationRequest command - see
Section 2.3.3).
A Call Agent may also request certain signals to be applied to an
endpoint (e.g., dial-tone) by supplying the name of the event in a
SignalRequests parameter.
Events and signals are grouped in packages, within which they share
the same name space which we will refer to as event names in the
following. Event names are case insensitive strings of letters,
hyphens and digits, with the restriction that hyphens SHALL NOT be
the first or last character in a name. Some event codes may need to
be parameterized with additional data, which is accomplished by
adding the parameters between a set of parentheses. Event names are
not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
equal.
Examples of event names can be "hu" (off hook or "hang-up"
transition), "hf" (hook-flash) or "0" (the digit zero).
The package name is OPTIONAL for events in the default package for an
endpoint, however it is RECOMMENDED to always include the package
name. If the package name is excluded from the event name, the
default package name for that endpoint MUST be assumed. For example,
for an analog access line which has the line package ("L") as a
default with dial-tone ("dl") as one of the events in that package,
the following two event names are equal:
L/dl
and
dl
For any other non-default packages that are associated with that
endpoint, (such as the generic package for an analog access
endpoint-type for example), the package name MUST be included with
the event name. Again, unconditional inclusion of the package name
is RECOMMENDED.
Digits, or letters, are supported in some packages, notably "DTMF".
Digits and letters are defined by the rules "Digit" and "Letter" in
the definition of digit maps. This definition refers to the digits
(0 to 9), to the asterisk or star ("*") and orthotrope, number or
pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
as the timer indication "T". These letters can be combined in "digit
string" that represents the keys that a user punched on a dial. In
addition, the letter "X" can be used to represent all digits (0 to
9). Also, extensions MAY define use of other letters. The need to
easily express the digit strings in earlier versions of the protocol
has a consequence on the form of event names:
An event name that does not denote a digit MUST always contain at
least one character that is neither a digit, nor one of the letters
A, B, C, D, T or X (such names also MUST NOT just contain the special
signs "*", or "#"). Event names consisting of more than one
character however may use any of the above.
A Call Agent may often have to ask a gateway to detect a group of
events. Two conventions can be used to denote such groups:
* The "*" and "all" wildcard conventions (see below) can be used to
detect any event belonging to a package, or a given event in many
packages, or any event in any package supported by the gateway.
* The regular expression Range notation can be used to detect a range
of digits.
The star sign (*) can be used as a wildcard instead of a package
name, and the keyword "all" can be used as a wildcard instead of an
event name:
* A name such as "foo/all" denotes all events in package "foo".
* A name such as "*/bar" denotes the event "bar" in any package
supported by the gateway.
* The name "*/all" denotes all events supported by the endpoint.
This specification purposely does not define any additional detail
for the "all packages" and "all events" wildcards. They provide
limited benefits, but introduce significant complexity along with the
potential for errors. Their use is consequently strongly
discouraged.
The Call Agent can ask a gateway to detect a set of digits or letters
either by individually describing those letters, or by using the
"range" notation defined in the syntax of digit strings. For
example, the Call Agent can:
* Use the letter "x" to denote" digits from 0 to 9.
* Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
sign.
The individual event codes are still defined in a package though
(e.g., the "DTMF" package).
Events can by default only be generated and detected on endpoints,
however events can be also be defined so they can be generated or
detected on connections rather than on the endpoint itself (see
Section 6.6). For example, gateways may be asked to provide a
ringback tone on a connection. When an event is to be applied on a
connection, the name of the connection MUST be added to the name of
the event, using an "at" sign (@) as a delimiter, as in:
G/rt@0A3F58
where "G" is the name of the package and "rt" is the name of the
event. Should the connection be deleted while an event or signal is
being detected or applied on it, that particular event detection or
signal generation simply stops. Depending on the signal, this may
generate a failure (see below).
The wildcard character "*" (star) can be used to denote "all
connections". When this convention is used, the gateway will
generate or detect the event on all the connections that are
connected to the endpoint. This applies to existing as well as
future connections created on the endpoint. An example of this
convention could be:
R/qa@*
where "R" is the name of the package and "qa" is the name of the
event.
When processing a command using the "all connections" wildcard, the
"*" wildcard character applies to all current and future connections
on the endpoint, however it will not be expanded. If a subsequent
command either explicitly (e.g., by auditing) or implicitly (e.g., by
persistence) refers to such an event, the "*" value will be used.
However, when the event is actually observed, that particular
occurrence of the event will include the name of the specific
connection it occurred on.
The wildcard character "$" can be used to denote "the current
connection". It can only be used by the Call Agent, when the event
notification request is "encapsulated" within a connection creation
or modification command. When this convention is used, the gateway
will generate or detect the event on the connection that is currently
being created or modified. An example of this convention is:
G/rt@$
When processing a command using the "current connection" wildcard,
the "$" wildcard character will be expanded to the value of the
current connection. If a subsequent command either explicitly (e.g.,
by auditing) or implicitly (e.g., by persistence) refers to such an
event, the expanded value will be used. In other words, the "current
connection" wildcard is expanded once, which is at the initial
processing of the command in which it was explicitly included.
The connection id, or a wildcard replacement, can be used in
conjunction with the "all packages" and "all events" conventions. For
example, the notation:
*/all@*
can be used to designate all events on all current and future
connections on the endpoint. However, as mentioned before, the use
of the "all packages" and "all events" wildcards are strongly
discouraged.
Signals are divided into different types depending on their behavior:
* On/off (OO): Once applied, these signals last until they are
turned off. This can only happen as the result of a reboot/restart
or a new SignalRequests where the signal is explicitly turned off
(see later). Signals of type OO are defined to be idempotent, thus
multiple requests to turn a given OO signal on (or off) are
perfectly valid and MUST NOT result in any errors. An On/Off
signal could be a visual message-waiting indicator (VMWI). Once
turned on, it MUST NOT be turned off until explicitly instructed to
by the Call Agent, or as a result of an endpoint restart, i.e.,
these signals will not turn off as a result of the detection of a
requested event.
* Time-out (TO): Once applied, these signals last until they are
either cancelled (by the occurrence of an event or by not being
included in a subsequent (possibly empty) list of signals), or a
signal-specific period of time has elapsed. A TO signal that times
out will generate an "operation complete" event. A TO signal could
be "ringback" timing out after 180 seconds. If an event occurs
prior to the 180 seconds, the signal will, by default, be stopped
(the "Keep signals active" action - see Section 2.3.3 - will
override this behavior). If the signal is not stopped, the signal
will time out, stop and generate an "operation complete" event,
about which the Call Agent may or may not have requested to be
notified. If the Call Agent has asked for the "operation complete"
event to be notified, the "operation complete" event sent to the
Call Agent SHALL include the name(s) of the signal(s) that timed
out (note that if parameters were passed to the signal, the
parameters will not be reported). If the signal was generated on a
connection, the name of the connection SHALL be included as
described above. Time-out signals have a default time-out value
defined for them, which MAY be altered by the provisioning process.
Also, the time-out period may be provided as a parameter to the
signal (see Section 3.2.2.4). A value of zero indicates that the
time-out period is infinite. A TO signal that fails after being
started, but before having generated an "operation complete" event
will generate an "operation failure" event which will include the
name of the signal that failed. Deletion of a connection with an
active TO signal will result in such a failure.
* Brief (BR): The duration of these signals is normally so short
that they stop on their own. If a signal stopping event occurs, or
a new SignalRequests is applied, a currently active BR signal will
not stop. However, any pending BR signals not yet applied MUST be
cancelled (a BR signal becomes pending if a NotificationRequest
includes a BR signal, and there is already an active BR signal). As
an example, a brief tone could be a DTMF digit. If the DTMF digit
"1" is currently being played, and a signal stopping event occurs,
the "1" would play to completion. If a request to play DTMF digit
"2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
would become pending.
Signal(s) generated on a connection MUST include the name of that
connection.
2.2 Usage of SDP
The Call Agent uses the MGCP to provide the endpoint with the
description of connection parameters such as IP addresses, UDP port
and RTP profiles. These descriptions will follow the conventions
delineated in the Session Description Protocol which is now an IETF
proposed standard, documented in RFC2327.
2.3 Gateway Control Commands
2.3.1 Overview of Commands
This section describes the commands of the MGCP. The service
consists of connection handling and endpoint handling commands.
There are currently nine commands in the protocol:
* The Call Agent can issue an EndpointConfiguration command to a
gateway, instructing the gateway about the coding characteristics
expected by the "line-side" of the endpoint.
* The Call Agent can issue a NotificationRequest command to a
gateway, instructing the gateway to watch for specific events such
as hook actions or DTMF tones on a specified endpoint.
* The gateway will then use the Notify command to inform the Call
Agent when the requested events occur.
* The Call Agent can use the CreateConnection command to create a
connection that terminates in an "endpoint" inside the gateway.
* The Call Agent can use the ModifyConnection command to change the
parameters associated with a previously established connection.
* The Call Agent can use the DeleteConnection command to delete an
existing connection. The DeleteConnection command may also be used
by a gateway to indicate that a connection can no longer be
sustained.
* The Call Agent can use the AuditEndpoint and AuditConnection
commands to audit the status of an "endpoint" and any connections
associated with it. Network management beyond the capabilities
provided by these commands is generally desirable. Such
capabilities are expected to be supported by the use of the Simple
Network Management Protocol (SNMP) and definition of a MIB which is
outside the scope of this specification.
* The Gateway can use the RestartInProgress command to notify the
Call Agent that a group of endpoints managed by the gateway is
being taken out-of-service or is being placed back in-service.
These services allow a controller (normally, the Call Agent) to
instruct a gateway on the creation of connections that terminate in
an "endpoint" attached to the gateway, and to be informed about
events occurring at the endpoint. An endpoint may be for example:
* A specific trunk circuit, within a trunk group terminating in a
gateway,
* A specific announcement handled by an announcement server.
Connections are logically grouped into "calls" (the concept of a
"call" has however little semantic meaning in MGCP itself). Several
connections, that may or may not belong to the same call, can
terminate in the same endpoint. Each connection is qualified by a
"mode" parameter, which can be set to "send only" (sendonly),
"receive only" (recvonly), "send/receive" (sendrecv), "conference"
(confrnce), "inactive" (inactive), "loopback", "continuity test"
(conttest), "network loop back" (netwloop) or "network continuity
test" (netwtest).
Media generated by the endpoint is sent on connections whose mode is
either "send only", "send/receive", or "conference", unless the
endpoint has a connection in "loopback" or "continuity test" mode.
However, media generated by applying a signal to a connection is
always sent on the connection, regardless of the mode.
The handling of the media streams received on connections is
determined by the mode parameters:
* Media streams received through connections in "receive",
"conference" or "send/receive" mode are mixed and sent to the
endpoint, unless the endpoint has another connection in "loopback"
or "continuity test" mode.
* Media streams originating from the endpoint are transmitted over
all the connections whose mode is "send", "conference" or
"send/receive", unless the endpoint has another connection in
"loopback" or "continuity test" mode.
* In addition to being sent to the endpoint, a media stream received
through a connection in "conference" mode is forwarded to all the
other connections whose mode is "conference". This also applies
when the endpoint has a connection in "loopback" or "continuity
test" mode. The details of this forwarding, e.g., RTP translator
or mixer, is outside the scope of this document.
Note that in order to detect events on a connection, the connection
must by default be in one of the modes "receive", "conference",
"send/receive", "network loopback" or "network continuity test". The
event detection only applies to the incoming media. Connections in
"sendonly", "inactive", "loopback", or "continuity test" mode will
thus normally not detect any events, although requesting to do so is
not considered an error.
The "loopback" and "continuity test" modes are used during
maintenance and continuity test operations. An endpoint may have
more than one connection in either "loopback" or "continuity test"
mode. As long as there is one connection in that particular mode,
and no other connection on the endpoint is placed in a different
maintenance or test mode, the maintenance or test operation shall
continue undisturbed. There are two flavors of continuity test, one
specified by ITU and one used in the US. In the first case, the test
is a loopback test. The originating switch will send a tone (the go
tone) on the bearer circuit and expects the terminating switch to
loopback the tone. If the originating switch sees the same tone
returned (the return tone), the COT has passed. If not, the COT has
failed. In the second case, the go and return tones are different.
The originating switch sends a certain go tone. The terminating
switch detects the go tone, it asserts a different return tone in the
backwards direction. When the originating switch detects the return
tone, the COT is passed. If the originating switch never detects the
return tone, the COT has failed.
If the mode is set to "loopback", the gateway is expected to return
the incoming signal from the endpoint back into that same endpoint.
This procedure will be used, typically, for testing the continuity of
trunk circuits according to the ITU specifications. If the mode is
set to "continuity test", the gateway is informed that the other end
of the circuit has initiated a continuity test procedure according to
the GR specification (see [22]). The gateway will place the circuit
in the transponder mode required for dual-tone continuity tests.
If the mode is set to "network loopback", the audio signals received
from the connection will be echoed back on the same connection. The
media is not forwarded to the endpoint.
If the mode is set to "network continuity test", the gateway will
process the packets received from the connection according to the
transponder mode required for dual-tone continuity test, and send the
processed signal back on the connection. The media is not forwarded
to the endpoint. The "network continuity test" mode is included for
backwards compatibility only and use of it is discouraged.
2.3.2 EndpointConfiguration
The EndpointConfiguration command can be used to specify the encoding
of the signals that will be received by the endpoint. For example,
in certain international telephony configurations, some calls will
carry mu-law encoded audio signals, while others will use A-law. The
Call Agent can use the EndpointConfiguration command to pass this
information to the gateway. The configuration may vary on a call by
call basis, but can also be used in the absence of any connection.
ReturnCode,
[PackageList]
<-- EndpointConfiguration(EndpointId,
[BearerInformation])
EndpointId is the name of the endpoint(s) in the gateway where
EndpointConfiguration executes. The "any of" wildcard convention
MUST NOT be used. If the "all of" wildcard convention is used, the
command applies to all the endpoints whose name matches the wildcard.
BearerInformation is a parameter defining the coding of the data sent
to and received from the line side. The information is encoded as a
list of sub-parameters. The only sub-parameter defined in this
version of the specification is the bearer encoding, whose value can
be set to "A-law" or "mu-law". The set of sub-parameters may be
extended.
In order to allow for extensibility, while remaining backwards
compatible, the BearerInformation parameter is conditionally optional
based on the following conditions:
* if Extension Parameters (vendor, package or other) are not used,
the BearerInformation parameter is REQUIRED,
* otherwise, the BearerInformation parameter is OPTIONAL.
When omitted, BearerInformation MUST retain its current value.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.3 NotificationRequest
The NotificationRequest command is used to request the gateway to
send notifications upon the occurrence of specified events in an
endpoint. For example, a notification may be requested for when a
gateway detects that an endpoint is receiving tones associated with
fax communication. The entity receiving this notification may then
decide to specify use of a different type of encoding method in the
connections bound to this endpoint and instruct the gateway
accordingly with a ModifyConnection Command.
ReturnCode,
[PackageList]
<-- NotificationRequest(EndpointId,
[NotifiedEntity,]
[RequestedEvents,]
RequestIdentifier,
[DigitMap,]
[SignalRequests,]
[QuarantineHandling,]
[DetectEvents,]
[encapsulated EndpointConfiguration])
EndpointId is the identifier for the endpoint(s) in the the gateway
where the NotificationRequest executes. The "any of" wildcard MUST
NOT be used.
NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
RequestIdentifier is used to correlate this request with the
notifications that it triggers. It will be repeated in the
corresponding Notify command.
RequestedEvents is a list of events, possibly qualified by event
parameters (see Section 3.2.2.4), that the gateway is requested to
detect and report. Such events may include, for example, fax tones,
continuity tones, or on-hook transition. Unless otherwise specified,
events are detected on the endpoint, however some events can be
detected on a connection. A given event MUST NOT appear more than
once in a RequestedEvents. If the parameter is omitted, it defaults
to empty.
To each event is associated one or more actions, which can be:
* Notify the event immediately, together with the accumulated list of
observed events,
* Swap audio,
* Accumulate the event in an event buffer, but don't notify yet,
* Accumulate according to Digit Map,
* Keep Signal(s) active,
* Process the Embedded Notification Request,
* Ignore the event.
Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
Notification Request, and Ignore is REQUIRED. Support for Accumulate
according to Digit Map is REQUIRED on any endpoint capable of
detecting DTMF. Support for any other action is OPTIONAL. The set
of actions can be extended.
A given action can by default be specified for any event, although
some actions will not make sense for all events. For example, an
off-hook event with the Accumulate according to Digit Map action is
valid, but will of course immediately trigger a digit map mismatch
when the off-hook event occurs. Needless to say, such practice is
discouraged.
Some actions can be combined as shown in the table below, where "Y"
means the two actions can be combined, and "N" means they cannot:
--------------------------------------------------------------
| | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor |
|--------------------------------------------------------------|
| Notif | N | Y | N | N | Y | Y* | N |
| Swap | - | N | Y | N | N | N | Y |
| Accum | - | - | N | N | Y | Y | N |
| AccDi | - | - | - | N | Y | N | N |
| KeSiA | - | - | - | - | N | Y | Y |
| EmbNo | - | - | - | - | - | N | N |
| Ignor | - | - | - | - | - | - | N |
--------------------------------------------------------------
Note (*): The "Embedded Notification Request" can only be
combined with "Notify", if the gateway is allowed to issue more
than one Notify command per Notification request (see below and
Section 4.4.1).
If no action is specified, the Notify action will be applied. If one
or more actions are specified, only those actions apply. When two or
more actions are specified, each action MUST be combinable with all
the other actions as defined by the table above - the individual
actions are assumed to occur simultaneously.
If a client receives a request with an invalid or unsupported action
or an illegal combination of actions, it MUST return an error to the
Call Agent (error code 523 - unknown or illegal combination of
actions, is RECOMMENDED).
In addition to the RequestedEvents parameter specified in the
command, some MGCP packages may contain "persistent events" (this is
generally discouraged though - see Appendix B for an alternative).
Persistent events in a given package are always detected on an
endpoint that implements that package. If a persistent event is not
included in the list of RequestedEvents, and the event occurs, the
event will be detected anyway and processed like all other events, as
if the persistent event had been requested with a Notify action. A
NotificationRequest MUST still be in place for a persistent event to
trigger a Notify though. Thus, informally, persistent events can be
viewed as always being implicitly included in the list of
RequestedEvents with an action to Notify, although no glare
detection, etc., will be performed.
Non-persistent events are those events that need to be explicitly
included in the RequestedEvents list. The (possibly empty) list of
requested events completely replaces the previous list of requested
events. In addition to the persistent events, only the events
specified in the requested events list will be detected by the
endpoint. If a persistent event is included in the RequestedEvents
list, the action specified will replace the default action associated
with the event for the life of the RequestedEvents list, after which
the default action is restored. For example, if "off-hook"was a
persistent event, the "Ignore off-hook" action was specified, and a
new request without any off-hook instructions were received, the
default "Notify off-hook" operation would be restored.
The gateway will detect the union of the persistent events and the
requested events. If an event is not included in either list, it
will be ignored.
The Call Agent can send a NotificationRequest with an empty (or
omitted) RequestedEvents list to the gateway. The Call Agent can do
so, for example, to a gateway when it does not want to collect any
more DTMF digits. However, persistent events will still be detected
and notified.
The Swap Audio action can be used when a gateway handles more than
one connection on an endpoint. This will be the case for call
waiting, and possibly other feature scenarios. In order to avoid the
round-trip to the Call Agent when just changing which connection is
attached to the audio functions of the endpoint, the
NotificationRequest can map an event (usually hook flash, but could
be some other event) to a local swap audio function, which selects
the "next" connection in a round robin fashion. If there is only one
connection, this action is effectively a no-op. If there are more
than two connections, the order is undefined. If the endpoint has
exactly two connections, one of which is "inactive", the other of
which is in "send/receive" mode, then swap audio will attempt to make
the "send/receive" connection "inactive", and vice versa. This
specification intentionally does not provide any additional detail on
the swap audio action.
If signal(s) are desired to start when an event being looked for
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new digit map as well. The
semantics of the embedded NotificationRequest is as if a new
NotificationRequest was just received with the same NotifiedEntity,
RequestIdentifier, QuarantineHandling and DetectEvents. When the
"Embedded NotificationRequest" is activated, the "current dial
string" will be cleared; however the list of observed events and the
quarantine buffer will be unaffected (if combined with a Notify, the
Notify will clear the list of observed events though - see Section
4.4.1). Note, that the Embedded NotificationRequest action does not
accumulate the triggering event, however it can be combined with the
Accumulate action to achieve that. If the Embedded
NotificationRequest fails, an Embedded NotificationRequest failure
event SHOULD be generated (see Appendix B).
MGCP implementations SHALL be able to support at least one level of
embedding. An embedded NotificationRequest that respects this
limitation MUST NOT contain another Embedded NotificationRequest.
DigitMap is an optional parameter that allows the Call Agent to
provision the endpoint with a digit map according to which digits
will be accumulated. If this optional parameter is absent, the
previously defined value is retained. This parameter MUST be
defined, either explicitly or through a previous command, if the
RequestedEvents parameter contains a request to "accumulate according
to the digit map". The collection of these digits will result in a
digit string. The digit string is initialized to a null string upon
reception of the NotificationRequest, so that a subsequent
notification only returns the digits that were collected after this
request. Digits that were accumulated according to the digit map are
reported as any other accumulated event, in the order in which they
occur. It is therefore possible that other events accumulated are
found in between the list of digits. If the gateway is requested to
"accumulate according to digit map" and the gateway currently does
not have a digit map for the endpoint in question, the gateway MUST
return an error (error code 519 - endpoint does not have a digit map,
is RECOMMENDED).
SignalRequests is an optional parameter that contains the set of
signals that the gateway is asked to apply. When omitted, it
defaults to empty. When multiple signals are specified, the signals
MUST be applied in parallel. Unless otherwise specified, signals are
applied to the endpoint. However some signals can be applied to a
connection. Signals are identified by their name, which is an event
name, and may be qualified by signal parameters (see Section
3.2.2.4). The following are examples of signals:
* Ringing,
* Busy tone,
* Call waiting tone,
* Off hook warning tone,
* Ringback tones on a connection.
Names and descriptions of signals are defined in the appropriate
package.
Signals are, by default, applied to endpoints. If a signal applied
to an endpoint results in the generation of a media stream (audio,
video, etc.), then by default the media stream MUST NOT be forwarded
on any connection associated with that endpoint, regardless of the
mode of the connection. For example, if a call-waiting tone is
applied to an endpoint involved in an active call, only the party
using the endpoint in question will hear the call-waiting tone.
However, individual signals may define a different behavior.
When a signal is applied to a connection that has received a
RemoteConnectionDescriptor, the media stream generated by that signal
will be forwarded on the connection regardless of the current mode of
the connection (including loopback and continuity test). If a
RemoteConnectionDescriptor has not been received, the gateway MUST
return an error (error code 527 - missing RemoteConnectionDescriptor,
is RECOMMENDED). Note that this restriction does not apply to
detecting events on a connection.
When a (possibly empty) list of signal(s) is supplied, this list
completely replaces the current list of active time-out signals.
Currently active time-out signals that are not provided in the new
list MUST be stopped and the new signal(s) provided will now become
active. Currently active time-out signals that are provided in the
new list of signals MUST remain active without interruption, thus the
timer for such time-out signals will not be affected. Consequently,
there is currently no way to restart the timer for a currently active
time-out signal without turning the signal off first. If the time-
out signal is parameterized, the original set of parameters MUST
remain in effect, regardless of what values are provided
subsequently. A given signal MUST NOT appear more than once in a
SignalRequests. Note that applying a signal S to an endpoint,
connection C1 and connection C2, constitutes three different and
independent signals.
The action triggered by the SignalRequests is synchronized with the
collection of events specified in the RequestedEvents parameter. For
example, if the NotificationRequest mandates "ringing" and the
RequestedEvents asks to look for an "off-hook" event, the ringing
SHALL stop as soon as the gateway detects an off-hook event. The
formal definition is that the generation of all "Time Out" signals
SHALL stop as soon as one of the requested events is detected, unless
the "Keep signals active" action is associated to the detected event.
The RequestedEvents and SignalRequests may refer to the same event
definitions. In one case, the gateway is asked to detect the
occurrence of the event, and in the other case it is asked to
generate it. The specific events and signals that a given endpoint
can detect or perform are determined by the list of packages that are
supported by that endpoint. Each package specifies a list of events
and signals that can be detected or performed. A gateway that is
requested to detect or perform an event belonging to a package that
is not supported by the specified endpoint MUST return an error
(error code 518 - unsupported or unknown package, is RECOMMENDED).
When the event name is not qualified by a package name, the default
package name for the endpoint is assumed. If the event name is not
registered in this default package, the gateway MUST return an error
(error code 522 - no such event or signal, is RECOMMENDED).
The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when a time-out signal(s)
should stop.
If signal(s) are desired to start as soon as a "looked-for" event
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new Digit Map as well. The
embedded NotificationRequest action allows the Call Agent to set up a
"mini-script" to be processed by the gateway immediately following
the detection of the associated event. Any SignalRequests specified
in the embedded NotificationRequest will start immediately.
Considerable care must be taken to prevent discrepancies between the
Call Agent and the gateway. However, long-term discrepancies should
not occur as a new SignalRequests completely replaces the old list of
active time-out signals, and BR-type signals always stop on their
own. Limiting the number of On/Off-type signals is encouraged. It
is considered good practice for a Call Agent to occasionally turn on
all On/Off signals that should be on, and turn off all On/Off signals
that should be off.
The Ignore action can be used to ignore an event, e.g., to prevent a
persistent event from being notified. However, the synchronization
between the event and an active time-out signal will still occur by
default (e.g., a time-out dial-tone signal will stop when an off-hook
occurs even if off-hook was a requested event with action "Ignore").
To prevent this synchronization from happening, the "Keep Signal(s)
Active" action will have to be specified as well.
The optional QuarantineHandling parameter specifies the handling of
"quarantine" events, i.e., events that have been detected by the
gateway before the arrival of this NotificationRequest command, but
have not yet been notified to the Call Agent. The parameter provides
a set of handling options (see Section 4.4.1 for details):
* whether the quarantined events should be processed or discarded
(the default is to process them).
* whether the gateway is expected to generate at most one
notification (step by step), or multiple notifications (loop), in
response to this request (the default is at most one).
When the parameter is absent, the default value is assumed.
We should note that the quarantine-handling parameter also governs
the handling of events that were detected and processed but not yet
notified when the command is received.
DetectEvents is an optional parameter, possibly qualified by event
parameters, that specifies a list of events that the gateway is
requested to detect during the quarantine period. When this
parameter is absent, the events to be detected in the quarantine
period are those listed in the last received DetectEvents list. In
addition, the gateway will also detect persistent events and the
events specified in the RequestedEvents list, including those for
which the "ignore" action is specified.
Some events and signals, such as the in-line ringback or the quality
alert, are performed or detected on connections terminating in the
endpoint rather than on the endpoint itself. The structure of the
event names (see Section 2.1.7) allows the Call Agent to specify the
connection(s) on which the events should be performed or detected.
The NotificationRequest command may carry an encapsulated
EndpointConfiguration command, that will apply to the same
endpoint(s). When this command is present, the parameters of the
EndpointConfiguration command are included with the normal parameters
of the NotificationRequest, with the exception of the EndpointId,
which is not replicated.
The encapsulated EndpointConfiguration command shares the fate of the
NotificationRequest command. If the NotificationRequest is rejected,
the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.4 Notify
Notifications with the observed events are sent by the gateway via
the Notify command when a triggering event occurs.
ReturnCode,
[PackageList]
<-- Notify(EndpointId,
[NotifiedEntity,]
RequestIdentifier,
ObservedEvents)
EndpointId is the name for the endpoint in the gateway which is
issuing the Notify command. The identifier MUST be a fully qualified
endpoint identifier, including the domain name of the gateway. The
local part of the name MUST NOT use any of the wildcard conventions.
NotifiedEntity is a parameter that identifies the entity which
requested the notification. This parameter is equal to the
NotifiedEntity parameter of the NotificationRequest that triggered
this notification. The parameter is absent if there was no such
parameter in the triggering request. Regardless of the value of the
NotifiedEntity parameter, the notification MUST be sent to the
current "notified entity" for the endpoint.
RequestIdentifier is a parameter that repeats the RequestIdentifier
parameter of the NotificationRequest that triggered this
notification. It is used to correlate this notification with the
request that triggered it. Persistent events will be viewed here as
if they had been included in the last NotificationRequest. An
implicit NotificationRequest MAY be in place right after restart -
the RequestIdentifier used for it will be zero ("0") - see Section
4.4.1 for details.
ObservedEvents is a list of events that the gateway detected and
accumulated. A single notification may report a list of events that
will be reported in the order in which they were detected (FIFO).
The list will only contain the identification of events that were
requested in the RequestedEvents parameter of the triggering
NotificationRequest. It will contain the events that were either
accumulated (but not notified) or treated according to digit map (but
no match yet), and the final event that triggered the notification or
provided a final match in the digit map. It should be noted that
digits MUST be added to the list of observed events as they are
accumulated, irrespective of whether they are accumulated according
to the digit map or not. For example, if a user enters the digits
"1234" and some event E is accumulated between the digits "3" and "4"
being entered, the list of observed events would be "1, 2, 3, E, 4".
Events that were detected on a connection SHALL include the name of
that connection as in "R/qa@0A3F58" (see Section 2.1.7).
If the list of ObservedEvents reaches the capacity of the endpoint,
an ObservedEvents Full event (see Appendix B) SHOULD be generated
(the endpoint shall ensure it has capacity to include this event in
the list of ObservedEvents). If the ObservedEvents Full event is not
used to trigger a Notify, event processing continues as before