1. Introduction

This document describes an application programming interface (SGCI) and a corresponding protocol (SGCP) for controlling Telephony Gateways from external call control elements. A telephony gateway is a network element that provides conversion between the audio signals carried on telephone circuits and data packets carried over the Internet or over other packet networks. Example of gateways are:

• Trunking gateways, that interface between the telephone network and a Voice over IP network. Such gateways typically manage a large number of digital circuits.
• Residential gateways, that provide a traditional analog (RJ11) interface to a Voice over IP network.
• Network Access Servers, that can attach a "modem" to a telephone circuit and provide data access to the Internet. We expect that, in the future, the same gateways will combine Voice over IP services and Network Access services.

The SGCP assumes a call control architecture where the call control "intelligence" is outside the gateways and handled by external call control elements. The SGCP assumes that these call control elements, or Call Agents, will synchronize with each other to send coherent commands to the gateways under their control. The SGC defined in this document does not define a mechanism for synchronizing Call Agents.

The SGCP assumes a connection model where the basic constructs are endpoints and connections. Endpoints are sources or sinks of data and could be physical or virtual. Examples of physical endpoints are:

• An interface on a gateway that terminates a trunk connected to a PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that terminates trunks is called a trunk gateway.
• An interface on a gateway that terminates an analog POTS connection to a phone, key system, PBX, etc. A gateway that terminates residential POTS lines (to phones) is called a residential gateway.

An example of a virtual endpoint is an audio source in an audio-content server. Creation of physical endpoints requires hardware installation, while creation of virtual endpoints can be done by software.

Connections may be either point to point or multipoint. A point to point connection is an association between two endpoints with the purpose of transmitting data between these endpoints. Once this association is established for both endpoints, data transfer between these endpoints can take place. A multipoint connection is established by connecting the endpoint to a multipoint session.

1.1 Relation with the H.323 standards

The SGCP is designed as an internal protocol within a distributed system that appears to the outside as a single VoIP gateway. This system is composed of a Call Agent, that may or may not be distributed over several computer platforms, and of a set of gateways. In a typical configuration, this distributed gateway system will interface on one side with one or more telephony (i.e. circuit) switches, and on the other side with H.323 conformant systems, as indicated in the following table:

In the SGCP model, the gateways focus on the audio signal translation function, while the Call Agent handles the signaling and call processing functions. As a consequence, the Call Agent implements the "signaling" layers of the H.323 standard, and presents itself as a "Gatekeeper" to the H.323 systems. Calls are established using the "Gatekeeper Routed" call model.

1.2 Relation with the IETF standards

While H.323 is the recognized standard for VoIP terminals, the IETF has also produced specifications for other types of multi-media applications. These other specifications include:

• the Session Description Protocol (SDP), RFC 2327,
• the Session Announcement Protocol (SAP),
• the Session Initiation Protocol (SIP),
• the Real Time Streaming Protocol (RTSP), RFC 2326.

The latter three specifications are in fact alternative signaling standards that allow for the transmission of a session description to an interested party. SAP is used by multicast session managers to distribute a multicast session description to a large group of recipients, SIP is used to invite an individual user to take part in a point-to-point or unicast session, RTSP is used to interface a server that provides real time data. In all three cases, the session description is described according to SDP; when audio is transmitted, it is transmitted through the Real-time Transport Protocol, RTP.

The distributed gateway systems and SGCP will enable PSTN telephony users to access sessions set up using SAP, SIP or RTSP. The Call Agent provides for signaling conversion, according to the following table:

The SDP standard has a pivotal status in this architecture. We will see in the following description that we also use it to carry session descriptions in SGCP.

1.3 Definitions

Trunk: A communication channel between two switching systems. E.g., a DS0 on a T1 or E1 line.

2. Simple Gateway Control Interface

The interface functions provide for connection control and endpoint control. Both use the same system model and the same naming conventions.

2.1 Model and naming conventions.

The SGCP assumes a connection model where the basic constructs are endpoints and connections. Connections are grouped in calls. One or more connections can belong to one call. Connections and calls are set up at the initiative of one or several Call Agents.

2.1.1 Names of endpoints

Endpoints names have two components:

• the domain name of the gateway that is managing the endpoint ,
• a local name within that gateway.

In the case of trunking gateways, endpoints are trunk circuits linking a gateway to a telephone switch. These circuits are typically grouped into a digital multiplex, that is connected to the gateway by a physical interface. Such circuits are named in three contexts:

• In the ISUP protocol, trunks are grouped into trunk groups, identified by the SS7 point codes of the switches that the group connects. Circuits within a trunk group are identified by a circuit number (CIC in ISUP).
• In the gateway configuration files, physical interfaces are typically identified by the name of the interface, an arbitrary text string. When the interface multiplexes several circuits, individual circuits are typically identified by a circuit number.
• In SGCP, the endpoints are identified by an endpoint name.

The Call Agents use configuration databases to map ranges of circuit numbers within an ISUP trunk group to corresponding ranges of circuits in a multiplex connected to a gateway through a physical interface. The gateway will be identified, in SGCP, by a domain name. The local name will be structured to encode both the name of the physical interface, for example X35V3+A4, and the circuit number within the multiplex connected to the interface, for example 13. The circuit number will be separated from the name of the interface by a fraction bar, as in:

X35V3+A4/13

The circuit number can be omitted when the physical interface only includes one circuit, or if the Call Agent requests the gateway to choose one available circuit within a multiplex.

Other types of endpoints will use different conventions. For example, an endpoint that produces an "all lines busy" announcement could be named:

all-lines-busy/loop@announce-23.whatever.net

The exact syntax of such names should be specified in the corresponding server specification.

2.1.2 Names of calls

Calls are identified by unique identifiers, independent of the underlying platforms or agents. These identifiers are created by the Call Agent. They are treated in SGCP as unstructured octet strings.

Call identifiers are expected to be unique within the system. When a Call Agent builds several connections that pertain to the same call, either on the same gateway or in different gateways, these connections will all be linked to the same call through the globally unique identifier. This identifier can then be used by accounting or management procedures, which are outside the scope of SGCP.

2.1.3 Names of connections

Connection identifiers are created by the gateway when it is requested to create a connection. They identify the connection within the context of an endpoint and a call. They are treated in SGCP as unstructured octet strings. The gateway should make sure that a proper waiting period, at least 3 minutes, elapses between the end of a connection that used this identifier and its use in a new connection.

2.1.4 Names of Call Agents and other entities

The simple gateway control protocol has been designed to allow the implementation of redundant Call Agents, for enhanced network reliability. This means that there is no fixed binding between entities and hardware platforms or network interfaces.

Reliability can be improved by the following precautions:

• Entities such as endpoints or Call Agents are identified by their domain name, not their network addresses. Several addresses can be associated with a domain name. If a command or a response cannot be forwarded to one of the network addresses, implementations should retry the transmission using another address.
• Entities may move to another platform. The association between a logical name (domain name) and the actual platform are kept in the domain name service. Call Agents and Gateways should keep track of the time-to-live of the record they read from the DNS. They should query the DNS to refresh the information if the time to live has expired.

2.1.5 Digit maps

The Call Agent can ask the gateway to collect digits dialed by the user. This facility is intended to be used with residential gateways to collect the numbers that a user dials; it may also be used with trunking gateways and access gateways alike, to collect the access codes, credit card numbers and other numbers requested by call control services.

An alternative procedure is for the gateway to notify the Call Agent of the dialed digits, as soon as they are dialed. However, such a procedure generates a large number of interactions. It is preferable to accumulate the dialed numbers in a buffer, and to transmit them in a single message.

The problem with this accumulation approach, however, is that it is hard for the gateway to predict how many numbers it needs to accumulate before transmission. For example, using the phone on our desk, we can dial the following numbers:

The solution to this problem is to load the gateway with a digit map that 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 following BNF notation:

Digit ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

Timer ::= "T"-- matches the detection of a timer

Letter::= Digit | Timer | "#" | "*" | "A" | "B" | "C" | "D"

Range ::= "x" -- matches any digit

| "[" Letters "]" -- matches any of the specified letters

Letters ::= Subrange | Subrange Letters

Subrange ::= Letter -- matches the specified letter

| Digit "-" Digit -- matches any digit between first and last

Position ::= Letter

| Range

StringElement::= Position-- matches an occurrence of the position

| Position "."-- matches an arbitrary number of occurrences

-- of the position, including 0

String::= StringElement | StringElement String

StringList::= String | String "|" StringList

DigitMap::= String | "(" StringList ")"

A DigitMap, according to this syntax, is defined either by a "string" or by a list of strings. Each string in the list is an alternative numbering scheme. A gateway that detects digits, letters or timers will:

1. Add the event parameter code as a token to the end of an internal state variable called the "current dial string"
2. Apply the current dial string to the digit map table, attempting a match to each regular expression in the Digit Map in lexical order
3. If the result is under-qualified (partially matches at least one entry in the digit map), do nothing further.

If the result matches, or is over-qualified (i.e. no further digits could possibly produce a match), send the current digit string to the Call Agent.

Digit maps are provided to the gateway by the Call Agent, whenever the Call Agent instructs the gateway to listen for digits.

2.2 Usage of SDP

The Call Agent uses the SGCP to provision the gateways 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 RFC 2327.

SDP allows for description of multimedia conferences. This version limits SDP usage to the setting of audio circuits and data access circuits. The initial session descriptions contain the description of exactly one media, of type "audio" for audio connections, "nas" for data access.

2.3 Gateway Control Functions

This section describes the commands of the SGCP. The service consists of connection handling and endpoint handling commands. There are five commands in the protocol:

• 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 to 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.

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 grouped into "calls". Several connections may belong to the same call, terminating in the same endpoint . Each connection is qualified by a "mode" parameter, which can be set to "send", "receive", "send/receive", data, "inactive", "loopback" or "continuity test."

The handling of the audio signals received on these connections is determined by the mode parameters:

• Audio signals received in data packets through connections in "receive" or "send/receive" mode are mixed and sent to the endpoint.
• Audio signals originating from the endpoint are transmitted over all the connections whose mode is "send" or "send/receive."

The "loopback" and "continuity test" modes are used during maintenance and continuity test operations. 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 expect the terminating switch to loopback the circuit. 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. The gateway will place the circuit in the transponder mode required for dual-tone continuity tests.

2.3.1 NotificationRequest

The NotificationRequest commands are 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 decide to use a different type of encoding method in the connections bound to this endpoint.

        NotificationRequest( EndpointId,
                             NotifiedEntity,
                             RequestedEvents,
                             RequestIdentifier,
                             DigitMap,
                             SignalRequests)

EndpointId is the identifier for the endpoint in the gateway where NotificationRequest executes.

RequestIdentifier is used to correlate this request with the notifications that it triggers.

RequestedEvents is a list of events that the gateway is requested to detect and report. Such events include, for example:

• fax tones,
• modem tones,
• continuity tone,
• continuity detection (as a result of a continuity test),
• on-hook transition (occurring in classic telephone sets when the user hangs-up the handset),
• off-hook transition (occurring in classic telephone sets when the user lifts the handset),
• flash hook (occurring in classic telephone sets when the user briefly presses the hook that holds the handset),
• wink,
• DTMF digits (or pulse digits).

To each event is associated an action, which can be:

• Notify the event immediately, together with the accumulated list of observed events,
• Swap audio,
• Accumulate according to Digit Map,
• Ignore the event.

Events that are not specified in the list will, by default, be ignored.

The Swap Audio action can be used when a gateway handles more than one active connection on an endpoint. This will be the case for three-way calling, 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 function swap audio, which selects the "next" connection in a round robin fashion. If there is only one connection, this action is effectively a no-op.

Hook transition events are normally observed only by access gateways. Tone detection can be done by any gateway, including trunking gateways. The Call Agent can send a NotificationRequest whose RequestedEvent list is empty. It will do so, for example, to an access gateway when it does not want to collect any more DTMF digits.

DigitMap is an optional parameter that allows the Call Agent to provision the gateways with a digit map according to which digits will be accumulated. This parameter must be present if the RequestedEvent parameters contain an 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.

SignalRequests is a parameter that contains the set of actions that the gateway is asked to perform on the endpoint , such as, for example:

• Ringing,
• Distinctive ringing, which can occur in 8 variants numbered 0 to 7,
• Ring back tone,
• Dial tones,
• Intercept tone,
• Network Congestion tone,
• Busy tone,
• Confirm tone,
• Answer tone,
• Call waiting tone,
• Off hook warning tone,
• Preemption tone,
• Continuity tones,
• Continuity test,
• Announcement (completed by an announcement name and a parameter),
• ASDI display (and string to be displayed).

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 event request ask to look for an "off-hook" event, the ringing shall stop as soon as the gateway detect an off hook event.

The specific definition of actions that are requested via these SignalRequests, such as the duration of and frequency of a DTMF digit, is outside the scope of SGC. This definition may vary from location to location and hence from gateway to gateway.

The RequestedEvents and SignalRequests refer to the same events. 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. There are only a few exception to this rule, notably the ASDI display, which can only be signaled but not detected, and the fax and modem tones, which can be detected but can not be signaled. However, we cannot necessarily expect all endpoint s to detect all events. For example, a digital trunk interface would normally not detect hook events, and may not be able to modulate DTMF digit; signals such as "wink" only make sense in some types of trunks. Gateways that receive a request to detect an event that they are not equipped to detect, or to generate a signal that they are not equipped to generate, should refuse the request and return an error code. Similarly, gateways that are equipped to generate announcements but cannot generate the specific announcement that the Call Agent requested, should return an appropriate error code.

The Call Agent can send a NotificationRequest whose requested signal list is empty. It will do so for example when tone generation should stop.

2.3.2 Notifications

Notifications are sent via the Notify command and are sent by the gateway when the observed events occur.

           Notify( EndpointId,
                   NotifiedEntity,
                   RequestIdentifier,
                   ObservedEvents)

EndpointId is the identifier for the endpoint in the gateway which is issuing the Notify command.

NotifiedEntity is an optional parameter that identifies the entity to which the notifications is sent. 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. In this case, the notification is sent to the entity from which the request was received.

RequestIdentifier is parameter that repeats the RequestIdentifier parameter of the NotificationRequest that triggered this notification. It is used to correlate this notifications with the request that triggered it.

ObservedEvents is a list of events that the gateway detected. A single notification may report a list of events that will be reported in the order in which they were detected. The list may 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 detection or provided a final match in the digit map.

2.3.3 CreateConnection

This command is used to create a connection.

    ConnectionId,
    [SpecificEndPointId,] 
    [LocalConnectionDescriptor] 
             <--- CreateConnection(CallId, 
                                      EndpointId,
                                      NotifiedEntity,
                                      LocalConnectionOptions, 
                                      Mode, 
                                      RemoteConnectionDescriptor,
                                      RequestedEvents,
                                      RequestIdentifier,
                                      DigitMap,
                                      SignalRequests)

This function creates a connection between two endpoints. A connection is defined by its endpoints. The input parameters in CreateConnection provide the data necessary to build a gateway's "view" of a connection.

CallId is a globally unique parameter that identifies the call (or session) to which this connection belongs. This parameter is unique within the whole network of gateways; connections that belong to the same call share the same call-id. The call-id can be used to identify calls for reporting and accounting purposes.

EndpointId is the identifier for the connection endpoint in the gateway where CreateConnection executes. The EndpointId can be fully-specified by assigning a value to the parameter EndpointId in the function call or it may be under-specified and the full value will be assigned by the gateway and its complete value returned in the SpecificEndPointId parameter of the response.

The NotifiedEntity is an optional parameter that specifies where the Notify or DeleteConnection commands should be sent. If the parameter is absent, the Notify or DeleteConnection commands should be sent to the originator of the CreateConnection command.

LocalConnectionOptions is a structure that describes the characteristics of the data communications from the point of view of the gateway executing CreateConnection. The fields contained in LocalConnectionOptions are the following:

• Encoding Method
• Packetization period
• Bandwidth
• Type of Service,
• Usage of echo cancellation

The values of these fields are defined in the SDP standard. For each of the first three fields, the Call Agent has three options:

• It may state exactly one value, which the gateway will then use for the connection,
• It may provide a loose specification, such as a list of allowed encoding methods or a range of packetization periods,
• It may simply provide a bandwidth indication, leaving the choice of encoding method and packetization period to the gateway.

The bandwidth specification shall not contradict the specification of encoding methods and packetization period. If an encoding method is specified, then the gateway is authorized to use it, even if it results in the usage of a larger bandwidth than specified.

The LocalConnectionOptions parameter may be absent in the case of a data call.

The Type of Service specifies the class of service that will be used for the connection. When the connection is transmitted over an IP network, the parameters encodes the 8-bit type of service value parameter of the IP header. When the Type of Service is not specified, the gateway shall use a default or configured value.

By default, the telephony gateways always perform echo cancellation. However, it is necessary, for some calls, to turn off these operations. The echo cancellation parameter can have two values, "on" (when the echo cancellation is requested) and "off" (when it is turned off.)

RemoteConnectionDescriptor is the connection descriptor for the remote side of a connection, on the other side of the IP network. It includes the same fields as in the LocalConnectionDescriptor, i.e. the fields that describe a session according to the SDP standard. This parameter may have a null value when the information for the remote end is not known yet. This occurs because the entity that builds a connection starts by sending a CreateConnection to one of the two gateways involved in it. For the first CreateConnection issued, there is no information available about the other side of the connection. This information may be provided later via a ModifyConnection call. In the case of data connections (mode=data), this parameter describes the characteristics of the data connection.

Mode indicates the mode of operation for this side of the connection. The options are: FullDuplex, ReceiveOnly, SendOnly, Inactive, Loopback, Continuity Test and Data

• FullDuplex indicates that this side of the connection sends data generated by the associated endpoint and can receive data from the remote end and feed it to the endpoint.
• ReceiveOnly indicates this connection end can only receive data form the remote end and feed it to the associated endpoint.
• SendOnly indicates that this connection end can only send data from its endpoint to the other end of the connection.
• Inactive indicates that this connection end does not send or receive any data. The connection exists but is not active. It has to be explicitly activated by a ModifyConnection command.
• Loopback indicates that the circuit to which the endpoint refers should be placed in loopback mode, so that audio signals received from the phone circuit are sent back on this same circuit.
• Continuity Test indicates that the other end of the circuit has initiated a continuity test procedure according to the GR specification. The gateway will place the circuit in the transponder mode required for dual-tone continuity tests.
• Data indicates that the circuit to which the endpoint refers will used for network access (such as Internet access) rather than for Internet telephony.

The gateway returns a ConnectionId, that uniquely identifies the connection within one endpoint , and a LocalConnectionDescriptor, which is a session description that contains information about addresses and RTP ports, as defined in SDP. The LocalConnectionDescriptor is not returned in the case of data connections. The SpecificEndPointId is an optional parameter that identifies the responding endpoint. It can be used when the EndpointId argument referred to a generic endpoint name. When a SpecificEndPointId is returned, the Call Agent should use it as the EndpointId value is successive commands referring to this call.

After receiving a "CreateConnection" request that did not include a RemoteConnectionDescriptor parameter, a gateway is in an ambiguous situation. Because it has exported a LocalConnectionDescriptor parameter, it can potentially receive packets. Because it has not yet received the RemoteConnectionDescriptor parameter of the other gateway, it does not know whether the packets that it receives have been authorized by the Call Agent. It must thus navigate between two risks, i.e. clipping some important announcements or listening to insane data. The behavior of the gateway is determined by the value of the Mode parameter:

• If the mode was set to ReceiveOnly, the gateway should accept the voice signals and transmit them through the endpoint .
• If the mode was set to Inactive, Loopback or Continuity Test, the gateway should refuse the voice signals.

Note that the mode values FullDuplex and SendOnly don't make sense in this situation. They should be treated as ReceiveOnly and Inactive.

The RequestedEvents, RequestIdentifier, DigitMap, and SignalRequests parameters are optional. They can be used by the Call Agent to transmit a NotificationRequest that is executed simultaneously with the creation of the connection. For example, when the Call Agent wants to initiate a call to an residential gateway, it should:

• ask the residential gateway to prepare a connection, in order to be sure that the user can start speaking as soon as the phone goes off hook,
• ask the residential gateway to start ringing,
• ask the residential gateway to notify the Call Agent when the phone goes off-hook.

This can be accomplished in a single CreateConnection command, by also transmitting the RequestedEvent parameters for the off hook event, and the SignalRequest parameter for the ringing signal.

When these parameters are present, the creation and the NotificationRequests should be synchronized, which means that both should be accepted, or both refused. In our example, the CreateConnection may be refused if the gateway does not have sufficient resources, or cannot get adequate resources from the local network access, and the off-hook NotificationRequest can be refused in the glare condition, if the user is already off-hook. In this example, the phone should not ring if the connection cannot be established, and the connection should not be established if the user is already off hook.

2.3.4ModifyConnection

This command is used to modify the characteristics of a gateway's "view" of a connection. This "view" of the call includes both the local connection descriptors as well as the remote connection descriptor.

   [LocalConnectionDescriptor] 
            <--- ModifyConnection(CallId, 
                                  EndpointId, 
                                  ConnectionId, 
                                  NotifiedEntity,
                                  LocalConnectionOptions, 
                                  Mode,
                                  RemoteConnectionDescriptor,
                                  RequestedEvents,
                                  RequestIdentifier,
                                  DigitMap,
                                  SignalRequests)

The parameters used are the same as in the CreateConnection command, with the addition of a ConnectionId that identifies the connection within the call. This parameter is returned by the CreateConnection function, as part of the local connection descriptor. It uniquely identifies the connection within the call.

The ModifyConnection command can be used to affect parameters of a connection in the following ways:

• Provide information on the other end of the connection, through the RemoteConnectionDescriptor.
• Activate or deactivate the connection, by changing the value of the Mode parameter. This can occur at any time during the connection, with arbitrary parameter values.
• Change the sending parameters of the connection, for example by switching to a different coding scheme, changing the packetization period, or modifying the handling of echo cancellation.

Connections can only be activated if the RemoteConnectionDescriptor has been provided to the gateway.

The command will only return a LocalConnectionDescriptor if the local connection parameters, such as RTP ports, were modified. (Usage of this feature is actually for further study.)

The RequestedEvents, RequestIdentifier, DigitMap, and SignalRequests parameters are optional. They can be used by the Call Agent to transmit a NotificationRequest that is executed simultaneously with the modification of the connection. For example, when a connection is accepted, the calling gateway should be instructed to place the circuit in send-receive mode and to stop providing ringing tones.

This can be accomplished in a single ModifyConnection command, by also transmitting the RequestedEvent parameters, for the on hook event, and an empty SignalRequest parameter, to stop the provision of ringing tones.

When these parameters are present, the modification and the NotificationRequests should be synchronized, which means that both should be accepted, or both refused.

2.3.5 DeleteConnection (from the Call Agent)

This command is used to terminate a connection. As a side effect, it collects statistics on the execution of the connection.

   Connection-parameters <-- DeleteConnection(CallId,
                                              EndpointId,
                                              ConnectionId,
                                              NotifiedEntity,
                                              RequestedEvents,
                                              RequestIdentifier,
                                              DigitMap,
                                              SignalRequests)

In the general case where a connection has two ends, this command has to be sent to both gateways involved in the connection. Some connections, however, may use IP multicast. In this case, they can be deleted individually.

After the connection has been deleted, the endpoint should be placed in inactive mode. Any loopback that has been requested for the connection should be cancelled.

In response to the DeleteConnection command, the gateway returns a list of parameters that describe the status of the connection. These parameters are:

For a detailed definition of these variables, refer to RFC 1889.

The NotifiedEntity, RequestedEvents, RequestIdentifier, DigitMap, and SignalRequests parameters are optional. They can be used by the Call Agent to transmit a NotificationRequest that is executed simultaneously with the deletion of the connection. For example, when a user hangs up is accepted, the gateway should be instructed to delete the connection and to start looking for an off hook event.

This can be accomplished in a single DeleteConnection command, by also transmitting the RequestedEvent parameters, for the off hook event, and an empty SignalRequest parameter.

When these parameters are present, the DeleteConnection and the NotificationRequests should be synchronized, which means that both should be accepted, or both refused.

2.3.6DeleteConnection (from the VoIP gateway)

In some circumstances, a gateway may have to clear a connection, for example because it has lost the resource associated with the connection, or because it has detected that the endpoint no longer is capable or willing to send or receive voice. The gateway terminates the connection by using a variant of the DeleteConnection command:

        DeleteConnection( CallId, 
                          EndpointId,
                          ConnectionId,
                          Reason-code,
                          Connection-parameters)

In addition to the call, endpoint and connection identifiers, the gateway will also send the call's parameters that would have been returned to the Call Agent in response to a DeleteConnection command. The reason code indicates the cause of the disconnection.

2.3.7 DeleteConnection (multiple connections, from the Call Agent)

A variation of the DeleteConnection function can be used by the Call Agent to delete multiple connections at the same time. The command can be used to delete all connections that relate to a Call for an endpoint :

        DeleteConnection( CallId, 
                          EndpointId)

It can also be used to delete all connections that terminate in a given endpoint :

        DeleteConnection( EndpointId)

After the connections have been deleted, the endpoint should be placed in inactive mode. Any loopback that has been requested for the connections should be cancelled.

This command does not return any individual statistics or call parameters.

2.4 Race conditions

In order to implement proper call signalling, the Call Agent must keep track of the state of the endpoint. A key element of this state is the position of the hook. A race condition may occur when the user decides to go off-hook before the Call Agent has the time to ask the gateway to notify an off hook event (the "glare" condition well known in telephony), or if the user goes on-hook before the Call Agent has the time to request the event's notification.

To avoid this race condition, the gateway should check the condition of the endpoint before acknowledging a NotificationRequest. It should return an error:

1. If the gateway is requested to notify an "off hook" transition while the phone is already off hook,
2. If the gateway is requested to notify an "on hook" or "flash hook" condition while the phone is already on hook.

The other state variables of the gateway, such as the list of RequestedEvent or list of requested signals, are entirely replaced after each successful NotificationRequest, which prevents any long term discrepancy between the Call Agent and the gateway.

When a NotificationRequest is unsuccessful, the list of RequestedEvents and requested signals are emptied. They must be reinstated by a new request.

Another race condition may occur when a notification is issued shortly before the reception by the gateway of a NotificationRequest. The RequestIdentifier is used to correlate Notify commands with NotificationRequest commands.

2.5 Return codes and error codes.

All SGCP commands are acknowledged. The acknowledgment carries a return code, which indicates the status of the command. The return code is an integer number, for which three ranges of values have been defined:

• values between 200 and 299 indicate a successful completion,
• values between 400 and 499 indicate a transient error,
• values between 500 and 599 indicate a permanent error.

The values that have been already defined are listed in the following table:

3. Simple Gateway Control Protocol

The SGCP implements the simple gateway control interface as a set of transactions. The transactions are composed of a command and a mandatory response. There are five types of command:

• CreateConnection
• ModifyConnection
• DeleteConnection
• NotificationRequest
• Notify

The first four commands are sent by the Call Agent to a gateway. The Notify command is sent by the gateway to the Call Agent. The gateway may also send a DeleteConnection as defined in 2.3.6.

3.1 General description

All commands are composed of a Command header, optionally followed by a session description.

All responses are composed of a Response header, optionally followed by a session description.

Headers and session descriptions are encoded as a set of text lines, separated by a line feed character. The headers are separated from the session description by an empty line.

SGCP uses a transaction identifier to correlate commands and responses. The transaction identifier is encoded as a component of the command header and repeated as a component of the response header (see section 3.2.1, 3.2.1.2 and 3.3).

Transaction identifiers have values between 1 and 999999999. An SGCP entity shall not reuse an identifier sooner than 3 minutes after completion of the previous command in which the identifier was used.

3.2 Command Header

The command header is composed of:

• A command line, identifying the requested action or verb, the endpoint towards which the action is requested, and the SGC protocol version,
• A set of parameter lines, composed of a parameter name followed by a parameter value.

3.2.1 Command line

The command line is composed of:

• The name of the requested verb,
• The identification of the transaction,
• The name of the endpoint that should execute the command (in notifications, the name of the endpoint that is issuing the notification),
• The protocol version.

These four items are encoded as strings of printable ASCII characters, separated by white spaces, i.e. the ASCII space (0x20) or tabulation (0x09) characters. It is recommended to use exactly one ASCII space separator.

3.2.1.1 Coding of the requested verb

The five verbs that can be requested are encoded as four letter upper or lower case ASCII codes (comparisons should be case insensitive) as defined in the following table:

The transaction identifier is encoded as a string of up to 9 decimal digits. In the command lines, it immediately follows the coding of the verb.

New verbs may be defined in further versions of the protocol. It may be necessary, for experimentation purposes, to use new verbs before they are sanctioned in a published version of this protocol. Experimental verbs should be identified by a four letter code starting with the letter X, such as for example XPER.

3.2.1.2 Coding of the endpoint names

The endpoint names are encoded as e-mail addresses, as defined in RFC 821. In these addresses, the domain name identifies the system where the endpoint is attached, while the left side identifies a specific endpoint on that system.

Examples of such addresses can be:

The name of notified entities is expressed with the same syntax, with the possible addition of a port number as in:

Call-agent@ca.whatever.net:5234

3.2.1.3 Coding of the protocol version

The protocol version is coded as the key word SGCP followed by a white space and the version number. The version number is composed of a major version, coded by a decimal number, a dot, and a minor version number, coded as a decimal number. The version described in this document is version 1.1.

In the initial messages, the version will be coded as:

Version 1.1 is a superset of version 1.0. Gateways that implement version 1.1 shall accept commands coded according to version 1.0. They should refrain from using version 1.1 extensions (See the section on compatibility with version 1.0).

3.2.2 Parameter lines

Parameter lines are composed of a parameter name, which in most cases is composed of a single upper case character, followed by a colon, a white space and the parameter value. The parameter that can be present in commands are defined in the following table:

Call-agent@ca.whatever.net:5234

The parameters are not necessarily present in all commands. The following table provides the association between parameters and commands. The letter M stands for mandatory, O for optional and F for forbidden.

Note (*) that the RequestedEvents and SignalRequests parameters are optional in the NotificationRequest. If these parameters are omitted, the corresponding lists will be considered empty.

If implementers need to experiment with new parameters, for example when developing a new application of SGCP, they should identify these parameters by names that start with the string "X-", such as for example:

     X-FlowerOfTheDay: Daisy

The parameters will only be recognized by applications that have been upgraded to become part of the experiment. A gateway that receives an extension that it cannot understand should refuse to execute the command. It should respond with an error code 511 (Unrecognized extension.)

3.2.2.1 Local connection options

The local connection options describe the operational parameters that the Call Agent suggests to the gateway. These parameters are:

• The packetisation period in milliseconds, encoded as the keyword "p", followed by a colon and a decimal number. If the Call Agent specifies a range of values, the range will be specified as two decimal numbers separated by an hyphen.
• The preferred type of compression algorithm, encoded as the keyword "a", followed by a character string. If the Call Agent specifies a list of values, these values will be separated by a semicolon.
• The bandwidth in kilobits per second (1000 bits per second), encoded as the keyword "b", followed by a colon and a decimal number. If the Call Agent specifies a range of values, the range will be specified as two decimal numbers separated by an hyphen.
• The echo cancellation parameter, encoded as the keyword "e", followed by a colon and the value "on" or "off".

Each of the parameters is optional. When several parameters are present, the values are separated by a comma.

Examples of connection descriptors are:

     L: p:10, a:G.711
     L: p:10, a:G.711;G.726-32
     L: p:10-20, b: 64
     L: b:32-64, e:off

3.2.2.2 Connection parameters

Connection parameters are encoded as a string of type and value pairs, where the type is a two letter identifier of the parameter, and the value a decimal integer. Types are separated from value by an `=' sign. Parameters are encoded from each other by a comma.

The connection parameter types are specified in the following table:

An example of connection parameter encoding is:

     P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48

3.2.2.3 Connection mode

The connection mode describes the mode of operation of the connection. The possible values are:

3.2.2.4 Coding of event names

Event names are mentioned in the RequestedEvents, SignalRequest and Observed Events parameter. Each event is identified by a code, as indicated in the following table. These ASCII encodings are not case sensitive. Values such as "hu", "Hu", "HU" or "hU" should be considered equal.

The following codes are used to identify events:

Code	Event
A string of digits [0-9], hash and star marks [*,#], the letters A, B, C, D and the timer indication T	DTMF tones
ann	Announcement
asdi	ASDI display
aw	Answer tone
bz	Busy tone
cf	Confirm tone
cbk	Call back request
cg	Network Congestion tone
co	Default continuity tone
co1	Continuity tone (single tone)
co2	Continuity test (go tone, in dual tone procedures)
cl	Carrier lost
cv	Continuity verified (response tone, in dual tone procedures)
dl	Dial tone
ft	Fax tones
hd	Off-hook transition
hf	Flash hook
hu	On-hook transition
it	Intercept tone
ld	Long duration connection
mt	Modem tones
ot	Off hook warning tone
pa	Packet arrival
pt	Preemption tone
r0, r1, r2, r3, r4, r5, r6 or r7	Distinctive ringing (0 .. 7)
rg	Ringing
rt	Ring back tone
t	Timer
wk	Wink
wt	Call waiting tone

3.2.2.5 RequestedEvents

The RequestedEvent parameter provides the list of events that have been requested. The event codes are described in the previous section. In addition to this coded events, the list may also include a parameter specifying digit collection. This parameter could take one of the following forms:

• individual digits, pound sign (octothorpe), star sign or letters A, B, C or D.
• timer mark T,
• ranges of digits, enclosed within square brackets, e.g. "[0-9]" or "[0-9*#T]"

Each event can be qualified by a requested action, or by a list of actions. The actions, when specified, are encoded as a list of keywords, enclosed in parenthesis and separated by commas. The codes for the various actions are:

When no action is specified, the default action is to notify the event. This means that, for example, ft and ft(N) are equivalent. Events that are not listed are ignored.

The digit-map action can only be specified for the digits, letters and timers.

The requested list is encoded on a single line, with event/action groups separated by commas. Examples of RequestedEvents encoding are:

R: hu(N), hf(S,N)
R: hu(N), [0-9#T](D)

3.2.2.6 SignalRequests

The SignalRequests parameter provides the name of the signals that have been requested. Each signal is identified by a code, as indicated in the previous section.

Two events, announcement and ASDI display, can be qualified by additional parameters:

• the name and parameters of the announcement,
• the string that should be displayed.

These parameters will be enclosed within parenthesis, as in:

S: asdi(123456 Your friend)
S: ann(no-such-number, 1234567)

When several signals are requested, their codes are separated by a comma, as in:

S: asdi(123456 Your friend), rg

3.2.2.6 ObservedEvent

The observed event parameters provides the list of events that have been observed. The event codes are the same as those used in the NotificationRequest. Events that have been accumulated according to the digit map are grouped in a single string. Examples of observed actions are:

O: hu
O: 8295555T 
O: hf, hf, hu

The packet arrival event is used to notify that at least one packet was recently sent to an Internet address that is observed by an endpoint. The event report includes the Internet address, in standard ASCII encoding, between parenthesis:

O: pa(192.96.41.1)

The call back event is used to notify that a call back has been requested during the initial phase of a data connection. The event report includes the identification of the user that should be called back, between parenthesis:

O: cbk(user25)

3.3 Format of response headers

The response header is composed of a response line, optionally followed by headers that encode the response parameters.

The response line starts with the response code, which is a three digit numeric value. The code is followed by a white space, the transaction identifier, and an optional commentary.

In the case of a CreateConnection message, the response line is followed by a Connection-Id parameter. It may also be followed a Specific-Endpoint-Id parameter, if the creation request was sent to a generic Endpoint-Id.

In the case of a DeleteConnection message, the response line is followed by a Connection Parameters parameter, as defined in section 3.2.2.2.

A LocalConnectionDescriptor should be transmitted with a positive response (code 200) to a CreateConnection. It may be transmitted in response to a ModifyConnection command, if the modification resulted in a modification of the session parameters. The LocalConnectionDescriptor is encoded as a "session description," as defined in section 3.4. It is separated from the response header by an empty line.

3.4 Encoding of the session description

The session description is encoded in conformance with the session description protocol, SDP. SGCP implementations are expected to be fully capable of parsing any conformant SDP message, and should send session descriptions that strictly conform to the SDP standard. The usage of SDP actually depends on the type of session that is being, as specified in the "mode" parameter:

• if the mode is set to "data", the session description describes the configuration of a data access service.
• if the mode is set to any other value, the session description is for an audio service.

For an audio service, the gateway will consider the information provided in SDP for the "audio" media. For a data service, the gateway will consider the information provided for the "network-access" media.

3.4.1 Usage of SDP for an audio service

In a telephony gateway, we only have to describe sessions that use exactly one media, audio. The parameters of SDP that are relevant for the telephony application are:

• The IP address of the remote gateway (in commands) or of the local gateway (in responses), or multicast address of the audio conference, encoded as an SDP "connection data" parameter. This parameter specifies the IP address that will be used to exchange RTP packets.

• Media description field (m) specifying the audio media, the transport port used for receiving RTP packets by the remote gateway (commands) or by the local gateway (responses) , the RTP/AVP transport, and the list of formats that the gateway will accept. This list should normally always include the code 0 (reserved for G.711).
• Optionally, RTPMAP attributes that define the encoding of dynamic audio formats,
• Optionally, a packetization period (packet time) attribute (Ptime) defining the duration of the packet,
• Optionally, an attribute defining the type of connection (sendonly, recvonly, sendrecv, inactive)
• The IP address of the remote gateway (in commands) or of the local gateway (in responses), if it is not present at the session level.

There is a request, in some environments, to use the SGCP to negotiate connections that will use other transmission channels than RTP over UDP and IP. This will be detailed in an extension to this document.

3.4.2 Usage of SDP in a network access service

The parameters of SDP that are relevant for a data network access application are:

• Media description field (m) specifying the network access media, identified by the code "m=nas/xxxx", where "xxxx" describes the access control method that should be used for parametrizing the network access, as specified below. The field may also specify the port that should be used for contacting the server, as specified in the SDP syntax.
• Connection address parameter (c=) specifying the address, or the domain name, of the server that implement the access control method. This parameter may also be specified at the session level.
• Optionally, a bearer type attribute (a=bearer:) describing the type of data connection to be used.
• Optionally, a framing type attribue (a=framing:) describing the type of framing that will be used on the channel.
• Optionally, attributes describing the called number (a=dialed:), the number to which the call was delivered (a=called:) and the calling number (a=dialing:).
• Optionally, attributes describing the range of addresses that could be used by the dialup client on its LAN (a=subnet:).
• Optionally, an encryption key, encoded as specified in the SDP protocol (k=).

The connection address shall be encoded as specified in the SDP standard. It will be used in conjunction with the port specified in the media line to access a server, whose type will one of:

Method name	Method description
radius	Authentication according to the Radius protocol.
tacacs	Authentication according to the TACACS+ protocol.
diameter	Authentication according to the Diameter protocol.
l2tp	Level 2 tunneling protocol. The address and port are those of the LNS.
login	Local login. (There is normally no server for that method.)
none	No authentication required. (The call was probably vetted by the Call Agent.)

If needed, the gateway may use the key specified in the announcement to access the service. That key, in particular, may be used for the establishment of an L2TP tunnel.

The valid values of the bearer attribute are defined in the following table:

Type of bearer description	Example of values
ITU modem standard	V.32, V.34, V.90.
ISDN transparent access, 64 kbps	ISDN64
ISDN64 + V.110	ISDN64/V.110
ISDN64 + V.120	ISDN64/V.120
ISDN transparent access, 56 kbps	ISDN56
Informal identification	(Requires coordination between the Call Agent and the gateway)

The valid values of the framing attribute are defined in the following table:

Type of framing description	Example of values
PPP, asynchronous framing	ppp-asynch
PPP, HDLC framing	ppp-hdlc
SLIP, asynchronous	slip
Asynchronous, no framing	asynch

The network access authentication parameter provides instructions on the access control that should be exercized for the data call. This optional attribute is encoded as:

"a=subnet:" <network type> <address type> <connection address> "/" <prefix length>

Where the parameters "network type", "address type", and "connection address" are formatted as defined for the connection address parameter (c=) in SDP, and where the "prefix length" is a decmal representation of the number of bits in the prefix.

Examples of SDP announcement for the network access service could be:

v=0
m=nas/radius
c=IN IP4 radius.example.net
a=bearer:v.34
a=framing:ppp-asynch
a=dialed:18001234567
a=called:12345678901
a=dialing:12340567890

v=0
m=nas/none
c=IN IP4 128.96.41.1
a=subnet:IN IP4 123.45.67.64/26
a=bearer:isdn64
a=framing:ppp-sync
a=dialed:18001234567
a=dialing:2345678901

v=0
c=IN IP4 access.example.net
m=nas/l2tp 
k=clear:some-shared-secret
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901

3.5 Transmission over UDP

SGCP messages are transmitted over UDP. Commands are sent to one of the IP addresses defined in the DNS for the specified endpoint . The responses are sent back to the source address of the commands.

When no port is specified for the endpoint, the commands should be sent to the default SGCP port, 2427.

SGCP messages, being carried over UDP, may be subject to losses. In the absence of a timely response, commands are repeated. SGCP entities are expected to keep in memory a list of the responses that they sent to recent transactions, i.e. a list of all the responses they sent over the last 30 seconds, and a list of the transactions that are currently being executed. The transaction identifiers of incoming commands are compared to the transaction identifiers of the recent responses. If a match is found, the SGCP entity does not execute the transaction, but simply repeats the response. The remaining commands will be compared to the list of current transaction. If a match is found, the SGCP entity does not execute the transaction, which is simply ignored.

It is the responsibility of the requesting entity to provide suitable time outs for all outstanding commands, and to retry commands when time outs have been exceeded. Furthermore, when repeated commands fail to be acknowledged, it is the responsibility of the

requesting entity to seek redundant services and/or clear existing or pending connections.

The specification purposely avoids specifying any value for the retransmission timers. These values are typically network dependent. The retransmission timers should normally estimate the timer by measuring the time spent between the sending of a command and the return of a response. One possibility is to use the algorithm implemented in TCP-IP, which uses two variables:

• the average acknowledgement delay, AAD, estimated through an exponentially smoothed average of the observed delays,
• the average deviation, ADEV, estimated through an exponentially smoothed average of the absolute value of the difference between the observed delay and the current average

The retransmission timer, in TCP, is set to the sum of the average delay plus N times the average deviation.

After the any retransmission, the SGCP entity should do the following:

• It should double the estimated value of the average delay, AAD
• It should compute a random value, uniformly distributed between 0.5 AAD and AAD
• It should set the retransmission timer to the sum of that random value and N times the average deviation.

This procedure has two effects. Because it includes an exponentially increasing component, it will automatically slow down the stream of messages in case of congestion. Because it includes a random component, it will break the potential synchronization between notifications triggered by the same external event.

4. Security requirements

If unauthorized entities could use the SGCP, they would be able to set-up unauthorized calls, or to interfere with authorized calls. We expect that SGCP messages will always be carried over secure Internet connections, as defined in the IP security architecture as defined in RFC 1825, using either the IP Authentication Header, defined in RFC 1826, or the IP Encapsulating Security Payload, defined in RFC 1827. The complete SGCP protocol stack would thus include the following layers:

Adequate protection of the connections will be achieved if the gateways and the Call Agents only accept messages for which IP security provided an authentication service. An encryption service will provide additional protection against eavesdropping, thus forbidding third parties from monitoring the connections set up by a given endpoint .

The encryption service will also be requested if the session descriptions are used to carry session keys, as defined in SDP.

These procedure do not necessarily protect against denial of service attacks by misbehaving gateways or misbehaving call agents. However, they will provide an identification of these misbehaving entities, which should then be deprived of their authorization through maintenance procedures.

5. Example of call flows

In order to understand the way the SGCP interface will be used, we have described here two possible call flows between a TGW, which is a trunking gateway that implements SGCP, and an RGW, which is a residential gateway that implements SGCP, as well as four call flows describing how SGCP could be used to control a network access service.

The diagrams also show a Common Database (CDB) that can be queried for authorization and routing information, and an Accounting Gateway (ACC) that collects accounting information at the start and the end of calls.

These diagrams are solely meant to exhibit the behavior of the SGCP, and to help understanding this protocol. They are not meant as a tutorial on the implementation of a Call Agent. They may very well include miscellaneous errors and imprecisions.

5.1 Basic call, RGW to TGW

RQNT 1201 endpoint-1@rgw-2567.whatever.net SGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AB
R: hd

200 1201 OK

NTFY 2001 endpoint-1@rgw-2567.whatever.net SGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AB
O: hd

200 2001 OK

RQNT 1202 endpoint-1@rgw-2567.whatever.net SGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: hu, [0-9#*T](D)
D: (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
S: dt

200 1202 OK

NTFY 2002 endpoint-1@rgw-2567.whatever.net SGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
O: 912018294266

200 2002 OK

RQNT 1203 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789AD
R: hu

200 1203 OK

CRCX 1204 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: recvonly

200 1204 OK
I:FDE234C8

v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

CRCX 1205 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: sendrecv

v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

200 1205 OK
I:32F345E2

v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

MDCX 1206 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0 
I:FDE234C8
M: recvonly

v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

200 1206 OK

RQNT 1207 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789AE
R: hu
S: rt

200 1207 OK

RQNT 1208 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789AF
R: hu
MDCX 1209 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0 

I:FDE234C8
M: sendrecv

The residential gateway will acknowledge these two commands:
200 1208 OK
200 1209 OK

At this point, the connection is established.
When the Call Agent receives the REL message from the CO switch, it will have to tear down the call. It will do so by sending to both gateways a DeleteConnection command:
DLCX 1210 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:FDE234C8

DLCX 1211 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:32F345E2

The gateways will respond with acknowledgements that should include a "call parameters" header fields:
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

250 1211 OK
P: PS=790, OS=45700, PR=1230, OR=61875, PL=15, JI=27, LA=48


At this point, phone attached to the residential gateway, in our scenario, goes on-hook. This event is notified to the Call Agent, according to the policy received in the last NotificationRequest by sending a Notify command: 
NTFY 2005 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789AF
O: hu

After this notification, the Call Agent should send an acknowledgement:
200 2005 OK

It should then issue a new NotificationRequest, to be ready to receive the next off-hook detected by the residential gateway: 
RQNT 1212 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789B0
R: hd

The gateway will acknowledge this message:
200 1212 OK

Both gateways, at this point, are ready for the next call.
5.2 Basic call, TGW to RGW


CO

SS7/ISUP

TGW

CA

CDB

ACC

RGW

Usr


IAM

->

 

 

 

 

 

 


 

IAM

- -

->

 

 

 

 


 

 

 

Check

->

 

 

 


 

 

 

<-

IP

 

 

 


 

 

<-

Create
Connection

 

 

 

 


 

 

Ack

->

 

 

 

 


 

 

 

Create
Connection

- -

- -

->

 


 

 

 

<-

- -

- -

Ack

 


 

 

<-

Modify
Connection

 

 

 

 


 

 

Ack

->

 

 

 

 


 

 

 

Notification
Request

- -

- -

->

ring


 

 

 

<-

- -

- -

Ack

 


 

<-

- -

ACM

 

 

 

 


<-

ACM

 

 

 

 

 

 


 

 

 

 

 

 

 

off hook


 

 

 

<-

- -

- -

Notify

 


 

 

 

Ack

- -

- -

->

 


 

 

 

Notification
Request

- -

- -

->

 


 

 

 

<-

- -

- -

Ack

 


 

 

<-

Modify 
Connection 

 

 

 

 


 

 

Ack

->

 

 

 

 


 

 

(cut-in)

Call start 

- -

- >

 

 


 

<-

- -

ANM

 

 

 

 


<-

ANM

 

 

 

 

 

 


 

 

 

 

 

 

 

on hook


 

 

 

<-

- -

- -

Notify

 


 

 

 

Ack

- -

- -

->

 


 

 

 

Delete
Connection

- -

- -

->

 


 

 

<-

Delete
Connection

 

 

 

 


 

<-

- -

REL

 

 

 

 


<-

REL

 

 

 

 

 

 


 

 

Perf
data

->

 

 

 

 


 

 

 

<-

- -

- -

perf data

 


 

 

 

Call end

- -

- >

 

 


 

 

 

Notification
Request

- -

- -

->

 


 

 

 

<-

- -

- -

Ack

 



This diagram shows the various exchange of messages during a call from a telephone user on the circuit-switched PSTN to a residential user connected to a residential gateway. During these exchanges the Call Agent uses SGCP to control both the TGW and the residential gateway. The exchanges occur on two sides. 
Upon reception of the IAM message, the Call Agent immediately sends a CreateConnection request to the trunking gateway to connect to the incoming trunk, creating a connection: 
CRCX 1237 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: recvonly

The trunking gateway immediately acknowledges the creation, sending back the identification of the newly created connection and the session description used to receive audio data: 
200 1237 OK
I: FDE234C8

v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

The SDP announcement, in our example, specifies the address at which the gateway is ready to receive audio data (128.96.41.1), the transport protocol (RTP), the RTP port (3456) and the audio profile (AVP). The audio profile refers to RFC 1890, which defines that the payload type 0 has been assigned for G.711 transmission. The gateway is also ready to use ADPCM encoding at 32 kbps (G.726 –4). There is no standard payload type associated to ADPCM, so the gateway mentions its readiness to use a non standard payload associated to the dynamic type 96. The "rtpmap" attribute entry associates the payload type 96 to G726/4.
The Call Agent, having seized the incoming trunk, must now reserve the outgoing circuit. It does so by sending a connection command to the residential gateway:
CRCX 1238 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: sendrecv

v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

The CreateConnection command has the same parameters as the command sent to the ingress gateway, with two differences:


The EndpointId points towards the outgoing trunk, 
The message carries the session description returned by the ingress gateway, 
Because the session description is present, the "mode" parameter is set to "send/receive".

We observe that the call identifier is identical for the two connections. This is normal: the two connection belong to the same call, which has a global identifier in our system.
The trunking gateway will acknowledge the connection command, sending in the session description its own parameters such as address, ports and RTP profile:
200 1238 OK
I:32F345E2

v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

The Call Agent will relay the information to the ingress gateway, using a ModifyConnection command:
MDCX 1239 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:FDE234C8
M: recvonly

v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000

The trunking gateway immediately acknowledges the modification: 
200 1239 OK

At this stage, the Call Agent has established a half-duplex transmission path. The Call Agent must now tells the residential gateway to ring the called line. It will send a NotificationRequest, consisting of the following lines:
RQNT 1240 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789B1
R: hd
S: rg

The residential gateway, at that point, is instructed to look for an off-hook event, and to report it. It will first acknowledge the command, repeating in the acknowledgement message the transaction id that the Call Agent attached to the query.
200 1240 OK

Upon reception of this message, the Call Agent sends an address complete message (ACM) to the calling switch, which will generate ringing tones for the calling user.
When the gateway notices the off hook event, it sends a Notify command to the Call Agent:
NTFY 2001 endpoint-1@rgw-2567.whatever.net SGCP 1.0X: 0123456789B0
O: hd

The Call Agent immediately acknowledges that notification. 
200 2001 OK

The Call Agent now asks the residential gateway to send a Notify command on the occurrence of an on-hook event. It does so by sending a NotificationRequest to the residential gateway: 
RQNT 1241 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789B1
R: hu

The gateway acknowledges that command:
200 1241 OK

MDCX 1242 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:FDE234C8
M: sendrecv

200 1242 OK

NTFY 2005 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789B1
O: hu

200 2005 OK

DLCX 1243 endpoint-1@rgw-2567.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:FDE234C8

DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.0
C: A3C47F21456789F0
I:32F345E2

250 1243 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

250 1244 OK
P: PS=790, OS=45700, PR=1230, OR=61875, PL=15, JI=27, LA=48

RQNT 1245 endpoint-1@rgw-2567.whatever.net SGCP 1.0
X: 0123456789B2
R: hd

200 1245 OK