Protocols Supported by the Signaling Gateway for Signaling Gateways

The IETF's Sigtran protocol is a stack for transporting SCN signaling protocols over IP networks, called the Sigtran stack. It supports standard proto-language interfaces for SCN signaling applications without any modifications, thus ensuring that existing SCN signaling applications can be used unmodified, and it utilizes the standard IP transport protocol as the low-level transport signaling to meet the requirements of SCN signaling transport by adding functionality to the protocol itself.

Figure 5 Signaling Transport Functional Model

Figure 5 shows a signaling transport functional model based on the Sigtran protocol. In this case, the signaling gateway terminates the signaling from the circuit-switched network and then transmits the signaling content to the media gateway controller for processing via Sigtran, and the media gateway controller terminates the interconnecting trunks and controls the trunks according to the instructions of the control commands from the media gateway controller.

No.7 The signaling adaptation layer supports specific primitives, such as management instruction primitives, that are required for specific SCN signaling protocols. In IP networks, the Sigtran protocol utilizes IP as the underlying transport protocol for SCN signaling. It supports standard interfaces so that it does not require any modifications to existing SCN signaling, thus ensuring that existing SCN signaling applications can be used without modification.

Based on the Sigtran protocol stack, a protocol model for a No.7 signaling gateway can basically be divided into a signaling application layer (e.g., TUP, ISUP, etc.), a signaling adaptation layer (e.g., M2UA, M3UA, etc.), and a signaling transport layer (e.g., SCTP), as shown in Figure 6.

Figure 6 No.7 signaling gateway protocol model

(1) Q.931: ISDN subscriber's technical specification for basic call control at layer 3 of the network interface.

(2) QSIC: Link layer protocol for ISDN developed by the European Computer Manufacturers Association.

(3) IUA: ISUP User Adaptation Layer, IUA is a protocol developed for the implementation of ISDN services in softswitch networks and is the ISDN Q.921/Q.931 User Adaptation Layer protocol.

(4) MTP3: No.7 signaling messaging part layer 3.

(5) M2UA: Subscriber Part Adaptation Layer of MTP3, M2UA is the adaptation layer protocol for the second level of MTP subscribers, which allows the signaling gateway to transmit MTP3 messages to peer IPSPs (Direct Providers of Mobile Internet Services, Content Application Services) to provide seamless network management interoperability between No.7 signaling network and IP network.

(6) TCAP: Things processing application part.

(7) SCCP: Signaling Connection Control Part.

(8) ISUP: ISDN user part.

(9) TUP: Telephone User.

(10) M3UA: User Adaptation Layer for MTP3, M3UA is the adaptation layer protocol for MTP Level 3 users, which allows signaling gateways to transmit MTP3 user information (such as ISUP/SCCP messages) to media gateway controllers or IP databases, and provides seamless managed interoperability for SS7 signaling networks and IP networks.

(11) SUA: SCCP Subscriber Adaptation Layer, SUA is the adaptation layer protocol for SCCP subscribers, and its main function is to adapt the transmission of SCCP subscriber information to the IP database to provide network management interworking function of SCCP.

(12) SCTP: Stream Control Transmission Protocol, SCTP is a connection-oriented transport layer protocol, is a new generation of general purpose IP transport protocols, its basic function is to provide a reliable message-based transport service between SCTP users, which is realized through an SCTP connection (called SCTP coupling) established between two endpoints of SCTP. Relative to other transport protocols such as TCP, it has a small transmission delay and avoids blocking caused by certain large data, so it is more suitable for transmitting telecommunication network signaling that requires high real-time and reliability, and the protocol enables signaling messages to be exchanged over an IP-based public *** packet-switched network.

The No.7 stack corresponds to the Sigtran stack as follows.

In order to realize the interoperability of the softswitch with the existing No.7 signaling network for call connectivity control, the SG first needs to terminate the No.7 signaling link, and then use the Sigtran protocol to pass the contents of the call connectivity control messages of No.7 signaling to the softswitch for processing. Since the communication between the softswitch and the SG mainly relies on the No.7 signaling adaptation layer protocols, including M2UA, M2PA, M3UA and other protocols, the correspondence between the No.7 protocol stack and the Sigtran protocol stack is shown in Figure 7.

Figure 7 Correspondence between No.7 signaling protocol stack and Sigtran protocol stack

Often said Sigtran protocol mainly refers to the SCTP and No.7 signaling adaptation sublayer, the signaling gateway according to the function it realizes, the No.7 signaling adaptation layer can be used as the second level of the second level of user adaptation layer of the No.7 signaling messaging part (M2UA), Messaging Part Level 2 Peer Adaptation Layer (M2PA), Messaging Part Level 3 User Adaptation Layer (M3UA), or Signaling Connection Control Part User Adaptation Layer (SUA), and so on. The public *** signaling transport protocol supports the public *** and reliable transport functions required for signaling transport, and it uses the StreamControlTransmissionProtocol (SCTP) to provide these functions.

The basic transmission process of signaling is: first, the ISUP message at the application layer passes through the local No.7 stack and is transmitted over the SS7 signaling network; when it arrives at the SG, it passes through one of the No.7 stacks to the NIF, then through the M3UA/SCTP/IP, and is transmitted over the IP network; when it arrives at the soft-switching device, it first passes through the Sigtran stack and breaks down the ISUP message. Up to this point, the ISUP signaling in the SP application layer is transparently transmitted to the application layer of the softswitch, realizing the interoperability between No.7 signaling network and IP network.

The Sigtran protocol stack is the core part of the No.7 signaling gateway software system, which is used to realize the protocol conversion between No.7 signaling and IP packets. Based on the protocol model proposed above, the original No.7 signaling system can be added on the basis of signaling gateway-specific signaling adaptation layer and transport layer processing. According to the system function, the signaling protocol processing system can be divided into three subsystems: No.7 signaling network interface subsystem, protocol processing subsystem, and IP network interface subsystem. the No.7 signaling network interface subsystem mainly completes the interface between the signaling gateway and the No.7 signaling network, realizes the message transmission with the No.7 signaling network, and belongs to the signaling application layer in the protocol model, such as TUP, ISUP and so on. The protocol processing subsystem realizes the reliable and efficient transmission of No.7 signaling in IP network, which is the key technology to realize the function of signaling gateway, and the signaling adaptation layer protocols in the protocol model must be completed in the development, including the protocols such as M3UA, M2UA, etc., as well as the SCTP protocol in the transport layer.The IP network interface module realizes the interface between the signaling gateway and the IP network, and it should be able to provide the interface with standard TCP/IP protocol. interface.

The protocol stack structure of the signaling gateway is shown in Figure 8.

The signaling from the PSTN side and IP side of the signaling gateway are discussed below.

1. Signaling part of PSTN side

As the part of SoftSwitch system to access PSTN signaling, signaling gateway must support SS7. from the protocol stack structure of signaling gateway, it can be seen that, in order to support various types of No.7, the signaling gateway on the PSTN side needs to provide the support for various types of MTP1, MTP2, MTP3, that is to say, the signaling gateway just needs to provide the support for various types of MTP1, MTP2, MTP3, and so on, which means that, as long as the signaling gateway can support the PSTN system, it will be able to support the PSTN system. support, which means that the signaling gateway only needs to support the specification parts of various signaling systems corresponding to MTP1, MTP2 and MTP3.

2. Signaling on the IP side

On the IP side, the signaling gateway must support the Sigtran protocol stack shown in Figure 7, and obey the corresponding IETFSigtran specification.
The protocol used by the SG for No. 7 signaling network interworking with the IP network is the Signaling Transport Protocol (Sigtran). stack of signaling protocols in a circuit-switched network SCN. In the soft-switching system, No.7 signaling on the SCN side and the Sigtran adaptation layer protocol on the IP side are converted by the SG system to realize the transmission of No.7 signaling in the IP network, so as to achieve the standard proto-language interface supported by Sigtran, and there is no need to make any modification to the existing SCN signaling application to ensure that the existing SCN signaling application can be used directly without modification. use without modification.

The Sigtran stack consists of the SCTP protocol and the No.7 signaling adaptation sublayer protocol.

(1) Public *** signaling transport protocol: it uses the Stream Control Transmission Protocol (SCTP) to provide these functions.

SCTP, or Stream Control Transmission Protocol, is a new end-to-end transmission control protocol based on IP networks, and its most fundamental purpose is to transmit signaling data originating from traditional PSTN networks on the basis of IP networks, and by transmitting PSTN signaling messages over connectionless IP networks, it can provide reliable data transmission over IP networks, and it is a new type of IP transport protocol, which is in the same layer with TCP and UDP. SCTP can transmit user data without error or duplication by means of acknowledgement; segment user data according to the MTU (Message Transfer Unit) limit of the path, and ensure the sequential delivery of user messages on multiple streams; multiplex multiple user messages into SCTP data blocks; and utilize the mechanism of SCTP coupling to provide network-level Fault guarantee, while SCTP also has the characteristics of avoiding congestion, and can avoid suffering from flooding and anonymity attacks. It is because SCTP has good security, reliability and flexibility, as well as some of its own advanced protocol mechanisms, such as selective acknowledgement, fast retransmission, out-of-order delivery, multi-homing mechanism, etc., so that SCTP can to a certain extent to meet the needs of high-performance transmission in complex network environments, which also brings SCTP a broader application space.

The SCTP protocol enables signaling messages to be exchanged over an IP-based public **** packet-switched network, with flow control and error control performed end-to-end, using a set of "application server processes" and "multi-homing nodes". By using a set of "Application Server Processes" and "Multi-Attribution Nodes", the effectiveness is improved. In this way, SCTP is a reliable transport protocol, and SCTP connections are called couples. SCTP provides highly reliable redundant transport ports between two endpoints.

(2) No.7 signaling adaptation layer: this layer supports specific primitives, which are necessary for specific SCN signaling protocols.SG Depending on the functions it implements, the No.7 signaling adaptation sublayer can be used as the No.7 signaling messaging portion (MTP) second level user adaptation layer (M2UA), and the messaging portion second level peer-to-peer adaptation layer (M2PA), Messaging Part Second Level User Adaptation Layer (M3UA), Signaling Connection Control Part User Adaptation Layer (SUA), and so on, and the No.7 signaling adaptation sublayer has the following adaptation layer protocols.

①M2PA (MTPlevel2UserPeer-to-PeerAdaptation).

② M2UA (MTPlevel2UserAdaptation).

③ M3UA (MTPlevel3UserAdaptation).

④SUA (SCCPUserAdaptation).

⑤IUA (ISDNUserAdaptation).

Based on the different ways of realizing No.7 signaling and IP interworking, these adaptation protocols have different application occasions, and the following is a detailed introduction to these protocols.

The function of each layer of the protocol model is described below.

(1) Characteristics of the M2PA protocol

The M2PA protocol can be used to transport No.7 signaling MTP3 messages over IP bearers, and the protocol uses SCTP to provide reliable transport, confirmation, and congestion avoidance functions.

The application of M2PA ensures that any signaling node using IP as a bearer can directly use the signaling message processing functions and signaling network management functions of MTP3. functions and signaling network management functions of MTP3. In addition, it also utilizes the functions provided by the SCTP protocol to achieve many of the same functions as MTP2, and the following are the characteristics of the M2PA protocol.

①SG adopts the standard No.7 signaling signaling protocols (MTP1, MTP2, MTP3 protocols) on the TDM side, and encapsulates the MTP3 and MTP3 upper layer signaling in the M2PA/SCTP protocol on the IP side. In addition, the SG can have SCCP layer function, and GT translation function.

②The SG supports MTP3 layer protocol and has signaling network management function, which can transmit IP signaling link status information between the SG and the soft switch to the TDM switch, and also transmit TDM signaling link status information between the SG and the TDM switch to the soft switch.

3) The SG is equipped with the signaling forwarding function, which supports the forwarding of No.7 signaling messages among multiple SGs connected through the IP network.

④Adopting M2PA protocol, from the viewpoint of SG's protocol stack, SG can be completely regarded as STP (signaling transfer point) between TDM telephone network and soft-switching network. Based on the completion of the physical connection, SG converts the bearer of signaling link (the standard defined in the MTP1 layer) to be based on the IP bearer, and converts the frame structure defined in the MTP2 layer to be based on the frame structure of M2PA, and also converts the SLC signaling link to the frame structure of M2PA. At the same time, the SLC signaling link is adapted to one or more SCTP connections, and at the same time, the M2PA must keep a correspondence table of the SCTP couples and the corresponding destination IP addresses of each No.7 signaling link to it.

Therefore, it can be said that the M2PA protocol can be used to build a signaling network that seamlessly connects TDM signaling links with IP signaling links, and the SG is the STP that connects TDM signaling links with IP link links, and, as an optional feature, the SG can have the SCCP layer function that realizes the transmission of MAP messages between TDMSPs and IPSPs. With the M2PA protocol, both the SG and the softswitch need to have independent signaling point coding, and the SG can only work in the signaling transfer point mode.

(2) M2UA Protocol Characteristics

In addition to the M2PA protocol, the IETF defines a protocol for the MTP2 user adaptation layer, which is also used to transmit signaling messages for MTP2 users over the SCTP. In this case, the M2UA protocol functions in the same way as the M2PA protocol. Since the concepts of AS and ASP are introduced in No.7 Signaling User Adaptation Protocol when applying the M2UA protocol, the state management functions for AS and ASP should also be included in the M2UA protocol.The application of the M2UA protocol is mainly to include both the signaling gateway (SG) function and the media gateway (MG) function in the same physical entity.

The functions of the M2UA protocol are as follows.

1) Support for MTP2 and MTP3 border interface primitives.

② Supports full MTP3 message processing and network management functions.

③ Supports inter-module communication for layer management between SG and MGC.

④Supports management of activated connections between SG and MGC.

The features of the M2UA protocol are as follows.

① The SG must have the MTP1 and MTP2 layer protocols, the M2UA/SCTP/IP protocols, and the soft switch must have the MISUP/MTP3/M2UA/SCTP/IP protocols, and must also support the interlayer primitive between the MTP2 and MTP3 layers.

②Since the SG does not handle the MTP3 layer protocol, it does not have the signaling network management function, and it cannot tell the TDM switch the status of the IP link between the SG and the soft switch, i.e., the TDM switch and the soft switch are connected to each other through more than two SGs, so that the TDM switch cannot be notified to reverse all the signaling messages to the other SG when the SCP/IP link between one SG and the soft switch fails. messages are inverted to another SG.

3) Since the SG does not have the MTP3 layer function, it is impossible for the SG to have the SCCP function.

④Since the SG does not need to have signaling point coding, the number of signaling point codes required by the softswitch network elements is less in the case that the softswitch does not use multiple signaling point codes.

⑤ SGs can only work in signaling proxy mode, and an SG can only proxy one softswitch, while a softswitch can be proxied by multiple SGs.

⑥When a soft switch is connected to a TDM switch through an SG via a direct-connect signaling link, the soft switch can use different SGs as proxies to connect to different TDM switches; when a soft switch is connected to a TDM switch through multiple SGs via quasi-direct-connect signaling links organized by an STP, the STP treats multiple SGs connected to it as a single SG, and the maximum number of SGs connected to a pair of STPs can be at most one SG, and the maximum number of SGs connected to an STP can be one SG, and the maximum number of SGs connected to a pair of STPs is one SG. A maximum of 16 signaling links can be set up between multiple SGs and a pair of STPs.

7 From the SG protocol stack, the SG is completely equivalent to a soft switch at the junction of the TDM and IP networks to set up a proxy point, and only the proxy MTP1 and MTP2 layers, the SG will signaling link bearer (MTP1 layer defined standards) based on the conversion of IP bearer, the signaling message MTP2 layer decapsulation, the need to invoke the interlayer between MTP2 and MTP3 layers, the MTP2 and MTP3 layers will be called. After the MTP2 layer decapsulation of the signaling message, the interlayer primitive that needs to be invoked between the MTP2 and MTP3 layers is encapsulated in the M2UA message packet, and then it is sent to the soft switch it represents through the SCTP connection. Therefore, the SG and the soft switch it represents can be regarded as the same signaling point, and the signaling point encoding is in the soft switch, and the SG does not need to have the signaling point encoding.

Adopting the M2UA protocol, it can be regarded as the IPSP (SS) extends the MTP1 and MTP2 layer functions to the SG located between the TDM telephone network and the softswitch network, and interchanges signaling with the TDM telephone network, and is responsible for accomplishing the MTP1 and MTP2 layer functions by the SG, and is responsible for accomplishing the MTP3 and the upper layer functions of MTP3 by the IPSP (SS); the signaling point of MTP3 and the upper layer functions of MTP3 is the same signaling point, and the signaling point is encoded in the softswitch. MTP3 and MTP3 upper layer signaling messages are adapted in the M2UA/SCTP protocol and transmitted in the IP network between the SG and the IPSP (SS).

(3) M3UA Protocol Features

The M3UA (MTP3UserAdaptationLayer) protocol is one of the adapted protocols in this system, and the M3UA protocol allows the signaling gateway to transmit MTP3 user information to the Media Gateway Controller or the IP database, and provides a seamless interworking function for the No.7 signaling network and the IP network. The following are the features of the M3UA protocol.

1) The SG must have the MTP1, MTP2, MTP3 layer protocols, and M3UA/SCTP/IP protocols, and the softswitch must have the MISUP/M3UA/SCTP/IP protocols, and must also support the interlayer proto-language between the MTP3 and ISUP layers.

②When the SG operates in the signaling transfer point mode, the functions that the M3UA can realize are basically the same as those of the M2PA, except that because the M3UA terminates the MTP3 layer protocol at the SG, signaling messages cannot be forwarded between SGs that are connected using an IP network.

3 from the SG protocol stack, on the surface of the SG is completely equivalent to a soft switch at the junction of the TDM and IP networks set up with a MTP1, MTP2, MTP3 layer protocol agent, but because the SG has the function of MTP3 layer protocol processing, so the SG can serve as a soft switch agent, but also as a soft switch STP with signaling point coding to provide signaling message function for soft switch. softswitch with signaling point encoding to provide signaling message functionality.

④When the SG works as a proxy, the functions that the M3UA can realize are basically the same as those of the M2UA, and the M3UA has the MTP3 layer function and can realize the signaling network management function. In addition, because the SG has MTP3 layer function, the SCCP function is also an optional function of the SG.

With the M3UA protocol, the SG can be used as a proxy for SS to connect to TDM signaling networks as well as for TDM signaling networks; it has almost all the advantages of M2PA and M3UA, but its only shortcoming is that, compared with M2PA, the signaling messages cannot be forwarded by multiple SGs in the IP network.

In addition to adapting the data in the MTP3 user part for transmission in the IP network, M3UA also provides part of the signaling network management function corresponding to the MTP3 layer, i.e., mapping the management primitives in the MTP3 to the messages in M3UA and transmitting them between the MTP3 of the SG (signaling gateway) and the user part of the MTP3 in the softswitch to realize the following end-to-end signaling transport and management from No.7 to the IP network.

The M3UA provides an interface between the Flow Control Transmission Protocol and applications using the MTP3 service, whereby MTP3 user messages in No.7 signaling are adapted and then transported in the IP network using the SCTP protocol, allowing nodes in the SS7 and nodes in the IP network to transmit MTP3 user messages to each other. In the IP network, the node that uses M3UA to send and receive MTP3 user messages is called the application server (AS), which is equivalent to the signaling point in SS7. M3UA supports all MTP3 users, and the primitive it provides to the upper-layer users is the same as the primitive provided by MTP3, and the bottom-layer uses SCTP to transmit on the IP network. M3UA and SCTP make it possible to transmit between the MTP3 users on the IP network and the M3UA and SCTP enable seamless peer-to-peer communication between the MTP3 users on the IP network and the corresponding users on the SS7. The signaling gateway that uses the M3UA protocol can adopt the signaling point proxy or signaling transfer point method when it is grouped into a network, and when it is grouped into a network by using the signaling point proxy method, the signaling gateway and the signaling points in the IP network to which it is connected will use the same signaling point code. When using the signaling transfer point method, the signaling gateway is assigned a separate signaling point code and can serve multiple nodes with different signaling point codes in the IP network.

The M3UA protocol can be used to support the delivery of MTP3 subscriber messages for No.7 signaling over an IP network. MTP3 subscriber messages can be either ISUP/TUP messages based on call connectivity or SCCP messages based on connectionless applications. This protocol can be used between a signaling gateway and an MGC or IP database node, or between two IP signaling points. With the adaptation of the M3UA protocol for MTP3 user messages, MTP3 user messages can be transported between a legacy No.7 signaling node and an IP signaling point. the application of the M3UA protocol can also include both direct or quasi-direct connections.

The M3UA protocol functions as follows.

①Supports all SS7MTP3 user part messaging (e.g., ISUP, SCCP, TUP, etc.).

②Supports seamless operation of the MTP3 user protocol peer layer.

③ Supports SCTP transport connections and session management.

④ Supports MGC failover and load sharing.

⑤ Supports asynchronous state change reporting for management entities.

⑥The delivery function of M3UA subscriber messages with routing labels.

⑦ M3UA layer management functions.

⑧ MTP3 inter-network management function in IP networks.

9 Support SCTP connection management function.

⑩Protection of protocols during system inversion switching.

Real-time observation of important data at the protocol layer.

(4) Comparison of application analysis of M2PA, M2UA and M3UA protocols

The following conclusions can be drawn from the analysis of M2PA, M2UA and M3UA protocols.

①The M2PA protocol was produced earlier, and its purpose is to completely transform the traditional TDM-based No.7 signaling network into IP-based No.7 signaling network, so M2PA has a strong IP signaling network networking capabilities, applicable to the use of IP networks to set up the traditional No.7 signaling network with basically the same function of the signaling network, but for the relatively simple structure of the softswitch convergence network, it is basically the same. But for the relatively simple network structure of the softswitch convergence network is basically not used, due to this application in the actual networking rarely have the opportunity to apply, so the development is relatively slow.

②M2PA protocol is the only protocol in the current Sigtran stack that can form IP signaling networks. m2UA protocol is simpler, M2UA protocol reduces the requirements for SG processing capability, but at the same time increases the requirements for the processing capability of the softswitch, the agent of the softswitch SG and the TDMSP/STP connection of all the signaling links need to be configured in the SS, which can be seen as the SS through the softswitch convergence network. The SS can be regarded as directly connected to all TDMSPs/STPs connected to the SG through the SG; at the same time, the signaling network management function is poor; therefore, the M2PA protocol is suitable for the networking mode in which the soft switch sets direct signaling links directly with a small number of TDM switches through the SG, or with a small number of STPs in the TDM switching network, and for the networking mode in which the soft switch is connected to TDM switches through the SG through the STP through the SG, but for the soft switch, it is not suitable for the network mode in which the soft switch is connected to TDM switches through the STP. For the networking mode in which soft switches are connected to TDM switches through STP via quasi-directly connected signaling links, there are certain shortcomings.

3) In the case of M3UA protocol, SG can work in both signaling transfer point mode and signaling agent mode, which has greater flexibility in network organization. When working in the signaling transfer point mode, it has almost all the advantages of the M2PA protocol, although it is not able to realize the signaling messages to be transferred through multiple SGs in the IP network, but this function is not needed in the softswitch convergence network that does not need to support the MAP and CAP protocols. Since the No.7 signaling link number (SLC) on the TDM side of the SG does not have a one-to-one correspondence with the IP signaling link (SCTP link) on the IP side, there is no longer a limitation of 16 signaling links between the softswitch and the SG (at this time, the SG can be regarded as an IPSTP) in the case of the SG working in the signaling transfer point mode, i.e., the conveyor width between the softswitch and the SG no longer becomes the "bottleneck"; in signaling network management, there is no problem when the SG works in the signaling transfer point mode; at the same time, the SG supports the pair-configured networking mode similar to the STP in the TDM switching network, and C-links can be set up between the paired SGs, which improves the security and reliability of the signaling network; however, for the case when the SG works in the signaling proxy mode, although it is slightly better than M2UA, the difference is not significant.

(5) Characteristics of the SUA protocol

The SUA protocol can be used to support the transmission of No.7 signaling SCCP user messages, which can be either TCAP messages or RANAP messages, over the IP network. This protocol can be used between a signaling transfer point and an IP database node, or between two IP signaling points, but its main application is for non-circuit call-type services, and the application of the protocol can also include two modes of signaling relay and non-relay methods.

The SUA protocol functions as follows.

l Supports No.7 user part messaging (e.g., TCAP, etc.).

l Support SCCP connectionless service.

lSupport seamless operation of SCCP user part peer layers.

l Support for distributed IP-based signaling nodes.

The IUA protocol features are as follows.

l Support for Q.921 and Q.931 interface interface transport primitives.

l Support inter-layer management module communication between SG and MGC.

l Supports management of activated connections between SG and MGC.

lSupports point-to-point and point-to-multipoint communication.

The SCTP protocol functions are as follows.

lSCTP connection establishment and dismantling.