TUP, NUP and ISUP are protocols designed for user part delivery. These can be used for the establishment, maintaining and ending of the calls. The protocol layer structure of the signaling is presented in Figure 4.7.
In fixed network signaling, the TUP/NUP/ISUP protocols are utilized as well as the underlying MTP protocol stack, the same principle has been adopted by mobile communications.
4.3.2 SIGTRAN
The transfer of information consists of the message and related signaling, or control part. The practical means of how these two parts are treated within the network depends on the system. In the earlier days of telecommunications, the information and control part were tight together and were sent to the receiving end physically via the same resources. Some examples of this principle are the first fixed telephone networks, and first, analog mobile telecommunications networks. A concrete example of the latter is NMT, which had a frequency modulated voice channel that was put on hold whilst the related signaling, for example, due to the handover procedure, was performed.
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Layer 1 Layer 2 Layer 3 TUP/NUP/ISUP
Figure 4.7 The fixed telephony network’s SS7 consists of MTP and TUP/NUP/ISUP layers.
Nowadays, modern telecommunications systems are based on independent physical resources for the delivery of information and related control data. This separate signaling can, logically, be delivered also within the same physical level, for example, via radio link or optical fiber, but there is no dependency on these two which provides, for example, a fluent continuum of the voice call even during the handover procedure.
SS7 has been useful for a long time for circuit switched telephony networks. Nevertheless, as the develop- ment has taken a major leap towards all IP concepts for both fixed and wireless networks, the basic form of SS7 is not optimal in the packet-based environment.
Various intentions were thus made, and many proposals for the solution have been presented, including TIPHON (Telecommunications and IP Harmonization on Networks). One of the major solutions for updating the signaling in the packet switched networks has been developed by IETF under the name SIGTRAN (Signaling Transport). It has been created in the working group with the same name, and it defines a set of protocols that are able to make the SS7 of the public telephone networks transported also over the IP networks. SIGTRAN was published in IETF RFC 2719 with the name “ Architectural Framework for Signaling Transport.” Figure 4.8 shows the idea of SIGTRAN protocol family as a replacement of the OSI model’s lower layers.
The set of protocols thus forms a new transport layer, SCTP (Stream Control Transmission Protocol) with a set of user adaptation layers that are capable of functioning like the services of the lower layers of SS7 and ISDN.
SIGTRAN can thus be considered as an evolved version of the original SS7. It defines adaptors and transport capacity by mixing protocols of SS7 and IP in such a way that it offers the essential from both technologies. The SIGTRAN applications include Dial-Up via Internet, IP telephony connected between IP and PSTN, among other services.
The components of SIGTRAN are presented in Figure 4.9. They are:
r Signaling Gateway (SG). This takes care of the interconnection of SS7 networks, and transmission of signaling messages of the IP nodes.
r Media Gateway (MG). This takes care of the encapsulation of the voice traffic as well as the transmission of traffic towards the destination.
r Media Gateway Controller (MGC). This takes care of the call control between SG and MG. It also controls the access of IP towards PSTN.
r IP Service Control Point (IP SCP). This is an integral part of IP networks, but this makes it possible to map it with SS7 networks.
Protocols 85 MTP2 IP SCCP ISUP MTP3 ISDN MTP1 SCTP M2UA, M3UA, M2PA
SUA, IUA TCAP
Ethernet T1/E1
Figure 4.8 The mapping of the SIGTRAN and T1/E1 protocol layers. SIGTRAN protocol family is designed as a replacement of lower-level OSI network type.
The set of protocols related to SIGTRAN includes the following:
r SCTP (Stream Control Transmission Protocol). This is the main protocol of SIGTRAN, and is essentially for the transport. This protocol has been first published in RFC 2960, and was later updated in RFC 3309. SCTP is used by one of the following protocols for the layer adaptation: SUA, IUA, M3UA, M2UA, M2PA, V5UA or DPNSS/DASS2. The explication of these is in the following bullet points.
r SUA (Signaling Connection Control Part User Adaptation Layer). An example of this is TCAP. It provides SCCP services in the case of peer-to-peer, like SG-to-IP SCP. Its user is, for example, TCAP.
MGC SG MG IP Phone MGC SG MG IP SCP SS7 Voice SS7 Voice IP network
Figure 4.9 The high-level presentation of the interfaces between IP networks and PSTN for the provision of voice service.
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r IUA (ISDN Q.921-User Adaptation Layer). This has been defined in RFC 4233 and RFC 5133. It provides LAPD (ISDN Data Link Layer) services. Its user is ISDN layer 3 entity (Q.931).
r M3UA (SS7 Message Transfer Part 3 (MTP3) User Adaptation layer). An example is ISUP and SCCP. This has been defined in RFC 4666. It provides MTP3 services in the cases of client-server and peer-to-peer. Its users are SCCP and ISUP.
r M2UA (SS7 Message Transfer Part 2 (MTP2) User Adaptation layer). This has been defined in RFC 3231. It provides MTP2 services in a setup consisting client and server, including SG (server) to MGC (client). The user of this layer is MTP3. M2UA transfers MTP2 user data SG (MTP2 instance) and MGC (MTP3 instance), and M2UA makes it possible for MGC to provide MTP2 service.
r M2PA (MTP2 Peer-to-peer user Adaptation layer). This has been defined in RFC 4165. It provides MTP2 services in a case of peer-to-peer, including SG-to-SG. Its user is MTP3.
r V5UA (V5.2-User Adaptation Layer). This has been defined in RFC 3807. It provides V.5.2 protocol services.
r DPNSS/DASS2 User Adaptation (DUA). This has been defined in RFC 4129.
These protocols are basically utilized in order to replace the original SS7 stacks described in Section 4.3.1 for the transport of the contents in the IP environment. This provides flexible routing of signaling in IP networks. The typical use cases of SIGTRAN are related to VoIP, and in general, multimedia over IP, including video and music.
Figure 4.10 shows the protocol layer structure of SIGTRAN. In the figure, the red elements are new, along with the definition of SIGTRAN, whilst the other elements have already been defined earlier. In addition to the elements shown in the figure, there is also ISUP for the ISDN user part. It would typically be located over MTP3 or M3UA.
The adaptation layers of SIGTRAN serve, for example, the following situations:
r They carry upper layer signaling protocols over IP transport, which is assumed to be reliable.
r They are transparent for the users and services, and thus provide the same services as is the case in PSTN.
IP MTP3
M3UA
SCTP SCCP
M2UA M2PA SUA IUA
ISDN TCAP
Protocols 87 Signaling gateway MTP2 M2UA SCTP IP MTP1 NIF SS7 links Media gateway controller M2UA SCTP IP MTP3
Figure 4.11 The architecture of M2UA. 4.3.2.1 M2UA
M2UA provides a link between remotely located MTP2 and MTP3 instances. In the case of M2UA, one user is MTP3 and the other is an SG IWF. M2UA functions according to a client-server model between SG (having MTP2 instance) and MGC (having MTP3 instance). In this model, MGC represents the client and SG the server. Figure 4.11 presents the architectural model of M2UA.
The functionality of M2UA is applicable, for example, in the case where there is a low density of SS7 links at a certain point of network, or if there are a large number of SG functions that are separate.
This SIGTRAN configuration is common in European networks as the SS7 links and voice circuits are typically shared physically.
4.3.2.2 M2PA
M2PA is similar to M2UA, but is meant for the peer-to-peer cases, and replaces the MTP2 link under MTP3. In the case of M2PA, the user is MTP3 at both ends of the connection. M2PA makes it possible for MTP3 layers in SGs to communicate directly.
M2PA allows communications between SS7 systems over IP instead of the use of TDM links based on T1 or E1. The practical benefit of M2PA is that it can be used instead of the MTP2 link in such a way that there is no need to invest in a dedicated SS7 HW.
The architectural model of M2PA is presented in Figure 4.12. This solution is applicable in SG-SG connection, and is functioning in such a way that it bridges two isolated SS7 parts of the network.
4.3.2.3 M3UA
The functioning of M3UA is similar to M2UA, that is, it functions in the same way as a client-server between SG and MGC, in order to provide the upper layer SS7 with protocol remote access to lower layers.
M3UA signaling gateways run lower levels of SS7 protocol stack, that is, in MTP3 and MTP2. On the other hand, M3UA functions as application servers at higher levels, that is, in ISUP and SCCP. M3UA provides reliable and flexible system architecture.
88 The Telecommunications Handbook Signaling gateway MTP2 M2PA SCTP IP MTP1 SS7 links MTP3 Signaling gateway MTP2 M2PA SCTP IP MTP1 MTP3 SS7 links
Figure 4.12 The architectural model of M2PA. The architectural model of M3UA is presented in Figure 4.13.
The M3UA is suitable when there is a high density of SS7, making it possible to use a standalone SG. This architecture is applicable when SS7 links are physically accessible as a single point.
The model is thus suitable in North America as the SS7 links are physically separated from the voice circuits.
4.3.2.4 SUA
Via SUA, it is possible for Application part, like TCAP, of IP SCP to be reached via an SG. The architecture of SUA is presented in Figure 4.14.
SUA makes it possible to make the mapping between SCCP addresses and IP addresses at the SG.
Signaling gateway MTP2 M3UA SCTP IP MTP1 NIF SS7 links Media gateway controller M3UA SCTP IP MTP3 ISUP
Protocols 89 Signaling gateway MTP2 SCTP IP MTP1 NIF SS7 links Media gateway controller SUA SCTP IP MTP3 SCCP SUA TCAP
Figure 4.14 The SUA architecture.