SS7 Protocol layers

The SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions. The SS7 protocol is designed to both facilitate these functions and to maintain the network over which they are provided. Like most modern protocols, the SS7 protocol is layered.

Physical Layer (MTP-1) This defines the physical and electrical characteristics of the signaling links of the SS7 network. Signaling links utilize DS–0 channels and carry raw signaling data at a rate of 56 kbps or 64 kbps (56 kbps is the more common implementation). Message Transfer Part—Level 2 (MTP-2) The level 2 portion of the message transfer part (MTP Level 2) provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling messages. It incorporates such capabilities as error checking, flow control, and sequence checking. Message Transfer Part—Level 3 (MTP-3) The level 3 portion of the message transfer part (MTP Level 3) extends the functionality provided by MTP level 2 to provide network layer functionality. It ensures that messages can be delivered between signaling points across the SS7 network regardless of whether they are directly connected. It includes such capabilities as node addressing, routing, alternate routing, and congestion control.

Signaling Connection Control Part (SCCP)

The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.

While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.

ISDN User Part (ISUP)

ISUP user part defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched network (PSN), and to manage the trunk network on which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes.

Transaction Capabilities Application Part (TCAP)

TCAP defines the messages and protocol used to communicate between applications (deployed as subsystems) in nodes. It is used for database services such as calling card, 800, and AIN as well as switch-to-switch services including repeat dialing and call return. Because TCAP messages must be delivered to individual applications within the nodes they address, they use the SCCP for transport.

Operations, Maintenance, and Administration Part (OMAP)

OMAP defines messages and protocol designed to assist administrators of the SS7 network. To date, the most fully developed and deployed of these capabilities are procedures for validating network routing tables and for diagnosing link troubles. OMAP includes messages that use both the MTP and SCCP for routing.

The History of SS7

The History of SS7

 

The SS7 standard defines the procedures and protocol by which network elements in the PSTN exchange information over a digital signaling network to perform call setup, routing and control.
Throughout the years, different variants, or flavors, of SS7 have been developed. In fact, today there are more than 100 different variants of SS7 worldwide. Of course, differences in protocols are certainly not specific to SS7 networks, there are multiple IP-side protocols being developed as well, including SCTP, M3UA, SUA, MGCP, H.248, MEGACO, SIP and H.323.

Because of these different SS7 flavors, signaling networks in one geographic area may not be able to interwork with a signaling network in another. Protocol conversion between these two flavors is necessary for intelligent routing to occur. In VoIP applications, this would ideally occur in the same signaling gateway that allows SS7 to communicate to IP. The SEGway Signaling Gateway does exactly this.

important features

Some of the important features:

• SS7 (SSP) simulation over TDM (E1 and T1)
• Multiple E1 and T1 line interfaces supported
• User-friendly GUI for configuring the SS7 MTP Layers
• User configurable signaling links
• User-configured Circuit Mapping, i.e defines Circuit Identification Codes (CIC) and map these CICs to Timeslots/Trunks in order to enable Voice/Data traffic
• Supports MTP2 and MTP3 protocol machine
• Multiple MTP links
• Access to all ISUP Message Parameters Initial Address, Subsequent Address, Release messages, and more
• User controlled access to optional ISUP parameters such as timers

SS7 Over IP

Benefits:

• Ease of deployment: When using signaling gateways (such as access service group [ASG]), there is no need to disrupt the existing SS7 network, and future enhancements are transparent.
• 
  Less costly equipment: There is no need for further expensive investments in the legacy signaling elements.
• Better efficiency: SIGTRAN over an IP network doesn't require the physical E1/T1 over synchronous digital hierarchy (SDH) rings. Using new technologies like IP over SDH and IP over fiber, for instance, can achieve much higher throughput.
• Higher bandwidth: SIGTRAN information over IP does not constrain to link capacity as it does in the SS7 network. The IP network is much more flexible than the TDM-based legacy network.
• Enhanced services: Implementing a core IP network facilitates a variety of new solutions and value-added services (VAS).

SS7 and Internet Protocol (IP)

The signaling system 7 (SS7) control system that is used in public switched telephone networks (PSTN) can be interconnected to other types of systems and networks using Internet Protocol (IP). Some of the interconnection issues relate to how the control of devices can be performed using dissimilar systems (e.g. mixing voice and data systems). Another key reason for interconnecting SS7 network devices to IP data networks is the cost savings that result from avoiding the access charges for connect equipment to SS7 systems and databases. This excerpted chapter from SS7 Basics, 3rd Edition provides an overview of how SS7 and Internet Protocol (IP) can be integrated. It is an introduction on how SS7 messages can be transported over IP networks (even the Internet in some cases) and how SS7 can interface with other systems (such as IP Telephony systems). It provides a description of the signaling transport (SIGTRAN) system and how it uses SCTP packet transmission protocol to reliably send telephone call control messages across connectionless data networks. Explained are the adaptation layers M3UA, M2UA, M2PA, SUA, and IUA that are used to interconnect SS7 nodes via IP systems. Overviews of SIP, MGCP, MEGACO, and H.323 IP telephone systems are provided.