The SS7 protocol stack
Signaling System No. 7 (SS7) uses a layered protocol stack for call control, routing, and service signaling. Its organization is similar in concept to the . However, SS7 defines three Message Transfer Part levels and user parts rather than seven OSI layers. MTP provides signaling transport across the network, while the user parts define call-control, database, and service procedures. The division separates common routing functions from application-specific signaling. MTP-3 delivers network-service primitives to user parts, while SCCP adds addressing and connection services for transaction-oriented applications.
The stack contains the Message Transfer Part (MTP) and user parts that use its transport service. MTP-1, MTP-2, and MTP-3 cover the physical, link, and network functions of signaling transport. Together they provide the common transport service used by ISUP, SCCP, and other user parts. User parts exchange messages for functions such as call control and subscriber-data queries.
Message Transfer Part (MTP)
MTP transfers signaling messages between network nodes and is divided into three levels. The levels cover the signaling data link, signaling link, and signaling network functions. Together they provide the transport service used by ISUP, SCCP, and other user parts. Signal units also carry signaling-network management messages.
- MTP Level 1, The Signaling Data Link (Physical Layer):
This physical layer defines the electrical and physical characteristics of the signaling link, including connectors, cabling, E1/T1 links, and transmission rates. It transfers bits across the physical medium and presents the interface used by MTP Level 2.
- MTP Level 2, The Signaling Link (Data Link Layer):
This layer provides reliable communication between two directly connected signaling points. It organizes bits from MTP-1 into signal units, detects errors, uses retransmission for damaged units, preserves sequence, and controls flow. Link status is supervised with LSSU and FISU signal units.
- MTP Level 3, The Signaling Network (Network Layer):
This layer routes signaling messages through the SS7 network from the originating point to the destination point, which may be several hops away. It uses point-code routing, selects links, shares load, manages congestion, and reroutes traffic if a link or node fails. Changeover and changeback procedures support link restoration and signaling network management. Its transport function is independent of the user-part application, so the same hierarchy can carry circuit-related and non-circuit-related signaling.
SS7 user parts
User parts use MTP or SCCP transport services to exchange messages for particular network functions. They include call-control and transaction-oriented protocols, and their messages are transported as signaling units by the lower MTP levels. The selected user part determines the application semantics of each message. ISUP normally uses MTP services directly, while TCAP-based applications normally use SCCP for subsystem addressing.
- ISUP (ISDN User Part):
The primary user part for call control. ISUP defines messages and procedures for setting up, managing, and releasing voice and data calls in PSTN and ISDN networks. Initial Address Message (IAM) carries call setup information, Answer (ANM) indicates answer, and Release (REL) starts call clearing.
- SCCP (Signalling Connection Control Part):
SCCP operates above MTP-3 as a level-4 function and extends it with subsystem addressing, , and connectionless and connection-oriented network services. SCCP can use a subsystem number together with a point code to address an application at a signaling point. These functions support database-oriented applications.
- TCAP (Transaction Capabilities Application Part):
TCAP enables transaction-based communication between network nodes, typically for database queries. It provides a standardized way for an application to invoke functions on another node and correlate a response with a request. TCAP supplies the transaction framework used to exchange operations and results and to manage the state of each transaction.
Services using TCAP
Several application-level protocols use TCAP for service procedures and exchange operations with remote service logic or databases:
- INAP (Intelligent Network Application Part):
Used to implement intelligent network services. When a toll-free number is called, INAP can query a database at an SCP to determine how the call should be routed, for example to a selected call center. It also enables services such as number portability and prepaid calling cards.
- MAP (Mobile Application Part):
Used in mobile networks such as GSM and UMTS. MAP handles mobility management, allowing the network to update a subscriber's location as they move between cells. It also supports authentication, SMS (Short Message Service) delivery, subscriber data, and between different operators.
SS7 signaling over IP with SIGTRAN
SIGTRAN is a suite for transporting SS7 signaling messages over IP networks instead of traditional dedicated links. It combines a transport protocol with adaptation layers for SS7 users, so existing signaling applications can be mapped to an IP transport association.
- SCTP (Stream Control Transmission Protocol):
SIGTRAN uses SCTP as its transport protocol over IP. SCTP provides reliable, message-oriented transport similar to TCP, with multiple ordered streams and multihoming. Independent streams limit head-of-line blocking between separate message sequences, while multihoming provides path redundancy. SCTP can preserve message boundaries that are useful for signaling protocols.
- Adaptation Layers (e.g., M2UA, M3UA):
Adaptation layers allow upper-level SS7 protocols such as ISUP and SCCP to operate over SCTP/IP. M2UA carries MTP-2 services to a remote MTP-3 user and forwards their primitives, M3UA provides an interface for MTP-3 users such as ISUP, and SUA provides an interface for SCCP users such as TCAP. They carry the SS7 user messages in formats suitable for IP transport.