SS7 Protocol Stack

The Signaling System 7 (SS7) protocol stack is a set of telephony signaling protocols used worldwide for setting up and tearing down telephone calls, as well as providing various other services in mobile and fixed-line networks.

SS7 Protocol Stack Layers

The SS7 stack is divided into four levels, which roughly correspond to the OSI model layers.

OSI Layer	SS7 Level	Protocols
Application (Layer 7)	Level 4	TCAP, MAP, INAP, IS-41
Presentation (Layer 6)	Level 3	SCCP
Session (Layer 5)	Level 3	SCCP
Transport (Layer 4)	Level 3	MTP3
Network (Layer 3)	Level 3	MTP3
Data Link (Layer 2)	Level 2	MTP2
Physical (Layer 1)	Level 1	MTP1

Layer Descriptions

Physical Layer (MTP1)

Defines the physical and electrical characteristics of the signaling links. It's responsible for the actual transmission of SS7 messages across the network.

Data Link Layer (MTP2)

Ensures reliable transfer of signaling messages between two adjacent signaling points. It handles error detection, error correction, and flow control.

Network Layer (MTP3)

Responsible for routing messages to their correct destinations. It handles network management functions, traffic management, and routing.

SCCP (Signaling Connection Control Part)

Provides additional networking functions, including the ability to route messages based on logical addresses. It's crucial for supporting advanced services and mobile communications.

Application Layer

TCAP (Transaction Capabilities Application Part): Supports the exchange of non-circuit related data between applications across the SS7 network.
MAP (Mobile Application Part): Used for mobile services, such as location updates, SMS, and roaming.
INAP (Intelligent Network Application Part): Supports advanced telephony services like toll-free calling and call forwarding.
IS-41: Used in North American cellular networks for mobility management and inter-system handoff.