The CCS7 network is a logically separate network within a telecommunication network. It consists of signaling points or signaling nodes connected with the signaling links. The CCS7 network has four distinct signaling points.
Service signaling points (SSPs) are network nodes that generate signaling messages to transfer call- or transaction- (non-call-) related information between different CCS7 nodes. In wireline networks, a local switch may have SSP capabilities. In wireless networks, the BSCs and MSCs are the SSPs.
Signaling transfer points (STPs) are network nodes that relay signaling information from one signaling node to another.
A combined SP/STP is a node that has capabilities of both SP and STP; i.e., it can originate or accept CCS7 signaling messages as well as transfer messages from one SP to another SP.
Signaling control points (SCPs) are nodes that contain databases that enable enhanced services.
Signaling links interconnect two signaling points. A signaling linkset is made up of multiple signaling links. It is recommended to have at least two signaling links in a linkset for reliability purposes. A linkset can have a maximum of 32 links. A route is defined as a collection of links between originating and terminating SPs via intermediate nodes. There may be several routes that a message can traverse between the originating and terminating SP. These signaling routes are collectively called a signaling routeset.
Figure 2-1 shows a simplified CCS7 signaling network architecture. As we will learn later in this topic, the CCS7 protocol has a built-in error recovery mechanism to ensure reliable transfer of signaling messages.
Figure 2-1 CCS7 signaling network architecture.
To take full advantage of the built-in recovery mechanism, the STPs and SCPs are generally provided in mated pairs. In addition, redundant links are provided to transfer the signaling messages using alternate routes in case of link failure.
CCS7 has a layered protocol architecture, as shown in Figure 2-2. The protocol stack consists of four levels. These levels are loosely related to Open System Interconnects (OSI) Layers 1 to 7. The lower three levels, referred to as the Message Transfer Part (MTP), provide a reliable service for routing messages through the CCS7 network.
The Signaling Data Link (referred to as MTP Level 1) corresponds to the Physical Layer of the OSI model. It defines the physical and electrical characteristics of the signaling link connecting two signaling nodes.
The Signaling Link (MTP Level 2) corresponds to the Layer 2 of the OSI model. It is responsible for error-free transmission of messages between two adjacent signaling nodes.
The Signaling Network (MTP Level 3) provides the functions related to message routing and network management.
MTP Levels 1, 2, and 3 together do not provide a complete set of functionalities as defined in OSI Layers 1 to 3. The Signaling Connection and Control Part (SCCP) offers enhancements to the MTP Level 3. The SCCP and MTP together are referred as the Network Service Part (NSP).
At Level 4, there are several user parts or application parts. The user parts use the transport capabilities of MTP or NSP. ISDN User Part (ISUP) provides for the control signaling needed to support ISDN calls. The Transaction Capabilities Application Part (TCAP) provides the control signaling to connect to centralized databases. The Mobile Application Part, which is the user of TCAP, provides the ability to support user mobility in wireless networks.
Figure 2-2 CCS7 protocol architecture.
