SS7 is a widely-used communications protocol that is used within and between telephony networks for system management, call setup and tear-down and other network functions. Although there are slight variations in the fine detail of SS7 implementations used around the world, all telephone companies use the same set of protocols known generically as SS7.
Because the signaling uses a separate network to the voice traffic, SS7 allows the use of features such as automatic callback when a called number is busy. Messages about the states of the voice lines are sent and received on the signaling network so voice lines remain available until both parties are known to be ready to speak.
SS7 links and network components
SS7 uses discrete messages to exchange information, the messages being passed between switches and other end points using packet switched data links. There are two types of connections in a telephony network:
- Packet switched links used for SS7 messages (but not voice data).
- Circuit switched links used for voice data (but not SS7 messages).
The SS7 protocol stack
SS7 is structured in a multi-layered stack which corresponds closely to the layers of the standard OSI model, although some SS7 components span a number of layers, as shown in Figure 1.
- Layer 1 (Physical): MTP-1 (Message Transfer Part-1)
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MTP-1 defines the physical means by which SS7 messages are transferred from one node to another. For E1 ot T1 networks, the physical layer is usually a timeslot of an E1 or T1 frame respectively.
The physical layer specifies only how a sequence of bits is conveyed from one SS7 node to another. It says nothing about the actual meaning of the bits or how they are grouped together to form a message.
- Layer 2 (Data Link): MTP-2
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MTP–2 defines how an MTP-1 bit transfer mechanism is used to reliably pass variable length messages from one SS7 node to another. MTP-2 uses a variant of the High level Data Link Control (HDLC) used in most modern data transfer protocols. This uses a delimiter to define the start and end of a data frame, prevents flags occurring in a frame (bit-stuffing) and protection for the entire frame (CRC at the end). It also defines how CRC errors are handled (by error response and retransmission).
MTP-2 says nothing about the actual content of a message. It simply defines a mechanism by which a message of any length can be sent 100% reliably between SS7 nodes and can be used by higher layers of the SS7 protocol.
MTP-2 knows nothing beyond the single point-to-point link it operates on.
- Layer 3 (Network Layer): MTP–3
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MTP-3 builds on top of the lower-level MTP layers to allow the creation of a network of telephony network nodes interconnected by SS7 links. Each node is assigned a unique address in the network (known as a Signaling Point Code or SPC). Messages can be sent at the MTP-3 level in one node to a topologically distant node (that is with one or more intermediate SS7 nodes) simply by specifying the Destination Point Code (DPC). MTP-3 entities on the SPC node, the DPC node, and all intermediate nodes coordinate the transfer of a higher-layer message through the network.
MTP-3 can use multiple parallel routes from SPC to DPC through the network to take account of link loading and availability (there should always be more than one way to get from any SPC to any DPC).
- Upper Layers: ISUP (Integrated Services User Part)
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The ISUP handles the setup and tear-down of telephone calls. Its functions include calling and called number notification (or suppression), the ability to control billing rates, and advanced telephony functions such as transfer and control over whether the voice channel is used for voice, fax, or data.
ISUP messages flow only during the setup and tear-down phases of a call.
For more detail on the SS7 protocol stack and its components, see the SS7 Support for Blueworx Voice Response: SS7 User's Guide information.
Using SS7 with Blueworx Voice Response
SS7 Support for Blueworx Voice Response is implemented as follows:
- An SS7 hardware adapter is plugged into the PCI bus of the pSeries computer. A separate E1 or T1 trunk carrying the signaling traffic for one or more voice trunks is connected to the SS7 adapter. The adapter handles the MTP-1 layer of the SS7 stack and contains firmware supporting the MTP-2 layer.
- An SS7 software stack runs on top of the PCI adapter and provides standards-compliant support for the MTP-1 and MTP-2 layers of the SS7 protocol stack. Although the stack is able to support most components of the stack above MTP-3, only ISUP is used for the Blueworx Voice Response basic call signaling application.
- The server component of the SS7 Support for Blueworx Voice Response product provides the support that allows the SS7 Server machine to connect to one or more Blueworx Voice Response SS7 Clients and provides the system configuration utilities.
- The client component of the SS7 Support for Blueworx Voice Response product contains a Blueworx Voice Response signaling process (a special Blueworx Voice Response custom server) and the SS7 Support for Blueworx Voice Response software provides the interface between the top of the ISUP layer of the SS7 protocol stack and the Blueworx Voice Response Signaling Library. This allows Blueworx Voice Response applications to receive incoming calls and to make outgoing calls without knowing the type of signaling being used so they do not need to change between CAS, ISDN, and SS7.
Although the above describes a logical view of the structure of Blueworx Voice Response’s SS7 support, the actual implementation is more complicated in that the SS7 stack and signaling processes can run on different pSeries computers connected by a fast TCP/IP LAN. This allows for a distributed system where a single SS7 controller machine running the SS7 adapter and stack (the SS7 Server) can control up to six voice processor machines each running Blueworx Voice Response and the SS7 signaling process (the SS7 Client).
The SS7 Server can be duplicated in a load-sharing configuration to provide high reliability.