Signaling System 7
Signaling System 7 (SS7) is a critical component in today’s telephone network, for it is responsible for handling many of the services we take for granted. For example, 800 service, credit card phone call charging, Centrex-like services across the city or country, and custom local area signaling services (CLASS) all rely on SS7 databases and related functions to work. New applications, such as local number portability, will also be implemented via SS7. We’ll look at a simple example to see why SS7 is such an important ingredient.
Suppose a call is made to a customer service representative of the fictitious Clamex Corporation. The end office switch invokes SS7 to place the call and simultaneously provide the caller’s name and account history to the Clamex representative. Let us examine the steps involved in such a call.
- The user dials 1-800-INEEDHELP. The originating end office switch recognizes the need to invoke SS7 upon receiving the dialed digits, 1-800-463-3343. (The 1+800 sequence is the actual trigger. Also, note that the excess digits, “LP,” are ignored by the switch). Further call processing is suspended until the receipt of call processing instructions from the network database.
- The originating switch requests routing instructions. The switch generates a routing request message, inserting the dialed 800+ number and the calling party’s identification. The message is sent to the signaling system’s network database.
- The network database responds with dialing information. The network database initiates a search for the dialed 800 number. When the number is found, either locally or via another carrier’s database, a routable 10 digit number is returned to the originating central office. The originating switch can now determine the location of the destination end office switch.
- The local end office routes the call to the destination. The originating end office generates a message that informs the destination end office of the incoming call. This message is carried over the signaling network and is delivered to the destination switch. Not only is the notification about the incoming call delivered, but also the calling party’s number and the dialed digits (800 number).
- At the destination, the call is processed to the corporation’s automatic call distributor. Although this step could be performed by the signaling network, most users have an automatic call distributor (ACD) computer to handle the distribution of calls within their organization. Any information delivered to the destination end office can be handed to the ACD. In our example, Mr. Smith’s number is delivered along with the call, enabling a database lookup based on his telephone number. The ACD determines the appropriate call service attendant, processes the voice call to the attendant, and coordinates a “screen pop” to the attendant’s computer.
The SS7 (CCS) Network
The visual illustrates the structure of a full SS7 network, also called the common channel signaling network (CCS). The network is composed of signaling points and signaling links as defined by the ITU-T.
- Service switching points (SSP) are the access points to the SS7 network for the telephone switches. They generate and terminate call processing and database query messages.
- Signaling transfer points (STP) are fundamentally packet switches. They could also provide services typically assigned to higher layers, in which case they are said to have “integrated SCP function.”
- Service control points (SCP) are typically host computers and provide database applications such as number translation.
As is typical of the telephone network, high degrees of redundancy and fault tolerance are built into the SS7 structure. SSPs and SCPs attach to SS7 via multiple A-links. B-links and C-links connect STPs into quads.
SS7 Protocol Structure
The visual relates SS7's protocol layers to those of the OSI Reference Model.
The Message Transfer Part (MTP) corresponds to the lower three layers of the OSI model. These include the Signaling Data Link Layer, the Signaling Link Layer, and the Signaling Network Layer. The Signaling Data Link encompasses the functions of the OSI Physical Layer. The Signaling Link provides a subset of the functions found in the OSI Data Link Layer. The Signaling Network corresponds to the OSI Network Layer.
The Signaling Connection Control Part (SCCP) builds additional function on top of the MTP. Examples include “basic connectionless class” and “flow control connection-oriented class” transport services.
Although the ISDN User Part (ISUP) is shown to be able to interface to either the SCCP or MTP, the ANSI specifications only define ISUP use directly with MTP to support basic bearer services and supplementary services. Transactions Capabilities describe the set of Application Layer protocols used by distributed applications within the network. Examples of applications include 800 database and alternative billing, as well as intelligent network services.
SS7 Protocol Components
The function of the MTP is to serve as a connectionless transfer system. Each signaling message takes its own route through the network with no prior connection establishment. The MTP provides reliable transfer of signaling messages between communicating users or application functions. The notion of providing reliable service over a connectionless network seems to contradict normal Network Layer service in a packet network. Normally, reliable service is provided over connection-based networks and unreliable service is provided over connectionless networks. The reasons for deviating from this maxim, however, are straightforward. Connectionless service avoids the overhead associated with establishment of signaling connections, thus streamlining the protocol in an attempt to yield high performance. On the other hand, providing reliable service is consistent with the need for high reliability in the telephone signaling system. The MTP corresponds to the OSI reference model’s Physical and Data Link Layers and a subset of the Network Layer.
The SCCP completes the Network Layer function as defined by OSI, and allows access to the database query functions afforded by the Transaction Capabilities Application Part (TCAP).
The ISUP provides circuit related functions. Call processing applications in end-office switches use ISUP to set up and tear down trunks used for POTS and ISDN calls. Thus, SS7 replaces in-band signaling on those interoffice trunks. ISUP is used to relay supervisory information needed for call establishment and disconnect, and for billing functions. SS7 performs these functions faster than in-band signaling, providing better utilization of message trunks and faster call setup. ISUP also allows addressing information to be passed in the signaling message. This includes calling number as well as called number, and makes possible several new types of services.
The TCAP enables signaling points to exchange information for service provision or network administration. It is designed to support database query/response by switches. Applications in switching machines can use TCAP to access centrally stored call processing and routing information or customer-specific information. TCAP also supports the communication necessary to support distributed processing in the network.
Upper Layers: User Parts and Application Parts
With MTP and SCCP providing transport services, SS7 must have upper layer protocols to provide meaningful communication-related tasks. These are separated into two main categories: User Parts (UP) and Application Parts (AP).
Three UPs are identified. These are ISDN User Part (ISUP, sometimes abbreviated ISDN-UP), Telephone User Part (TUP), and Data User Part (DUP). Due to the fact that ISUP includes the functions of both TUP and DUP, it has been chosen as the only user part for SS7 networks in North America. ISUP was developed to meet ISDN service needs with sufficient flexibility to support a broad range of future services. It was designed to interwork with ISDN’s access signal interface (DSS1). ISUP also supports “supplementary services” such as user-to-user signaling, closed user group, call forwarding, and calling line identification.
Examples of APs include TCAP for database queries, and Operations, Maintenance, and Administration Part (OMAP) for network management features.
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