Signaling System 7

Next time you pick up your telephone handset, think about what happens after you dial but before you hear the telephone ringing at the other end.

If you're making a call using a regular area code, the local phone exchange can determine the call's destination simply by looking at the first six digits of the 10-digit phone number.

However, if you're calling a toll-free number, the numbers themselves mean nothing. For this type of call, your local phone service provider first needs to query a database to determine where that number is physically located. The provider then contacts phone switches along the destination to set up the call.

If you use a calling card during a business trip, the local telephone central office needs to communicate with that calling card provider's database, as well as with phone switches between you and your call's destination central office.

If you use caller ID or call forwarding, other control signals must be communicated through the Public Switched Telephone Network (PSTN) as the call is set up. Billing information also must be tracked and sent to the appropriate database after the call.

The common thread that ties the PSTN and these services together is Signaling System 7 (SS7), a communications protocol first defined by the ITU-T in 1980.

SS7 Signaling Points

SS7 is an out-of- band protocol, meaning its signals travel on their own data paths from the actual voice or data call. The protocol is used within the PSTN to communicate between three classes of devices: Service Switching Points (SSPs), Signal Transfer Points (STPs), and Service Control Points (SCPs). Collectively, these devices are referred to as signaling points, or SS7 nodes.

SS7 messages originate at an SSP, which is a telephone switch that places or receives a call. The SSP is usually found at a telco's central office, but SS7 messages may also be used by an enterprise PBX. An ISDN PRI's D channel can also send messages that are compatible with SS7. To set up the call, the SSP sends an SS7 message to a directly connected STP (see Figure 1).

Figure 1: Signaling System 7 (SS7) consists of Service Switching Points (SSPs), which originate or terminate calls; Switching Control Points (SCPs), which offer acess to databases; and Switching Transfer Points (STPs), which route SS7 information between SSPs and SCPs. Note the many redundant links, particularly between mated STPs.

Not all STPs are identical. Local STPs handle only domestic traffic within a Local Access and Transport Area (LATA), while internetwork STPs provide connectivity between LATAs. International STPs provide translation between the slightly different American version of SS7, defined by ANSI (T1.111), and the international version, defined by the ITU-T (Q.700-Q.741). Gateway STPs provide the interface between the PSTN and other services, such as cellular telephone service providers.

If the call-originating SSP knows how to route the call, it will ask the STP to create a connection to the receiving SSP. If the route is unknown, such as with toll-free numbers or local number portability (a proposed way of taking your phone number with you, even if you move across the country), the STP will contact an SCP, which has access to databases with call-routing information.

The STP uses these databases to perform a function called global title translation, which determines a call's destination SSP by translating the global title digits (the number dialed by the caller, including toll-free numbers, 900 numbers, calling-card numbers, or cellular telephone numbers) into a route to the destination SSP. In the case of cellular telephones, the global title translation process also determines the mobile identification number of the receiving phone, as cellular telephones don't really have telephone numbers. SSPs can also send billing information to the SCPs using SS7.

The robustness of the PSTN is partly due to the variety of redundant links between SS7 nodes. Nearly all STPs and SCPs are deployed in pairs, and most SSPs are linked to two or more STPs. In many cases, multiple connections between various STPs run over separate physical media. The SS7 links themselves have special names , as shown in "Linking SS7 Signaling Points (Figure 2)."

Linking SS7 Signaling Points
Link type	Description
Access (A) link	Connects a Signal Transfer Point (STP) to either a Service Switching Point (SSP) or to a Service Control Point (SCP).
Bridge (B) link	Connects one STP to another STP.
Cross (C) link	A pair of redundant bridge links between two STPs; one link is used, the other is for failover. STPs linked in this way are known as mated STPs.
Diagonal (D) link	Similar to a bridge link, but connects STPs within a Local Access and Transport Area (LATA) with an internetwork gateway STP (used to bridge LATAs).
Extended (E) link	Connects an SSP to an STP that's in a different LATA to increase failover redundancy.
Fully associated (F) link	Connects two SSPs directly, bypassing STPs, to accommodate a high volume of traffic.

Figure 2

The SS7 Protocol Stack

The SS7 protocol stack consists of four layers , or levels (see Figure 3). The lowest three levels are combined into one set of protocols, referred to as the Message Transfer Part (MTP). The MTP is split into levels 1 through 3, corresponding to the lowest three levels in the OSI seven-layer protocol stack.

Figure 3: The SS7 protocol stacks consists of four layers, which roughly correspond to the OSI model.

MTP level 1, analogous to the OSI Physical layer, defines various physical interfaces between signaling points. Between STPs and their local SSPs and SCPs, the MTP-level-1 links are primarily DS0A (56Kbit/sec) and V.35 (64Kbit/sec) links; DS1 (1.544Mbit/sec) and faster are often used for links between STPs.

Corresponding to OSI's Data-link layer, the MTP level 2 provides node-to-node message error detection and correction (using a 16-bit Cyclic Redundancy Check, or CRC), as well as message sequencing. If an error is detected , level 2 requests a retransmission.

MTP level 3 performs similar functions to the OSI Network layer. This protocol level handles message discrimination to determine who the SS7 message is for. If the message's destination is within the local signaling point, level 3 performs message delivery; if not, it performs message routing to determine the next step toward the destination.

Level 3 is responsible for detecting whether SS7 nodes or links have failed, or whether they are unreliable, congested , or have been shut down or restarted. It determines alternate routes for messages, and sends management messages to adjacent signaling points about changes in link status.

At level 4 in the SS7 stack, two sets of protocols span OSI layers 4 to 7: ISDN User Part (ISUP), for managing phone connection-oriented communications; and Signaling Connection Control Part/Transaction Capabilities Application Part (SCCP/TCAP), to handle all other messaging.

ISUP originates, manages , and terminates ISDN and non-ISDN (also known as POTS, or Plain Old Telephone Service) connections between telephone devices. Thanks in part to ISUP, the days when a caller forgot to hang up the phone-and thus tied up the recipient's phone until the problem was solved -are over. That's because either the originating or destination SSPs can send ISUP messages to terminate the connection. ISUP also transmits caller ID information.

SCCP is used for services between STPs and databases. Corresponding to the OSI Transport layer, SCCP provides more detailed addressing information than the MTP, which only identifies a signaling point; SCCP also indicates which database within the SCP is to be queried.

The database query itself, addressed by SCCP, is sent and returned by TCAP. TCAP messages can send and receive database information, such as a credit card validation or routing information, before a call is placed using ISUP. After a call is completed, TCAP can send billing information to the appropriate accounting database. For mobile users, TCAP transmits user-authentication messages and sends messages notifying SCP databases about the location of cellular telephones. With certain new phone features, such as repeat dialing (which redials a call if the line is busy), the caller's SSP can notify the destination's SSP that this feature is requested ; when the line is no longer busy, the destination's SSP notifies the caller's SSP, which redials the call.

Intelligent Network

SS7's advanced features, including TCAP messaging and more sophisticated databases connected to SCPs, are part of the phone system's evolution to a set of functions referred to by the industry as the Intelligent Network . The Intelligent Network goes beyond simple telephony, with new features such as caller ID, selective call blocking, and local number portability (all of which depend on SS7 messages). Another Intelligent Network feature enables subscribers to instantly reconfigure their own services (for example, setting up call forwarding using a Touch-Tone phone). Many more features will be phased in over the next few years , thanks to prodding from the FCC-and SS7.

Resources

A good online introduction to SS7, with details about the format of the MTP, ISUP, SCCP, and TCAP protocols, is at www.microlegend.com/whatss7.htm.

Bell Atlantic offers a short self-study course on SS7, complete with quiz questions, at www.webproforum.com/bell-atlantic2/full.html.

For an overview of the telephone network, including SS7, SONET, and broadband ISDN, check out Telecommunications Protocols , by Travis Russell (McGraw-Hill, 1997).

For more about the specifics of SS7's components , the best in-depth reference is Travis Russell's book Signaling System #7, 2 ^nd Edition , (McGraw-Hill, 1998).

This tutorial, number 125, by Alan Zeichick, was originally published in the December 1998 issue of Network Magazine.