What is SS7 ? Signaling System No. 7	Presented by:	Copyright 2000©

SS7 is an out-of-band signaling system for the exchange of call control information between network switching offices, in support of voice and nonvoice services. SS7 was originally designed to replace previous network signaling systemps and in-band signaling methods and to provide higher utilization of network trunks. By adding various databases to the network, SS7 is also able to provide additional revenue-producing business and residential services, such as 800 numbers, end-to-end user signaling, and caller line identification. Although originally intended for circuit-related users, SS7 can also serve non-circuit-related applications.

SS7 is a signaling system that allows network switches and data-bases to communicate, and its implementation is essential for communication between ISDN LEs.

A telecommunication net-work's signaling system is an essential component that provides a way for switches within the network to exchange routing, link status, and connection control information.

Prior to the 1970s, a telephone call was set up through the network using in-band signaling. Network signals shared the same physical channel as the call that was being established and were carried with in the user's 300 to 3400-Hz voiceband.

Network signals and user data did not interfere with each other since they usually occurred at different times; most network signaling typically occurred prior t users actually talking to each other.

Network signaling performs three major functions:

* Supervisory. Monitors circuit status, such as the on or off hook signal to indicate an idle or in-use local loop, respectively.

* Addressing. Provides routing information, such as the called party's number.

* Call information. Provides call status and progress information, such as ringback, busy, and reorder ("fast busy") signals.

Early common control switches used a variety of tones for these puposes, called single-frequency (SF)/multifrequency (MF) signaling. DTMF tones are used to send information from CPE to the network, while MF tones are used within and between networks.

Calls were typically set up one circuit at a time. Trunks along the physical route were allocated sequentially. Therefore, even if the final trunk had no available capacity for this call, all of the other network resources had to be allocated before the network knew that the call could not be completed.

These in-band schemes had a narrow range of functionality because they were limited to the voiceband and because they resulted in rela-lively long call setup times (10 to 15 s).

As an aside, in-band signaling was not limited only to the analog environment. Early T1 carriers used a signle bit from each of the twenty-four 8-bit time slots in every sixth frame to indicate whether the channel was on-or off-hook.

An out-of-band, or common channel signaling (CCS) network, is designed to exchange signaling information between processor-equipped switching offices, using signaling channels that are separate from the user's voice channel. This allows network facilities to be quickly allocated, tested, and released. The CCS network can examine all parts of the route of a call to determine if facilities are available; if they are, the signaling network can allocate all of the necessary resources, as well. This allows for fast fall setup, minimizes the amount of time wasted on retries for fast call setup, minimizes the amount of time wasted on retries, and allows much more efficient routing. With CCS, the average call setup time for a toll call dropped to approximately 3 s.

CCS networks offer a number of benefits to the user and the network provider. First, long-distance bandwidth is conserved since signaling is out of band and signaling for several trunks can be multi-plexed on a signal signaling channel. Second, costs are kept down since less equipment is needed. In-band signaling requires separate signaling facilities for each user circuit, while a signle out-of-band signaling can support many user circuits. Third, additional user services can be offered with CCS, such as 800 service, closed user groups (CUGs), credit card verification, and calling party identification.

CCS networks have, in general, two signaling modes. The signaling mode refers to the relationship between the actual path of signaling messages and the path of the information flow to which the signals refer.

In associated signaling mode, signaling messages related to a given information flow between two signaling points are carried on a signaling trunk directly interconnecting the two signaling points. In non-associated signaling mode, the signaling path does not necessarily follow the same physical path as the user trunk groups that it support.

A limited case of non-associated signaling is called quasi-associated signaling mode, where all messages relating to a given call will follow the same non-associated path through the signaling network.

CCITT Recommendation Q.700 provides an overview of common channel signaling and SS7; detailed descriptions of SS7 protocols and procedures are contained in the remaining Q.700-series recommendations (and ANSPs T1.110-series standards). The SS7 protocol architecture has the following main components.

* Message Transfer Part (MTP). Comprises protocols roughly corresponding to the lower three OSI layers, providing physical, data link, and connections network functions. The MTP provides reliable transfer and delivery of signaling messages.

* Signaling Connection Control (SCCP). Provides some network layer protocol functions, including full OSI addressing capabilities and connection-oriented information transfer.

* User and application parts. Provides the end-to-end signaling function for switched voice and nonvoice services in the ISDN; several different user and/or application parts can operate in parallel (multiplexed) over a signal MTP and/or SCCP protocol implementation.

The MTP comprises three protocol levels corresponding to the OSI chained layers. It provides a connectionless service to the SCCP and other user and application parts, ensuring reliable transport and delivery of signaling messages. The MTP also has mechanisms to detect and respond to switch and link failures within the network.

The MTP level 1 protocol, the Signaling Data Link, corresponds to the OSI physical layer. The signaling system is designed to be user on full-duplex digital links operating at speeds up to 64 kbps, although rates of 1.544, 2.048, and 8.448 Mbps are also possible.

To ensure the appropriate ones density on 1.544-Mbps links, bits are inverted prior to transmission.

By inverting the bits prior to transmitting them, the networks is assured that no more than six contiguous 0 bits occur on the line, thus providing a sufficient number of 1 bits for timing and synchronization. Bit inversion is not employed on 2.048- and 8.448-Mbps signaling links.

The MTP level 2 protocol, the Signaling Link, corresponds to the OSI data link layer. MTP level 2 uses a bit-oriented protocol.

As in other bit-oriented protocols, MTP level 2 frames, called signaling, begin and end with a flag bit pattern (01111110); zero-bit insertion and removal (bit stuffing) is used for transparency.

The Backward Sequence Number is a 7-bit subfield, it indicates the sequence number of the signal unit being acknowledged. The Forward Sequence Number is a 7-bit subfield contains the sequence number of this signal unit. The Forward and Backward Indicator Bits are part of the basic error-correction method.

The Length Indicator (LI) field is a 6-bit value specifying the number of octets following the LI field and preceding the check bits. The value of this field also indicates the type of signal unit being transmitted.

The Check (CK) bits contain the remainder from the CRC calculation used to detect bit errors.

MTP level 2 defines two types of error-correction procedures. In basic error correction, frames without bit errors are acknowledged, frames with bit errors arignored, and out-of-sequence frames are rejected. Frames with errors are corrected using a "go-back-N" scheme; the bad frame and all subsequently transmitted frames are retransmitted.

Preventive cyclic retransmission users acknowledgments but no negative responses. Instead, whenever the transmitter has no new frames to send, or when as acknowledgment is overdue, it retransmits all unacknowledged frames. While "go-back-N" transmission is commonly employed in most data link protocols, it can have a very high overhead on links with long delays because of the large number of frames that might require retransmission even if there is only a signal bit error.

The MTP level 3 protocol defines the Signaling Network functions and corresponds to the lower half to the OSI network layer. It has the responsibility to transport messages between the signaling point of the network. There are two broad functional categories performed by this layer, message handling and network management.

Signaling message handling refers to those functions that actually move messages through the network. These functions are invoked based upon a part of the message called the routing label, which comprises a destination point code (DPC), origination point code (OPC), and a signaling link selection (SLS) code. The DPC and OPC, which identify the message destination and origination points, respectively, are 14 bits in length; the SLS, which identifies the specific signaling link to the employed, is 4 bits in length. The SLS is used as part of SS7's load-balancing scheme. The route to a particular DPC from this signaling point may use more than one signaling link in an effort to balance the load as evenly as possible across the available network resources.

The signaling message functions are:

* Message discrimination. Accepts a message from MTP level 2 and determines whether that message belongs at this signaling point or another, based upon the DPC. If it belongs here, pass it to the distribution function; if it is to be relayed to another signaling point, pass it to the routing function.

* Message distribution. If a message received from MTP level 2 belongs at this signaling point, pass the message to the appropriate MTP user or MTP level 3 function, based upon the SIO field.

* Message routing. It the message received from MTP level 2 is to be relayed to another signaling point, or if the message originated at this signaling pint, forward the message based upon the DPC and SLS.

The MTP level 3 also handles signaling network management functions, including traffic, route, and link management. The purpose of these functions is to provide reconfiguration of the signaling network in the case of link or signaling point failures and control traffic in case of congestion. Reconfiguration of the network in response to these conditions must be accomplished without causing lost, duplicate, or out-of-sequence messages or excessive delays.

The Signaling network management functions are:

* Signaling traffic management. Redirects signaling traffic to alternative paths in response to link failure or congestion; ensures there is no loss or duplication during this process.

* Signaling route management. Distributes information about the status of the signaling network to other signaling points to block or unblock signaling routes.

* Signaling link management. Reinitializes failed signaling links, activates new links, and removes errored links from operation. 

A large part of the signaling network functional specification is concerned with procedures for overcoming link failures and congestion. Procedures are specified for quickly determining when a link has failed, removing it from service, rerouting traffic, and bringing the link back into service after repair.

The MTP was originally designed to meet the real-time requirements of telephone network signaling and, for that reason, provides a connectionless network network service. Some applications, however, require a connection-oriented transfer capability and a larger, more complete address space than the MTP makes available. The SCCP provides these enhancements to the MTP to make it functionally equivalent to the OSI network layer, and the MTP and SCCP together form SS7's Network Services Part.

One major enhancement provided by the SCCP is in its expanded addressing functionality. MTP message addressing capability provides delivery of a message to a node (using the DPC and OPC) and has a limited distribution capability at the node using a 4-bit indicator in the SIO field of an MSU. As the number of applications grows, the MTP's limited address space will only be capable of delivering a message to a node but will be unable to specify the application associated with that message.

The SCCP supplements MTP addressing by defining an additional field called the Subsystem Number (SSN), which consists of local addressing information used to identify SCCP users at each node. The combination of OPC plus SSN forms the calling party address, and the DPC plus SSN number is the called party adderss.

Another SCCP enhancement is that it provides four classes of network service:

* Class O, basic connectionless class. A pure datagram service, where SCCP messages are transported independently and may arrive out-of-sequence.

* Class 1, sequenced (MTP) connectionless class. Also a datagram service but will an optional feature so that related messages will be assigned the same SLS and, therefore, delivered in sequence.

* Class 2, basic connection-oriented class. A service class where temporary of permanent signaling connections are established. As in class 1, related messages may be assigned the same SLS to ensure sequentially. This service class also provides a segmentation and reassembly capability for messages greater than 255 octets in length.

* Class 3, flow control connection-oriented class. Includes the features of class 2 service plus flow control and expedited data transfer capability. This service class can also detect out-of-sequence or lost messages, in which case the signaling connection is reset and higher levels are notified.

The SS7 user and application parts correspond to the higher, end-to-end layers of the OSI model. User and application part are each complete and independent of each other. Users communicate directly with the MTP or the SCCP, providing an end-to-end signaling service. Several user and/or application parts can oprate in parallel over a signal MTP or SCCP.

The two user parts originally specified for SS7 were the Telephone User part (TUP) and the Data User Part (DUP). The TUP specifies the signaling necessary for the control of ordinary domestic and international telephone communications. The DUP is designed for circuit-mode data networks and is not intended for ISDN. 

The ISDN User Part provides the signaling necessary for basic ISDN circuit-mode bearer services. The ISUP provides the same voice-oriented signaling services as the TUP but also provides additional functions for the support of nonvoice calls and those ISDN supplementary services having end-to-end significance.

The ISUP may use the transport services provided by either the MTP or SCCP. MTP services are used for the transport of signaling messages between exchanges, while the SCCP may be employed for additional connectivity services as well as end-to-end signaling.

SS7 is a network within a network. End-users access ISDN services via their Les. The Les, in turn, establish circuit-switched connections using the ISUP services of the SS7 network. The ISUP is totally transparent, then, to ISDN end users.

The ISUP, using the services of the MTP and SCCP, provides a logical connection between two ISDN switches. This connection is identified by a circuit identification code (CIC), which is analogous to an X.25 logical channel identifier of frame relay DLCI; the dialed number is used only for routing by the SCCP and/or MTP during connection establishment, and the CIC is used by the ISUP to refer to the connection.

ISUP signaling proce may also be employed to provide several of ISDN supplementary services, including:

* Call forwarding

* Calling line identification

* Closed user group

* Direct dialing in

* User-to-user signaling

The Transaction Capabilities Application Part (TCAP) provides a general purpose, remote operation function for SS7. It provides the capability for an application at one node to invoke the execution of an operation at another node and to receive the results from that remote process.

TCAP was originally designed to support queries into databases, such as those supporting telephone calling cards and 800 numbers, although its role can include additional functions. Current uses for the TCAP include carrying special billing instructions and customer network control and management information.

In the SS7 vernacular, the term transaction capabilities (TC) refers to an application layer protocol (such as TCAP) plus any underlying protocols. TCAP communicates directly with SCCP services and the OSI layers 4 through 6 are essentially null. A TC-user is the application that uses TCAP services.

The TCAP itself comprises two protocol sublayers called the transaction sublayer (TSL) and the component sublayer (CSL).

The transaction sublayer is the lower TCAP sublayer. A transaction, or dialogue, defines the context in which the remote operation will occur, such as the exchange of queries and responses between two TC-users. The TSL defines two types of dialogues, namely, unstructured and structured. 

In an unstructured dialogue, the TCAP provides a method for a TC-user to send one or more messages to its remote peer process that do not require responses. Since communication is considered to be one-way, there is no explicit association created between the two TC-user processes. TC-user messages are sent in a TSL UNIDIRECTIONAL message.

A structured dialogue is analogous to a virtual connection, where queries and responses exchanged between peer TC-users are associated with each other by assigning a unique transaction identifier (TID) to all messages. The TID is assigned during is terminated. Four types of TSL messages are defined for a structured dialogue:

* BEGIN. Used to initiate a transaction with another peer TC-user and to assign a TID to the dialogue.

* CONTINUE. Used to complete the establishment of a transaction and continue message exchange in an established dialogue.

* END. Used to terminate a dialogue.

* ABORT. Used to terminate a transaction following some sort of abnormal condition detected by the TSL or to abort a transaction by the TC-user.

The component sublayer is the upper TCAP sublayer, defining the actual messages, or components, that are contained in the TSL messages described above. The CSL models the behavior of the user, and its procedures and conventions are nearly identical to the OSI remote operations protocol, called the Remote Operation Service Element (ROSE). 

There are four types of CSL components:

* Invoke. A request to perform a remote operation.

* Return result. A reply to an Invoke component containing the response to the requested operation.

* Return error. A reply to an Invoke component containing an indication of some type of error, such as an invalid for non-accessible number.

* Reject. A reply to an Invoke component indicating that a syntax error occurred, such as an unrecognized message.

The transaction sublayer, then, provides a platform for the exchange of messages which are defined in the component sublayer. These TCAP services are provided to a TC-user application, called an application service element (ASE). An ASE is responsible for providing the information that a specific application needs, such as translating an 800 number into a routable telephone number or obtaining a billing number from a telephone calling card. The TCAP and ASE, taken together, correspond to the OSI application layer.

The operations, Maintenance and Administration Part (OMAP) provides the procedures for network management and supervision from central control points in the SS7 network. It defines application protocols and procedures to monitor, test, coordinate, and control SS7 network resources. OMAP, as a network management tool, is related network management protocol.

OMAP uses the connectionless services (unstructured dialogue) of TCAP. OMAP procedures are used for a number of functions, including:

* MTP route verification, to detect route loops, excessive delays, or inaccessibility of signaling points.

Much of the current SS7 work today is related to signaling for B-ISDN services and ATM networks. The current ISDN signaling systems, DSS 1 and the ISUP, logically associate call control with the physical path of the bearer service and assume that there is only a single, bi-directional bearer channel associated with a given call.

This is not necessarily the case with B-ISDN applications, however. Multimedia services may require multiple channels and services for a single B-ISDN call, such as one circuit-mode connection for voice, another circuit-mode connection for video, and a packet-mode connection for data. The possibility of a single call requiring multiple channels and services, plus the potential of multiparty calls, requires a new flexibility to the signaling network supporting B-ISDN. In addition, some type of service compatibility checking prior to call acceptance is also under study due to the wide variety of B-ISDN TE that may be deployed.

The CCTTTs Study Group X1, responsible for SS7, is examining an ISDN Service Control Part (ISCP), a new SS7 application layer structure that separates the call control and bearer control parts of the protocol. The result is that call control messages can take a different network path than the actual bearer channel. From this concept, a new Broadband Application Part (BAP) is being developed for B-ISDN signaling applications. Pending the initial availability of the BAP in the 1993-1994 time frame, SG XI has adopted a three-phase approach to B-ISDN signaling.

The initial plans, or Release 1, will use an extension to DSS 1 procedures for the user-network interface and extensions to the ISUP to handle ATM and AAL network connections. Only point-to-point connections will be supported and bandwidth will be allocated on a peakrate basis.

Release 2 will use the BAP, which will allow a wider variety of call topologies and more flexible bandwidth allocation mechanisms. By the end of 1992, six draft recommendations for the BAP had been proposed, describing such topics as general procedures, call control, bearer service control, and application context. Internet working between Release 1 and Release 2 signaling mechanisms will also be defined.

Release 3 protocols will add additional functions to support full multimedia communications and management.

The implementation of SS7 will involve a whole new set of protocols. The result for the end user, however, will be a large set of new potential services and network capabilities.

Out-patient monitoring, opinion pools, electronic mail, voice mail, catalog shopping, video services, telecommuting (commuting to the office via a communications link), inter reading, and information and database services are among the wide range of user services that can be made available from an ISDN C.O. using SS7.

800 service provides a mechanism whereby a long distance call is automatically charged to the 800 subscriber rather than the calling party. Since the 800 subscribers pay for the call, they, rather than the calling party, choose the long distance provider.

To allow the Ics to compete in the 800 service market, and to facilitate access to the appropriate IC by the local carrier, postdivestiture regulations in the United States mandated that the first three digits after the "800" identify the IC. Local switches, then, routed an 800 call based uponthe NXX portion of the 800 number and the IC completed thcall to the appropriate destination.

Many large service-oriented organizations, such as airline and hotel reservation and information systems, combine 800 service with automatic call distribution (ACD) equipment on their site to route calls to available customer service agents. Network based ACD service allows dynamic reconfiguration of the call distribution function as the number of lines expands and contracts. In addition, the people answering the 800 number telephones do not have to be located at the same site; network-based ACD allows the incoming calls to be distributed to many location in a given area.

Most people in the United State are familiar with 911, the universal telephone number for police, fire, and emergency medical services. Although mandated in the late 1600s, the implementation of 911 service is far from universal in this country. Emergency services are further complicated by the mismatch between emergency services jurisdictions and telephone switch boundaries, varying emergency services availability during a given time of day or day of week, lack of familiarity of a local emergency unit at the time of a call. Enhanced 911 (E911) allows Public Safety Answering Points (PSAPs) that usually operate autonomously to consolidate their traffic into one location. It can also remove the limitations of interoperability between different local C.O. equipment. In addition, new features are available to the 911 dispatchers. Databases can be created that will automatically switch calls to the correct PSAP based upon some set of parameters (e.g., time of day), aid in the dispatch of emergency personnel, and provide the dispatcher with the calling party's telephone number, address, and other pertinent location information (such as the nearest fire hydrant and any pertinent medical history of the residents).

A Line Information Data Base (LIDB) is a multipurpose database with information about individual customer lines. Initially, it can provide capabilities such as an alternate billing service to validate telephone companies. For example, another database can be used to validate credit and charge cards, such as VISA, MasterCard, or American Express, or to provide authorization for check-writing purposes. T & T has tested allowing some customers to use their long distance calling cards as a general-purpose credit card, a potentially lucrative business since they have issued over 40 million calling cards. MCI and Sprint are working with American Express to allow long distance calls to be charged to the card; in addition, American Express is responsible for billing their customers, collecting the fees, and turning the revenues over to the appropriate IC.

Another potential database service is an on-lien, local telephone book, possibly with automatic dialing. Suppose, for example, a customer needs emergency plumbing service on a Sunday morning. A call to the operator, or directly to the database service, can provide the customer with a list of the plumbers who are open, what forms of payment they take, and the numbers where they can be reached. Customers could also access up-to-date directory listings without an operator's intervention. With ISDN, these services could be accessed via the ISDN PC or telephone, merely pressing a button on the telephone could then dial the selected number automatically.

Citywide Centrex is an SS7 service that providers an alternative to private PBX networks. Many companies have employees scattered over several sites within a community. Real estate firms, for example, may have several small field offices throughout an area, each with just a few telephone lines. While the individual offices might not make good candidates for Centrex service, all of the offices taken together might be. Citywide Centrex could allow all of the offices to be tied together and listed under a single number in the telephone directional features include office-to-office calling and programmable call forwarding (e.g. at night or in case one office is closed). This also opens up the opportunity for telecommuting, where several members of a business actually work at home.

Another private network alternative is that of a private virtual network (PVN). The PVN concept is not new, but SS7 makes it feasible for the LEC to offer the service.

From the customer's point of view, PVN circuits are accessed exactly like regular private (leased) lines. The network, however, does not allocate any dedicated physical resource. Instead, ordinary network trunks are used, with the SS7 signaling and database capabilities monitoring which lines are being used by this "private line" customer. Users, then, have all of the features that they are used to with leased line service. In addition, they can request special voice and data services on an ad hoc basis to create custom-designed network services. Furthermore, customers can, theoretically, tailor the actual network that will carry their voice and data traffic. For example, the PVN customer can specify the IC to carry its traffic based upon characteristics such as voice versus data, time of day, day of week, and distance between the two parties, to obtain the best possible rates.

One of the most interesting aspects of SS7 is the set of customized services that can be brought to the business and residential customer, or what Bellcore calls Custom Local Area Signaling Services (CLASS). CLASS services differ somewhat from other SS7 services in that the service provision is handled on a call-by-call basis and is based upon data known at the C.O., such as the calling party's telephone number and the status of the called line. Depending upon the vendor and the network provider, there are a myriad of CLASS services that can be made available.

Automatic number identification (ANI), also known as calling party identification presentation (CLIP) or caller ID, displays the calling party's telephone number at the called party's telephone set during the ring cycle. ANI is the basis of all CLASS services.

ANI is subject of tremendous controversy in the United State. One viewpoint is that ANI helps protect the privacy of the called party; users may be able to identify unwanted calls during mealtime, for example, or be able to report the source of crank or obscene calls. Businesses may be able to offer faster, more efficient service by automatically looking up customer records during the incoming ring cycle based upon ANI.

The other viewpoint is that ANI invades the privacy of the calling party. For example, a vendor could collect telephone numbers and develop a telemarketing calling list from them, unlisted telephone numbers could become a waste of money, and callers might be deterred from calling certain hot lines (e.g., AIDS information or drug abuse assistance) for fear of being identified.

There are a number of possible solutions to handling the legitimate concerns on both sides of the ANI controversy. Some state have banned ANI outright. Significantly more states allow ANI, but customers can block their number identification from being forwarded with a service called caller identification blocking: this may be available on a per-call basis or, by subscription, for all calls. It might also be possible for customers to subscribe to a blocked caller identification rejection service which would automatically reject any incoming call where ANI has been blocked.

 Another solution would be to assign customers two numbers: one would be their real telephone number and the second would be their caller identification number. This would identify the calling party but would not supply a valid telephone number for a return call. While this protects those callers with unlisted numbers, it introduces the new problem of assigning the numbers in the first place. Further more, the NANP is running out of available address space. Forwarding the directory listing of the calling party rather than the telephone number might also be a solution.

Other CLASS services are not as controversial as ANI bethey do not involve the display of the calling party's number. Some otheCLASS services include:

* Automatic callback. Places a return call to the last incoming caller's number, whether or not that call was answered; if that line is busy, it can be automatically recalled.

* Automatic recall. Continually monitors a busy line until the call is completed and notifies the caller when the line is ringing.

* Computer access restriction. Provides additional security for computer dial-up systems by only allowing calls from a predefined list to be connected to a remote computer.

* Customer-originated trace. Allows the user to send the caller ID directly to the telephone company, even if ANI is blocked, in cases of crank, harassing, or threatening calls.

* distinctive ring. Provides a special ringing signal when an incoming call originates from a number on a predefined list provided to the telephone company.

* Important call waiting. Sends a special call waiting signal to the customer if an incoming call is received from a list of predefined numbers while the customer's line is busy.

*Selective call acceptance. Permits incoming calls only from those numbers on a predefined list.

*Selective call forwarding. Automatically forwards calls from a preselected list; the user is not notified of the incoming call and the telephone will not ring. 

The CLASS services described here require the availability of the SS7 services to the customer's LE. CLASS services, like the ISDN supplementary services described in Sec. 3.3.3, will be controlled and invoke by the end user using buttons on the ISDN TE.

Home | Search | About | Offices | Manufacturers | Products | Services | Ordering
 Data101 | Training | Mailing | Hot Products |  Digital HQ | Employment | Email