Sccp Protocol Descriptive Essay

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Tik-109/110.300 Telecommunications architectures (Assignment 10)

SS7 - Signaling System No. 7

Nov. 20, 1998

He Guoyou

Department of Computer Science and Engineering

Helsinki University of Technology


The Signaling systems No. 7 network refers to the interoffice signaling network in place on the public network. SS7 is an internationally recognized standard for interoffice signaling. This standard is a protocol procedure used to establish connections within and between networks. This essay presents an overview of the SS7's architecture, which includes the different components and protocols of SS7. The main components, SSP, STP, and SCP, of SS7 are described in a general level. The functionality and mapping onto OSI model of the SS7 protocol stack, MTP1, MTP2, MTP3, SCCP, TCAP, ISDN-UP, ASE, and OMAP,  are presented. The structures of the message unit for different layers are discribed individually. Finally some featrues and enhanced services made possible with SS7 signaling are discussed.

1 Introduction

SS7 is a signaling method in which a signaling channel conveys, by means of labeled messgaes, signaling information relating to call setup, control, network management, and network maintaince. Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by ITU-T. The standard defines the procedures and protocol by which network elements in PSTN exchange information over a digital signaling network to cellular and wireline call setup, routing and control. The ITU definition of SS7 allows for national variants such as ANSI and Bellcore standards used in North America and ETSI standard used in Europe.
The SS7 network and protocol are used for: a)basic call setup, management, and tear down, b) wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication, 3) local number portability (LNP), 4) toll-free (800/888) and toll (900) wireline services, e) enhanced call features such as call forwarding, calling party name/number display, and three-way calling, f)efficient and secure worldwide telecommunications[2][4].

2SS7 architecture

2.1 The components of SS7 [1][2][4]

A telecommunication network consists of a number of switches and application processors intrconnected by transmission circuits. The SS7 network exists within the telecommunication network and control it. SS7 schieves this control by creating and transfering call processing, network management, and maintenance message to the network's various components.
An SS7 network has three distinct components:
        1) SSP - Service Switching Point,
        2) STP - Signal Transfer Point;
        3) SCP - Service Control Point.

Figure 2. The components of an SS7 signalling network [4]

Service Switching Points

SSPs are telephone switches interconnected by SS7 links. The SSPs perform call processing on calls that originate, tandem, or terminate at that site. As part of this call processing, the SSP may generate SS7 messages to transfer call-related information to other SSPs, or to send a query to a SCP for instructions on how to route a call. An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number. An alternate routing number may be used by the SSP if the primary number is busy or the call is unanswered within a specified time. Actual call features vary from network to network and from service to service[3][4].

Signaling Transfer Points

STP are switches that relay messages between network switches and databases. Their main function is to route SS7 messages to the correct outgoing signaling link, based on information contained in the SS7 messages address fields. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signaling points. An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message. An STP can also act as a "firewall" to screen SS7 messages exchanged with other networks[3][4].

Service Control Points

SCPs contains centralized network databases for providing enhanced services. The SCP accepts queries from an SSP and returns the requested information to the originator of the query.

2.2 SS7 Reliability[4]

To meet the stringent reliability requirements of public telecommunications networks, a number of safeguards are built into the SS7 protocol:

  • SCPs and STPs are usually deployed in mated pair configurations in separate physical locations. On the failure of individual components, this duplication allows signaling traffic to be automically diverted to an alternate resource, minimizing the impact on service.
  • Links between signaling points are also provisioned in pairs. Signaling traffic is automatically diverted to alternate links in the case of link failures.
  • The SS7 protocol has built-in error recovery mechanisms to ensure reliable transfer of signaling messages in the event of a network failure.
2.3 ISDN Access Protocol[4]

SS7 is designed to provide an internationally standardized, general-purpose signaling system; however, SS7 was not intended to be used as the signaling standard for access to the telephone network from PBXs or from telephone sets. To satisfy this latter need, the  Integrated Services Digital Network - Access Protocol (ISDN-AP) has been developed. Together, SS7 and the ISDN-AP provide the end-to-end signaling required to deliver enchanced features to users. As an interim step, many telephone exchange carriers use proprietary access signaling to provide enchanced services.

2.4 SS7  Protocol Stack mapped onto the OSI model

The SS7 standard was designed to map onto the OSI 7-Layer Reference Model. The comparison of the OSI Reference Model and the SS7 protocol stack is illustrated in figure 2.4.
The bottom half of the SS7 protocol consists of the Massege Transfer Part (MTP). three are three levels to the MTP: Level 1 corresponds to the OSI Layer 1 (Physical Layer); Level 2 corresponds to OSI Layer 2 (Data Link Layer); and, Level 3 corresponds to the bottom of OSI Layer 3 (Network Layer).
The upper half of the SS7 protocol consists of several parts.The SS7 Signaling Connection Control Part (SCCP) corresponds to the top of OSI Layer 3. The ISDN-User Part (ISDN-UP) maps onto OSI Layer 3 as well, and, in addition it maps onto Layer 4 (Transport Layer), Layer 5 (Session Layer), Layer 6 (Presentation Layer), and Layer 7 (Application Layer). The transaction Capabilities Application Part (TCAP), the Application Service Elements (ASE), and the  Operations, Maintenance and Administration Part (OMAP) of SS7 protocol all map onto OSI Layer 7 as well[3][4].

 Figure 2.4 Similarities of the SS7 Protocol and the OSI model [4]

3 Message Transfer Part[2][3][4]

3.1 MTP Level 1

The MTP Level is known as the signaling data link. MTP Level 1 is equivalent to the OSI model Physical Layer (Layer 1). This layer defines the physical, electrical, and functional characteristics of the signaling inks connecting SS7 components.
A signaling link is a bidirectional transmission path for signaling, comprising two data channels operating together in opposite directions at the same data rate.

3.2 MTP Level 2

The MTP Level 2 together with MTP Level 1, provides signaling link for reliable transfer of signaling messgaes between two directly connected signaling points. MTP Level 2 maps onto Layer 2 of the OSI 7-Layer Model.
An SS7 message is called a signal unit (SU). There are three kinds of signal units: Fill-In Signal Units (FISUs), Link Status Signal Units (LSSUs), and Message Signal Units (MSUs). They are differentiated by the value contained in the length indicator (LI) field:

  • LI=0            Fill-in singal unit
  • LI=1 or 2    Link status signal unit
  • 2<LI<63    Message signal unit
The signal units are made up of several fields: F (flag), CK (cyclic redundancy check), SIF (signaling information field), SIO ( service information octet), BSN (backward sequence number), BIB (backward indicator bit), FSN (forward sequence number), FIB (forward indicator bit), and LI (length indicator).

3.3 MTP Level 3

The MTP Level 3 provides the functions and procedures related to message routing  and network management. MTP  Level 3 handles these functions, assuming that signaling points are connected with signaling links as described in MTP Level 1 and Level 2. The MTP Level 3 maps onto Layer 3 of OSI 7-Layer Model.
MTP Level 3 has signaling message handling functions, and signaling network management functions. The signaling message handling functions are made up of message routing, discrimination and distribution; these functions are performed at each signaling point in signaling network. The signaling network management functions provide the actions and procedures required to active and maintain signaling service, and to restore normal signaling conditions in the event disruption in the signaling network, either in signaling links or at signaling points..

4 SCCP - Signaling Connection Control Part [1][2][3]

SCCP provides enhancements to the MTP Level 3 to provide connectionless and connection-oriented network services, as well as to address translation capabilities. The SCCP enhancements to the MTP services provide a network service which is equivalent to the OSI Network Layer 3.
SCCP is used as the transport layer for TCAP-based services such as freephone (800/888), calling card, wireless roaming, and personal communications services (PCS).

4. 1 GTT - Global Title Translation

SCCP also provides the means by which an STP can perform global title translation, a procedure by which the destination signaling point and subsystem number (SSN) is determined from digits (i.e., the global title) present in the signaling message. The global title digits may be the dialed 800/888 number, calling card number, or mobile subscriber identification number depending on the service requested. Because an STP provides global title translation, originating signaling points do not need to know the destination point code or subsystem number of the associated service. Only the STPs need to maintain a database of destination point codes and subsystem numbers associated with specific services and possible destinations.

4.2 SCCP Message Format

SCCP messages are contained within the signaling information field (SIF) of an MSU. The SIF contains the routing label followed by the SCCP message contents. The SCCP message is comprised of a one-octet message type field followed by the mandatory fixed part (mandatory  fixed-length parameters), mandatory variable part (mandatory variable-length parameters), and  the optional part. Each optional part parameter is identified by a one-octet parameter code followed by a length indicator field.

5 Integrated Services Digital Network User Part (ISDN-UP)[2][4]

The basic functionof the ISDN-UP is the control of circuit switched network connections between subscriber line exchange terminations. This includes basic voice and data services, and supplementary services. The ISDN-UP provides some of the functionality of OSI Layers 3 through 7.
.The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls. Calls that originate and terminate at the same switch do not use ISUP signaling.

5.1 Basic ISUP Call Control

1.When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch. The IAM includes the originating point code, destination point code, circuit identification code, dialed digits and, optionally, the calling party number and name.

2.The destination switch examines the dialed number, determines that it serves the called party, and that the line is available for ringing. The destination switch transmits an ISUP address complete message (ACM) to the originating switch (via its home STP) to indicate that the remote end of the trunk circuit has been reserved. The destination switch rings the called party line and sends a ringing tone over the trunk to the originating switch. The STP routes the ACM to the originating switch  which connects the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The calling party hears the ringing tone on the voice trunk.

3.When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP. The STP routes the ANM to the originating switch which verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.

4.If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches. The STP routes the REL to the destination switch. If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).

5.Upon receiving the REL, the destination switch disconnects the trunk from the called party's line, sets the trunk state to idle, and transmits an ISUP release complete message (RLC) to the originating switch to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC, it terminates the billing cycle and sets the trunk state to idle in preparation for the next call.
ISUP messages may also be transmitted during the connection phase of the call (i.e., between the ISUP Answer (ANM) and Release (REL) messages.

5.2 ISUP Message Format

ISUP information is carried in the Signaling Information Field (SIF) of an MSU. The SIF contains the routing label followed by a 14-bit (ANSI) or 12-bit (ITU) circuit identification code (CIC). The CIC indicates the trunk circuit reserved by the originating switch to carry the call. The CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC) which defines the contents of the remainder of the message.
Each ISUP message contains a mandatory fixed part containing mandatory fixed-length parameters. Sometimes the mandatory fixed part is comprised only of the message type field. The mandatory fixed part may be followed by the mandatory variable part and/or the optional part.The mandatory variable part contains mandatory variable-length parameters. The optional part contains optional parameters which are identified by a one-octet parameter code followed by length indicator field.

5.2.1 Initial Address Message

An Initial Address Message (IAM) is sent in the "forward" direction by each switch needed to complete the circuit between the calling party and called party until the circuit connects to the destination switch. An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

5.2.2 Address Complete Message

An Address Complete Message (ACM) is sent in the "backward" direction to indicate that the remote end of a trunk circuit has been reserved. The originating switch responds to an ACM message by connecting the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The calling party hears the ringing tone on the voice trunk generated by the destination switch.

5.2.3 Answer Message

When the called party answers, the destination switch terminates the ringing tone and sends an Answer Message (ANM) to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk.

5.2.4 Release Message

A Release Message (REL) is sent in either direction indicating that the circuit is being released due to the cause indicator specified. An REL is sent when either the calling or called party "hangs up" the call. An REL is also sent in the backward direction if the called party line is busy.

5.2.5 Release Complete Message

A Release Complete Message (RLC) is sent in the opposite direction of the REL to acknowledge the release of the remote end of a trunk circuit and end the billing cycle as appropriate.

5.2.6 Telephone User Part

In some parts of the world (e.g., China), the Telephone User Part (TUP) supports basic call processing. TUP handles analog circuits only; digital circuits and data transmission capabilities are provided by the Data User Part.

6 Transaction Capabilities Application Part (TCAP)[1][2][3]

The TCAP is an SS7 application protocol which can be used by various applications. TCAP provides non-circuit related information transfer capabilities and generic services to applications, yet remains independent of application. TCAP maps into the OSI Layer 7.
TCAP enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signaling points using the SCCP connectionless service. An SSP uses TCAP to query an SCP to determine the routing number(s) associated with a dialed 800, 888, or 900 number. The SCP uses TCAP to return a response containing the routing number(s) (or an error or reject component) back to the SSP. Calling card calls are also validated using TCAP query and response messages. When a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register requests service profile information from the subscriber's home location register (HLR) using mobile application part (MAP) information carried within TCAP messages.
TCAP messages are contained within the SCCP portion of an MSU. A TCAP message is comprised of a transaction portion and a component portion.

6.1 Transaction Portion

The transaction portion contains the package type identifier. There are seven package types:

  • Unidirectional: Transfers component(s) in one direction only (no reply expected).
  • Query with Permission: Initiates a TCAP transaction (e.g., a 1-800 query). The destination node may end the transaction.
  • Query without Permission: Initiates a TCAP transaction. The destination node may not end the transaction.
  • Response: Ends the TCAP transaction. A response to an 1-800 query with permission may contain the routing number(s) associated with the 800 number.
  • Conversation with Permission: Continues a TCAP transaction. The destination node may end the transaction.
  • Conversation without Permission: Continues a TCAP transaction. The destination node may not end the transaction.
  • Abort: Terminates a transaction due to an abnormal situation.
The transaction portion also contains the Originating Transaction ID and Responding Transaction ID fields which associate the TCAP transaction with a specific application at the originating and destination signaling points respectively.

6.2 Component Portion

The component portion contains components. There are six kinds of components:

  • Invoke (Last): Invokes an operation. For example, a Query with Permission transaction may include an Invoke (Last) component to request SCP translation of a dialed 800 number. The component is the "last" component in the query.
  • Invoke (Not Last): Similar to the Invoke (Last) component except that the component is followed by one or more components.
  • Return Result (Last): Returns the result of an invoked operation. The component is the "last" component in the response.
  • Return Result (Not Last): Similar to the Return Result (Last) component except that the component is followed by one or more components.
  • Return Error: Reports the unsuccessful completion of an invoked operation.
  • Reject: Indicates that an incorrect package type or component was received.
Components include parameters which contain application-specific data carried unexamined by TCAP.

7. Operations, Maintaince and Administration Part (OMAP)[4]

The OMAP provides procedures related to operations and maintaince functions. OMAP corresponds to the OSI model's Application Layer.

7.1 Management Dodel

  • The SS7 management model represents the relationship of various management components: System Management Application Process (SMAP), Management Information Base (MIB), Layer Management Interface (LMI),Layer Management Entity (LME), and System Management Application Entity (SMAE).
    • System Management Application Process (SMAP): The SMAP monitors, controls and coordinates through the application layer protocols.
    • Management Information Base (MIB): The MIB contains the performance and alarm data collected from the SS7 network by OMAP and the configuration data accessed by OMAP to assist in the management of the SS7 network. The Layer Management Interface (LMI) is implementation-dependent and not subject to standardization. data collected from  the protocol layers is stored in MIB using the LMI. The Layer Management Entity (LME) deals with management functions of the corresponding SS7 layer.
    • System Management Application Entity (SMAE): The SMAE is the aspect of SMAP involved with communications. SMAE contains one or more communications functions for an application. Each communications function is called an Application Service Element (ASE)
    7.2 OMAP Application Service Elements (ASE)

    Two OMAP ASEs are defined: the Message Routing Verification Test, and the Circuit Validation Test. The OMAP ASEs use the services of TCAP to perform their functions.

    • Message Routing Verification Test (MRVT) is designed to verify that the routing tables in SS7 network nodes are consistent and that no routing loops or other anomalies are present.
    • Circuit Validation Test (CVT) is designed to ensure that two exchanges have consistent translation data for the circuits connecting the two exchanges.
    8 Applications[4][5]

    The ISDN-UP and TCAP have already covered some features and enhanced services made possible with SS7 signaling. From a broader, end-user perspective, SS7 and adunct services will enhance telecommunications capabilities by making features available on a network-wide basis.

    8.1 Voice Message Integration

    Calling an associate in another city may result in the call being answered by a voice messaging system. You may be prompted by one of the voice recordings:

            "Press 1 to leave a message."
            "Press 2 to activate ring again."
            "Press 3 to speak to an operator."

    Press "1" might store the calling line ID in the voice messging system. When your busy associate  checks her voice messaging system and hears your message, she may enter a one-digit code to transfer her call back to your number, simplying the return call process. Precess "2" would be a convenient way to activate Network Ring again, while pressing "3" would provide access to an operator.

    8.2 Network Automatic Call Distribution (NACD)

    NACD is a natural fit for organizations such as airlines that have customer service centers located across the country over different time zones. NACD  can improve customer service by pooling NACD agent at different locations into one large group. If one NACD location is very busy or out of service, incoming calls would be automatically routed to alternate sites. The same number of agents could be now handle more calls, with a reduced average answering time. The airline gains sales, since more customers get through to agents and fewer customers hang up and a call competitor. The airline would also have more flexibility in selecting customer service locations and setting staff levels at each site.

    8.3 Virtual Private Networking (VPN)

    VPN is a service which provides multi-site organization with the appearance of a private network with dedicated inter-switch circuits and a private dialing plan. However, in reality, the customer is part of a shared network within other VPN customer, or the customer is using the public network. VPNs provide users with a number of advantages of both private and public networks.
    Organizations with multiple sites which are not large enough to support dedicatedconnections can use shared VPN facilities to connect the sites.However, the use of the shared VPN facilities still provides the advantages of a private network, such as access to long-haul facilities and services, private numbering plan, and feature transparency.

    8.4 Customer Area Signaling Services (CLASS)

    CLASS  provides residential subscribers with many of the enhanced features previously available only with PBXs or Centrex service. The flexibility offered by these services can provide substantial benefits to busy users. From a distinctive ring for the in-laws, to the rejection of a particular telemarketing firm, there is a useful service for all.

    8.5 Enhanced 800 Service

    Enhanced 800 service options provide organizations with considerable flexibility in tailoring 800 services to their unique requirements. With the time-of-day and day-of-week routing options, organizations can set up full-time customer service centers backed up by part-time centers which only operate during peak business hours. All calls after hours, or on week-ends, would automatically be routed to the main business center.
    Call routing based on location of caller provides additional flexibility. With this option not only could the call be routed based on the ared code, but also on the NXX (the office code) as well. An organization could select an 800 number to be valid only in certain cities or regions within a state. When combined, 800 service with these enhanced features provide a powerful and flexible way to meet certain telecommunications requirements.

    9 Summary

    This essay has presented an overview of the SS7. ehanced services and telephone network features once viewed as luxuries are quickly besoming necessities in today's fast paced world. Residential telephone service customers as well increasingly are demanding the efficiency and convenience advanced telecommunications technology can provide. Naturally, telephone operating companies want to justify the demands efficiently. Tranditional signaling systems cannot provide the level of sophistication required to deliver much more than basic telephone service. To deliver the enhanced services, telecomminutions carriers need new, intelligent message-based signalling systems. The solution is SS7.
    This essay just presents a brief overview to SS7, and give a general appreciation and understanding of its structure and impact. It covers the necessarity and step by step action procedures that occur in the network when SS7 functionality is being used.


    [1] Denis Pierrot & Jean Pierre Allege, The HP OpenCall SS7 platform allows
          users to build computer-based signaling applications connected to SS7 signaling
          network, August, 1997 [referred 20. 11. 1998]
    [2] MicroLegend, MicroLegend SS7 Tutorial, February 6, 1998 [referred 20.11.1998]
    [3] Sudeep K. Palat, CCITT Signalling System No 7, Apr 6, 1997 [referred 20 .11.1998]
    [4] Toni Beninger, SS7 Basics, Telephony Div., Intertec Publishing Corp., 1991 [refferred 20.11.1998]
    [5] Walt Mansell, Telecommunications Signaling and the SS7 Network, 1998 [referred

    Further Information

    More information related to the topics of SS7 can be found in following link list.

    Feature Story
            An SS7 troubleshooting guide for LNP.
    Integrated SS7
            Description of the funcationality, features, and applications of the integrated SS7.
    Intelligent Network (IN) Tutial
            A tutorial discusses the networks' evolution from switch-based service logic services
            to service-independent Advanced Intelligent Networks (AIN)
            List of IntelliNet Technologies' SS7 API functions.
    Internet Telephony Over Frame Relay
            Information about frame relay technology which enabled network performance to move
            from kilobits per second (Kbps) to megabits per second(Mbps).
    Major Features of the DSC INfusion STP C32
            Describing the major features of DSC COMMUNICATION's INfusion Signal Transfer
            Point C32 (STP C32).
    Major Features Of The MegaHub STP
            Information about the major features of DSC COMMUNICATION's MegaHub Signal
            Transfer Point family.
    MicroLegend SS7/C7 - Protocol Converters
            Information of MicroLegend, MicroLegend's SS7/C7- Protocol Converters
    MicroLegend VSP - Versatile Signaling Point
            Information of MicroLegend, MicroLegend's Versatile Signalling Point
    Signaling protocol ushers in intelligent nets
            Discription of the model of Intelligent networks whch offers the control and
            invocation of services through the network.
    SS7 A-Link Concentration
            Information about DTI’s DXC Tandem Switch with SS7 which can be used to
            concentrate the SS7 A-Links in a network.
    SS7 networks in a PCS  World
            Describing the functionality of SS7 in the field of personal communication services.
    SS7 Overview
            An overview of different SS7 vendors's SS7 hardware and software platforms
    SS7 - Signaling System 7 Controller
            The functionality of Excel's Signaling System 7 controller hardware.
            The table of contents of The World SS7 Market.

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