VocalTec Architecture A Standards-based Platform for IP Telephony ______
VocalTec SS7 Solution White Paper
Release 1.4
January 2001 VocalTec Communication
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© 2001 VocalTec Communications Ltd. All rights reserved.
VocalTec®, VocalTec Gatekeeper, VocalTec Telephony Gateway and VocalTec Network Manager are trademarks or registered trademarks of VocalTec Communications Ltd. All other trademarks are the property of their respective owners.
SS7 White Paper, 3rd edition – January 2001
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Publication number: VTWP- SS7 -01-2001-V3-R1-E
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Contents
Introduction...... 4 Overview ...... 4 SS7 Network ...... 4 SS7 Advantages ...... 5 SS7 Market ...... 5 SS7 Technology/ Network Elements ...... 5 SS7 Signaling Points ...... 6 Service Switching Point (SSP) ...... 6 Signal Transfer Point (STP) ...... 6 Service Control Point (SCP)...... 6 SS7 Network Availability ...... 6 SS7 Signaling Links Types...... 7 SS7 Protocol Stack ...... 8 Message Transfer Part...... 8 ISDN User Part (ISUP)...... 9 Telephone User Part (TUP)...... 9 Signaling Connection Control Part (SCCP)...... 9 Transaction Capabilities Applications Part (TCAP)...... 9 ISUP Call Flow ...... 10
VocalTec’s SS7 Solution ...... 11
VocalTec’s SS7 Solution ...... 11 SS7 Introduction...... 11 Migrating to SS7...... 11 VocalTec SS7 Architecture ...... 11 VocalTec SS7 Signaling Gateway...... 11 SS7 Configurations ...... 12 Non-Associated SS7 Configuration...... 12 Associated SS7 Configuration...... 13 Multiple Media Gateways Configuration...... 14 Redundant Signaling Gateways Configuration ...... 14 SS7 Solution Details ...... 15 Signaling Gateways Features ...... 15 Network Redundancy/Availability...... 15
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Signaling Gateway hook-ups ...... 17 Media Gateway hook-ups...... 17 Support for Multiple Gateways ...... 17 SS7 Management ...... 18 Phone To Phone Call Flow ...... 19 Supported Services under SS7...... 16 Supported SS7 Standards...... 16 Supported SS7 Flavors ...... 16
Glossary...... 20
Figures Figure 1 - SS7 Signaling Points...... 6 Figure 2 - SS7 Network Design for Availability...... 7 Figure 3 - SS7 Signaling Link Types ...... 7 Figure 4 - SS7 Protocol stack...... 8 Figure 5 - SS7 ISUP Call Flow ...... 10 Figure 6 - VocalTec SS7 Architecture ...... 11 Figure 7 - Associated Configurations ...... 12 Figure 8 - Associated Configurations ...... 13 Figure 9 – Multiple Media Gateways Configurations...... 14 Figure 10 – Redundant Signaling Gateways Configurations ...... 14 Figure 12 – Phone-to-Phone Call Flow ...... 19 Tables Table 1 - ISUP Call Setup Messages...... 10 Table 2 - Supported SS7 Standards...... 16 Table 3 - Supported SS7 Flavors ...... 17
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Introduction
Overview SS7, also referred to as out-of-band signaling, is the common channel signaling protocol used for call handling within the Public Switched Telephone Network (PSTN). SS7 is the underlying data communications protocol used by telephone networks to control call set-up and call routing, and to provide services such as caller ID and CLASS features. SS7 offers telephone network management functions that are faster and more reliable, by managing voice circuit functions on a separate, fully redundant data network. SS7 was designed in the late 1970s and early 1980s for exchanging call control information between the various network switches and databases in the PSTN. It replaces an earlier Common Channel Inter-office Signaling system (CCIS, or SS6 in ITU terms) and offers several important advantages, including greater speed and flexibility. SS7 is now widely deployed and is used to support services such as ISDN (Integrated Services Digital Network). The global SS7 standard defined by ITU allows for national variants. Variants are defined by the national Telecommunications Companies (Telcos). There may be more than one national variant. ANSI is the international SS7 standard for T1 lines and ETSI is the standard for E1 lines. The SS7 standard allows network elements in the PSTN to exchange information over a digital signaling network to support wireless (cellular) call setup, routing and control.
SS7 Network An SS7 network is a packet network operating in parallel with the public switched telephone network. SS7 networks are designed for extremely high reliability using traditional telephone company techniques, i.e., providing full redundancy. An IP Network (such as the Internet) achieves high availability with routing protocols that route around failures. An SS7 network achieves high availability by having two or more copies of everything — every router and every connection (link). SS7 networks are typically engineered so that no link is loaded to more than 40 percent of its capacity. In the event of a link failure, the remaining link can carry all the traffic with margin to spare. SS7 network signaling uses relatively small messages that are efficiently packed. SS7 typically runs on 64-Kbps links. Since the messages are extremely compact, a single 64- Kbps-link set (that is, pair of links) is capable of carrying the message traffic for over 50,000 calls per hour. SS7 allows a single flag byte between packets to denote both the end of the preceding packet and the beginning of the next packet. It requires the constant transmission and reception of small “fill-in” signal units (packets), which can bombard an HDLC processor with interrupts. And, with SS7, one must constantly monitor the error rate on each to identify links that may need to be removed from service. In combination, these features generally require special-purpose hardware and/or firmware rather different from that used with Ethernet, X.25, or traditional IP transports. The SS7 network is basically a packet network that controls the operation of the PSTN and provides access for new services in the traditional telephone network.
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SS7 Advantages The advantages of using SS7 (over other PSTN signaling systems) are numerous: · Faster call set-up, answer detection and call tear down · One signaling channel can perform signaling for thousands of voice channels · Increased information associated with the call · Ability to reduce telecommunications costs by purchasing high capacity trunks · Ease of upgrade to new services · Suitable for mobile applications and wireless services · Efficient and secure Telecommunications · A high level of call security is provided through the monitoring of traps and call information
SS7 Market A growing numbers of service providers around the globe, from the major Local Exchange Carriers (LECs) and Inter Exchange Carriers (IXCs), to the wireless operators, mobile operators and smaller carriers, are opening up for those vendors who can connect to the SS7 network. In addition, corporate users are gaining indirect access to SS7 networks via gateway services offered by the major IXCs. As an example, a corporation with multiple, geographically dispersed call centers can improve operating efficiency by using their own computers and an SS7 gateway to specify routing for their inbound traffic on a call-by-call basis. SS7 is the interface to connect this new application logic to the traditional circuit- switched network. The following Telephony service providers commonly use SS7: · International and long distance carriers · Local carriers · Large Internet Telephony Service Providers (ITSPs)
SS7 Technology/ Network Elements SS7 messages are sent at 56-64 Kbps on bi-directional channels called signaling links. Each SS7 signaling link can support up to 4096 calls. Actual numbers depend on the network load and type of equipment. Out-of-band signaling allows for faster call setup and more efficient use of the voice channel. Out-of-band signaling also supports Intelligent Network (IN) services, which require signaling to network elements without voice trunks (e.g., database systems). Each signaling point is uniquely identified by a numeric point-code. These include: · Origination Point-Code (OPC) – address on the network of the originator of the signal · Destination Point-Code (DPC) – used to identify the destination point on the network Each signaling point uses a routing table (e.g., based on prefixes) to select the appropriate signaling path for each message.
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SS7 Signaling Points
Figure 1 - SS7 Signaling Points
A typical SS7 network consists of the following signaling points:
Service Switching Point (SSP) The SSP is a switch that originates, terminates, or tandems (transfers) calls. The SSP is used to set up, manage and release voice channels and to complete calls. The SSP may send query messages to a centralized database (SCP), e.g. to map a toll-free 1-800/888 virtual number to an actual number. The purpose of these queries is to enhance the functionality of the SSP by providing external information (such as routing information for the 800/888 destination mapping). A tandem SSP tunnels the call information to the outgoing trunk, while a terminating SSP (which ends the SS7 signaling section of the call) does not. A terminating SSP is for example, when the call continues over a PRI line or an analog line to a customer. An SSP can be terminating for one call and tandem for another destination (the call goes through to another SSP or CO). An SSP can be tandem for all calls.
Signal Transfer Point (STP) An STP acts as a hub or router for SS7 messages (including call setup, management and teardown messages, call information and all database queries). This improves network utilization and alleviates the need for a meshed network. An STP can also act as a firewall to screen SS7 messages exchanged with other networks. For that purpose it may perform global title translation – translating OPCs and DPCs between SS7 sub-networks.
Service Control Point (SCP) An SCP provides application services (such as 1-800 mapping). An SCP usually drives a database to hold the permanent data. The SCP services, known as Service Control Functions (SCF), provide data in response to queries.
SS7 Network Availability SS7 network elements (such as SCPs, STPs and links) commonly appear in mated pair configurations (e.g., a pair of STPs serve a PBX), so that there is always an alternative route for traffic in the event that one element fails. Elements are often placed at separate geographical locations. Each site is physically connected to at least two other sites. A link-set
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is the group of links that service the same connection between points A and B. If one of the links (in a link-set) fails, the signaling traffic is rerouted over another link in the set. Traffic is also load-balanced among all links in the link-set. Error correction and retransmission allow for continued service in the event of any signaling point or link failure.
Network 1 Network 2
Telephone
Telephone Telephone
Telephone SSP Voice Trunk Telephone
STP Signaling Link
SCP Subscriber Line
Figure 2 - SS7 Network Design for Availability
SS7 Signaling Links Types
Figure 3 - SS7 Signaling Link Types
The following signaling links are based on the SS7 signaling network: · A Link – access link; between the SCP/SSP and STP · B Link - bridge link, between STP and STP (quad B links between networks)
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· C Link – cross link; between STP and STP (mated pair). This is used only when an STP has no other route available to a destination signaling point due to link failure(s) · D Link - diagonal link; between secondary STP pair (e.g., local or regional) and primary STP pair (e.g., inter-network gateway) · E Link - extended link; between the SCP/SSP and STP · F Link - fully associated link; connects two signaling endpoints in association with the voice trunks. F links are commonly used in networks without STPs The VocalTec SS7 Architecture supports both A-links and F-Links
SS7 Protocol Stack Hardware and software functions of the SS7 protocol are divided into functional abstractions called “levels”. These levels map loosely to the Open Systems Interconnect (OSI) 7 layer model.
Figure 4 - SS7 Protocol stack
Message Transfer Part The message transfer part (MTP) is divided into three levels: · MTP Level 1 - defines the physical, electrical, and functional characteristics of the digital signaling link. Examples are: E-1 (2048 kb/s; 32 * 64 kb/s channels), T-1 (1536 kbps; 24 * 64 kb/s channels) and V.35 (64 kb/s). · MTP Level 2 - Ensures accurate end-to-end transmission of a message across a signaling link. Flow control, message sequence validation, and error checking. If there is an error on a signaling link this layer ensures that the message (or set of messages) is retransmitted. · MTP Level 3 - provides message routing between signaling points in the SS7 network. MTP Level 3 re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs.
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ISDN User Part (ISUP) Protocol used to set-up, manage, and release trunk circuits that carry voice and data between terminating line exchanges. Used for both ISDN and non-ISDN calls.
Telephone User Part (TUP) TUP is an older variant of ISUP. In some parts of the world (e.g., China, Brazil), TUP is used to support basic call setup and tear-down. TUP handles analog circuits only. In most countries, ISUP has replaced TUP for call management.
Signaling Connection Control Part (SCCP) Provides connectionless and connection-oriented network services and Global Title Translation (GTT) capabilities above MTP Level 3. SCCP is used as the transport layer for TCAP-based services.
Transaction Capabilities Applications Part (TCAP) Supports the exchange of non-circuit related data between applications across the SS7 network. TCAP is provided over SCCP. TCAP Example – Service Queries and responses sent between SSPs and SCPs are carried in TCAP messages, e.g., to determine the routing number associated with a dialed 800 number and to check the personal identification number (PIN) of a calling card user. TCAP Example – Roaming In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming. When a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register (VLR) requests service profile information from the subscriber's home location register (HLR) using mobile application part (MAP) information carried within TCAP messages.
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ISUP Call Flow
Figure 5 - SS7 ISUP Call Flow
The following table describes the ISUP Call Setup messages illustrated above. Message Description IAM Initial address Message – contains the dialed E164 number address. ACM Address Complete Message – voice channel open and setup is completed. ANM Answer Supervision Number Message – the destination user answered. REL Release Message - release the call. RLC Release Complete Message – call release complete.
Table 1 - ISUP Call Setup Messages
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VocalTec’s SS7 Solution
SS7 Introduction
Migrating to SS7 VocalTec’s SS7 solution is based on the VocalTec architecture, a standards-based multi- service IP Telephony platform in which VocalTec Telephony Gateways provide the bridge between the IP domain and the PSTN realm. The VocalTec Telephony Gateways interface to the regular (non-SS7) PSTN world for both voice and signaling. When connecting to an SS7 PSTN network, the signaling trunks carry SS7 control messages. Supporting the SS7 interface is accomplished by adding the VocalTec SS7 Signaling Gateways, which acts as the signaling front-end.
VocalTec SS7 Architecture The VocalTec Telephony Gateways (VGW 120 or VGW 2000) act as media gateways handling the voice trunks. The VocalTec SS7 Signaling Gateways acts as signaling gateways providing the signaling translation from SS7 to Media Gateway call control. SS7 support is provided by VocalTec SS7 Signaling gateways, which function as SS7 transit exchange and/or outgoing international gateway exchange.
Service Provider Network Operating Center
Figure 6 - VocalTec SS7 Architecture
VocalTec SS7 Signaling Gateway VocalTec SS7 Signaling Gateway is an essential component of the VocalTec SS7 solution. VocalTec SS7 Signaling Gateways process SS7 stacks - including Message Transfer Part (MTP) level 2/3 and TUP or ISUP messages – and convert them to SS7 over IP messages. The signaling gateway connects to the traditional SS7 PSTN via E1/T1 or V.35. A Signaling gateway can connect to multiple VocalTec Telephony (media) Gateways over the IP network.
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SS7 Configurations
Non-Associated SS7 Configuration
Single Span Signaling (ISUP over IP)
SS7 ISUP VoIP Signaling GW PCM (E1/T1)
VGW VGW w/ SS7 SSP
Quad Span Signaling (ISUP over IP)
SS7 ISUP VoIP Signaling GW PCM (E1/T1)
VGW VGW w/ SS7 SSP
Figure 7 – Non-Associated Configurations
In this configuration the signaling and PCM packets (audio) are carried via two separate trunks. Signaling passes directly to the signaling gateway, which is connected to the media gateway over IP. PCM packets pass directly to the media gateway. In the trunk used for signaling, one or more channels carry the signaling data (depending on configuration). The other channels are unused. In SS7 terms, the signaling trunk is known as an "A-Link" (See SS7 Signaling Links Types on page 7 in this document). It is possible to transmit more than one signaling channel over a trunk or multiple trunks for creating link-sets. Having the signaling go through a separate media, allows supporting 31/24 voice circuits in each E1/T1.
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Associated SS7 Configuration
Single Span Signaling (ISUP over IP)
PCM (E1/T1) PCM + VoIP SS7 ISUP Signaling GW
VGW VGW w/ SS7 SSP
Quad Span Signaling (ISUP over IP)
PCM (E1/T1) PCM + VoIP SS7 ISUP PCM (E1/T1) Signaling GW
VGW VGW w/ SS7 SSP
Figure 8 - Associated Configurations
This configuration shows the signaling trunk used to transfer both PCM audio and SS7 signaling. In SS7 terms, this trunk is known as an "F-Link" (See SS7 Signaling Links Types on page 7 in this document ). On the mixed signaling/PCM trunk, PCM and signaling pass through the signaling gateway. The signaling gateway separates the signaling channel from the PCM channels. The PCM is then sent over a PCM trunk to the media gateway, while the signaling travels over IP to the media gateway for call control. In configurations with more than one voice trunk, only one trunk carries mixed PCM + Signaling packets. The other (PCM only) trunks go directly from the SSP to the media gateway. The mixed trunk supports 30/23 voice circuits (may be less if multiple circuits are used for SS7 signaling). The PCM only trunks support 31/24 voice circuits in each E1/T1.
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Multiple Media Gateways Configuration
VoIP PCM (E1/T1)
VoIP PCM (E1/T1)
VoIP PCM (E1/T1) PCM + SS7 ISUP VGW(s) Signaling SSP
Signaling GW VGW w/ SS7
Figure 9 – Multiple Media Gateways Configurations In a configuration with multiple media gateways, the Signaling Gateway routes the signaling information to the proper media gateway according to the trunk configuration.
Redundant Signaling Gateways Configuration
STP A
POP A
STP B
POP B
Figure 10 – Redundant Signaling Gateways Configurations
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SS7 Solution Details
Signaling Gateways Features The Signaling Gateway configuration determines the link-set division, the individual links, user parts (ISUP/TCAP and the variant that is running on each link set), the point-code, and additional configuration parameters. The signaling gateway supports the following features: · Supports E1/T1 interfaces (V.35 line can be supplied on demand). Support up to 24 signalling trunk interfaces (depending on type of SS7 Signaling gateway model) · Supports multiple distributed hosts (media gateways) - up to 128 hosts (connecting more than 32 media gateways requires a proxy) · Supports fault tolerance, high availability and redundancy, through Load sharing signaling gateways. Supports multiple link redundancy, signaling card redundancy and signaling gateway redundancy · Supports ITU-T and ANSI standards as well as over 20 country variants. Other variants can be provided upon request · Support for ISUP, including Basic call control, Circuit Management (circuits un/blocking, circuit groups and resetting circuits) · Supports supplementary services · Functions as SS7 transit exchange and/or outgoing international gateway exchange · Supports multiple SS7 standards (see Table 2 - Supported SS7 Standards Table) · Alarm capabilities on all MTP levels 1-3 and on ISUP
Network Redundancy/Availability Network redundancy is implemented at the following levels: · Link level · Having every two elements connected using two or more links, which together can be defined as a link set – thus providing fail over. · Having each node (such as STP) connect with two other nodes thus providing fail over via an alternate node. If one link or node fails then the other links on the link set can take over the load. · Board level – The signaling gateway can have up to 12 signaling boards. Board failure generates the same effect as link failure – causing other links in the link-set to take over. · Signaling gateway level redundancy – two signaling gateways can act in a load- sharing mode. If one of the signaling gateways fails, then the other can take over. All active calls remain intact.
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Supported Services under SS7 All VocalTec services are all supported within the VocalTec SS7 architecture, including: · Phone-to-Phone, Fax-to-Fax · Post Paid and Pre-paid Calling card · Web-to-Phone, PC-to-Phone · Virtual Private Networks (VPN) · Interdomain (IDM)
Supported SS7 Standards SS7 Standard Description Supported Q.700-Q.709 T1.111.3 - ANSI Messaging Transfer Part (MTP) Yes Q.710 PBX Application -- Q.711-Q.716 T1.112- ANSI Signaling Connection Control Part (SCCP) Yes Q.721-Q.725 Telephone User Part (TUP) Yes Q.730 ISDN Supplementary Services Yes Q.741 Data User Part (DUP) -- Q.761- Q.766 ISDN User Part (ISUP) Yes T1. 113- ANSI 1988/1992 Q.771-Q.775 T1.114- ANSI Transaction Capabilities Application Part -- (TCAP) Q.791- Q.795 Monitoring, Operations, and Maintenance Partial Q.780- Q.785 Test Specifications Yes Table 2 - Supported SS7 Standards
Supported SS7 Flavors VocalTec SS7 architecture supports the following country variants. Other variants can be provided upon request. Country SS7 Flavor Details Argentina ITU ISUP Australia BTNUP + ITU ISUP Belgium ITU ISUP Canada ITU ISUP Chile ITU ISUP China ITU ISUP TUP version France ITU ISUP French TUP Germany ITU ISUP Hong Kong ITU ISUP Hungary ITU ISUP Indonesia ITU ISUP Israel ITU ISUP Italy ITU ISUP Japan MTP (Q.701), Signaling Network (Q.704) Korea New Zealand BTNUP + ITU ISUP Netherlands ITU ISUP Peru ITU ISUP
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Country SS7 Flavor Details Portugal ITU ISUP Singapore ITU ISUP South Africa ITU ISUP Spain ITU ISUP Sweden ITU ISUP Taiwan ITU ISUP UK BTNUP + ITU ISUP United Emirates ITU ISUP USA ANSI ISUP Table 3 - Supported SS7 Flavors
Signaling Gateway hook-ups
Connection with SSP(s) and/or STP(s) The signaling gateway connects to the SS7 network over E1/T1/V.35 lines using the ITU-T ISUP protocol (ANSI-ITU and TUP can be supported upon request). The physical connection is over T1/E1. V.35 can be provided upon request.
Connection with Media Gateway The signaling gateway can be connected to media gateways as follows: · IP connection only – This connection type is used in a non-associated SS7 mode (A- Link). The signaling is transmitted over a dedicated trunk (E1/T1) from the PSTN to the Signaling Gateway. The signaling is then transmitted from the Signaling Gateway to the media gateway over an IP connection. · IP connection + E1/T1 trunk – This connection type is used in a full-associated SS7 mode (F-Link). The trunk used for signaling is also used to pass PCM audio (the timeslots, besides those used for signaling, are used for audio). The Signaling Gateway performs a drop & insert function, extracting/inserting the signaling timeslot. The signaling is transmitted to the media gateway over IP. The audio is passed through (without modification) to an outgoing trunk that carries the PCM to the media gateway.
Media Gateway hook-ups The connection between the switch and the gateway is a regular PCM E1/T1 connection for voice timeslots. The switch can pass voice lines that have no signalling directly to the gateway and visa versa.
Support for Multiple Gateways If connected via the Ethernet, each gateway is considered as a Signaling gateway host. A single Signaling Gateway can support up to 128 media gateway hosts (connecting more than 32 gateways requires a proxy).
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SS7 Management The Signaling gateway needs no direct management since it is part of the Media gateway. All management take place from the VNM as part of the Media Gateway management. SS7 Monitoring All SS7 alarms are displayed on the Media Gateway machine console as well as on the VNM. Two types of alarms are generated: · Internal alarms – these are generated by the internal stacks/engines/application. For example, when the NIC reports a lost connection, the PCM interface reports CRC error on one of the channels. · External alarms – these are received via the interfaces and report problems. For example, VGK reports login denied or the SS7 network reports lost trunk synchronization. The Signaling Gateway is a logical part of the Media Gateway. It relays any problems (such as internal problems) or errors (such as SS7 network errors) to the Media Gateway it is connected to.
SS7 Configuration SS7 configuration is available through the media gateway configuration. When the media gateway uses an SS7 signaling connection to the PSTN, a special SS7 configuration tab provides control over SS7 related aspects. The following SS7 parameters can be configured using VNM for a specific media gateway: · VGW host ID - this identifies the specific media gateway when more than one media gateway is connected to the same signaling gateway. The first media gateway is numbered zero (0), the next one is numbered one (1) and so on. · SIU IP address – Defines the signaling gateway IP address to enable communication between the media gateway and the signaling gateway. · Channel mapping - when the signaling gateway transports voice as well as signaling, sometimes the channels on the trunk from the SS7 network are mapped to another channel number on the trunk between the signaling gateway and the VGW. When this happens, there is a need to inform the VGW of this mapping. This mapping data is used by VGW to match messages received from the Telephony interface in the VGW to messages received from the SS7 network. · Additional SS7 parameters – these are for advanced use only (e.g., gateway timeout timers). The SS7 network configuration does not require daily maintenance unless deployment is changed (e.g., add trunks, change point codes, etc). All other low level SS7 parameters can be configured via TELNET (over IP), FTP (over IP) or a VT-100 Terminal connected directly (RS-232) to the signaling gateway, which uses its own command-line configuration. Low-level parameters should only be modified by field technicians only.
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Phone To Phone Call Flow
Signaling Signaling Phone A STP Gatekeeper STP Phone B Gateway Gateway
Dial
IAM ACM
ARQ ACF Set-up ARQ ACF IAM
ACM
Ring Answer
ANM Connect
ANM
Voice
Figure 11 – Phone-to-Phone Call Flow
1. The user picks up the phone and dials the destination number. 2. The PSTN switch routes the call to the gateway using ISUP Initial Address Message. 3. The gateway sends an Address Complete Message back to the PSTN and passes the destination number to the gatekeeper using the RAS ARQ message. 4. The gatekeeper authenticates and authorizes the call, responding with ACF message that includes the destination gateway IP address. 5. The gateway sends a H.225 Setup message to the destination gateway. 6. The termination gateway requests admission from the gatekeeper (RAS ARQ message). 7. The gatekeeper grants permission for the call (RAS ACF message). 8. The termination gateway sends an IAM message to the PSTN. 9. The switch sends a ring signal to the phone. 10. When the user picks up the phone, the switch sends an ANM to the originating gateway. 11. The termination gateway sends a Connect message to the originating gateway (both gateway’s send IRR (call start) message to the VGK). 12. The originating gateway sends an ANM message to the origination switch. 13. Voice connection is established through the Audio trunk.
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Glossary AIN Advanced Intelligent Network - a separate data network that provides the telephone network with advanced call control and enhanced services features (American version of IN). ANI Automatic Number Identification - provides the called party with the calling party’s number. CCS7 Common Channel Signaling 7 - European name for SS7. CLASS Customer Local Area Signaling Services - a Bellcore definition of local calling services including call forwarding, caller ID, and call waiting. CO equipment is usually marked with CLASS number, such as CLASS 4 or CLASS 5. This number defines what kind of services this CO offers. DNIS Dialed Number Identification Service - This is a feature that provides the destination number. It is used by the 800-services for real-destination resolution. However DNIS is also available for other non-800 services (e.g. premium-rate-service). IN Intelligent Network - a separate data network to provide the telephone network with advanced call control and enhanced services features all IN services require SS7 signaling as underlay foundation. However SS7 does not imply IN and vice versa. INAP Intelligent Network Application Part - an IN application layer. ISUP ISDN User Part - an SS7 layer which supports implementation of ISDN. IXC Inter Exchange Carrier - a long distance phone company. LEC Local Exchange Carrier - the local phone company. MTP Message Transfer Part - lower layers of the SS7 protocol stack (physical, link network). OPC Origination Point Code – address on the network of the originator of the signal. DPC Destination Point Code – used to identify the destination point on the network. SCP Service Control Point - an IN definition of a control element, typically a database which is used to determine how to handle the call. SCCP Signaling Control Connection Part - an SS7 protocol layer, which provides data services used to support TCAP and other higher-level layers. SLK Signaling Link - the physical transmission line connecting the individual node in an SS7 Network. SP Signaling Point - an SS7 signaling point. SS7 Signaling System 7 - the international data communications protocol standard which supports advanced telephone services and IN. SSP Service Switching Point - an IN definition for an originator or terminator of calls and SS7 messages, typically a central office (CO). STP Signal Transfer Point - an IN definition for a network node in an SS7 network that does not terminate calls. TCAP Transaction Capabilities Application Part - a higher-level SS7 protocol layer, which supports transaction-based applications. VOIP Voice Over Internet Protocol. A standard term referring to transferring voice over the open Internet or IP networks.
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