The topology of a network describes the various network nodes and how they interconnect. Regulatory policies play a major role in exactly how voice network topologies are defined in each country, but general similarities exist. While topologies in competitive markets represent an interconnection of networks owned by different service providers, monopolistic markets are generally an interconnection of switches owned by the same operator.

Depending on geographical region, PSTN nodes are sometimes referred to by different names. The three node types we discuss in this chapter include:

• End Office (EO)— Also called a Local Exchange. The End Office provides network access for the subscriber. It is located at the bottom of the network hierarchy.

• Tandem— Connects EOs together, providing an aggregation point for traffic between them. In some cases, the Tandem node provides the EO access to the next hierarchical level of the network.

• Transit— Provides an interface to another hierarchical network level. Transit switches are generally used to aggregate traffic that is carried across long geographical distances.

There are two primary methods of connecting switching nodes. The first approach is a mesh topology, in which all nodes are interconnected. This approach does not scale well when you must connect a large number of nodes. You must connect each new node to every existing node. This approach does have its merits, however; it simplifies routing traffic between nodes and avoids bottlenecks by involving only those switches that are in direct communication with each other. The second approach is a hierarchical tree in which nodes are aggregated as the hierarchy traverses from the subscriber access points to the top of the tree. PSTN networks use a combination of these two methods, which are largely driven by cost and the traffic patterns between exchanges.

Posted in Chapter 5 Tagged: C7, network, signaling, SS7, telecommunication ]]> http://ss7bible.wordpress.com/2009/12/12/network-topology/feed/ 0 The Author http://ss7bible.wordpress.com/2009/11/28/the-public-switched-telephone-network-pstn/ http://ss7bible.wordpress.com/2009/11/28/the-public-switched-telephone-network-pstn/#comments Sat, 28 Nov 2009 16:05:39 +0000 The Author http://ss7bible.wordpress.com/?p=48 ]]>

The term Public Switched Telephone Network (PSTN) describes the various equipment and interconnecting facilities that provide phone service to the public. The network continues to evolve with the introduction of new technologies. The PSTN began in the United States in 1878 with a manual mechanical switchboard that connected different parties and allowed them to carry on a conversation. Today, the PSTN is a network of computers and other electronic equipment that converts speech into digital data and provides a multitude of sophisticated phone features, data services, and mobile wireless access.

At the core of the PSTN are digital switches. The term “switch” describes the ability to cross-connect a phone line with many other phone lines and switching from one connection to another. The PSTN is well known for providing reliable communications to its subscribers. The phrase “five nines reliability,” representing network availability of 99.999 percent for PSTN equipment, has become ubiquitous within the telecommunications industry.

This chapter provides a fundamental view of how the PSTN works, particularly in the areas of signaling and digital switching. SS7 provides control signaling for the PSTN, so you should understand the PSTN infrastructure to fully appreciate how it affects signaling and switching. This chapter is divided into the following sections:

• Network Topology

• PSTN Hierarchy

• Access and Transmission Facilities

• Network Timing

• The Central Office

• Integration of SS7 into the PSTN

• Evolving the PSTN to the Next Generation

We conclude with a summary of the PTSN infrastructure and its continuing evolution.

SS7 is a data communications network that acts as the nervous system to bring the components of telecommunications networks to life. It acts as a platform for various services described throughout this book. SS7 nodes are called signaling points (SPs), of which there are three types:

• Service Switching Point (SSP)

• Service Control Point (SCP)

• Signal Transfer Point (STP)

SSPs provide the SS7 functionality of a switch. STPs may be either standalone or integrated STPs (SSP and STP) and are used to transfer signaling messages. SCPs interface the SS7 network to query telecommunication databases, allowing service logic and additional routing information to be obtained to execute services.

SPs are connected to each other using signaling links. Signaling links are logically grouped into a linkset. Links may be referenced as A through F links, depending on where they are in the network.

Signaling is transferred using the packet-switching facilities afforded by SS7. These packets are called signal units (SUs). The Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP) provide the transfer protocols. MTP is used to reliably transport messages between nodes, and SCCP is used for noncircuit-related signaling (typically, transactions with SCPs). The ISDN User Part (ISUP) is used to set up and tear down both ordinary (analog subscriber) and ISDN calls. The Transaction Capabilities Application Part (TCAP) allows applications to communicate with each other using agreed-upon data components and manages transactions.

The number of possible protocol stack combinations is growing. It depends on whether SS7 is used for cellular-specific services or intelligent network services, whether transportation is over IP or is controlling broadband ATM networks instead of time-division multiplexing (TDM) networks, and so forth. This requires coining a new term—traditional SS7—to refer to a stack consisting of the protocols widely deployed from the 1980s to the present:

• Message Transfer Parts (MTP 1, 2, and 3)

• Signaling Connection Control Part (SCCP)

• Transaction Capabilities Application Part (TCAP)

• Telephony User Part (TUP)

• ISDN User Part (ISUP)

Such a stack uses TDM for transport. This book focuses on traditional SS7 because that is what is implemented. Newer implementations are beginning to appear that use different transport means such as IP and that have associated new protocols to deal with the revised transport.

The SS7 physical layer is called MTP level 1 (MTP1), the data link layer is called MTP level 2 (MTP2), and the network layer is called MTP level 3 (MTP3). Collectively they are called the Message Transfer Part (MTP). The MTP protocol is SS7’s native means of packet transport. In recent years there has been an interest in the facility to transport SS7 signaling over IP instead of using SS7’s native MTP. This effort has largely been carried out by the Internet Engineering Task Force (IETF) SigTran (Signaling Transport) working group. The protocols derived by the SigTran working group so far are outside the scope of this introductory chapter on SS7. However, full details of SigTran can be found in Chapter 14, “SS7 in the Converged World.”

TUP and ISUP both perform the signaling required to set up and tear down telephone calls. As such, both are circuit-related signaling protocols. TUP was the first call control protocol specified. It could support only plain old telephone service (POTS) calls. Most countries are replacing TUP with ISUP. Both North America and Japan bypassed TUP and went straight from earlier signaling systems to ISUP. ISUP supports both POTS and ISDN calls. It also has more flexibility and features than TUP.

With reference to the Open System Interconnection (OSI) seven-layer reference model, SS7 uses a four-level protocol stack. OSI Layer 1 through 3 services are provided by the MTP together with the SCCP. The SS7 architecture currently has no protocols that map into OSI Layers 4 through 6. TUP, ISUP, and TCAP are considered as corresponding to OSI Layer 7 [111]. SS7 and the OSI model were created at about the same time. For this reason, they use some differing terminology.

SS7 uses the term levels when referring to its architecture. The term levels should not be confused with OSI layers, because they do not directly correspond to each other. Levels was a term introduced to help in the discussion and presentation of the SS7 protocol stack. Levels 1, 2, and 3 correspond to MTP 1, 2, and 3, respectively. Level 4 refers to an MTP user. The term user refers to any protocol that directly uses the transport capability provided by the MTP—namely, TUP, ISUP, and SCCP in traditional SS7. The four-level terminology originated back when SS7 had only a call control protocol (TUP) and the MTP, before SCCP and TCAP were added.

The following sections provide a brief outline of protocols found in the introductory SS7 protocol stack, as illustrated in Figure 4-18.

MTP

MTP levels 1 through 3 are collectively referred to as the MTP. The MTP comprises the functions to transport information from one SP to another.

The MTP transfers the signaling message, in the correct sequence, without loss or duplication, between the SPs that make up the SS7 network. The MTP provides reliable transfer and delivery of signaling messages. The MTP was originally designed to transfer circuit-related signaling because no noncircuit-related protocol was defined at the time.

The recommendations refer to MTP1, MTP2, and MTP3 as the physical layer, data link layer, and network layer, respectively. The following sections discuss MTP2 and MTP3. (MTP1 isn’t discussed because it refers to the physical network.) For information on the physical aspects of the Public Switched Telephone Network (PSTN), see Chapter 5, “The Public Switched Telephone Network (PSTN).”

MTP2

Signaling links are provided by the combination of MTP1 and MTP2. MTP2 ensures reliable transfer of signaling messages. It encapsulates signaling messages into variable-length SS7 packets. SS7 packets are called signal units (SUs). MTP2 provides delineation of SUs, alignment of SUs, signaling link error monitoring, error correction by retransmission, and flow control. The MTP2 protocol is specific to narrowband links (56 or 64 kbps).

MTP3

MTP3 performs two functions:

• Signaling Message Handling (SMH)— Delivers incoming messages to their intended User Part and routes outgoing messages toward their destination. MTP3 uses the PC to identify the correct node for message delivery. Each message has both an Origination Point Code (OPC) and a DPC. The OPC is inserted into messages at the MTP3 level to identify the SP that originated the message. The DPC is inserted to identify the address of the destination SP. Routing tables within an SS7 node are used to route messages.

• Signaling Network Management (SNM)— Monitors linksets and routesets, providing status to network nodes so that traffic can be rerouted when necessary. SNM also provides procedures to take corrective action when failures occur, providing a self-healing mechanism for the SS7 network.

SS7 can employ different types of signaling network structures. The choice between these different structures can be influenced by factors such as administrative aspects and the structure of the telecommunication network to be served by the signaling system.

The worldwide signaling network has two functionally independent levels:

• International

• National

This structure makes possible a clear division of responsibility for signaling network management. It also lets numbering plans of SS7 nodes belonging to the international network and the different national networks be independent of one another.

SS7 network nodes are called signaling points (SPs). Each SP is addressed by an integer called a point code (PC). The international network uses a 14-bit PC. The national networks also use a 14-bit PC—except North America and China, which use an incompatible 24-bit PC, and Japan, which uses a 16-bit PC. The national PC is unique only within a particular operator’s national network. International PCs are unique only within the international network. Other operator networks (if they exist) within a country also could have the same PC and also might share the same PC as that used on the international network. Therefore, additional routing information is provided so that the PC can be interpreted correctly—that is, as an international network, as its own national network, or as another operator’s national network. The structure of point codes is described in Chapter 7, “Message Transfer Part 3 (MTP3).”

Signaling Links and Linksets

SPs are connected to each other by signaling links over which signaling takes place. The bandwidth of a signaling link is normally 64 kilobits per second (kbps). Because of legacy reasons, however, some links in North America might have an effective rate of 56 kbps. In recent years, high-speed links have been introduced that use an entire 1.544 Mbps T1 carrier for signaling. Links are typically engineered to carry only 25 to 40 percent of their capacity so that in case of a failure, one link can carry the load of two.

To provide more bandwidth and/or for redundancy, up to 16 links between two SPs can be used. Links between two SPs are logically grouped for administrative and load-sharing reasons. A logical group of links between two SP is called a linkset. Figure 4-2 shows four links in a linkset.

A number of linksets that may be used to reach a particular destination can be grouped logically to form a combined linkset. For each combined linkset that an individual linkset is a member of, it may be assigned different priority levels relative to other linksets in each combined linkset.

A group of links within a linkset that have the same characteristics (data rate, terrestrial/satellite, and so on) are called a link group. Normally the links in a linkset have the same characteristics, so the term link group can be synonymous with linkset.

Routes and Routesets

SS7 routes are statically provisioned at each SP. There are no mechanisms for route discovery. A route is defined as a preprovisioned path between source and destination for a particular relation. Figure 4-3 shows a route from SP A to SP C.

The first specification (called a recommendation by the CCITT/ITU-T) of CCITT Signaling System No. 7 was published in 1980 in the form of the CCITT yellow book recommendations. After the yellow book recommendations, CCITT recommendations were approved at the end of a four-year study period. They were published in a colored book representing that study period.

Under the CCITT publishing mechanism, the color referred to a published set of recommendations—that is, all protocols were published at the same time. The printed matter had the appropriate colored cover, and the published title contained the color name. When the ITU-T took over from the CCITT, it produced single booklets for each protocol instead of producing en bloc publications as had been the case under the supervision of the CCITT. Under the new mechanism, the color scheme was dropped. As a result, the ITU-T publications came to be known as “White Book” editions, because no color was specified, and the resulting publications had white covers. Because these publications do not refer to a color, you have to qualify the term “White Book” with the year of publication.

As Table 4-1 shows, when SS7 was first published, the protocol stack consisted of only the Message Transfer Part 2 (MTP2), Message Transfer Part 3 (MTP3), and Telephony User Part (TUP) protocols. On first publication, these were still somewhat immature. It was not until the later Red and Blue book editions that the protocol was considered mature. Since then, the SS7 protocols have been enhanced, and new protocols have been added as required.

Figure 4-1 shows how many pages the ITU-T SS7 specifications contained in each year. In 1980, there were a total of 320 pages, in 1984 a total of 641 pages, in 1988 a total of 1900 pages, and in 1999 approximately 9000 pages.

The following are the main systems that preceded SS7:

• CCITT R1 (regional 1) was deployed only on a national level. R1 is a Channel Associated Signaling (CAS) system that was employed in the U.S. and Japan. It uses multifrequency (MF) tones for signaling. It is no longer in general operation, although some remnants might remain in the network.

• CCITT R2 (regional 2) was deployed only on a national level. R2 is a CAS system that was employed in Europe and most other countries. It used Multifrequency Compelled (MFC) for signaling; it compelled the receiver to acknowledge a pair of tones before sending the next pair. It is no longer in general operation, although some remnants might remain in the network.

• Signaling systems that have been deployed for both national and international (between international switches) signaling have progressed from CCITT #5 (C5) to CCITT #6 (C6) and finally to CCITT #7 (C7):

  - C5 (CCITT Signaling System No. 5) is a CAS system standardized in 1964 that has found widespread use in international signaling. It is still in use today on a number of international interfaces. National implementations are now scarce, except in less-developed regions of the world, such as Africa, which makes extensive use of the protocol. C5 can be used in both analog and digital environments. In an analog setting, it uses tones for signaling. In a digital setting, a digital representation of the tone is sent instead (a pulse code modulation [PCM] sample).

  - C6 (CCITT Signaling System No. 6), also called SS6, was the first system to employ Common Channel Signaling (CCS). It was standardized in 1972. (CAS and CCS are explained in Chapter 1, “The Evolution of Signaling.”) C6 was a pre-OSI model and as such had a monolithic structure as opposed to a layered one. C6 was a precursor to C7 and included the use of data links to carry signaling in the form of packets. It had error correction/detection mechanisms. It employed a common signaling channel to control a large number of speech circuits, and it had self-governing network management procedures. C6 had a number of advantages over C5, including improvements in post-dial delay and the ability to reject calls with a cause code. The use of locally mapped cause codes allowed international callers to hear announcements in their own language. Although C6 was designed for the international network, it was not as widely deployed as C5. However, it was nationalized for the U.S. network and was deployed quite extensively under the name Common Channel Interoffice Signaling System 6 (CCIS6) in the AT&T network. C6 was introduced into the Bell system in the U.S. in 1976, and soon after, Canada. All deployments have now been replaced by SS7.

The next section provides a brief history of SS7.

The International Telecommunication Union (ITU) is the international governing body for Signaling System No. 7. More specifically, it is governed by the Telecommunication Standardization Sector of the ITU (ITU-TS or ITU-T for short). Formerly it was governed by the ITU’s Consultative Committee for International Telegraph and Telephone (CCITT) subcommittee until that was disbanded in 1992 as part of a process to speed up the production of recommendations (as well as other organization changes). See Chapter 2, “Standards,” for more information on standards-making bodies.

Signaling System No. 7 is more commonly known by the acronyms SS7 and C7. Strictly speaking, the term C7 (or, less commonly, CCS7) refers to the international Signaling System No. 7 network protocols specified by the ITU-T recommendations as well as national or regional variants defined within the framework provided by the ITU-T. The term C7 originates from the former title found on the specifications—CCITT Signaling System No. 7. The term SS7 tends to specifically refer to the North American regional standards produced by Telcordia (formerly known as Bell Communications Research or Bellcore) and the American National Standards Institute (ANSI). The North American standards themselves are based on the ITU-T recommendations but have been tailored outside the provided framework. The differences between ITU and Telcordia/ANSI are largely subtle at the lower layers. Interaction between ANSI and ITU-T networks is made challenging by different implementations of higher-layer protocols and procedures.

For the purpose of this book, we will use the term SS7 to refer generically to any Signaling System No. 7 protocol, regardless of its origin or demographics. An overview of SS7 by the ITU-T can be found in recommendation Q.700 [111], and a similar overview of SS7 by ANSI can be found in T1.110 [112].

Chapter 3, “The Role of SS7,” provides a comprehensive list of the functions and services afforded by SS7. These can be summarized as follows:

• Setting up and tearing down circuit-switched connections, such as telephone calls made over both cellular and fixed-line.

• Advanced network features such as those offered by supplementary services (calling name/number presentation, Automatic Callback, and so on).

• Mobility management in cellular networks, which permits subscribers to move geographically while remaining attached to the network, even while an active call is in place. This is the central function of a cellular network.

• Short Message Service (SMS) and Enhanced Messaging Service (EMS), where SS7 is used not only for signaling but also for content transport of alphanumeric text.

• Support for Intelligent Network (IN) services such as toll-free (800) calling.

• Support for ISDN.

• Local Number Portability (LNP) to allow subscribers to change their service, service provider, and location without needing to change their telephone number.

After reading the preceding chapters, you know that signaling serves the requirements of the telecommunications service being delivered; it is not an end in itself. Signaling enables services within the network.

This chapter makes you familiar with the SS7 network, protocols, fundamental concepts, and terminology so that the topics covered in the rest of the book will be more accessible if you’re unfamiliar with the subject. This chapter begins with a brief description of pre-SS7 systems and SS7 history. The chapter then presents the protocol stack, showing how SS7 protocols fit together. It concludes with a discussion of the relevant protocols.

This chapter has shown that, although it is transparent, SS7/C7 plays a role in the lives of virtually every individual in developed countries. It is also the key to new, revenue-generating services and is crucial to the QoS as perceived by subscribers—both of which lie at the very heart of success in a fiercely competitive telecommunications market. Furthermore SS7/C7 is a common thread that ties fixed-line, cellular, and IP networks together, and it is a key enabler for the convergence of the telecommunications and data communications industries.

Telecommunications network operators can realize increased investment returns by marrying existing SS7/C7 and intelligent networking infrastructures with Internet and other data-centric technologies. SS7/C7 is a key protocol for bridging the telecom and datacom worlds.

The following sections describe the exemplar hybrid network services that SS7/C7 enable:

• Internet Call Waiting

• Internet Calling Name Services

• Click-to-Dial Applications

• Web-Browser-Based Telecommunication Services

• WLAN “Hotspot” Billing

• Location-Based Games

Internet Call Waiting and Internet Calling Name Services

Internet call waiting is a software solution that alerts online Internet users with a call-waiting message on their computer screens when a telephone call enters the same phone line they use for their Internet service. The user can then send the call to voice mail, accept the call, or reject it.

Some providers linking it to CNAM, as mentioned in Calling Name (CNAM), have enhanced the Internet call-waiting service. This service is known as Internet calling name service, and it provides the calling party’s name and number.

Click-to-Dial Applications

Click-to-dial applications are another SS7-IP growth area. An example of a click-to-dial application is the ability to click a person’s telephone number in an email signature to place a call. These types of services are particularly beneficial to subscribers because they do not require them to change their equipment or access technologies; a POTS and a traditional handset are the only requirements.

Web-Browser-Based of Telecommunication Services

Over the coming decade, we are likely to witness an increase in web based telecommunications services. An example is customer self-provisioning via the Internet, a practice that has been in the marketplace for some time and is likely to increase in both complexity and usage. A customer can already assign himself a premium or toll-free “number for life” via the Internet. The customer can subsequently use a Web interface to change the destination number it points to at will, so that during the day it points to the customer’s office phone, and in the evening it points to the customer’s cell phone, and so forth.

Another example is the “call me” service, which allows a customer to navigate a Web page to arrange a callback from a department, rather than navigating interactive voice response (IVR) systems through the use of voice prompts and a touch-tone phone.

The potential extends far beyond traditional telecommunications services, to the point where the distinction between Web and telecommunications services is blurred. An example of such an enabling technology is Voice Extensible Markup Language (VoiceXML), which extends Web applications to telephones and shields application authors from low-level, platform-specific interactive voice response (IVR) and call control details.

The marriage is not only between SS7/C7, the Internet, and fixed-line networks—it also extends to cellular networks. Plans are underway to put the location-based information and signaling found in cellular networks into hybrid use. For example, Web-based messenger services could access cellular network home location registers (HLRs) to enable a user to locate a friend or relative in terms of real-time geographic location.

WLAN “Hotspot” Billing

SS7/C7 has recently begun playing a role in the marriage of wireless (WLANs) and cellular networks. A subscriber can use a cellular subscriber identity module (SIM) card for authentication and billing purposes from a WLAN hotspot. For example, if a subscriber is at a café with WLAN facilities (typically wi-fi), the subscriber can request permission to use the service via a laptop screen. This request triggers a short cellular call to authenticate the subscriber (using SS7/C7 signaling). The usage is then conveniently billed to the subscriber’s cellular phone bill.

Location-Based Games

SS7/C7 is not only used to deliver games to cell phones, but it also plays a role in the creation of a new genre of location-based games and entertainment. Cellular games incorporate the player’s location using SS7/C7 to provide mobility information a dedicated web site as a central point. Some of the games that are emerging at the time of this writing are using global positioning system (GPS), WLAN support, and built-in instant messaging capabilities (to help tease your opponents) to blend higher location accuracy.