72 Chapter 4: S87 Network Architecture and Protocols Introduction
`
`where it also has direct trunks. It has no STP connection at all. SSP 2 and SSP 3 are
`connected to the STP pair via nailed-up connections at SSP 1.
`
`Figure 4-17 Example of Direct and Indirect SSP Interconnections to STPs
`
`Physical View
`
`Logical View
`
`.----8-----
`...
`
`·•.
`
`.... e .. __
`e-··
`.~:e
`
`'
`
`.
`~
`·,
`II( ~ Signaling link connections
`., ~
`•
`~ ~ to STPs transported with
`'
`voice bearers on common
`carriers.
`
`STP
`
`STP
`
`STP
`
`STP
`
`No STP Connection
`Indirect STP Connection
`Direct STP Connection
`
`No STP Connection • • • • • •
`STP Connection
`
`Normally within networks that do not use STPs, circuit-related (call-related) signaling
`takes the same path through the network as user traffic because there is no physical need to
`take a different route. This mode of operation is called associated signaling and is prevalent
`outside North America. Referring back to Figure 4-14, both the user traffic and the
`signaling take the same path between SSP Band SSP C.
`
`Because standalone STPs are used to form the SS7 backbone within North America, and
`standalone STPs do not support user traffic switching, the SSP's signaling mode is usually
`quasi-associated, as illustrated between SSP A and SSP Bin Figure 4-14.
`
`In certain circumstances, the SSP uses associated signaling within North America. A great
`deal of signaling traffic might exist between two SSPs, so it might make more sense to place
`a signaling link directly between them rather than to force all signaling through an STP.
`
`S57 Protocol Overview
`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
`
`
`
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`

`

`S87 Protocol Overview
`
`73
`
`(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)
`
`Figure 4-18 shows a common introductory SS7 stack.
`
`Figure 4-18 Introductory SS7 Protocol Stack
`
`I TCAP I ISUP
`
`TUP
`
`B
`
`j
`
`MTP Level 3
`
`Level4
`
`Level3
`
`Level2
`
`Level 1
`
`j
`
`I
`
`MTP Level 2
`
`MTP Level 1
`
`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 (MTPl), 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 intro(cid:173)
`ductory 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
`
`
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`74 Chapter 4: S87 Network Architecture and Protocols Introduction
`
`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 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 MTPl, MTP2, and MTP3 as the physical layer, data link
`layer, and network layer, respectively. The following sections discuss MTP2 and MTP3 .
`(MTPl 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)."
`
`Signaling links are provided by the combination of MTPl and MTP2. MTP2 ensures reli(cid:173)
`able 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) .
`
`MTP
`
`MTP2
`
`
`
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`

`S87 Protocol Overview
`
`75
`
`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, provid(cid:173)
`ing 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(cid:173)
`healing mechanism for the SS7 network.
`Figure 4-19 shows the relationship between levels 1, 2, and 3.
`
`Figure 4-19 A Single MTP3 Controls Many MTP2s, Each of Which Is Connected to a Single MTP 1
`
`-ER
`
`us
`PA RT/S
`
`M
`T
`p
`
`3
`
`SPA
`
`=1 MTP2 I MTP 1
`=1 MTP2 i MTP 1
`=1 MTP2 I MTP 1
`
`SPB
`
`I MTP2
`
`I
`
`I
`
`1=
`1=
`MTP2 1=
`
`I MTP2
`
`I
`
`I
`
`I
`
`I
`
`I
`
`---USE
`
`R
`PAR
`TIS
`
`M
`T
`p
`
`3
`
`TUP and ISUP
`TUP and ISUP sit on top of MTP to provide circuit-related signaling to set up, maintain,
`and tear down calls. TUP has been replaced in most countries because it supports only
`POTS calls. Its successor, ISUP, supports both POTS and ISDN calls as well as a host of
`other features and added flexibility. Both TUP and ISUP are used to perform interswitch
`call signaling. ISUP also has inherent support for supplementary services, such as
`automatic callback, calling line identification, and so on.
`
`SCCP
`
`The combination of the MTP and the SCCP is called the Network Service Part ( NSP) in the
`specifications (but outside the specifications, this term is seldom used) .
`
`
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`

`76 Chapter 4: S87 Network Architecture and Protocols Introduction
`
`The addition of the SCCP provides a more flexible means of routing and provides mecha(cid:173)
`nisms to transfer data over the SS7 network. Such additional features are used to support
`noncircuit-related signaling, which is mostly used to interact with databases (SCPs) . It is
`also used to connect the radio-related components in cellular networks and for inter-SSP
`communication supporting CLASS services. SCCP also provides application management
`functions. Applications are mostly SCP database driven and are called subsystems. For
`example, in cellular networks, SCCP transfers queries and responses between the Visitor
`Location Register (VLR) and Home Location Register (HLR) databases. Such transfers
`take place for a number of reasons. The primary reason is to update the subscriber's HLR
`with the current VLR serving area so that incoming calls can be delivered.
`
`Enhanced routing is called global title (GT) routing. It keeps SPs from having overly large
`routing tables that would be difficult to provision and maintain. A GT is a directory number
`that serves as an alias for a physical network address. A physical address consists of a point
`code and an application reference called a subsystem number (SSN) . GT routing allows SPs
`to use alias addressing to save them from having to maintain overly large physical address
`tables. Centralized STPs are then used to convert the GT address into a physical address;
`this process is called Global Title Translation (GTT). This provides the mapping of tradi(cid:173)
`tional telephony addresses (phone numbers) to SS7 addresses (PC and/or SSN) for
`enhanced services. GIT is typically performed at STPs.
`
`NOTE
`
`It is important not to confuse the mapping of telephony numbers using GTT with the
`translation of telephony numbers done during normal call setup. Voice switches internally
`map telephony addresses to SS7 addresses during normal call processing using number
`translation tables . This process does not use GIT. GIT is used only for noncircuit-related
`information, such as network supplementary services (Calling Name Delivery) or database
`services (toll-free).
`
`In addition to mapping telephony addresses to SS7 addresses, SCCP provides a set of
`subsystem management functions to monitor and respond to the condition of subsystems .
`These management functions are discussed further, along with the other aspects of SCCP,
`in Chapter 9, "Signaling Connection Control Part (SCCP):'
`
`TCAP
`
`TCAP allows applications ( called subsystems) to communicate with each other ( over the
`SS7 network) using agreed-upon data elements. These data elements are called compo(cid:173)
`nents. Components can be viewed as instructions sent between applications. For example,
`when a subscriber changes VLR location in a global system for mobile communication
`(GSM) cellular network, his or her HLR is updated with the new VLR location by means
`
`
`
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`

`S87 Protocol Overview
`
`77
`
`of an UpdateLocation component. TCAP also provides transaction management, allowing
`multiple messages to be associated with a particular communications exchange, known as
`a transaction.
`
`There are a number of subsystems; the most common are
`
`• Toll-free (E800)
`
`• Advanced Intelligent Network (AIN)
`•
`Intelligent Network Application Protocol (INAP)
`• Customizable Applications for Mobile Enhanced Logic (CAMEL)
`• Mobile Application Part (MAP)
`Figure 4-20 illustrates these subsystems as well as another protocol that uses SCCP, the
`Base Station Subsystem Application Part. It is used to control the radio-related component
`in cellular networks.
`
`Figure 4-20 Some Protocols That Might Exist on Top of the SCCP, Depending on the Application
`
`Radio-Interface
`Related
`
`Database-Driven
`Application Support
`
`Call Control
`
`BSS- ii
`----------MAP_!
`
`TCAP
`
`SCCP
`
`ISUP
`
`TUP
`
`:
`;
`
`! I
`
`Native
`Packet(cid:173)
`Switched
`Transport
`
`MTP Level 3
`
`MTP Level 2
`
`MTP Level 1
`
`;
`
`;
`~
`
`It is highly unlikely that a protocol such as the one shown in Figure 4-20 would exist at any
`one SP. Instead, protocol stacks vary as required by SP type. For example , because an STP
`is a routing device, it has only MTPl, MTP2, MTP3, and SCCP. A fixed-line switch without
`IN support might have only MTPl, MTP2, MTP3, and ISUP, and so forth . A diagram
`showing how the SS7 protocol stack varies by SP can be found in Chapter 13.
`
`
`
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`

`78 Chapter 4: S87 Network Architecture and Protocols Introduction
`
`Summary
`SS7 is a data communications network that acts as the nervous system to bring the compo(cid:173)
`nents 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.
`
`
`
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`

`CHAPTER 5
`
`The Public Switched Telephone
`Network (PSTN)
`
`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 elec(cid:173)
`tronic equipment that converts speech into digital data and provides a multitude of sophis(cid:173)
`ticated phone features, data services, and mobile wireless access.
`
`TIP
`
`PSTN voice facilities transport speech or voice-band data (such as fax/modems and digital
`data), which is data that has been modulated to voice frequencies.
`
`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 subscrib(cid:173)
`ers. 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
`
`
`
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`

`82 Chapter 5: The Public Switched Telephone Network (PSTN)
`
`• 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.
`
`Network Topology
`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 rep(cid:173)
`resent 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.
`'Iransit-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 exist(cid:173)
`ing 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 communi(cid:173)
`cation with each other. The second approach is a hierarchical tree in which nodes are aggre(cid:173)
`gated 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.
`
`Figure 5-1 shows a generic PSTN hierarchy, in which End Offices are connected locally and
`through tandem switches. Transit switches provide further aggregation points for connect(cid:173)
`ing multiple tandems between different networks . While actual network topologies vary,
`most follow some variation of this basic pattern.
`
`
`
`Page 90 of 156
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`

`

`Figure 5-1 Generic PSTN Hierarchies
`
`PSTN Hierarchy
`
`83
`
`PSTN Hierarchy
`The PSTN hierarchy is implemented differently in the United States and the United
`Kingdom. The following sections provide an overview of the PSTN hierarchy and its
`related terminology in each of these countries.
`
`PSTN Hierarchy in the United States
`In the United States, the PSTN is generally divided into three categories:
`
`• Local Exchange Networks
`•
`lnterExchange Networks
`•
`International Networks
`Local Exchange Carriers (LECs) operate Local Exchange networks, while lnterExchange
`Carriers (IXCs) operate InterExchange and International networks.
`
`The PSTN hierarchy in the United States is also influenced by market deregulation, which
`has allowed service providers to compete for business and by the divestiture of Bell.
`
`
`
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`

`84 Chapter 5: The Public Switched Telephone Network (PSTN)
`
`Local Exchange Network
`The Local Exchange network consists of the digital switching nodes (EOs) that provide
`network access to the subscriber. The Local Exchange terminates both lines and trunks,
`providing the subscriber access to the PSTN.
`
`A Tandem Office often connects End Offices within a local area, but they can also be con(cid:173)
`nected directly. In the United States, Tandem Offices are usually designated as either Local
`Tandem (LT) or Access Tandem (AT). The primary purpose of a Local Tandem is to provide
`interconnection between End Offices in a localized geographic region. An Access Tandem
`provides interconnection between local End Offices and serves as a primary point of access
`for IXCs . Trunks are the facilities that connect all of the offices, thereby transporting inter(cid:173)
`nodal traffic.
`
`lnterExchange Network
`The InterExchange network is comprised of digital switching nodes that provide the con(cid:173)
`nection between Local Exchange networks. Because they are points of high traffic aggre(cid:173)
`gation and they cover larger geographical distances, high-speed transports are typically
`used between transit switches . In the deregulated U.S . market, transit switches are usually
`referred to as carrier switches. In the U.S., IXCs access the Local Exchange network at
`designated points, referred to as a Point of Presence (POP) . POPs can be connections at the
`Access Tandem, or direct connections to the End Office.
`
`International Network
`The International network consists of digital switching nodes, which are located in
`each country and act as international gateways to destinations outside of their respective
`countries. These gateways adhere to the ITU international standards to ensure interopera(cid:173)
`bility between national networks. The international switch also performs the protocol con(cid:173)
`versions between national and international signaling. The gateway also performs PCM
`conversions between A-law and µ-law to produce compatible speech encoding between
`networks, when necessary.
`
`Service Providers
`Deregulation policies in the United States have allowed network operators to compete for
`business, first in the long-distance market (InterExchange and International) beginning in
`the mid 1980s, and later in the local market in the mid 1990s. As previously mentioned,
`LECs operate Local Exchange networks, while IXCs operate the long-distance networks .
`Figure 5-2 shows a typical arrangement of LEC-owned EOs and tandems interconnected to
`
`
`
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`

`

`PSTN Hierarchy
`
`85
`
`IXC-owned transit switches. The IXC switches provide long-haul transport between Local
`Exchange networks, and international connections through International gateway switches.
`
`Figure 5-2 Generic U.S. Hierarchies
`
`Over the last several years, the terms ILEC and CLEC have emerged within the Local
`Exchange market to differentiate between the Incumbent LECs (ILECS) and the Competitive
`LECs (CLECS). ILECs are the incumbents, who own existing access lines to residences
`and corporate facilities; CLECs are new entrants into the Local Exchange market. Most
`of the ILECs in the United States came about with the divestiture of AT&T into the seven
`Regional Bell Operating Companies (RBOC) . The remainder belonged to Independent
`Operating Companies (I OCs). Most of these post-divestiture companies have been signifi(cid:173)
`cantly transformed today by mergers and acquisitions in the competitive market. New com(cid:173)
`panies have experienced difficulty entering into the Local Exchange market, which is
`dominated by ILECs. The ILECs own the wire to the subscriber's home, often called the
`"last mile" wiring. Last mile wiring is expensive to install and gives the ILECs tremendous
`market leverage. The long-distance market has been easier for new entrants because it does
`not require an investment in last mile wiring.
`
`
`
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`

`86 Chapter 5: The Public Switched Telephone Network (PSTN)
`
`Pre-Divestiture Bell System Hierarchy
`Vestiges of terminology relating to network topology remain in use today from the North
`American Bell System's hierarchy, as it existed prior to divestiture in 1984. Telephone
`switching offices are often still referred to by class. For example, an EO is commonly called
`a class 5 office, and an AT is called a class 4 office. Before divestiture, each layer of the
`network hierarchy was assigned a class number.
`
`Prior to divestiture, offices were categorized by class number, with class 1 being the highest
`office category and class 5 being the lowest (nearest to subscriber access). Aggregation of
`transit phone traffic moved from the class 5 office up through the class 1 office. Each class
`of traffic aggregation points contained a smaller number of offices. Table 5-1 lists the
`class categories and office types used in the Bell System Hierarchy.
`
`Table 5-1
`
`Pre-Divestiture Class Categories and Office Types
`
`Class
`
`1
`
`2
`
`3
`
`4
`
`5
`
`Office Type
`
`Regional Center
`
`Sectional Center
`
`Primary Center
`
`Toll Center
`
`End Office
`
`Local calls remained within class 5 offices, while a cross-country call traversed the hierarchy
`up to a regional switching center. This system no longer exists, but we included it to give
`relevance to the class terminology, which the industry still uses often.
`
`PSTN Hierarchy in the United Kingdom
`Figure 5-3 shows the PSTN topology used in the United Kingdom. End Offices are referred
`to as Digital Local Exchanges (DLE). A fully meshed tandem network of Digital Main
`Switching Units (DMSU) connects the DLEs. Digital International Switching Centers
`(DISC) connect the DMSU tandem switches for international call connections.
`
`
`
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`

`

`Access and Transmission Facilities
`
`87
`
`Figure 5-3 U.K. PSTN Hierarchy
`
`Access and Transmission Facilities
`Connections to PSTN switches can be divided into two basic categories: lines and trunks.
`Individual telephone lines connect subscribers to the Central Office (CO) by wire pairs,
`while trunks are used to interconnect PSTN switches. Trunks also provide access to corpo(cid:173)
`rate phone environments, which often use a Private Branch eXchange (PBX)-or in the
`case of some very large businesses, their own digital switch. Figure 5-4 illustrates a number
`of common interfaces to the Central Office.
`
`
`
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`

`88 Chapter 5: The Public Switched Telephone Network (PSTN)
`
`Figure 5-4 End Office Facility Inteifaces
`
`~ Remote
`~ ~oncentrator
`~~
`
`PBX
`
`ISDN Phone
`
`Digital Trunks
`
`Lines
`
`Lines are used to connect the subscriber to the CO, providing the subscriber access into the
`PSTN. The following sections describe the facilities used for lines, and the access signaling
`between the subscriber and the CO.
`
`• The Local Loop
`• Analog Line Signaling
`• Dialing
`
`• Ringing and Answer
`• Voice Encoding
`ISDNBRI
`•
`
`The Local Loop
`The local loop consists of a pair of copper wires extending from the CO to a residence or
`business that connects to the phone, fax, modem, or other telephony device. The wire pair
`
`
`
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`

`Access and Transmission Facilities
`
`89
`
`consists of a tip wire and a ring wire. The terms tip and ring are vestiges of the manual
`switchboards that were used a number of years ago; they refer to the tip and ring of the
`actual switchboard plug operators used to connect calls. The local loop allows a subscriber
`to access the PSTN through its connection to the CO. The local loop terminates on the Main
`Distribution Frame (MDF) at the CO, or on a remote line concentrator.
`
`Remote line concentrators, also referred to as Subscriber Line Multiplexers or Subscriber
`Line Concentrators, extend the line interface from the CO toward the subscribers, thereby
`reducing the amount of wire pairs back to the CO and converting the signal from analog to
`digital closer to the subscriber access point. In some cases, remote switching centers are
`used instead of remote concentrators.
`
`Remote switching centers provide local switching between subtending lines without using
`the resources of the CO. Remotes, as they are often generically referred to, are typically
`used for subscribers who are located far away from the CO. While terminating the physical
`loop, remotes transport the digitized voice stream back to the CO over a trunk circuit, in
`digital form .
`
`Analog Line Signaling
`Currently, most phone lines are analog phone lines. They are referred to as analog lines
`because they use an analog signal over the local loop, between the phone and the CO. The
`analog signal carries two components that comprise the communication between the phone
`and the CO: the voice component, and the signaling component.
`
`The signaling that takes place between the analog phone and the CO is called in-band
`signaling. In-band signaling is primitive when compared to the out-of-band signaling used
`in access methods such as ISDN; see the "ISDN BRI" section in this chapter for more
`information. DC current from the CO powers the local loop between the phone and the CO.
`The voltage levels vary between different countries, but an on-hook voltage of-48 to -54
`volts is common in North America and a number of other geographic regions, including the
`United Kingdom.
`
`TIP
`
`The actual line loop voltage varies, based on the distance and the charge level of the batteries
`connected to the loop at the CO. When the phone receiver is on-hook, the CO sees practi(cid:173)
`cally no current over the loop to the phone set. When the phone is off-hook, the resistance
`level changes, changing the current seen at the CO. The actual amount of loop current that
`triggers an on/off-hook signal also varies among different countries. In North America, a
`current flow of greater than 20 milliamps indicates an off-hook condition. When the CO has
`detected the off-hook condition, it provides a dial tone by connecting a tone generation
`circuit to the line.
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`90 Chapter 5: The Public Switched Telephone Network (PSTN)
`
`Dialing
`
`When a subscriber dials a number, the number is signaled to the CO as either a series of
`pulses based on the number dialed, or by Dual Tone Multi-Frequency (DTMF) signals. The
`DTMF signal is a combination of two tones that are generated at different frequencies . A
`total of seven frequencies are combined to provide unique DTMF signals for the 12 keys
`( three columns by four rows) on the standard phone keypad. Usually, the dialing plan of the
`CO determines when all digits have been collected.
`
`Ringing and Answer
`To notify the called party of an incoming call, the CO sends AC ringing voltage over the
`local loop to the terminating line. The incoming voltage activates the ringing circuit within
`the phone to generate an audible ring signal. The CO also sends an audible ring-back tone
`over the originating local loop to indicate that the call is proceeding and the destination
`phone is ringing. When the destination phone is taken off-hook, the CO detects the change
`in loop current and stops generating the ringing voltage. This procedure is commonly
`referred to as ring trip. The off-hook signals the CO that the call has been answered; the
`conversation path is then completed between the two parties and other actions, such as
`billing, can be initiated, if necessary.
`
`Voice Encoding
`An analog voice signal must be encoded into digital information for transmission over
`the digital switching network. The conversion is completed using a codec (coder/decoder),
`which converts between analog and digital data. The ITU G.711 standard specifies the
`Pulse Coded Modulation (PCM) method used throughout most of the PSTN. An analog(cid:173)
`to-digital converter samples the analog voice 8000 times per second and then assigns a
`quantization value based on 256 decision levels . The quantization value is then encoded
`into a binary number to represent the individual data point of the sample. Figure 5-5
`illustrates the process of sampling and encoding the analog voice data.
`
`Figure 5-5 Voice Encoding Process
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`ISDN BRI
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`Access and Transmission Facilities
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`91
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`Two variations of encoding schemes are used for the actual quantization values: A-law and
`µ -Law encoding. North America uses µ-Law encoding, and European countries use A-law
`encoding. When voice is transmitted from the digital switch over the analog loop, the digital
`voice data is decoded and converted ba