The PSTN Infrastructure

Views about what a network should be designed to support and what the infrastructure should be composed of have changed quite a bit over the years, as applications and technology have changed. Before discussing what is needed in a network today, this chapter takes a look at how the PSTN infrastructure evolved and where it is now.

The traditional PSTN infrastructure was specifically designed to support only continuous, real-time voice communications. At the time this infrastructure was being designed, we had no notion of data communications or, indeed, notions of bursty communications or long-duration conversations.

The length of calls is an important variable in the design of the PSTN. Most voice calls are quite short, so the circuit switches in the PSTN are engineered for call durations of three minutes or less, and only a small percentage of subscribers off-hook at any time. The average Internet session, on the other hand, lasts around an hour. This means that increased Internet access through the PSTN has, in some locales, put a strain on the local exchanges. If a circuit switch is blocked because it is carrying a long Internet session, people may not be able to get a dial tone. There are several solutions to this problem. For example, as discussed in Chapter 10, "Next-Generation Networks," we can apply intelligence in front of some exchanges so that calls destined for ISPs can be diverted over a packet-switched network to the ISP rather than being completed on a circuit-switched basis through the local exchange.

Yet another variable that's important to the design of the PSTN has to do with what it was designed to support. The capacities of the channels in the PSTN are of the narrowband generationthey are based on 64Kbps channels. The worldwide infrastructure to accommodate voice communications evolved to include a series of circuit switches. Different switches are used based on the locations to which they connect. The switches have a high degree of intelligence built into them, both for establishing the communications channels and for delivering the service logic to activate a growing array of features. In the traditional framework, the monolithic switches in the network had all the smarts. The switch manufacturer and the carrier worked together very closely, and the carrier couldn't introduce new features and services into a particular area until a software release was available for the switch platform through which the neighborhood was being serviced. Thus, carriers were often unable to roll out new services and features because they hadn't yet received the new software releases from the switch manufacturers. Over time, we have separated the functions of switching and connection establishment from the functions involved in the intelligence that enables various services and features to be activated.

The traditional PSTN is associated with highly developed, although not necessarily integrated, operational support systems (e.g., billing systems, provisioning systems, network management systems, customer contact systems, security systems). These systems have very well-developed business processes and techniques for managing their environments. But the various systems' databases still do not always speak to one another to give one comprehensive view. In the IP world, the operational support systems are relatively new and are generally no more integrated than those found in the PSTN.

The backbone of the traditional PSTN was largely based on a generation called the Plesiochronous Digital Hierarchy (PDH), which includes the T-carrier, E-carrier, and J-carrier standards, depending on the country the system is deployed in. The local loop of the PSTN was provisioned as a twisted-copper-pair analog subscriber line.

Service Providers

Many abbreviations and acronyms are used to define the various players and the parts of the network in which they play. Some telcos can and do fulfill more than one of these functions, depending on the policy, regulatory, and licensing conditions that prevail in the area. The following terms are largely used in the United States, but they are important to the discussion in this chapter because they illustrate the functions service providers are addressing:

• PTO PTO stands for public telecommunications operator, which is the name for an incumbent carrier in places other than the United States. Often, these are government owned or controlled. In some countries, the more proper term is PT&T, for post, telegraph, and telephone.
• VAN provider VAN stands for value-added network. The term VAN provider originated around 1970 and was applied to companies competing to provide telecommunications services, specifically with offerings focused on data communications and data networking. VANs provided more than a simple pipe from point A to point B. They provided some additional intelligence in the network, to, for example, perform error detection and correction, or to convert protocols or languages that different computers speak so that you could have interoperability across the network. Value-added services (VAS) provider is another term used to define such providers.
• LEC In the local environment we use the acronym LEC for local exchange carrier. There was originally no competition among LECs, but as soon as competition in the local loop picked up, LECs were segmented into ILECs, CLECs, and DCLECs.
• ILEC The ILEC is the incumbent local exchange carrier, the original common carrier that either once had or, in some countries, still has monopoly rights in the local loop. For most residents in the United States, this would be one of the three "baby Bells"Qwest Communications International, AT&T, and Verizon Communications.
• CLEC The CLEC is the competitive local exchange carrier. CLECs came about as a result of the Telecommunications Act of 1996, which opened up competition in the local loop. The CLEC is the competitor to the ILEC. Although the decline of the telecommunications economy in 2000 and 2001 forced several CLECs out of business, or made some ripe for acquisition by ILECs, there are still some CLECs in the United States, and they currently focus on delivering dial tone to business customers.
• DCLEC (or DLEC) DCLEC stands for data competitive local exchange carrier. A DCLEC is a company specifically focused on supporting data services (e.g., providers that offer DSL services to end users). Most of these providers have ceased operating.
• ELEC ELEC stands for Ethernet local exchange carrier. The ELEC specializes in providing Ethernet solutions in the local loop and metro area.
• IXC The interexchange carrier is the carrier for long-distance and international communications. AT&T Corporation, MCI (now part of Verizon), Sprint, Qwest, and Verizon are the primary IXCs in the United States. Unless certain stringent requirements imposed by the Federal Communications Commission are met, an IXC cannot offer long-distance services in the areas where it is also the ILEC.
• SP Because so many lines are being blurred today by bundled services and bundled territories of operation, the basic term service provider is commonly used to refer generically to providers of different types of services.

Network Access

Figure 4.1 is a simple diagram that shows network access. On the left-hand side is the customer environment, which includes residences (single-line instruments being served by an access line) and business premises (with onsite telephone systems such as private branch exchange [PBX] or key telephone systemssmaller site systems for installations where there are 50 or fewer employees). Those in the customer environment are connected to the PSTN via access lines. The access network, also called the local loop, includes the network interface at the premises (also referred to as the demarcation point), the access line leading to the local exchange, the outside plant (e.g., telephone poles, drop lines), the components at the local exchange on which those access lines terminate (e.g., the line cards with a port associated with each telephone line), and the logic used to help control the flow of traffic over the access lines. Where competition is allowed in the local loop, a myriad of players may be interested in owning the local loop (e.g., ISPs, wireless operators, cable TV companies, power utilities). However, worldwide, the incumbent local providers continue to dominate the local loop, and, as usual, politics and economics are principal factors in delaying the mass deployment of high-speed residential access.

Figure 4.1. Network access

The local exchange, in the center of Figure 4.1, provides access to the backbone, or the core, of the network, which begins with the tandem switch and includes toll and international exchanges. From the local exchange, connections are established with the other providers, such as IXCs for long distance, international carriers for overseas calls, mobile providers, and ISPs.

The underlying network access facilities can be either analog or digital loops, and they connect the exchanges to the customer premises. At the customer premises there are the network interfaces, customer premises equipment (CPE), premises distribution systems where wiring is cross-connected, and network interfaces. The equipment for providing switch access services includes line-termination cards, carrier and multiplexer equipment, and local exchange switching capabilities that support addressing, supervisory alerting, call progress, and other signaling functions.

Access Services

The main categories of access services are trunks, business lines for key telephone systems, centrex service, leased lines, and residential subscriber lines.

Trunks are used to provide connections into the PBX environment. There are three subcategories of trunks:

• Two-way local exchange trunks On these trunks, traffic flows in both the incoming and outgoing directions.
• DID trunks Direct inward dialing (DID) trunks are designed for only incoming calls. A benefit of DID trunks is that they enable the dialed number to ring directly on a user's phone rather than having to go through a centralized attendant, with the connection being established within the PBX switch. If people know whom they want to call directly, this can be a very useful feature to ease the process of connecting the call. Another benefit of DID trunks is that they make it seem like a private line goes directly to the user, but DID can support perhaps 100 different numbers with a group of only 25 to 35 trunks (traffic engineering is used to determine the proper number of trunks).
• DOD trunks Direct outward dialing (DOD) trunks are used specifically for outgoing calls. DOD trunks are used when you dial an access code such as the number 9 or the number 8 to get an outside-line dial tone before you can dial the actual number that you want to reach.

To service the key telephone systems, business lines connect the network termination at the user end to the local exchange. Users who want to use the local exchange as if it were their PBX rent centrex trunks on a monthly basis. Large companies often access the network via leased lines, which can be a very expensive solution, and home users access the network via residential subscriber lines.

Access lines can be either analog facilities or digital carrier services. Analog transmission is often called plain old telephone service (POTS). Three main types of digital services are offered over twisted-pair cable:

• Digital services involving T-1 access (at 1.5Mbps), E-1 access (at 2.048Mbps), and J-1 access (at 1.544Mbps)
• Narrowband ISDN (N-ISDN) services, including Basic Rate Interface (BRI) for residences and small businesses and Primary Rate Interface (PRI) for larger businesses
• xDSL services, including high-speed lines that enable the all-important applications of Internet access and multimedia exploration

Chapter 2, "Traditional Transmission Media," describes these types of access in more detail.

Transport Services

Transport services are the network switching, transmission, and related services that support information transfer between the originating and terminating access facilities. The underlying facilities include local exchanges and tandem switches, toll and transit switches, international gateways, and interoffice transmission equipment. Transport services include switched services, nonswitched services, and virtual private networks (VPNs).

Switched Services

There are two main types of switched services: public and private. Switched public services include local calling, long-distance calling, toll-free calling, international calling, directory assistance, operator assistance, and emergency services.

Switched private services can be switchable either because they are deployed within the CPE or because they are deployed on a carrier basis. With CPE-based services, you can add capabilities to the telephone systems onsite in the PBXsa feature called electronic tandem networking. For example, you can use electronic tandem networking to gain some flexibility in routing around congestion points: If the preferred leased line from switch A to switch B is occupied or not available, the switch can decide how to reroute that traffic to still reach switch B, but through a different series of leased lines. However, because leased lines (also referred to as tie trunks) are mileage sensitive and dedicated to individual customers, they are very expensive; thus, not much private voice networking is done over tie trunks because there are several more attractive solutions, such as VPNs, which are discussed shortly.

With carrier-based switched private services, a centrex customer could partition and implement extensions across multiple local exchanges and thereby be able to switch traffic between those locations.

Nonswitched Services

Nonswitched services include leased lines, foreign exchange (FX) lines, and off-premises extensions (OPXs). With leased lines, two locations or two devices are always on, using the same transmission path.

FX lines enable a toll call to appear to be a local call. For example, you might have a dedicated leased line that runs from your customer premises to a local exchange in a distant area where you call large numbers of customers. When anyone behind your PBX dials a number associated with that foreign local exchange, the PBX automatically selects the FX line. The dial tone the caller receives is actually coming from the distant local exchange, and the call proceeds as if it were a local call. The tradeoff with FX lines is that although you are not charged per call for your long-distance calls to the specified exchange, you pay a flat monthly fee for the leased line, and you have to apply some traffic engineering to ensure that you're not making people wait for the FX line to become available. So with FX lines, you need to find the right balance between reducing costs and ensuring a high level of service.

OPXs are used in distributed environments, such as a city government. Say that the city government has public works stations, libraries, fire stations, and parks and recreation facilities that are too far from the PBX to be served by the normal cabling. The city uses an OPX setup: It connects a leased circuit from the PBX to the off-premises location and ties it in as if it were part of that PBX. City government employees can then call one another, using their normal extension plan, their call accounting information can be accumulated so that cost allocations can be performed, and the employees can access the full suite of features that a business PBX offers.

VPNs

Although you might think that VPNs are related to the Internet or to Internet Protocol (IP) and are a somewhat new development, they actually originated in the circuit-switched network environment, with AT&T's software-defined network (SDN) in the early 1980s. A VPN is a concept, not a technology platform or a set of networking techniques. A VPN defines a network in which customer traffic is isolated over shared-service provider facilities; as more customers share the same facilities, their costs go down. The purpose of a VPN, then, is to reduce the high cost of leased lines while still providing high quality of service and guaranteeing that private traffic has capacity between locations. Figure 4.2 shows an example of a VPN.

Figure 4.2. An example of a VPN

The underlying facilities of a VPN include the carrier public network, augmented by network control programs (NCPs) and service management systems (SMSs). Under computer control, the traffic is then routed through the public network in a manner that makes the VPN service seem like a facilities-based private network. Access to the VPN can occur via dedicated access, leased lines, or carrier-switched access, using either an analog or a digital carrier.

The NCP represents a centralized database that stores a subscriber's unique VPN information. The NCP screens every call and then applies call processing in accordance with the customer-defined requirements. A common-channel signaling network connects the various network elements so that they can exchange information with each other in real-time. (Common-channel signaling is discussed later in this chapter, in the section "Signaling Systems.")

An SMS is used to build and maintain the VPN database. It allows customers to program specific functions to accommodate their particular business applications. It transmits information to the NCPs, with important instructions on a customer-by-customer basis. Thus, VPNs introduce to the realm of the PSTN a lower-cost alternative to building a private voice network.

PSTN Architecture

The PSTN includes a number of transmission links and nodes. There are basically four types of nodes: CPE nodes, switching nodes, transmission nodes, and service nodes.

CPE Nodes

The term CPE node generally refers to equipment located at the customer site. The main function of CPE nodes is to transmit and receive user information. The other key function is to exchange control information with the network. In the traditional realm, this equipment includes PBXs, key telephone systems, and single-line telephones.

Switching Nodes

Switching nodes interconnect transmission facilities at various locations and route traffic through a network. They set up the circuit connections for a signal path, based on the number dialed. To facilitate this type of switching, the ITU standardized a worldwide numbering plan (based on ITU E.164) that essentially acts as the routing instructions for how to complete a call through the PSTN. The switching nodes include the local exchanges, tandem exchanges (for routing calls between local exchanges within a city), toll offices (for routing calls to or from other cities), and international gateways (for routing calls to or from other countries). Primary network intelligence is contained in the Class 4 switches (i.e., toll offices switches) and Class 5 switches (i.e., local exchange switches). The Class 4 switches provide long-distance switching and network features, and the Class 5 switches provide the local switching and telephony features that subscribers subscribe to. Figure 4.3 shows where the types of telephone exchanges are located.

Figure 4.3. Types of telephone exchanges

The Local Exchange

The local exchange (also called the Class 5 office or central office) is where communications common carriers terminate customer lines and locate the switching equipment that interconnects those lines. The local exchange represents the local network. Every subscriber line location in a local exchange is assigned a number, generally 7 digits (in the United States) or 8 digits (in many other countries). The first 3 (or 4) digits represent the exchange and identify the local exchange switch that serves a particular telephone. The last 4 digits identify the individual line number, which is a circuit that is physically connected from the local exchange to the subscriber. Some areas have gone to 10-digit dialing for local calls, meaning the area code must be included even when dialing a local number.

The traditional local exchange switch can handle one or more exchanges, and each exchange is capable of handling up to 10,000 subscriber lines, numbered 0000 to 9999. In large metropolitan areas, it is common to find one local exchange building that houses more than one local exchange switch and for each switch to handle five or more exchanges. These offices are sometimes referred to as multientity buildings.

The Tandem Office

The tandem office, or junction network, is an exchange used primarily as a switching point for traffic between local exchanges in a metropolitan area. It is an office used to interconnect the local end offices over tandem trunks in a densely settled exchange area where it is not economical for a telephone company to provide direct interconnection between all end offices. The tandem office completes all calls between the end offices but is not directly connected to subscribers.

The Toll Office

The toll office (also called the trunk exchange or transit switch) is a telephone company switching center where channels and toll message circuits terminatein other words, where national long-distance connections are made. This is usually one particular exchange in a city, but larger cities may have several exchanges where toll message circuits terminate.

The International Gateway

The international gateway is the point to and from which international services are available in each country. Protocol conversion may take place in the gateway; in ITU terminology, this is called a centre de transit (CT). CT1 and CT2 international exchanges connect only international circuits. CT2 exchanges switch traffic between regional groups of countries, and CT1 exchanges switch traffic between continents. CT3 exchanges connect switch traffic between the national PSTN and the international gateway.

Transmission Nodes

Transmission nodes, which are part of the transport infrastructure, provide communications paths that carry user traffic and network control information between the nodes in a network. The transmission nodes include the transmission media discussed in Chapter 2 as well as transport equipment, including amplifiers and/or repeaters, multiplexers, digital cross-connect systems, and digital loop carriers.

Service Nodes

Service nodes handle signaling, which is the transmission of information to control the setup, holding, charging, and releasing of connections, as well as the transmission of information to control network operations and billing. A very important area related to service nodes is the ITU standard specification Signaling System 7 (SS7), which is covered later in this chapter.