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 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.

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 electronic equipment that converts speech into digital data and provides a multitude of sophisticated 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 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 Network Architecture and Protocols Introduction Summary

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.