The Public Switched Telephone Network (PSTN)

Telecommunications networks rely heavily on the Public Switched Telephone Network (PSTN), which was originally created many decades ago for the transmission of human speech. The telephone system now provides many of the links that connect wide area networks, and is an integral part of the Internet itself. For about a hundred years, analogue signalling was used throughout the telephone system, which made it unsuitable for the transmission of digital data. Although modulation techniques were developed that allowed the transmission of digital data on an analogue telephone lines, there were severe limitations in terms of the data rates achievable. The latter part of the twentieth century, however, has seen the introduction of digital telephone exchanges and fibre-optic trunk exchange lines that have vastly increased the capacity of the telephone network.

When Bell first patented the telephone in 1876, telephones were sold in pairs, and had to be connected directly together using a pair of wires. It was obviously impractical to connect every telephone to every other telephone, so the Bell Telephone Company was established, and ran a wire to each customer's premises from a central office. To make a call, the customer would crank the phone. This caused a ringing sound in the telephone company office which attracted the attention of an operator, who would then manually connect the caller to the person being called using a jumper cable. It soon became possible to make long distance calls as connections were set up between telephone company offices in different cities. It soon became apparent that to connect every switching office to every other switching office directly was also impractical, so a second level of switching offices was introduced. The system has evolved into a highly redundant multi-level hierarchy, as illustrated below.

The telephone network hierarchy

The telephone network hierarchy

Each subscriber telephone has two copper wires coming out of it that run to the telephone company's local exchange office, a distance of up to ten kilometres. This part of the system is known as the subscriber loop, and in most areas of the world is now the only part of the telephone system that is still mostly analogue. If a subscriber calls another subscriber attached to the same local exchange, the switching mechanism inside the exchange sets up a direct connection between the two local loops, which remains in place for the duration of the call. If the two subscribers are connected to different local exchanges however, the call is routed from one local exchange to the other via one or more exchange trunk line.

The connections between exchanges are predominantly optical fibre, and signalling between exchanges is entirely digital. The subscriber loop, on the other hand, is likely to remain analogue for the forseeable future, due mainly to the huge cost involved in replacing it. Consequently, the analogue voice and data signals transmitted over a subscriber loop must be digitised before they can be relayed over exchange trunk lines, using a technique called pulse code modulation. The resulting digital channel has a data rate of 64 kbps.

The trunk lines between exchanges have a capacity that is a binary multiple of 64 kbps, so multiple incoming voice and data channels can be multiplexed at the local exchange onto an outgoing trunk using Time Division Multiplexing (TDM). Because of a failure to agree on an international standard for digital signalling hierarchies, the systems used in Europe and North America are different. The North American standard is based on the 24-channel T1 carrier, with a gross line bit-rate of 1.544 Mbps, whereas the European system is based on the 32-channel E1 carrier, with a gross line bit-rate of 2.048 Mbps. Trunk networks multiplex the basic rate channels (T1 or E1) together, forming higher capacity trunk lines that are multiples of these basic units. The standard channel rates used in Europe and North America are shown in the table below.

T-Carrier and E-Carrier Digital Signalling (DS) Levels
LevelNorth AmericanEuropean (CEPT)
DS0 (channel data rate)64 kbps64 kbps
DS11.544 Mbps (24 channels) - T12.048 Mbps (32 channels) - E1
DS1c (U.S. only)3.152 Mbps (48 channels) - T1c-
DS2 6.312 Mbps (96 channels) - T28.448 Mbps (128 channels) - E2
DS3 44.736 Mbps (672 channels) - T334.368 Mbps (512 channels) - E3
DS4 274.176 Mbp (4032 channels) - T4139.264 Mbps (2048 channels) - E4
DS5400.352 Mbps (5760 channels) - T5565.148 Mbps (8192 channels) - E5

The Plesiochronous and Synchronous Digital Hierarchies

The technologies used for the bulk transfer of digital data over telephone system core networks are Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH) or Synchronous Optical Network (SONET). The term plesiochronous is derived from the Greek words plesio, meaning near, and chronos, meaning time. This refers to the fact that different parts of a PDH network are almost synchronised, but not perfectly. Although PDH data streams are nominally transmitted at the same bit-rate, some variation in speed is allowed. One consequence of this variation in bit rate is that, in order to access a single channel within the data stream, it must be de-multiplexed completely back to the constituent channels. PDH is now being replaced by SDH in most European telecommunications networks, and by SONET (from which SDH is derived) in the United States. SDH allows individual channels within the data stream to be extracted or inserted at a network node without the need to completely de-multiplex the carrier. SDH also provides management featurers such as remote reconfiguration and monitoring. Although SDH and SONET are not directly compatible, they have been harmonised to facilitate inter-working between the two. The following table shows some of the common carrier rates for the two technologies.

SONET and SDH Transmission Rates
Optical LevelElectrical LevelLine Rate (Mbps)Payload Rate (Mbps)Overhead Rate (Mbps)SDH Equivalent
OC-3STS-3155.520150.336 5.184STM-1

Other rates (OC-9, OC-18, OC-24, OC-36, OC-96) are referenced in some of the standards. OC stands for Optical Carrier and defines the optical signal, STS stands for Synchronous Transport Signal and defines the equivalent electrical signal, and STM stands for Synchronous Transmission Module.