The PSTN was the earliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878–1929) is credited
with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and the number of
personnel required to deliver a specific level of service.
In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived
a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been
overtaken by the disruptive technology of the Internet. Only the oldest parts of the telephone network still use analog technology for anything
other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services
such as DSL, ISDN, FTTP and cable modem systems.
Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way
to go before this transition can be made. The QoS guarantee is one aspect that needs to be improved in the Voice over IP (VoIP) technology.
There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private
networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX).
Telephone exchange:
A telephone operator manually connecting calls with patch cables at a telephone switchboard.
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls.
A central office is the physical building used to house inside plant equipment including telephone switches, which make phone calls "work" in
the sense of making connections and relaying the speech information.
The term exchange can also be used to refer to an area served by a particular switch (typically known as a wire center in the US telecommunications
industry). More narrowly, in some areas it can refer to the first three digits of the local number. In the three-digit sense of the word, other
obsolete Bell System terms include office code and NXX. In the United States, the word exchange can also have the legal meaning of a local access
and transport area under the Modification of Final Judgment (MFJ).
Electromechanical Network signaling:
Circuits connecting two switches are called trunks. Before Signalling System 7, Bell System electromechanical switches in the United States communicated with one another over trunks using a variety of DC voltages and signaling tones. It would be rare to see any of these in use today.
Some signalling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between a
Panel switch and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending
dial pulses, equivalent to a rotary dial's pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20
pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. Using the
faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk
signaling. Multi-frequency (MF) was the last of the pre-digital methods. It used a different set of tones sent in pairs like DTMF. Dialing was
preceded by a special keypulse (KP) signal and followed by a start (ST). Variations of the Bell System MF tone scheme became a CCITT standard.
Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated
to further improve utilization. For example, one switch might send only the last four or five digits of a telephone number. In one case, seven
digit numbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-per-call savings). This
improved revenue per trunk and reduced the number of digit receivers needed in a switch. Every task in electromechanical switches was done in
big metallic pieces of hardware. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic.
Examples of signals communicating supervision or call progress include E and M signaling, SF signaling, and robbed-bit signaling. In physical
(not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require a hundred pair cable between switches, for example.
Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). In two-way trunks with E and M signaling, a
handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time. By changing the state of
these leads from ground to -48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would
send a signal to get ready for a call and the remote switch would reply with an acknowledgment to go ahead with dial pulsing. This was done
with relay logic and discrete electronics. These voltage changes on the trunk circuit would cause pops or clicks that were audible to the
subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a
second set of clunks when the called party answered. A second common form of signaling for supervision was called single-frequency or SF
signaling. The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for
a certain duration would go idle. (The duration requirement reduced falsing). Some systems used tone frequencies over 3,000 Hz, particularly
on SSB frequency division multiplex microwave radio relays. On T-carrier digital transmission systems, bits within the T-1 data stream were
used to transmit supervision. By careful design, the appropriated bits did not change voice quality appreciably. Robbed bits were translated
to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling,
or dial pulses, to be sent between electromechanical switches over a digital carrier which did not have DC continuity.
Problems listening to the file? See media help.A characteristic of electromechanical switching equipment is that the maintenance staff could
hear the mechanical clattering of Strowgers or crossbar relays. Most Bell System central offices were housed in reinforced concrete buildings
with concrete ceilings and floors. In rural areas, some smaller switching facilities, such as Community Dial Offices (CDOs), were sometimes
housed in prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflected sounds.
During heavy use periods, it could be hard to talk over the clatter of calls being processed in a large switch. For example, on Mother's Day in
the US, or on a Friday evening around 5pm, the metallic rattling could make raised voices necessary. For wire spring relay markers these noises
resembled hail falling on a metallic roof.
On a pre-dawn Sunday morning, call processing might slow to the point that one might be able to hear individual calls being dialed and set up.
There were also noises from whining power inverters and whirring ringing generators. Some systems had a continual, rhythmic "clack-clack-clack"
from wire spring relays that made reorder (120 ipm) and busy (60 ipm) signals. In Bell System installations, there were typically alarm bells,
gongs, or chimes. These would annunciate alarms calling attention to a failed switch element. Another noisemaker: a trouble reporting card
system was connected to switch common control elements. These trouble reporting systems would puncture cardboard cards with a cryptic code
that logged the nature of a failure. Remreed technology in Stored Program Control exchanges finally quieted the environment.
Maintenance Tasks:
The maintenance of electromechanical systems was partly DC electricity and partly mechanical adjustments. Unlike modern switches, a circuit
connecting a dialed call through an electromechanical switch actually had DC continuity. The talking path was a physical, metallic one.
In all systems, subscribers were not supposed to notice changes in quality of service because of failures or maintenance work. A variety
of tools referred to as make-busys were plugged into electromechanical switch elements during repairs or failures. A make-busy would
identify the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Subscribers
who got behind in payments would have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's
office equipment (Crossbar) or line group (step). The subscriber could receive calls but could not dial out.
Strowger-based, step-by-step offices in the Bell System were under continual maintenance. They required constant cleaning. Indicator lights
on equipment bays in step offices alerted staff to conditions such as blown fuses (usually white lamps) or a permanent signal (stuck off-hook
condition, usually green indicators.) Step offices were more susceptible to single-point failures than newer technologies.
Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an Originating Register)
would be connected to a call just long enough to collect the subscriber's dialed digits. Crossbar architecture was more flexible than step
offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, Automatic number identification had been
retrofitted to nearly all step-by-step and crossbar switches in the Bell System.
Electronic Network switches:
The first Electronic Switching Systems were not entirely digital. The Western Electric 1ESS switch still had reed relay metallic paths. It
was stored-program-controlled. Equipment testing, changes to phone numbers, circuit lockouts and similar tasks were accomplished by typing
on a terminal. Northern Telecom SP1, Ericsson AKE, Philips PRX/A, ITT Metaconta, British Telecom TXE series and several other designs were
similar. These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. They also
introduced a new form of data communications: two 1ESS exchanges could communicate with one another using a data link called Common Channel
Interoffice Signaling, (CCIS). This data link was based on CCITT 6, a predecessor to SS7.
Digital Network switches:
A typical satellite PBX with front cover removed.Digital switches work by connecting two or more digital circuits together, according to a
dialed telephone number. Calls are set up between switches using the Signalling System 7 protocol, or one of its variants. In U.S. and military
telecommunication, a digital switch is a switch that performs time division switching of digitized signals. This was first done in a few
small and little used systems. The first product using a digital switch system was made by Amtelco. Prominent examples include Nortel DMS-100,
Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephone exchange. With few exceptions, most switches built since the 1980s are digital,
so for practical purposes this is a distinction without a difference. This article describes digital switches, including algorithms and equipment.
A digital exchange (Nortel DMS-100) used by an operator to offer local and long distance services in France. Each switch typically serves
10,000-100,000+ subscribers depending on the geographic areaDigital switches encode the speech going on, in 8000 time slices per second. At
each time slice, a digital PCM representation of the tone is made. The digits are then sent to the receiving end of the line, where the
reverse process occurs, to produce the sound for the receiving phone. In other words, when you use a telephone, you are generally having
your voice "encoded" and then reconstructed for the person on the other end. Your voice is delayed in the process by a small fraction of
one second — it is not "live", it is reconstructed — delayed only minutely. (See below for more info.)
Individual local loop telephone lines are connected to a remote concentrator. In many cases, the concentrator is co-located in the same
building as the switch. The interface between remote concentrators and telephone switches has been standardised by ETSI as the V5 protocol.
Concentrators are used because most telephones are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated
into only tens or hundreds of shared connections.
Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone
switches. These complex machine (or series of them) in a central exchange building are referred to as "carrier-level" switches or tandems.
Some telephone exchange buildings in small towns now house only remote or satellite switches, and are homed upon a "parent" switch, usually
several kilometres away. The remote switch is dependent on the parent switch for routing and number plan information. Unlike a digital loop
carrier, a remote switch can route calls between local phones itself, without using trunks to the parent switch.
Telephone switches are usually owned and operated by a telephone service provider or carrier and located in their premises, but sometimes
individual businesses or private commercial buildings will house their own switch, called a PBX, or Private branch exchange.
Fully-connected mesh network:
One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully-connected mesh
network. This is the method usually used in central office switches, which have low utilization of their resources.
Mobile (Cell) Phones,
Mobile phone features,
Cell Phone Usage,
Cell Phone Business Models,
History of Cell Phones, 3G,
History of the Telephone,
History of the Telephone Controversy Debate and Patents
Source References and additional reading:
- National Park Service "first switchboard".
- Calvert, J. B. (2003-09-07). "Basic Telephones".
- Calvert, J. B. (2003-09-07). "Basic Telephones, The Switchboard (ringdown is near bottom)".
- Connected to a switch, an off-hook condition operates a relay to connect a dial tone and a device to collect dialed digits.
- Federal Standard 1037C and from MIL-STD-188.
- Wikipedia.org, the free encyclopedia
