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Telephone exchange

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Note regarding usage: many of the terms in this article have conflicting UK and US usages.

central office = Exchange building in the U.S.
telephone exchange = Exchange building in the UK, and is also the UK name for a telephone switch, and also has a technical meaning in U.S. telecoms
telephone switch is the U.S. term, but is in increasing use in technical UK telecoms usage, to make the CO/switch/concentrator distinction clear

The term exchange can also be used to refer to an area served by a particular switch. And more narrowly, in U.S. usage can refer to the first three digits of the local number. In the past, the first two digits would map to a mnemonic exchange name, e.g. 869-1234 was TOwnsend 9-1234.

In the United States, the word exchange can also have the technical meaning of a local access and transport area under the Modification of Final Judgment (MFJ).

In U.S. and military telecommunication, a digital switch is a switch that performs time-division-multiplexed switching of digitized signals. Source: from Federal Standard 1037C and from MIL-STD-188. All switches built since the 1970s are digital, so for practical purposes this is a distinction without a difference. This article describes digital switches, including algorithms and equipment.

This article will use the terms:

telephone exchange for the building
telephone switch for the switching equipment
concentrator for the concentrator, whether or not it is co-located with the switch

Automatic telephone exchanges came into existence in the early 1900s. They were designed to replace the need for human telephone operators[?]. Before the exchanges became automated, operators had to complete the connections required for a telephone call. Almost everywhere, operators have been replaced by computerized exchanges.

An exchange automatically senses an off hook (tip) telephone condition, provides dial tone to that phone, receives the pulses or DTMF tones generated by the phone, and then completes a connection to the called phone within the same exchange or to another distant exchange.

The exchange then maintains the connection until a party hangs up, and the connection is disconnected. Additional features, such as billing equipment, may also be incorporated into the exchange.

Early exchanges used motors, shaft drives, rotating switches and relays. Some types of automatic exchanges were Strowger, All Relay, X-Y, Panel and Crossbar.

A telephone switch is the brains of an exchange. It is a device for routing calls from one telephone to another, generally as part of the public switched telephone network). They work by connecting two or more digital virtual circuits together, according to a dialed telephone number.

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 concentrators and telephone switches has been standardised by ETSI as the V5 protocol.

Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. Usually a complex machine (or series of them) in a central exchange building, these are referred to as "carrier-level" switches.

Most former telephone exchange buildings now house only remote concentrators for the "parent" switch, usually several kilometres away. In certain cases an concentrator may be "remote parented" on an switch hundreds of kilometres away. This is done to increase the resiliency of the network, so that even a total regional outage of the telephone system will leave some essential telephone lines working.

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 (which may well be owned and operated by a telephone service provider still).

The switch's place in the system

Telephone switches are a small part of a large network. The most expensive thing is to rewire. Much of the modern switching fabric is actually outside the telephone exchange building.

Up to several hundred telephones attach to a remote concentrator. In the U.S., you and your neighbors share a concentrator in a little box near your houses. In Europe, the building that once contained your local Strowger telephone exchange is usually empty except for a remote concentrator -- the switching mostly occurs elsewhere.

When you pick up the phone, the concentrator produces dial tone. When you dial, it reads the tones. When you're done dialing, the concentrator's microcomputer sends the dialing data to the central switch, which allocates a time slot for the dialing phone on the wire pairs that pass through the concentrator and through the switch.

The trick is that after the central switch tells the concentrator which time slot to use, the concentrator "opens" a time-slot on the loop to a local phone. The allocated time slot on the wiring into the concentrator is used to send data from the remote telephone's microphone to the local telephone's speaker. The allocated time slot on the wiring out of the concentrator (with the same time slot number) carries data from the local microphone to the remote speaker.

So, to arrange a connection, the switch just completes the circuit between your phone and the remote phone. It interchanges the data from one to the other. Telephone "exchange" is exactly correct terminology.

Switches are used in both local central offices and in long distance centers.

Table of contents

1 Switch design

2 Switch control algorithms

3 Topological sort

4 Fault tolerance

Switch design

Long distance switches may use a slower, more efficient switch-allocation algorithm than central offices, because they have near 100% utilization of their input and output channels. Central offices have more than 90% of their channel capacity unused.

While traditionally, telephone switches connected physical circuits (e.g., wire pairs), modern telephone switches use a combination of space- and time-division switching. In other words, each voice channel is represented by a time slot (say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, the telephone switch interchanges the information between A1 and B2. It switches both the time slot and physical connection. To do this, it exchanges data between the time slots and connections 8000 times per second, under control of digital logic that cycles through electronic lists of the current connections. Using both types of switching makes a modern switch far smaller than either a space or time switch could be by itself.

The structure of a switch is an odd number of layers of smaller, simpler subswitches, interconnected by a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In most designs, a physical (space) switching layer will alternate with a time switching layer. The layers are symmetric, because every call is symmetric (there's a connection in both directions).

A space-division subswitch uses digital multiplexers controlled by a cyclic memory. This takes physical space for the wiring.

A time-division subswitch reads a complete cycle of time slots into a memory, and then writes it out in a different order, also under control of a cyclic computer memory. This causes some delay in the signal.

Switch control algorithms

The scarce resources in a telephone switch are the connections between layers of subswitches. The control logic has to allocate these connections.

The connections consist of both time slots and wires. The first thing to try is to search for a subswitch that contains the needed in and out connections. There are two design paths to go if this simple search fails.

One way is to have enough switching fabric to assure that the pairwise allocation will always succeed. This is the method usually used in central office switches, which have low utilization of their resources.

Topological sort

Another way is to have a minimal switching fabric that still can theoretically make all the connections, and reorganize the switch's connections when a new connection won't fit.

If a subswitch with the needed pair of connections can't be found, a pair of subswitches will still have the necessary in and out, because there has to be at least the same number of connections between each layer of the switch, or else the switch will not be able to complete a full set of connections.

The pair of subswitches' connections can be reorganized with a clever algorithm called a topological sort, so that all the existing connections continue, though they might migrate between the two different subswitches. This is the method usually used in long distance switches, which have high utilization of their switching fabric.

A topological sort picks two subswitches. One has a needed input connection. The other has a needed output connection. The connections of both subswitches are placed in a list that also includes the desired new connection.

In the list, the basic trick is to trace connections. Starting from some input or output, the computer traces a connection to an output, then traces the other connection at that output to an input, and so forth, until it comes to an end. Each time it traces from input to output, the connection is placed in one subswitch, and removed from the list. When it traces from output to input, the connection is placed in the other subswitch and removed from the list. To complete correctly, tracing must begin with single connection inputs and outputs, and only then trace double-ended inputs and outputs, which might form loops.

Fault tolerance

Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt old calls that remain working. As calls are ended, the subswitch then becomes unused. Some time later, a technician can replace the circuit board. The next test succeeds, the connections to the repaired subswitch are marked "not in use", and the switch returns to full operation.

To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using first-in-first-out lists. That way, when a disgusted customer hangs up and redials, they will get a different set of connections and subswitches. A last-in-first-out allocation of connections might cause a continuing string of very frustrating failures.