Inside View of BSNL (Telephone Exchange Server room)
Local Exchange Components
The Telephone System Hierarchy
The public-switched telephone network (PSTN) is organized as a multilevel hierarchy. The original telephone system used five numbered levels, as shown on the Original Telecommunications Hierarchy Diagram.
Class 1: Regional Centers
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Class 2: Sectional Centers
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Class 3: Primary Centers
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Class 4: Toll Offices
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Class 5: End Offices Tandem Offices
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Customers
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Original Telecommunications Hierarchy
While Class 2 and Class 3 offices are seldom used in today's system, the original numbering system has survived. Therefore, each top-level Class 1 office usually connects to multiple Class 4 offices, skipping the old Classes 2 and 3. Each Class 4 office, in turn, connects to multiple Class 5 offices. The Class 5 offices, or end offices, connect to individual subscribers, as shown on the Current Telecommunications Hierarchy Diagram.
Current Telecommunications Hierarchy
Class 5 Central Office: The Local Exchange
The Class 5 CO is also called the end office or local office. It is the local workhorse for the telephone and data communications traffic in one local exchange. When you pick up your telephone at home, you receive dial tone from a Class 5 CO. There are currently about 1,500 Class 5 local exchanges in the United States.
The Class 5 office is the only office that connects to individual or business subscribers. Offices higher in this hierarchy have only lower level COs as their subscribers. However, each Class 5 CO also connects to other nearby Class 5 offices, as well as its "parent" Class 4 office one level up in the hierarchy.
If a subscriber places a call to another subscriber connected to the same Class 5 office, that office makes the connection directly, as shown on the Call Within the Same Exchange Diagram.
Call Within the Same Exchange
If the caller's Class 5 CO is directly connected to the destination Class 5 CO, the calling CO passes the call directly to the destination CO, which completes the call to the destination subscriber, as shown on the Calling Handoff Diagram.
Calling Handoff
However, if the destination CO is not directly connected to the calling CO, or if that connection is too busy, the caller's Class 5 CO passes the call up the hierarchy to its parent Class 4 office, as shown on the Tandem Switching Diagram.
Tandem Switching
Class 4 Central Office
Each Class 4 CO connects to multiple Class 5 offices. Each Class 4 office also connects to nearby Class 4 offices as well as its parent Class 1 office. This interconnection provides alternate paths for calls in the event of a cable outage or, more commonly, congestion on the network. For example, AT&T network engineers can reroute traffic, even down to the telephone number, when an event such as a football game or conference dramatically increases the number of calls to/from a specific location.
AT&T, MCI, Sprint, and other IXCs, for the most part, use only Class 4 CO systems. Therefore, we often call these companies Class 4 networks. AT&T's network includes approximately 140 Class 4 CO systems.
Class 4 offices work together in a similar way as Class 5 offices. If two Class 5 COs are both connected to the same Class 4 office, the Class 4 office can switch calls between them. In addition, if the caller's Class 4 CO is directly connected to the destination Class 4 CO, the calling Class 4 CO passes the call directly to the destination Class 4 CO, which passes it to the destination Class 5 CO and the destination subscriber.
However, if the destination Class 4 CO is not directly connected to the calling Class 4 CO, or if that connection is overused, the caller's Class 5 CO passes the call up the hierarchy to its parent Class 1 office.
Switch Hierarchy
Tandem Central Offices
As the number of Class 5 COs in a region increases, it becomes impractical to connect every Class 5 CO to every other Class 5 CO. Some COs carry too little traffic between them to justify the cost of a direct connection. Also, the number of connections becomes unmanageable as the number of offices grows, as shown on the Direct Connections Diagram. As you can see from the Switching Hierarchy Diagram, six direct connections are needed to link four offices; each office also needs a separate connection to its parent Class 4 office.
Direct Connections
To solve this problem, tandem offices are used to connect Class 5 COs. Toll tandem offices, also called interexchange tandem offices, connect Class 5 COs to the Class 4 offices of the IXCs. A special type of tandem switch, known as a gateway, interconnects the telephone networks of different countries when their networks are not compatible.
As you can see on the Tandem Central Offices Diagram, tandem offices provide a more efficient way to connect multiple Class 5 COs to one another and interexchange carrier switches. Each tandem office connects to other nearby tandem offices and COs, forming a web of interconnections.
Tandem Central Offices
Tandem COs are considered a type of Class 5 office; however, they do not have direct connections to end users. They serve as intermediaries, switching high volumes of calls between Class 4 and 5 offices.
Class 1 Offices
Class 1 offices, or regional toll telephone offices, formed the backbone of the telecommunications system and appear in only a few places in the country. However, purely Class 1 COs are older model switches, and are being replaced by a newer generation of Class 4/Class 5 switches. As these upgrades continue, and Class 1 switches are replaced, the telephone network hierarchy is becoming more "shallow." Rather than reserving the biggest and fastest transmission connections for a few Class 1 offices, telecommunications companies are providing those high-speed connections to their Class 4 COs.
Each of the lower class offices can pass calls up the hierarchy if a connection has failed or is overloaded. However, calls must be handled at the Class 1 level, because there is no higher level to take them. Therefore, a Class 1 office is also known as an "office of last resort."
Evolution of Telephone Switching
Central Office Switching have gone through a number of fundamental technological changes, as illustrated in the Telephone Switching Evolution Table.
Telephone Switching Evolution
Switching System
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Operation
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Method of Switching
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Type of Control
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Type of Network
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1878 Manual Operator
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Manual
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Space/analog
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Human
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Plug/cord/jack
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1892 Step-by-Step
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Elecromechanical
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Space/analog
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Distributed stage-by-stage
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Stepping switch train
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1918 Crossbar
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Electromechanical
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Space/analog
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Common Control
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X-bar switching
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1960 Lucent ESS - First Generation
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Semielectronic
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Space/analog
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Common Control
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Reed switch
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1972 Lucent ESS - Second Generation
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Semielectronic
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Space/analog
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Stored program control
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Reed Switch
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1976 Lucent ESS - Third Generation
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Electronic
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Time/digital
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Stored program common control
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Pulse code modulation
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Manual Switching System, circa 1880
One hundred years ago, all calls were connected manually. In the late 1800s, each pair of wires from a customer's telephone (each local loop) was terminated in a plug at an operator's console as illustrated on the Tip and Ring Plug Diagram.
Tip and Ring Plug
Each wire of the local loop pair was connected to a different part of the operator's plug. One wire attached to the tip of the plug, and the other to a ring slightly farther back. Each jack, or socket, of the operator's switchboard contained two metal contacts. When the plug was inserted into the jack, the electrical circuit was completed.
The terms "tip" and "ring" are still used to describe each wire of a local loop, and the analog local loops that provide most "plain old telephone service" (POTS) are commonly called tip and ring circuits.
Operator Plug/Tip and Ring
To place a call, a customer lifted the handset to request service from the CO. When the operator answered, the caller would request the called party by name. The attendant would then plug the caller's wire pair into a horizontal bar line as illustrated on the Operator Plug/Tip and Ring Diagram. He (the first operators were young men) would then yell to the operator who handled the called customer; that operator would connect to the bar and finish setting the call. When the call was completed, another operator would yell to all in the room that the line was clear again.
Of course, this process was slow, prone to error, and labor intensive. It was only practical when relatively few people had telephones. Some experts estimate that, to manually process today's volume of calls, half the people in the world would have to be telephone operators. Fortunately, someone came up with much better ideas before that was necessary.
Step-by-Step Switching and Rotary Dial Telephones
The first automated telephone switching was invented in 1891 (or 1893 depending on which reference book you read) by Almon B. Strowger, a funeral director in St. Louis. He was losing business to another funeral company, because the telephone operators were being bribed to manually switch calls to his competitor.
His telephone switch, called a "step-by-step," "stepper," or "Strowger switch," was actually a system that combined a new type of telephone set with an automated switch. To make the two of these work together, each telephone was assigned a unique number.
The new telephone featured a 10-digit rotary dial. To call another party, a caller would first lift the handset to establish the circuit, then dial the digits of the telephone number one at a time. The rotary dial would send a series of pulses, or clicks, down the circuit to the switch: one pulse for the number "1," three pulses for the number "3," and so on.
At the CO, the Strowger switch contained banks of 10-level relays. Each bank of relays represented one of the numbers in a telephone number, as shown on the Step-by-Step Switching Diagram.
Step-by-Step Switching
Each pulse from the caller's telephone would advance a relay bank by one step. For example, if a customer first dialed a "4," the first relay bank would advance four steps. During the pause between sets of pulses, the next relay bank would be connected, ready for the second digit. When the last digit was dialed and recorded, the combination of the various relay connections formed the talking path to the called party.
Step-by-Step Switching System
The step-by-step switch brought greater speed, reliability, and privacy to telephone use. These switches began to be phased out in the 1950s, but a few steppers are still in use today.
Crossbar Switching
The crossbar switch, installed for the first time in 1937 in Brooklyn, N.Y., used pairs of electromechanical relays (horizontal and vertical) to form a gridwork of potential talking paths. The Crossbar Switching Diagram is a highly simplified description of this complex switch. In the diagram, you can see how contact is made at point A3 by activating the relays for "A" and "3." This common control could be used repeatedly to set up and tear down calls and never sit idle.
Crossbar Switching
After reaching its peak of installed lines in 1983, the crossbar switch is now largely obsolete because it takes up a lot of space and is not programmable.
Crossbar Switching System
Later Electromechanical Switching
When electronics came along, the electromechanical control of the common control system was replaced with electronics; the network, or switch matrix, was usually replaced with tiny glass-encapsulated reed switches. Only a part of the switch was electronic.
In the next generation, the stored program operation of a digital computer was applied to the switch, although the network remained a complex of reed switches.
Digital Switches
In the next generation of fully digital switches, the talking path through the switch was no longer an electrically continuous circuit. Instead, the sound pattern of speech was digitized into a digital stream of 1s and 0s. However, the local loop between the CO switch and customer remained an electrically continuous analog circuit. (We will explain the difference between analog and digital signals in a later lesson.)
Nortel invented the first all-digital CO switching system in 1979, followed by the Lucent (then AT&T) 4ESS switching system.
Prior to Divestiture in 1984, AT&T set telephony standards by means of its research arm, Bell Laboratories. All CO switches, and all lines that carried calls, had to meet prescribed standards. Today's competing hardware manufacturers continue to follow those standards, and cooperate to extend them, so that anyone with a telephone can talk to anyone else. Dialing, ringing, routing, and telephone numbering all conform to uniform standards and plans.
DTMF Signaling
As you saw above, the first step-by-step switches were designed to work with rotary-dialed telephones. Those telephones used "dial pulse" signaling, which produced short, regular interruptions of the direct current flowing between a telephone and switch. The number of interruptions, or pulses, corresponded to the value of the digit. In other words, when you dial the number 5, you hear five clicks.
As CO switching went digital, telephone sets also improved the way they transmitted telephone numbers. The dual tone multifrequency (DTMF) system, commonly called touch tone, uses a pad of 12 buttons. When pressed, each button sends out a combination of two pure tones not found in nature: one high-frequency and one low-frequency. The DTMF Touchpad and Tones Diagram illustrates this concept.
DTMF Touchpad and Tones
By assigning one tone to each row and column, only seven unique tones are needed to identify each of the 12 buttons. These tones can easily be detected by a telephone switching system.
Namdev Pratap is Engineering student ,he is an active blogger and a Space enthusiast, He has Involved in many theoretical research in astronomy and space science,he has served as Volunteer for SETI Home for a period of 5 years in BOINC(Berkley Open Infrastructure for Network Computing).
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