Public Switched Telephone Network: Switching from the 1900s to 2000 - Coursework Example

Computer Sciences and Information Technology Public Switched Telephone Network: Switching from 1900s to 2000 Purcell Donna Order# 546558 Theinformation covers a brief history of POTS telephone systems and its transition to the PSTN telephone system from the early 1900s until present. The information also covers the various switching systems and how they have been modified over the years and upgraded from manual, automatic electromechanical, electronic and digital. Information in the essay covers AT&T’s panel switches, crossbar switches, the Strowger switch and the early phones used by Bell Telephone Company. Protocols are mentioned in relation to switch operations and how they are used. A definition of multiplexing and SONET are given in relation to the Internet. Switch algorithms and fault tolerance are briefly covered as well as the present and future outlook for Internet VoIP systems. Keywords: dial-tone availability, five nines, supervision, electromechanical, trunk, Panel call indicator pulsing, encoded, remote, satellite, parent, multiplexing Sonet. Computer Sciences and IT: Switching from 1900s to 2000 Western Union’s greatest threat became reality when a new technology the “talking telegraph” was patented in 1876. Alexander Graham Bell offered the patent to Western Union for $100,000; however, the company opted for higher dividends instead of expansion (Tomas Nonnenmacher, 02 January 2010). “The Bell Telephone Company was formed in 1877, which became the first predecessor company to AT&T, issuing stock to seven original shareholders.”(AT&T History, 2011). In 1879 the first telephone exchange or switch opened in New Haven, Connecticut under the license of Bell Telephone (AT&T History, 2011). The first systems, most of which have since been replaced by newer equipment, was called POTS or Plain Old Telephone Service. This was and still is, in some parts of the world, voice-grade telephone service that remains the basic form of residential and small business service in the telephone network. (Wikipedia, 17 May 2011). POTS systems have been available almost since the introduction of the public telephone system in the late 1900s. These systems are still used as the basic telephone service even after the convergence of more advanced forms of telephony such as ISDN (Integrated Services Digital Network), mobile phones, and VoIP (Voice Over Internet Protocol). (Wikipedia, 17 May 2011). The POTS system has remained virtually unchanged to the normal user even with the development of touch-tone dialing, electronic telephone exchanges and fiber-optics integration into the public switched telephone network (PSTN). Originally the system was known as the Post Office Telephone Service or System internationally (POTS). As telephone services were removed from the control of national post offices, the term was dropped (Wikipedia, 17 May 2011). In the United States the pairs of wire that connection from the central switch office to a subscribers home is called a subscriber loop. Typically this is powered by a 48V direct electrical current and backed by a bank of batteries in the central office. The loop usually carries a load of about 300 ohms and does not pose a threat of electrocution to human beings. (Wikipedia, 17 May 2011). PSTNs circuits are continually being advanced by digital updates; however, these changes have been basically unknown to the POTS customer. In most cases, the operation of the POTS loop is practically unchanged and remains compatible with old pulse dialing (landline) telephones, even some dating back to the early 20th century. Even though POTS provides limited features, low bandwidth and no mobile service, it is more reliable than other updated systems. (Wikipedia, 17 May 2011). Many service providers attempt to achieve “dial-tone availability” more than 99.999 percent of the time the phone is off the hook. This is called the “five nines” reliability standard, which equates to being able to obtain a dial tone all but less than five minutes each year (Wikipedia, 17 May 2011). Within the telecommunications field, a telephone exchange or telephone switch consists of a system of electronic components that allows telephone calls to connect. The physical building, or the central office, holds the inside plant equipment. Exchange area refers to the area that handles the particular switch, and is sometimes know as the wire center in the US. The exchange code or Central Office Code is the first three digits of the local number (NXX). This is sometimes confused with the area code (NPA). Local access and transport areas (LATA) make up the legal entity for local exchange areas within the United States under regulations of Modification of Final Judgement (MFJ). (Wikipedia, 31 May 2011). Manual Service Exchanges were the first switch systems that required the customer to lift the receiver off the hook and ask an operator to connect the call. If the number was in the same central office, the call was connected by plugging into the jack corresponding to the called customer’s line. However, if the call were in another central office, the operator plugged into the trunk for the other office and requested a connection. This operator was known as the “inward operator.”(Wikipedia, 32 May 2011). In 1879, the first telephone exchange was opened in Atlanta as the National Bell Telephone Company. The switchboard was on the top floor of the Kimball House, which made it easier for wires to be strung out of the building to telephone poles. The exchange handled about 25 lines and most of the lines were shared party lines with 2 or 3 subscribers, for a total of over 60 subscribers. At first there were no numbers; the operators memorized the names of the subscribers. As subscribers increased, this system became obsolete, and numbers were introduced (Atlanta Telephone History, Part 1, Early Telephone Service). The first phones were called a Butterstamp phone, which had a receiver/transmitter. You had to talk into it and quickly place it to your ear to listen; the button was to signal the operator. The sound quality was very poor and was soon replaced with the Magneto Crank Phone and the Blake Transmitter. Two large batteries replaced by a telephone employee every six months powered the Magneto Phone. The first batteries were wet cell and would sometimes leak, but were replaced later with dry cell batteries. (Atlanta Telephone History, Part 1, Early Telephone Service). Most exchanges were common-battery; meaning the central office provided power for the circuits. The pair of wires from the manual exchange to the subscriber’s phone carried a –48VDC across the conductors. When the subscriber was off-hook, the phone incorporated a DC resistance short across the line. In Manual Service, the current would flow through a relay coil activating a buzzer and lamp on the operator’s switchboard, telling the operator the subscriber needed service. (Wikipedia, 31 May 2011). Smaller towns with Manual service used magneto phones featuring up to 10 or more subscribers sharing a single line. Anyone could pick up and listen if they wanted as everyone could hear the rings. One American small town, Bryant Pond, Woodstock, Maine, were still using the Magneto system as late as 1983. (Wikipedia, 31 May 2011). Automatic Exchanges or dial service were introduced in the early 1900s. Dial service eliminated the need for many operators. The telephone switch is the central brain of an automatic exchange. This device is used for routing calls from one phone to another and is usually part of the PSTN. An off hook condition is automatically sensed, a dial tone is provided to that phone, and DTMF tones are generated by the phone, and the connection to the called phone in the same exchange or to another exchange is completed. (Wikipedia, 31 May 2011). This tracking is called supervision. In Bell System dial service a feature called automatic number identification (ANI) was started allowing service for automatic billing, toll free 800 numbers, and 911 service. In Manual Service the operator knew where the call originated by the light on the switchboard. ANI did not exist in early dial service. (Wikipedia, 31 May 2011). Multiple switchboards were introduced around 1887 to solve the problems of shouting at more than one operator across the switchboard. Long distance calls were sent to an operator queue, the operator asked the party’s number, and it was written on a toll ticket. On February 2, 1890 the first long distance line to Chicago was completed with Southern Bell in Atlanta. It was now possible to call Chicago, St. Louis, Kansas City, Indianapolis, and Pittsburgh (Atlanta Telephone History, Part 1, Early Telephone Service). Relays, shaft drives, motors and rotating switches were used at first. Switches were relay-logic computers in a very basic sense. Automatic exchanges were Strowger, All Relay, X-Y, Panel and crossbar switches. All of these are referred to as “electromechanical” switches. Almon Strowger obtained a patent in 1891 for the first practical automatic telephone switch. (Hochheiser, Sheldon, 15 November 2010). The Strowger switch allowed the subscriber to dial directly and established the circuit for the duration of the call. The switch had 10 x 10 bank of contacts; therefore, the earliest switch could handle only 100 telephones. After 1910, Strowger switches began spreading to Europe. The Strowger switch, also known as the step-by-step switch, remained as the most widely used switch until the 1960s. Further improvements were made by Keith and Erickson in switches as well as Erickson’s patent for the telephone dial (Sheldon Hochheiser, 22 September 2010). Electromechanical Signaling is very rarely seen at all today. These switches were used before Bell’s Signaling System 7. The circuits connecting two switches are called trunks and electromechanical switches connected these trunks using a variety of DC voltages and signal tones. Early forms of electromechanical switches were called Panel Call Indicator Pulsing that used pulses to set up calls between a panel switch and a manual switchboard. The most common form of dialed digits between electromechanical switches was sending dial pulses, which were the same as rotary dial pulsing. However, they were sent over trunk circuits between switches. DTMF or dial tone multi-frequency was not used for trunk signaling, however, MF or multi-frequency was used last before digital began. (Wikipedia, 31 May 2011). Examples of signals communicating supervision include E and M signaling, SF signaling and robbed-bit signaling. In E and M trunk circuits the trunks were four wire and fifty trunks would require a hundred pair cable between switches. (Wikipedia, 31 May 2011). Conductors in one circuit configuration were named tip, ring, ear and mouth. However, in two-way trunks with E and M signaling, a handshake took place as a preventative measure to keep both switches from colliding by dialing on the same truck at the same time. The ground leads were changed to –48 volts so that the switches stepped through a handshake protocol. The DC voltage changes allowed the switch signal to get ready for a call and the remote switch would reply with a go ahead with dial pulsing. The procedure was done with relay logic and discrete electronics. (Wikipedia, 31 May 2011). One characteristic of electromechanical switches was that the maintenance staff could hear the clattering of Stowgers panel switches or crossbar relays. Wire spring relay marker noises resembled hail falling on a metallic roof. (Wikipedia, 31 May 2011). More shared or common control circuits were used by the crossbar offices. The first crossbar switches went into service in 1938 in New York City. (Hochheiser, Sheldon, 15 November 2010). J. N. Reynolds of Western Electric invented the crossbar selector. This changed from the large movement of the Strowger switches to small mechanical motions. (Hochheiser, Sheldon, 15 November 2010). The crossbar switch remained largely confined to the United States, however, the design of the #5 crossbar gained interest globally. Beginning in the 1950s manufacturers all over the world began producing their own crossbar switches adapted from the American design. (Hochheiser, Shelton, 15 November 2010). Up until then the Strowger switches were the backbone of the world’s telephone exchanges. This achieved AT&T’s goal of reducing cost for manufacturing and maintenance, plus it had more features that made it flexible and adaptable (Sheldon Hochheiser, 22 September 2010). AT&T’s Panel Switch was an extremely complex device. The tall panels covered 500 rows of terminals and each panel housed an electric motor to operate its 60 electromagnetically-controlled clutches. The change to fully automatic urban switching was made possible by a plan in 1916 by AT&T engineer W. G. Blauvelt. Automatic dialing was able to take place without the customer having to obtain a new telephone number. The customer did receive a new dial but letters were simply added to the numbers on the dial. (Hochheiser, Sheldon, 15 November 2010). Electronic switches were not entirely digital when they first appeared. “The Western Electric 1ESS switch had reed relay metallic paths which were stored-program-controlled.” (Wikipedia, 31 May 2011). Any maintenance of phone numbers or circuits was accomplished by typing on a terminal. Other companies such as Northern Telecom SPI, Ericsson AKE, Philips PRX/A, ITT Metaconta, and British Telecom TXE designs were similar. These systems could use the old electromechanical signaling methods from crossbar and Strowger. This system introduced a new form of data communication, the 1ESS, which allowed two exchanges to communicate through a data link called Common Channel Interoffice Signaling (CCIS). This link was based on CCITT 6, which preceded SS7. As the 1970s approached, it was clear that the days of the electromechanical switches were numbered. (Wikipedia, 31 May 2011). In 1965 AT&T installed the first 1ESS (electronic switching system) in a local exchange in Succasunna, New Jersey. The 1ESS was an analog electronic switch, which involved no mechanical motions. (Hochheiser,Sheldon 15 November 2010) “The analog technology converts air vibrations into an analogous electrical frequency. Unless a key telephone system, PBX or VoIP is used, most homes and small offices still use analog phones, and the typical loop is mostly analog.” (PCMAG Encyclopedia, 2011). Electronic switches were faster and much easier to maintain and were essentially special purpose computers. The electronic switches were much more flexible and allowed advanced features to be added such as call waiting (Hochheiser, Sheldon, 15 November 2010). Digital switches were first installed in 1975 and 1976, the 4ESS, in Chicago as a digital electronic toll switch. These switches were much faster and could handle as many as 350,000 calls her hour with more flexibility and speed than the electromechanical switches (Milestones in AT&T History, 2011). Digital switches connect two or more digital circuits together by use of the dialed telephone number. Switches set up calls by using the Signaling System 7 protocol or one of the variants. The first product using a digital switch system was produced by Amtelco. Other examples include Nortel DMS-100, Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephones exchanges. (Wikipedia, 31 May 2011). The greatest number of switches made since the 1980s are digital with only a few exceptions. Digital switches enclose the speech in 8000 time slices per second. A digital PCM (Pulse Code Modulation) representation of the tone is made at each time slice. Digits are then sent to the end of the line where they are reversed and thus produce the sound for the receiving phone. Essentially, when you use a phone your voice is being “encoded” and reconstructed into recognizable sounds for the other person. This encoding has a delay of a fraction of a second. A remote concentrator is used to connect individual local loop telephone lines, and the concentrator is located many times in the same building as the switch. The connection between the remote concentrators and the switches are standardized by ETSI as the V5 protocol. (Wikipedia, 31 May 2011). The purpose of concentrators are to handle the high volume of calls during the evening hours, as most phones are idle most of the day. This way the traffic from hundreds of thousands of phones can be concentrated into a lower number of shared connections. Some switches do not use concentrators but rather connect to other switches directly. The complex machines that handle this maneuver are called “carrier-level” switches or tandems. (Wikipedia, 31 May 2011). “Multiplexing is a form of data transmission in which one communication channel carries several transmissions at the same time.” (Data Communication Multiplexing, ligaturessoft.com). The access network and inter-exchange use optical transmission in synchronized form. These are SONET and Synchronous Digital Hierarchy (SDH) technologies. Some parts still use the older PDH systems. (Wikipedia, 31 May 2011). Small towns sometimes now have only remote or satellites switches and are linked to a “parent” switch usually several miles away. Remote switches and parent switches are dependent on each other for routing and number plan information. A remote switch can route calls between local phones without using trunks to the parent switch, which is unlike a digital loop carrier. The structure of a switch is an odd number of smaller layers of simpler subswitches. (Wikipedia, 31 May 2011). A web of wires that link from each subswitch to a set of subswitches on another layer interconnects these layers. Most designs include a switching layer that alternates with another time switching layer. The layers are symmetic, because callers and callees in a phone system are equal. A time-division reads a cycle of time slots into memory and writes it in a different order under the control of a cyclic computer memory. This produces a delay in the signal. A space division subswitch switches electrical paths, sometimes using a variant of a non-blocking minimal spanning switch, or sometimes a crossover switch. (Wikipedia, 31 May 2011). Switch control algorithms produce a fully connected mesh network when enough switching fabric is used to assure that pairwise allocation will always succeed. This method is normally used in central office switches that have low utilization of their resources. (Wikipedia, 31 May 2011). The Charles Clos algorithm has been a very important aspect to the telephone industry as it is used in many telephone switches. The scarce resources in a telephone switch are its connections between subswitches and layers. Therefore, the control logic has to allocate these connections, and most do it in a way that is fault tolerant (Wikipedia, 31 May 2011). Fault tolerance is a feature of composite switches. In the event that a subswitch fails, the controlling computer can sense it during a test. All the connections to the subswitch are marked as “in use” by the computer. This procedure prevents new calls and does not interrupt old calls that are working. As the calls end, the subswitch becomes unused and new calls bypass the subswitch because it is already “in use.” Later the technician will replace the circuit board. When the new test succeeds, the connection to the repaired subsystem is marked “not in use” and the switch is put back into full operation. (Wikipedia, 31 May 2011). This brings us up-to-date with Internet exchanges. The phone exchange concept has been adapted to use in Internet exchanges. Voice over IP (VoIP) traffic may pass through both kinds of exchanges, however, it depends on what kind of service the caller and the callee is using. (Wikipedia, 31 May 2011). Since discussing AT&T and Bell South previously, continuing with AT&T seems appropriate. “AT&T Internet Services (ATTIS) is based on a Cisco-Powered Network. All ATTIS POPs are designed to Carrier-Class standards. ATTIS has a completed state of the art OC-192c National IP Network that provides for diversely routed OC-192c circuits engineered to facilitate fiber and hardware redundancies and accommodate any network or hardware facility failures.” (AT&T Dedicated Internet Access, 2011). VoIP obviously differs from POTS in many ways. Basically, the end result is the same. Both systems allow you to pick up a phone, dial a number, and talk to the other party, however, they are extremely different. VoIP does not use wires to physically connect devices. VoIP digitizes your voice, sends it over the Internet to the destination and then it returns to audio on the other side. It only requires two players, your Internet provider and your VoIP provider. (VoIP vs POTS, 28 May 2008). For reliability the better choice is POTS, which is why many people keep a landline active for 911 service. However, if you are an average phone user and weighing cost against reliability, VoIP might be your best choice. In conclusion, telephone switches are a small part of the large network. However, the majority of work that goes into the expense of a phone system is the wiring that is outside the central office or plant. The mid 20th century subscribers required an individual pair of wires from the switch to the phone. A typical telephone company today has a large database that tracks information about each subscriber pair and the status of each jumper. Before the days of digital switches Bell System records recorded this information in pencil by hand in an accounting ledger (1980s). (Wikipedia, 31 May 2011). It is a far cry from the POTS historical systems. “Internet telephone calling has been identified as a leading disruptive technology that has a promising and potentially profitable future. However, the leading public US companies in Internet telephone calling have seen their stocks drop 80 percent or more over the past year.”(Business Wire, 14 June 2001). Even internationally in today’s global telecommunications marketplace, traditional cents-per-minute revenue models are being rendered obsolete. (Business Wire, 14 June 2001). References AT&T Dedicated Internet Access – Network. (2011). Global Customer Service Center. Web. 14 June 2011. http://dedicated.sbcis.sbc.com/. Atlanta Telephone History. (Part 1). Early Telephone Service. Web. 14 June 2011. http://atlantatelephonehistory.info/. Business Wire. (14 June 2001). What is the Future of Internet Telephone Calling? When will The Bloodbath End? Asks CoolCall.com founder. 14 June 2011. http://thefreelibrary.com/. Data Communication Multiplexing, Web. 14 June 2011. http://ligaturesoft.com/. Hochheiser, Sheldon. ( 22 September 2010) Stars: Electromechanical Telephone-Switching. Web. 15 June 2011. http://ieeehgn.org/. Milestones in AT&T History. Web. 15 June 2011. http://corp.att.com/history/. Nonnenmacher, Tomas. Allegheny College. History of the U. S. Telegraph Industry. Web. 15 June 2011. http://eh.net.encyclopedia/article/. PCMAG.COM Encyclopedia. Definition of Analog Phone. Web. 15 June 2011. http://pcmag.com/. Public Switched Telephone Network. Web. 14 June 2011. http://en.wikipedia.com/. Plain Old Telephone Service. Web. 14 June 2011. http://en.wikipedia.org/. Telecom Dictionary – Definitions of terms. Higgins International. Web. 14 June 2011. http://faxswitch.com/. Telephone Exchange. Web. 14 June 2011. http://en.wikipedia.org/. What is POTS? (28 May 2008). Web. 14 June 2011. http://voiptechchat.com/. Read More