POTS & PABX

PAMANTASAN NG LUNGSOD NG MAYNILACollege of Engineering and Technology

Bachelor of Science in Electronics Engineering (BS ECE)Intramuros, Manila

Plain Old Telephone Service (POTS) &Private Branch Exchange (PBX)

Submitted by:Cerbito, Keil T.

Santos, Maverick Z.

Submitted to:Engr. Joan del Espiritu

Submitted Date:February 2, 2015

TABLE OF CONTENTSI. Plain Old Telephone Service (POTS) 1

A. The Subscriber Loop 2B. The Local Loop 2C. Reliability 3

II. Private Branch Exchange 4A.History 5B. System components 7C. Current trends 7D. Functions 8E. Interface Standards 8

1. Interfaces for connecting extensions to a PBX 82. Interfaces for connecting PBXs to each other 83. Interfaces for connecting PBXs to trunk lines 84. Interfaces for collecting data from the PBX 9

F. Hosted PBX systems 9

1POTS & PABX | Plain Old Telephone Service

Plain Old Telephone Service (POTS) POTS, is a term which describes the voice-grade telephone service that remains the basic

form of residential and small business service connection to the telephone network in mostparts of the world. The system was originally known as the Post Office Telephone Serviceor Post Office Telephone System. Today the term Plain Old Telephone Service is used,after the services were removed from the control of national post offices.

The pair of wires from the central switch office to a subscribers home was called thesubscriber loop. It was typically powered by 40VDC and backed up by a large bank ofbatteries in the central office, resulting in continuation of service during most commercialpower outages.

This 64Kbps service is a bi-directional, or full duplex, voice path with limited frequencyrange of 300 to 3400 Hz: in other words, a signal to carry the sound of the human voiceboth ways at once. Today, it is also used for internet access via a dial modem, DSL, fax,credit card terminals, etc.

Figure 1: Block Diagram of POTS

The standard two-wire telephone-set connection known as analog PSTN (Public SwitchedTelephone Network) (loop start) or POTS (Plain Old Telephone Service) is the oldest but still mostwidely used service offered by the telephone companies.

Other types of analog services offered by the telephone companies include: Four-wire services, using separate pairs of wires for transmitting and receiving provides

improved fidelity; Reverse battery services allow automated PBXs to act as localized central offices;


Ground start and E and M tie trunks offer more reliable methods of signaling than the loopstart systems.

In addition to the public switched services, where the user can dial different numbers, privatepoint-to-point services are provided in both two-wire and four-wire formats.

The Subscriber LoopThe simplest and most straightforward form of telephone service is the PLAIN OLD

TELEPHONE SERVICE (POTS), which involves subscribers accessing the public telephonenetwork through a pair of wires called the local subscribers loop or simply the local loop.

The subscriber loop provides the means to connect a telephone set at a subscriber`s locationto the closest telephone office, which is commonly call an end office, local exchange office, orcentral office. Once in the central office, the subscriber loop is connected to an electronic switchingsystem (ESS), which enables the subscribers to access the public telephone network.

It was typically powered by 40VDC and backed up by a large bank of batteries in the centraloffice, resulting in continuation of service during most commercial power outages.

The Local LoopLocal Loop is the most fundamental component of a telephone circuit. A local loop is

simply an unshielded twisted-pair transmission line (cable pair), consisting of two insulatedconductors twisted together.

A customer equipment (also called subscriber or terminal equipment) is usually connectedto the telephone company exchange (Central Office) by on average about 5 kilometers (3 miles)of a twisted pair of No. 22 (AWG) or 0.5 mm copper wires, known as the subscriber (local) loop.The resistance of 0.5 mm wire (single lead) is 16.5 ohm per thousand feet (54 ohm per 1 km). Thetwisted pair is used to provide a balanced line which reduces common mode interference(crosstalk) from adjacent pairs in the cable and RFI (Radio Frequency Interference). Balance is ameasure of equality of impedance between each lead in the pair (called Tip and Ring) and theground. In order to keep the balance of the line, the terminal equipment should be balanced also.


Figure 2: Local Loop

POTS is characterized by several aspects: Bi-directional (full duplex) communications. Using balanced signaling of voltage analogs of sound pressure waves on a two-wire copper

loop Restricted to a narrow frequency range of 300 to 3300 Hz, called the (voiceband), which

is much less than the human hearing range of 20 - 20,000 Hz Call-progress tones, such as dial tone and ringing signal. Dial pulse signaling of addresses. BORSCHT functions.

ReliabilityWhile POTS provides limited features, low bandwidth, and no mobile capabilities, it

provides greater reliability than other telephony systems (mobile phone, VoIP, etc.). Manytelephone service providers attempt to achieve dial-tone availability more than 99.999% of thetime the telephone is taken off-hook. This is an often cited benchmark in marketing and systems-engineering comparisons, called the "five nines" reliability standard. It is equivalent to having adial-tone available for all but about five minutes each year.

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POTS & PBX | Private Branch Exchange

Private Branch Exchange (PBX)A private branch exchange (PBX) is a telephone exchange or switching system that serves

a private organization and performs concentration of central office lines or trunks and providesintercommunication between large numbers of telephone stations in the organization. The centraloffice lines provide connections to the public switched telephone network and the concentrationaspect of a PBX permits the shared use of these lines between all stations in the organization. Theintercommunication aspect allows two or more stations to establish telephone or conferencing callsbetween them without using the central office equipment.

Figure 3: Avaya G3si PBX (front cover removed)

Each PBX-connected station, such as a telephone set, a fax machine, or a computer modem,is often referred to as an extension and has a designated extension telephone number that may ormay not be mapped automatically to the numbering plan of the central office and the telephonenumber block allocated to the PBX.

Initially, the primary advantage of a PBX was the cost savings for internal phone calls:handling the circuit switching locally reduced charges for telephone service via the central officelines. As PBX systems gained popularity, they were equipped with services that were not availablein the public network, such as hunt groups, call forwarding, and extension dialing. In the 1960s asimulated PBX known as Centrex provided similar features from the central telephone exchange.

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A PBX is differentiated from a key telephone system (KTS) in that users of a key systemmanually select their own outgoing lines on special telephone sets that control buttons for thispurpose, while PBXs select the outgoing line automatically or, formerly, by an operator. Thetelephone sets connected to a PBX do not normally have special keys for central office line control,but it is not uncommon for key systems to be connected to a PBX to extend its services.

A PBX, in contrast to a key system, employs an organizational numbering plan for itsstations. In addition, a dial plan determines whether additional digit sequences must be prefixedwhen dialing to obtain access to a central office trunk. Modern number analysis systems permitusers to dial internal and external telephone numbers without special codes to distinguish theintended destination.

Figure 4: Block Diagram of Small PBX SystemHistory

The term PBX was first applied when switchboard operators managed companyswitchboards manually using cord circuits. As automated electromechanical switches and laterelectronic switching systems gradually replaced the manual systems, the terms private automaticbranch exchange (PABX) and private manual branch exchange (PMBX) were used to differentiatethem. Solid state digital systems were sometimes referred to as electronic private automatic branch

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exchanges (EPABX). Today, the term PBX is far the most widely recognized. The acronym is nowapplied to all types of complex, in-house telephony switching systems.

Two significant developments during the 1990s led to new types of PBX systems. One wasthe massive growth of data networks and increased public understanding of packet switching.Companies needed packet switched networks for data, so using them for telephone calls wastempting, and the availability of the Internet as a global delivery system made packet switchedcommunications even more attractive. These factors led to the development of the voice over IPPBX, or IP-PBX.

Figure 5: PBX Switchboard (1975)The other trend was the idea of focusing on core competence. PBX services had always

been hard to arrange for smaller companies, and many companies realized that handling their owntelephony was not their core competence. These considerations gave rise to the concept of thehosted PBX. In wireline telephony, the original hosted PBX was the Centrex service provided byTelcos since the 1960s; later competitive offerings evolved into the modern competitive localexchange carrier. In voice over IP, hosted solutions are easier to implement as the PBX may belocated at and managed by any telephone service provider, connecting to the individual extensionsvia the Internet. The upstream provider no longer needs to run direct, local leased lines to theserved premises.

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System components Cabinets, closets, vaults and other housings. Console or switchboard allows the operator to control incoming calls. Interconnecting wires and cables. Logic cards, switching and control cards, power cards and related devices that facilitate

PBX operation. Microcontroller or microcomputer for arbitrary data processing, control and logic. Outside Telco trunks that deliver signals to (and carry them from) the PBX. Stations or telephone sets, sometimes called lines. The PBXs internal switching network. Uninterruptible power supply (UPS) consisting of sensors, power switches and batteries.

Current trendsSince the advent of Internet telephony technologies, especially Voice over IP, PBX

development has tended toward the Voice-Over-IP (VoIP) PBX (or IP-PBX), which uses theInternet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems alsoreplaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling,call forwarding, and programmable caller ID. However, recent open source projects combinedwith cheap modern hardware are sharply reducing the cost of PBX ownership.

Originally having started as an organization's manual switchboard or attendant consoleoperated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centersthat are hosted by the operators or even manufacturers.

Even though VoIP is considered the future of telephony, the circuit switched network isstill the core of communications, and the already bought PBXs are competitive in services withmodern IP systems. Five distinct scenarios may be recognized:

Hosted/virtual PBX (hosted and circuit-switched) or traditional Centrex IP Centrex or hosted/virtual IP (hosted and packet-switched) IP PBX (private and packet-switched) Mobile PBX solution (mobile phones replacing or used in combination with fixed phones) PBX (private and circuit-switched)

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FunctionsThe PBX performs four main call processing duties:

Establishing connections (circuits) between the telephone sets of two users (e.g. mappinga dialed number to a physical phone, ensuring the phone isn't already busy)

Maintaining such connections as long as the users require them (i.e. channeling voicesignals between the users)

Disconnecting those connections as per the user's requirement Providing information for accounting purposes (e.g. metering calls)

Interface StandardsInterfaces for connecting extensions to a PBX:

DECT a standard for connecting cordless phones. Internet Protocol For example, H.323 and SIP. POTS (plain old telephone service) the common two-wire interface used in most homes. Proprietary the manufacturer has defined a protocol. One can only connect the

manufacturer's sets to their PBX, but the benefit is more visible information displayedand/or specific function buttons.

Interfaces for connecting PBXs to each other: DPNSS for connecting PBXs to trunk lines. Standardized by British Telecom, this usually

runs over E1 (E-carrier) physical circuits. Internet Protocol H.323 and the Session Initiation Protocol (SIP) are IP based solutions

for multimedia sessions. Primary rate interface (ISDN) Provided over T1 (23 bearer channels and 1 signaling

channel) or E1 carriers. Proprietary protocols if equipment from several manufacturers is on site, the use of a

standard protocol is required. QSIG for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-

carrier) physical circuits.Interfaces for connecting PBXs to trunk lines:

Internet Protocol H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate overIP and are supported by some network providers.

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ISDN the most common digital standard for fixed telephony devices. This can be suppliedin either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Mostmedium to large companies would use Primary ISDN circuits carried on T1 or E1 physicalconnections.

RBS (robbed bit signaling) delivers 24 digital circuits over a four-wire (T1) interface Standard POTS (plain old telephone service) lines This is adequate only for smaller

systems, and can suffer from not being able to detect incoming calls when trying to makean outbound call (commonly called glare).

Interfaces for collecting data from the PBX: File the PBX generates a file containing the call records from the PBX. Network port (listen mode) an external application connects to the TCP or UDP port. The

PBX streams information to the application. Network port (server mode) the PBX connects to another application or buffer. Serial interface historically used to print every call record to a serial printer. In modern

systems a software application connects via serial cable to this port.A data record from a PBX or other telecommunication system that provides the statistics for atelephone call is usually termed a call detail record (CDR) or a Station Messaging Detail Record(SMDR).

Hosted PBX systemsVirtual PBX systems or hosted PBX systems deliver PBX functionality as a service,

available over the public switched telephone network (PSTN) or the Internet. Hosted PBXs aretypically provided by a telephone company or service provider, using equipment located in thepremises of a telephone exchange or the provider's data center. This means the customer does notneed to buy or install PBX equipment. Generally the service is provided by a lease agreement andthe provider can, in some configurations, use the same switching equipment to service multiplehosted PBX customers.

The first hosted PBX services were feature-rich compared to most premise-based systemsof the time. Some PBX functions, such as follow-me calling, appeared in a hosted service beforethey became available in hardware PBX equipment. Since introduction, updates and new offeringshave moved feature sets in both directions. It is possible to get hosted PBX services that include

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feature sets from minimal functionality to advanced feature combinations. In addition to thefeatures available from premises-based PBX systems, hosted-PBX:

Allows a single number to be presented for the entire company, despite its beinggeographically distributed. A company could even choose to have no premises, withworkers connected from home using their domestic telephones but receiving the samefeatures as any PBX user.

Allows multimodal access, where employees access the network via a variety oftelecommunications systems, including POTS, ISDN, cellular phones, and VOIP. Thisallows one extension to ring in multiple locations (either concurrently or sequentially).

Allows scalability so that a larger system is not needed if new employees are hired, and sothat resources are not wasted if the number of employees is reduced.

Eliminates the need for companies to manage or pay for on-site hardware maintenance. Supports integration with custom toll plans (that allow intra company calls, even from

private premises, to be dialed at a cheaper rate) and integrated billing and accounting.

1 Front Page.pdf2 Table of Contents.pdf3 POTS.pdf4 PABX.pdf

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POTS & PABX