Optical Transmission System

Optical transmission systems use strands of glass or plastic fiber to transfer optical energy between points. For most optical transmission systems, the transmitting end-node uses a light amplification through stimulated emission of radiation (LASER) device to convert digital information into pulsed light signals (amplitude modulation). The light signals travel down the fiber strand by bouncing (reflecting) off the sides of the fiber (called the cladding) until they reach the end of the fiber. The end of the fiber is connected to a photo-detector that converts these light pulses back into their electrical signal form.

Synchronous optical transmission systems used a specific frame structure and the data transmission through the transmission line is synchronized to a precise clock. This eliminates the signaling overhead requirement for framing or timing alignment messages. The basic frame size used in optical transmission systems is 125 usec frames.

Optical transmission systems are characterized by their carrier level (OCx) where the basic carrier level 1 is 51.84 Mbps. Lower level OC structures are combined to produce higher-speed communication lines. There are different structures of OC used in the world. The North American optical transmission standard is called synchronous optical network (SONET) and the European (world standard) is synchronous digital hierarchy (SDH).

Signals are applied to and are extracted from optical transmission systems using an optical add/drop multiplexer (OADM). The OADM is a network element that provides access to all or some subset synchronous transport signal (STS) line signals contained within an optical carrier level N (OC-N). The process used to direct a data signal or packet to a payload of an optical signal is called mapping. The mapping table is contained in the OADM. A copy of the OADM mapping is kept at other locations in the event of equipment failure. This allows the OADM to be quickly reprogrammed.

Synchronous digital hierarchy (SDH) is an international digital transmission format used in optical (fiber) networks standardized that is similar (but not identical) to SONET. SDH uses standardized synchronous transmission according to CCITT standards G.707, G.708, and G.709. These standards define data transfer rates, defined optical interfaces, and signal structure formats.

Some of the key differences between SONET and SDH include differences in overhead (control) bits and minimum transfer rates. The first level available in the SONET system is OC1 and is 51.84 Mbps. The first level in the SDH system starts at STM-1 and has a data transmission rate of 155.52 Mb/s. SONET also multiplexes synchronous transport signal level 1 (STS-1s) to form multiple levels of STS. The SDH system divides the channels into multiple DS0s (64 kbps channels). This is why the overhead signaling structures are different.

Figure 1 shows the optical standards for both SONET and SDH. This table shows that the first common optical level between SONET and SDH is OC3 or STS-1. STS-x and STM-x are the standards that specify the electrical signal characteristics that are input to the respective optical encoding/multiplexing processes.

Figure 1: Optical Transmission Systems

High Speed Packet Switching Technology

High-speed backbone networks are networks that provide rapid variable data rate (dynamic bandwidth) transport between switching centers. Traditional switching systems have been limited to low-speed fixed bandwidth connections.

High-speed switching systems are telecommunications infrastructures composed of circuits and equipment capable of near-instantaneous connection of end points at near-perfect efficiency and required data transmission throughput.

High-speed packet switching technology allows multiple communication channels to share the resources of a data communication network. This allows the same network to integrate voice, data, and video signals. In addition to the rapid switching of packets, packet switches are designed to handle different types of packets in different ways. Packet switches receive incoming packets, update the address information of the packets with their new destination address, temporarily store the packets until the next path and channel becomes available, and then transfers the packet to the appropriate communication line (Path) and channel (time slot or portion of a time slot).

Figure 1 shows a high-speed data packet switching system. This diagram shows that several high-speed data transmission lines are providing packet to the packet switch. The packet switch uses a routing table to search for the incoming address and then it replaces the address with the new packet destination address (the next switch or end point). The data packet is then stored in buffer memory where it waits for availability of its destination path and channel. This diagram also shows how the packet switch manages excessive network switch activity. As the packet switch gets busy (receives more data than it can process), the buffer memory begins to fill. As the buffer memory nears exhaustion, packets within the memory will be reviewed for priority and low-level priority data packets will be discarded.

Figure 1: High-Speed Packet Switching

Market Growth

The market for IXC communications is becoming more competitive through the introduction of more efficient technologies and lower barriers to entry for new carriers. Although the total usage of IXC communication is increasing, the average revenue pre minute is decreasing. As a result, the total industry revenue for the IXC market is decreasing.

Figure 1 shows the growth in number of inter-exchange carriers in the United States. This diagram shows that the deregulation of the telecommunications industry and advances in cost effective technology have increased the number of interexchange carriers from less than 80 in 1993 to more than 200 in 2000.

Figure 1: Number of IXC Carriers in the United States.

Figure 2 shows the decline in IXC revenue in the United States starting in 2000. This diagram shows that total IXC revenues in the United States increased over 6% per year from 1985 through 1995. Since 1996, revenue of the IXC market has been decreasing and total revenues for 2000 was only $108 billion, less than revenue in 1999. This decrease in revenue has occurred despite a sharp increase in the amount of IXC network usage from 1996 through 2000.

Figure 2: United States Inter-Exchange Market Growth

International Interconnection (Inter Exchange Networks)

IXC backbone carrier facilities primarily use microwave and fiber transmission lines. Microwave systems offer medium capacity of up to several hundred Mbps communications with a range of 20-30 miles between towers. Fiber optic communication systems offer data transmission capacity of over one million Mbps (million million bits per second).

Microwave transmission systems transfer signal energy through an unobstructed medium (no blocking buildings or hills) between two or more points. In 1951, microwave radio transmission systems became the backbone of the telecommunications infrastructure. Microwave systems require a transducer to convert signal energy of one form into electromagnetic energy for transmission. The transducer must also focus the energy (using an antenna dish) so it may launch the energy in the desired direction. Some of the electromagnetic energy that is transmitted by microwave systems is absorbed by the water particles in the air.

Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable such as in developing countries.

Figure 1 shows a terrestrial microwave system-connecting IXC switches in Philadelphia and New York City. The microwave signals are moved between the two switching offices through a series of relay microwave systems located approximately 30 miles apart. Microwave is a line-of-sight technology that must take the earth’s curvature into consideration. Also note that microwave towers are not limited to only facing one or two directions. A single tower can be associated with several other towers by positioning and aiming additional transceiver antennas at other microwave antennas on other towers.

Figure 1: Long Haul Microwave

Fiber optic transmission is the transfer of information (usually in digital form) through the use of light pulses. Fiber optic transmission can be performed through glass fiber or through air. Fiber optic transmission lines are capable of extending up to 1200 km without amplifiers. Each fiber optic strand can carry up to 10 Gbps optical channels and a fiber can have many optical channels (called DWDM). Each fiber cable can have many strands of fiber.

Fiber cable is relatively light, low cost, and can be easily installed in a variety of ways. It does not experience distortion from electrical interference and this allows it to be installed on high voltage power lines or in other places that have high levels of electromagnetic interference.

Figure 2 shows common installations of fiber optic cable. This diagram shows that fiber transmission systems are installed along railway and natural gas pipelines, under water, and along high voltage lines.

Figure 2: IXC Fiber Optical Cable Installation Options

High Speed Switching Systems
IXCs use high speed switching systems to interconnect transmission lines. The key high speed switching system used in IXC networks is asynchronous transfer mode (ATM). ATM is a fast packet switching technology that transports information through the use of small fixed length packets of data (53 byte cells).

The ATM system uses high-speed transmission facilities (155 Mbps/OC-3 and above). OC-3 is the entry-level speed for commercial ATM. Higher speeds (such as OC-192) are used in backbone networks of IXC’s and other specialized service providers. ATM service was developed to allow one communication technology (high-speed packet data) to provide for voice, data and video service in a single offering.

International Interconnection
International interconnection issues include converting transmission line and control signaling formats, transcoding different types of digital voice signals, and rating billing records.

IXC networks must be capable of converting transmission line formats. These include digital signaling standards (e.g., T1 to E1), different optical standards (SONET and SDH), and command signaling protocols differences such as ISDN signaling differences.

Transcoding is the conversion of digital signals from one coding format to another. Transcoding is necessary because the digital signal companding process that is used for encoding/decoding signals is different throughout the world. This companding process increases the dynamic range of a binary signal by assigning different weighted values to each bit of information than is defined by the binary system. The A-law encoding system is an international standard and the uLaw standard is used in the Americas.

IXC systems must be capable of creating billing record in different formats. Billing systems in different countries use different rating systems (e.g., flat rate compared to time usage). It may be necessary for IXCs to receive and pay in different currencies and currency exchange rates for different countries rapidly vary. The payment or receipt of payments for calls routed through the IXC must be settled through clearinghouse companies that have relationships with many IXC, LEC, and PTT operators.

Several independent companies have installed or operate international transmission lines. These international circuits may be leased to IXCs or to independent corporations. Companies that operate these international transmission lines are often called international carriers (IC’s) or international record carriers (IRC’s).

Figure 3 shows an IXC network that has many international interconnections. This diagram shows that various transmission systems are used for interconnection. There are several high capacity switching points in these networks with redundant links between them. Some of the interconnection lines are operated by satellite and transoceanic cable/fiber carrier services provided IC/IRCs.

Figure 3: International IXC Interconnections

Inter Exchange Networks

Inter-exchange networks (IXCs) are telecommunications networks that connect local exchange carriers (LECs), competitive local exchange carriers (CLECs), local post, or telephone and telegraph (PTT) with each other. IXCs provide long distance bearer service communication and may provide other value-added teleservices. IXC’s are regulated by governmental commissions but are not usually government-owned. In other parts of the world the government may own and operate LECs and PTTs.

Some IXC’s provide interconnection for the Internet through their high-speed links and switching nodes. IXC networks use meshes of microwave, fiber, copper, coaxial cable, and satellite links to interconnect their switching systems.

Figure 1 shows a diagram of an inter-exchange carrier network. This diagram shows that the IXC interconnects LECs and CLECs with teach other through POP switching points. Access lines connect the IXC POP switching centers with LEC and CLEC tandem switching systems. These interconnection lines are typically dedicated high-speed carrier transmission lines such as DS3 or OC3 lines.

Figure 7.1: Inter-Exchange Carrier Network

IXC networks use high-speed switching systems to interconnect high-capacity transmission lines. End users connect to IXC networks either through local telephone systems or through direct connection using customer provided equipment (CPE). Network interconnections are the points where IXCs connect to other networks. Transmission lines transport signals through the IXC network. High-speed switching systems provide interconnections between transmission lines and individual channels on those transmission lines. IXCs have multiple types of international interconnection issues to adapt telecommunication formats between different types of systems.

The overall operation of services, switches, and transmission lines in an IXC is coordinated by network operations centers (NOCs). NOC’s continuously monitor the status and performance of all network nodes and links. If a network transmission or equipment fails, most networks will automatically reconfigure to (reroute) communication lines or automatically switch to backup systems. Practically all network components have redundant assemblies that will automatic switch into service on detection of equipment failure. Multiple routes are required between all switching facilities. These facilities are hardened with all support systems such as power, water, local emergency access, security redundant, and sabotage-proof.

NOC’s management systems are usually distributed to multiple locations. These management centers contain information related to addressing, routing, and reroute scenarios. These regional centers are capable of distributing the network configuring information to remote switching nodes through communication links. Through this application of decentralized control and operations combined with an extensive data base maintenance and support activity, the utilization, efficiency, and security of network capacity can be maximized.

The actual placement of circuits and switching equipment is confidential information when viewed as an operational system. This is because of the critical nature of this type information to all countries. Major damage to a country’s telecommunications infrastructure could easily cripple an area or even a whole country. Telecommunications is considered a vital part of national security and special requirements exist to the protection and reliability of telecommunications networks.

Network Interconnection Points

IXCs connect to LECs, CLECs, PTTs, and other networks through access lines and network interconnection points. Network interconnection points link networks to an IXC through the IXC’s point of presence (POP). POPs are the switching points in an IXC network that are located on the edge of the network (end switching points). A POP can be a switching location (like an end office (EO) where direct access to the IXC’s high-speed infrastructure is available. POP’s can also be simply access nodes (multiplexers) that are co-located with the LEC/CLEC for convenience and logistics.

Some IXCs connect directly with end users to provide high-speed communication services. When an end user directly connects to an IXC, facilities such as T-1’s may be installed directly tying the customer to the IXC’s POP without connecting through the LEC, CLEC or PTT. This is often the case when a business contractually receives discounts for the amount of long distance the IXC can bill to the customer business per month.

Multimedia Communication

Multimedia communication is the delivery of different types of information such as voice, data, or video. Communication systems may separately or simultaneously transfer multimedia information.

Video conferencing is an application of multimedia communication technology that merges voice and video via the use of microphones, video cameras, and special multiplexers. Routinely companies set up certain conference rooms at their various sites and equip them with video conferencing equipment. There are various video conferencing standards including the International Telecommunications Union (ITU) H.323 and standard T.120 for multipoint data conferencing.

Video conferencing standards may allow for the use of whiteboards. Whiteboards are devices that can capture images or hand drawn text so they can be displayed in a window in at the connected video conferencing system. Whiteboards allow video conferencing users to place share documents, images, and/or hand written diagrams with one (or more) video conference call attendees.

Figure 1 shows the basic operation of sending video over an Internet connection. This diagram shows a computer with video conferencing capability that calls a destination computer. Computer #1 initiates a video conference call to computer #2 using the address When computer #2 receives a data message from computer #1, a message is displayed on the monitor and an audio tone (ring alert) occurs. If the user on computer #2 wants to receive the call, they select the answer option (via the mouse or keyboard) that is generated by the software. Computer #1 then initiates a data connection with computer #2. The video conferencing software and data processing software in the computers (e.g., USB data bus and sound card) convert the analog audio signal from the microphone and digital video signal into a digital form that can be transmitted via the data link between the computers.

Figure 1: Video Conferencing through the Internet

LAN Telephony

Local access network (LAN) telephony (sometimes called TeLANophy) use LAN systems to transport voice communications. LAN telephone technology is an evolution of voice over IP (VoIP) and the rapid acceptance of virtual private networks (VPN’s) as an alternative to leased line private networks. The ability to share data networks with voice systems offers significant cost reduction for telephone services.

Figure 1 shows a LAN telephony system. This diagram shows that a LAN telephone system consists of LAN telephones, a data network, a LAN call processing system, and a voice gateway to the PSTN. LAN telephones convert audio into digitized packets that are transferred on the LAN to the call processing computer (CTI system). Each LAN telephone has its own network data address. The call processing system communicates with LAN telephones over the same high-speed LAN data network that communicates with computers. When calls are received from the PSTN, the call processing system looks in the database to find the associated LAN telephone address (data address) and this address is used to alert the LAN telephone of an incoming call. When calls are originated from the LAN telephone, the dialed telephone number is passed to the call processing system. This system determines if the call is routed within the data network or if the voice gateway must be used to connect the call to the PSTN.

Figure 1: LAN Telephony

Call Centers (Telecom Made Simple)

Call Centers
A call center is a place where calls are answered and originated, typically between a company and a customer. Call centers assist customers with requests for new service activation and help with product features and services. A call center usually has many stations for call center agents that communicate with customers. When call agents assist customers, they are typically called customer service representatives (CSRs).

Call centers use telephone systems that usually include sophisticated automatic call distribution (ACD) systems and computer telephone integration (CTI) systems. ACD systems route the incoming calls to the correct (qualified) customer service representative (CSR). CTI systems link the telephone calls to the accounting databases to allow the CSR to see the account history (usually producing a “screen-pop” of information).

Call centers are typically established as either incoming or outgoing. Seldom are they set up together. The main exception is debt collection where there are representatives making outgoing calls and other taking incoming calls. Still, in most cases, the functions are really separate although to the outside client they appear as one.

Incoming (inbound) call centers are set up primarily for some sort of customer service function such as catalogues sales, service or billing inquiries, or technical support. They may be front-ended by an interactive voice response (IVR) systems that take care of customer questions and inquiries that can be handled via computer database look-up’s or via general information recordings.

Traffic monitoring in such centers via ACD and IVR reporting is critical in order to detect and correct bottlenecks and lost calls before such situations becomes crises. Where the representatives are geographically spread, much of this analytical support may be contracted to the carrier that supplies the inbound telephone service. As small sales/service offices become less profitable and are closed, less on-site technical support is available from manufacturers/vendors, and less people are available to provide customer interface, the need for such incoming call centers increases. Consequently this type operation will flourish for some time to come even in face of the Internet.

Figure 1 list the typical costs associated with a call center used for order fulfillment. This table shows that the cost of inbound call center order fulfillment may include a minimum call processing charge in addition to a percentage of sales.

Figure 1: Cost of Inbound Call Center Service

Outbound call centers are primarily geared to two businesses: telephone sales (telemarketing) and debt collection. Many systems use special computer software that dials numbers from a database and once the call is answered passes the call off to an attendant who actually speaks to the person called. The timing of the pass off is critical. Older systems dialed a number and when answered mechanically switched the call causing significant delays between the person answering the call and the representative speaking. Many people routinely hang up on these type calls. Some outbound call centers are designed to deliver a pre-recorded message until an available CSR can be connected. Call center telemarketing services are heavily regulated in the United States and in many other countries. There may be restrictions on whom the call center can contact, the times of day calls can be originated, what the CSR can say, and what they must disclose to the prospective customer.

Services : Direct Marketing Broadcast Services

Private telephone systems can generate revenue for companies by offering broadcast services or operating as call centers. Direct marketing broadcast services provide revenue by using an existing telephone system to broadcast a customer’s message to a list of hundreds or perhaps thousands of recipients. The customer is then charged for the usage of the telephone system to broadcast their message. Call centers provide revenue by processing customer service requests (such as placing orders) or by providing sales and marketing services (telemarketing). In either case, call centers may charge fee on a per-call or percentage of sales basis.

Direct Marketing Broadcast Services
Direct marketing broadcast services include sending fax and e-mail messages to qualified groups of people. Fax and email broadcast services are distribution service that can use CTI technology to delivery the same message or even adapt each message for a list of recipients. Clients use e-mail and fax broadcast to reach a targeted audience with a sales message. Clients may provide a list of recipients for the fax broadcast, or for an additional fee, broadcasters will supply the customer with a list of names to target. Typically, clientele are charged on a per fax basis, with discounts for high volume broadcasts. Some broadcasters charge a per-minute usage fee rather than billing for services on a per fax basis.

Figure 1 shows the typical charges for direct fax market broadcasting. This table shows that prices for this service range from a few cents to 50 cents per fax drop as quantity ordered increases to beyond 5,000 faxes. This chart also shows that there is usually a restriction on the minimum amount of information that a customer can send.

Figure 1: Direct Market Broadcast Cost

Wireless Private Branch Exchange (WPBX)

Wireless Private Branch Exchange (WPBX)
WPBX systems integrate wireless telephones with a PBX switching system. Wireless PBX telephones (handsets) communicate through wired base stations (fixed radio transmitters) to the WPBX switching system. Most WPBX systems have automatic switching call transfer that allows wireless handsets to transfer their calls to other base stations as the move through the WPBX radio coverage areas. Base stations are strategically located around the served area (both inside and/or outside) to provide contiguous radio coverage. WPBX systems can be completely, or partially, wireless between the system and the telephone instruments.

WPBX systems fill a need where all, or part, of the work force is highly mobile in a relatively small area such as a building/plant or a small commercial campus. Hospitals and manufacturing plants tend to have several types of personnel that tend to be constantly on the move: medical emergency personnel, maintenance personnel, and production-line supervisors to name a few. Such people are frequently away from their desk or other fixed telephone station set location; however, it is often quite important that they be contacted quickly.

There are several different types of WPBX systems industry standard systems and proprietary systems. Some of the standard WPBX systems include digital enhanced cordless telephone (DECT) and cordless telephony second generation (CT2). A WPBX radio system allows for voice or data communications on either an analog (typically FM) or digital radio channel. The radio channel typically allows multiple mobile telephones to communicate on the same frequency at the same time by special coding of their radio signals.

The wireless office base station is the link between the radio transmissions sent to and received from the wireless telephone and the WPBX switching system. Wireless office base stations are similar to cell sites used in mobile telephone systems as they regularly communicate directly with the WPBX switching system. Because these base stations are fairly close to the switching system, they are directly connected by cable. This allows power to be supplied by the WPBX switching system and no battery backup power supply system is required.

The cable that connects the base station to the switching system typically carries multiple voice and/or data channels. The power and data signals may be supplied over a single twisted pair or dedicated lines may be used for data and power. As the signals arrive at the base station, a communications controller divides the multiple channels, processes their signals, and routes them to the base station radio signal amplifier.

The design objectives of a WPBX base station are similar to those of a general mobile telephone system, but there are several additional requirements. WPBX base stations must be much simpler to install, relocate and service (diagnose or debug). Operations without skilled or highly trained staff are very desirable. Many WPBX base stations are almost “self configuring,” implying that the system sets the frequencies of each base station automatically, to both optimize the overall frequency plan and to avoid interference with non-radio RF sources which may be present.

The WPBX switching system coordinates the operation of all the base stations and wireless handsets in the system. The switching hardware and software for the WPBX may be incorporated into the main office telephone system (integrated), may reside in a separate switching and/or control module (external), or be completely separate from any wired system (independent). Integrated systems allow one switch to serve all the base stations and wired telephones connected to the system. An external system is used when a radio system is added to an existing system or the older system cannot be directly upgraded to support handoff switching inside the main switch. Independent systems may be used when there is no wired system installed. An independent system may only consist of WPBX handsets that can access a public cellular system for office use at a reduced billing rate.

In a WPBX installation that has handoff (call transfer between base stations) capability, there is a continual process of signaling which occurs between all the handsets which are powered up but idle and the nearest base station(s). This allows the wireless handsets to handover (call transfer) between base stations as the move to other radio coverage areas.

Figure 1 shows a sample WPBX radio system. A WPBX system typically has a switching system that is located at the company. The WPBX switch interfaces a PSTN communication line and multiple radio base stations. Radio base stations communicate with wireless office telephones that can move throughout the system. A control terminal is used to configure and update the WPBX with information about the wireless office telephones and how they can be connected to the PSTN.

Figure 1: Wireless Private Branch Exchange (WPBX)

Systems : Computer Telephony Integration (CTI)

The different types of systems used in private telephone networks include key telephone systems (KTS), private branch exchange (PBX), Centrex, and computer telephony integration (CTI). Key telephone and PBX systems often use proprietary specifications. There are several industry standards that are used for computer telephony and LAN telephony system.

Computer Telephony Integration (CTI)
CTI is the integration of computer processing systems with telephone technology. Computer telephony provides PBX functions along with advanced call processing and information access services. These services include, pre-paid telephony access control, interactive voice response (IVR), call center management, and private PBX.

CTI uses a system of interfaces between telephone switching systems (typically PBX’s) and computer systems. It is through these interfaces that information is exchanged that causes actions by the receiving system in coordination with the sender. These industry standard interfaces include telephone application programming interface (TAPI), telephony services application programming interface (TSAPI), and Java TAPI (JTAPI).

Telephony API (or TAPI) is a standard for communication between computer systems and telephone systems. Most telephone PBX manufactures provide TAPI via special interface cards that directly network with computer systems. TSAPI is a software communication standard developed primarily by the companies Lucent and Novell to allow PBX or Centrex systems to communicate through the use of NetWare communications software. JTAPI is a software communication standard based on Java programming language that allows computers to control PBX systems using Java programming language. Through the use of these standard interfaces, IVR and ACD systems can exchange information with PBX and CTI systems.

At the core of most CTI systems is a voice board installed in a Unix or Windows based computer system. The voice board is a small switch that contains line interfaces. One of the voice board line interfaces connect to a trunk line (such as a T1 or E1 line). Voice boards usually have multiple telephone extension line interfaces. These line interfaces can be for analog or digital telephones. A single CTI computer may contain multiple voice boards or expansion assemblies may be connected CTI systems can use standard or advanced digital telephones.

CTI systems can hold several software programs that are capable of different applications such as voice mail, IVR, ACD, fax, and email broadcast. CTI systems can use hard disk memory to store voice mail and fax mailboxes.

Figure 1 shows a sample CTI system computer that contains a voice card. This voice card is connected to a multiple channel T1 line. The voice card connects digital PBX stations through the voice card to individual DS0 channels on the T1 line when calls are in progress. Several software programs are installed on this system that provide for call processing, IVR, ACD, voice mail, fax, and email broadcasting. The monitor shows a directory of extensions. The advanced call processing feature shows text names along with the individual extensions to allow callers to automatically search through a company’s directory without the need to use an operator.

Figure 1: Computer Telephony Integration (CTI)

Systems : Central Exchange (Centrex) Features

The different types of systems used in private telephone networks include key telephone systems (KTS), private branch exchange (PBX), Centrex, and computer telephony integration (CTI). Key telephone and PBX systems often use proprietary specifications. There are several industry standards that are used for computer telephony and LAN telephony system.

Central Exchange (Centrex) Features
Centrex is a service offered by a local telephone service provider that allows the customer to have features that are typically associated with a PBX. These features include 3 or 4 digit dialing, intercom features, distinctive line ringing for inside and outside lines, voice mail waiting indication, and others. Centrex services are provided by the central office switching facilities in the telephone network.

Centrex systems are EO switches that have software installed to enable advanced call processing features. The software applies to specific ports on the EO switch. This allows these ports on the standard EO switch to operate more like private telephone systems.

Figure 1 shows a typical Centrex system. This diagram shows that the EO switch is equipped with Centrex software. Individual ports from the switch are connected to individual telephones at a company. The public telephone company programs the Centrex features for specific companies into the switch software. The Centrex software monitors the ports so advanced features such as abbreviated dialing can be performed. This example shows that a telephone that is used in a Centrex system can dial a 4 digit dialing number to reach a telephone connected within the companies Centrex telephone network.

Figure 1: Centrex System

Systems : Private Branch Exchange (PBX)

The different types of systems used in private telephone networks include key telephone systems (KTS), private branch exchange (PBX), Centrex, and computer telephony integration (CTI). Key telephone and PBX systems often use proprietary specifications. There are several industry standards that are used for computer telephony and LAN telephony system.

Private Branch Exchange (PBX)
PBX systems are small private telephone systems that are used to provide telephone service within a building or group of buildings in a small geographic area. PBX systems contain small switches that use advanced call processing software to provide features such as speed dialing or call transfer. PBX systems connect local PBX telephones (stations) with each other and to the public switched telephone network (PSTN).

While a PBX is similar to a miniature telephone company EO, PBX systems typically offer more features than public telephone system. The primary function of a PBX is to receive call requests (outgoing calls) from telephone stations users as well as routing incoming calls to specific extension.

Figure 1 shows a private branch exchange (PBX) system. This diagram shows a PBX with telephone sets, voice mail system, and trunk connections to PSTN. The PBX switches calls between telephone sets and also provides them switched access to the PSTN. The voice mail depends on the PBX to switch all calls needing access to it along with the appropriate information to process the call.

Figure 1: Private Branch Exchange (PBX)

Systems : Key Telephone System (KTS)

The different types of systems used in private telephone networks include key telephone systems (KTS), private branch exchange (PBX), Centrex, and computer telephony integration (CTI). Key telephone and PBX systems often use proprietary specifications. There are several industry standards that are used for computer telephony and LAN telephony system.

Key Telephone System (KTS)
A key telephone systems (KTS or key systems) is a multi-line private telephone network that allows each key telephone station to select one of several telephone lines. Key systems contain a key service unit (KSU) that coordinates status lights and lines to key telephones (Key Sets). Key systems have some advanced call processing features such as call hold, busy status, and station-to-station intercom.

KTS are relatively simple non-switching telephone systems. The KSU only interfaces (connects) key sets to the public telephone lines allow calls to directly pass through. The KSU does sensing and provide display status lines to each key service unit. The first generation key systems allowed multi-button telephones to have an appearance (e.g., a button) for multiple end office lines. When the incoming telephone line received a ringing signal, the key system flashed the appropriate button. To answer the call, the user picked up the handset and pressed the flashing button. This off-hook indication is sensed by the KSU which results in the the key set’s line status light to become solid. This indicated to other telephone users that the line was being used.

To place a call, the user would first view the lights on telephone line buttons. If a button was not lit, the user pressed the button. Again, the KSU sensed the off-hook condition and a solid light came on on all key sets.

To allow key sets to talk with each other without connecting through the public telephone network, most KTS systems included an intercom feature. The intercom feature allowed a key set to call one or all the key sets that are connected to the KSU.

Figure 1 shows a typical key telephone system. This diagram shows telephones wired to a key service unit (KSU) that is connected to the PSTN. The KSU allows the telephones to have access to the outside lines to the PSTN. The KSU controls lights on the telephone sets, intercom access, and call hold.

Figure 1: Key Telephone System

Telecom Made Simple

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