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

Systems : Computer Telephony Integration (CTI)

Systems
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

Systems
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)

Systems
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)

Systems
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

Private Telephone Networks : Switching Systems, Numbering Plan

Switching Systems
Private telephone switching systems are network devices that are small versions of telephone switching systems. Early key telephone switches used mechanical levers (crossbars) to interconnect lines. These were called key service units (KSUs). PBX systems use a time slot interchange (TSI) memory matrix to dynamically connect different communications paths through software control. Computer telephony and LAN telephony systems use packet switching systems to interconnect one of more telephone station with each other.

For large private telephone systems, some of the switching functions may be distributed to remote points. An example of distributed switching is the Nortel RPE that allows the Meridian PBX to remote a portion of its station interface to a remote site via a pair T1’s or E1’s.

Numbering Plan
Each extension in a private telephone system has a unique extension number. The station numbering plan for private telephone systems is controlled by the owner of the private telephone system. Many private systems have a limited range for extension numbers (e.g., 1000 -1999. This extension range is restricted due to hardware configurations.

When private telephone systems are interconnected to the public telephone network, the CCITT world numbering plan (E.164) and national numbering plans are used. PBX call processing systems are able to filter numbers to enable least cost routing (LCR). LCR is a telephone system feature that routes the connection of a call over the least expensive route available at the time the call is originated.

To allow automatic routing of incoming calls, direct inward dialing (DID), or higher-level trunk lines (e.g., T1 or E1) with advanced signaling may be used. DID connections are 2-wire trunk-side (network side) EO connections that provide additional information to the PBX to allow the automatic routing of calls within the PBX system. Although network signaling on incoming 2-wire circuits is primarily limited to one-way, incoming service, DID connections employ different supervision and address pulsing signals than dial lines. Typically, DID connections use a form of loop supervision called reverse battery, which is common for one-way trunk-side connections. Until recently, most DID trunks were equipped with either Dial Pulse (DP) or dual tone multifrequency (DTMF) address pulsing. While many carriers would have preferred to use multifrequency (MF) address pulsing, a number of LEC’s prohibited the use of MF on DID trunks.

PSTN : Switching Systems

Switching Systems
Switching systems are assemblies of equipment that setup, maintain, and disconnect connections between multiple communication lines. Switching systems are often classified by the type of network they are part of (e.g., packet or circuit switched) and the methods that are used to control the switches. The term “switch” is sometimes used as a short name for switching system. Public telephone switching systems have many switches within their network. A typical switch can handle up to 10,000 communication lines each.

Early switches used mechanical levers (crossbars) to interconnect lines. Modern switches use computer systems to dynamically setup, maintain, and disconnect communication paths through one or more switches. True computer-based switching came about through the introduction of the electronic switching systems (ESS’s). ESS EOs did not require a physical connection between incoming and outgoing circuits. Paths between the circuits consisted of temporary memory locations that allowed for the temporary storage of traffic. For an ESS system, a computer controls the assignment, storage, and retrieval of memory locations so that a portion of an incoming line (time slot) could be stored in temporary memory and retrieved for insertion to an outgoing line. This is called a time slot interchange (TSI) memory matrix. The switch control system maps specific time slots on an incoming communication line (e.g., DS3) to specific time slots on an outgoing communication line.

The public telephone network switching system architecture uses a distributed switching system that has a hierarchy of switching levels. Distributed switching systems connect calls through the nearest switching system. With distributed network architecture, the call processing requirements are distributed to multiple points. Using a multilevel hierarchy structure for switching systems allows switching to occur at lower levels of switching unless the telephone call must pass between multiple switches. At that point, the call is passed up to a higher-level switch for transfer to more distant locations.

In conjunction with distributed network architecture, the ability to perform “dynamic routing” furthers the network’s resiliency to faults. Sometimes called “adaptive routing”, dynamic routing automatically re-routes communication paths or circuits as the network traffic levels (e.g., levels of congestion) change or as paths go in or out of service.

A key part of public telephone networks is system reliability. As a result, in the event of equipment failure in such a network, backup (redundant) equipment must provide for continued service. Although this increases the reliability of switching systems, it also increases the system cost (for additional backup equipment) and complexity (recovery management systems).

Public telephone switching systems use EO telephone switches to connect the telephone network to end customers. These switches serve as an end node switch that and provides local dial and access to local and long distance services. Switches that are used to interconnect switches to each other are called tandem switches.

Some systems use mini-switches called remote digital terminals that are located near the EO switch. These mini-switches act as concentrator lines of voice channels between the end customers and the EO switching system. Concentrators grouping multiple communication lines into more efficient trunked (multi-channel) lines.