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 SystemsIXCs 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 InterconnectionInternational 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
