4.5. Point-to-Point Trunking
Besides the trunks that supply a PBX with a dial-tone from the telephone company CO, it is common to have trunk connections between multiple PBXs. These types of connections, which are often high-density copper cables if the PBXs are within a few hundred meters , are called point-to-point trunks or private trunks . Often, these trunks are established in order to forego the PSTN, and any associated fees, for calls between disparate business locations, as in Figure 4-9.
Figure 4-9. A private trunk connects two switches on the same private voice network
When privately owned cable can't be run between locations due to excessive distance, telco-owned facilities, like T1s, T3s, or wireless solutions can be used for private trunking. BRI-ISDN is often used for low-density switch-to-switch trunking across relatively short distances, but its high cost is a discouraging factor. BRI-ISDN allows two voice calls to be carried at once.
T1 circuits are far more abundant than BRI-ISDN in trunking situations, because they offer 12 times the capacity. Even though the cabling used to carry the T1 circuit is the phone company's property, and the phone company charges a fee for the use of that pathway , the T1 circuit used to connect two switches can still be called a private trunk because the voice calls it carries are not considered PSTN traffic.
The ITU's recommendation for T1 technology describes both the transport and data link layers . The physical layer for a T1 circuit tends to be two pairs of copper wiring. The phone company may connect the T1 through its local access transport areas (LATAs) using fiber, but the part of the circuit that connects to the customers' locations is almost always copper.
Even higher-density T1-like circuits are available for private trunking if the need exists. DS3 circuits multiplex many times the capacity of a T1. You can find out about even higher-capacity telephone circuits by studying the ITU-T's recommendations or by asking a local telephone company's presales engineer.
Aside from ISDN lines, T1, and DS3, there are four ways private voice trunks can be carried between locations: copper, fiber optics, radio waves, and free-space optics. The two most dominant, in traditional telephony, are copper and fiber- optic cabling, but wireless technologies are becoming more popular for trunking because of their low cost.
4.5.1. Copper Cabling
Copper is the material that is found in most local area data and voice network wiring. It's a flexible, resilient material that is highly conductive of electricity. It carries electrical signals via direct current over short distances, say from the CO to your house or from an Ethernet switch to a desktop PC or IP phone. Copper is the most common type of material used to wire the connections between analog and digital phones and their switches.
Copper is excellent and cheap for short-distance transmission. But over long hauls, it suffers from attenuation. This is why Ethernet twisted-pair data links have a length limit.
4.5.2. Fiber-Optic Cabling
Fiber-optic cables, or fibers, are very, very long strands of glass that are almost optically perfect. That is, when light beams shine into one end of a fiber-optic strand , they tend to emit from the other end without losing any power. Switching the light source on and off allows you to transmit a digital bit stream over the fiber. Attenuation is still a factor for fiber optics as with copper, but the length limits imposed are much highermiles instead of meters.
Fiber itself is rather expensive and more fragile than copper, making it a poor choice for short-distance connectivity. But there are some local area applications where fiber beats copper. In areas with very high electrical noise, fiber will work where copper often fails, because fiber cabling carries optical, not electrical, signals.
Because fiber doesn't use electricity for signaling, it doesn't require a physical loop. This means that it can still operate with one strand of fiber, rather than two, intact, while a copper segment needs two strands intact in order to function at all.
Radio waves can be used to carry voice and data, though in modern enterprise networks, their use for data is far more common. 802.11b and 802.11g are common standards for deploying Ethernet wirelessly using radio. They offer short-distance (typically twenty-five to a few hundred meters, depending on the antenna and base) connectivity for many client devices using a single radio base station. These standards can also be used for simple Ethernet bridging.
Other types of radio can be used to connect, or trunk, between locations in a metro area network (MAN). These radio technologies use high-power microwave transmitter/receiver stations with a very narrow focus to transmit data across relatively great distancesas much as 15 or 20 miles in enterprise networks. If you've ever spotted the dish-shaped antenna that are sometimes used to connect skyscrapers in a downtown skyline, then you've seen how this technology facilitates point-to-point trunking.
Radio is great for ease of deployment. Many manufacturers of radio datacom devices support license-free radio spectrum. The products themselves tend to be Ethernet bridges, making them largely configuration-free. But radio is less reliable than copper or fiber. Microwave radio needs a clear "line of sight" between connected locations, which often rules it out in mountainous regions or terrains with very tall trees. While antenna towers can solve this problem, they are often cost prohibitive, and an eyesore, too.
4.5.4. Free-Space Optics
A relatively new development, free-space optical (FSO) uses light beams transmitted over the open air, like a television remote control, to connect remote locations. Like radio, FSO requires a line of sight and is subject to the same terrain challenges. Like microwave radio antenna, FSO transmitters and receivers must be carefully kept in alignment.
But unlike radio products, FSO links are capable of much higher data throughput approaching 3 gbps. FSO's range is limited to about a mile. Where radio is susceptible to RF interference, FSO is susceptible to light interferenceespecially from the sun.
More information about metro area networking is available in "Metro Ethernet," available at O'Reilly's Safari Bookshelf (http://safari.oreilly.com), and in O'Reilly's 802.11 Wireless Networks: The Definitive Guide .