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Broadband Tutorial

Broadband networks offer much greater ‘bandwidth’ than the older narrowband technologies. By bandwidth we mean the bit-rate, or number of bits per second that can be transmitted. The International Telecommunications Union (ITU) has defined a broadband connection as any rate higher than the standard rate, T-1.

SPEEDS AND FEEDS. Bit rates come in increments called Kilobits (thousands), Megabits (millions) and Gigabits (billions) of bits per second. The fastest modems connecting a PC to the Internet operate at 56 Kbps (Kilobits per second), while a typical fiber-optic cable carries 2.5 Gbps (Gigabits per second). Here are some examples of typical narrowband rates:

Narrowband communications normally operate over copper wires or coaxial cables. Broadband transmission technologies were intended to leverage the vast bandwidth of fiber optics. The Optical Carrier (OC) hierarchy, is used to describe broadband rates:

Recent advances in optical transmission technology, known as Dense Wavelength Division Multiplexing (DWDM), promise dramatic increases in the capacity of a fiber line. DWDM technology splits a beam of light into multiple colors, or wavelengths, each of which can operate at 10 Gbps. The technology is rapidly advancing, with the number of possible wavelengths exceeding 100 per fiber.

CIRCUITS AND PACKETS. There are two fundamental types of networks, circuit, and packet. The telephone network is circuit switched, while data networks, such as the Internet, are packet switched. When a telephone call is made, a live circuit is set up through the network, and a fixed amount of bandwidth, typically 64 Kbps, is reserved for the duration of the call.

Packet technology breaks data into small pieces, each containing an address. Sending a packet is much like mailing a letter; many envelopes of data enter the network at the same time, where they may travel over the same or different routes. Eventually, most of them arrive at the destination.

Packets are more efficient than circuits, because a single line can carry multiple messages simultaneously. The problem is ‘real-time’ communications, like voice or video, doesn’t work well on packet networks because there is no way to know when the packets will arrive or in what order.

Packet technology is improving, however, and it is only a matter of time before packet switched networks can support telephone calls, with an acceptable level of quality, at a fraction of the cost of a circuit switched call.