Hello, everyone!
Today, I would like to share with you an article on DSL technology and its limits.
Digital subscriber line (DSL) technology was developed to reuse existing copper telephone cables for broadband services, with the minimum use of optical fibers. This approach reduced costs, but limitations imposed by the use of twisted pairs as transmission medium are severe:
Figure 1. Typical reach vs. bit rate in DSL systems using twisted pairs in telephone cables.
Attenuation and cross talk rise with frequency, reducing bit rate and reach (Figure 1).
Cross talk between adjacent pairs prevents NTs from simultaneously operating at the same frequencies, making the cable a medium with shared bandwidth.
Old telephone cables are frequently in poor condition (deteriorated insulation, entry of moisture, deformations), and failures are frequent.
Copper cables are often stolen for metal scrap, increasing maintenance costs.
Problem (d) can be eliminated with digital cancelation of cross talk, known as vectoring. Twisted pairs exhibit rise of attenuation and cross talk with frequency, resulting in interdependence between bit rate B and reach L of DSL link, in accordance with a formula:
B = kL-1.5
where k is a factor dependent on wire diameter, design, and condition of cable, as well as signal processing employed in active equipment. This dependence is presented in Figure 1.
Higher bit rate requires shorter copper loops and larger number of remote units, which in turn need construction permits and electric power. The acute need for remote units becomes clear after comparing reach values presented in Figure 1 and distances from the CO to 90 or 95% of customers.
Development of DSL systems is focused on increasing bit rates, combined with attempts to deliver a good fraction of full capacity at longer distances. Equipment must dynamically adopt to a frequency-dependent, variable transmission characteristics of copper loops, including attenuation, cross talk, and external interference.
This is achieved by employing a discrete multitone (DMT) modulation and division of occupied bandwidth into a large number of equally spaced narrow sub-bands, e.g., 2048 in the 2.2–106.0 MHz band with 51.75 kHz spacing in the G.fast system.
Only the sub-bands with sufficient signal-to-noise ratio are used, resulting in variable bit rate and “up to” service specifications.
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