Hello, everyone!
Today, I would like to share with you an article on Hybrid networks - DSL technology.
DSL (digital subscriber line) technology was created to allow broadband services to be delivered over existing copper telephone lines with little utilization of optical fibers. This technique decreased expenses, but it also imposed significant constraints due to the usage of twisted pairs as a transmission medium:
a. With increasing frequency, attenuation and cross talk increase, lowering bit rate and range ( Below figure 1).
Figure 1. In DSL systems using twisted pairs in telephone wires, the typical reach vs. bit rate is shown.
b. Cross talk among adjacent pairs precludes NTs from running at the same frequency at the same time, resulting in a shared bandwidth medium on the cable.
c. Old phone wires are generally in bad shape (degraded insulation, moisture intrusion, deformations), and breakdowns are common.
d. Copper wires are frequently stolen for scrap metal, resulting in higher maintenance expenses.
The digital cancellation of cross talk, also known as vectoring, can solve problem (b).
Twisted pairs show an increase in attenuation and cross talk with frequency, resulting in an interdependence between bit rate B and DSL link reach L, according to the formula:
B ¼ kL-1.5
where k is a variable that is affected by wire diameter, cable design, and condition, along with signal processing in active equipment. Figure 1 depicts this relationship.
Greater bit rates necessitate shorter copper loops and a greater number of remote units, both of which necessitate building permissions and electricity. When examining the reach values in Figure 1, it becomes evident that distant units are desperately needed.
The goal of DSL system development is to increase bit rates while also attempting to supply a good portion of full capacity over greater distances. Attenuation, cross talk, and external interference are all frequency-dependent, changing transmission properties of copper loops that equipment must dynamically adapt to.
This is accomplished in the G.fast system by using discrete multitone (DMT) modulation and dividing the occupied bandwidth into a large number of evenly spaced tiny sub-bands, such as 2048 in the 2.2–106.0 MHz region with 51.75 kHz spacing. Only sub-bands with a high enough signal-to-noise ratio are used, resulting in varying bit rates and "up to" service standards.
The statistics in Table 1 for bandwidth and bit rate are maximum values defined in standards. The length of time a cable can be used in service is determined by its design and technical condition; the numbers in Figure 1 and Table 1 are just suggestive.
Table 1. A comparison of DSL systems that are now in use.
Pair bonding: the use of two or more copper pairs for parallel transmission, which is most successful when combined with vectoring, can improve the performance of a DSL link.
Figures 1 and 2 show that future upgrades to legacy copper networks are not viable since system reach has become too short for most purposes, with the exception of unique instances such as historic buildings in Europe, where new cable installation is fiercely opposed.
Figure 2. Residential clients get the best download rates.
The gigabit version of the G.fast technology has already demonstrated this. Despite the fact that standards were published in 2014, large-scale deployments did not begin until 2017, and operators choose 250 Mb/s products with a range of around 200 meters.
Nevertheless, work on the system continues; in 2014, a prototype XG-FAST system capable of transmitting 10 Gb/s over one or two pairs at a distance of 30 m and with a bandwidth of 500 MHz was demonstrated.
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