Asymmetric Digital subscriber Line (ADSL) (1)

ADSL can provide users with more than 6Mbps of data transmission bandwidth, this condition is enough to achieve Internet access, video on demand and access to the local area network. ADSL can achieve bidirectional transmission rate of 640 Kbps in interactive mode. ADSL technology has increased the bandwidth of the existing public telephone network data transmission 50 times times. The transmission of voice, text, and low-resolution images is no longer limited by bandwidth.

ADSL technology can be a unified network to the entire country to provide multimedia (including full-frame video program) services. ADSL performance parameters are shown in table 1.

Table 1. ADSL Performance Parameters

Transfer Rate (Mbps)	Line number specification (AWG)	Distance (ft)	Line width (mm)	Distance (km)
1.5.2.0	24	18,000	0.5	5.5
1.5.2.0	26	15,000	0.4	4.6
6.1	24	12,000	0.5	3.7
6.1	26	9,000	0.4	2.7

1. Analog Modem History

The term modem (modem) is actually an acronym for the term modulation/demodulation (modulation/demodulation). Modems allow two computers to communicate with each other using a public switched telephone network. The public switched telephone network can only transmit voice signals, so modems need to convert the computer's digital information into a series of high-frequency voice signals that can be transmitted over a telephone line. When the speech signal arrives at the destination, it is then demodulated, which is then converted to digital information that can be received by the computer (see Figure 1).

P2 Figure 1.

All modems use some form of compression algorithm and error correction algorithm. The compression algorithm can increase the rate of data transfer under normal conditions by 2 to 4 times times. The error-correcting algorithm is responsible for checking the integrity of the introduced data and requesting the source to resend the packet when it detects a problem with the data.

2. Analog Modem Market

The origins of the analog modem market can be traced back to the July of 1968. At the time, in a landmark carterfone resolution, the US Communications Commissioner stated: "It is unreasonable to prohibit users from using their own interconnected equipment." ”

January 1, 1969, at&t revised its procedures to allow user-owned devices (such as modems) to connect to public switched networks, but must comply with the following three important conditions:

• User-owned equipment in the output power and other performance indicators must be limited so that they do not interfere with or in any way damage the telephone network.

• Only the protection device provided by the telephone company can interconnect with the public switched network, sometimes the protection device refers to the data access device (DAA).

• All network control signaling, such as dial tone, busy signal and so on, must be handled by the phone company's device at the device interconnection point.

In 1976, the United States Communications Commission proposed a proposed programme under which current protection equipment should be phased out to support the so-called registration programme. The registration programme permits the use of equipment that is electrically connected to a direct and switched network, provided that such equipment is inspected and registered by an independent body such as the United States Communications Commissioner, who is responsible for technical testing of these equipment and verifying their safe use in the Exchange network.

After World War II, a strong focus on the importance of information science prompted Claude Shannon to publish a landmark paper in 1948, through the scientific principles elaborated in this important paper, To deepen our understanding of the communication channel power capacity and the channel that is limited by Gaussian noise (i.e. analog telephone channel in our daily life):

C = Bw * LOG2 (1+s/n)

The above simple formula clearly illustrates the relationship between the various factors that determine the capacity of the channel, in which C equals the available channel bandwidth, bw being a multiple of the bandwidth signal-to-noise ratio plus 1 followed by the 2 base logarithm. This formula does not explain how to implement the bandwidth of the channel in engineering, it simply shows that the capacity of the channel can be achieved by taking appropriate technical measures.

As more and more users start buying and using modems, issues such as speed and reliability of modems become even more important. Product suppliers are trying to get their products up to the bandwidth limit set by Shannon Law. Until v.32 recommended, all modem standards seemed to be less than 9 to 10 db bandwidth capacity. The estimation of channel capacity is based on the following assumptions: Bandwidth 2400 to 2800Hz, signal-to-noise ratio from 24 db to 30 db, the transmission rate is about 24,000bps. It is clear that, before this gap is reduced, practical error-correcting techniques must be developed.

The the 1950s modems used proprietary FSK (300 to 600bps) and residual edge bands (1200 to 2400 bps) technology. These devices either adopt or directly build on the wireless frequency technology developed during World War II, and then apply the wireless frequency technology to the wired communication field.

The international standardization of modems began in the 1960s. 1964 first passed the first CCITT modem recommended v.21 (1964), a FSK modem with a rate of bps (now the bps), which is still used in the V.34/v.8 handshake protocol. In the 1984, the better performance of the 4-phase (or 2x2-QAM) modulation and the 4x4 QAM V.22bis appeared. In addition, the same year also introduced a new generation of modems recommended v.32 and echo suppression, Trellis coding technology. The trellis code, which was first established by Dr. Gottfredungerboeck, is a significant technological breakthrough that enables modems to obtain forward error-correcting capabilities while also achieving 3.5 db coding gain, nearly 3 to 1 of the maximum Shannon channel capacity. V.32bis recommended that the above technology and the general connection signal to noise ratio is promoted, the transmission rate is increased to the 14,400bps.

It is recognized that telephone networks in many parts of the world need to be further improved, so that the 1989/90-year V.34 standard was set to work. Subsequently, the original 19,200bps design goals for the new standard were quickly modified to 24,000bps and subsequently corrected to 28,800bps. The updated V.34 (1996) modem supports the 33,600bps rate. The bandwidth of such a modem reaches 10 bits per hertz, almost to the theoretical limit. Recently, many companies have introduced 56.6kbps analog modems that connect to standard telephone lines. However, the modem's operating mode is asymmetrical (it works at the upstream end at the speed of a normal modem) and requires a dedicated T1/E1 line to connect to the ISP to reach its theoretical limit. It is reported that the performance of this modem is inconsistent for users who do not have such a line. However, the bandwidth limitation of The voice band is not inherent to the user line itself, but is limited by the function of the core network. The filter at the edge of the core network restricts the speech-level bandwidth to approximately 3.3kHz. If it is not for this filter, the frequency of the copper telephone line can be reached at a MHz order of magnitude. First, the attenuation of the signal determines the transmission rate on the twisted pair, which is determined by the length and frequency of the wires. Table 1 shows the actual limits of one-way data transfer rates in different line lengths.