The dial-up connection has become too slow, with a theoretical speed of 56 kbps but actual speeds being much lower. Nowadays, with ADSL technology, internet cafes are popping up everywhere to meet the information-seeking needs… especially for the communication and entertainment demands of young people. Moreover, Internet Service Providers (ISPs) offer affordable ADSL service packages to cater to the diverse needs of the public. So, what is ADSL? Is ADSL a branch of xDSL technology?
Alexander Graham Bell invented the telephone in 1875, and since then, the telephone has become an indispensable tool in human life. The continuous development of the telephone network has motivated telecommunications developers to aspire to ensure that telephone lines are not only used for voice signal transmission but can also transmit many other services, to meet the growing needs of people such as TV, video conferencing, and internet access…
Previously, telephone lines could only transmit one voice channel with a bandwidth of 3.4 kHz. Thanks to the application of digital processing technologies, attenuation compensation, and noise reduction, xDSL technology can transmit 100 digital voice channels or one high-quality video channel over a single telephone line. DSL (Digital Subscriber Line) is a technology that uses complex modulation methods to convert data into packets for transmission over telephone lines.
DSL modems transmit data between two endpoints of the copper cable in a different way. The signal does not pass through the telephone switching system, and therefore does not interfere with the voice signal. In fact, the voice bandwidth on copper cables is only 0 – 4 kHz, while DSL technology typically uses frequencies above 100 kHz.
The significant advantage of xDSL technology when it was introduced was its ability to transmit various applications that were previously unachievable, while also leveraging the existing and widespread telephone network. A technology is considered effective when it utilizes available resources and infrastructure.
xDSL is categorized as follows:
ISDN – Integrated Services Digital Network
ISDN is considered the precursor of xDSL. It was introduced in 1976 with the ambition to unify data and voice transmission. In ISDN, the Basic Rate Interface (BRI) provides 2 channels of 64 kbps (B channels) for voice or data and one 16 kbps channel (D channel) for control signaling information. The drawback of this technology is that it only transmits voice services and low-speed packet switching, making it unsuitable for high-speed packet switching and long hold times, which characterizes the current Internet. Therefore, ISDN is not widely adopted and is mainly used by households or small businesses. However, for ISDN users in the United States (the birthplace of ISDN), the benefits provided by ISDN are hard to forget, as it was the initial technology for all types of integrated services. IDSL (ISDN Digital Subscriber Line) is an xDSL technology based on ISDN, ensuring a speed of 144 kbps over both B and D channels.
HDSL – High-Bit-Rate Digital Subscriber Line
HDSL was developed in a laboratory in 1986. Essentially, HDSL transceivers are an evolution of ISDN but more complex. HDSL is based on the T1/E1 standard of the US/Europe. HDSL1 allows for the transmission of 1.544 Mbps or 2.048 Mbps over 2 or 3 pairs of wires. HDSL2, which came later, allows for the use of one pair of wires to transmit 1.544 Mbps symmetrically. HDSL2 incorporates many ideas from ADSL. The advantage of HDSL is that it is a technology that does not require repeaters, meaning it has lower loss than other types over the transmission line. Therefore, HDSL can transmit further while maintaining signal quality. HDSL is favored for its diagnostic features (SNR measurement) and minimal crosstalk interference. HDSL is used by local operators (telephone companies) or to provide high-speed connections between multiple buildings or office complexes.
VDSL – Very-High-Bit-Rate Digital Subscriber Line
VDSL is an xDSL technology that provides symmetric transmission over a pair of copper wires. The download bit rate of VDSL is the highest among all xDSL technologies, reaching up to 52 Mbps, while the upload rate can reach 2.3 Mbps. VDSL typically operates effectively only in short-loop networks. It primarily uses fiber optic cables for transmission, with copper cables utilized only at the endpoints.
ADSL – Asymmetrical DSL
ADSL is a branch of xDSL technology. ADSL provides asymmetric bandwidth over a pair of wires. The term “asymmetric” here refers to the imbalance in download (downstream) and upload (upstream) data flow. The download data flow has a larger bandwidth than the upload data flow. ADSL was developed in 1989 in a laboratory setting. ADSL1 provides 1.5 Mbps for the download data flow and 16 kbps for the upload data flow, supporting the MPEG-1 standard. ADSL2 can provide up to 3 Mbps for the download and 16 kbps for the upload, supporting 2 streams of MPEG-1. ADSL3 can offer 6 Mbps for the download and at least 64 kbps for the upload, supporting the MPEG-2 standard. The ADSL service we commonly use today theoretically provides 8 Mbps for the download and 2 Mbps for the upload; however, due to various reasons from the ISPs, the quality of ADSL service at our endpoints often does not meet the initial advertised standards.
RADSL – Rate-Adaptive Digital Subscriber Line
RADSL is a version of ADSL where modems can check the transmission line upon initialization and adjust during operation to the fastest speed the line can provide. RADSL is also known as ADSL with variable speed.
