DSL (Digital Subscriber Line) is a combination of transmission technologies using copper telephone lines as the transmission medium, which includes HDSL, SDSL, VDSL, ADSL, and RADSL, etc. and is generally referred to as xDSL. The main difference between the two is in the difference in the signaling speeds and distances, as well as the difference in symmetry between the upstream and downstream rates. and downlink rate symmetry.
HDSL and SDSL support symmetrical T1/E1 (1.544Mbps/2.048Mbps) transmission. HDSL has an effective transmission distance of 3-4 kilometers and requires two to four pairs of copper twisted-pair telephone wires; SDSL has a maximum effective transmission distance of 3 kilometers and requires only one pair of copper wires. In comparison, symmetric DSL is more suitable for enterprise point-to-point connectivity applications, such as file transfers, video conferencing, and other tasks where the amount of data sent and received is roughly proportional. The market for symmetric DSL is much smaller compared to asymmetric DSL.
VDSL, ADSL and RADSL are asymmetric transmissions. Among them, VDSL technology is one of the fastest among xDSL technologies. On a pair of copper twisted-pair telephone lines, the rate of uplink data is 13 to 52 Mbps, and the rate of downlink data is 1.5 to 2.3 Mbps. However, the transmission distance of VDSL is only within a few hundred meters, and VDSL can be a cost-effective alternative to fiber-to-the-home, and at present, Shenzhen's VOD ( VOD (Videodemand) in Shenzhen is realized by using this access technology; ADSL supports uplink rate of 640Kbps to 1Mbps and downlink rate of 1Mbps to 8Mbps on a pair of copper wires, with an effective transmission distance of 3-5 kilometers; RADSL can provide the same range of speeds as ADSL but it can be used according to the quality of the twisted copper wires. RADSL can provide the same speed range as ADSL, but it can dynamically adjust the user's access speed according to the quality of the twisted pair copper wire and the distance of transmission. It is these features that make RADSL an ideal technology for high-speed surfing on the Internet, video-on-demand (IAV), and remote local area network (LAN) access, where users often download more information than they upload (send commands).
According to CNNIC (China National Network Information Center), as of October 31, 1997, the number of Internet users in China reached 620,000, of which 49.1% of the users thought that the most disappointing thing about Internet was that the online speed was too slow, while 36.2% of the users thought it was that the charges for Internet access were too expensive; and as of June 30, 1998, in less than one year As of June 30, 1998, in less than a year, the above three statistics have nearly doubled, respectively: 1.175 million, 88.9%, 61.2%. This shows that with the rapid expansion of the number of Internet users in China, the slow speed of the Internet and the expensive charges have become two major obstacles to the development of the Internet in China.
ADSL access service can do a higher performance-price ratio of this point, and ADSL access technology than other access technologies have their unique technical advantages are inseparable. Here's how ADSL compares with other access services:
Comparison of ADSL and CableModem:
Compared with CableMode, ADSL technology has considerable advantages.CableModem's HFC access program uses a hierarchical tree-type structure, which has the advantage of higher bandwidth (10M), but the technology itself However, the technology itself is a crude bus-based network, which means that users have to share a limited amount of bandwidth with neighboring users, which slows down when there is a surge of users on a line. In addition, the information shows that, in most cases, the HFC program must take into account the existing cable TV programs, and take up part of the bandwidth, only a part of the remaining for the transmission of other data signals, so the theoretical transmission rate of CableModem can only reach a small half. Overseas company experiments show that its rate is reduced to 1M-2Mbps, more common is 400K-500Kbps. comprehensive view, even in the ideal state, HFC is only equivalent to a 10Mbps **** enjoy the type of bus-type Ethernet, and ADSL access program in the network topology is more advanced, because each user has a separate line connected with the ADSL bureau, its structure can be seen as a star-type. Its structure can be regarded as a star structure, and its data transmission bandwidth is exclusively shared by each user.
Comparison of ADSL with ordinary dial-up Modem and N-ISDN:
A) Compared with ordinary dial-up Modem's maximum rate of 56K and N-ISDN's rate of 128K, ADSL's rate advantage is self-evident.
B)Compared with ordinary dial-up Modem or ISDN, ADSL is more attractive because it transmits data and voice signals on the same copper line, and the data signals do not pass through the telephone exchange equipment, which reduces the load of the telephone exchange, and does not require dialing, always online, belonging to the private line Internet access. This means that you don't need to pay a separate phone bill to use ADSL to access the Internet.
Asymmetric Digital Subscriber Line (ADSL)
ADSL can provide users with a data transmission bandwidth of more than 6Mbps, which is more than enough for Internet access, video-on-demand, and access to LANs. In interactive mode ADSL is capable of reaching bi-directional transmission rates of 640 kbps. ADSL technology increases the data transmission bandwidth of the existing public **** telephone network by a factor of 50.
ADSL Technology
ADSL transmits data over ordinary telephone lines almost 200 times faster than today's modems and 90 times faster than ISDN. Early tests and experiments worldwide have shown ADSL to be a promising access technology.