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What is an SDSL Modem?
An SDSL modem is a device that transmits and receives data through regular telephone lines at speeds that far exceed traditional analog modem technology. It allows your workstation to connect to a remote site using a network protocol such as IP or IPX. The OfficeConnect Remote 840 contains one SDSL modem port which is the WAN interface. http://www.3com.com/solutions/en_US/home/dsl_faq.html I hear a lot about different DSL technologies. What are the differences among these DSL technologies and why would I choose one over the other?
Often, one will see reference to "xDSL" where x is replaced by a particular letter indicating the type of DSL technology. The available DSL technologies today include: ADSL, SDSL, IDSL, HDSL/HDSL2 and VDSL. ADSL: Asymmetrical Digital Subscriber Line: A next-generation modem technology that allows up to 8 Mbps downstream and up to 1 Mbps upstream. ADSL comprises ADSL full-rate and G.lite, which are covered in this document. SDSL: Symmetric Digital Subscriber Line: Is used often for symmetric T1/E1 speeds up to 1.544/2.048 Mbps. SDSL makes an ideal and cost-effective replacement for T1/E1 service. IDSL: ISDN Digital Subscriber Line: A DSL flavor that uses 2B1Q line coding on ISDN basic rate circuits. It is used for data applications only and typically operates up to 144 Kbps. IDSL is like a nailed-up ISDN circuit. HDSL: High Bit Rate Digital Subscriber Line: A mature, medium-speed, symmetric technology. This technology is often used to implement T1 data circuits over phone lines. HDSL requires two pairs of wire for transmitting and receiving. HDSL2 is a new proposed standard for running T1 data speeds over a single pair of copper wires. VDSL: Very High Speed Digital Subscriber Line: A future technology to watch. It supports dedicated high speeds (10 to 50 Mbps downstream and up to 8 Mbps upstream) over short distances of 1 to 4,000 feet or 1.2 Km. VDSL is only in limited trial deployment today. Standards bodies are beginning work on an industry-standard VDSL implementation. The choice of which DSL technology is your best option depends on 1) your bandwidth requirements, 2) how far you are from your central office and 3) what equipment the service provider has installed in that central office. ADSL will function out to a maximum of about 18,000 feet. While SDSL, which only provides speeds of up to 1.5 Mbps symmetric, will only work out to about 15,000 feet. The same holds true for HDSL and HDSL2. IDSL, however, can be extended out to 36,000 feet from the central office by using U-loop repeatersùdouble the distance of an ADSL line. The other benefit of IDSL is that the signal can pass through a Digital Loop Carrier which typically terminates other DSL flavors. The downside is that the maximum speed for IDSL is 144 Kbps. VDSL is the technology that works at only short distances from the central office. The chart below summarizes the DSL technologies by maximum distance and maximum speed. Technology Maximum Distance from CO Maximum Speed ADSL / G.lite ~18,000 Feet / 5.4 Km 8 Mbps Down / 1 Mbps Up SDSL ~18,000 Feet / 5.4 Km 1.5 Mbps Symmetric IDSL ~ 18,000 to 36,000 Feet / 10.8 Km 144 Kbps Symmetric HDSL ~15,000 Feet / 4.5 Km 1.5 Mbps Symmetric VDSL ~4,000 Feet / 1.2 Km ~50 Mbps Down / ~8 Mbps Up
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