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2. Symmetric DSL
Symmetric types of DSL connections provide equal bandwidth
for both uploads and downloads. Symmetric DSL technology is popular for
business-class DSL services as companies often have greater needs for
transferring data. Typical symmetric DSL connections support 1.5 Mbps for
downloads and uploads.
(Symmetric Digital Subscriber Line): SDSL possesses all of the common
characteristics of DSL, including an "always on" combination of voice
and data services, availability limited by physical distance, and high speed
access compared to analog modems. SDSL supports data rates up to 3,088 Kbps.
SDSL works by sending digital pulses in the high-frequency area of telephone wires and cannot operate simultaneously with voice connections over the same wires.SDSL requires a special SDSL modem.
II. HDSL (High-data-rate DSL): HDSL service provides equal bandwidth for both downloads and uploads, offering data rates up to 2,048 Kbps. Note that HDSL requires multiple phone lines to accomplish this.
Like other forms of DSL, HDSL features "always on" combined voice and data services. HDSL enjoyed some popularity in years past, but the technology has effectively been replaced by alternatives like SDSL that only require one phone line and offer similar performance and reach.
HDSL can carry both voice and data over a single communication link. The maximum distance for HDSL transmission is generally 3700 meters when running over unconditioned copper twisted-pair wiring; some vendors claim that their devices support twice this distance. This maximum distance from the central office (CO) is sometimes called the Carrier Service Area (CSA). HDSL is typically used for connecting bridges, routers, and telephone equipment such as Private Branch Exchanges (PBXs) over a campus using HDSL line drivers with built-in CSU/DSU (Channel Service Unit/Data Service Unit) functionality. It can also be used for private data networks, connections between points of presence (POPs), and other services. It is not widely implemented at the customer premises level, which uses the more popular ADSL or G.Lite for providing customers with high-speed Internet access.
3. ISDN DSL or IDSL
Another type of DSL Internet Service is the so-called IDSL, which is actually a hybrid DSL (ISDN technology) that was designed at the same time when the other forms of DSL Internet technologies were made. IDSL however is not used that much because of the low speeds that it offers, actually only a maximum of 128 Kbps.
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