DSL

DSL

Today the two most common types of broadband residential access are digital subscriber line (DSL) and cable. In most developed countries today, more than 50% of the households have broadband access, with South Korea, Iceland, Netherlands, Denmark, and Switzerland leading the way with more than 74% penetration in households as of 2008 [ITIF 2008]. In the United States, DSL and cable have about the same market share for broadband access [Pew 2008]. Outside the United States and Canada, DSL dominates, mostly in Europe where more than 90% of the broadband connections are DSL in many countries.

A residence typically obtains DSL Internet access from the same company that provides it wired local phone access (i.e., the local telco). Therefore, when DSL is used, a customer's telco is also its ISP. As shown in the following figure, each customer's DSL modem uses the existing telephone line (twisted-pair copper wire) to exchange data with a digital subscriber line access multiplexer (DSLAM). Normally located in the telco's CO. The telephone line carries simultaneously both data and telephone signals, which are encoded at different frequencies:

● A high-speed downstream channel, in the 50 kHz to 1 MHz band
● A medium-speed upstream channel, in the 4 kHz to 50 kHz band
● An ordinary two-way telephone channel, in the 0 to 4 kHz band

This approach makes the single DSL link appear as if there were three separate links, so that a telephone call and an Internet connection can share the DSL link at the same time, (We'll explain this technique of frequency-division multiplexing in "Circuit Switching and Packet Switching"). On the customer side, for the signals arriving to the home, a splitter separates the data and telephone signals and forwards the data signal to the DSL modem. On the telco side, in the CO, the DSLAM separates the data and phone signals and sends the data into the Internet. Hundreds or even thousands of households connect to a single DSLAM [Cha 2009, Dischinger 2007].

DSL Internet access

DSL has two major advantages over dial-up Internet access. First, it can transmit and receive data at much higher rates. Normally, a DSL customer will have a transmission rate in the 1 to 2 Mbps range for downstream (CO to residence) and in the 128 kbps to 1 Mbps range for upstream. Because the downstream and upstream rates are different, the access is said to be asymmetric. The second major advantage is that users can simultaneously talk on the phone and access the Internet. Unlike dial-up, users do not dial an ISP phone number to get Internet access; instead, they have an "always-on" permanent connection to the ISP's DSLAM (and hence to the Internet).

The actual downstream and upstream transmission rate available to the residence is a function of the distance between the home and the CO, the gauge of the twisted-pair line and the degree of electrical interference. Engineers have expressly designed DSL for short distances between the home and the CO, allowing for substantially higher transmission rates than dial-up access. To boost the data rates. DSL relies on advanced signal processing and error correction algorithms, which can lead to high packet delays. However, if the residence is not located within 5 to 10 miles of the CO, DSL signal-processing technology is no longer effective, and the residence must resort to an alternative form of Internet access.

There are also a variety of higher-speed DSL technologies enjoying penetration in a handful of countries today. For example, very-high speed DSL (VDSL), with highest penetration today in South Korea and Japan, provides impressive rates of 12 to 55 Mbps for downstream and 1.6 to 20 Mbps for upstream [DSL 2009].


Tags

transmission rate, isp, telco

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