ADSL System Basics: Overview and Types

This article provides an overview of ADSL system basics, including typical ADSL setup, ADSL types, and ADSL standards. Applications like video streaming, VoIP, online video gaming, online shopping, and TV programs require high-speed internet for effective operation. Existing systems like coaxial cable, ISDN, and analog modems often can’t provide the necessary data rates. This demand for high-speed internet services has driven the development of technologies like ADSL (Asymmetric Digital Subscriber Line).

ADSL leverages existing unshielded copper lines, making it usable with the millions of telephone lines already in place. The development of low-cost DSP chips, facilitated by advancements in VLSI, has further popularized this technology.

Fig-1 depicts a typical ADSL setup.

It consists of various subsystems such as DSLAM (Digital Subscriber Line Access Multiplexer), voice switch, frequency selector switch, main distribution panel, DP Box, Splitter, and ADSL modem. Let’s explore these systems.

To provide ADSL connections, the existing telephone network is used. A DSLAM is installed at the telephone exchange or CO (Central Office), and an ADSL modem is placed at the subscriber’s premise.

• DSLAM: It interfaces with the internet backbone and manages the routing of data traffic. Functioning like a concentrator, it provides multiple DSL connections to access internet bandwidth. The connection to the subscriber is established via copper lines.

• Frequency Selector Switch: Installed at the telephone exchange, this filter separates incoming ADSL signals, directing voice traffic to the voice switch and data traffic to the DSLAM.

• Voice Switch: Connected to the PSTN (Public Switched Telephone Network), it provides analog voice connections.

• Main Distribution Panel (MDP): DSLAM pairs terminate at the MDP, from where connections to subscribers are provided using twisted pairs of copper lines.

• Splitter: The twisted pair terminates at the DP box located at the subscriber’s premise, connecting first to the splitter. The splitter is basically a Low Pass filter which removes low frequency POTS voice signals within the band 300Hz to 3400Hz. It passes high frequency ADSL signals to the ADSL modem.

• ADSL Modem: Finally, a connection to the user’s computer is established using an Ethernet cable connected to the Ethernet port of the ADSL modem. The physical layer portion in the ADSL modem converts the signal to a format understandable by the computer.

There’s also a variant of ADSL known as ADSL-Lite, based on G.992.2 or G.Lite. This version does not require a POTS splitter. ADSL-Lite delivers a downstream speed of 1.5Mbps and an upstream speed of 512Kbps.

ADSL is a full-duplex communication system employing either TDD (Time Division Duplex), FDD (Frequency Division Duplex), or ECD (Echo Cancelling Duplex). FDD utilizes separate frequency bands for downstream and upstream communication. ECD stands for Echo Cancelling Duplex.

ADSL employs modulation techniques like OFDM (Orthogonal Frequency Division Multiplexing) and DMT (Discrete Multi-tone).

ADSL Types

The DSL family is collectively referred to as xDSL. Numerous variations exist, including IDSL, HDSL, SHDSL, RADSL, UDSL, VDSL, and GDSL. These differ in data rates, bandwidth allocations, and maximum coverage distances.

The most popular version, ADSL (Asymmetric ADSL), provides a downstream data rate of approximately 8Mbps and an upstream data rate of around 500Kbps.

• IDSL: (Integrated DSL) delivers a data rate of 144Kbps over a maximum distance of about 5.4 Km.

• HDSL: (High bit rate DSL) is a symmetric technology, delivering a data rate of 2.048Mbps for a maximum distance of approximately 4.6 Km, utilizing a pair of copper wires.

• SHDSL: (Single pair high speed DSL) delivers data rates from about 384Kbps to 4.608Mbps, with a step size of approximately 128Kbps.

• RADSL: (Rate Adaptive DSL) adjusts its downstream and upstream data rates based on line noise and the distance to be covered.

• UDSL: (Uni-DSL or Universal DSL) delivers downstream and upstream data rates of about 200Mbps.

• VDSL: (Very High Speed DSL) is symmetric, delivering a downstream data rate of about 13 to 52Mbps and an upstream data rate of about 1.5Mbps to 2.3 Mbps using a single copper line, covering distances from about 300 meters to 1.4 Km from the exchange.

• GDSL: (Gigabit DSL) is based on the MIMO concept, delivering a data rate of approximately 1Gbps over a distance of around 300 meters. Ethernet local loops incorporate features of both Ethernet and DSL, delivering a data rate speed of about 6Mbps over a distance of 6.5Km.

ADSL Standards or versions

The following table outlines various ADSL standards or versions, along with their respective standard, upstream data rate, and downstream data rate.