Dec 11, 2014

Understanding ADSL Technology

An acronym for Asymmetric Digital Subscriber Line, ADSL is the technology that allows high-speed data to be sent over existing POTS (Plain Old Telephone Service) twisted-pair copper telephone lines. It provides a continuously available data connection whilst simultaneously providing a continuously available voice-grade telephony circuit on the same pair of wires.

ADSL technology was specifically designed to exploit the "one-way" nature of most internet communications where large amounts of data flow downstream towards the user and only a comparatively small amount of control/request data is sent by the user upstream. As an example, MPEG movies require 1.5 or 3.0 Mbps down stream but need only between 16kbps and 64kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth. The ADSL specification supports data rates of 0.8 to 3.5 Mbit/s when sending data (the upstream rate) and 1.5 to 24 Mbit/s when receiving data (the downstream rate). The different upstream and downstream speeds is the reason for including "asymmetric" in the technology's name.

ADSL Standard Common Name Downstream rate Upstream rate
ANSI T1.413-1998 Issue 2
8 Mbit/s
1.0 Mbit/s
ITU G.992.1
8 Mbit/s
1.0 Mbit/s
ITU G.992.1 Annex A
8 Mbit/s
1.0 MBit/s
ITU G.992.1 Annex B
8 Mbit/s
1.0 MBit/s
ITU G.992.2
ADSL Lite G.Lite)
1.5 Mbit/s
0.5 Mbit/s
ITU G.992.3/4
12 Mbit/s
1.0 Mbit/s
ITU G.992.3/4 Annex J
12 Mbit/s
3.5 Mbit/s
ITU G.992.3/4 Annex L
5 Mbit/s
0.8 Mbit/s
ITU G.992.5
24 Mbit/s
1.0 Mbit/s
ITU G.992.5 Annex L
24 Mbit/s
1.0 Mbit/s
ITU G.992.5 Annex M
24 Mbit/s
3.5 Mbit/s

The downstream and upstream rates displayed in the above table are theoretical maximums. The actual data rates achieved in practice depend on the distance between the DSLAM (in the telephone exchange) and the customer's premises, the gauge of the POTS cabling and the presence of induced noise or interference.

Broadband is generally defined as a connection which is greater than 128kbs (kilo-bits per second).

Voice-grade telephony uses a bandwidth of 300Hz to 3.4kHz. The sub 300Hz bandwidth can be used for alarm-system data-transfer/monitoring. Bandwidth above 3.4kHz can be used to carry ADSL traffic.

Analogue voice circuits have a nominal 600 ohms impedance at the VF frequency range but exhibit an impedance of around 100 ohms at the frequency range used by ADSL.

DMT Discrete MultiTone modulation technology is used to superimpose the ADSL bandwidth on top of the telephony bandwidth.ADSL typically uses frequencies between 25 kHz and around 1.1 MHz. The lower part of the ADSL spectrum is for upstream tansmission (from the customer) and the upper part of the spectrum is for downstream (towards the customer) transmission.

The ADSL standard allows for several spectra divisions but the upstream band is typically from 25 to 200 kHz and the downstream band is typically 200kHz to 1.1MHz. in a FDM Frequency Division Multiplexed system, different frequency ranges are used for upstream and downstream traffic. Echo-cancelled ADSL allows the downstream band to overlap the upstream band, significantly extending the available downstream bandwidth and extends the upstream bandwidth to provide faster upstream data rates.

POTS/ADSL spectrum allocation is represented in the following diagram.

A DSLAM Digital Subscriber Line Access Multiplexer is installed at the telephone exchange. and has a modem for each customer and network interface equipment. A POTs Splitter Rack is used to separate voice traffic and data traffic on the customers telephone line.

ADSL filters and filter/splitters are used in the customer's premises to separate ADSL data from analogue speech signals and prevent interference between the two types of service. It's important that the specifications of the filters and filter/splitter you use are checked to ensure that effective filtering and equipment isolation and protection are achieved.

The ADSL standard (G.99x.x series) covers several xDSL systems, protocols and tests. They encompass a framework for operation with individual networks and providers free to adapt their system within the framework guidelines. The standards provide the boundaries for equipment manufacturers.

ADSL Physical (PHY) Layer Parameters

Overall symbol rate 4kHz
Number of carriers per DMT symbol 256
Subcarrier spacing 4.3125kHz
Cyclic prefix length 32 samples
Operational modes FDM or Echo Cancelled
FDM Mode frequency range 64 to 1100kHz
Echo Cancelled Mode frequency range 13 to 1100kHz
Number of bits assigned per subcarrier 0 to 15 (no bits assigned to 64k QAM)*
Synchronisation Pilot tone at subcarrier 64, f = 276kHz
Number of subcarriers per DMT symbol 32
Cyclic prefix length 4 samples
FDM Mode frequency range 11 to 43 kHz
Echo Cancelled Mode frequency range 11 to 275 kHz
Synchronisation Pilot Tone at subcarrier 16, f = 69kHz
Handshake/initialisation Per G.994.1

* The lower three to six subcarriers are set to a gain of "0" (turned off) to permit the simultaneous operation of a POTS service provided that a filter/splitter is installed at the customer's premises telephone line entry point.


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