Communicate Better with ADSL2

ADSL technology uses existing copper telephone lines to provide better data transfer rates than dial-up. Though the standard is being widely used, it has one major drawback. It’s unable to provide consistent performance over longer distances. So, you’d get more out of an ADSL connection sitting close to an ADSL exchange than if you were sitting very far from it. To tackle this, a new version of the technology, called ADSL2, has been defined. ADSL2 is approved by ITU (International Telecommunication Union), and is designated as G.999.3 and G.999.4. Besides improvements in performance and data rates, the technology has features like rate adaptation, diagnostic and power management.

ADSL2 tackles inconsistent performance over long distances by making improvements in modulation. It has a 16-state four-dimensional trellis-code and 1-bit QAM (Quadrature Amplitude Modulation) constellation. Trellis-code is a combination of coding and modulation, which increases the available bandwidth and keeps it constant during transmission.

Crosstalk in telephone lines caused ADSL to drop connections. ADSL 2 has taken care of this

Lower frame overhead



In first generation ADSL, overhead bits per frames were fixed to 32 kbps, and these 32 kbps were consumed from the actual payload data. In ADSL2, this can be reduced from 32 to just 4 kbps. This is also programmable as per requirements, so the overhead frame can save bandwidth while transmitting data.

Reed-Solomon code



ADSL2 adopted RS (Reed-Solomon) code for achieving higher coding gain when data rates are low. RS code is a block-based error correction scheme, which takes blocks of digital data and adds an extra redundant bit to it, while encoding. At the receiving end, RS decoder is used to recover the actual data, and kicks out the noise and other disturbance, which accrue during transmission. This brings significant reduction in frame errors. 

Diagnostic capabilities



ADSL2 has diagnostic capabilities on its transceivers, which were not there in ADSL, making it easy to detect problems on networks in case of a breakdown. It has tools for troubleshooting the problems, during and after installations. It also does real-time performance monitoring while giving service and provides information like line quality and noise conditions at both ends of the line. 

Power management



Power management is another enhancement in ADSL2. In first generation ADSL system, transceiver operates in full-power mode round the clock, even when ADSL is not in use. Imagine the amount of power used up with several million ADSL modem installations worldwide. A significant amount of electricity can be saved if these devices could go in a stand-by/sleep mode, just like computers. ADSL2 provides two power management modes L2 and L3 for doing just this.

Advantage ADSL2

Provides faster startup, taking less than 3 secs compared to 10 secs in ADSL  Supports canalization and can split bandwidth into different channels. Can split voice and data applications on different channels Can transmit data on the voice bandwidth channel, which adds 256 kbps of bandwidth to your upstream data Supports packet-based services such as Ethernet to transfer data

L2 low power mode: An ADSL2 transceiver unit is smart enough to enter or exit from this mode, depending up the data traffic over the ADSL connection. However, ADSL2 works on full power mode (L0) during heavy usage to give maximum speed. Otherwise it will automatically go to L2 low power mode when Internet traffic decrease.

L3 low power mode: This mode is total sleep mode, where no traffic can be communicated on the ADSL connection. When

the user is not online, ADSL2 goes to sleep mode and shuts down the power from both sides. As soon as the user gets back online, ADSL2 transceiver gets re-initialed within 3

secs. 

Rate adaptation 



Telephone wires are bundled together in multi-pair binders containing 25 or more twisted pair wires. This can create interference from one pair of wires to another, due to electrical signals, which create electromagnetic couple on adjacent pair.

This phenomenon is known as crosstalk and can affect ADSL data rates. As a result, ADSL system drops the connection.

However, ADSL2 has arrangements to tackle this problem. SRA (Seamless Rate Adaptation) is a technique that can change the data rate of the connection, while in operation, without any disruption in the service. It simply determines the channel conditions, changes a data rate accordingly, and transmits that rate. This is done by initiating a message to the transmitter to change the data rate. The message contains information about new adopted data rate, transmission parameters, etc. This technique helps reduce the number of disconnections.

Bounding



Bandwidth management is a major area, where bandwidth is distributed according to the customers’ needs and charges are made in terms of the bandwidth being used.

To provide bandwidth to different customers, ADSL2 adopted bounding, where multiple phone lines are bounded together to throttle up the data rates. (ADSL did not support bounding.) ADSL2 uses the IMA (Inverse Multiplexing from the ATM) standard, which specifies a new sub layer (IMA sub layer) that resides between the ADSL physical layer and the ATM layer.

At the transmitter side, the IMA sub layer takes in a single ATM stream from the ATM layer and distributes this to multiple ADSL physical layers. On the receiver side, the IMA sub-layer takes in ATM calls from multiple ADSL physical layers, and reconstructs the original ATM stream. This provides the required amount of bandwidth to heavy applications like online video and gaming. Plus, ISPs can allocate bandwidth to different user levels (home, corporate).

Later this year, vendors are expected to provide dual-mode solutions, which will support both legacy ADSL and ADSL2 equipments. 

Sanjay Majumder