As members of the broadband design community have been perfecting their designs based on the first generation of ADSL standards (ITU G.992.1 and G.992.2), the ITU has quietly been putting the finishing touches on the latest revision of the ADSL standards, referred to at the ITU as "G.dmt.bis" and "G.lite.bis", will be better known as ADSL2.
ADSL2 (ITU G.992.3 and G.992.4) adds new features and functionality targeted at improving performance and interoperability, and adds support for new applications, services, and deployment scenarios. Among the changes are improvements in data rate and reach performance, rate adaptation, diagnostics, and stand-by mode, to name a few. The article that follows takes an in-depth look at the ADSL2 spec by examining some of the opportunities it provides to improve upon existing ADSL system and silicon designs.
Extending Reach
Mention the words broadband connection in a room and you'll hear plenty of cheers and—depending on the neighborhood—a few jeers. The cheers are from those fortunate DSL customers living close enough to a central office (CO) to quickly download flashy web pages. The jeers are likely from the few remaining subscriber-wannabes still waiting for the phone company to provide DSL connectivity in their neighborhood.
Of course, the issues here are rate and reach. ADSL2 has been specifically engineered to enable designers to improve the rate and reach of ADSL largely by achieving better performance on long lines in the presence of narrowband interference. ADSL2 accomplishes this by improving modulation efficiency, reducing framing overhead, achieving higher coding gain, improved the initialization state machine, and providing enhanced signal processing algorithms. As a result, ADSL2 mandates higher performance for all standard-compliant devices.
ADSL2 provides better modulation efficiency by mandating a four-dimensional, 16-state trellis-coded and 1-bit quadrature amplitude modulation (QAM) constellations which provide higher data rates on long lines where the signal-to-noise ratio (SNR) is low. In addition, receiver determined tone-reordering enables the receiver to spread out the non-stationary noise due to AM radio interference to get better coding gain from the Viterbi decoder.
ADSL2 systems reduce framing overhead by providing a frame with a programmable number of overhead bits. Therefore, unlike the first-generation ADSL standards where the overhead bits per frame were fixed and consumed 32 kbps of actual payload data, in the ADSL2 standard the overhead bits per frame can be programmed from 4 to 32 kbps. In the first-generation ADSL systems on long lines where the data rate is low (.e.g < 128 kbps), a fixed 32 kbps (or 25% of the total data rate) is allocated to overhead information. In ADSL2 systems the overhead data rate can be reduced to 4 kbps, which provides an additional 28 kbps for payload data.
ADSL2 also achieves higher coding gain from the Reed-Solomon (RS) code when the data rates are low on long lines. This higher coding gain results from improved flexibility and programmability in the construction of RS codewords that results also from the improvements in the ADSL2 framers.
Additionally, the initialization state machine has numerous improvements that provide increased data rates in ADSL2 systems. Examples of these improvements include:
- Power cutback capabilities at both ends of the line to reduce near-end echo and the overall crosstalk levels in the binder.
- Receiver determines the location of the pilot tone in order to avoid channel nulls from bridged taps or narrow band interference from AM radio.
- Receiver and transmitter control the length of certain key initializations states in order to allow optimum training of receiver and transmitter signal processing functions.
- Receiver determines the carriers used for initialization messages in order to avoid channel nulls from bridged taps or narrow band interference from AM radio.
- Improvement in channel identification for training receiver time domain equalizer with spectral shaping during Channel Discovery and Transceiver Training phases of initialization.
- Tone blackout (disabling tones) during initialization to enable Radio Frequency Interference (RFI) cancellation schemes.
Figure 1 shows the rate and reach of ADSL2 as compared to the first-generation ADSL standard. On longer phone lines, ADSL2 will provide data rate increase of 50 kbps — a significant increase for those customers who need it most. This data rate increase results in an increase in reach of about 600 feet, which translates to an increase in coverage area of about 6%, or 2.5 square miles.
Figure 1: ADSL2 systems can deliver an improvement in reach of about 600 feet over existing copper networks.
Tracking Down Problems
Determining the cause of problems in consumer ADSL service has at times been a challenging obstacle in ADSL deployments. To tackle the problem, ADSL2 transceivers have been enhanced with extensive diagnostic capabilities. These diagnostic capabilities provide tools for trouble resolution during and after installation, performance monitoring while in service, and upgrade qualification.
In order to diagnose and fix problems, ADSL2 transceivers provide measurements for line noise, loop attenuation, and signal-to-noise ratio (SNR) at both ends of the line. These measurements can be collected using a special diagnostic testing mode even when line quality is too poor to actually complete the ADSL connection.
Additionally, ADSL2 includes real-time performance monitoring capabilities that provide information on line quality and noise conditions at both ends of the line. This information is interpreted by software and then used by the service provider to monitor the quality of the ADSL connection and prevent future service failures. It can also be used to determine if a customer can be offered higher data rate services.
Power Enhancements
First-generation ADSL transceivers operate in full-power mode day and night, even when not in use. With several millions of deployed ADSL modems, a significant amount of electricity can be saved if the modems engage in a stand-by/sleep mode just like computers. This would also save power for ADSL transceivers operating in small remote units and digital loop carrier (DLC) cabinets that operate under very strict heat dissipation requirements.
To address these concerns, the ADSL2 brings in two power management modes that help reduce overall power consumption while maintaining ADSL's "always-on" functionality for the user. These modes include:
- L2 low-power mode: This mode enables statistical powers savings at the ADSL transceiver unit in the central office (ATU-C) by rapidly entering and exiting low power mode based on Internet traffic running over the ADSL connection.
- L3 low-power mode: This mode enables overall power savings at both the ATU-C and the remote ADSL transceiver unit (ATU-R) by entering into sleep/stand-by mode when the connection is not being for extended periods of time (i.e. user asleep, modem asleep).
The L2 power mode is one of the most important innovations of the ADSL2 standard. ADSL2 transceivers can enter and exit the L2 low power mode based on the Internet traffic over the ADSL connection. When large files are being downloaded, ADSL2 operates in full power mode (called "L0" power mode) in order to maximize the download speed. When Internet traffic decreases, such as when a user is reading a long text page, ADSL2 systems transition into L2 low power mode, in which the data rate is significantly decreased and overall power consumption is reduced.
While in L2, the ADSL2 system can instantly re-enter L0 and increase to the maximum data rate as soon the user initiates a file download. The L2 entry/exit mechanisms and resulting data rate adaptations are accomplished without any service interruption or even a single bit error, and as such, are not noticed by the user (Figure 2).
Figure 2: ADSL's L2 power mode allows a broadband modem to quickly move from L2 to L0 operation and back without limited bit errors.
The L3 power mode is a total sleep mode where no traffic can be communicated over the ADSL connection when the user is not on-line. When the user returns to go on-line the ADSL transceiver require approximately three seconds to re-initialize and enter into steady state communication mode.
Rate Adaptation
Telephone wires are bundled together in multi-pair binders containing 25 or more twisted wire pairs. As a result, electrical signals from one pair can electromagnetically couple onto other adjacent pairs in the binder (Figure 3). This phenomenon is known as "crosstalk" and can impede ADSL data rate performance. As a result, changes in the crosstalk levels in the binder can cause an ADSL system to drop the connection.
Crosstalk is just one reason that ADSL lines drop connections. Others include changes in the narrowband, AM radio disturbers; temperature changes, and water in the binder.
Figure 3: When adjacent pairs couple together they can cause crosstalk on an copper line, thus forcing the ADSL system to drop a connection.
ADSL2 addresses this problem by including the ability to seamlessly adapt the data rate on-line. This new innovation, called seamless rate adaptation (SRA), enables the ADSL2 system to change the data rate of the connection while in operation without any service interruption or bit errors. ADSL2 simply detects changes in the channel conditions — for example, a local AM radio station turning off its transmitter for the evening — and adapts the data rate to the new channel condition transparently to the user.
SRA is based on the decoupling of the modulation layer and the framing layer in ADSL2 systems. This decoupling enables the modulation layer to change the transmission data rate parameters without modifying parameters in the framing layer which would cause the modems to loose frame synchronization resulting in uncorrectable bit errors or system restart. SRA uses the sophisticated online reconfiguration (OLR) procedures of ADSL2 systems to seamlessly change the data rate of the connection. The protocol used for SRA works as follows:
- The receiver monitors the SNR of the channel and determines that a data rate change is necessary to compensate for changes in channel conditions.
- The receiver sends a message to the transmitter to initiate a change in data rate. This message contains all necessary transmission parameters for transmitting at the new data rate. These parameters include the number of bits modulated and transmit power on each subchannel in ADSL multicarrier system.
- The transmitter sends a "Sync Flag" signal which is used as a marker to designate the exact time at which the new data rate and transmission parameters are to be used.
- The Sync Flag signal is detected by the receiver and both transmitter and receiver seamlessly and transparently transition to the data rate.
Bonding For higher Data Rates
One of the big requests by today's carriers is the ability to provide different service level agreements (SLAs) to different customers. For example, carriers want to deliver a standard bandwidth requirement to the majority of home users while providing higher bandwidth offerings for corporations.
The original ADSL standard did not support bonding, so designers did not have the ability to deliver these advanced SLA capabilities in equipment designs. However, through the support for bonding, ADSL2 is addressing these requests.
Bonding multiple phone lines together is a relatively simple way to significantly increase data rates to homes and businesses. To provide bonding, the ADSL2 spec taps into the inverse multiplexing for ATM (IMA) standard developed for traditional ATM architectures. Through IMA, equipment designs can bind copper pairs in an ADSL link (Figure 4) . The result is a far greater deal of flexibility with downstream data rates:
- 20 Mbps on 2 bonded pairs
- 30 Mbps on 3 bonded pairs
- 40 Mbps on 4 bonded pairs
Figure 4: Through bonding, design engineers can increase the data throughput on an ADSL channel to as high as 40 Mbps.
The IMA standard specifies a new sublayer that resides between the ADSL physical layer (PHY) and the ATM layer. At the transmitter side, this sublayer, called the IMA sublayer, takes in a single ATM stream from the ATM layer and distributes this stream to multiple ADSL PHYs. At the receiver side, the IMA sublayer takes in ATM cells from multiple ADSL PHYs and reconstructs the original ATM stream.
The IMA sublayer specifies IMA framing, protocols and management functions that are used to perform these operation when the PHYs are lossy (bit errors), asynchronous, and have different delays. In order to work under these conditions, the IMA standard also requires modifications to some of the standard ADSL PHY functions such as the discarding of idle cells and errored cells at the receiver. The ADSL2 includes an IMA operation mode to provide the necessary PHY modifications for IMA to work in combination with ADSL.
Bonding using ADSL2 is expected to play a role in helping carriers to provide bandwidth-hungry applications such as video and gaming to residential customers. It will also be effective in delivering high-bandwidth services to businesses who prefer asymmetric over symmetric data rates.
Some Added Benefits
Clearly, the above benefits provide some nice enhancements during the design and development of ADSL equipment. The ADSL2 specification, however, does not stop there. In addition to the above benefits, ADSL 2 delivers:
- Improved Interoperability: Clarifications and additions to the initialization state machine improve interoperability and provide better performance when connecting ADSL transceivers from different chip suppliers.
- Fast startup: ADSL2 provides a fast startup mode that reduces initialization time from more 10 seconds (as is required for ADSL) to less than 3 seconds.
- Channelization: ADSL2 provides the ability to split the bandwidth into different channels with different link characteristics for different applications. For example, ADSL2 enables simultaneous support of a voice application, which might have low latency but a higher error rate requirement, and a data application, which might have high latency but lower error rate requirement. ADSL2 also provides support for channelized voice-over-DSL (VoDSL), a method to transport derived lines of TDM voice transparently over DSL bandwidths.
- All Digital Mode: ADSL2 enables an optional mode that allows for transmission of ADSL data in the voice bandwidth, adding 256 kbps of upstream data rate. This is an attractive option for businesses that have their voice and data services on different phone lines, and value the extra upstream bandwidth.
- Support of Packet-Based Services: ADSL2 includes a packet mode transmission trans-convergence layer (PTM-TC) that enables packet-based service (such as Ethernet) to be transported over ADSL2.
By the first quarter of next year, leading silicon vendors are expected to have dual-mode solutions available, supporting both legacy ADSL equipment as well as new ADSL equipment.
About the Authors
David Benini is the director of product marketing at Aware. He holds a BS Degree in electrical engineering from Penn State University and an MBA Degree from Cornell University. Dave can e reached at dave@aware.com.
David Krinsky is the senior director of software development at Aware, Inc. He holds a BS from Cornell University, an MSEE from Northeastern University, and a Ph.D. from Northwestern University. David can be reached at dkrinsky@aware.com.
