ADSL modems

G.992.1

VOCAL Technologies, Ltd. modem software libraries include a complete range of ETSI / ITU / IEEE compliant modulations, optimized for execution on ANSI C and leading DSP architectures (ADI, AMD-Alchemy, ARM, CEVA, Infineon, LSI Logic ZSP, MIPS and TI). This software is modular and can be executed as a single task under a variety of operating systems or it can execute standalone with its own kernel.

ITU GDMT (ITU G.992.1) describes Asymmetric Digital Subscriber Line (ADSL) Transceivers on a metallic twisted pair that allows high-speed data transmission between the Central Office (ATU-C) and the customer end remote terminal (ATU-R). G.DMT can provide ADSL transmission simultaneously on the same pair with voice (band) service, ADSL transmission simultaneously on the same pair with ISDN services (G.961 Appendix I or II); or ADSL transmission on the same pair with voiceband transmission and with TCM-ISDN (G.961 AppendixIII)in an adjacent pair.

GDMT allows approximately 6 Mbps downstream and approximately 640 kbps upstream data rates depending on the deployment and noise environment. An ADSL transmission unit can simultaneously convey all of the following: downstream simplex bearers, duplex bearers, a baseband analogue duplex channel, and ADSL line overhead for framing, error control, operations and maintenance. Systems support a minimum of 6.144 Mbps downstream and 640 kbps upstream.

GDMT Terminology:

GDMT uses discrete multitone (DMT) line code. DMT is based in the use of the IFFT to generate a set of sub-channels, and transmit information in each sub-channel independently.
Figure 1 shows the G.DMT spectrum with indication of the POTS, upstream pilot tone, downstream pilot tone, subcarrier spacing, and number of subcarriers for the upstream and downstream direction. Dividing the available bandwidth into a set of independent, orthogonal subchannels are the key to DMT performance. By measuring the SNR of each subchannel and then assigning a number of bits based on its quality, DMT transmits data on subcarriers with good SNRs and avoids regions of the frequency spectrum that are too noisy or severely attenuated. The underlying modulation technique is based on quadrature amplitude modulation (QAM). Each subchannel is 4.3125 kHz wide and is capable of carrying up to 15 bits. The downstream is up to 1104 kHz, offering 249 subchannels, and the upstream from 26 to 138 kHz, offering 25 upstream subchannels.

Figure 1. GDMT subcarriers distribution

GDMT Features:

G.DMT has the option to allow for data transmission using bit serial synchronous transfer mode (STM) as an alternative to ATM cell transport. ATM, and its associated ATM adaptation layers (AAL0, AAL1, AAL2, AAL3/4, and AAL5), allows all types of traffic to be carried. The different AAL mechanisms permit the delivery of real-time and non-real-time video, and the transport of Internet Protocol (IP) traffic, using the point-to-point protocol (PPP) over ATM over ADSL protocol stacks. Designating ATM as the transport mechanism ensures the robustness of G.DMT.
ATU-R transmission of timing, synchronized to the ATU-C, is the classic method of timing a network.
GDMT supports four framing structures. The ATU-C negotiates which framing structure to use with the ATU-R during activation, and sets itself to the reported framing structure accordingly.
Transport of Network timing Reference (NTR). G.DMT has the capability of transport the NTR from the CO side to the CPE side.
G.DMT can transport two latency paths, allowing different applications, performance and robustness services.
Optional TCM as a combination of convolutional coding and QAM. The redundancy of convolutional coding requires more states in the TCM QAM constellation than with QAM alone. TCM uses an encoding scheme similar to the one used for QAM, but adds extra bits for its error correction work. If trellis coding had been selected on the transmit side, the data would pass through the Viterbi decoder on the receive side. The Viterbi algorithm for decoding uses the structure of the trellis (the allowable transitions) and the input data to determine the most likely path through the trellis. VOCAL G.DMT support Trellis and Viterbi.

Implementations:

Full Software Implementation. Figure 2 shows the task partition for the receiver (RX) and the Transmitter (TX) for the case of all software implementation.
Software with hardware acceleration Implementation. The hardware implementation of the TEQ needs 32 taps for the scalar transversal filter in 16 points fixed point arithmetic in a 2.208 MHz clock frequency. This implementation is easy using serial multipliers with a low cost and low risk.
Full hardware Implementation. Hardware implementation of the TEQ needs 32 taps for the scalar transversal filter in 16 points fixed point arithmetic in a 2.208 MHz clock frequency. This implementation is easy using serial multipliers with a low cost and low risk. Hardware implementation of the 512 points real, 64 points complex FFTs is in 16 points fixed point arithmetic with a 2.208 MHz clock frequency using efficient Radix 8 with serial multipliers with a low cost and is available as a core from many vendors. Hardware implementation of the Reed-Solomon decoder uses the Berlekamp-Massey decoder with a standalone syndrome calculation. the algorithm uses a log domain table look-ups to implement the GF(256) multiplies with a low cost and is available as a core from many vendors.

RX Processing

TX Processing

Figure 2. GDMT task partition for full software implementation

ITU Recommendation G.992.1 (G.dmt)

ITU Recommendation G.992.2 (G.lite)

ITU Recommendation G.992.3 (ADSL G.dmt 2)

ITU Recommendation G.992.4 (ADSL G.lite 2)

ITU Recommendation G.992.5 (ADSL2+)

Datasheet