The Discrete Multi-tone DMT Technique

270 ACCESS NETWORKS ADSL2 ADSL2 provides a number of features that increase the performance and functionality of ADSL. The following are some of these features: • Rate and reach improvement : ADSL2 increases the data rate and reach of ADSL by doing the following: improving modulation efficiency, reducing framing overhead, achieving a higher coding gain, improving the initialization state machine, and providing enhanced signal processing algorithms. • Diagnostics : ADSL transceivers have been enhanced with extensive diagnostic capa- bilities that provide troubleshooting tools for installation during and after and perfor- mance monitoring. • Low powerSleep mode : ADSL transceivers operate continuously in full power mode, even when they are not in use. The ADSL2 ATU-C is equipped with a low power mode, which it can enter and exit rapidly based on the traffic running over the ADSL connection. In addition, both ATU-C and ATU-R transceivers have a sleep mode that is activated when the ADSL connection is not used for a long time. These two features permit saving a significant amount of electricity, particularly since there are millions of deployed ADSL modems. • Rate adaptation : Telephone wires are bundled together in multi-pair binders, contain- ing 25 or more twisted wire pairs. As a result, electrical signals from one pair might electro-magnetically couple onto adjacent pairs in the binder. This phenomenon, called crosstalk , can impede the data rate of ADSL. ADSL2 addresses this problem by seam- lessly adapting the data rate. Specifically, the receiver monitors the link’s signal-to-noise ratio SNR , and if it determines that a data rate change is necessary, then it sends a message to the transmitter. The transmitter then sends a Sync Flag signal, which is used as a marker to designate the exact time at which the new data rate is to be used. The Sync Flag signal is detected by the receiver, and the transmitter and receiver both change to the new data rate. • Bonding for higher data rates : Data rates can be significantly increased by bonding mul- tiple phone links together. ADSL2 supports the ATM Forum’s inverse ATM multiplexer standard, which was developed for transporting ATM traffic over multiple low-speed links. This standard permits ADSL2 to bind two or more twisted pairs to a single ADSL connection, which results in higher downstream data rates. In addition to the above features, ADSL2 has a channelization capability, whereby it can split the bandwidth into different channels for different applications. ADSL channeliza- tion capability provides support for Channelized Voice over DSL CVoDSL. Specifically, ADSL2 reserves a number of subchannels tones in the upstream and downstream spec- trum for the transport of PCM 64-Kbps voice data see Figure 11.8. With the current ADSL standard, voice can be transmitted over IP VoIP by encapsulat- ing the IP packets in PPP and subsequently transmitting the PPP frames over AAL5ATM ADSL. Voice can also be carried over ATM VoATM by transmitting the voice pack- ets over AAL2ATMADSL. The protocol stacks for VoATM and VoIP are shown in Figure 11.9. VoIP and VoATM is in contrast with CVoDSL which is transmitted directly over the ADSL physical layer, without having to use any higher-level protocols. Recall that POTS is transmitted directly over the twisted pair using subchannels 1 to 6. THE ADSL-BASED ACCESS NETWORKS 271 KHz . . . . . . POTS Upstream Downstream . . . 64 Kbps DS0 Figure 11.8 Channelized voice over DSL CVoDSL. IP PPP ATM AAL2 IP PPP ATM Customer Premises Network Baseband POTS ADSL Physical layer VoIP VoATM CVoDSL POTS AAL5 AAL2 AAL5 Figure 11.9 CVoDSL, VoATM, VoIP. 1.1 MHz . . . . . . POTS Upstream Downstream . . . . . . ADSL2+ ADSL2 2.2 MHz KHz 0.14 MHz Figure 11.10 The ADSL2+ downstream bandwidth. ADSL2+ ADSL2+ doubles the downstream bandwidth thereby increasing the downstream data rate on telephone lines shorter than 5000 feet. ADSL and ADSL2 use a bandwidth of 1.1 MHz, whereas ADSL2+ specifies a bandwidth of 2.2 MHz see Figure 11.10. This results in a significant increase in the downstream speed on short lines.