Addressing THE JUMPSTART PROJECT

262 ACCESS NETWORKS are: asymmetric DSL ADSL, high data rate DSL HDSL, symmetric DSL SDSL, ISDN DSL IDSL, and very high data rate DSL VDSL. Some of the xDSL technologies use analog signaling methods to transport analog or digital information over the twisted pair, while others use true digital signaling to transport digital information. A list of speci- fications for the xDSL family technologies is given in Table 11.1. In access networks, downstream means from the network to the user, and upstream means from the user to the network. These specifications are likely to change as the technology evolves. The very high data rate DSL VDSL, as its name implies, achieves very high data rates over the twisted pair. However, the distance over which such rates can be transported is limited. Currently, it can achieve a downstream data rate of 52 Mbps and an upstream data rate of 6 Mbps over a distance of up to 1000 feet. For the same distance, it can also provide symmetric rates of 26 Mbps downstream and 26 Mbps upstream. The longest distance it can be transported is currently 5000 feet, for which it can achieve 13 Mbps downstream and 1.6 Mbps upstream. VDSL can be used to deliver high quality video together with access to the Internet and regular telephone services. Because of the distance limitation, it is envisioned that it will be used to deliver information from a cabinet in the street which is connected to an APON. The asymmetric digital subscriber line ADSL technology utilizes the existing twisted pair from the central office to the home to transport data in addition to the basic telephone services. It was originally designed to provide video on demand services transported over switched DS1 or E1 links. This type of traffic is referred to in the ADSL standard as the synchronous transfer mode STM traffic. In its current standard ITU-T G.992.1 full rate ADSL has been defined to carry either ATM or STM traffic or both. ADSL is primarily used for ATM traffic, and there is a limited number of applications for STM traffic. As its name implies, ADSL provides asymmetrical data rates with the downstream rate being considerably higher than the upstream rate. The data rate depends on the length of the twisted pair, the wire gauge, presence of bridged taps, and cross-couple interference. Ignoring bridged taps, currently ADSL can deliver a full DS1 or E1 signal downstream over a single unloaded 24 gauge twisted pair for a maximum distance of 18,000 feet. Up to 6.1 Mbps is possible for a maximum distance of 12,000 feet, and 8.128 Mbps for a maximum distance of 9000 feet. Upstream data rates currently range between 64 Kbps and 800 Kbps. The ADSL data rates and reach have recently been improved with two new standards: ADSL2 and ADSL2+ see Section 11.1.5. The deployment of ADSL over the twisted pair, requires an ADSL transmission unit at either end of the line. The ADSL transmission unit at the customer site is referred to Table 11.1 xDSL maximum data rates. xDSL type Maximum data rate Usage Downstream Upstream ADSL 8.128 Mbps 800 Kbps Data HDSL 1.544 Mbps 2.048 Mbps T1E1 replacement SDSL 2.3 Mbps 2.3 Mbps Data ISDL 144 Kbps 144 Kbps Data VDSL 52 Mbps 6 Mbps Videodata THE ADSL-BASED ACCESS NETWORKS 263 Telephone wires Telephone wire PC Filter ATU-R Figure 11.1 Deployment of ADSL at the customer site. as the ADSL transceiver unit, remote terminal ATU-R, and the ADSL transmission unit at the central office is referred to as the ADSL transceiver unit, central office ATU-C. The signal transmitted over the twisted pair, which contains both ADSL data and voice, is propagated throughout the home telephone wires see Figure 11.1. The voice signal is filtered out using a high pass filter inside the ATU-R. On the other hand, the ADSL signal can cause a strong audible noise through the telephone set. Therefore, each phone is attached to a telephone plug through a filter, which filters out the ADSL signal and at the same time isolates voice events, such as ring and onoff hook, from the ADSL signal. The ATU-R can be plugged in to any telephone plug. Consider the ATU-C at the central office. In the downstream direction, the voice signal is added after the ADSL signal leaves the ATU-C see Figure 11.2. In the upstream direction, the voice signal is extracted from the ADSL signal, before the ATU-C. The ATU-C generates the ADSL signal in the downstream direction, and terminates the ADSL signal in the upstream direction. A number of ATU-Cs are serviced by an ADSL access multiplexer , known as DSLAM, which provides connectivity to IP and ATM networks. The DSLAM is an ATM switch. It has an OC-3STM-1 or higher link to an ATM access backbone network, and has ADSL links serving a number of customer sites. Each ADSL link at the DSLAM is associated with an ATU-C, which is the physical layer associated with the link. We now proceed to describe how an ATU-C or an ATU-R works. The protocols used to provide IP and ATM services over ADSL are described in Section 11.1.4.

11.1.1 The Discrete Multi-tone DMT Technique

The discrete multi-tone DMT technology is the standardized line coding technique used for ADSL. DMT devices can easily adjust to changing line conditions such as moisture or interference. ATM switch ATU-C ATU-C POTS POTS ADSL access multiplexer . . . . . . ATU-R ATU-R Figure 11.2 ADSL access multiplexer DSLAM.