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VDSL, The Copper SuperhighwayBuilding on its excellent know-how and experience in ADSL (Asymmetric Digital Subscriber Line), Alcatel is making VDSL a reality today. The VDSL (Very high-speed Digital Subscriber Line) technology transforms the existing telephone twisted pair access network into an information superhighway with speeds up to 58 Mbit/s. Alcatel's VDSL solution is based on DMT (Discrete MultiTones), ensuring spectral compatibility with existing services, robustness to radio interference and ADSL interoperability. Why VDSL?VDSL is the ultimate solution for broadband services over copper telephone lines. VDSL gives telecom operators the opportunity to offer existing and emerging services at unequalled speeds, up to 58 Mbit/s, over the existing twisted pairs. Symmetrical and asymmetrical services are supported. Examples are fast Internet access for users and content providers, digital video broadcasting, interactive video, LAN interconnection, teleworking, and high quality videoconferencing. VDSL in the networkVDSL is deployed from the cabinet or from the central office exchange, referred to as Fiber to the Cabinet (FTTCab) and Fiber to the Exchange (FTTEx) respectively. VDSL is essentially a fiber to the node architecture with an optical network unit sited in the access network. The VDSL Line Terminations (LT) are grouped so that they share the same optical interface to the broadband network. At the customer's premises, the VDSL Network Termination (NT) provides a set of standardized user network interfaces. VDSL allows shared use of the telephone pair for broadband and narrowband services such as Plain Old Telephone Service (POTS) or Integrated Services Digital Network (ISDN). Simultaneous operation of VDSL with legacy narrowband services requires a splitter filter at both ends of the line to separate the higher frequency VDSL signal from the lower frequency POTS or ISDN signal (out-of-band transmission). Alternatively, the narrowband services can be carried in-band, as part of the digital data.
Figure 1 situates VDSL in the network. VDSL system featuresA VDSL transceiver can transport ATM (Asynchronous Transfer Mode) or STM (Synchronous Transfer Mode) services with associated network timing references. Furthermore, it can offer delay-sensitive services (e.g. telephony and videoconferencing) as well as services with less stringent latency requirements but that are sensitive to impulsive noise (e.g. digital video). Symmetric and asymmetric modes of operation are possible. The up- and downstream rates depend on the loop length, the noise environment and the selected downstream-to-upstream ratio. Some typical rates (not exclusive) are listed in table 1. The aggregate transmit power is limited to 11.5 dBm (14 mW) over a band from DC up to a maximal 30 MHz. For the power spectral density (psd) limit, different mask options exist depending upon the topology (FTTCab or FTTEx) and the environment. The key parameters characterizing the mask options are ADSL compatibility, psd enhancement and radio interference emission notching. In addition, regional differences exist. Table 1: Typical VDSL rates
VDSL has to be spectrally compatible with existing xDSL systems like ADSL, ISDN-BRA (Basic Rate Access), ISDN-PRA (Primary Rate Access) and HDSL (High-speed DSL). In addition the VDSL system should not interfere with wireless communication systems within its bandwidth. Without precautions, such interference would be caused by poor cable balance resulting in unwanted emission and susceptibility to radio interference. Of particular concern is egress into and ingress from radio amateurs (HAM radio). VDSL standardization is in progress in ITU (International Telecommunication Union), study group SG15/Q4, as well as in the regional bodies ANSI/T1E1.4 (American National Standards Institute) and ETSI/TM6 (European Telecommunications Standards Institute). DMT-VDSL: the perfect matchIn the Alcatel solution, the VDSL signal is modulated into Discrete MultiTones (DMT). The basic principle of DMT, as depicted in figure 2, is to transmit the information bits in parallel over a large number of carriers (tones), each of which is Quadrature Amplitude Modulated (QAM). The carrier frequencies are multiples of some basic frequency, called the tone spacing.
Figure 2 - DMT-based VDSL The multiplicity of independent and finely spaced carriers results in better performance with higher robustness to line and other impairments, easy rate adaptation and greater flexibility for special band allocation and spectral shaping than a single carrier solution. Indeed, the DMT system can allocate bits to carriers individually according to the signal to noise ratio on each, thus optimizing performance. Carriers that are subject to external interference like Radio Frequency Interference (RFI) and crosstalk can be turned off for higher robustness. This, in combination with other spectral shaping techniques, will also reduce the egress noise from the twisted pairs to the environment at these frequencies. By adjusting the number of bits per carrier, the DMT system can automatically adjust the data rate. Further, the combination with digital duplexing allows to allocate carriers to up- or downstream transmission, resulting in extreme flexibility in frequency band allocation. With DMT being the standard linecode for ADSL, the choice of DMT for VDSL also gives the possibility of insuring interoperability between both services. This means that an ADSL modem at one end of the line, whether LT or NT, can communicate with a VDSL modem at the other end. As it is expected that ADSL will be widely deployed, interoperability of VDSL with ADSL could pave the way for a smooth migration from ADSL to VDSL (and vice versa) with cost benefits for operators and end-users. Alcatel, your partner for xDSL solutionsAs the established world leader in ADSL, Alcatel has shown its expertise in DMT technology. Alcatel is the major ADSL chip vendor, dominates the worldwide ADSL-DSLAM market and leads the market in ADSL modems. Alcatel also plays a major role in other domains of high-speed access over twisted pairs, such as ISDN and HDSL. Alcatel offers complete solutions, from the network terminal at the end-user to the access multiplexer at the operator, including network management. Alcatel also offers the right equipment to interface with the backbone network and to offer end-to-end services. With the addition of VDSL to its xDSL product range, Alcatel shows its commitment to continuous technology improvement. | |||||||||||||||||||||||
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