VDSL is the highest-rate DSL technology available. Running at speeds of up to 52 Mbps, VDSL is the next generation of DSL, with higher throughput and simpler implementation requirements than ADSL. VDSL began its life being called VADSL but was renamed VDSL by the ANSI T1E1.4 working group. The principal reason T1E1.4 decided on VDSL over VADSL was that, unlike ADSL, VDSL is both symmetric and asymmetric. VDSL is nearly ten times faster than ADSL and is over thirty times faster than HDSL. The tradeoff for increased speed is loop length: VDSL has a shorter reach in the loop. Table 3 provides a comparison of the various DSL technologies available today. We see that VDSL is the highest-bandwidth technology, supporting both asymmetric and symmetric applications, and is ideal for full-service broadband networks.

DSL Type	Symmetric/Asymmetric	Loop Range (kft)	Downstream (Mbps)	Upstream (Mbps)
IDSL	symmetric	18	0.128	0.128
SDSL	symmetric	10	1.544	1.544
HDSL (2 pairs)	symmetric	12	1.544	1.544
ADSL G.lite	asymmetric	18	1.5	0.256
ADSL	asymmetric	12	6	0.640
VDSL	asymmetric	3	26	3
	asymmetric	1	52	6
	symmetric	3	13	13
	symmetric	1	26	26

Table 3. xDSL Types

Like other DSL technologies, VDSL uses the higher-frequency spectrum available over standard copper above the frequencies used for lifeline plain old telephone service (POTS) and integrated services digital network (ISDN) services. This is commonly referred to as data- and video-over-voice technology. This technology enables telcos to utilize existing copper infrastructure for the delivery of broadband services over the same physical plant. The VDSL spectrum is specified to range from 200 kHz to 30 MHz. Actual spectral allocation varies based on line rates and whether or not asymmetric or symmetric rates are being used. The baseband for lifeline POTS and ISDN service is preserved by using passive filters commonly known as splitters. Figure 2 illustrates an example of spectral allocation for asymmetric VDSL running at 25.92 Mbps downstream and 3.24 Mbps upstream.

Figure 2. Example Single-Carrier VDSL Asymmetric Spectral Allocation

Asymmetric VDSL
VDSL has been designed to deliver a host of asymmetric broadband services, including digital broadcast TV, video on demand (VoD), high-speed Internet access, distance learning, and telemedicine, to name a few. Delivery of these services requires the downstream channel to have a higher bandwidth than the upstream channel and is thus asymmetric. For example, HDTV requires 18 Mbps for downstream video content. Upstream, however, it only requires the transmission of signaling information (i.e., channel changing or program selection), which is on the order of kbps. Tables 4 and 5 clarify the VDSL line-rate standards established in the ANSI T1/E1.4 specification. Downstream rates are derived from submultiples of the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) canonical speed of 155.52 Mbps, namely 51.84 Mbps, 25.92 Mbps, and 12.96 Mbps.

Typical Service Range	Bit Rate (Mbps)	Symbol Rate (Mbps)	Comments
short range, 1 kft	51.84	12.96	baseline
	38.88	12.96
	29.16	9.72	optional
	25.92	12.96
medium range, 3 kft	25.92	6.48	baseline
	22.68	5.67
	19.44	6.48
	19.44	4.86	optional
	16.20	4.05
	14.58	4.86
	12.96	6.48
long range, 4.5 kft	12.96	3.24	baseline
	9.72	3.24	optional
	6.48	3.24

Table 4. Downstream Line Rates for Asymmetric VDSL Services (ANSI T1E1.4)

Typical Service Range	Bit Rate (Mbps)	Symbol Rate (Mbps)	Comments
short range, 1 kft	6.48	0.81	baseline
	4.86	0.81	optional
	3.24	0.81
medium range, 3 kft	3.24	0.405	baseline
	2.43	0.405	optional
	1.62	0.405
long range 4.5 kft	3.24	0.405	baseline
	2.43	0.405	optional
	1.62	0.405

Table 5. Upstream Line Rates for Asymmetric VDSL Services (ANSI T1E1.4)

Symmetric VDSL
VDSL has also been designed to provide symmetrical services for small and medium business customers, the corporate enterprise, high-speed data applications, video teleconferencing and teleconsulting applications, and etc. Symmetric VDSL can be utilized to provide short-haul T1 replacements at nxT1 rates, plus support a host of other business applications. Table 6 contains the VDSL line standards for symmetric service established in the ANSI T1E1.4 specification. At rates from 6.48 Mbps to 25.92 Mbps, it should be noted that VDSL provides symmetrical service between the standard T1 (1.536 Mbps) and T3 (44.376 Mbps) rates, bridging the gap over single twisted-pair copper. Although ANSI has not specified the distance and rates for long-range symmetric services, 6 Mbps to 1.5 Mbps over loops up from 3 kft to 10 kft are likely to be supported.

Typical Service Range	Bit Rate (Mbps)	Downstream Symbol Rate (Mbps)	Upstream Symbol Rate (Mbps)
short range, 1 kft	25.92	6.48	7.29
	19.44	6.48	7.29
medium range, 3 kft	12.96	3.24	4.05
	9.72	3.24	2.43
	6.48	3.24	3.24

Table 6. Line Rates for Symmetric VDSL Services (ANSI T1E1.4)