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Service Downstream Data Rate (Mbps) Distance (meters)
1/4 STS-1 12.96 1,500
1/2 STS-1 25.92 1,000
STS-1 51.84 300

Table 9-4. VDSL modem characteristics

VDSL has an effective range of 1.5 kilometer radius from the local exchange. The transmission rate for duplex operation is 13 to 52 Mbps downstream and 1.5 to 2.3 Mbps upstream. VDSL is used for Internet access, VOD, remote LAN access, interactive multimedia, and HDTV. Future VDSL lines may have equal upstream/downstream rates, albeit on shorter lines.

VDSL initially was known as VADSL with the A designating an asymmetrical flavor of VDSL. The data rates are higher than ADSL but the modems are limited to shorter distances.

VDSL is designed for ATM networks, eliminating the channelization and packet handling requirements ADSL must cope with in dealing with circuit switched and packet switched networks. Since VDSL is limited to shorter lines than ADSL, fewer transmission constraints are involved, resulting in simpler transceiver algorithms for VDSL modems. Since VDSL modems can interface with passive line terminations, more than one VDSL modem can be connected to a line. So, any user can connect three VDSL modems to one twisted pair and receive signals per the frequencies listed in Table 9-4.

VDSL operates in conjunction with POTs and ISDN with the differing signals separated at the opportune moment with passive filtering. Passive filtering is like a sand filter with one size of holes in the filter material, perhaps a wire mesh. Everything smaller than the hole gets through.


Figure 9-6.   DMT spectrum

Four data/information coding methods are proposed for VDSL: Carrierless Amplitude/Phase Modulation (CAP), Discrete Multitone (DMT), Discrete Wavelet Multitone (DWMT), and Simple Line Code (SLC).

SDSL

Single Line Digital Subscriber Line (SDSL) supports POTS and DS-1 simultaneously. The transmission rate is either 1.544 Mbps or 2.048 Mbps duplex over a single copper line. SDSL is used for DS-1 service, feeder plant, LAN/WAN access, server access, and premises access. Symmetric, meaning as much data flows one way as the other, services such as remote LAN users and servers are good candidates for SDSL.


Figure 9-7.   ADSL FDM channelization bandwidth utilization

Discrete Multi-Tone

Discrete Multi-Tone (DMT) coding divides the available bandwidth into channels. Some of the channels are used for upstream data transport and some of the channels are used for downstream transport. One example reserves the frequency spectrum from 0 to 4 KHz for the POTS channel and divides the spectrum from 26 KHz to 1.1 MHz into 246 distinct and separate channels. Each channel carries its information as a discrete analog signal. DMT can dynamically adapt to the changing line conditions to deliver the maximum throughput per channel with the least amount of signal degradation. DMT was chosen over Carrierless Amplitude/Phase Modulation (CAP) and Quadrature Amplitude/ Phase Modulation (QAM) because it is more robust over long distances. Robust means it could go the distance and still deliver the goods.


Figure 9-8.   ADSL echo cancellation bandwidth utilization

Two different techniques to divide the bandwidth are used. Frequency division multiplexing and echo cancellation are used depending upon the modem manufacturer and the users needs. The two techniques are discussed below.

FDM

FDM channelization consists of separate upstream and downstream paths. TDM techniques are used to divide the upstream path and the downstream path into multiple channels. The downstream path is multiplexed into both high-speed and low-speed channels while the upstream path is just multiplexed into slow-speed channels. With FDM channelization, there is a guard band between POTS and the upstream channels and another guard band between the upstream channels and the downstream channels. The guard bands are necessary to ensure that the data stream from each path does not interfere with the data in the other paths.


Figure 9-9.   Signal adaptation

Echo Cancellation

Echo cancellation utilizes the available bandwidth between the upstream and downstream paths, resulting in an overlap of paths. Echo cancellation can accomplish this feat of electronic dexterity by the well-known technique of local echo cancellation. Local echo cancellation is used in the V.32 and V.34 analog modem technology.

High-Level Online Adaptation

High-level online adaptation is also called bitswapping in some literature circles. Bitswapping is the technique of determining which channels are usable due to line noise conditions and using only those channels for communications.

The ADSL modem monitors the line and adapts its bit rate to the line conditions to provide the most error free connection possible. Here is how signal adaptation occurs. The first diagram in Figure 9-9 shows individual channels with the same amplitude level. The level of each channel is sufficient for the equipment to recognize the signals as valid ADSL signals. The second diagram in the figure shows a typical burst of noise on the line. As long as the desired signal is sufficiently more (10 dB or more) than the noise signal, the equipment can recognize the desired signal. When the noise signal rises higher and the ratio between the noise signal and desired signal is sufficiently low (less than 10 dB), the desired signal becomes indistinguishable from the noise signal. The third diagram shows the desired signal level after combining with the noise signal. For those channels dipping down in the low spot, the signal-to-noise ratio is too low for acceptable use. So, the equipment will recognize those channels as unusable and will not transmit any data on the channels. The ADSL modem will monitor the line periodically and mark the unusable channels as necessary.


Figure 9-10.   ADSL network termination


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