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Wire and Wireless Communication Applications Chap. 8
most cases, twisted-pair lines are already installed, and short lines of, say, up to 1,500 ft, have
useful bandwidths as large as 30 MHz. In conventional applications, the POTS line cards in
the RT restrict the bandwidth to the VF range of 3.3 kHz. This limits the data rate to around
50 kbs for a SNR of 50 dB (V.90 modem). But, even worse, the line may have a lower SNR,
so that the VF modem typically falls back to an operating speed of about 24 kbs in practice
(as demonstrated by SA1–3). However, digital subscriber line (DSL) cards can be used in the
RT to take advantage of the wide bandwidth of short “last mile” twisted pair lines and provide
data rates on the order of 6 Mbs. There are many DSL methods, so we will denote this family
of methods by the notation xDSL. All use one or two twisted pairs to serve the “last mile”
customer from an RT.
1. HDSL (high-bit-rate digital subscriber line) uses two twisted pairs (one transmit and
one receive) to support 1.544 Mbs at full duplex for a distance of up to 12,000 ft from
the RT. It uses a 2B1Q line code of ISDN (described below) or a suppressed carrier version
of QAM called carrierless amplitude-phase (CAP).
2. SDSL (symmetrical digital subscriber line) is a one-pair version of HDSL. It provides
full duplex to support 768 kbs in each direction using a hybrid or echo canceller to
separate data transmitted from data received.
3. ADSL (asymmetrical digital subscriber line) uses one twisted pair to support 6 Mbs
sent downstream to the customer and 640 kbs sent upstream over a distance of up to
12,000 ft. The ASDL spectrum is above 25 kHz. The band below 4 kHz is used for a VF
POTS signal. Two variations of ADSL—called G.DMT and G.Lite—are discussed in
the next section.
4. VDSL (very-high-bit-rate digital subscriber line) uses one pair of wires to support
25 Mbs downstream for distances of up to 3,000 ft from the RT or 51 Mbs downstream
for distances of up to 1,000 ft from the RT. Up to 3.2 Mbs can be supported
upstream.
5. ISDN (integrated service digital network) uses one twisted-pair line to provide a subscriber
data rate of up to 144 kbs in each direction at distances of up to 18,000 ft from
the RT. This technology has been available since 1990 and has been popular in Europe
and Japan, but not too popular in the United States. More details about ISDN are discussed
in a later section.
G.DMT and G.Lite Digital Subscriber Lines
G.DMT and G.Lite use “always on” (packet) ADSL technology with a spectrum above the
4 kHz voice band. This allows a VF POTS signal to be supported along with the data signal
on a single twisted-pair line.
As shown in Fig. 8–5a, G.DMT uses a splitter so that the telephone handsets will not
“short out” the data signal and vice versa. G.DMT uses discrete multitone modulation
(DMT) with up to 256 carriers and up to 15 bits of data, 32,768 QAM, modulated on each
carrier. † This provides a maximum data rate of 6.1 Mbs downstream and 640 kbs
† DMT is Orthogonal Frequency Division Multiplexing (OFDM) and is described in Sec. 5–12.