ZXDSL 9210 (V3.1) Broadband Universal Access Equipment Technical Manual
1-6
Table 1.2-2 Transmission Rates of High-Speed Channels and Medium-Speed Channels
Channel Average
Rate
Lowest Rate
Highest Rate
High-speed downlink channel
6 Mbit/s
1.5 Mbit/s
8 Mbit/s
Medium-speed duplex channel
512 kbit/s
64 kbit/s
1 Mbit/s
Note: This table is for reference only. Actual rate is subject to the physical cable length, dimension, and interference.
In the past years, the hardware technology of the telephone system has improved a lot
while ADSL achieves a surprising rate very simply: Compression. It uses very
advanced DSP and algorithms to compress as much information as possible in the
telephone line (twisted pair). ADSL products employ FDM to multiplex
uplink/downlink channels, and isolate POTS channels. In order to fully utilize
spectrum, ADSL generally adopts the DMT modulation. DMT divides 1 MHz spectrum
into 256 sub-channels with the bandwidth of 4.3125 kHz. The number of bits over each
channel is determined by the actually measured channel quality, so as to shunt those
noisy sub-channels with too much damage, thus realizing reliable communication.
ADSL working between points needs no medium access control, so each subscriber can
work in the continuous full-speed environment. The ADSL rate depends fully on line
distance, that is, the longer a line is, the lower rate it has. The rate may be lower than
1.5 Mbit/s, but the average rate may amount to 6 Mbit/s. Fig. 1.2-4 illustrates the
allocation of the channel frequency of ADSL over POTS.
Uplink channel
POTS
4kHz
138kHz
1.104MHz
30kHz
P (f)
f
Downlink channel
Fig. 1.2-4 Allocation of ADSL Channel Frequency
ZXDSL 9210 uses the Frequency-Division Multiplexing (FDM) technology, and spares
0 to 4 kHz for ordinary telephone signals by closing low-end sub-channels. The uplink
signals use the frequency bands of 30 kHz to 138 kHz while the downlink signals
occupy those of 138 kHz to 1.104 MHz.
