In the upstream direction, the digital sig-
nal is encoded to QPSK or 16 QAM, con-
verted into an analog signal, and sent to the
tuner for transmission in the 5 to 65 MHz
bandwidth. Figures 11 and 12 show the up-
stream symbol mappings. Figure 12 repre-
sents differential-coded symbol mapping.
Table 4 summarizes the characteristics of
the upstream packets generated by the cable
modem.
Downstream traffic
The RF signals from the CMTS to the cable
modem are encoded in either 64 or
256 QAM. The symbol mappings resemble
those shown in Figure 12, but with six-bit
encoding for 64 QAM with 16 codes per
quadrant, and eight-bit encoding for
256 QAM with 64 codes per quadrant.
There are several stages in the downstream
transmission of packets from the CMTS to
the cable modem.
When powering up, the cable modem ex-
tracts information that regulates when and
how it is to communicate with the head-
end. The information needed for the initial
contact is available in the frames that are
broadcast on the network.
During the ranging process, information
is transmitted to help the cable modem ad-
just its timing and upstream transmission
levels. After the modem has been registered,
the downstream traffic will contain the
download data and administrative informa-
tion that the cable modem requested or in-
formation that the CMTS wants to distrib-
ute. Every cable modem can listen to the sig-
nals broadcast from the CMTS, but only the
modem with the correct destination address
can access the information contained in the
payload section of the packet.
Ordinarily, one downstream channel is
paired with multiple upstream channels to
achieve the requisite balance in data band-
width. Each modem transmits bursts in
timeslots (reserved, contention or ranging).
The cable modem must accept a modulated
RF signal (Table 4).
Basic operation of the
CMTS
A head-end cable modem termination sys-
tem communicates with cable modems lo-
cated in subscribers’ homes, to create a vir-
tual local area network (LAN) connection.
40
Ericsson Review No. 1, 2001
Figure 12
16 QAM symbol mapping.
Parameter North America Europe
Frequency 5 to 42 MHz edge to edge 5 to 65 MHz edge to edge
Level range +8 to 55 dBmV (16 QAM) +68 to 115 dBµV (16 QAM)
+8 to +58 dBmV (QPSK) +68 to +118 dBµV (QPSK)
Modulation type QPSK and 16 QAM QPSK and 16 QAM
Symbol rate (nominal) 160, 320, 640, 1,280 and 160, 320, 640, 1,280 and
2,560 ksym/sec 2,560 ksym/sec
Bandwidth 200, 400, 800, 1,600 200, 400, 800, 1,600
and 3,200 kHz and 3,200 kHz
Output impedance 75 ohms 75 ohms
Output return loss > 6 dB (5-42 MHz) > 6 dB (5-65 MHz)
Connector F-connector per [IPS-SP-406] F-connector per [IPS-SP-406]
(in common with the input) (in common with the input)
TABLE 4, ELECTRICAL OUTPUT FROM THE CABLE MODEM
(20 pagine)
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