Ericsson HM200c/HM201c Manual de usuario descargar pdf (Pagina 4)

Ericsson Review No. 1, 2001

35

system with 400 MHz of downstream band-

width can carry the equivalent of 50 to 60

analog TV channels, and a modern hybrid

fiber-coax system with 700 MHz of down-

stream bandwidth has capacity for some 80

to 110 channels.

To deliver data services over a cable net-

work, one television channel (in the 50 to

865 MHz range) is typically allocated for

downstream traffic to homes, and one or

more channels (in the 5 to 42 MHz band)

are used to carry upstream signals. Depend-

ing on the availability and the business vi-

ability of other channels, the number of

cable modem users supported by a head-end

can be incremented by commandeering

other channels for data and IP transmission.

When a channel is used for data, it cannot

be used for other conventional, revenue-

generating broadcasts, such as commercial

TV or pay-per-view services. However, new

Inernet-based revenue-generating services

can now be offered on that channel.

Figure 4 shows the modulation and de-

modulation protocols as well as frequency

ranges for the CATV system. It also shows

the bandwidth and effective bit rates. Fig-

ure 5 compares upstream and downstream

transmission.

Using 64 quadrature amplitude modula-

tion (QAM) transmission technology, a sin-

gle downstream 6 MHz television channel

can support up to 27 Mbit/s of downstream

data throughput from the cable head-end.

Speeds can be boosted to 36 Mbit/s using

256 QAM. Depending on the spectrum al-

located for service, upstream channels from

the home can deliver 0.5 to 10 Mbit/s using

16 QAM or quadrature phase shift key

(QPSK) modulation techniques. The up-

stream and downstream bandwidth is

shared by active data subscribers who are

connected to a given cable network segment,

typically 500 to 2,000 homes on a modern

HFC network.

Wide-area

network

Cable

network

Cable

modem



Cable modem

termination

system



NSI RFI

Transparent IP traffic through the system

CMCI

Customer

premises

equipment



Figure 3

Data traffic through data-over-cable sys-

tem.

CMTS head-end

Upstream demodulator

QPSK/16QAM

Freq. 5–65MHz BW 2MHz

Rate:¯ ~3Mbit/s

Downstream modulator

64QAM/256QAM

Freq. 65–850MHz BW: 6/8MHz

Rate:¯ 27–56Mbit/s

Upstream modulator

QPSK/16QAM

Freq. 5–65MHz BW 2MHz

Rate:¯ ~3Mbit/s

Downstream demodulator

64QAM/256QAM

Freq. 65–850MHz BW: 6/8MHz

Rate:¯ 27–56Mbit/s

Cable modem

Figure 4

Functional block diagram of the cable modem termination system.

565

Upstream 2MHz Downstream 8MHz

850 MHz

Figure 5

Frequency domains of the upstream and downstream channel positions.

1 2 3 4 5 6 7 8 9 10 11 12

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