Ericsson HM200c/HM201c Manual do Utilizador descarregar pdf (Página 3)

comparison of DOCSIS and Euro-DOCSIS,

see Tables 1, 2 and 3).

The cable modem

termination system

A CATV network consists of six major parts:

• a cable modem termination system;

• a trunk cable;

• a distribution system or feeder in the

neighborhood;

• the drop cable to the home and in-house

wiring;

• a cable modem, which is connected via

coaxial cable and optical fibers; and

• customer premises equipment (CPE).

Figure 3 shows a simple diagram of the data

traffic through a data-over-cable system as

well as the external interfaces of the key com-

ponents. The interface between the wide-

area network (WAN) and the CMTS is

called the network-side interface (NSI); the

interface between the CMTS and the cable

modem is called the radio frequency inter-

face (RFI); and the interface between the

cable modem and customer premises equip-

ment is called the cable modem-to-CPE in-

terface (CMCI). The user connects to the In-

ternet service provider through the WAN

interface.

The system shares media for upstream and

downstream transmissions. The two most

common problems in this architecture are

• the need to control access to shared re-

sources, particularly upstream band-

width; and

• the injection of ingress noise in the up-

stream direction.

Traditional coaxial cable systems typically

operate with 330 or 450 MHz of capacity,

whereas modern hybrid fiber-coax systems

have been expanded to 850 MHz. The terms

CMTS and head-end are commonly inter-

changed in contexts that refer to the equip-

ment responsible for communications in the

cable network. In reality, the CMTS could

be part of the head-end—the equipment

from which multiservice operators broad-

cast television content.

Logically, downstream video program-

ming signals—that is, from the CMTS to

the cable modem—begin around 50 MHz,

the equivalent of channel 2 for over-the-air

television signals. The portion of the spec-

trum between 5 and 42 MHz is usually re-

served for upstream communication—that

is, from the cable modem to the CMTS. In

North America, each standard television

channel occupies 6 MHz of spectrum (in Eu-

rope, 7 or 8 MHz). Thus, a traditional cable

Ericsson Review No. 1, 2001

Frequency 42 to 850 MHz in North

America; 65 to 850 MHz in

Europe

Bandwidth 6 MHz in North America;

8 MHz in Europe

Modulation 64 QAM with 6 bits per

symbol (normal)

256 QAM with 8 bits per

symbol (faster, but more

sensitive to noise)

TABLE 1, PROPERTIES OF

DOWNSTREAM SIGNALS

64 QAM 256 QAM

6 MHz 31.2 Mbit/s 41.6 Mbit/s

8 MHz 41.4 Mbit/s 55.2 Mbit/s

TABLE 2, DATA RATES PER

MODULATION SCHEME

Parameter North American value European value

Center frequency 91 to 857 MHz ± 30 kHz 112 to 858 MHz ± 30 kHz

Level range (one channel) -15 dBmV to 15 dBmV 43 to 73 dBµV for 64 QAM

47 to 77 dBµV for 256 QAM

Modulation type 64 QAM and 256 QAM 64 QAM and 256 QAM

Symbol rate (nominal) 5.056941 Msym/sec (64 QAM) 6.952 Msym/sec (64 QAM) and

and 5.360537 Msym/sec 6.952 Msym/sec (256 QAM)

(256 QAM)

Bandwidth 6 MHz (18% square root 8 MHz (15% square root raised

raised cosine shaping for 64 QAM cosine shaping for 64 QAM

and 12% square root raised and 15% square root raised

cosine shaping for 256 QAM) cosine shaping for 256 QAM)

Total input power <30 dBmV < 90 dBµV

(40-900 MHz)/

(80-862 MHz) for Europe

Input (load) impedance 75 ohms 75 ohms

Input return loss > 6 dB (88-880 MHz) > 6 dB (85-862 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 3, ELECTRICAL INPUT TO CABLE MODEM

1 2 3 4 5 6 7 8 ... 11 12

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