Ericsson HM200c/HM201c Manual de usuario descargar pdf (Pagina 5)

Depending on the network architecture

and traffic load, an individual subscriber

might experience access speeds from

500 kbit/s to 1.5 Mbit/s or more. Compared

to dial-up alternatives, this is blazing per-

formance.

Cable modem details

Description

The cable modem (CM) is a modem in the

truest sense of the word—that is, it modu-

lates and demodulates signals. Among its

key components are

• a tuner;

• a demodulator;

• an encryption/decryption unit; and

• an upstream modulator.

Cable modems typically send and receive

data in two slightly different ways. In the

downstream direction, digital data is mod-

ulated and then placed on a 6 MHz (North

America) or 8 MHz (Europe and PAL sys-

tem) channel somewhere between 65 and

850 MHz. Upstream transmission is more

challenging, since it tends to be very noisy

in the 5 to 65 MHz region. Noise or inter-

ference is generated by amateur radio oper-

ators, citizen band (CB) radios, home appli-

ances, loose connectors and poor cabling.

Since cable networks form a tree or branch

architecture, noise is aggregated as the sig-

nals travel upstream.

When a cable modem is installed, a power

splitter and a high-pass filter might be nec-

essary to isolate the TV set from “strong”

signals from the cable modem. The filter

also blocks upstream ingress noise in the low

frequency band. Figure 6 shows how the sig-

nal is split from the main cable to TVs and

cable modems in the home.

Figure 7 shows the major components of

a cable modem implementation. The tuner,

including the diplexer, connects directly to

the CATV outlet, which provides upstream

and downstream traffic to the rest of the

cable modem. The tuner solution, which in-

tegrates the diplexer into a dual-conversion

tuner and digital signal processor, demod-

ulates 64 and 256 QAM signals. The out-

put is a 44 MHz intermediate frequency (IF)

signal that is fed into the analog-to-digital

converter (ADC) inputs for demodulation

and error correction. The media access con-

troller then extracts data and sends it to the

customer premises equipment via a univer-

sal serial bus (USB) or Ethernet interface.

The tuner also includes a line amplifier

for the transmit function, controlled by the

media access controller (MAC), which sends

upstream signals at the level negotiated by

the cable modem and head-end. According

to DOCSIS, Reed-Solomon forward error

correction (FEC) is recommended. This adds

robustness as well as physical layer overhead,

which translates into delay.

The heart of the protocol implementation

resides in the MAC. The CMTS has control

over the allocation of upstream and down-

stream bandwidth in a dynamic mix of con-

36

Ericsson Review No. 1, 2001

Cable modem

10 Mbit/s Ethernet

or USB

TV

Set-top

box

Coax splitter

Coax

Computer

Figure 6

Home environment.

Tuner

Demodulator

error

correction

Burst

modulator

Media

access

controller

Interface

USB-Ethernet

Figure 7

Cable modem architecture.

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