Ericsson HM200c/HM201c Manual de usuario descargar pdf (Pagina 8)

Ericsson Review No. 1, 2001

39

comes operational. The CMTS must autho-

rize the cable modem before it can send traf-

fic into the network.

Each cable modem should contain a

unique 48-bit MAC address (IEEE 802) that

is assigned during the manufacturing

process, and security information necessary

to authenticate the cable modem.

When the initialization process has been

completed, the customer premises equip-

ment can communicate with the outside

world through the CATV installation. The

communication is bidirectional—down-

stream and upstream.

Upstream traffic

The term upstream is used to indicate the

signal transmitted by the cable modem. Up-

stream traffic always occurs in bursts, so that

many cable modems can transmit in the

same frequency. Of the two modulation

schemes (QPSK and 16 QAM), 16 QAM

(four bits per symbol) is the fastest, but it is

also the most sensitive to ingress noise. The

upstream direction is characterized by

• a flexible and programmable cable

modem under the control of the CMTS;

• frequency agility;

• time-division multiple access;

• QPSK and 16 QAM modulation formats;

• support for fixed-frame and variable-

length packet data unit (PDU) formats;

• multiple symbol rates;

• programmable Reed-Solomon block cod-

ing; and

• programmable preambles.

When an Ethernet packet arrives at the cable

modem from customer premises equip-

ment, it is encapsulated in a MAC packet

with a PDU header. The cable modem as-

sesses bandwidth allocation for transmission

opportunities and sends the MAC frame

when allowed. Transmission opportunities

have two basic components: number of mini-

slots, and physical layer characteristics.

At the other end, the CMTS receives the

packet, removes the header, and then for-

wards the packet to another cable modem

through the radio frequency interface or to

the WAN through the network side inter-

face. Figure 9 shows a generic frame format;

Figure 10 shows a generic MAC frame for-

mat. There are three types of MAC header:

packet PDU MAC header, ATM cell MAC

header, and MAC-specific header.

MAC header

Packet PDU header, ATM cell header, ...

MAC-specific header

PMD overhead

(upstream)

MPEG header

Data PDU optional

Data, MAC mangement packet ...

Guard band, preamble, etc.

Figure 10

Generic MAC frame format.

01 11

00 10

Figure 11

QPSK symbol mapping.

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

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