Ericsson HM200c/HM201c Manuel d'utilisateur télécharger pdf (Page 10)

Ericsson Review No. 1, 2001

Signals from various sources, including

broadcast transmissions, satellite-delivered

programming, and local television broad-

casts, are received and processed in the head-

end. Each television signal travels on a dif-

ferent frequency that acts as a self-contained

spectrum inside the cable.

Network-layer requirements for the

CMTS extend beyond transparency to IP

traffic. The CMTS must also support

• variable-length subnet masks;

• classless addressing;

• IP multicast addressing and forwarding;

• Internet group management protocol

(IGMP);

• proxy ARP; and

• the filtering of DHCP downstream-

bound broadcast packets to protect

against BOOTP server spoofing.

The data-over-cable protocol relies heavily

on the CMTS for its implementation. Each

node on the head-end is capable of support-

ing between 1 and 2,000 cable modems. The

average number of cable modems per node

is expected to be around 500. The CMTS is

responsible for the initialization, ranging

and maintenance of the network formed by

the cable modems. The initialization

process, which is managed by the CMTS,

can be divided into the following phases:

• synchronization—the CMTS sends tim-

ing and frequency information to the cable

modem, to establish synchronization;

• ranging—the CMTS guides the cable

modem through the ranging process;

• IP connectivity—the CMTS establishes

IP connectivity;

• time—the CMTS establishes the time of

day;

• settings—the CMTS transfers operational

parameters; and

• baseline privacy initialization (BPI)—the

CMTS initializes baseline privacy if the

cable modem can run it.

The CMTS and the cable modem operate as

forwarding agents and as hosts. Figure 13

shows the protocol stack used by these com-

ponents. The data forwarded through the

cable modem is link-layer transparent bridg-

ing and supports multiple network layers.

The main function of the CMTS is to

transmit IP packets transparently between

the head-end and end-user. Management

functions, such as support for spectrum

management and software downloads, are

sent as IP packets as demonstrated in Fig-

ure 13. Both the CMTS and the cable

modem operate as IP and logical link con-

trol (LLC) hosts according to the IEEE 802

standard for communication over the cable

network. The CMTS must support the

transport of IP traffic and must be able to

restrict the network layer to a single proto-

col, such as IP.

Conceptually, the CMTS forwards data

packets at two interfaces between the

CMTS-RFI and the CMTS-NSI, and be-

tween the upstream and downstream chan-

nels (Figure 14).

The cable modem access network operates

at layers 1 (physical) and 2 (media access con-

trol/logical link control) of the open systems

interconnection (OSI) reference model.

Thus, layer 3 (network) protocols, such as

UDP

IP, ICMP

ARP

LLC/DIX

Link security

MAC

Transmission convergence

(downstream only)

Cable physical media stream (PMD)

sublayer

SNMP TFTP DHCP Security

Figure 13

Protocol stack on the RF interface.

IP over IEEE 802 or

IP over ATM

IP IP

Forwarding Bridging

Data

link 

layer

PHY

layer

Data

link 

layer

PHY

layer

LLC

Link security

Cable network

transmission

Cable modem termination

system stack

Cable modem stack

CMTS network

system interface

Cable modem

CPE interface

Cable MAC

Downstream 

trans. conv

Cable PMD

LLC

Link security

Cable MAC

Downstream 

trans. conv

Cable PMD

IP over Ethernet or 

IP over USB

Figure 14

Data forwarding through the CMTS and cable modem.

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