Ericsson HM200c/HM201c Manual de usuario descargar pdf (Pagina 6)

Ericsson Review No. 1, 2001

tention- and reservation-based transmission

opportunities. Each cable modem has a

unique 48-bit MAC address that is used for

registration and authentication. This is en-

tered in a table with the primary service ID

(SID) and IP address, once the IP address is

assigned. The CMTS then communicates to

the modem with the assigned ID.

Significant processing power is needed for

• converting encoded signals into the for-

mat needed by the customer premises

equipment; and

• conveying messages from the end-user to

the destination point through the CATV

system.

All available silicon solutions currently im-

plement digital signal processors (DSP) for

demodulating downstream transmissions,

which are expected to have low bit error rate

(BER). The DSPs handle demodulation,

whereas regular processing power is devot-

ed to implementing the key components of

the cable modem transmission protocol.

Powerful RISC processors enforce protocol

functions.

Downstream transmission takes place in

one of the 6 or 8 MHz channels between 65

and 850 MHz at 25 to 56 Mbit/s, as deter-

mined by the modulation scheme (Table 1).

The raw data rate depends on the modula-

tion and bandwidth (Table 2).

A symbol data rate of 6.9 Msym/s is used

for 8 MHz bandwidth and 5.2 Msym/s is

used for 6 MHz bandwidth. Due to error

correction, framing and other overhead, the

effective data rate is somewhat slower than

the raw data rate. Since downstream data is

received by all cable modems, the total

bandwidth is shared by all active cable

modems on the system. This is similar to

Ethernet, except that the wasted bandwidth

is much greater in Ethernet.

Operation

Initialization

Cable modem communications are set up

through a series of initialization steps. After

power-up, the modem scans for a down-

stream channel with which it can synchro-

nize. The CMTS sends synchronization

packets to generate a timing reference. Cable

modems are synchronized and ranged so that

they know when to begin transmission—in

order to hit a specific minislot provided by

the head-end. The CMTS controls access to

slots by assigning specific “transmit oppor-

tunities” to ranges of minislots (a transmit op-

portunity can be contention- or reservation-

based). A reserved slot is a timeslot that is

reserved for a particular cable modem—that

is, no other cable modem may transmit in

that timeslot. The CMTS allocates time-

slots through a bandwidth-allocation algo-

rithm. The algorithm is vendor-specific and

might differ considerably from vendor to

vendor. Reserved slots are generally suited

for longer data transmissions. After syn-

chronization is complete, the cable modem

receives the upstream parameters it needs to

inform the CMTS of its presence on the net-

work. The cable modem receives the up-

stream allocation information—which it

uses to start the ranging process.

Due to the physical distance between the

head-end and cable modem, the time delay

(in milliseconds) can vary significantly. To

compensate for delay, each cable modem

employs a ranging protocol that effectively

adjusts its internal clock. To do so, a num-

ber of consecutive timeslots (normally three)

is reserved for each ranging process. The

cable modem is instructed to transmit in the

second timeslot. The CMTS measures the

transmission and instructs the modem to ad-

just its clock as necessary. The two time-

slots before and after create a gap to ensure

that the ranging burst does not collide with

other traffic.

Ranging is also used to coordinate the

transmission power level of all cable

modems, so that the upstream bursts arrive

at the CMTS at the same level. A balanced

transmission power level is essential for

maintaining optimum performance of the

upstream demodulator in the head-end. The

attenuation from the cable modem to the

head-end can vary by more than 15 dB.

Contention slots are open for all cable

modems to transmit in. If two cable modems

transmit in the same timeslot, their packets

collide and the data is lost. When this oc-

curs, the lack of positive ACK from the

CMTS serves to indicate that the CMTS did

not receive any data, and the cable modems

retransmit at another, randomly selected

time. Contention slots are normally used for

very short data transmissions, such as a re-

quest for reserved slots, in order to transmit

more data.

Ranging registration allows the cable

modem to identify itself to the CMTS. It

also allows the head-end

• to assign downstream and upstream fre-

quencies;

• to set power levels; and

• to distribute other administration infor-

mation necessary to manage the network.

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