The time-division multiple access (TDMA)
transmissions from cable modems at various
distances from the head-end must be coor-
dinated so that all transmissions align with
the boundaries of the head-end minislots.
Discrepancies between individual modems
and the head-end are caused by propagation
delays in the CATV plant, the FEC inter-
leaving function (a variable-depth inter-
leaver supports latency-sensitive and
latency-insensitive data), and processing
time.
The CMTS informs the modem of the
propagation delay after it receives the rang-
ing request. Upstream frequency assign-
ments can change at any time. The head-end
ensures that the cable modem receives the
new frequency assignment before listening
for the modem’s transmission on the fre-
quency. After ranging is complete, the cable
modem must invoke dynamic host config-
uration protocol (DHCP) mechanisms to
obtain an IP address and continue the reg-
istration/configuration process (provision-
ing process).
The DHCP server responds with IP ad-
dresses, the name of the configuration file,
and server addresses. After establishing a se-
curity association, the cable modem must
download (via TFTP) a file with configura-
tion parameters from the configuration serv-
er. The CMTS then checks that the config-
uration was obtained from a legitimate con-
figuration server. Finally, the cable modem
registers (Figure 8) with the CMTS and be-
38
Ericsson Review No. 1, 2001
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