User's Manual

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The Modem Initialization Process – p. 29
3.1.2 Downstream Synchronization
The modem listens for downstream transmissions from the WMTS (which
are broadcast at frequent intervals for exactly this purpose) according to its
pre-configured or default frequency plan. This may be a single fixed
frequency or a sequence of frequencies, depending on the Network RF Plan
and/or the internal programming of the modem.
If the modem is programmed for a single fixed downstream frequency and
fails to receive an WMTS downstream transmission after three attempts it
switches to its additional backup frequencies. See Setting the Modem’s
Downstream Frequency for a description of how to set these parameters.
Synchronization is obtained when the modem locks onto the WMTS
downstream signal and is able to recognize downstream SYNC messages
which are periodically broadcast: these are crucial to calculating the time
slots available for upstream transmission.
3.1.3 Obtaining Upstream Parameters
The modem must now wait to collect all the Upstream Channel Descriptors
(UCDs) for all upstream channels that are connected to the current
downstream channel.
3.1.4 Ranging
The modem then sends a “ranging request” message on one or more of the
Upstream frequencies, corresponding to a UCD, at different power levels in
an attempt to establish communication with the WMTS. Eventually the
WMTS decodes a ranging request from the modem and sends a ranging
response.
Ranging is now complete. Whenever the modem needs to send MAC layer
data for IP to the WMTS it sends a “request” to the WMTS. The WMTS
schedules a time slot for the modem and sends a “grant” message back to
the modem that contains all the necessary information telling the modem
when it can transmit data. A similar method is used to send E1 or T1 data
except that the request sets up periodic time slots that the modem uses to
transmit data.
3.1.5 Establish IP Connectivity
The modem uses the “request-grant” (“best effort”) mechanism to send a
standard DHCP request to the Network DHCP server. The server will
recognize the modem via its MAC (Ethernet) address and assign it a
temporary IP address. This response also contains the default gateway and
the IP subnet mask. The IP addresses of the TFTP and ToD servers, and
the name of the modem configuration file are also included. The response
also includes the local time offset from Universal Coordinated Time (UTC).
Once the modem receives this response it sets its IP address to the
designated value and uses the default gateway to talk to devices using its IP
stack.