User Manual

ADCP-XX-XXX • November 2000 • Section 3: WMTS Functional Description
Page 3-14
 2000, ADC Telecommunications, Inc.
shifting the upstream frequency by 5kHz. In preparing the ranging response message, the WMTS
notes:
The time offset of when it receives the ranging request from the actual beginning of the initial
maintenance transmit opportunity.
The exact frequency of the transmission.
The incoming transmission power level.
Based on this data, the WMTS determines the corrections and send them back to the WMU in the
ranging response message. The WMU adjusts its parameters based on these corrections and transmits
a second ranging request to the WMTS. The WMTS once again returns the ranging response with
time, frequency, and power corrections, as needed. This process continues until the WMTS is satisfied
with the timing, frequency, and the power settings being used by the WMU. When finished, the timing
is synchronized to within less than 1 microsecond, frequency is transmitted to within 10 Hz, and the
transmit power level is adjusted to within 1/4dB. The ranging process is first performed during an
Initial maintenance transmit opportunity, when a WMU is first connected to the network. Once
booted, the ranging process is repeated for each WMU at regular intervals during periodic
maintenance opportunities that the WMTS schedules. Periodic tuning of timing, transmit frequency,
and transmit power, ensures continued reliable communications between the WMUs and the WMTS.
The time synchronization is critical to the function of the MAC protocol. Time synchronization
requires that the two-way round trip transmission delay be compensated, since all upstream
transmissions are aligned with the mini-slot timing as viewed at the WMTS. Communication is
delayed on the downstream direction by the latency, downstream propagation delay and the
processing overhead. When the wireless modem sends a transmission upstream, it is delayed by
upstream propagation and processing overhead. The sum of these delays is effectively removed by the
ranging process. The timing offset causes the wireless modem to transmit at an earlier time than the
assigned mini-slot time (as measured by the WMTS). This is in order to compensate for the delays
caused by interleaving latency in the downstream propagation in the system, in addition to fixed
processing overload in both the WMTS and the WMU. All further transmissions on the return path
which take place, are based on the requests from the WMU to the WMTS, and grants from the WMTS
to the WMU. Whenever the modem has data to send, it requests a time slot from the on return path,
during a request period (as defined on a MAP message). The WMTS grants this request and
schedules the WMU, to transmit the data. Initial ranging can be considered analogous to the
AutoDiscovery feature. AutoDiscovery is essentially the modem being given an opportunity to
transmit without first being given a grant to transmit. The MAC allows this to occur, by offering
periodic initial maintenance (paragraph 7.3.3) transmit opportunities on the return channel that can be
used by any modem just connected to the network.
6.6 IP Layer Establishment
Once timing, frequency, and power are set, the modem must establish the IP protocol. This is achieved
in conjunction with the MAC address of the WMU. An entry of the MAC address in the address
translation table at the WMTS, provides the relationship of the MAC address to the IP address. The IP
address is established by invoking the Dynamic Host Configuration Protocol (DHCP) which assigns
the WMU an IP Address. DHCP runs between the WMU and DHCP server, administered by either the
operator or some other service provider. As long as the WMU is active it is leased an IP address to