User Manual

ADCP-XX-XXX • November 2000 • Section 3: WMTS Functional Description
Page 3-11
 2000, ADC Telecommunications, Inc.
a certain amount of data. As the WMTS receives requests from all the modems with data to transmit,
it reserves mini-slots (essentially transmit opportunities) on the return path accordingly. Periodically
(every few milliseconds), the WMTS sends a message to the WMUs (over the downstream path)
indicating the specific mini-slots granted to each modem. As a result of reserving bandwidth, modems
are guaranteed a collision-free interval in which to transmit. The WMTS is responsible for allocating
bandwidth to modems based on the type of service to which the user is subscribed. In order for a
modem to request bandwidth without first having been allocated a time to send the message, the
WMTS periodically allocates a portion of the return channel for any modem to send a request. These
requests may collide with a request from another modem. However, the WMTS varies both the length
and the frequency of the contention request intervals to keep the number of collisions to a minimum.
If collision is detected, the WMUs that sent the messages, back-off random intervals and then
retransmit their requests.
5.6 Bandwidth Allocation Map Parameters
The WMTS informs WMU of the allocation of its upstream channel using a MAP (Upstream
allocation map). This is a MAC Management message, sent from the WMTS to the WMU, via a
downstream channel. The map contains an Allocation Start Time that indicates the time when this
MAP takes effect. It also contains a list of Information Elements describing consecutive allocations.
The unit of time used in a MAP is called a minislot. Parameters used to configure the MAP affect its
effectiveness. These parameters can be updated from the SNMP MIB. A suggested configuration will
be offered along with an explanation of each parameter. For each parameter, there is an explanation,
location in SNMP MIB table and description of its effect on a MAP.
6 MODEM CONFIGURATION
6.1 Overview
This section describes the procedures required to operate a modem, after being connected to the
network and before the green receive lock LED is lit. The modem performs many functions, and each
one is briefly described in the following sections.
6.2 Obtain Upstream Parameters
During this phase, the WMU looks for three MAC messages that are repeatedly sent downstream by
the WMTS to provide a common time reference to all the modems. The first message is the Time
Synchronization message (SYNC). The SYNC message is sent by the WMTS to provide a common
time reference to all modems. The second message defines the Upstream Channel Descriptor (UCD).
The WMU must find a UCD that describes an upstream channel that matches its own capabilities. For
instance, if the WMU could only transmit at certain symbol rates or is limited in its frequency range
and modulation types, the WMU may not be able to transmit in a particular upstream channel. The
final message can be to find an Upstream Bandwidth Allocation Map (MAP) that matches the
upstream channel described in the UCD. The MAP message contains the mini-slot information that
indicates when a modem transmits and for how long. The SYNC message provides the time reference
for these transmissions. For its transmission the WMU looks in the MAP for the transmit opportunity