User's Manual Chapter 13

Chapter 7: Operational Description
AT&T Wireless Services
7-103
FCC Type Acceptance Application
10991 Rev. 1.0 7/6/99
based resource shared by the active RU population served by the Base.
Currently, the maximum data rate on the uplink primary LDC is 72
kbps. Future releases of the product may support higher uplink data
rates.
Higher downlink data rates are possible by dynamically recovering
resources. To support this, the Base will utilize the resources of LDCs
other than the primary to increase the downlink or uplink speed in 72
kbps increments. Since the use of LDCs other than the primary implies
the potential creation of co-channel interference (due to the lower reuse
factor), it would be advantageous if the base had some knowledge of
the interference environment at each of the RUs. Future releases of the
PWAN will enable RUs to provide the Base with information regarding
their interference environment. With this knowledge, the Base may
select additional downlink or uplink LDCs to maximize bandwidth
usage while minimizing the generation of co-channel interference.
7.2.6.3 High-Speed Data Medium Access Control
Although the Digital Medium Access Control (D-MAC) is not part of
the PWAN physical layer, it is briefly described here in order to
differentiate the access method used for packet-switched data services
from that employed for circuit-switched voice traffic.
The D-MAC method used for the uplink is a variant of the Digital
Sense Multiple Access with Collision Detection (DSMA/CD) scheme.
DSMA/CD provides contention control through the use of two flags
that signal the status of the uplink. These status flags are broadcast on
the downlink at specified intervals. One status flag indicates whether
the reverse channel is idle or busy, and the other indicates packet
decoding success or failure. This scheme allows the RU population to
not only recognize when the reverse channel is idle or busy, but it also
allows them to determine whether or not a transmission was
successfully decoded. For example, a channel status flag of busy
followed by a decode failure status flag indicates that channel
contention took place, and the RUs that transmitted should back off a
random amount of time before a retransmission is attempted.
7.2.7 Network Synchronization
Time and frequency synchronization is extremely important to the
proper operation of an OFDM system. Consequently, the airlink