User Guide

AB-Access Config & User Guide 5.5 Axxcelera Broadband Wireless
July 27, 2004 Company Confidential Page 25 of 129
5.4.1.4 Downstream Acknowledgement (DACK)
The Downstream Acknowledgement (DACK) is an acknowledgement sent from the AP,
which contains bit maps corresponding to the success or failure of individual cells sent to an SU in
the previous frame. If any cells were missed or dropped they will be resent in the next frame.
5.4.1.5 Downstream Data Cells (DCELL)
The Data Cells are the actual ATM cells that contain data. The maximum ATM cells per
frame are 32. The maximum ATM cells that can be sent to an individual SU is 6 per frame.
5.4.1.6 Subscriber Turnaround Time (STT)
AB-Access system is TDD (Time Division Duplexing), meaning that the AP and SUs
transmit and receive on the same frequency. It is therefore necessary to have a small delay between
transmit and receive processes, because it is using the same hardware to perform both functions.
There is also a set delay for each individual SU. This delay in turnaround is to compensate for
propagation delay.
5.4.1.7 Reservation Request (RR)
The Reservation Grant Request (RR) is a request sent by the SU to the AP when it has data to
send. The RGR is a contention based request, meaning it performs like an Ethernet network were
there is no guarantee that it will be received by the AP on the first try. This would happen when
another SU wants to transmit at the same time and would cause a collision. If a collision occurs the
SU will try again until it is acknowledged with a Reservation Grant Response.
5.4.1.8 Upstream Acknowledgement (UACK)
The Upstream Acknowledgement (UACK) contains bit maps corresponding to the success or
failure of individual cells from an AP. If any cells were missed or dropped they will be resent in the
next frame.
5.4.1.9 Upstream Cell (UCELL/UCELLR)
The UCELL’s are the actual ATM cells that contain data. The maximum ATM cells per
frame are 32. The maximum ATM cells that can be sent to an AP by an individual SU is 6 per frame.
The UCELLR is a data cell that also has a Reservation Grant Request if the SU has more than 6 data
cells to send. The reason for this is so that the SU doesn’t have to contend for another Reservation
Grant in the contention slot.
5.5 Delay Compensation
During the upstream portion of the MAC several SUs transmit in sequence. Since SUs are
separated from the AP by anywhere from 0m to 16km, there is a wide range of propagation times for
the full path for the AP to the SUs and back. In order for the SUs transmissions to arrive at the AP
properly aligned, some active compensation of propagation delay is performed. An SU must delay