Trouble Shooting Guide

Axxcelera Broadband
Troubleshooting-Installing an RF link - 29 - Issue: 5.3.x
Rev 2
The upstream cell retransmission rate (i.e. average for this SU-AP link) can be calculated approximately
from the “tx” and “CELL TX” counts, as follows:
Upstream Cell Re-Transmission Rate = (( CELL TX – tx ) / tx ) x 100%
A value of 0% means that cells are only sent once, a value of 100% means that on average each cell is sent
twice, and a value of 500% means that every cell is sent six times - the maximum.
Note – This formula is only valid for a large number of cells (minimum recommended = 10000), because
of a 64 cell buffer that can distort the cell re-transmission calculation for small numbers of cells.
9.1.14 inseq
The “inseq” count is the number of ATM cells that the UTOPIA interface has received from the MAC for
this MID (i.e. this SU). This number represents the number of unique, error-free ATM cells that the SU has
received in the correct sequence. For an SU this number may be much lower than the number of CELL
RXOK, because CELL RXOK relates to the cells for all SUs, whereas inseq relates to the cells for this
specific SU.
9.1.15 miss
The “miss” count represents the number ATM cells the UTOPIA interface has received with the sequence
number wildly out. These represent errors. The missed count is usually close to zero, and should be orders
of magnitude lower than the inseq count.
9.1.16 dup
The “dup” count represents cells that have been received with their sequence number to low, i.e. cells that
have been received multiple times (because they were re-transmitted over the RF link). There are two
mechanisms that cause duplicates; CELL errors and ACK errors. These are described below.
Duplicates caused by CELL errors
Up to 6 cells can be sent to a specific SU in a single burst. If the last cell is received in error, then only that
cell will be repeated in another frame. However, if the first cell is received in error, then all of the cells in
that burst will be repeated in another frame (i.e. six cells may be resent, even though the last five were
received correctly). This results in duplicates. If the errored cell is in the middle of the burst, then cells
from that one on will be re-transmitted.
Duplicates caused by ACK errors
Another cause of duplicates is failure of the ACKs to be received by the AP at the other end of the RF link.
All cells may be received correctly by the SU, but if the ACKs sent as a result of receiving the cells are not
received correctly by the AP then all the cells will be resent. This results in duplicates.
A high duplicate count but low CELL RxMissed and CELL RxBad counts indicates ACK problems at the
other end of the link (i.e. at the AP receiver). If the majority of SUs have high duplicate counts due to
ACK errors at the AP receiver then the problem may lie with the AP receiver or the AP’s transmitter: if the
AP transmits at the wrong power downstream then the upstream power is also wrong, causing errors at the
AP’s receiver. If only one SU has a high duplicate count due to ACK errors at the AP then the problem
may lie with this SU’s transmitter.