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Chapter 4 - Operation and Administration
BreezeACCESS VL System Manual
194
Under normal conditions, the actual Information Rate (IR) is between the
applicable CIR and MIR values, based on the formula IR=CIR+K(MIR - CIR).
In this formula K is between 0 and 1 and is determined dynamically by the AU
according to overall demand in the cell and the prevailing conditions that
influence the performance of the wireless link. In some situations the minimum
rate (CIR) cannot be provided. This may result from high demand and poor
wireless link conditions and/or high demand in over-subscribed cells. When this
occurs, the actual information rate is lower than the CIR, and IR = (1+K)*CIR,
where K < 0. The K value to be used in the cell is advertised by the AU in every
beacon, and it is changed every second based on comparison of the traffic during
the last one second interval with the traffic during the previous previous one
second interval. The advertised K value is used by each SU to calculate the
amount of data that can be transmitted. This algorithm ensures fair resource
distribution among SUs, based on their configured CIR/MIR values.
The MIR Threshold Percent parameter determines the level of wireless link
utilization above which the MIR/CIR mechanism is activated. A Threshold of 0%
allows CIR only. A threshold of 100% means MIR only. For other values, if the
actual wireless link utilization is below the threshold, K is set to 1. As the link
utilization increases above the threshold, K is decreased as described above.
The simple solution for managing the information rate in such cases can result in
an unfair allocation of resources, as subscribers with a higher CIR actually
receive an IR lower than the CIR designated for subscribers in a lower CIR
bracket.
A special algorithm for graceful degradation is incorporated into the AU, ensuring
that the degradation of performance for each individual Subscriber Unit is
proportional to its CIR.
The MIR/CIR algorithm uses buffers to control the flow of data. To balance the
performance over time, a special Burst Duration algorithm is employed to enable
higher transmission rates after a period of inactivity. If no data intended for a
certain SU (in the AU) or for the AU (in an SU) is received from the Ethernet port
during the last N seconds, the unit is allowed to transmit to this destination N
times its allowed IR value without any delay. For example, if the Burst Duration
is set to 0.5 second (or more), then after a period of inactivity of 0.5 seconds up to
128 Kbits x 0.5 = 64 Kbits may be transmitted to a unit whose IR is 128 Kbps,
without any delay (provided overall conditions in the wireless link allow this
burst).
4.2.6.6.2.1 MIR: Downlink (SU only)
Sets the Maximum Information Rate of the downlink from the AU to the SU. The
MIR value cannot be lower than the corresponding CIR value.
Available values range and default value are shown in Table 4-14.