User manual

Chapter 2 - Managing a Single Device/Cell Service Parameters Page
AlvariCRAFT Device Manager 122 User Manual
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 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 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 2048 Kbits x 0.5 = 1024 Kbits may be
transmitted to a unit whose applicable MIR value is 2048 Kbps, without any delay
(provided overall conditions in the wireless link allow this burst).
The MIR/CIR section includes the following parameters: