User's Manual

Chapter 4 - Operation and Administration
BreezeACCESSS VL System Manual
4-86
MIR and CIR Parameters
The CIR (Committed Information Rate) specifies the minimum data rate
guaranteed to the relevant subscriber. The MIR (Maximum Information
Rate) value specifies the maximum data rate available for burst
transmissions, provided such bandwidth is available.
Under normal conditions, the actual Information Rate (IR) is between
the applicable CIR and MIR values, based on the following 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.
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 is received from the Ethernet port during the last N
seconds, the unit is allowed to transmit N times its CIR value without
any delay. For example, after a period of inactivity of 0.5 seconds, a unit
with CIR = 64Kbps can transmit up to 64Kbits x 0.5 = 32Kbits without
any delay.
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 4-10 on
page 4-87.
The actual value will be the entered value rounded to the nearest
multiple of 128 (N*128).