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ATM Features and Implementation Notes

5-15

• The PCR speciﬁes the desired rate for the attached physical media (that is,

SONET/SDH, DS-3, or E-3). It does not specify the rate for the ATM network

as a whole. For example, you can specify a full 149.76 Mb/s for each PVC or

Control VC on a SONET/SDH connection. However, if the VC ultimately

connects to a lower-speed link (for example, T1 or E1), your PCR is limited to

the maximum rate of that media.

• ATM FRE2 ILI pairs require a minimum PCR value of 300 cells/s. VCs may

fail to operate with PCR values less than 300 cells/s.

• ATM ARE ILI pairs require a minimum PCR value of 128 cells/s. VCs may

fail to operate with PCR values less than 128 cells/s.

• The E-3 framing mode setting affects the maximum PCR setting. Refer to

Table 5-1 for additional information.

Sustainable Cell Rate and Minimum Cell Rate

For ATM FRE2 ILI pairs, the sustainable cell rate (SCR) is the upper bound on the

conforming average rate of an individual PVC or Control VC. In this deﬁnition,

average rate is the number of cells transmitted over the link divided by the

duration of the connection. The duration of the connection equals the total amount

of time it takes from connection setup to connection release.

The SCR allows you to deﬁne future cell ﬂow on a PVC or Control VC in greater

detail than by just using the PCR. For ATM FRE2 ILI pairs, the SCR controls the

rate over time — not at a speciﬁc instant of time — and can help you more

efﬁciently use your network resources. In other words, the SCR allows sufﬁcient

bandwidth for operation, but does not allow a bandwidth as high as the PCR.

For ATM ARE ILI pairs, the SCR value maps directly to a minimum cell rate

(MCR) value. In other words, when you conﬁgure the SCR on an ATM ARE ILI

pair, you actually conﬁgure the MCR. Instead of deﬁning the upper bound of an

average rate, like the SCR, the MCR deﬁnes the minimum amount of guaranteed

bandwidth allowed for PVCs and Control VCs on the ATM line.

The MCR (that is, SCR) not only controls the rate over time, it guarantees this

rate. In other words, the SCR provides sufﬁcient bandwidth for SVC operation

but, at the same time, guarantees the bandwidth for PVCs and Control VCs on the

ATM line.

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Avaya Configuring ATM Services Manual de usuario Pagina 99