Specifying SCR and PCR

When ordering CBR service, only PCR is defined. When pricing CBR flat rated service, the SCR pricing per 64 kbps and per 1 Mbps should be used since PCR equals SCR for CBR service. CBR PVCs should specify a PCR value in 64 kbps or in 1 Mbps increments at 3 Mbps and higher. SCR is also not assigned to UBR service. Thus, SCR is only specified for VBR service.

VBR-rt and VBR-nrt SCR must be between 20% and 70% of PCR.   Likewise, PCR must be between 143% and 500% of SCR. SCR and PCR are rounded to the nearest 64 kbps or 1 Mbps increments.

NOTE:   SCR is not a guarantee ! !  It means that the customer can expect to be able to push through that much data “on average”.  The leaky bucket theorem uses “credits” to  police the traffic at the SCR level.  There are 3 scenarios :

1)      exceeding SCR – if the customer exceeds SCR – they will be allowed to exceed SCR until their “credits” run out – and will then  be pulled back down below SCR for a time to even it out.

2)      SCR rate -  if the customer does not exceed SCR, but is running steadily at the SCR rate, they can expect that rate to continue.

3)      Less than SCR - if the customer runs below SCR, they build up “credits” to allow them to burst over SCR until the credits are depleted.

PCR is also a VBR term,however, it is sometimes stated that "the CBR is equal to the PCR".  PCR is the setting for the maximum data rate, for a burst of data.  It is basically telling the customer, "this is the maximum data transfer rate you can possibly reach".

SCR vs PCR  - SCR is a fraction of PCR, and can be anywhere from .2 to .7, therefore :

                   SCR = N x PCR,  where   .2 <= N  <= .7

The following chart provides the maximum PCR rates for CBR service:

Specifying VBR-rt/VBR-nrt PCR and SCR

When pricing and ordering VBR-rt and VBR-nrt service, the SCR and PCR must be defined per PVC. VBR-rt and VBR-nrt SCR must be between 20% and 70% of the PCR. The SCR must be in 64 kbps increments up to 2.944 kbps or in 1 Mbps increments at 3 Mbps and higher. VBR-rt and VBR-nrt PCR must be ordered in 64 kbps or 1 Mbps increments and should be between 143% to 500% of SCR. (SCR must be between 20% and 70% of PCR.)

The Access Type (ATM Port) limits the VBR-rt and VBR-nrt PVC's SCR and PCR value. The PCR can not be higher than the applicable line rate. The SCR can not be higher than 70% of the PCR rounded down to the nearest 64 kbps increment under 3 Mbps and rounded down to the nearest 1 Mbps increment at 3 Mbps and higher. (See below chart.)

VBR-rt and VBR-rt PCR can be ordered at the below Maximum PCR (Line Rates). Ordering PCR at Line Rate is the only exception to the 64 kbps increment and 1 Mbps increment SCR and PCR ordering requirements.

The chart below provides the maximum SCR and PCR rates for VBR-rt and VBR-nrt service. PCR must then be set between 143% and 500% of SCR. Please remember, maximums for PCR still apply for FRASI (up to 1.39 Mbps since this is the maximum information rate (without an SCA) allowed for FRASI). Please ensure PCR for FRASI does not exceed the line rate of the Frame Relay side of the connection.

Once the maximum VBR-rt/VBR-nrt SCR and PCR value is determined based on Access Type, the below chart should help determine the available PCR if SCR is known or help determine the available SCR is PCR is known. The below chart only considers the relationship between SCR and PCR. It does not consider Access Type and Line Rates.