Oversubscription

 VBR (Variable Bit Rate) - at any given port, the maximum allowed over-subscription is 200% SCR and 500% PCR.  This of course includes the sum of all PVC's.

 CBR (Constant Bit Rate) - no over-subscription is allowed.  CBR rates can be selected at up to an aggregate of approximately 95% of the port speed.

 UBR (Unspecified Bit Rate) - the oversubscription is theoretically unlimited, but you need to be practical, and design for the actual bandwidth the customer requires

Oversubscription of an ATM access port occurs when the sum of all the PCRs associated with the PVCs on a port exceeds the maximum information rate of the ATM port.  Due to the intermittent, or bursty nature of most data traffic, it is unlikely that all PVCs assigned to a single access port will simultaneously transmit during the same millisecond.  Therefore, sometimes more bandwidth can be subscribed to a port than the maximum information rate, while still maintaining the specified Quality of Service (QoS) for each PVC.  Thus, customers may more fully utilize the ATM port capacity, reducing overall network costs.

The provider currently permits access port oversubscription within certain limits for VBR-nrt service.  Oversubscription of an ATM access port should only be considered if the customer has reasonable knowledge of the their traffic patterns.  Customers must consider the delay sensitivity of their applications and the probability of simultaneous activity from the transmitting locations­–generally, the more intermittent the application, the greater the level of oversubscription that may be tolerated.

Oversubscription of an ATM Port may cause degradation in the QoS of the customer’s ATM Service. This is especially true when inbound PVCs are simultaneously attempting to deliver more traffic than the port can handle.  In this case, there is a high probability that cells will be discarded.  This directly impacts The provider’s QoS objectives related to cell loss and cell delay. Therefore, while The provider will allow oversubscription as per the rules stated below, our stated QoS objectives as well as Service Level Agreements (SLAs) for the VBR service category will not  apply when a customer oversubscribes to an ATM port.

The following oversubscription rules must be followed when subscribing to The provider’s ATM Service.  These rules apply separately for customer premises ingress (into the The provider ATM port) and egress (into the customer’s ATM port) PVCs.

** NOTE: Violations of the following rules will result in refused orders **

Oversubscription of CBR:

No oversubscription of CBR service is permitted.

Oversubscription of VBR-nrt:

Oversubscription of VBR-nrt service is permitted based upon the following two limits, whichever occurs first:

1. The sum of all PCRs must not exceed 500% of the maximum Information Rate of the port in which it is subscribed and,

2. The sum of all SCRs must not exceed 200% of the maximum Information Rate of the ATM port in which it is subscribed. 

When VBR-nrt traffic is mixed with CBR traffic on the same ATM Port, the CBR bandwidth is reserved and must be deducted from the maximum Information Rate of the ATM Port when calculating the above VBR-nrt oversubscription limits.  For example, if the customer subscribed the maximum of 18 Mbps of CBR on a DS3 port, VBR-nrt PCR oversubscription would be limited to five times the remaining 18.86 Mbps bandwidth (about 94 Mbps; ordered in 1 Mbps increments).

Oversubscription of UBR:

No oversubscription limits currently apply to UBR traffic.

Maximum Port Informatioin Rate for a DS3

A DS3 ATM Port has the following capacity constraints:

·        Port Access Rate: 45 Mbps

·        Port Transfer Rate: 40.7 Mbps (53 byte cells)

·        Port Information Rate:

·        CBR -  34.291 Mbps (47 byte user payload)

·        VBR-nrt -  36.86 Mbps (48 byte user payload)

Oversubscription Calculation Example

Below is the calculation of oversubscription, based on the above information to insure oversubscription rules are followed:

CBR Subscription:                                                                                                                

= (Sum of CBR PCRs / Port Information Rate) x 100%

= (6 Mbps / 36.10 Mbps) x 100%

= 16.6% (within the 50% limit for mixed traffic configurations)

VBR-nrt PCR Subscription:

= (Sum of VBR-nrt PCRs / (Port Information Rate—CBR PCRs)) x 100%

= (106.5 Mbps / (36.86 Mbps -- 6 Mbps)) x 100%

= 345% (within the 500% limit)

VBR-nrt SCR Subscription:

= (Sum of VBR-nrt SCRs / (Port Information Rate—CBR PCRs)) x 100%

= (56 Mbps / (36.86 Mbps—6 Mbps)) x 100 %

= 181% (within the 200% limit)