Network design

As Figure 80: Committed information rate (burst range) shows, traffic up to the CIR is guaranteed, whereas traffic beyond the CIR usually is not. This is how Frame Relay is intended to work. CIR is a committed and reliable rate, whereas burst is a bonus when network conditions permit it without infringing upon the CIR of any user. For this reason, burst frames are marked as discard eligible (DE), and are queued or discarded when network congestion exists. Although experience has shown that customers can achieve significant burst throughput, it is unreliable and unpredictable, and not suitable for real-time applications like IP Telephony.

Therefore, the objective is to prevent voice traffic from entering the burst range and being marked DE. One way to accomplish this is to prohibit bursting by shaping the traffic to the CIR and setting the excess burst size (Be – determines the burst range) to zero. However, this also prevents data traffic from using the burst range.

Additional frame relay information

One interesting piece of knowledge is that most IXCs convert the long-haul delivery of Frame Relay into ATM. That is, the Frame Relay PVC is converted to an ATM PVC at the first Frame Relay switch after leaving the customer premise. It is not converted back to Frame Relay until the last Frame Relay switch before entering the customer premise. This is significant because ATM has built- in Class of Service (CoS). A customer can contract with a carrier to convert the Frame Relay PVC into a constant bit rate (CBR) ATM PVC. ATM CBR cells are delivered with lower latency and higher reliability.

Finally, under the best circumstances, Frame Relay is still inherently more susceptible to delay than ATM or TDM. Therefore, after applying the best possible queuing mechanism, one should still expect more delay over Frame Relay than is present over ATM or TDM.

MPLS

MultiProtocol Label Switching (MPLS) VPN service from service providers is commonly used by enterprises for WAN connectivity. The service is often available over different types of access links, and usually offers multiple classes of service. MPLS service is typically expected to provide good QoS and therefore to satisfy VoIP requirements, though this often depends on the Service Layer Agreement (SLA) and the actual quality delivered by the service provider.

With MPLS service, unlike private WAN, the enterprise controls QoS explicitly only on the access link —- that is, on the connection from each enterprise site to the MPLS network. Within the MPLS network QoS is controlled by the service provider. The enterprise affects the service given to its traffic by assigning the traffic to appropriate classes of service in the service provider’s network. This is done with DiffServ Code Point (DSCP) marking in the packet’s IP header. DSCP remarking by the enterprise edge routers may be required, mapping the DSCPs of enterprise traffic to the DSCP values designated by the MPLS service provider for the different classes of service in their service offering.

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Avaya 555-245-600 manual Mpls, Additional frame relay information