Buildout Rule

The size and depth of commercial networks is a never-ending battle for providers.  As more and more customers sign up, of course, there must be capacity . . . circuits, switches, and PVC availability is a key issue.

So most providers monitor the network utilization and "build out" the network as needed.  But they must stay ahead of the growth curve, and be proactive . . . not reactive.

As a general rule of thumb, build out the network so that the average peak utilization remains below 70% (70% is the utilization value above which there are noticeable performance degradations).  If you added up all of the port speeds of all of our customers, you would find that in actuality, the network is oversubscribed (about 2 to 1, but it varies).  Oversubscription is inherent and expected with Frame Relay Networks.  In fact, that is what makes the "shared" feature of the protocol so attractive to vendors - the facilities are shared in such a way as to allow oversubscription without compromising throughput.

All of this means that there will be some time periods where the network becomes congested to the point that it must discard traffic.  The provider should be able to guarantee that over a period of a MONTH, 99.0% (assuming 0 CIR PVCs) of all frames transmitted onto the network will be delivered the first time through.  During short periods of congestion, some frames will be discarded. It is entirely possible, that in any short time period, more than 1% of the frames will be discarded.

Essentially, it would be too costly to build out the network so that there is no over-subscription (shared resources is one of the reasons that Frame Relay was invented- without over-subscription, we would basically have private lines).  The consequence is that there may be some discarded data, regardless of whose Frame Relay network you use (even our competitors' CIR SLAs are only good for 99.9% or 99.99%, so there is no 100% guarantee).  

For any provider to guarantee 100% DDR, they would probably have to charge Private Line prices (expensive !!).  Most customer applications are designed to recover from a certain degree of data loss, and for those that are less tolerant to loss, we have SNA and Voice grade classes of service, as well as the availability of CIR on all of our PVCs.  Applications such as Voice and Video are good examples of more sensitive types of traffic.