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Different service providers offer different service level agreements. Many carriers offer a zero committed information rate (CIR) service because it is relatively inexpensive when compared with service level agreements involving a nonzero CIR. If Sprint guarantees 99.9% delivery, it may be sufficient for many users and meet with their expectations.
People wanting a lower likelihood of dropped packets will want to obtain a service level agreement for a specific CIR. If a business is sending important data or VoIP traffic across a Frame Relay interface, they will typically want to use a separate data link connection identifier (DLCI) which has its own CIR.
When the network becomes congested, the carrier will discard frames with the DE bit set before discarding those that do not. The carrier can discard frames without the DE bit set if the congestion level completely overwhelms their equipment. This, however, is a very unlikely event. That is why you would still pay more for a 'guaranteed' CIR.
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