This read-write value sets the portion of the port's overall data rate specifically reserved for ATM QoS Class 1 traffic, typically the highest-priority class such as CBR voice, carving out a guaranteed slice of bandwidth from the port's total capacity for that traffic category. An admin sets this to guarantee capacity for the most latency and jitter-sensitive traffic class regardless of how much lower-priority traffic is competing for the link. For example, reserving a specific bits-per-second value in lportAtmDataRateQoS1 for voice traffic ensures that even during heavy data traffic bursts on QoS classes 2 through 4, voice quality-of-service traffic still has its guaranteed share of the port's capacity.