A packet communications system includes a method for maintaining quality of service by dynamically determining a maximum packet size for a communications system that transports at least one of real-time and time-sensitive data and other data in packets. Accordingly, a plurality of packets is received for transmission over a communications link. Either an encoder type or a packet size for the time-sensitive or real-time data packets is determined. A link speed for the communications link is determined. Based on the link speed and either the packet size or encoder type, a maximum packet size for any packets that do not include time-sensitive or real-time data is determined. Nominally, prior to transmission on the communications link, packets with non-real-time and non-time-sensitive data are fragmented into packets equal to or less than the maximum packet size. The maximum packet size may be dynamically reevaluated if the communications traffic or link characteristics change.