Data Over Cable Systems Interface Specification (DOC-SIS) is the leading standard for data over cable networks. We consider the problem of scheduling Constant Bit Rate (CBR) flows in a DOCSIS compliant cable network. CBR flows are required to support the delivery of voice and other real-time applications that generate fixed size data packets on a periodic basis. The primary application of CBR flows is Voice over IP (VoIP) which cable operators intend to use in order to provide cable telephony services. DOCSIS 1.1 is enhanced with Quality of Service (QoS) capabilities; it defines the Unsolicited Grant Service as the mechanism for supporting CBR flows. The scheduling algorithms, however, are not defined by the standard.

We present the scheduling problem and examine several interesting special cases of it. We show that deciding whether a set of CBR flows can be legally scheduled is NP-complete whenever there are two or more different grant intervals. We model the scheduling problem as a variant of bin packing where bin sizes can be modified in a constrained manner. This model enables the development of scheduling algorithms which are based on known algorithms for bin packing. We present an algorithm based on the Next-Fit algorithm and investigate its performance. We show that under certain assumptions which typically hold for VoIP and many other practical applications, a simple polynomial time scheduling algorithm is sufficient.