ied is either 64-QAM or 256-QAM, and upstream it is either QPSK or 16-QAM. Along with cable modems, DOCSIS 1.1 also supports VoIP phones and residential gateways. To deploy DOCSIS 1.1 (or higher), the cable operator must have in place a two-way HFC network that supports a return path for the upstream traffic. The DOCSIS 1.1 standard addresses real-time applications such as IP telephony. The key issues in cable-based IP telephony include voice quality and how to guarantee it in terms of latency, fidelity, jitter, packet loss, and reliability at the customer end. Other issues are legacy signaling support, data security, scalability, and feature deployment at the service provider's end. Finally, there are a number of provider- specific issues, such as implementation of systems for PSTN gateways and gate- keepers, provisioning, billing, and network maintenance. DOCSIS 1.1 enables time-sensitive voice and multimedia packets to share in HFC networks with time-insensitive pure data packets. DOCSIS 1.1 enables a node to recognize a nondata packet and switch to it instantaneously from whatever data packet it is working on. It requires a CMTS at the edge of the cable access network and a DOCSIS 1.1-compliant cable modem at the customer premises. Edge cable CMTSs need the intelligence to isolate traffic flows and apply policy-based QoS treatments in real-time. Traffic flows need to be isolated by service provider, appli- cation, and subscriber so that during times of congestion, flows within the service- level agreement (SLA) are maintained and flows that exceed the SLA are discarded first. Operators then map the DOCSIS-based flows to IP specifications such as DiffServ and MPLS, which are discussed in Chapter 10, to manage the handoff to the core network. Figure 12.5 shows an example of using DOCSIS 1.1 for cable-based IP tele- phony. Considerations for cable-based IP telephony include technical architecture, achieving PSTN-level reliability (i.e., five-nines reliability, or 99.999%), being able to accommodate the same PSTN