change, but that is not the case in using a shared coax system. Also, HFC faces bandwidth constraints: Given that cable modems have caught on and there are increasingly more subscribers to such services, more homes within a neighborhood are making use of their Internet access channel, and every- one's downloads and bandwidth are minimized. Subdividing the nodes can help Chapter 12 Broadband Access Alternatives alleviate bandwidth constraints, and it can also help reduce ingress noise. If the ser- vice provider continues to subnet, it can keep performance high for the subscribers. Cable Modems and CMTSs A cable modem is needed in order to support high-speed data access over HFC, using the cable TV infrastructure. Cable modems function like special-purpose routers, linking the cable network's Layer 3 to another network or device. Gener- ally, this requires an external box with cable and Ethernet connections. Figure 12.4 illustrates cable modem connectivity. On the left side of the figure is an individual neighborhood with users attaching to the shared coax system via their cable modems. These various coax trunks then come into the headend facility, where they terminate on a CMTS. The CMTSs are linked by a common Ethernet hub, which in turn feeds into the IP router, which determines the optimum path to take over an optical backbone onto the ISP. CMTS functions include providing QoS, allocating bandwidth, classifying packets, policing packets for Type of Service (ToS) fields, adjusting the ToS fields as needed, performing traffic shaping, forwarding packets, converting and classifying QoS parameters, handling signaling and reservation of backbone QoS, and record- ing call resource usage. Cable modems provide downstream data rates of up to 36Mbps, and the down- stream rates are generally supported in the frequency band 42MHz to 750MHz. Coaxial Headend Facility Video Receivers Modulators SDH/SONET Backbone Ethernet IP Router Internet CM = Cable Modem CMTS = Cable Modem Termination System Figure 12.4 Cable modems: LAN-o