While most of the world is deploying xDSL for broadband, the majority of broadband users in North America use cable modems on cable networks, which gives very different packet-voice capabilities. Figure 3 shows a typical example of a cable setup.
There is no ATM capability, so VOIP is the only possibility. Although there are some similarities in capability to DSL, there are probably more differences. The major similarity is that cable operators can deploy an MGCP ATA that has access to a guaranteed QOS channel to the cable modem termination system (CMTS) and core network. This uses the CableLabs DOCSIS 1.1 QOS system between the cable modem and the CMTS. The only standardized system is the CableLabs PacketCable system, illustrated in the top section of Figure 3, and this uses the PacketCable network-based call signaling protocol, which is a profile of MGCP. The standard specifies client ATAs or multimedia terminal adapters (MTAs), which can be standalone or more often embedded with a cable modem in an integrated telephony cable modem (ITCM). And, like the residential routing gateway, the NAT/firewall router can be integrated as well, creating what at least one vendor calls a cable access router. At the edge of the HFC access network, the CMTS terminates the DOCSIS systems and forwards traffic to a B-RAS or edge router.
An interesting point appears to be that only PacketCable MTAs can access the DOCSIS 1.1 QOS system for guaranteed voice quality. An earlier proprietary approach used an H.323 MTA with DOCSIS 1.0. So SIP phones and SIP softphones accessing the QOS system would appear to be ruled out, which would restrict the attractiveness of packet voice over cable services.
Another approach, shown in the lower section of Figure 3, is used by Vonage Holdings Corp. and others. With Vonage, a SIP MTA is embedded in a voice-terminal/router. The MTA could be connected by Ethernet into a regular home router, but that would jeopardize voice quality and raise the NAT-traversal issues. In the configuration in Figure 3, the MTA still cannot access the cable modem QOS capability, but it can give voice traffic from the MTA absolute priority over other data traffic, and it can bypass the NAT.