Frank, I wanted you to know that you have captured the gist of the matter regarding cable MSO IP Telephony (we don't even call it VoIP anymore BTW).
The IP Telephony monikor is meant to signify that cable "VoIP" is carrier-grade. That is, that it has equivalent quality with PSTN voice traffic. That is not to say that CableLabs (which is as much a political institution as much as a technical one) has "given up" on the idea of carrier grade primary line service in its PacketCable initiative. To satisfy the requirements of all its constituents though, it has to incorporate many compromises, which starts to move us away from primary line service. Suffice it to say, the individual MSOs have not abandoned the IP Telephony carrier grade primary line model at all. Far from it.
As to how IP Telephony QoS is maintained throughout the entire network connection, you have nailed it down well. First, IP Telephony was never intended to operate over the public Internet. It has 2 distinct flavors, as you point out. The "easier" IP implementation is actually only IP Telephony in the access network. Once at the headend, the voice signal is converted to TDM by use of an IP to TDM Gateway device, usually installed at the headend. Then the transport network brings the voice traffic to the existing Class 5 switch (either owned by the MSO or a partner) for connection to the traditional PSTN. The call is terminated at the far end by either the MSO's IP network or the PSTN. Because service providers like AT&T can provide both the local access, the transport, switching and LD network, the economics of this type of solution are well grounded.
The other type of IP Telephony is what is dubbed "end-to-end IP Telephony". In this case, you install a soft switch and various other server mechanisms to take the place of the monolithic Class 5 PSTN switch. The functions are not so much physical as logical (that is, it does not matter where in the network the box resides as long as it has logical connectivity to the rest of the network). There are several ways we control QoS and obvious impairments such as latency and jitter. The main component is what we call the "managed IP backbone". This type of private IP network has network management that determines the route the voice call will take through the network, determine how many router hops it will take and automatically reroute the call if it will take too many hops and thereby introduce too much delay. It thereby restricts the number of routers the call goes through on a managed basis, such as specifying the call cannot traverse more than 7 backbone routers (as an example only). Of course, on the far end, the terminating point may not be on an IP Telephony enable access network, so an IP to TDM Trunk Gateway is needed to access the local PSTN switch on the far end.
There are currently trials underway and planned deployments of both types of IP Telephony which are set to be carrier grade primary line service right now at several major MSOs.
Hope that clarifies things a bit more, let me know if you have any other questions on this. |
