I want to run a PBX behind a firewall behind a DOCSIS modem. Our ISP does not want to deploy a TA / gateway at our site. Our ISP only uses the modem for analog telephony and nothing else. QoS for voice on DOCSIS lines is achieved by having two MACs for each modem. One address for access, one address for voice. Because of that it seems to me that an ISP cannot deliver QoS for voice.
However, we want to have a PBX. There are cable companies which provide my desired service. I suspect the reason for not providing a SIP trunk is that there is no direct routing to the customer's site. Is it possible to run a SIP trunk under these circumstances?
WAN —> DOCSIS 3 —> modem as bridge —> service IP by DHCP —> customer's own device. Subnet routed via service IP.
The customer's router is connected to a DOCSIS modem in bridge mode. By requesting an IP via DHCP on the WAN interface, a varying service IP is assigned which is the last hop to the customer's booked subnet. The subnet is statically routed via a service address. Therefore, no public IP can be assigned to a WAN interface.
Usually, a VoIP solution independent from the ISP requires the reservation of an IP within the customer's network at the SBC for billing and QoS – as it is common practice via DSL lines. Considering the indirect routing and DOCSIS in between, is a SIP trunk possible?
- Parties involved: ISP running the WAN and the SBC. Customer running the router and the PBX.
- 200.0.8.0/28 is a public network.
- Router's LAN interface and the PBX share the same network permanently routed routed via the service IP by the ISP. PBX's address is supposed to be listed by ISP's SBC to allow VoIP.
- Router requests an address via DHCP and is assigned the service address which basically is the same one but may change due to technicalities, meaning it is not a static public IP in the narrow sense of the word.
- Modem is invisible to the Internet and inaccessible to Router.
Best Answer
To answer your question: it is possible to run a SIP trunk under your circumstances because your circumstances are no different than those of any other cable subscriber.
DOCSIS is a Layer 1 & 2 protocol, while SIP trunks essentially operate over IP & UDP which are Layer 3 protocols. I don't think it helps to mix them.
With most SIP trunking providers, when the PBX on your LAN registers with the SBC, the SBC uses NAT punching techniques to find the right private IP, if there is a NAT to cross.
Your deployment concerns are going to be:
Since you are concerned about QOS - this is something that is simply not possible with packets over the internet. Many SIP trunking providers will sell a managed service product that comes with private data T1 connectivity to their backbone - so essentially they're taking & owning the QOS problem.