Firewall and NAT traversal (part 5)

For equipment that is resident at customer premises, such as IP phones and Subscriber Gateways it is likely that there will be a firewall at the edge of the customer premises. In addition, Network Address Translation (NAT) may be used to convert internal IP addresses to external IP addresses.  Therefore it is important that both the RTP media traffic and the signaling flows (SIP, H.248, MGCP) can negotiate both NAT and the firewall. For the firewall to be effective it needs to ensure that only authorized flows enter or leave the networks.  There are working groups within the IETF, including Midcom and NSIS, who are addressing the issue of communications with firewalls and network address translators.

Billing and Reconciliation
The PSTN has extensive and accurate mechanisms for billing both subscribers and reconciliation between service providers. Currently most billing mechanisms are based on usage, e.g. per minute billing, although some services are charged on a flat-rate basis, e.g. local calls in the US. 

Service Providers generate Call Detail Records (CDRs) for traffic entering or leaving their networks and generate bills based on these.    A VoIP network must provide similar mechanisms to allow service providers to generate revenue. At least in the short-term it is likely that the existing billing mechanisms will remain in place both for inter-carrier reconciliation and subscriber billing, which requires generation of equivalent CDR records. Longer-term billing could move to be based on the bandwidth used, requiring alternative record keeping mechanisms such as those specified by IPDR.

Network Interconnection
The PSTN is not a single network but a collection of networks operated by thousands of service providers. At each network boundary a network interconnection is required. Network interconnection agreements are put in place to cover items such as interconnection points, signaling, timing, billing and tariffs, bearer transport, regulatory requirements, etc. In addition these normally require approval from the relevant regulator.  Constraints of scalability and established business models mean the next generation VoIP network will, like the PSTN, be a collection of networks and network interconnection agreements will still be required. For example in figure 1, Service Provider 1 and 2 will need an interconnect agreement as will Service Providers 2 and 3. This will need to cover similar topics to existing interconnect agreements but in addition address items such as security, QoS, signaling protocols (SIP, SIP-T, BICC). These additions may require regulatory approval as well.