The first step towards next generation networks was to replace the core TDM-based networks with SIP components (please note that most of the argumentation here applies just as well to BICC networks). Strangely enough, the usage of IP-based technology usually stops at the borders of the network. TDM is still the basis for interconnection with other operators or even with other divisions of the same company. This is rather inefficient due to multiple reasons:
- QoS reductions: Moving from IP to TDM and possibly back to IP means that the voice communication will have to be transcoded from IP packets to TDM and back to IP packets. This will introduce delays and possibly losses.
- Service limitations: As TDM networks are designed for voice only, any other service such as video or messaging can not be exchanged transparently between operators. Actually, not even high quality voice can be used in an end-to-end manner as both operators will have to agree on G.711.
- Costs: One of the main reasons for moving to IP-based core networks was to reduce costs. The same should apply to peering.
Monetary aspects certainly play a role in delaying the all-IP interconnection. Especially smaller providers of VoIP services seem to be making a living off of the termination fees they receive from their TDM peering partners. Hence, they do not show much willingness to move to a pure IP-based interconnection which is usually though to be free or based on some flat rates as is the case with interconnection of pure IP packets.
However, this does not explain why TDM is still the prevailing interconnection technology between large operators or between the divisions of the same operator.
Looking deeper into this issue we can identify the following reasons for why VoIP peering is not so popular yet:
- Timing: Lots of operators have just finished igrating their core networks to SIP, and hence it is only natural that they are taking their time to go into the next step.
- Organization: Interconnection infrastructure is often managed by a different organization than the mobile or fixed divisions inside an operator. In order to go IP, the interconnection division would have to be convinced to move to IP. as well.
- Security: VoIP as an IP based application still has the taste of something unknown und insecure to many operators. With TDM, there is already a long history and extensive experience in all kinds of troubles that might arise. Operators know how to deal with fraud and interoperability problems in TDM and have established solutions to deal with these issues. VoIP technology poses new threats and problems that require new solutions and often a non-negligible learning curve.
- Interoperability: Even after years of standardization and interoperability testing it is not uncommon to have interoperability issues between SIP equipment from different vendors. Solving the interoperability issue when rolling out a VoIP core network is rather straightforward. The vendors selected by the operators will most likely not get paid until all interoperability issues are solved. In the case of a peering scenario with other operators this is not as simple. Different operators and even different divisions of the same operator will have their own equipment. The willingness to change the configuration of one’s own equipment or to pressure a vendor to update an implementation will not be too high.
- Monetary: Especially in the case of large operators interconnection costs over TDM are predictable and low. Hence, the costs of moving to a new technology and establishing new business relations might not be economically justifiable.