Paul Brittain, Metaswitch: core TDM networks are very extensive
and aren’t going to go away in a hurry. There’s too much
invested and they still work 
                                          
                                  

                                  
Mark Lewis, Interoute: migrating through a standard upgrade
cycle will put operators at a competitive disadvantage 
                                  
                                  
The motives for operators migrating from TDM to IP network technology are clear. IP-based traffic increasingly dominates network payloads and as consumption of video and applications such as unified communications continues to accelerate more and more traffic will be IP-based. That has had an inevitable influence on operators’ technology decisions.
In tandem with that has been the imperative for operators to cut their operational costs as their traditional services become commoditised.
TDM technology is notoriously expensive to deploy and operate. It continues to be a better proposition for some services — notably voice — but as current equipment retires it will be replaced with IP hardware.
However, operators have invested heavily in TDM networks and won’t simply rip and replace them until those assets have become fully depreciated and lived out their useful lives. In some networks that point could be at least a decade away.
Approaches to migration will differ from operator to operator depending on their existing assets and their customer base.
“It varies a lot depending on what assets they already have and the vintage of those assets,” says Paul Brittain, vice president of sales engineering at Metaswitch.
“There’s also a split between the access and the core market. In the access market operators have to do what their customers want and I can’t see anyone ordering new T1/E1 capacity from an operator. Core TDM networks are very extensive and aren’t going to go away in a hurry. There’s too much invested and they still work.”
There’s no problem when it’s data to data but the voice market is much more interesting, he adds.
“The big push is already happening at operators. They’ve built out mobile core networks and will need media gateways for the next 10 years plus because of the sheer volume of TDM interconnects. Altering those would mean changing the connection agreement and persuading the other side to move. Even if a commercial incentive can be given for them to do so, it will take substantial time.”
Stephen Sherry, head of IP Exchange at BT, concurs. “We’ve seen the trend of operators with traditional TDM networks which want to migrate to the IP world,” he says. “The technology is all doable — there’s no technical reason why you can’t do it. Softswitches are different but the services are inherently the same to start with.
“The biggest problem I have is services down at the IP level — things like ringback when free. IP protocols don’t have ways of doing that. It may be supported with other signalling protocols but you don’t get other applications if you do that, you’re just replicating functionality for a single app.” 
                                  
                                  
Headache services 
                                  
He adds: “Migration from TDM to IP has been a reality check in terms of carriers needing to understand what they need to support in the IP world.” Even though 95% of services are supported “it’s the 5% that give me a headache”.
Staged migrations will be the pattern followed because the economics of total network replacement don’t stack up and the issues surrounding media gateways and continued service provision are prohibitively challenging to resolve.
A likely approach for wholesale providers is to begin migration as part of their cycle of site upgrades.
“The migration is first handled with new rollouts and site upgrades,” explains Mike Haugh, senior product manager at Ixia. “For example, once IP/ethernet services are available in an area they would most likely not continue to roll out new sites connected via TDM based services.
“Another example is if a customer was using the network connection for data and now is looking to support data, voice and video. They will be likely to upgrade the connection from TDM based services to IP/ethernet. The majority of the network has already migrated away from TDM although there is still a large install based of Sonet/SDH in the metro [market]. The migration challenge is more at the edge and in the last mile out to customer locations. In some cases the fibre infrastructure is not there to upgrade.”
Mark Lewis, enterprise service development director at Interoute, identifies that the migration has been under way for several years and thinks passively migrating through a standard upgrade cycle will put operators at a competitive disadvantage. IP can be rolled out as required with TDM kept running until it reaches the end of its natural life. 
                                  
                                  
Wholesale transition 
                                  
“The transition from TDM to ethernet wholesale has been accelerating over the last three years and today our wholesale demands are almost entirely ethernet-based,” he says.
“The traditional TDM circuit switched networks cannot cost in versus a packet switched overlay and operators who do not make the switch will not be able to compete. In terms of replacement, it is simpler: operators are buying ethernet as they grow and leaving TDM to rot as the volume of traffic it carries becomes increasingly inconsequential.
“TDM is still great for voice, which is still the same size, so our belief is that our customers are re-grooming voice onto TDM for existing contracts, and then simply shifting to ethernet when they come to renew platforms and providers contracts.”
That voice strength is one of the main reasons TDM will continue to be operated. IT also has a market in sensitive situations such as industries that sensitive to network issues such as latency.
Haugh adds: “At this point, with home services like VoIP from Skype, video from YouTube and other IP-based services widely available, the benefit of the speed and quality [of IP] are pretty widely accepted and understood. The issue for large enterprises, like financial networks, is their concerns with the performance such as loss and latency.”
Such concerns are addressed by service level agreements on TDM networks but IP will have to rise to the challenge. In addition, it’s not just high-end financial communications that suffer from quality impacts. Even voice calls have suffered in recent years as operators turn to least cost routing on long distance routes. That introduces greater delay as voice passes through a greater number of switches and codecs as it seeks out the cheapest route.
Brittain points out that the migration to IP will ultimately address users’ quality concern because voice will be packetised once and then remain as IP packets as it crosses multiple networks. 
                                  
                                  
Packet voice 
                                  
“The human ear is very sensitive to loss and getting that interactive voice service across many IP connections is something operators still struggle with. The loss in quality on transatlantic calls isn’t to do with a single operator getting it wrong but because least cost routing means traffic goes through codecs and additional delay is introduced. Once you get to the point that the network is all-IP you packetise voice once and can cross multiple carriers with no problem.”
The situation will improve quickly as more of the networks go to direct IP interconnects and the crystal clear connection returns.
“Certainly the larger operators are all very keen to push new IP interconnect services,” adds Brittain. “BT’s IP Exchange is an example and there are initiatives similar to that in other markets. That activity is still small change in volume compared to what they do with their TDM interconnects.”
Sherry explains why BT launched its IP Exchange. “IP Exchange in the UK was a foray by BT Wholesale into the world of IP interconnect,” he says. “Launched in 2007, it was aimed at new and emerging communications providers entering telecoms without all the TDM issues. It was aimed purely at the emerging market to start with and, in the four and a half years we’ve been in business, that market has grown enormously. We’ve dealt with international traffic and we’ve dealt with customers who are dealing in premium services and want to offer advanced services or premium rate services.”
There is an industry-wide move towards direct IP interconnects that is reflected in the emergence of IP exchanges and as the industry moves towards rich communication services enabled by IP that trend is likely to continue as the market matures and deployments accelerate.
“It depends on where you’re looking in the network but he momentum behind direct IP interconnects is growing very fast,” says Brittain. “In the 18 months to five years timeframe there will be a lot of movement. We are at the point now where the industry is coming together better than ever in the past. It’s an exciting time and the market is looking towards the rich communication service sets you can get in a pure IP world.” GTB