Ethernet over TDM: Maintaining Legacy Networks

Wireless carriers need to maintain legacy networks even as they move toward more broadly deploying LTE. Ethernet over TDM is a conversion technology that allows them to provide 2G and 3G services, and analysts expect it to gradually shrink – but still expect it to be around for years to come.
Maintaining Legacy Networks
The move to packet-based networks has been in the works for years. But many more remain before legacy networks can be completely replaced or turned down.
Historically, according to Eitan Schwartz, vice president of Carrier Ethernet of RAD Data Communications’ U.S. operations, network operators haven’t wanted to push their core all the way to the network edge – so network architectures have included an access network. Previous access networks were heavily based on T1 lines – “a very deterministic, consistent pipe that carried the bits,” prior to packet-based technology. And they didn’t scale very well.
“Traditionally, your networks were TDM: CDMA, GSM were all TDM. So wholesale backhaul networks were TDM,” said Schwartz. What happened is that providers of the cell backhaul equipment started to handle Ethernet, started the migration toward packet, but the infrastructure was still TDM.
“So what you needed was Ethernet over the existing TDM infrastructure,” Schwartz added.
One solution that evolved was a converter that was a Carrier Ethernet interface on one side, and TDM on the other and could convert between the two – aka, Ethernet over TDM.
Now, Schwartz said, serving a cell site likely means being able to handle both TDM interfaces with 2g and 3G traffic, as well as 4G equipment with Ethernet interfaces. All of the data still needs to be transported back to the core network, which is typically MPLS.
Wholesalers have considered T1s with TDM service to be their bread and butter, Schwartz said. “They have to get on the bandwagon of delivering Ethernet. They can’t be content with the fact that they’re the incumbent – there are alliances forming that compete with them.”
Cable and power companies can pair up, Schwartz said, and a telephone company may suddenly find its traffic going over someone else’s fiber network.
“In the mobile backhaul space, Ethernet over TDM/SONET seems to be shrinking. It’s pretty mature,” said Schwartz. RAD provides Ethernet over TDM services, among others.
“Ultimately, it’s going to get smaller and smaller because of the scalability requirements for backhaul,” Schwartz added.
Ethernet over TDM: A Slow Evolution
According to Nir Halachmi, product line manager for TelCo Systems, the use of TDM over Ethernet is dependent on the region of the world. In the U.S., he said, operators often still maintain a separate network for TDM and voice, and also a packet network.
Once mobile operators make large-scale investments in Voice over LTE, though, the voice traffic will be able to be carried via the packet network instead of relying on TDM.
“I think that 4G will have to go to VoLTE eventually,” Halachmi said. “Today they’re doing tricks in the 3G and 4G, so they’re moving voice on 3G infrastructure using TDM or circuit emulation. But everything is going toward packet.
“I think we’ll still have 2G and 3G within the next decade, for sure. It’s a slow evolution,” Halachmi said.
Customers are steadily upgrading to 4G, but legacy systems will still need to be maintained by technologies like Ethernet over TDM.
“The mobile operators may be looking at the customers who are trading in old phones and getting updated phones; however, the old technologies seem to stay around for a very long time,” Schwartz affirmed. He noted that enterprise customers such as home alarm companies still have large-scale existing deployments of 2G technologies, where neither the company nor the homeowner is eager to upgrade.
“So the mobile operators will probably see 2G traffic for a very long time, because no one wants to change out equipment in their home or business.”