Meeting Network Needs: Both Old And New

In the heady days of the telecom boom, hype led many to believe a full-scale network revolution was underway. Circuit-switched infrastructure was going the way of the dinosaur, the forklifts were ready and running on the frontlines, and IP networks were on the rise.

While the evolutionary pace has slowed from what was predicted after the telecom crash, carriers still face the same challenges when planning to migrate from a circuit-based to packet-switched-network (PSN) infrastructure. Primarily, they need an efficient and cost-effective solution for transporting revenue-generating legacy services over the PSN.

One emerging solution that meets the economic and competitive issues, thus pushing carriers toward network convergence, is Circuit Emulation Services over Packet (CESoP) technology.

The carrier crunch For decades, the time division multiplexing(TDM)-based circuit-switched network has been the heart of the Public Switched Telephone Network (PSTN). Some think TDM’s days are numbered, while others believe this venerable technology will coexist alongside IP for years to come. Much of the world’s voice traffic is still carried over the circuit- switched network, accounting for a significant portion of carrier revenues. However, data traffic now outpaces voice traffic on the network.

Carriers are caught in an economic and competitive crunch. Recognising the tremendous growth in data traffic, carriers have started to build out IP networks on top of their circuit infrastructure. Fiscally, this is only a short-term solution. The bottom line makes it impossible to maintain two disparate networks for voice and data traffic, each with separate support staff, unique hardware, and different management systems. Cable providers and wireless operators are also aggressively moving off the sidelines, and are now circling the carriers’ once-secure voice dollars.

Carriers are fighting back, and clearly the path forward is convergence toward one single PSN. They’re installing new data networks with VoIP to transfer revenue-generating services to a less-costly network, and announcing more IP or Ethernet services based on true Ethernet networks, not on circuit-switched networks. This trend will continue as carriers endeavour to reduce costs. But the major issue remains—carriers need to transport legacy traffic originating from the home, enterprise, and campuses over the new network.

Introducing CESoP The challenge of transporting circuit-switched trunks over connection-less networks was met by both IC manufacturers and equipment vendors. Their solution combines old and new approaches to transport TDM traffic over new IP, Ethernet, or multiprotocol-label-switching (MPLS) networks.

The agreed approach is patterned after Circuit Emulation Service over ATM. Some took ATM cells and packetised the payloads into IP packets. Others put the TDM channels into the IP payload directly, thus eliminating the segmentation and reassembly step of ATM. CESoP has parallels to ATM AAL1 transmission. Both are constant bit rate, both transmit uncompressed voice, and both support structured and unstructured circuit transport.

Synchronisation is a critical issue when converging circuitswitched services on a packet-based infrastructure. In the circuit-switched network, timing and synchronisation is inherent in the technology’s design. However, when this service is carried over a PSN, the timing element is lost and must be transported by other means.

With CESoP technology, all associated timing and signalling information is “tunnelled” alongside TDM voice, video, and data traffic across the packet network.

Standardisation work is well underway on CESoP technology. The ITU’s recommendation ITU Y.1413 deals with TDM over MPLS networks. The MPLS and Frame Relay Alliance and the MEF Forum recently released their implementation agreements for Circuit Emulation Services over MPLS and Metro Ethernet network. The IETF is also actively working on TDM-over-Packet standardisation.

Potential opportunities Now let’s turn to the opportunities for this technology. There are many potential applications in different networks, since the technology can run over IP, MPLS, or Ethernet networks.

One main benefit of the technology is its ability to transport many TDM trunks from one location to another over a packet network. The technology is initially being adopted in the access part of the network, primarily to transfer multiple trunks across a medium such as a gigabit fibre or a fixed-radio access network.

The simplest application for CESoP technology is to provide packet interconnection for existing TDM equipment, enabling legacy traffic to move across an IP/MPLS/Ethernet network. By doing this, TDM vendors can extend the lifespan of their equipment, and generate years of income from older products. Evolving installed equipment to support new services, rather than a forklift upgrade, is something carriers are asking for now. Network applications for CESoP CESoP technology is being used to carry Sonet/SDH traffic across an IP or Ethernet network, such as a metro Ethernet network or a Resilient Packet Ring. The Sonet/SDH sub-rates are packetised and carried across the packet network. Thus, a VT1.5 can be carried across the packet network and be reintegrated into a Sonet/SDH ring.

We can see that there are many applications for this technology in existing and new networks. Despite the best wishes of some, legacy TDM traffic won’t disappear overnight, meaning carriers must find innovative and cost-effective ways to carry this traffic over the PSN. CESoP is one viable approach. It simpler than VoIP, and can be complementary where VoIP technology is too complex or expensive to use.

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