TDM's demise is greatly exaggerated

Since data traffic first showed signs of eclipsing voice in the late 1980s, telecom service providers have considered assassinating time-division multiplexing in broad daylight or gently euthanizing the time-slotting technology when no one was looking. But, like the constantly reviving undead, TDM networks have a way of coming back again and again after being declared dead and buried.
Time slotting was conceived as a way to channelize 64-kbit/second voice traffic, and the efficiency of multiplexing such traffic has been one of the factors keeping circuit-switched voice traffic reliable and resilient in the face of network breakdowns. But TDM also has played a role in aggregation at the network core.
Digital leased-line circuits, the T1/T3 hierarchy in North America and E1/E3 equivalents in Europe, are based on TDM. So are the Sonet payloads used in the fiber-based hierarchy of dual rings used throughout metropolitan areas. Again and again, packet afficionados have advocated schemes for cutting TDM out of the network, only to learn that carriers aren't prepared to make a complete cutover to Internet Protocol (IP) packets.
Asynchronous transfer mode (ATM) was the first carrier-designed concept meant to bring together data packets and circuit-emulated voice, though the size of ATM cells-53 bytes-represented a compromise that satisfied no one. To be sure, core networks transitioned to ATM across the board, but these ATM networks used the hit-and-miss available-bit-rate service and ATM Adaptation Layer 5 designed specifically for data packets. In almost all circumstances, continuous-bit-rate and other services designed for voice were talked about endlessly, and never implemented.
Using ATM bought the carriers some time in the 1990s for conversion to IP, and represented a better switching infrastructure at the core than frame relay. But there is some degree of irony in the fact that an updated frame-relay switching method, committed information rate, brought the older frame-relay switches closer to coexistence with TDM than most of the ATM networks that still exist today.
ATM's demise was due largely to the dominance of Ethernet traffic at the desktop: If LAN users and broadband-access clients would not accept ATM in the last mile, its role in the core was doomed. Since then, developers have attempted to address low-latency traffic from two directions.
One concept sought to make a packet-based datagram service "look and feel" like a circuit through clever traffic shaping and quality-of-service (QoS) prioritization. At the core of large metro and interregional networks, router makers advocated Layer 3 IP routing along with a new "tag switching" concept of defining IP flows called Multiprotocol Label Switching, or MPLS. Meanwhile, the manufacturers of the largest Layer 3 Ethernet switches for the enterprise tried out various forms of Ethernet-based quality-of-service queuing so that time-sensitive traffic could be carried from enterprise-switched LANs to core MPLS routers.
Reluctant to switchIf the corporate world had cut straight over to 100 percent voice-over-IP (VoIP) use, this topology would have made sense for everyone. Many networks are being installed in metropolitan areas today, and they could represent the future carrier topology of choice. But carriers told equipment designers that customers were reluctant to give up T1 and T3 services, and some of them were adamant about not giving up circuit-switched voice.
Continuing limitations of VoIP in handling 911 calls, fax services and network-powered phone calls has also made corporate clients more adamant about retaining TDM. So members of the Metro Ethernet Forum began rethinking the problem, working backward from the central office to the customer site.
Architectures from the likes of Overture Networks Inc. and Ceterus Networks Inc. are based on bonding TDM channels into Ethernet transport services, thus providing the same types of encapsulation offered in modern Sonet services, such as generic framing procedure and virtual concatenation.
There are also schemes for bringing timing information directly to packet switching. In early June, Zarlink Semiconductor Inc. (Ottawa) introduced a patented technology the company calls Timing Over Packet, in which a hardware processing engine adds timing and synchronization information to Ethernet, IP, ATM and MPLS packets. The Zarlink processors encode and transmit a master clock over the packet network and recover the clock at client nodes. Since Zarlink, formerly Mitel Semiconductor, was responsible for developing controllers for the time-slotted H.110 bus used in older enterprise telephony networks, OEMs are giving the Zarlink proposal careful consideration.
The ultimate irony of this reaching back to TDM comes in the effort to define next-generation backplanes for communication equipment. The Advanced Telecommunication Computing Architecture, a project promoted by the PCI Industrial Computer Manufacturers Group and other trade organizations, originally was intended to define architectures for the largest routers and Layer 3 switches at the network core. With the addition of the mezzanine-card-based MicroTCA, the ATCA family has been extended to all kinds of devices functioning at the network edge, including access routers, IP PBX systems, DSL access multiplexers and Ethernet switches.