TranSwitch has already done a great job in supporting the growing movement to Ethernet-based access services with its EtherMap Ethernet over TDM products. Now with the introduction of their PacketTrunk-4 internetworking gateway chip, they are throwing their weight behind the TDM over IP (TDMoIP) protocol, MPLS, or switched Ethernet. The device trunks four T1/E1 (or freeform serial) channels onto a 10/100 Ethernet MAC which is intended as a metro Ethernet-based WAN connectionIf your application requires it, you can also use the chip to bridge a single T3/E3/STS-1 connection. Depending on how it's configured, the chip provides TDM trunking services for circuit or loop emulation, and can also support HDLC payloads for transporting SS7 over IP networks.

Because TranSwitch's press release provides an excellent description of its product, I'll only give you a few of the basic facts behind the chip, and spend most of the review talking about why I think TransSwitch's vision is so powerful.

When trying to understand why the PacketTrunk-4 is such a significant product, you have to sort of invert the thinking you use with VoIP. The device employs the TDM over IP (TDMoIP) protocol developed by RAD Data to encapsulate traditional TDM services within IP packets. Now this sounds a lot like VoIP, but here's the difference: While VoIP digitizes and compresses an analog voice channel before encapsulating it into an IP-based format, TDMoIP lets the telephony traffic remain in its "native" TDM format. The packetized TDM is then trunked along with data traffic and passed out to an IP-based WAN connection.

While both VoIP and TDMoIP deliver traditional services using Ethernet or IP-based access services, TDMoIP allows a subscriber to easily transition from T1/T3-based access services. This is because IP-based loop and circuit emulation allows you to "hollow-out" a PSTN leaving a "PSTN-flavored wrapper" around an IP network. When done properly, you can replace your T1/T3 connection by plugging most of your existing equipment into a relatively inexpensive trunking box before it heads off to the metro Ethernet connection.

RAD Data and other companies have been producing TDMoIP equipment like this for several years, but until now they used costly custom circuits. Equipment manufacturers should be able to significantly cut both costs and development time when they replace the ASICs and FPGAs with the PacketTrunk-4 device.

While it's relatively simple in the abstract, actually getting such a device to work across such dissimilar environments was a significant challenge. In developing the PacketTrunk-4, one of the biggest challenges RAD and TranSwitch faced was to keep the data synchronized while flowing between two completely different timing environments that had no direct way of relating to each other.

This is accomplished using some very clever latency management techniques on the IP side of the chip. Part of their secret is an adaptive clock recovery block that monitors buffer fill and inter-packet arrival time to manage arrival times and buffers inter-packet jitter. RAD has found that solid clock recovery and good network design can eliminate the need for the overhead associated with the RTP protocol normally used to guarantee quality of voice over IP connections.

By preserving most customer side equipment, and simplifying the provisioning requirements, a PacketTrunk-4-powered TDMoIP device makes the transition to last-mile Ethernet a whole bunch less disruptive. It should also find a home replacing expensive T1-based backhaul lines in cellular base stations, as well as IP-CENTREX units, IP-based DSLAMs or other multi-service access equipment.

Rather than compete head-on with networking powerhouses like Agere, Cypress, Conexant, and PMC, for its SONET/SDH products, TranSwitch is artfully crafting its own niche in the emerging multi-service broadband market space. And with the introduction of the PacketTrunk-4, they are addressing one of the previously-invisible stumbling blocks that hampers the migration of voice services to lower-cost IP-based carrier technologies.

The TranSwitch PacketTrunk-4 will sample in December 2003, is packaged in a 27 x 27 mm PBGA-256, and will be priced at $70 in volume. Power supply voltages are 2.5-V core and 3.3-V I/O.