SONET/SDH Framer and Time Slot Interchange ICs

Slot

SONET and SDH are synchronous optical transport technologies used for carrier networks, metro rings, leased-line services, mobile backhaul, utility communications, and digital cross-connect systems. A framer integrated circuit terminates and processes the SONET/SDH frame structure. A time slot interchange, or TSI, rearranges synchronous payload time slots so traffic can be groomed, switched, protected, and cross-connected inside the network element.

In the early 2000s, these chips were critical to building add/drop multiplexers, digital cross-connects, and high-density transport shelves. The networking world was moving toward Ethernet and packet services, but carriers still needed to aggregate DS1/E1, DS3/E3, OC-n/STM-n, ATM, private-line, and early Ethernet-over-SONET services over reliable synchronous optical infrastructure.

What a SONET/SDH Framer Does

Frame alignment: finds and maintains the SONET/SDH frame boundary.
Overhead processing: terminates transport overhead and path overhead used for operations, alarms, maintenance, and performance monitoring.
Pointer processing: tracks payload location within synchronous frames and handles timing differences.
Serialization and deserialization: connects high-speed line-side signals to parallel or lower-speed system interfaces.
Clock recovery and timing: supports synchronous operation, jitter requirements, and network timing.
Protection switching support: participates in automatic protection switching and fault indication.

What a TSI Does

A time slot interchange is the digital switching fabric for TDM payloads. It lets a system take time slots arriving on one port and place them into different time slots on another port. This is how a cross-connect can groom lower-rate tributaries into higher-rate optical channels or rearrange traffic without converting everything into packet flows.

High-density TSIs were important because carrier shelves had to handle thousands of STS-1 or equivalent tributary units without blocking. Non-blocking operation, hitless switching, concatenated payload support, and stable timing were essential. A TSI problem could become a service outage even if the optical line was clean.

TFI-5 and Interoperability

TFI-5, the TDM Fabric-to-Framer Interface from the Optical Internetworking Forum, was designed to let framers and switch fabrics from different vendors interoperate. The OIF implementation agreement defined mapping, link integrity monitoring, connection management, and mechanisms for SONET/SDH and non-SONET/SDH clients such as Ethernet and Fibre Channel.

This mattered because high-density systems were too complex for every vendor to build as one closed silicon island. If a framer and a TSI could interoperate over a standard interface, equipment makers could build line cards and switch cards with less custom engineering risk.

The 2003 Infineon and Vitesse Collaboration

Infineon Technologies AG and Vitesse Semiconductor announced a collaboration to establish interoperability between SONET/SDH framer integrated circuits and Time Slot Interchange ICs.

By working together, Infineon and Vitesse aimed to provide system designers with a complete end-to-end silicon solution for SONET/SDH systems. Interoperability between the Infineon framers and Vitesse TSIs covered products from each company's SONET/SDH roadmap for 160 Gbit/s, 320 Gbit/s, and 640 Gbit/s systems, along with system-level reference designs for rapid development and deployment.

The first result was proven interoperability between the Infineon Tethys SONET/SDH Multi-Rate Framer IC and the Vitesse VSC9185 TSI. Interoperability with the VSC9185 was important because it belonged to a widely used Vitesse TSI product family.

Test results from the Tethys framer and VSC9185 TSI reference design demonstrated interoperability in signal integrity, clock synchronization, frame alignment, and automated, manual, and hitless switching. Electrical and logical compatibility, including AC and DC coupling, optical connectivity, and SONET framing at 2.5 Gbit/s, were shown to work error-free. In a test configuration using an FR4 backplane, a zero bit-error rate was achieved over a 15-hour operating period.

The reference design, software, and documentation were made available for SONET/SDH telecommunications equipment manufacturers and operators seeking to increase circuit-handling capacity by integrating Tethys framer-based line cards into existing hardware.

"This is an extremely important event," said Jeff Camillo, product marketing manager for the Infineon North America Communications Business Group, "because it means our customers can be assured of compatibility between our framers and one of the most widely used TSI product families in the industry, both now and in the future. We expect the relationship with Vitesse to be beneficial for everyone involved, but especially for the system OEM, who can now more easily achieve seamlessly integrated, high-density systems."

The companies then focused on interfacing TFI-5-compliant devices, including Vitesse's VSC9195, described as a 340 Gbit/s, 136-channel, non-blocking TSI, with the Tethys Multi-Rate Framer. Use of TFI-5 was intended to support next-generation SONET/SDH system deployments, addressing automatic protection switching and performance monitoring. Used with bit-slicing functionality, the design could scale to high-density switch matrices in a lower-power, denser form factor than a traditional three-stage architecture.

"Infineon's collaboration with Vitesse illustrates that the major players in the SONET/SDH market are committed to supporting their customers with complete, interoperable solutions rather than just point products," said Andy Ebert, product marketing manager for Vitesse. "Combining our experience and mastery of signal integrity and SONET/SDH system issues, customers gain a significant reduction in both development costs and time-to-market."

The Tethys and Vitesse Devices

The Tethys SONET/SDH Multi-Rate Framer family provided functions needed in add/drop multiplexers and digital cross-connect switches, including aggregation of up to 16 channels, pointer processing, SerDes functions, clock and data recovery, and transport and path overhead processing. The framers allowed each port to be independently configured for different data rates, supporting an "any port, any rate" model with direct optical-module interfaces. The Tethys 448 supported 16 channels of OC/STS-3/STM-1, OC/STS-12/STM-4, or OC/STS-48/STM-16 in mixed combinations. The Tethys 4192 added support for four channels of OC/STS-192/STM-64.

Vitesse's TSI family was designed for metro and access switching applications. The VSC9185 was a 64 x 64 TSI supporting STS-12/STM-4 or STS-48/STM-16 on each input and output independently, providing 160 Gbit/s of non-blocking STS-1 connectivity with support for concatenated tributaries. Inputs and outputs used differential serial signals at 2.488 Gbit/s or 622 Mbit/s. The VSC9195 was a 340 Gbit/s TSI with TFI-5 compliance, described as a 136 x 136 device supporting STS-48/STM-16 and non-blocking STS-1 connectivity.

What Changed Since 2003

SONET and SDH remain important in legacy transport networks, but they are no longer the center of new optical-network design. Packet transport, Carrier Ethernet, MPLS, OTN, coherent DWDM, and IP-over-DWDM carry much of the growth. ITU-T G.709 Optical Transport Network became the main digital wrapper for carrying Ethernet, Fibre Channel, SDH/SONET, and other client signals over optical wavelengths with strong operations and forward-error-correction support.

The semiconductor company lineage also changed. Microsemi completed its acquisition of Vitesse in 2015, and Microchip completed its acquisition of Microsemi in 2018. Infineon remains active in semiconductors, though its current focus is much broader than the SONET/SDH framer market that shaped this 2003 announcement.

Where SONET/SDH Still Appears

Legacy carrier rings and metro transport systems.
Utility, rail, government, defense, and industrial networks with long asset lifecycles.
Private-line services and TDM circuit emulation that have not yet migrated.
Mobile networks with older 2G/3G backhaul or protected timing requirements.
Interworking shelves that translate between TDM, Ethernet, OTN, and packet networks.

Planning Checklist

Inventory every legacy circuit before retiring SONET/SDH equipment; many low-speed services have hidden dependencies.
Check timing, synchronization, jitter, and wander requirements when moving TDM services to packet or OTN.
Validate protection switching behavior, not only steady-state throughput.
For circuit emulation, test alarms, clocking, slips, framing, and voice quality under packet loss and failover.
Keep spare optics, cards, power modules, and management access plans for equipment that is out of mainstream support.
When building new transport, evaluate OTN, packet transport, coherent DWDM, or IP-over-DWDM rather than recreating a SONET/SDH architecture by habit.

SONET/SDH framer and TSI chips solved a hard problem: how to switch and groom huge numbers of synchronous circuits reliably inside carrier equipment. Even as new networks move toward packet and OTN designs, understanding these devices helps explain why legacy transport systems are so resilient, and why replacing them cleanly takes more than pulling out old cards.

SONET/SDH Framer and Time Slot Interchange ICs - Yenra