Time-driven priority

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Time-driven priority (TDP) [1] is a synchronous packet scheduling technique that implements UTC-based pipeline forwarding [2] and can be combined with conventional IP routing to achieve the higher flexibility than another pipeline forwarding implementation known as time-driven switching (TDS) or fractional lambda switching (FλS). [3] [ clarification needed ] Packets entering a switch from the same input port during the same [time frame] (TF) can be sent out from different output ports, according to the rules that drive IP packet routing. Operation in accordance to pipeline forwarding principles ensures deterministic quality of service and low complexity packet scheduling. Specifically, packets scheduled for transmission during a TF are given maximum priority; if resources have been properly reserved, all scheduled packets will be at the output port and transmitted before their TF ends.

Various aspects of the technology are covered by several patents issued by both the United States Patent and Trademark Office and the European Patent Office.[ citation needed ]

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  1. High efficiency in utilization of network resources, which enables accommodating a larger amount of traffic on the network, thus lowering operation cost and being the foundation for accommodating the exponential growth of modern networks.
  2. Low implementation complexity, which enables the realization of larger and more powerful networking systems at low cost, thus offering further support to network growth.
  3. High scalability, which is an immediate consequence of the above two features.
  4. Deterministic and predictable operation with minimum delay and no packet loss even under full load condition, which is key in supporting the demanding requirements of the new and valuable services that are being deployed, or envisioned to be deployed, on modern networks, such as telephony, videoconferencing, virtual presence, video on demand, distributed gaming.

When realizing pipeline forwarding a predefined schedule for forwarding a pre-allocated amount of bytes during one or more time frames along a path of subsequent switches establishes a synchronous virtual pipe (SVP). The SVP capacity is determined by the total number of bits allocated in every time cycle for the SVP. For example, for a 10 ms time cycle, if 20,000 bits are allocated during each of 2 time frames, the SVP capacity is 4 Mbit/s.

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Fractional lambda switching (FλS) leverages on time-driven switching (TDS) to realize sub-lambda switching in highly scalable dynamic optical networking, which requires minimum buffers. Fractional lambda switching implies switching fractions of optical channels as opposed to whole lambda switching where whole optical channels are the switching unit. In this context, TDS has the same general objectives as optical burst switching and optical packet switching: realizing all-optical networks with high wavelength utilization. TDS operation is based on time frames (TFs) that can be viewed as virtual containers for multiple IP packets that are switched at every TDS switch based on and coordinated by the UTC signal implementing pipeline forwarding. In the context of optical networks, synchronous virtual pipes SVPs typical of pipeline forwarding are called fractional lambda pipes (FλPs).

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Yoram Ofek was a Marie Curie Chair and full professor in the Information Engineering and Computer Science Department at the University of Trento, Italy. He was the inventor of 45 US and European patents and published more than 120 journal and conference papers. He invented several novel architectures for networking, computing and storage. He was elected IEEE Fellow in 2006 for his contributions to switching, scheduling and synchronization in data networks.


  1. Li, Chung-Sheng; Ofek, Yoram; Yung, Moti (1996), ""Time-driven Priority" Flow Control for Real-time Heterogeneous Internetworking", IEEE Int. Conf. on Computer Communications (INFOCOM 1996) (PDF), IEEE
  2. Baldi, Mario; Marchetto, Guido; Ofek, Yoram (10 October 2007), "A Scalable Solution for Engineering Streaming Traffic in the Future Internet", Computer Networks (COMNET), 51 (14): 4092–4111, CiteSeerX , doi:10.1016/j.comnet.2007.04.019
  3. Baldi, Mario; Ofek, Yoram (2004), "Fractional Lambda Switching - Principles of Operation and Performance Issues" (PDF), SIMULATION: Transactions of the Society for Modeling and Simulation International, 80 (10): 527–544, CiteSeerX , doi:10.1177/0037549704046461