Melbourne Neuropsychiatry Centre

Dr Andrew Zalesky

Dr Andrew Zalesky, BEng, PhD

Post Doctoral Fellow

I am Australian Research Council International Fellow at the University of Melbourne, Australia.

I am member of the Melbourne Neuropsychiatry Centre and the Centre for Ultra-Broadband Information Networks.

I am working on:
(i) queueing models to evaluate the impact of design choices in telecommunications networks
(ii) tracking algorithms for reconstructing neural pathways in the human brain using diffusion-MRI.

I am grateful for the support provided by the Melbourne Neuropsychiatry Centre, Australian Research Council, American Australian Association, Australian Academy of Science and CASS Foundation.


Contact

E-mail: azalesky (at] unimelb.edu.au

Phone: (+613) 8344 1839

Mailing Address:
Neuropsychiatry Imaging Laboratory
National Neuroscience Facility
Level 2 & 3, Alan Gilbert Building
The University of Melbourne
Victoria, 3010, Australia

 

Recent Work

"To burst or circuit switch?," A. Zalesky, IEEE/ACM Trans. Networking, in press

    Optical burst switching (OBS) and optical circuit switching (OCS) are rival switching protocols devised to expedite the deployment of next-generation telecommunications networks.

    This paper quantifies which of OBS or OCS yields higher utilisation for a prescribed worst-case packet queueing delay and maximum packet blocking probability.

    The first approximations for end-to-end blocking probabilities in OBS networks were based on Erlang's fixed-point approximation (EFPA). EFPA makes the assumption that links are mutually independent. This paper parts with that assumption. Parting with the assumption of link independence is shown to yield higher accuracy. The accuracy of EFPA has been improved in the same manner for conventional circuit switching (Coyle et al.) and OCS (Zhu et al.).

    This paper is reminiscent of earlier evaluative studies (e.g. Widjaja) of the old tell-and-go and tell-and-wait protocols.

    The ubiquity of modern computing has led to the gradual demise of the teletrafficist.

"DT-MRI fiber tracking: a shortest paths approach," A. Zalesky, IEEE Trans. Medical Imaging, in press

    The brain is interconnected via a network of pathways that enable communication between functionally distinct regions. Pathways in the living human brain can be reconstructed virtually with diffusion-MRI and an appropriate tracking algorithm. Virtually tracking pathways in the brain is called tractography.

    This paper casts tractography as a problem in computing shortest paths in a weighted graph. Shortest paths are computed with Dijkstra's algorithm.

    Tractography algorithms in prevalent use today are locally greedy. The follow-the-principal-eigenvector rule is one example. In contrast, the tractography algorithm devised in this paper preserves global optimality. This ensures one 'wrong turn' does not spell disaster.

    Sethian's fast marching algorithm has been adapted to perform tractography (Parker et al., Staempfli et al.). An adaption is required because the front propagation rate now depends on the direction of propagation. Fast marching and Dijktra's algorithm are closely allied but not identical. Fast marching returns a continuous path that is computed using gradient descent, while Dijkstra's algorithm returns a discretised path. Therefore, fast marching is sub-optimal due to the inherent sub-optimality of gradient descent, whereas Dijkstra's algorithm is sub-optimal due to discretisation error.

    Tuch was the first to put forth the notion of computing globally optimal pathways in the brain. Dijkstra's algorithm has also been used by Iturria-Medina et al.

    • Y. Iturria-Medina et al., "Characterizing brain anatomical connections using diffusion weighted MRI and graph theory," NeuroImage, vol. 36, no. 3, 2007, pp. 645-660.
    • G.J.M. Parker, C.A.M. Wheeler-Kingshott, G.J. Barker, "Estimating distributed anatomical connectivity using fast marching methods and diffusion tensor imaging," IEEE Trans. Medical Imaging, vol. 21, no. 5, May 2002, pp. 505-512.
    • P. Staempfli, T. Jaermann, G.R. Crelier, S. Kollias, A. Valvanis, P. Boesiger, "Resolving fiber crossing using advanced fast marching tractography based on diffusion tensor imaging,” NeuroImage, vol. 30, no. 1, 2006, pp. 110-120.
    • D.S. Tuch, "Mapping cortical connectivity with diffusion MRI," in Proc. IEEE Int. Symp. Biomedical Imaging, 2002, pp. 392-394.
"A new method for approximating blocking probability in overflow loss networks," E.W.M. Wong, A. Zalesky, Z. Rosberg, M. Zukerman, Computer Networks, vol. 51, no. 11, August 2007, pp. 2958-2975.

    Erlang's fixed-point approximation (EFPA) was devised in 1964 by Katz and Cooper to estimate end-to-end blocking probabilities in circuit-switched telephony networks. In the ensuing years, EFPA has been revamped again and again to estimate blocking probabilities in a diverse range of loss networks.

    EFPA is notoriously inaccurate in the case of overflow loss networks. EFPA relies on the assumption that links are offered an independent Poisson-distributed arrival process. This assumption is violated most in overflow loss networks, which may render EFPA in error by several orders of magnitude.

    This paper proposes a very simple modification to EFPA termed overflow priority classification (OPC). OPC has since been proved to yield higher accuracy than conventional EFPA for a particular kind of overflow loss network. This proof would not have been possible without the persistence of Eric Wong, Bill Moran, Moshe Zukerman and Zvi Rosberg.

    Particular kinds of overflow loss networks can be concocted for which OPC fails dismally. OPC is therefore not intended to replace EFPA.

    Traditionally, the accuracy of EFPA has been improved by fitting higher moments of the arrival process. Wilkinson's method and the equivalent random method are examples.

"A DTI-derived measure of cortico-cortical connectivity," A. Zalesky, A. Fornito, under review.

    The brain is interconnected via a network of pathways that enable communication between functionally distinct regions. Pathways in the living human brain can be reconstructed virtually with diffusion-MRI and an appropriate tracking algorithm. Virtually tracking pathways in the brain is called tractography.

    Even though hundreds of tractography algorithms have been devised over the past decade (e.g. deterministic, stochastic, shortest paths) a 'wiring diagram' for the human brain remains to be charted. This may be due to an emphasis on visualisation rather than quantification of brain connectivity.

    Current quantitative measures of brain connectivity (e.g. the fraction of intersecting streamlines) suffer three impediments: (i) distance bias, (ii) invariance to a pathway's cross-sectional area, and (iii) sensitivity to an exchange in seed and target. This paper devises a connectivity measure that is endowed with the precise converse of these three undesirable properties.

    This paper models the brain as a network. Distinct brain regions serve as nodes. Links are drawn between pairs of nodes for which connectivity exceeds a certain threshold.

    Functional brain networks have been charted with functional-MRI (Achard et al.) and complement the structural brain networks considered in this paper. We have recently shown that correspondence between an individual's structural and functional network is attributable to more than chance alone.

    Hagmann et al. and Iturria-Medina et al. have recently charted structural brain networks using diffusion-MRI.

    • S. Achard, R. Salvador, Whitcher, J. Suckling, E. Bullmore, "A resilient, low-frequency, small-world human brain functional network with highly connected association hubs," J. Neuroscience, vol. 26, no. 1, 2006, pp. 63-72.
    • P. Hagmann, M. Kurant, X. Gigandet, P. Thiran, V.J. Wedeen, R. Meuli, J.-P. Thiran, "Mapping human whole-brain structural networks with diffusion MRI," PLoS ONE, vol. 2, no. 7, 2007, e597.
    • Y. Iturria-Medina et al., "Characterizing brain anatomical connections using diffusion weighted MRI and graph theory," NeuroImage, vol. 36, no. 3, 2007, pp. 645-660.

     

  1. Journal Articles

    1. "A DTI-derived measure of cortico-cortical connectivity,"
      A. Zalesky, A. Fornito,
      under review
    2. "Cost comparison of optical burst switched and automatically switched optical networks,"
      R. Parthiban, C. Leckie, A. Zalesky, M. Zukerman, R. Tucker,
      under review
    3. "DT-MRI fiber tracking: a shortest paths approach,"
      A. Zalesky,
      IEEE Trans. Medical Imaging, in press
    4. "Designing an optimal scheduler buffer in OBS networks,"
      A. Zalesky, H.L. Vu,
      IEEE/OSA J. Lightwave Technology, in press
    5. "To burst or circuit switch?,"
      A. Zalesky,
      IEEE/ACM Trans. on Networking, in press
    6. "Stabilizing deflection routing in optical burst switched networks,"
      A. Zalesky, H.L. Vu, Z. Rosberg, M. Zukerman, E.W.M. Wong,
      IEEE J. Selected Areas in Communications, vol. 25, no. 6, Aug. 2007, pp. 3-19.
    7. "A new method for approximating blocking probability in overflow loss networks,"
      E.W.M. Wong, A. Zalesky, Z. Rosberg, M. Zukerman,
      Computer Networks, vol. 51, no. 11, Aug. 2007, pp. 2958-2975.
    8. "Enhanced blocking probability estimation in trunk reservation networks,"
      G. Raskutti, A. Zalesky, M. Zukerman,
      IEEE Communications Letters, vol. 11, no. 6, June 2007, pp. 543-545.
    9. "OBS contention resolution performance,"
      A. Zalesky, H.L. Vu, Z. Rosberg, E.W.M. Wong, M. Zukerman,
      Performance Evaluation, vol. 64, no. 4, May 2007, pp. 357-373.
    10. "On generalizations of the Engset model,"
      E.W.M. Wong, A. Zalesky, M. Zukerman,
      IEEE Communications Letters, vol. 11, no. 4, April 2007, pp. 360-362.
    11. "Analysis of OBS networks with limited wavelength conversion,"
      Z. Rosberg, A. Zalesky, H.L. Vu, M. Zukerman,
      IEEE/ACM Trans. Networking, vol. 14, no. 5, Oct. 2006, pp. 1118-1127.
    12. "Packet delay in optical circuit switched networks,"
      Z. Rosberg, A. Zalesky, M. Zukerman,
      IEEE/ACM Trans. Networking, vol. 14, no. 2, April 2006, pp. 341-354.
    13. "Automatic laser shutdown implications for all-optical data networks,"
      K. Hinton, P. Farrell, A. Zalesky, L.H. Andrew, M. Zukerman,
      IEEE/OSA J. Lightwave Technology, vol. 24, no. 2, Feb. 2006, pp. 674-680.
    14. "Dynamically concatenated wavelength converters,"
      A. Zalesky, R.S. Tucker,
      IEEE Photonics Technology Letters, vol. 18, no. 2, Jan. 2006, pp. 352-354.
    15. "Scalable performance evaluation of a hybrid optical switch,"
      H.L. Vu, A. Zalesky, E.W.M. Wong, Z. Rosberg, S.H. Bilgrami, M. Zukerman, R.S. Tucker,
      IEEE/OSA J. Lightwave Technology, Special Issue on Optical Networks, vol. 23, no. 10, Oct. 2005, pp. 2961-2973.
    16. "Prioritized deflection routing in optical burst switched networks,"
      C. Cameron, A. Zalesky, M. Zukerman,
      IEICE Trans. Communications, vol. E88-B, no. 5, May 2005, pp. 1861-1867.
    17. "Reducing spare capacity through traffic splitting,"
      A. Zalesky, H.L. Vu, M. Zukerman,
      IEEE Communications Letters, vol. 8, no. 9, Sep. 2004, pp. 594-596.
    18. "Performance analysis of an OBS edge router,"
      A. Zalesky, E.W.M. Wong, M. Zukerman, H.L. Vu, R.S. Tucker,
      IEEE Photonics Technology Letters, vol. 16, no. 2, Feb. 2004, pp. 695-697.
    19. "A framework for solving logical topology design problems within constrained computation time,"
      A. Zalesky, H.L. Vu, M. Zukerman, I. Ouveysi,
      IEEE Communications Letters, vol. 7, no. 10, Oct. 2003, pp. 499-501.

Refereed Conference Papers

    1. "Blocking probability estimation for trunk reservation networks,"
      G. Raskutti, A. Zalesky, E.W.M. Wong, M. Zukerman,
      IEEE International Conference on Communications, ICC, Glasgow, Scotland, June 2007
    2. "Effect of traffic shifts on the economics of telecommunications competition,"
      J. Baliga, A. Zalesky, M. Zukerman,
      IEEE International Conference on Communications, ICC, Glasgow, Scotland, June 2007
    3. "Optimizing an OBS scheduler buffer,"
      A. Zalesky,
      First International Conference on Performance Evaluation Methodologies and Tools, Valuetools, Pisa, Italy, Oct. 2006
    4. "Waveband burst switching-a new approach to networking,"
      R. Parthiban, C. Leckie, A. Zalesky, R.S. Tucker,
      Optical Fiber Communication Conference, OFC/NFOEC, poster session JThB47, Anaheim, California, March 2006
    5. "Performance evaluation of a hybrid optical switch,"
      A. Zalesky, H.L. Vu, E.W.M. Wong, M. Zukerman, Z. Rosberg, S.H. Bilgrami,
      19th International Teletraffic Congress, ITC, Beijing, China, Aug./Sep. 2005
    6. "Models and approximations for bufferless optical networks,"
      H.L. Vu, A. Zalesky, Z. Rosberg, M. Zukerman,
      10th Opto-Electronics and Communications Conference, OECC, Seoul, Korea, July 2005
    7. "A novel method for modeling and analysis of distributed video-on-demand systems,"
      E.W.M Wong, M.Y.M. Chiu, M. Zukerman, Z. Rosberg, S. Chan, A. Zalesky,
      IEEE International Conference on Communications, ICC, Seoul, Korea, May 2005, vol. pp. 88-92
    8. "Delay analysis of optical burst switching networks,"
      H.L. Vu, A. Zalesky, M. Zukerman, Z. Rosberg, J. Guo, T.W. Um,
      IEEE International Conference on Communications, ICC, Seoul, Korea, May 2005, vol. 3, pp. 1656-1662
    9. "Does optical burst switching have a role in the core network?,"
      R. Parthiban, R.S. Tucker, C. Leckie, A. Zalesky, A.V. Tran,
      Optical Fiber Communication Conference, OFC/NFOEC, Anaheim, California, March 2005, vol. 3, pp. 50-52
    10. "Comparison of OBS resolution policies,"
      A. Zalesky, H.L. Vu, Z. Rosberg, E.W.M. Wong, M. Zukerman,
      International Conference on Optical Communications and Networks, ICOCN, Hong Kong, China, Nov. 2004, pp. 99-102
    11. "Does optical burst switching have a role in the core network?,"
      R. Parthiban, R.S. Tucker, C. Leckie, A. Zalesky, A.V. Tran,
      Optical Fiber Communication Conference, OFC/NFOEC, Anaheim, California, March 2005, vol. 3, pp. 50-52
    12. "Shortest path prioritized random deflection routing in optical burst switched networks,"
      C. Cameron, A. Zalesky, M. Zukerman,
      1st International Conference on Broadband Networks and 2nd Workshop on Optical Burst Switching, Broadnets/WOBS, San Jose, California, Oct. 2004
    13. "Shared risk link protection in IP over optical transport network,"
      A.V. Tran, H.L. Vu, A. Zalesky,
      30th European Conference on Optical Communication, ECOC, poster session We.4.P.151, Stockholm, Sweden, Sep. 2004
    14. "Optical burst switching - prioritized deflections,"
      C. Cameron, A. Zalesky, M. Zukerman,
      9th Opto-Electronics and Communications Conference and 3rd International Conference on Optical Internet, OECC/COIN, Yokohama, Japan, July 2004
    15. "IP over optical transport network: cost and scalability study,"
      H.L. Vu, A.V. Tran, A. Zalesky,
      9th Opto-Electronics and Communications Conference and 3rd International Conference on Optical Internet, OECC/COIN, Yokohama, Japan, July 2004\
    16. "Laser safety issues in dynamically reconfigurable all-optical networks,"
      K. Hinton, P. Farrell, A. Zalesky,
      9th Opto-Electronics and Communications Conference and 3rd International Conference on Optical Internet, OECC/COIN, Yokohama, Japan, July 2004
    17. "Evaluation of limited wavelength conversion and deflection routing as methods to reduce blocking probability in optical burst switched networks,"
      A. Zalesky, H.L. Vu, M. Zukerman, Z. Rosberg, E.W.M. Wong,
      International Conference on Communications, ICC, Paris, France, June 2004, vol. 3, pp. 1543-1547
    18. "Modelling and performance evaluation of optical burst switched networks with deflection routing and wavelength reservation,"
      A. Zalesky, H.L. Vu, Z. Rosberg, E.W.M. Wong, M. Zukerman,
      23rd Joint Conference of IEEE Computer and Communications Societies, INFOCOM, Hong Kong, China, March 2004, vol. 3, pp. 1864-1871
    19. "Reduced load Erlang fixed point analysis of optical burst switched networks with deflection routing and wavelength reservation,"
      A. Zalesky, H.L. Vu, Z. Rosberg, M. Zukerman, E.W.M. Wong,
      1st International Workshop on Optical Burst Switching, WOBS, Dallas, Texas, Oct. 2003
    20. "Performance analysis of optical burst switching networks with deflection routing,"
      A. Zalesky, H.L. Vu, Z. Rosberg, M. Zukerman, E.W.M. Wong,
      2nd Conference on the Optical Internet and Australian Conference on Optical Fibre Technology, COIN/ACOFT, Melbourne, Australia, July 2003
    21. "Routing and wavelength assignment over multi-fibre optical networks with no wavelength conversion,"
      A. Zalesky, H.L. Vu, M. Zukerman, C. Leckie, I. Ouveysi,
      7th IFIP Working Conference on Optical Network Design and Modelling, ONDM, Budapest, Hungary, Feb. 2003, vol. 3, pp. 1155-1169
    22. "A lifetime quantification tool for WDM networks,"
      N. Maxemchuk, T. Shum, A. Zalesky, M. Zukerman,
      Workshop on Optical Networking: Technologies, Architectures and Management, Melbourne, Australia, Nov. 2001

     

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