Frame of

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WDM links
Concentrator
Packet from a host
Lower bit-rate host interface (e.g. Gig-Ethernet)
Frame of time slots
TSOBS Concept

• Time Sliced Optical Burst Switching (TSOBS)
• Eliminates the need for expensive wavelength
  conversion by switching data in the time domain.
• Each wavelength channel organized into a series
  of frames, each of which into timeslots.
• A few wavelength channels are reserved to carry
  control information (Burst Header Cells).
• TSOBS routers can potentially be
  cost-competitive with electronic routers.

Statistical Multiplexing Performance

• Performance determined by number of timeslots
  per frame (N)
• With single timeslot length packets, good
  performance achieved at 64 timeslots/frame.
• As the average number of timeslots/frame
  increases, performance degrades.
• For good performance, we need 128-256
  timeslots/frame.

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TSOBS Router Design
OTSI Design

• TSOBS Router Design
• Each wavelength channel is demultiplexed and
  switched independently.
• Uses high-speed crossbars to switch data into
  timeslots.
• Each router is equipped with a few regeneration
  ports to regenerate signals as necessary.
• OTSIs use fiber delay lines to provide the time
  domain switching.
• OTSIs use WDM for the delay lines, thus sharing
  the cost of fiber among the wavelengths.
• SYNCs synchronize data to a common timing
  reference using a structure similar to OTSIs.

Optical Time Slot Interchanger Design

• Nonblocking OTSIs provide the best performance,
  however they are significantly more expensive.
• Key performance criteria
• Num. of switching operations per timeslot
• Scheduling complexity of timeslots
• Number of delay lines (size of crossbar)
• Natural choice of delay values for blocking
  OTSIs 1, 2, 3,, N/2 timeslots.
• Blocking OTSIs give good performance using at
  most 3 switching operations for most timeslots.

1 Time Sliced Optical Burst Switching,
J.Ramamirtham and J. Turner, Proceedings of
INFOCOM 2003, April 2003.
3
Component Cost Requirements

• Cost/Gbps changes linearly with cost of
  crossbars and multiplexors.
• Cost of the system scales linearly with number
  of wavelengths.
• The cost of the fiber is not a big factor for a
  large number of wavelengths.
• Above estimates are for a system with 16
  input/output fibers, 128 wavelengths per fiber,
  and 128 timeslots per frame.

1 µs Timeslot
Synchronizer Design

• A timeslot has a guard time with a minimum guard
  time for switching operations.
• For a system with 1 µs timeslots each having 100
  ns guard times and 20 ns minimum guard time, the
  900 ns long data can be placed anywhere in a 960
  ns period.

Minimum guard time20ns
960 ns
Data900ns

• Two parameters affect the cost
• Precision difference between successive delay
  values (60 ns)
• Range maximum delay it provides
• By operating with links synchronized to timeslot
  boundaries instead of frame boundaries, the range
  can be reduced.
• For a network with 1 µs timeslots each having 100
  ns guard times, the synchronizer can be operated
  with a precision-to-range ratio of 110.

Incoming Signals
Passive Coupler
Optical Crossbar
Optical Crossbar
Optical Crossbar

• Two switching stages delay lines of 1, 2,,
  ??S?-1 followed by ??S?, 2??S?,, (??S? -1) ??S?.
• Two switching operations needed.
• Two crossbars of size ??S? x ??S?
• Good for moderate number of delay values.

• One delay line for each delay value, 1, 2,, S
  (S range-precision ratio)
• One switching operation needed.
• Good for small number of delay values.

Department of Computer Science And Engineering
4
Design Issues for Time Sliced Optical Burst
Routers
Jeyashankher Ramamirtham, Jonathan Turner jai,
jst _at_arl.wustl.edu Washington University in St.
Louis