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Dan Marconett by mikesanye


									    A Survey of Architectural Design and
Implementation Tradeoffs in Network on Chip

                       Dan Marconett
             Next-Generation Networking Systems Lab
                  University of California, Davis


• Introduction SoC/NoC
• Architectures
• Routing Strategies
• Energy Dissipation
• Conclusion


What is System-on-Chip (SoC)?
• Integration of multiple computer components
(i.e. microcontroller, memory blocks, timers,
etc.) onto a single silicon chip
• Each on chip component
referred to as a block
• Block abstraction enables
component-level design of
SoC containing multiple
proprietary elements


What is Network-on-Chip (NoC)?
• Leveraging existing computer networking
principles to improve inter-component intra-chip
communications for SoC
• Each on chip component
connected by switch to a
particular comm wire(s)
• Improvement over standard
bus based interconnections
for SoC architectures in terms
of throughput


• Introduction SoC/NoC
• Architectures
• Routing Strategies
• Energy Dissipation
• Conclusion

           Architectures: CLICHE

CLICHÉ: Chip-Level Integration of
Communicating Heterogeneous Elements
• Two-dimensional mesh network layout for NoC
• All switches are connected
to the four closest other
switches and target resource
block, except those switches              on
the edge of the layout

• Connections are two
unidirectional links

           Architectures: Folded Torus

Similar to mesh based architectures
• Wires are wrapped around from the top
component to the bottom and rightmost to
• Smaller hop count
• Higher bandwidth
• Decreased Contention
• Increased chip space usage

            Architectures: BFT

BFT: Butterfly Fat Tree
• Each node in tree model has coordinates
(level, position) where level is depth and
position is from left to right
• Leaves are component blocks
• Interior nodes are switches
• Four child ports per switch
and two parent ports
•LogN levels, ith level has
n/(2^i+1) switches, n = leaves (blocks)
• Use traffic aggregation to reduce congestion

            Architectures: SPIN

SPIN: Scalable, Programmable, Integrated
• Leverages the Butterfly Fat Tree design
• Now every level has                       same
number switches
• Network grows like
• Trades area overhead and
decreased power efficiency for higher
• Illustrative of performance vs. power
           Architectures: Octagon

Standard model: 8 components, 12
• Design complexity increases linearly with
number of nodes
• Largest packet travel
distance is two hops
• High throughput
• Shortest path routing
easy to implement


 • Introduction SoC/NoC
 • Architectures
 • Routing Strategies
 • Energy Dissipation
 • Conclusion

     Routing: Circuit/Packet Switching

Circuit Switching
• Dedicated path, or circuit, is established over which
data packets will travel
• Naturally lends itself to time-sensitive guaranteed
service due to resource allocation
• Reservation of bandwidth decreases overall throughput
and increases average delays

Packet Switching
• Intermediate routers are now responsible for the
routing of individual packets through the network, rather
than following a single path

• Provides for so-called best-effort services

Routing: Wormhole/Virtual Cut Through

  Wormhole Switching
  • Message is divided up into smaller, fixed length flow
  units called flits
  • Only first flit contains routing information, subsequent
  flits follow
  • Buffer size is significantly reduced due to the limitation
  on the number of flits needed to be buffered at any given

  Virtual Cut Through Switching
  • Much like Wormhole switching
  • Header flit can travel ahead and undergo processing
  while remaining flits are still navigating the network
  • Higher acceptance rates and lower latencies than
           Routing: Contention

Contention occurs when routers or IP
blocks attempt to send data over the
same link at the same time
• For Circuit Switching, contention is resolved
at the time of actual connection setup
• For packet switching, contention resolution is
handled at a much finer level, by the router
buffering and scheduling individual packets of
• Better overall performance for packet
switched networks at the cost of lack of
service guarantee

 • Introduction SoC/NoC
 • Architectures
 • Routing Strategies
 • Energy Dissipation
 • Conclusion

Energy Dissipation: Architectures

Two causes for dissipation, switches and
wire segments
Many parameters in the architectural
design phase which affect the key trade-
off of performance vs. power dissipation
• Length of physical wires
• Switching techniques
• Buffer allocation
• Types of guaranteed service

• The topology itself

Energy Dissipation: Architectures (2)
 Pande et al. [10] used a simulator to investigate various
 metrics, including energy dissipation, with respect to the
 five main architectures
 • Average dynamic energy dissipated per event, each
 layout containing 256 functional blocks
 • Energy dissipation increases linearly with the increase
 of virtual channels for all five architectures
 • Small number (4) of virtual channels will keep energy
 dissipation low without giving up throughput
 • When the traffic load was analyzed, it was found that
 the energy dissipation reached an upper limit when
 throughput was maximized
 • Architectures with more elaborate topologies, and
 therefore higher degrees of connectivity (such as SPIN
 and Octagon) have a higher much greater energy
 dissipation on average (~60 nj vs. 250-350 nj)
    Energy Dissipation: Switching

How to route information from block A to block
B in such a way that the constraints on energy
consumption are maintained
Banerjee et al. [9] address this issue through a
modeling approach based on a 4x4 mesh layout
• Virtual-cut Through Switching versus Wormhole
• For both routing techniques, energy dissipation rises
linearly with the injection rate of data packets until the
network is fully congested, after which it is constant
• Both techniques yield same power consumption
• Virtual-Cut Through switching produces higher
acceptance rates and lower latencies than Wormhole
Switching, therefore VCT is preferred

 • Introduction SoC/NoC
 • Architectures
 • Routing Strategies
 • Energy Dissipation
 • Conclusion

1. More elaborate layouts with higher degrees of
   connectivity (SPIN and Octagon) were seen to have
   much higher rates of energy dissipation, however,
   they also yield increased throughput
2. Elaborate architectures also take up more space on
   the silicon chip
3. VCT is preferred to Wormhole due to decreased
   latency, though both have same energy dissipation for
   given traffic loads
4. Decide on priorities; communication reliability, energy
   efficiency, increased throughput, decreased

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