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OptimalComplexity Optical Router

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Title: OptimalComplexity Optical Router


1
Optimal-Complexity Optical Router
  • Hadas Kogan, Isaac Keslassy
  • Technion (Israel)

2
Router schematic representation
Router
Optic to electronic
Electronic to optic


Optic to electronic
Electronic to optic
  • Problem - electronic routers do not scale to
    optical speeds
  • Access to electronic memory is slow and power
    consuming.
  • Data conversions are power consuming as well.

3
Power consumption per chassis
Nick McKeown, Stanford
4
How about an optical router?
  • No electronic memory bottleneck
  • No O/E/O conversions
  • BUT
  • An optical router is thought to be too complex.
  • Is it?

5
  • Objective quantify the fundamental complexity of
    an optical router

6
Quantifying complexity
  • Quantify the fundamental complexity of an
    optical router ? reduce into most basic
    building blocks
  • Switching 2x2 switches(and input/output lines)

7
Basic optical buffering component
  • Buffering 2x2 switches (and input/output/fiber
    delay lines)
  • Mode of operation
  • (a) Write
  • (b) Circulate
  • (c) Read

(a)
(b)
(c)
1
1
1
8
  • The complexity of a system is the minimal number
    of 2x2 switches needed to implement it.

9
Quantifying complexity
  • Complexity lower-bound To get a state-space of
    size K in time T, the minimal number C of 2x2
    switches needed is
  • Examples
  • NxN switch
  • Time Slot Interchange with time frame N

10
Quantifying complexity
  • A construction algorithm is said to be optimal if
    its number of 2x2 switches grows like the
    construction complexity.
  • Examples
  • NxN switch (Benes is optimal)
  • Time Slot Interchange

Benes 65
Jordan et al. 94.
11
Optimal buffer emulation

12
Emulation definition
  • Original buffer
  • Buffer emulation (with delay D)

6
5
4
3
2
1
t
D
13
Emulation idea
  • Objectif emulate buffer of size B
  • Universal buffer any policy
  • Idea schedule using frames of size B
  • During any frame of B slots, observe which
    packets leave the original buffer and color them
    in blue
  • After some pipeline delay, send these blue
    packets in the same order

F - Frame of size B
Frame-based scheduling
Optical buffer
14
Algorithm
departure
4
B
5
2
  • Algorithm is optimal
  • Complexity T(ln B)
  • Complexity lower-bound T(ln B) (the Time Slot
    Interchange is a special case)

15
Optimal router emulation

16
What we want an ideal router
  • An output-queued push-in-first-out (OQ-PIFO)
    switch.
  • OQ - Arriving packets are placed immediately in
    the queue of size B at their destination output.
  • PIFO packets departure ordering is according to
    their priority.

17
What we want an ideal router
  • Why it is ideal
  • OQ Work-conserving ? best throughput and average
    delay.
  • PIFO Enables FIFO, strict priority, WFQ
  • But up to N packets could be destined to the
    same output
  • Speed-up for switch
  • Speed-up for queue
  • PIFO is hard to implement.

18
Finding the complexity
  • Direct calculation of complexity seems impossible
    what are the states?
  • Alternative way of finding the complexity
  • Find a lower bound
  • Find an upper bound via algorithm
  • Algorithm complexity T (lower bound) ?
    algorithm is optimal

19
Lower bound - intuition
B
At least T(Nln(N))
At least T(ln(B))
  • Intuition the complexity of an OQ-PIFO switch is
    at least T(N ln(N) N ln(B)) T(N ln(NB))

20
Lower bound
  • A frames switch (time/space switch)

t1
t2
t3
t4
t5
t6
t3
t4
t5
t6
t7
t8
t9
Frames switch
1
2
3
7
8
9
N
6
2
5
11
9
10
4
5
6
10
11
12
3
4
1
12
7
8
B
  • A frames switch is a special case of an OQ-PIFO
    switch.
  • The practical complexity of a frames switch?
    Complexity OQ-PIFO T(Nln(NB))
  • Now, find an algorithm that reaches this
    lower-bound

21
Solving the speed-up problem
Leaves output A at time 1
  • Example Emulating a non-idling OQ-FIFO switch

D1
Output A
Input 1
A1

Output B
C1

A2
Input N
C2
  • Using parallel buffers to resolve conflicts
  • At most one packet can enter a buffer at each
    time slot (N-1 constraints).
  • A packet departing at time T should not enter a
    buffer with a packet departing at T (N-1
    constraints).
  • ? 2N-1 buffers are enough.

22
The pigeonhole principle
  • Proof intuition
  • Pigeons ? Packets
  • Holes ? Buffers
  • For emulating PIFO behavior
  • The departure process is slightly modified
  • 4N-2 parallel buffers are required

Input 1
Output 1



Input N
Output N
23
An optical emulation of an OQ-PIFO switch
The pigeonhole principle Our emulation of an
optical buffer An optical OQ-PIFO switch
Optical buffer
Output 1


Output N
Optical buffer
24
An optical emulation of an OQ-PIFO switch
(4N-2)xN switch
Nx(4N-2) switch
B
pB
pB
B
B
pB
pB
B
...
B
pB
pB
B
  • Number of 2x2 switches T(NlnNNlnB)
    T(Nln(NB)) T(lower bound)? algorithm is
    optimal

25
Conclusion
  • Buffer fundamental complexity T(lnB)
  • OQ-PIFO router fundamental complexity T(NlnNB)
  • Both can be reached using given algorithms

26
Thank you!
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