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On trains and wagons: switching variable length packets in slotted OPS Chris Develder Mario Pickavet Piet Demeester Dept. of Information Technology (INTEC) Ghent University - IMEC, Belgium UNIVERSITEIT GENT Outline Intro Slotted

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UNIVERSITEIT GENT

On trains and wagons: switching variable length packets in slotted OPS

Chris Develder Mario Pickavet Piet Demeester

  • Dept. of Information Technology (INTEC)

Ghent University - IMEC, Belgium

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SLIDE 2

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

2

Outline

  • Intro
  • Slotted variable length packets
  • Switch architecture
  • Performance criteria
  • Simulation set-up
  • Trains or wagons?
  • influence of load
  • influence of granularity
  • service differentiation
  • Conclusions
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COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

3

Optical switching

  • Optical switching:
  • direct light from an

input port to an output port

  • possibly wavelength conversion
  • circuit-switching:
  • continuous bit-stream
  • pre-established light-paths
  • set-up: “manually” or automatic
  • packet/burst switching
  • chunks of bits, encapsulated in packets
  • packet header determines forwarding
  • e.g. label switching, GMPLS based

f f c c b a d b e c f

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COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

4

Variable length packets in OPS

  • Segmentation & reassembly:
  • chop variable length packets into

OPS slots

  • calls for extra S&R info in header
  • S&R functionality resides at edges
  • “Trains or wagons”:
  • trains: treat train as a whole
  • S&R trivial since wagons are kept together

and in sequence

  • only a single header, i.e. minimal control
  • verhead
  • wagons: treat each wagon individually
  • simpler scheduling algorithms

1 2 3 4 1 2 3 4 1 2 3 4

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SLIDE 5

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

5

Switch Architecture

  • Node in core OPS network (backbone)
  • Switch functionality:
  • slotted operation
  • WDM ports
  • fully non-blocking switching matrix (SOA based)
  • wavelength conversion to solve contention
  • FDLs to provide buffering

FDL delay = D all-optical space switch F fibers W wavelengths B buffer ports

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SLIDE 6

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

6

Scheduling

  • Scheduling: each timeslot:

(0) collect packets (from inputs + FDLs) per destination output port (1) select packets for forwarding along outgoing fibres; (2) elect packets for buffering from excess packets; drop remaining packets

switch matrix buffer drop

2 1

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SLIDE 7

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

7

Simulation set-up

  • Parameters:
  • F=6 input/output fibres
  • W=8 wavelengths per i/o fibre
  • B=0..8 recirculating buffer ports
  • D=2L delay in buffer
  • L=1.5…20 wagons per train (average)
  • Traffic model:
  • train length: neg. exponential distribution rounded to slot length
  • train inter-arrival: Poisson process
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SLIDE 8

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

8

Performance criteria

  • (byte) loss rate:
  • amount of data lost / amount of data sent
  • main indicator of service quality for end user
  • delay:
  • of secondary importance

(delay in OPS switches only small fraction of end-to-end delay)

  • fairness:
  • large trains should not be discriminated against
  • service differentiation:
  • the scheduling mechanism should allow for efficient class of service

differentiation

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SLIDE 9

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

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Influence of load (1)

1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0.2 0.4 0.6 0.8 1 load loss rate trains, B=0 wagons, B=0 trains, B=4 wagons, B=4 trains, B=8 wagons, B=8 no buffer B=4 B=8

loss rate loss wagons / loss trains

0% 100% 200% 0.2 0.4 0.6 0.8 1 load loss rate no buffer 4 buffer ports 8 buffer ports wagons better trains better

  • no buffer: trains better
  • wagon approach results in losing parts of multiple overlapping trains
  • with buffer: wagons can be better for medium loads
  • buffer allows to store wagons for multiple overlapping trains; wagon-

approach allows to exploit buffer more efficiently than train approach

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SLIDE 10

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

10

Influence of load (2)

load=0.62 B=4

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 1 3 5 7 9 11 13 15 train size train loss rate total loss, trains total loss, wagons

  • fairness:
  • wagon approach seriously discriminates against longer trains
  • wagons can reach lower overall loss rate if sufficient buffer,

and for medium load, but at the price of more unfairness (and possibly higher delays)

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SLIDE 11

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

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Influence of granularity

1.E-04 1.E-03 1.E-02 1.E-01 5 10 15 20 25 meanpkt loss rate

trains, load 0.5 w agons, load 0.5 trains, load 0.6 w agons, load 0.6 trains, load 0.7 w agons, load 0.7

trains better wagons better

B=4

  • granularity:
  • performance of trains/wagons depends on ratio train length and OPS

slot length

  • wagon approach better if trains are short

(cross-over point shifts to slightly larger lengths for lower loads)

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SLIDE 12

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

12

Service differentiation (1)

  • Scheduling: each timeslot:

(0) collect packets (from inputs + FDLs) per destination output port (1) select packets for forwarding along outgoing fibres; (2) elect packets for buffering from excess packets; drop remaining packets

⇒ simple priority mechanism: first high priority packets

switch matrix buffer drop

2 1

“priority queue”: 1) first higher priority packets; 2) same priority: first “oldest” 3) same timestamp: random (uniform over same pri and tstamp) tstamp = when packet enters switch

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SLIDE 13

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

13

Service differentiation (2)

1.E-06 1.E-04 1.E-02 1.E+00 0.2 0.4 0.6 0.8 1 load loss rate high pri, trains high pri, wagons low pri, trains low pri, wagons

  • service differentiation:
  • train approach does not allow strong service differentiation with a

simple differentiation mechanism without preemption of earlier arrived low priority trains

  • wagon approach achieves strong separation with very simple

differentiation mechanism

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COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

14

Conclusions

  • wagon approach is advantageous…
  • …to achieve strong service differentiation
  • …to achieve lower overall loss for medium loads if there is a buffer
  • …to slightly reduce average delay when load is limited
  • … but at the price of
  • …stronger discrimination against long trains
  • …increased control overhead (header information + higher load on

scheduler)

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UNIVERSITEIT GENT

That’s all, folks!

… thanks for your attention … any questions?

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COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

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Influence of load (3)

  • low loads: wagon approach has slightly lower delays
  • only a few of the train’s wagons need to be buffered, whereas the train

approach buffers complete trains (thus also the last wagon)

  • high loads: train approach has lower average delays
  • in wagon approach under high loads, the chance of having trains with

no buffered wagons is substantially reduced

expo 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0.2 0.4 0.6 0.8 1 load traindelay_recv trains, B=0 wagons, B=0 trains, B=4 wagons, B=4 trains, B=8 wagons, B=8 expo 0% 20% 40% 60% 80% 100% 120% 140% 160% 0.2 0.4 0.6 0.8 1 load traindelay_recv no buffer B=4 buffer ports B=8 buffer ports

delay delay wagons / delay trains

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SLIDE 17

COIN, Tu.A2-6, 15 July 2003

  • C. Develder, et al., "On trains and wagons: switching variable packets in a slotted OPS network"

17

Delay measurement

3 2 1

  • delay:
  • delay induced by buffering
  • time elapsed between end of transmission of packet and completion
  • f its reception
  • we account for possible re-ordering (with wagon approach)

3 2 1

Tx

delay = 2 slots delay = 3 slots 3 2 1 3 2 1

Rx