A Survey of Optical Burst Switching in the Next-Generation Optical Internet

Outline

Introduction Switching Techniques Optical Burst Switching QoS Support in All-Optical Networks Performance Issues Conclusions

Introduction

Introduction– IP runs over all-optical WDM layer– Challenging issues– How to support QoS?

Introduction

New optical switches/routers (hardware) are being built for the next-generation optical Internet.

The huge bandwidth of fiber optic networks– Due to DWDM (dense wavelength-division

multiplexing ) technique

Data transmitted optically has to be slowed down at each node if it is to be switched electronically.

Introduction

IP over WDM– Transport IP packets directly over the optical layer

without any O/E/O conversion.

No optical form of RAM available today

Novel protocols (software) running on top of optical switches/routers are needed.

Introduction

Challenging issues– The current lack of optical random acces

s memory– The requirement for synchronization– How to provide basic QoS support?

Introduction

Switching Techniques

Switching Technique– Wavelength routing– Optical packet switching– Optical burst switching

Switching Techniques

Wavelength routing– Two-way reservation is needed to setup lightpaths

Advantages:– No optical buffer or O/E/O conversion of data is neede

d.

Limitations:– Low bandwidth utilization

– There are not enough wavelengths in the fiber to enable full mesh connectivity

– Setting up and tearing down a lightpath would take at least several tens of milliseconds

Switching Techniques

Optical packet/cell switching– The payload(data) is sent along with its header without

setting up a path

Each packet needs to be buffered– Due to the tight coupling in time between the payload

and header, store-and-forward nature

The size of the payload is too small given the high channel bandwidth of optical networks, resulting in relatively high control overhead.

Switching Techniques

Optical burst switching– Combines the best of circuit and packet switching while

avoiding their shortcomings.

– One-way reservation. (a data burst follows a corresponding control packet without waiting for an acknowledgment)

– Control can be performed electronically, but data can be switched optically.

– A burst will cut through intermediate nodes without being buffered.

Switching Techniques

Optical switching paradigm

Bandwidth utilization

Latency (including

setup)

Implementation difficulty

Adaptivity (to traffic and

fault)

Wavelength Low High Low Low

Packet/cell High Low High High

OBS High Low Medium High

OBS

Optical Burst Switching– Open-ended

• TAG ( tell-and-go )• IBT ( in-band-terminator )

– Close-ended• JET ( just-enough-time )

Differ mainly in the way that bandwidth release is triggered

OBS

TAG (tell-and-go)– The source node sends a release packet

IBT (in-band-terminator)– A burst contains an IBT (e.g., silence in a

voice circuit), and bandwidth is released as soon as the IBT is detected.

OBS

JET (just-enough-time)

T(i) = T - Σδ(h)

OBS

JET– Offset Time– Delayed Reservation (DR)

QoS Support

QoS Support in All-Optical Networks– Without FDLs– With FDLs

( FDL : fiber delay line )

QoS Support

QoS scheme– Critical data can be transported at the WD

M layer more reliably than noncritical data.

Intraclass contentions and interclass contentions

Without FDLs

t01 > l0

With FDLs

Multiple Classes

tdiff : the difference in the offset times assigned to class i and

class (i-1)

R : The lower bound of the isolation degree

Performance Issues

Performance Issues– Blocking probability– Queuing delay and end-to-end latency

Performance Issues

Performance Issues

Performance Issues

The impact of the extra offset time, depends on the number of classes, and the offset time difference (tdiff)used.

The mean burst size : 15 kbytes

10 Gb/s => L = 12 μs

Service classes (n) : 4

tdiff = 3L (at least 95% class isolation)

Maximum additional delay = 108 μs (n -1) * tdiff

Conclusions

The integration of IP and WDM Overview of Optical Burst Switching

– Achieving a balance between wavelength routing and optical packet switching

– Without requiring buffering at the WDM layer– Support QoS in optical networks

An OBS protocol : JET– The use of offset time and DR