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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