Download Generalized Multiprotocol Label Switching: An Overview of Routing

TTM1:
Approaches to Optical Internet Packet Switching
David K. Hunter and Ivan Andonovic
Abstract
• Increased capcity demands  WDM
• Next evolution after WDM: Optical
switching (OPS)
Introduction
• There is a ”mismatch” between the capacity
available through use of WDM and processing
capacities of routers.
• IP routers have the following tasks:
– Routing: Build connectivity through establishing routing tables in
a network. Different protocols used to spread information (e.g.
OSPF, IS-IS)
– Forwarding: Decide output port interface
– Switching: Transport a packet to the correct output port
– Buffering: If contention, store packet temporarily
Optical Packet Switching
• Transmission and switching in the optical
domain, but forwarding and routing in the
electronical domain.
• The next step would be to process packet
headers (forwarding) in the optical domain.
Then only the actual routing is left for the
elctronical domain.
Optical Packet Switching
• (At least) two alternative approaches to
OPS:
– Fixed Length Packets (FLP)
– Variable Length Packets (VLP)
– (But additionally one could distinguish
between Asynchronous or Syncronous
(Slotted) transport).
The design of Optical Packet Switches
• Three principal sub-blocks (Note: This is a
slotted network):
– Input interface: Alignment of packets i time. Why?
– Switching core: Transports packets to the correct
output port
– Output interface: Header insertion
The design of Optical Packet Switches
• Packet format defined in the KEOPS project
–
–
–
–
Sync.pattern. Why?
Guard time. Why?
More sync: payload sync. Why?
More guard. Why?
Wavelength in Contention Resolution
Two possible multiplexing schemes:
• Scattered Wavelength Path (SCWP)
– Packets are spread on random (”scattered”)
free wavelengths.
• Shared Wavelength Path (SHWP)
– Each path (=”virtual connection”) in the optical
packet layer is assigned a particular
wavelength. Wavelengths may be shared by
many paths. (But packets belonging to a path
will not change to another wavelength).
Wavelength in Contention Resolution
– Using SCWP there is one large buffer per fiber for all
wavelengths. Buffer depth per wavelength is size of
buffer divided by number of wavelengths.
– Using SHWP there is one buffer per wavelength.
Comparison of buffer depth for achieving PLR 10-9
Wavelength in Contention Resolution
• Broadcast and Select Switch (KEOPS)
– Wavelength encoder. N wavelength converters, one for each
input. Encoding each packet on a fixed wavelength with a
unique wavelength for each input.
– Buffer and broadcast section. Number of FDLs and a space
switch stage. Electronically controlled selection (full signal?).
– Wavelength selector block. N demultiplexers, followed by
electronically controlled selection.
• All packets available at all outputs => support multicast
Wavelength in Contention Resolution
WASPNET design:
• No large splitting losses as in KEOPS B&S
• Core components are Tuneable Wavelength Converters
(TWCs), and 2N x 2N Arrayed Waveguide Grating
(AWG). Pluss N*N space switch.
Variable-Length Optical Packet Switching
– S stages, D in/outputs in every stage except first (N
inputs) and last (N outputs); and D FDLs.
– Delay line granularity of each stage is N times that of
the next stage.
– Brute force algoritme controlling the switch is
computationally intensive.
Delay lines in units of packet granularity
Conclusions
• At the start of OPS (year 2000) Most OPS
approaches assumed fixed length packets
and synchronous operation of switches.
• If the goal is to carry variable length
packets (as in Ethernet) asynchronous
operation may be necessary.
• Use of wavelength dimension to resolve
contention is also shown to be useful.