- Coming technologies: DWDM and MPLS.
- Dense wavelength division multiplexing is WDM on steroids: closer
spacing between adjacent wavelengths (128 or so currently).
- Multi-protocol label-lambda switching tags IP packets with a label
that can then be mapped into extra information at nodes.
- The optical-electronic difference is speed.
- Light moves fast in a stright line, bit it's hard to get it to bend
(switch) or stand still (store).
- It's easy to get electrons to dance, or stand still, but it's hard to
get them to move fast.
- The fasest transister operates at 350 gHz (2002); the fasest
optical modulator operates at 1 gHz (2004).
- But wait - 1 gHz vs. 350 gHz? Why is that a problem?
- The network topology: the last mile, access networks, MANs, backbones.
- Cross-connects and add-drop multiplexers.
- A digital cross-connect is a combination switch-multiplexer.
- All optical packet switching provides fast packet (and lower-level)
switching with low power consumption.
- Three stages: input, switching, and output; controlled by a header
processing unit.
- Packet switching, the IP dominance.
- Packet-switching problems.
- Destination only routing is limited.
- Per-node and per-packet routing overhead.
- Slow electronics.
- MANs interface to two disparate networks: the campus (or client) network
and the backbone network.
- Reflected in the core-collector rings in the MAN architecture.
- The campus-side interface.
- Heterogeneous in technology and protocols.
- The campus network interfaces at layers 2 (vlans) or 3.
- Best performance from aggrigation into optical streams via LSR.
-
- WAN-side interface.
- The problem here is lable switching through the networks.
- Architectures.
- Slotted networks.
- Fixed-size packets.
- Variations in synchronization and transmission require buffering.
- Less contention due to greater organiation.
- Unslotted networks.
- Variable-sized packets; no fragmentation and re-assembly.
- Less synchronization at the end-points, but more in the switches.
- Contention increases at the end-points.
- Contention resolution: two (or more) packets, one output port, one
wavelength.
- Contention effects packet loss ratio, average packet delay, average hop
distance, and network utilization.
- Electrical routers deal with contention via buffering (RAM, time) or
rerouting (space).
- There are no optical RAMs.
- Also optical switches have wavelengths, which can also be used to
resolve contention.
- Wavelength conversion maintains hop counts and latency.
- Fiber delay lines allow buffering to resolve contention.
- They are hard to use, being fifo.
- There needs to be a lot of them.
- Rerouting around contention is the third option; increases hop count
and latency.
- Optical burst routing (recognized packet trains) provide contention
avoidance (at least for the burst).
- Send a warning through, then send the burst through.
- Similar to wormhole routing, but with explicit prior notification.
- Node architectures.
- Broadcast and select.
- Combined contention resolution mechanisms out-perform any single
mechanism.
- Trade-off delay lines with wavelength conversion and deflection
routing.
- Overall requirements are atmost 1% packet loss at 10s of gbit/sec
rates.
- Enabling technologies.
- You need optical buffering and optical routing tables.
- There aren't either of these things.
This page last modified on 30 November 2004.