WDM  Optical Packet Networking

C. Guillemot and F. Clerot
France Telecom  CNET/DTD/RTO
2 Avenue P. Marzin          22307 Lannion,         FRANCE

	All optical WDM networking makes available the huge fibre
bandwidth for point to point transmission, thus moving the potential
bottlenecks for high speed packet networking towards the forwarding
and the switching issues.
	It is now recognized that it is feasible to do a full routing
lookup for each IP packet at gigabit speeds. Forwarding tables can be
built small enough from the routing tables managed by the IP routers
to fit in the cache of a conventional general purpose processor
(1). By providing each wavelength carrier of a WDM link accessing a
router with its own forwarding engine, advantage is taken from the
parallelism peculiar to WDM to cope with the forwarding issue while
exploiting fibre links with transmission capacity in the range of
hundreds of Gbit/s.
	Regarding the switching issue, ATM is foreseen as the high
speed link layer technology able to carry IP datagrams between gigabit
links(2) .  However, the connection oriented nature of ATM leads
either to tag switching architectures that add redundancy to the
forwarding functionnality or to specific routing protocols such as
NHRP that give rise to potential inconsistency with the IP routing
protocols through the direct interconnection of routers that are not
aware of each other as neighbouring routers. In addition, the ATM cell
based link technology is precluded at gigabit rates so that an
additional high speed link layer such as SDH is mandatory to enable
the transport of ATM cells over the WDM wavelength carriers.
	Therefore we shall present a network architecture which allows
to directly interface IP with all optical WDM networks by means of
transparent optical packet networking. Here, transparency means that
the optical packet payloads carrying the encapsulated IP packets
remain in the optical domain within the WDM optical backbone while
headers are electronically processed consistently with the IP
protocols. As a result, futureproof scaling of the optical packet
switching capability is achieved with respect to the WDM optical
transmission bandwidth.  The overall architecture of the optical
packet network is discussed with respect to the interconnection of the
core switches by means of WDM links and regarding interworking with
electrical networks and the WDM transport network. As one of the major
issues that must be addressed is contention resolution, switch
architectures are presented where contention is solved along the time
dimension by means of fiber delay lines and along the wavelength
dimension thanks to wavelength conversion on a per packet basis.
Wavelength agility at the packet level allows to take advantage of the
wavelength resource that is specific to WDM networks and enables
statistical multiplexing at the fibre bandwidth capacity level. As a
result, the transfer delay and the packet delay variation issues are
strongly alleviated because of the small depth of the optical buffers
whereas the packet loss rate is kept similar to what is typically
expected from electronic switches even under bursty traffic streams.
Up to now, a time slotted operation is assumed for the optical
switches thanks to a fixed duration of the optical packets. The switch
design however naturally allows the use of variable duration packets
and further work will address this issue. Finally, let us mention that
a 4x4 demonstrator is being implemented as the EU ACTS project KEOPS
demonstrator. Its implementation demonstrates the feasibility of such
a switch with affordable optical components and electronics, both from
the general operation of the switch viewpoint and for switch
transparency feasibility.
	This work is funded by the European Union as the Advanced
Communication Technologies & Services KEOPS project (KEys to Optical
Packet Switching).

(1) M. Degermark & al., 'Small forwarding tables for fast routing
lookups', in Proceedings of ACM/SIGCOMM 97, Cannes, France, pp. 3-14,
1997.
(2) U. Briem & al., 'Trafic control for an ATM switch with per VC
queuing : concept and implementation', in Proceedings of International
Switching Symposium 97, Toronto, Canada, pp. 409-415, 1997.