New Vision for an All Optical Network
White paper for
Workshop on New Visions for Large-Scale Networks
submitted by
V. Anantharam, C. Chang-Hasnain, J. Kahn, D. Messerschmitt, D. Tse,
J. Walrand, and P. Varaiya
EECS Department, University of California, Berkeley
Optical packet switching needs to be fundamentally different in conception because of
the absence of a viable analog of the electronic packet buffer. We propose a vision for an
all optical network based on deflection routing, erasure codes at the packet level, and the
building of applications based on novel transport level primitives. Research is necessary
both at the physical layer, to design and develop the required optical switches, and at the
systems level, to define the necessary transport primitives and to develop the coding and
routing mechanisms that will realize them.
Contact information : Venkat Anantharam or Connie Chang-Hasnain, 231 Cory Hall,
EECS Department, University of California, Berkeley CA 94720.
ananth@eecs.berkeley.edu , 510-643-8435 (VA)
cch @eecs.berkeley.edu, 510-642-4315 (CCH)
New Vision for an All Optical Network
Optical packet switching needs to be fundamentally different in conception because of
the absence of a viable analog of the electronic packet buffer.
We envision research into a novel paradigm for design of an all optical network. The
research addresses both systems issues and physical layer issues. The physical layer
research is geared towards the design of faster optical switches with as much switching
flexibility (defined below) as feasible. The systems research is geared toward
incorporating ideas from coding theory into the networking arena and the building of
applications based on novel transport layer primitives.
We first describe the idea in general terms. Then we describe the physical layer
challenge and subsequently the systems level challenge.
We would like to create an optical packet switch without an optical buffer. Since
switching necessarily involves contention, this would seem to be an impossibility. To
understand why this is not the case, imagine the following simple problem : the design of
a 2 by 2 packet switch. Imagine that time is slotted. At each time slot, at each input, one
packet could arrive which might be destined to either one of the outputs. If the two
packets arriving in a time slot are both destined for the same output, one or the other
packet is going to lose.
However, imagine that the very same two packets show up once again in the next time
slot. Then the losing packet can now be the winner. Thus both packets have got through
to their desired output. More precisely, one copy of each packet has got through to the
desired output. The other copy has been misdirected and will eventually have to be taken
out of the network at subsequent switches. This can be done, for instance, by having the
notion of dead output directions at each switch which effectively bury packets.
One feature that emerges from the above discussion is the notion of redundancy. Indeed
packets need to be replicated and the guarantees that the network will have to provide at
the applications layer level is that the applications will work as long as a reasonable
fraction of the packets involved get through in the desired direction. This is part of the
systems challenge and will be discussed in that section. This a challenge both at the
application layer and at the transport layer : not only does one have to define what kinds
of transport functionality it is appropriate to treat as fundamental, but one also has to
define how such functionality is to be realized. Note that the way in which packet
redundancy is introduced will need to be much more sophisticated than mere packet
replication. This is because, first of all the traffic streams encounter multiple switches
between the source and the destination, and secondly while bandwidth on the links is
very large it might still pose a bottleneck for bandwidth hungry applications.
The second feature that emerges from the above discussion is the need for packet by
packet optical switching. At the current level of technology this is not yet feasible. The
physical layer challenge is to get as close to this goal as possible. This has two aspects to
it. One is to switch as small groups of packets as possible. This relates to the speed of the
switching (for instance the rate at which mirror angles can be changed in a MEMS mirror
based switch) relative to the packet rate. The second has to do with the topological
flexibility of the switch. This relates to which switching patterns one can move to from a
given switching pattern, and this is constrained by the technology, so it is a physical layer
issue. This is discussed in more detail in the section on the physical layer challenge.
The physical layer challenge
In spite of all the recent, greatly publicized research and product development activities,
there has not been successful demonstration of optical switches that have reasonable
dimension, i.e. larger than 32x32, and meet all the necessary performance, physical size
and power requirements. The necessary performance requirements include channel
crosstalk, insertion loss and reliability at the minimum. Most optical switching fabrics
suffer from scalability and/or crosstalk problems. The control electronics for the optical
switches, requiring multiple digital feedback loops for each physical switch, may be
larger and consume more power than the electronic switching fabric that the optics intend
to replace.
In addition to the above-mentioned issues, all of the all-optical switches are too slow for
all-optical packet-switched networks. Even though a single switch made of LiNbO3 or
semiconductor optical amplifier (SOA) can latch on within several nanoseconds, the
speed drastically slows down when the switch matrix dimension is increased. Simple
calculation shows that the speed is on microsecond scale for a 16x16 matrix. The
MEMS, liquid crystal, and thermal ink-jet based technologies are nearly three orders of
magnitude slower.
In this program, we propose to examine this problem by first separating an optical switch
into two major blocks of channel (wavelength) switch and fiber (space) switch, and
determining the application requirements for each. With the separation of channel-switch
and fiber-switch, the total hardware requirement for the switch fabric may be drastically
reduced, and thus potentially resolving the scalability problem. Crosstalk and speed
considerations will be examined separately for metro and WAN applications, where the
distance requirements and physical architectures are very different. By focusing on the
metro application, where the traffic is more bursty and LAN like and thus optical packet
switching can create a tremendous impact, we believe the performance requirements can
be greatly relaxed. Similar idea of focusing on a smaller size network for all-optical
packet switching is also discussed recently in the Optical Internetworking Forum.
Finally we address the speed of the optical switches. We propose to develop nanosecond
tunable VCSEL array technology, with full tuning range covering full C- and L- band.
These devices can perform cost-effective channel switching within a given fiber.
Although this form of wavelength switching is more optoelectronic rather than all-optical
in the strictest sense, it is generally believed that the performance and reliability can be
far better. Presently, the tuning and latching speed of commercial tunable lasers with a
wide tuning range is around 1-10 millisecond. In this program, we propose to explore
nanosecond tuning and latching design, including a proposal for nanosecond wavelength
locking. In addition, we plan to develop a space-based switch with nanosecond latching
time using either SOA or semiconductor electro-absorptive device technologies.
The systems level challenge
In our view this challenge is comprised of two interrelated parts :
(1) Defining appropriate transport layer primitives that facilitate the building of
applications over this radically different network.
(2) realizing these primitives in the architecture.
The issues related to the former will become clearer after a discussion of the latter,
so we discuss the latter challenge first.
Realizing transport layer primitives
As currently conceived the network supports two basic kinds of point to point transport
primitives : unreliable transport (UDP) and reliable end to end transport (TCP). In
addition there is a strong push to build in QoS related primitives, primarily using MPLS
(multiprotocol label switching), which is a link layer technology to set up label switched
paths and DiffServ, which defines per hop behaviour of packets through a DS field in the
packet. Further, there are some multicast primitives available. To succinctly illustrate that
challenges involved in realizing transport layer primitives in the new all optical network
we conceive of, let us focus on the basic example : achieving reliable end to end
transport.
As described in the introduction, packets in our all optical network will not be
guaranteed to leave a switch in the direction in which they would like to go. The way we
propose to overcome the problem posed by this is to introduce redundancy at the level of
packets. The controlled use of redundancy to overcome erasures of symbols is the
province of coding theory. A deep and extensive body of research has been built up over
the past few decades in this area. In brief coding for erasure permits the reliable transport
of a fixed number of symbols (packets representing the session in our case) by
transmitting a different, larger number of symbols, and providing schemes by which the
desired information can be recovered only if a fraction of the actually transmitted
symbols are received.
Coding to combat erasures has begun to make its appearance as a practical technique in
the networking arena. The primary driver behind this development, which is associated
with the concept of tornado codes and is being commercialized by a company called
Digital Fountain, is multicast streaming applications, where acknowledgement based
transport protocols like TCP lead to ack overload. Our conception of the use of erasure
correcting codes in the novel network is fundamentally different and poses several
significant new challenges.
To illustrate the issue, note that the applications we envision supporting are not just
streaming but also interactive applications and applications with stringent real time
constraints that cannot use buffers to smooth out jitter, as many streaming applications
do. This means that small bursts of packets will need to find their way through the
network from the source to destination in real time. Such a burst of packets will typically
encounter several switches along its route and the packets that comprise it face the
possibility of misdirection at each of the switches. Further, the switching speed of the
switches is expected to be slow relative to the packet rate (part of the physical layer
challenge is to mitigate this problem to the extent possible), so the misdirection will also
occur in bursts. How does one code to ensure that the packet burst nevertheless gets
through reliably ? This is a central problem we propose to tackle.
It should be noted that deflection routing has been proposed earlier before in the context
of interconnection networks. Here it is possible to reroute deflected packets by once again
injecting them into the network, giving them a second chance to go the correct way, as it
were. One does not expect this to be feasible in a large scale network. On the other hand,
there is the possibility of exploiting spatial redundancy. Indeed packets could be sent
along multiple paths from the source to the destination and the recovery of the intended
packet burst could be attempted based on the aggregate of all the packets received from
all the different paths. This possibility will play an important role in our research.
Defining appropriate transport layer primitives
A fundamental issue that arises in such any new network is : what are the appropriate
primitives to use on which to build applications ? In the network we envision, the
deflection of packets might be viewed not just as a disadvantage, but as a potential
advantage for realizing multiagent primitives. One of the most important emergent uses
of the network is in facilitating collaborative activities between small
(or occasionally large) groups of end users. This is radically different from the unicast
and multicast applications that have dominated so far. Such applications naturally need
the ability of work with primitives which ensure that information is replicated at certain
well defined subsets of hosts. Deflection in an all optical network along the lines we
propose gives a natural mechanism for establishing such consistency between the
participating hosts.
We propose to systematically study the issue of defining the best transport level
primitives in a deflection and erasure coding based all optical network. The research
approach we will follow is based on a compositional view of the function of a network,
as opposed to a decompositional view, i.e. we will work from the bottom up to define
mutually intercompatible basic objects from which larger scale applications of the desired
semantic characterisitcs can be easily created by a simple matter of picking and
interconnecting the appropriate components.