Optical interconnection technology in switches, routers and optical cross connects (länk till annan webbplats)

Optical interconnection technology in switches, routers and optical cross connects (länk till annan webbplats)
Jonsson, M., and U. Ohlin, "Optical interconnection technology in switches, routers and optical cross connects," Research Report
IDE - 0023, School of Information Science, Computer and Electrical Engineering (IDE), Halmstad University, Sweden, Nov. 2000.
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Optical interconnection technology in switches,
routers and optical cross connects
Magnus Jonsson and Ulf Olin1
School of Information Science, Computer and Electrical Engineering,
Halmstad University, Halmstad, Sweden, Box 823, S-301 18, Sweden
[email protected], http://www.hh.se/ide
1
Ericsson Generic Technologies, ERA/X/L
164 80 Stockholm
[email protected]
Abstract
The performance of data- and telecommunication equipment must keep up
with the increasing network speed. Moreover, to allow more input and
output ports on such equipment, the internal interconnection complexity
often grows exponentially with the number of ports. Therefore, new
interconnection technologies to be used internally in the equipment are
needed. Optic interconnection technology is a promising alternative and a
lot of work has been done. In this report, a number of optical and
optoelectronic interconnection architectures are reviewed, especially from
a data- and telecommunication equipment point-of-view. Three kinds of
systems for adoption of optical interconnection technology are discussed:
(i) optical cross connects (OXCs), (ii) switches and routers with some kind
of burst switching, and (iii) switches and routers which redirect traffic on
the packet or cell level. The reviewed interconnection technologies and
architectures are discussed according to their suitability of adoption in the
three mentioned systems.
The annex summarises manufacturers of devices required for optical
interconnects and backplanes, and needs for optical interconnection
technology in future Ericsson systems.
Keyword
Optical backplanes, optical interconnects, VCSELs, fibre-ribbon cables
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Contents
1
2
2.1
2.2
2.3
3
3.1
3.2
3.3
3.4
4
4.1
4.2
4.3
4.4
5
5.1
5.2
5.3
6
6.1
6.2
7
7.1
7.2
7.3
7.4
8
9
10
10.1
10.2
1
Introduction
Electronic switches and routers
Circuit switching
Packet switching
Switch fabrics
Optical link technologies
Fibre-ribbon links
Bit-parallel WDM links
Reliability of optoelectronics
Major research activities
Fibre-optic interconnection networks
Passive fibre-optic networks
WDM star
WDM ring
Fibre-ribbon pipeline ring network
Integrated optical interconnection systems
Integrated fibre and waveguide solutions
Planar free space optics
Free space optical backplanes
Optical and optoelectronic switch-fabrics
Optical interconnections and electronic crossbars
WDM/SDM switches
Commercial system implementations
Sycamore
Pluris
Sirocco Systems
InfiniBand Trade Association
Conclusions
References
Annex
Commercial solutions
Demands from future Ericsson products
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43
Introduction
Novel optical technologies result in the possibility of new solutions for the
increasing bandwidth demands of data communication and
telecommunication equipment. In this report, we explore the possibilities,
from an architectural and a device perspective, of using optical
interconnections in such equipment. Although there exist many other
optical interconnection architectures that might be candidates, only some
selected groups or concepts are selected here to give a reasonably broad
view of possible solutions.
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The interconnection architectures are evaluated according to three types of
systems, which mostly varies in terms of switching time requirements. The
systems are: (i) optical cross connects (OXCs), (ii) switches and routers
with some kind of burst switching, and (iii) switches and routers which
redirect traffic on the packet or cell level.
OXCs have relatively slow timing requirements, i.e., in the order of
milliseconds or even tens of milliseconds. On the other hand, it is valuable
if the signals remain optical all the way through the switch including
possible queuing systems, i.e., having optical transparency. In this way it is
easier to scale up the bit rate or change protocols.
When using burst switching, packets with the same destination (e.g., the
same exterior gateway) can be grouped together to reduce switching time
requirements. This has, for instance, been discussed for use in all optical
packet switching [Callegati et al. 1999] [Turner 1999]. The switching time
requirements are in the order of sub-microseconds.
For pure switching at the packet or cell level, the switching time
requirements are in the order of a nanosecond. With longer switching
times, the overhead between each packet will be too large. All-optical
packet switching has been proposed but is far from a mature technology
[Callegati et al. 1999]. However, some experiments have been done
[Blumenthal et al. 1999] [Chiaroni et al. 1998]. Another driving application
domain for optical system-level interconnections, in addition to data- and
telecommunication equipment, is parallel and distributed computing
systems.
The rest of the report is organised as follows. Electronic switches and
routers are briefly reviewed in Section 2. In Section 3, optical link
technologies are described, and in Section 4, fibre-optic interconnection
networks are presented. Integrated optical interconnection systems and
optical and optoelectronic switch-fabrics are presented in Sections 5 and 6,
respectively. The report is then concluded in Section 7. The annexes
summarise commercial devices of interest for optical backplanes and
interconnects and the needs from Ericsson product units for optical
interconnects and backplanes.
2
Electronic switches and routers
Data communication networks can be divided into circuit switching and
packet switching networks. These two categories will be treated in
Subsections 2.1 and 2.2, respectively. Then, in Subsection 2.3, switch
fabrics to be used as the core switching part of networking equipment will
be discussed.
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Circuit switching
When using circuit switching, a "physical" channel is allocated before the
communication between a pair of nodes can start. The "physical" channel
does not need to be a purely physical but can, e.g., be a cyclically
available time slot on a time multiplexed channel, i.e., Time Division
Multiplexing (TDM). SDH is an example of a communication standard that
relies on TDM. The advantage of circuit switching is the guaranteed
bandwidth, while disadvantages are long set-up times and low bandwidth
utilisation when the channel is idle for a long time, since the bandwidth
normally cannot be reused.
Figure 1: Some wavelengths can be dropped and/or added, while other
wavelengths just pass through.
The switching time requirements for circuit switching networks are typically
not so demanding because of the duration of channels. However, in, e.g.,
an SDH Add-Drop Multiplexer (ADM), all traffic (including bypassing traffic)
must be electronically processed in some sense. It can, for instance, be
needed to be able to separate traffic with different destinations (e.g., be
dropped or pass) from different time slots. By multiplexing in the
wavelength domain instead of the time domain, a wavelength only carrying
traffic which should pass do not have to be processed at all (see Figure 1).
2.2
Packet switching
When using packet switching, the data to be transferred are split into
packets that compete for bandwidth with packets from other nodes. Traffic
situations with temporary bursts of large volumes of data from one or a few
sources can therefore be handled better in a packet-switched network than
in the case that much of the bandwidth is allocated by other nodes using
circuit switching. Also, sporadic traffic often experiences a shorter latency
than in the case that a circuit must be set-up each time. Handling real-time
traffic is, however, harder in a packet-switched network. Two main kinds of
packet switches are ATM switches and IP routers.
Packet switches can be built in a number of ways for which we here will
give two examples of architectures. The first packet switch architecture
consist of input interfaces, input queues, switch fabric, output queues, and
output interfaces, where some or all of the listed units are coupled to a
central control unit (see Figure 2). The main function of the control unit is
to configure the switch fabric to pass packets queued in the input queues
to the correct output queues, based on some routing decision. Other
queuing strategies are possible too, e.g., only at the input side or the
output side, a larger shared queuing memory, or internally in the switch
fabric.
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Switch
fabric
Control unit
Figure 2: An example of switch architecture.
One of the simplest ways to implement a switch fabric is to have a shared
medium to which I/O interfaces or similar are attached. A common way of
implementing a shared-medium network is to use the bus topology, but it
can also be, e.g., a ring where only one node is allowed to send at a time.
The great advantage of a shared-medium network is the easy
implementation of broadcast, which is useful in many situations. The
disadvantage is that the bandwidth does not scale at all with the number of
nodes.
The second example of a packet switch architecture is a bus to which I/Ointerfaces and a control unit (routing processor), or units, are connected
(see Figure 3). Incoming packets are transferred from an I/O interface to
the control unit. The control unit makes routing decisions or similar and
redirects each packet to the correct I/O interface for transmission out on
the network again. Sometimes, the I/O interfaces have enough intelligence
to redirect packets directly to the destination I/O interface, at least for some
kind of traffic. More information on switch and router architectures is found
in [Kou 1999].
Control unit
(e.g., routing
processor)
Figure 3: Bus-based switch architecture.
2.3
Switch fabrics
The crossbar is the most flexible switch fabric and can be compared with a
fully connected topology, i.e., point-to-point connections between all
possible combinations of two nodes. The drawback, however, is the
increase by N2 in cost/complexity of the switch, where N is the number of
ports. Systems with a single true crossbar are therefore limited to small
systems.
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Receiver side
Figure 4: Omega network for an eight-node system. One path through the
network is highlighted.
In multistage shuffle-exchange networks, the cost function is reduced to
N log2 N, but where log2 N stages must be traversed to reach the desired
output port. An example of such a network is the Omega network (Figure
4) which provides exactly one path from every input to every output. The
four different switch functions of the 2 × 2 switch that is used as building
block are shown in Figure 5, where the two rightmost configurations are
used for broadcast. Switches larger than 2 × 2 can also be used. Each
stage of the switches in an Omega network is preceded by a perfectshuffle pattern. In contrast to a crossbar network, which is a nonblocking
network, an Omega network is a blocking network. This means that there
might not always exist a path through the network as a result of already
existing paths that block the way.
Figure 5: Possible states of a 2 × 2 switch.
Rearrangeable networks are another category of multistage networks
where it is always possible to find a path through the network. However, if
not all paths are routed at the same time, it may be necessary to reroute
already existing paths. An example of a rearrangeable network is the
Benes network shown in Figure 6. Other multistage networks include
Banyan networks [Goke and Lipovski 1973].
Figure 6: A rearrangeable Benes network.
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Optical link technologies
Historically, the use of optical fibres for signal interconnection began with
the simplest level of point-to-point fibres in telecommunications long-haul
networks. However, today, as the complexity and the bit rates increase in
switching and routing equipment, optical interconnects are starting to be
implemented in the equipment building practice to satisfy the distance and
bandwidth requirements.
Connections are usually made from the optoelectronics on printed wiring
boards or cards to those on other boards on the same or other shelves.
Other types of optical interconnections are between cabinets of
multicabinet equipment (such as digital crossconnect switches) or between
the central processing units and remote memories of high-performance
workstations and computers.
According to ElectroniCast Corp. [Montgomery 2000] the market for optical
backplanes is expected to grow from a total of $6.1 million in 1998 to $125
million in 2003 and $1 647 million in 2008. In 1998, two thirds of the total
global demand of optical backplanes came from telecom transport
terminals and switches.
The potential advantages with optical interconnects are that
• Higher interconnect bit rate per circuit area can be achieved
• Noise is not generated as for wire interconnects
• Scaleable interconnection bandwidths are possible
• The low loss in waveguides and free-space makes longer connections
possible
Today, for high data rate (100’s of Mbps to over 1 Gbps) electrical
connections, twinned-pair LVDS (Low Voltage Differential Signalling) and
related technologies are used. With improved impedance matching
between the driver and termination circuits and the cable, data rates up to
2-4 Gbps are anticipated to be achieved. For short distances, 0,5 – 1 m,
also bit rates up to 10 Gb/s are considered using new board materials.
In the table below is shown a comparison between a commercial parallel
optical fibre transmitter [Mitel] with 12 channels, each operating at 2.5
Gbps, and a 400 Mbps serial bus LVDS circuit [National]. The last column
shows a few years old goals from a DARPA program on optical
interconnection technologies [Towe]. It is evident that today’s VCSELbased fibre ribbon cable technology has properties comparable to those of
electrical interconnections, when the full capacity of the fibre-ribbon links
are utilised.
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VCSEL
array
LVDS
circuit
DARPA
goals 1997
14.7
18.8
10
Data rate/circuit area (Gb/s /
cm2)
4
0.07
10
Data rate/power consumption
(Gb/s / W)
15.8
3.8
20
Price/data rate ($ / Gb/s)
Table 1. Comparison of characteristics for VCSEL array and LVDS circuit
Below, two main categories of optical link technologies are discussed,
fibre-ribbon links (Subsection 3.1) and bit-parallel WDM links (Subsection
3.2). Single-channel single-fibre solutions are not treated.
3.1
Fibre-ribbon links
A system component that has reached the market recently [Bursky 1994]
[Fibre Systems 1998] is the fibre-ribbon link [Buckman et al. 1998]
[Engebretsen et al. 1996] [Hahn 1995] [Hahn 1995B] [Hahn et al. 1996]
[Hartman et al. 1990] [Jiang et al. 1998] [Karstensen et al. 1995]
[Karstensen et al. 1998] [Kuchta et al. 1998] [Nagarajan et al. 1998]
[Nishimura et al. 1997] [Nishimura et al. 1998] [Schwartz et al. 1996] [Siala
et al. 1994] [Wickman et al. 1999] [Wong et al. 1995]. Several links can be
used to build high bandwidth point-to-point linked networks [Hahn et al.
1995]. With ten parallel fibres, each carrying data at a bit rate of 400
Mbit/s, an aggregated bandwidth of 4 Gbit/s is achieved [Schwartz et al.
1996]. Bi-directional links, with some fibres in the fibre-ribbon cables
dedicated for each direction, are also possible [Jiang et al. 1995]. More
references to reports on fibre-ribbon links are found in [Tooley 1996].
Modules that support multiple high-speed channels but are not specifically
optimised for fibre-ribbons have been reported, e.g., receiver and
transmitter modules with five channels, each channel with a bit rate of 2.8
Gbit/s [Nishikido et al. 1995].
In addition to the high bandwidth offered by a fibre-ribbon cable, it also
offers a ten-fold increase in packing density as compared to electrical
cables, resulting in less rigid cables [Karstensen et al. 1995]. Furthermore,
it is not necessary for the designer to be concerned about electromagnetic
emissions.
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Skew in parallel links
In scaling up the bandwidth of a fibre-ribbon link where a dedicated fibre
carries the clock signal, the main problem is channel-to-channel skew. The
skew is mainly the result of differences in propagation delay between
different fibres and variations of lasing delay time among different laser
diodes [Kurokawa et al. 1998]. The 400 Mbit/s OPTOBUS has a specified
maximum skew of 200 ps, excluding the fibre-ribbon cable for which 6
ps/m is assumed for standard ribbons [Schwartz et al. 1996]. Even if it is
rather short distances in the kind of systems discussed in this report, the
scaling to higher speeds calls for the discussion of techniques to reduce
the effect of the skew below.
The skew is affected by intrinsic optical properties of the fibres in the
ribbon, but also by the effect of the ribbon process on the fibres. Ribbon
process parameters affecting the skew are fibre tension, fibre excess
length and winding. In short, you should have a small group index
difference, e.g. use fibres from the same preform or part of a preform, and
have good control of fibre tensions in the ribbon process.
One technique is to actually reduce the skew, either by using low skew
ribbons or employing skew compensation. Fibre-ribbons with about 1 ps/m
skew [Siala et al. 1994] and below [Kanjamala and Levi 1995] have been
developed, which essentially increases the possible bandwidth distance
product. All the fibres in the same ribbon were sequentially cut to reduce
the variation of refractive index among the fibres. In the fibre-ribbon link
described in [Wong et al. 1995], a dedicated fibre carries a clock signal
used to clock data on 31 fibres. The transmitter circuitry for each channel
has a programmable clock skew adjustment to adjust the clock in 80-ps
increments.
The problems with skew in modern ribbon fibres are now attracting large
R&D efforts, c.f. example [Jason and Arvidsson 2000].
3.1.2
Clock recovery
The clock-recovery circuit in high-speed serial links needs a data stream
with a high density of transitions. Commercially available
serialiser/deserialiser chip-sets use line-codes such as 8B10B to provide a
proper transition density. Both SAW devices and phase-locked loops are
used for clock extraction.
The disadvantage of extracting the clock signal from the bit flow on each
fibre is increased hardware complexity when adding a clock recovery
circuit and a buffer circuit for each channel in the receiver. A hybrid
solution is to skip the separate clock channel and encode clock information
on the data channels while still sending in bit-parallel mode, as reported in
[Yoshikawa et al. 1997] [Yoshikawa et al. 1997B]. In this case, a deskew
unit relying on FIFO registers (First In First Out) ensures that parallel data
words that are output from the receiver are identical to those which were
sent. A possible ± 15-ns deskew was reported. A similar system is
reported in [Fujimoto et al. 1998].
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The techniques mentioned above introduce either increased hardware
complexity or a more sophisticated fibre-ribbon manufacturing process. If
the manufacturing process allows for adding more fibres in each ribbon,
this may be a cheaper alternative. For example, 12 channel links with 1
Gbit/s per channel [Karstensen et al. 1995] and 2 Gbit/s per channel
[Karstensen 1995] have been reported, and array modules supporting 12 ×
2.4 Gbit/s for, e.g., fibre-ribbon links were described in [Peall 1995]. A
fibre-ribbon link with 32 fibres, each with a bit rate of 500 Mbit/s, was
described in [Wong et al. 1995], and researchers at NEC have developed
a module in which 8 × 2 lasers are coupled to two fibre-ribbons [Kasahara
1998]. Instead of fibre-ribbons, fibre-imaging guides (FIGs) with thousands
of pixels can be used [Li et al. 1995]. In the system described in [Li et al.
1998B], both a 14000-pixel FIG and a 3500-pixel FIG were coupled to an 8
× 8 VCSEL array in different set-ups.
3.2
Bit-parallel WDM links
Another way is to synchronously transmit on several channels in parallel,
i.e., bit-parallel byte-serial transmission [Loeb and Stilwell 1988] [Loeb and
Stilwell 1990]. However, compensation for bit-skew caused by group delay
dispersion (different wavelength channels travel at different speeds in the
fibre) may be needed in these systems [Jeong and Goodman 1996].
A dedicated wavelength in a bit-parallel WDM link can be used for clock
information. Wavelengths (or fibres in a fibre-ribbon cable) can also be
dedicated to other purposes such as frame synchronisation and flow
control. Significantly higher bandwidth distance products can be achieved
when using bit-parallel WDM over dispersion shifted fibre instead of fibreribbons [Bergman et al. 1998] [Bergman et al. 1998B]. If, however, there is
only communication over shorter distances (e.g., a few meters), the
bandwidth distance product is not necessarily a limiting factor.
Transmission experiments with an array of eight pie-shaped VCSELs
arranged in a circular area with a diameter of 60 µm, to match the core of a
multimode fibre, have been reported [Coldren et al. 1998]. Other work on
the integration of components for short distance (non-telecom) WDM links
has been reported, e.g., a 4 × 2.5 Gbit/s transceiver with integrated splitter,
combiner, filters, etc. [Aronson et al. 1998].
3.3
Reliability of optoelectronics
The main degradation modes of laser diodes are: dislocations that affect
the interior region, metal diffusion and alloy reaction that affect the
electrodes, solder instability that affect the bonding parts, facet damage,
etc. The degradation rate increases with increasing temperature and
current. By making life time tests at different temperatures and currents it
is possible to determine an acceleration factor
AF = TTF1 / TTF2 = (I F 1 / I F 2 ) ×exp[
E A / k B ×(1 / TJ 2 − 1 / TJ 1 )],
n
(1)
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where TTFi is Time To Failure at operation condition i, IFi is the current
through the device, n is an exponential in the range of 1.5 – 2 determined
from the test data, EA is an activation energy also determined from the test
data, kB is the Boltzmann constant, and TJi is the junction temperature. The
activation energy is in the range of 0.7 – 0.8 eV for lasers.
VCSELs from Honeywell have median lifetimes, for normal drive currents
and room temperature operation, of around 9 MHours [Honeywell]. That is,
on an average a device works without failure for 1 000 years! If the drive
current is increased 50 % and the ambient temperature is increased from
25 ºC to 70 ºC, the median lifetime is down to 10 years. Similar
performance has also been reported for VCSELs from Infineon and
Hewlett Packard [Wipiejewski et al. 1999, Lei et al. 1999].
The reliability of a system, like AXD 301, is normally determined using
analyses based on Markov models. Using this technique, it is possible to
estimate MTBSF (Mean Time Between System Failures), given the
individual components’MTTF (Mean Time To Failure), their redundancy
and the MDT (Mean Down Time) of the system at repairs.
Device lifetimes in the range of 10 – 40 years are normally sufficient to
keep the MTBSF at an acceptable level. It is concluded that the VCSEL
technology now is sufficiently mature to satisfy these requirements.
3.4
Major research activities
3.4.1
POLO
Hewlett-Packard Laboratories is leading an industrial consortium, the
Parallel-Optical Link Organization (POLO), which is engaged in the
development of economical, high-performance, parallel-optical links. Its
purpose includes the development of device, packaging, and
interconnection technologies and standards. Other members of POLO
working on the VCSEL-based link are AMP (develops connectors and
housings), DuPont (provides polymer waveguide technologies), and USC
(demonstrates workstation interfacing and networking). The consortium is
supported by DARPA.
3.4.2
POINT
The Polymer Optical Interconnect Technology (POINT) program is a
collaborative effort among GE, Honeywell, AMP, AlliedSignal, Columbia
University, and UC San Diego, sponsored by DARPA/ETO, to develop
affordable optoelectronic packaging and interconnect technologies for
board and backplane applications. Specifically, progress has been
reported on:
• Development of a plastic VCSEL array packaging technology using
batch and planar fabrication
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• Demonstration of high-density optical interconnects for board and
backplane applications using polymer waveguides to a length of 50 cm
at an I/O density of 250 channels per inch
• Development of low-loss optical polymer waveguide with loss less than
0.1 dB/cm at 850 nm
• Development of passively alignment processes for efficient coupling
between a VCSEL array and polymer waveguides
3.4.3
ChEEtah
Honeywell is involved in a number of DARPA-sponsored research projects
on optical interconnection technologies. Cost Effective Embedding of
Parallel Optical Interconnects (ChEEtah) is one of these projects. The
objective of the Honeywell-led ChEEtah program is to develop parallel
optical links achieving cost parity with copper interconnects, and that
address the need for a high bandwidth-density product across a backplane
over distances ranging from 0.3 to 1 meter, and between boxes over
distances ranging from 1 to 100 meters. The goal is to reduce the
optoelectronic transceiver function to the minimum number of parts, and to
leverage many of the advances in optical component, IC and optical and
electrical packaging technology that have recently occurred. Relevant
advances that will be incorporated into this program include: 1) low
threshold, high efficiency VCSEL designs, 2) yield, uniformity and reliability
demonstrated in VCSEL arrays, 3) the application of low power, high
speed CMOS I/O buffer circuits, 4) the development of high strength
optical fibre with a small bend radius and techniques for laminating fibre
arrays onto a printed circuit board, 5) low cost parallel fibre cabling and
connector assembly techniques, and 6) mechanical features for optical
self-alignment.
4
Fibre-optic interconnection networks
Fibre-optic network architectures, especially passive optical networks are
discussed below. First, in Subsection 4.1, different basic passive fibre-optic
network architectures are described. Then, in Subsections 4.2 and 4.3,
WDM star and WDM ring networks are respectively discussed. Fibreribbon ring networks are presented in Subsections 4.4.
4.1
Passive fibre-optic networks
In an all-optical network, the data stream remains in the optical form all the
way from the transmitter to the receiver. Three basic architectures for alloptical multi-access networks are the ring, the bus, and the star (see
Figure 7). These network architectures will be discussed below. Most work
on passive optical networks have been focused on LANs or similar but
they can be used as substitutes for switch fabrics in data- and
telecommunication equipment too. It should be noted that if one of these
networks is used in an OXC (or similar) as a switch-fabric, the signal is
converted to electrical and back to optical form at the entrance and the exit
of the switch-fabric.
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(a)
(b)
(c)
Figure 7: Three passive optical network architectures: (a) ring, (b) (dual)
bus, and (c) star.
An all-optical multi-access ring network differs from a traditional ring
network in the sense that all other nodes can be reached in a single hop
without any intermediate optoelectronic conversion. In contrast to the
repeating function of a node in, for example, an FDDI network, messages
simply pass a node through passive optics in an all-optical multi-access
ring network. This is true for all messages except a node's own messages
that should be removed from the ring after one round. Just a fraction of the
optical power contained in the bypassing fibre is tapped to the receiver,
which gives all nodes the opportunity to read the message, i.e., a multicast
(one to many) or a broadcast (one to all). Outgoing messages are inserted
into the ring and, in a multi-channel system, mixed together with bypassing
messages on other channels.
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Figure 8: Folded fibre-optic bus.
In an optical bus, the light travels only in one direction, making it necessary
to have two buses (upper and lower), one for each direction (higher or
lower node index of destination nodes). This kind of bus architecture is
called dual bus. The disadvantage of the dual bus is that two transceivers
are needed in each node. This is avoided in the folded bus, where the two
buses are connected with a wrap-around connection at one end of the
buses (see Figure 8) [Tseng and Chen 1982]. In the folded bus,
transmitters are connected to the upper bus while receivers are connected
to the lower bus. Several bus architectures and hybrids in which the bus is
part of the architectures are discussed in [Nassehi et al. 1985].
In a star network, the incoming light waves from all nodes are combined
and uniformly distributed back to the nodes. In other words, the optical
power contained in the middle of the star is equally divided between all
nodes.
All of the three basic network architectures have different advantages. The
ring has the least amount of fibres, a bus network’s medium access
protocol can utilise the linear ordering of the nodes [Nassehi et al. 1985]
and the attenuation for an (ideal) star only grows logarithmically with the
number of nodes. However, star networks are the most popular, judging
from the number of published papers.
The passive optical networks that only offer one shared channel are no
promising alternatives. However, they form the basis for more powerful
networks using WDM. These networks can be promising solutions as
switch fabrics in some kinds of data- and telecommunication equipment,
even though the main target is LANs and similar.
λ1
λ2
Node 1
Node 2
λ 1 ,λ 2 ... λ Ν
λ 1 ,λ 2 ... λ Ν
λΝ
Node N
λ 1 ,λ 2 ... λ Ν
Figure 9: WDM star network.
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WDM star
A passive fibre-optic star distributes all incoming light on the input ports to
all output ports. A network with the logical function of a bus is obtained
when connecting the transmitting and receiving side of each node to one
input and output fibre of the star, respectively. By using WDM, multiple
wavelength channels can carry data simultaneously in the network
[Brackett 1990]. In other words, each channel has a specific colour of light.
A flexible WDM network requires tunable receivers and/or transmitters, i.e.;
it should be possible to send/listen on an arbitrary channel [Mukherjee
1992].
Figure 9 shows an example of a WDM star network configuration. Each
node transmits on a wavelength unique to the node, while the receiver can
listen to an arbitrary wavelength. The configuration is used in the TDTWDMA network [Jonsson et al. 1996], which has support for
guaranteeing real-time services, both in single-star networks [Jonsson et
al. 1997] and star-of-stars networks [Jonsson and Svensson 1997]. One
can say that this kind of network architecture implements a distributed
crossbar. The flexibility is hence high, and multicast and single-destination
traffic can co-exist. The number of wavelengths is practically limited to 1632 [Brackett 1996], but, as stated above, hierarchical networks with
wavelength reuse can be built. WDM star networks have, in addition to
LANs and similar networks, been especially proposed for internal use in
packet switches [Eng 1988] [Brackett 1991] [Sadot and Elhanany 2000].
Tunable components (e.g., filters) with tuning latencies in the order of a
nanosecond have been reported, but they often have a limited tuning
range [Kobrinski et al. 1988]. At the expense of longer tuning latencies,
however, components with a broader tuning range can be used [Cheung
1990]. Such components can be used to achieve a cheaper network in
systems where much of the communication patterns remain constant for a
longer period, e.g., circuit-switching.
λ1
λ2
λ3
λ4
Node 1
Node 2
Node 3
Node 4
λ 2 ,λ 4
λ 1 ,λ 3
λ 2 ,λ 4
λ 1 ,λ 3
Figure 10: WDM star multi-hop network.
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Receiver side
λ2
Node 1
λ4
Node 2
Node 3
λ1
λ2
λ3
λ2
λ3
λ4
Node 2
Node 3
λ1
Node 4 λ
4
λ 3 Node 4
Figure 11: Multi-hop topology.
Complete removal of the ability to tune in a WDM star network gives a
multi-hop network [Mukherjee 1992B]. Each node in a multi-hop network
transmits and receives on one or a few dedicated wavelengths. If a node
does not have the capability of sending on one of the receiver wavelengths
of the destination node, the traffic must pass one or several intermediate
nodes. The wavelengths can be chosen to get, e.g., a perfect-shuffle
network [Acampora and Karol 1989]. One can also choose to have a
network in which several topologies are embedded, e.g., a ring and a
hypercube. An example of a multi-hop network is shown in Figure 10. This
configuration of wavelength assignments corresponds to the topology
shown in Figure 11. Dynamic real-time scheduling can be done in a multihop network [Yu and Bhattacharya 1997]. The method works like static
scheduling, but here a central node runs the scheduling algorithm and high
priority messages might pre-empt low priority messages. The highest
priority level is used for messages with hard deadlines, while the other
levels are used for messages with soft deadlines. Lower priority levels are
used for less important messages.
4.3
WDM ring
A WDM ring network utilises ADMs in all nodes to insert, listen, and
remove wavelength channels to/from the ring. In the WDMA ring network
described in [Irshid and Kavehrad 1992], each node is assigned a nodeunique wavelength on which to transmit. The other nodes can then tune in
an arbitrary channel on which to listen. This configuration is logically the
same as that for the WDM star network with fixed transmitters and tunable
receivers. The distributed crossbar again gives good performance for
general communication patterns.
Spatial wavelength reuse can be achieved by removing the transmitted
light at the destination node (last destination node for multicast). At high
degrees of nearest downstream neighbour communication, throughputs
significantly higher than 1 can be achieved for a single wavelength. In this
way, a smaller number of wavelength channels are needed.
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Fibre-ribbon pipeline ring network
Bit-parallel transfer can be utilised when fibre-ribbon cables/links are used
to connect the nodes in a point-to-point linked ring network. In such a
network, one of the fibres in each ribbon is dedicated to carry the clock
signal. Therefore, no clock-recovery circuits are needed in the receivers.
Other fibres can be utilised for, e.g., frame synchronisation. Figure 12
shows how a ring network is used as a switch-fabric.
Figure 12: Ring network as switch-fabric.
Node 1
Node 2
Node M
Node 3
Node 5
Node 4
Figure 13: Example of spatial bandwidth reuse. Node M sends to Node 1
at the same time as Node 1 sends to Node 2 and Node 2 sends a
multicast packet to Nodes 3, 4, and 5.
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As seen in Figure 13, aggregated throughputs higher than 1 can be
obtained in ring networks with support for spatial bandwidth reuse
(sometimes called pipeline rings) [Wong and Yum 1994]. This feature can
be effectively used when most of the communication is to the nearest
down stream neighbour. Two fibre-ribbon pipeline ring networks have
recently been reported [Jonsson 1998B]. The first network has support for
circuit switching on 8+1 fibres (data and clock) and packet switching on an
additional fibre [Jonsson et al. 1997B]. The second network is more flexible
but is a little more complex, and has support for packet switching on 8+1
fibres and uses a tenth fibre for control packets (see Figure 14) [Jonsson
1998]. The control packets carry MAC information for the collision-less
MAC protocol with support for slot reserving. Slot reserving can be used to
get RTVCs (Real-Time Virtual Channels) [Arvind et al. 1991] for which
guaranteed bandwidth and a worst-case latency are specified (compare
with circuit switching). The fibre-ribbon ring network can offer rather high
throughputs due to the aggregated bandwidth of a fibre-ribbon cable but
the spatial reuse will probably be limited due to the general traffic.
Node 1
Node 2
Node M
Node 3
= Packet-switched data
channel (8 fibers)
= Control channel (1 fiber)
Node 5
Node 4
= Clock channel (1 fiber)
Figure 14: Control channel based network built up with fibre-ribbon pointto-point links.
Another fibre-ribbon ring network is the PONI network (formerly USC
POLO), which is proposed to be used in clusters of workstations and
similar systems [Raghavan et al. 1999] [Sano and Levi 1998]. Integrated
circuits have been developed for the network, and tests have been
performed [Sano et al. 1996] [USC 1997].
5
Integrated optical interconnection systems
Below, three kinds of systems were optical interconnections are integrated
into a more or less pure interconnection system, are presented. Such an
interconnection system can be used, e.g. to interconnect electronic switch
chips or I/O interfaces. In Subsection 5.1, integrated fibre and waveguide
solutions are presented, while planar free space optics and free space
optical backplanes are discussed in Subsections 5.2 and 5.3, respectively.
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Integrated fibre and waveguide solutions
Fibres or other kinds of waveguides (hereafter commonly denoted as
channels) can be integrated to form a more or less compact system of
channels. Fibres can be laminated to form a foil of channels, for use as
intra-PCB (Printed Circuit Board) or back-plane interconnection systems
[Eriksen et al. 1995] [Robertsson et al. 1995] [Shahid and Holland 1996].
Fibre-ribbon connectors are applied to fibre end-points of the foil. An
example is shown in Figure 15, where four computational nodes are
connected in a ring topology. In addition, there is a clock node that
distributes clock signals to the four computational nodes via equal-length
fibres to keep the clock signals in phase. In other words, a fibre-optic clock
distribution network [Kiefer and Swanson 1995] and a data network are
integrated into one system. If one foil is placed on each PCB in a rack,
they can be passively connected to each other via fibre-ribbon cables.
Using polymer waveguides instead of fibres brings advantages such as the
possibility of integrating splitters and combiners into the foil, and the
potential for more cost-effective mass production [Eriksen et al. 1995].
Node 1
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Out
Node 4
In
Out
Node 2
In
In
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Node 3
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Figure 15: A foil of fibres connects four computational nodes. In addition, a
clock node distributes clock signals to the computational nodes.
Integrated systems of channels can be set-up and used in a number of
configurations, some of which are discussed below. One way is to embed
a ring with bit-parallel transmission and the possibility for spatial bandwidth
reuse, as described in Subsection 4.4; the medium is simply changed into
a more compact form. Besides pure communication purposes, channels
for, e.g., clock distribution (as seen in the example) and flow control can be
integrated into the same system.
Fiber-ribbon
Node 1
Node 2
Node M
Figure 16: Array of passive optical stars connects a number of nodes via
fibre-ribbon cables.
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Another way is to follow the proposed use of an array of passive optical
stars to connect processor boards in a multiprocessor system via fibreribbon links, for which experiments with 6 x 700 Mbit/s fibre-ribbon links
were done (see Figure 16) [Parker 1991] [Parker et al. 1992]. Of course,
the processor boards can be exchanged with, e.g., transceiver cards
and/or switch cards. As indicated above, such a configuration can be
integrated by the use of polymer waveguides. The power budget can,
however, be a limiting factor to the number of nodes and/or the distance.
Advantages are simple hardware owing to bit-parallel transmission (like
other fibre-ribbon solutions) and the broadcast nature, but the star array
can become a bottleneck as in all bus-like systems. In a similar system,
the star array is exchanged by a chip (with optoelectronics) that has one
incoming ribbon from each node and one output ribbon [Lukowicz et al.
1998]. The output ribbon is coupled to an array of 1 × N couplers so that
each node has a ribbon connected to its receiver. The chip couples the
incoming traffic together in a way that simulates a bus. At contention, the
chip can temporarily store packets.
Electronic crossbars can be distributed on the PCBs and/or placed on a
special switch PCB in a back-plane system, and be connected by
integrated parallel channels.
Other similar systems include the integration of fibres into a PCB for the
purpose of clock distribution [Li et al. 1998]. Distribution to up to 128 nodes
was demonstrated. The fibres are laminated on one side of the PCB, while
integrated circuits are placed on the reversed side. The end section of
each fibre is bent 90 degrees to lead the light through a so called via hole
to the reversed side of the PCB.
5.2
Planar free space optics
By placing electronic chips (including optoelectronic devices) and optical
elements on a substrate where light beams can travel, we get a planar free
space system (Figure 17) [Jahns 1994] [Jahns 1998] [Sinzinger 1998].
Electronic chips are placed in a two-dimensional plane, while light beams
travel in a three-dimensional space. In this way, optical systems can be
integrated monolithically, which brings compact, stable and potentially
inexpensive systems [Jahns 1998]. By using, e.g., Spatial Light Modulators
to dynamically direct the optical beams, a flexible interconnection network
can be obtained. Using only fixed interconnection patterns and electronic
switching can, however, give shorter switch times. A planar free space
optical crossbar switch has been reported [Reinhorn et al. 1999].
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Figure 17: Example of a planar free space system. The direction of the
beam is steered by the optical element on the way between two chips.
5.3
Free space optical backplanes
Several different optical backplanes have been proposed, three of which
are discussed below. As shown in Figure 18a, using planar free space
optics is one means of transporting optical signals between PCBs.
Holographic gratings can be used to insert and extract the optical signals
to/from the waveguide, which may be a glass substrate [Zhao et al. 1995].
Several beams or bus lines can be used, i.e., each arrow in the figure
represents several parallel beams [Zhao et al.1996].
(a)
(b)
(c)
Figure 18: Optical backplane configurations: (a) with planar free space
optics, (b) with smart pixel arrays, and (c) with a mirror.
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In the system shown in Figure 18b, two-dimensional arrays of optical
beams (typically 10 000) link neighbouring PCBs together in a point-topoint fashion [Szymanski 1995] [Hinton and Szymanski 1995]. Smart pixel
arrays then act as intelligent routers that can, e.g., bypass data or perform
data extraction operations where some data pass to the local PCB and
some data are retransmitted to the next PCB [Supmonchai and Szymanski
1998]. Each smart pixel array can typically contain 1 000 smart pixels
arranged in a two-dimensional array, where each pixel has a receiver, a
transmitter, and a simple processing unit. One way of configuring the
system is to connect the smart pixel arrays in a ring, where the ring can be
reconfigured to embed other topologies [Szymanski and Hinton 1995]
[Szymanski and Supmonchai 1996].
The configuration shown in Figure 18c is similar to the optical backplane
based on planar free space interconnects. The difference is the
replacement of the waveguide by a mirror [Hirabayashi et al. 1998]. An
optical beam leaving a transmitter is simply bounced once on the mirror
before it arrives at the receiver. A regeneration of the optical signal (multihop) might be needed on the way from the source to the final destination.
Of the three types of optical backplanes discussed, the one with smart
pixel arrays seems to be the most powerful. On the other hand, a simple
passive optical backplane may have other advantages. Other optical
backplanes have been proposed, e.g., a bus where optical signals can
pass through transparent photo detectors or be modulated by spatial light
modulators [Hamanaka 1991].
6
Optical and optoelectronic switch-fabrics
In this section, optical and optoelectronic switch-fabrics are introduced.
First, in Subsection 6.1, the combination of optical interconnections and
electronic crossbars is treated, while WDM/SDM switches are discussed in
Subsection 6.2.
6.1
Optical interconnections and electronic crossbars
Communication systems such as Myrinet [Boden et al. 1995], by which
arbitrary switched topologies can be built using electrical switches, have
been proposed for parallel computing systems and can be adopted for
data- and telecommunication equipment too. Fibre-ribbons can be used to
increase bandwidth, compared to electrical systems, while still sending in
bit-parallel mode. It is possible to have bit rates in the order of 1 Gbit/s
over each fibre in the ribbon over tens of meters using standard fibreribbons. As noted in Subsection 5.1, foils of fibres or waveguides (e.g.,
arranged as ribbons) can be used to interconnect nodes and crossbars on
the PCB and/or back-plane level.
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The switch itself can also be modified to increase performance or packing
density. A single-chip switch core where fibre-ribbons are coupled directly
to optoelectronic devices on the chip is possible [Szymanski et al. 1998].
Attaching 32 incoming and 32 outgoing fibre-ribbons with 800 Mbit/s per
fibre translates to an aggregated bandwidth of 204 Gbit/s through the
switch when eight fibres on each link are used for data.
A 16×16 crossbar switch chip, with integrated optoelectronic I/O was
implemented for switching packets transferred using bit-parallel WDM
[Krisnamoorthy et al. 1996]. Each node has two single-mode fibres
coupled to the switch, one for input and one for output.
6.2
WDM/SDM switches
The architecture with optical interconnections and electronic crossbars is
flexible and powerful. Optics and optoelectronics can however also be
used internally in a switch fabric, i.e., more than just in the I/O interface. A
broad spectrum of solutions has been proposed, and some examples are
given below.
SDM (Space Division Multiplexing) switches [Goh et al. 1998] [Guilfoyle et
al. 1998] [Kato et al. 1998] [Lai et al. 1998] [Moosburger and Petermann
1998] [Sawchuk et al. 1987] and WDM switches (consisting of, e.g.,
wavelength converters and wavelength selective components) [Pedersen
et al. 1998] [Flipse 1998] can be used both as stand-alone switches and as
building components in larger switch fabrics [Reif and Yoshida 1994]. As
an example, a Banyan multistage network built of 2 x 2 switch elements
has been described [Chamberlain et al. 1998]. Another multistage network
uses both WDM and SDM switches but in different stages [Kawai et al.
1995]. A multistage network can also be implemented using chips with
processing elements placed on a two-dimensional plane [Christensen and
Haney 1997]. The processors then communicate with each other by a
mirror that bounces back the beam to the plane but to another processor.
Switching is made on the chips while each pass between two switch
stages corresponds to a bounce on the mirror.
A multistage switch incorporating both electrical and optical switching, but
in different stages, has also been reported [Duan and Wilmsen 1998].
Some work has focused on the communication between stages, e.g.,
perfect shuffle with lenses and prisms [Lohmann et al. 1986]. Switch times
for SDM switches in the order of 1 ns have been reported [Kato et al.
1998], while some SDM switches have switch times in the order of 1 ms
[Tajima et al. 1998]. A switch can be placed on a dedicated board in a
cabinet and be connected to processor boards (or, e.g., line cards) via
fibres or via an optical backplane [Maeno et al. 1997].
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A system that implements a distributed crossbar, or a fully connected
system, connecting N nodes with only passive optics between the
transmitters and receivers has been demonstrated [Li et al. 1998B]. All
optical channels turned on from a transmitter's two-dimensional √N × √N
VCSEL array are inserted into a fibre image guide. The fibre image guides
from all transmitters end at a central free space system with lenses. The
lenses are arranged in such a way that the light from each VCSEL pixel in
a VCSEL array is focused on a single spot together with the corresponding
pixels in all other arrays. This gives N spots where each is focused into a
single fibre leading to a receiver. Hence, selecting a pixel in a VCSEL
array to be turned on corresponds to addressing a destination node.
Wavelength converters are important components in many WDM switches.
A way of building fast wavelength converters is to first have conversion to
the electrical domain and then back to the optical domain but on another
wavelength. However, this is not a valid solution if an all-optical switch is
desired.
Lately, a lot of focus has been paid on using MEMS
(microelectromechanical systems) technology to build all-optical SDM
switches. As reported in [Lin 1999], an array of electrically controlled
mirrors can be used to build an 8×8 OXC. Lucent Technologies has
already announced 256×256 OXCs to be released on the market [Kenward
2000]. Due to the relatively low loss possible in MEMS switches,
multistage MEMS switches are also possible. In this way, rather large
optically transparent switches can be built. In addition to pure SDM
switches, the MEMS technology can be used in equipment for wavelength
routing networks (wavelength routers), e.g., consisting of wavelength
splitters, a MEMS SDM switch, and wavelength combiners. More
information on all-optical switching is found in [Pattavina et al. 2000].
7
Commercial system implementations
In this section three products are described where optical interconnect
technology is used. The exact backplane technology is not stated in the
companies’material. The fourth section deals with InfiniBand, which is an
industry association to promote a common interconnect standard for
computer-related hardware.
7.1
Sycamore
The Sycamore SN 16000 is an optical switching platform that provides a
transition of the optical network from a ring-based architecture to a meshbased network topology. According to Sycamore, the SN 16000 delivers
automated provisioning, routing, and restoration of lightpaths.
VCSEL (Vertical Cavity Surface Emitting Laser) technology is used to
interconnect the switch fabric shelf with the port card shelf as well as to
interconnect card to card within a shelf. According to Sycamore, this
optical interconnect technology system enables scalability to large switch
matrix sizes, while maintaining a high port density.
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Pluris
The Pluris terabit network router architecture uses a distributed switching
fabric and an n-dimensional fibre optic interconnect structure to provide
scalability in terms of switching capacity, line capacity, port density and line
rate forwarding. The system supports thousands of IP enabled OC-12, OC48 and OC-192 interfaces by combining up to 1920 line cards within a
single system.
7.3
Sirocco Systems
The Sirocco Zephyr Optical Access Device is a network element, designed
to aggregate services onto the optical network. There are two Zephyr
models: the Zephyr Z-48 which provides aggregation up to OC-48/STM-16
and includes an integral optical backplane for support of multiple
wavelengths, and the Zephyr Z-12 offering aggregation up to OC-12/STM4 for entry-level applications. Zephyrs are designed for deployment in the
Metro or Access layer of the network and will typically be located in central
offices and multi-tenant buildings.
7.4
InfiniBand Trade Association
Seven computing companies, Compaq, Dell, Hewlett-Packard, IBM, Intel,
Microsoft and Sun Microsystems have joined together to develop a new
common I/O specification to deliver a channel based switched fabric
technology. This issue is addressed through an independent industry body
called the InfiniBand Trade Association. The specification will support both
copper and fibre implementations and the performance range will be
scalable from 500MB/s to 6GB/s per link.
8
Conclusions
Reviewed interconnection architectures are summarised in Table 2 with
remarks on their suitability in data- and telecommunication equipment from
different aspects. Switch time is marked as slow (ms), medium (submicrosecond), or fast (ns) in the table depending on the suitability of
adoption in OXCs, packet switches with burst switching, or true packet
switches, respectively.
Optical transparency is valuable to get protocol-independent OXCs. The
MEMS technology is promising for such systems, at least as long as the
requirements on switch times are moderate. Multistage networks using
MEMS technology can be especially good alternatives because of their
scalability. All-optical packet switches, however, will probably not be
mature technology in the near future. Also, one must think of the flexibility
and power of electronic switches, and of processors to control the
switches.
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Scalability is desired to be able to build equipment with many in-/output
ports. We state the scalability as poor if only tens of ports is realistic,
medium for hundred to a few hundred ports, and good for thousand ports
or more. Different multistage networks and free space optical backplanes
with a high density of optical channels seems to be good candidates from
a scalability point of view. When building smaller systems instead, e.g., the
WDM star distributed crossbar with its passive optical star can be a good
and simple alternative.
Switch
time
Optical
transparency
WDM star distributed crossbar
fast
internally
poor to
medium
nonSlow switching relaxes component’s
blocking tuning time requirements.
WDM ring
fast
internally
poor
nonSlow switching relaxes component’s
blocking tuning time requirements.
Fibre-ribbon pipeline ring
fast
no
poor
blocking Can be compared with a bus but with
spatial bandwidth reuse
internally
for some
systems
varies a
lot
varies a Scalable if many I/O channels on one
lot
card. Switching and line cards can be
mixed.
Free space optical backplanes varies a lot
Scalability
Nonblocking MEMS system
slow
yes
medium
(or better)
Multistage MEMS system
slow
yes
good
slow
(or better)
(yes)
fast
no
poor to
medium
fast if no
SLMs
no
good
WDM/SDM switches
Optical interconnections and
electronic crossbar
Planar free space optics
Blocking Notes
non
topology Scalable optically transparent for
dependent systems with relaxed switching time
requirements
varies a topology For optical transparency, only alllot
dependent optical wavelength conversion is
allowed.
nonMany optoelectronic I/O channels can
blocking be integrated on a switching
chip/module
topology Chips are only placed in two
dependent dimensions. Promising in terms of
assembly.
Table 2: Summarising evaluation of reviewed interconnection architectures.
If an interconnection network implements a true crossbar it is non-blocking
(e.g., WDM star distributed crossbar), while it is topology dependent for
many of the reviewed architectures whether they are blocking or nonblocking. It should, however, be noticed that the fibre-ribbon pipeline ring
network is blocking. On the other hand, the increasingly good
price/performance ratio for fibre-ribbon links indicates a great success
potential for interconnection systems using fibre-ribbon links.
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Having optics inside a switch gives the same flexibility as electronic
crossbars, but it might be possible to build larger switch fabrics with high
transmission capacities using optics. The suitability of the different free
space systems depends a great deal on the more detailed configurations
of the systems. For example, planar free space systems can be arranged
in arbitrary topologies.
Integrated fibre and waveguide solutions make the building of compact
systems possible, especially for networks such as those using fibreribbons. The same reasoning about compactness can be argued for free
space systems. Optical backplanes may earn their success from the
similarities with current rack-based systems, while future planar free-space
systems might give the possibility to integrate optics and electronics in a
compact way, easy to assemble.
9
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10
Annex
10.1
Commercial solutions
10.1.1
Backplanes
10.1.1.1
Optical
Notice that the companies presented below primarily focus on the
backplane connectors. The actual backplane technology that will be used
is more uncertain, whether it will be fibres stuck on plastic foil, fibres
mounted on the PCB or optical waveguides within the PCB.
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Amphenol
Amphenol has developed a modular optical backpanel interconnect system
capable of providing optical and electrical interconnections between
components housed within an integrated rack, as well as input/output
connections and rack-to-rack interconnections. The interconnect system is
particularly suited for military avionics applications.
10.1.1.1.2
Diamond
Diamond’s E-2000 Optical Backplane is targeted towards data
communication and telecommunication applications. Its architecture is
optimised to provide optical design flexibility through 2 and 6 channel
configurations for deployment of 8 to 40+ channel systems.
As part of the E-2000 product line, the Backplane System utilises
Diamond’s "Active Core Alignment" technology. This ensures, according to
Diamond, that mating fibres align with zero core offset resulting in low
back-reflection and low and repeatable insertion loss.
The E-2000 Backplane is available in multimode, single mode, convex
polished (PC), and 8 degree angle polished (APC) versions as well as a
field mountable version called the E-2000 FUSION.
10.1.1.1.3
AMP
The AMP Fibre-Optic Backplane Connector is used for optical
interconnection of daughter cards to motherboard. An SC connector
interface is used from motherboard, with 2 to 8 positions and a centreline
spacing of 12.5mm. Singlemode or multimode versions are available. The
daughter card floats and alignment pins on mother card assure mating.
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MOLEX
MOLEX Backplane Connector System (BCS) was designed to provide a
smooth transition from board mounted and pigtailed active devices (LEDs,
lasers and receivers) to backplane fibre optic components using a
standard SC interface. Pigtailed active devices, located anywhere on the
daughter card, are terminated to the card edge mounted BSC connectors.
These BSC connectors provide a blind mating interface through backplane
BSC adapters and NTT-SC standard input/output ports. A floating ferrule
design allows the connectors to self-align inside the adapter and provides
physical contact. The BSC backplane connector system provides a great
deal of flexibility for designing and servicing the daughter card. Its blind
mating interface provides the daughter card with a plug-in feature that
allows the card to be inserted, removed or exchanged without disrupting
the input/ output ports and the associated cabling. For multiple fibre
applications, alignment pins in the BSC housing allow the connectors to be
stacked. BSC to standard SC connector mating is accomplished through
the BSC adapters, which are available with either zirconia ceramic or
phosphor bronze, sleeves. In addition to the traditional SC style adapter, a
shuttered SC version is also available. A spring-loaded cover on the
shuttered version shields against exposure to laser radiation and dust
contamination.
10.1.1.1.5
FCI
The MACII is a multi-fibre (18 fibre positions) backplane optical connector
developed by AT&T. MACII was used in the first optical backplane
deployed in a large-scale telecommunications platform. It was deployed in
1994 by AT&T in the DACS VI-2000 digital access and cross-connect
system designed for the SDH synchronous transmission standard. Berg
Electronics has further developed it into the miniMAC connector. Berg was
subsequently acquired by FCI.
10.1.1.2
Electrical
10.1.1.2.1
National Semiconductor
Low Voltage Differential Signalling (LVDS) is a technology addressing the
needs of today's high performance data transmission applications. LVDS
technology features a low voltage differential signal of 330mV (250mV MIN
and 450mV MAX) and fast transition times. This allows the products to
address high data rates ranging from 100's Mbps to greater than 1 Gbps.
Additionally, the low voltage swing minimises power dissipation while
providing the benefits of differential transmission.
The Channel Link chipsets multiplex and demultiplex slow TTL signal lines
to provide a narrow, high speed, low power LVDS Interface. These
chipsets provide systems savings in cable and connector costs, as well as
a reduction in the amount of physical space required for the connector
footprint.
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Bus LVDS (BLVDS) is a new family of bus interface circuits based on
LVDS technology specifically addressing multi-point cable or backplane
applications. It differs from standard LVDS in providing increased drive
current to handle double terminations that are required in multi-point
applications.
10.1.1.2.2
Texas Instruments
Texas Instruments have both LVDS and pseudo-ECL (PECL) circuits for
high-speed data transmission.
10.1.1.2.3
Teradyne
Teradyne has an 8-row VHDM HSD (Very High Density Metric High-Speed
Differential) interconnect designed to meet the needs of the newly
emerging 2.5Gb/sec device families. Stripline shielding allows 100% of the
signal pins to be used for signal transmission, yielding 38 real signal pairs
per linear inch.
10.1.1.2.4
Litton
According to Litton, they are producing, in volume, boards at speeds of 2.5
GHz and 1.2 Gbit/s on standard and emerging PCB dielectric materials.
They are actively designing and testing for 2.4 Gbit/s and have design
activities for up to 6 GHz and 5 Gbit/s.
10.1.2
Link optoelectronics
10.1.2.1
Mitel Semiconductor
The MFT62340 and MFR62340 is a transmitter and receiver pair for
parallel fibre applications. This pair, together with a multimode parallel fibre
ribbon cable, constitutes a complete 12-channel parallel fibre link. The link
provides 2.5 Gbps interconnects for use within and between large capacity
switches, routers and data transport equipment. The transmitter and
receiver have a differential common mode logic interface and support
MPX, MPO/MTP and MT fibre connectors.
10.1.2.2
Gore
W. L. Gore & Associates, Inc. is completing the development of a family of
multichannel optical transmitters and receivers targeted at intra-system
and short reach inter-system high data rate communication links. The
transmitter module utilises Gore’s Vertical Cavity Surface Emitting Laser
(VCSEL) technology, which makes multi-Gigabit/sec data rates possible
while minimising space and cost. The nLIGHTENTM modules offer an 8X
improvement in density over standard 1x9 serial transceiver products and
reductions in costs at the same time. It utilises 62.5-mm multimode fibre
and its operational wavelength is 850 nm.
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Emcore
EMCORE has designed a new 2.5 Gbps 850nm oxide Vertical Cavity
Surface Emitting Laser (VCSEL) specifically for the next generation data
communications networks. The VCSEL could be used for applications
such as Storage Area Networks (SANs) and Local Area Networks (LANs).
10.1.2.4
Honeywell
The HFT428X is a single package transmitter and receiver designed to
interface with the MT-RJ style optical connectors. The transmitter is an
850nm VCSEL packaged for up to 2.5 Gbps data communications. The
PIN + pre-amplifier converts optical power into a differential output
electrical signal.
No VCSEL array devices were found on the Honeywell home page.
10.1.2.5
Infineon
Infineon has a parallel optical link product, denoted PAROLI, with 12
channels each operating at up to 2.5 Gbps. The footprint for the PAROLI
device is slightly larger than that of Mitel’s parallel link device.
10.1.2.6
Agilent
Agilent has several different single VCSEL devices for Fibre Channel and
Gigabit Ethernet applications. No VCSEL array devices were found on
Agilent’s home page.
10.1.2.7
Lucent
Lucent has made laboratory demonstrations of transmission of 10 Gbps
over a single channel, using a VCSEL and their new LazrSPEED
multimode fibre, for a distance of 1.6 km. The LazrSPEED fibre is for sale
now, but Lucent is not offering any VCSELs.
10.1.2.8
Cielo
Cielo recently demonstrated an 850nm Vertical Cavity Surface Emitting
Laser (VCSEL) operating at 12.5 Gbit/sec on link lengths of greater than
300 meters of Lucent LazrSPEED multi-mode fibre. The laser supports
direct modulation and was packaged in an optical subassembly (OSA)
designed to couple light efficiently into the new fibre.
10.1.2.9
New Focus
New Focus has presented a VCSEL transceiver operating at 10 Gbps.
These VCSEL transceivers have a power consumption of approximately
1.25 W, and require no active cooling.
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These 10-Gb/s transmitters and receivers are intended for, multimode,
fibre-optic links between network equipment within a central office. The
transmitter includes a directly modulated 850-nm VCSEL with laser-driver
circuitry and automatic power control; the receiver includes a
transimpedance amplifier with automatic gain control (AGC) and DC
restoration. The units use a differential CML electrical interface.
10.2
Demands from future Ericsson products
To clarify the needs for optical interconnection technology in future
Ericsson products, interviews were made with people involved in the
product development of certain Ericsson products.
10.2.1
AXD 301
The AXD 301 is a carrier class scalable multi-service ATM switching
system, enabling highly reliable edge and backbone network solutions.
The modularity and load sharing architecture, of the internal ATM switching
fabric, allows the switching capacity of the present system to be scaled
from 10 Gbps up to 160 Gbps. The full 160 Gbps system consists of nine
cabinets, with the switch core cabinet surrounded by four access subrack
cabinets on each side.
The channel data rate today is 500 Mb/s over a 1-metre backplane or 10
metres of cable. When considering optical interconnects for AXD 301, and
certainly for other products as well, the most important factors are power,
cost and bit error rates. Optical interconnects would be advantageous for
distributed switching architectures.
10.2.2
ERION OXC
Optical interconnects are evaluated for opto-electrical cross connects
(OEXCs) with hundreds of input and output ports. The technology chosen
must be “clean”, in the sense that dust and dirt in the air and from the
connectors themselves should not degrade the connector properties.
10.2.3
Cello
Cello is a general platform for radio base stations and media gateways.
The Cello backplane consists of two different buses: the common Cello
ATM bus and the application bus. The Cello ATM bus is the same for all
different products, whereas the design of the application bus depends on
the particular product requirements.
For the Cello platform, power consumption is more important than space,
as the costs for power supply and cooling is larger than the cost for floor
space. There should be a power-efficient electrical interface between the
ASICs and the optical interconnect drive modules.
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