3.9.2 The DAVID Project
The DAVID (Data and Voice Integration over DWDM) project [75–78] is part of
the Information Society Technology (IST) Program, a research program of the
European Union. DAVID is pursued by a fellowship of major operators, manufacturers,
as well as leading universities and research organizations from all over
Europe. The project aim is to study and demonstrate an agile, dynamically reconfigurable,
and intelligent optical layer based on a packet-over-DWDM solution,
that offers capabilities of traffic engineering and network management, covering
the entire area form MAN to WAN. The proposed solution seeks the optimum
balance between the “optical” and the “electronic” layers. The long term design
goals of DAVID include the formulation of a multi-QoS, service-transparent,
flexible optical layer.
The proposed network architecture covers both metropolitan area networks and
wide area networks. In both domains, fixed-length packets are used in a synchronous
mode of operation [78]. In order to reduce the optical complexity, the optical part of
the MAN ring is bufferless, something that requires (1) aggressive exploitation of
electronic buffering and advanced traffic engineering in the network periphery
and (2) the deployment of a MAC protocol for collision-free accessing of the
optical layer.
The WAN is envisaged as a meshed network of interconnected optical packet
routers. A link is a set of one or more fibers, each carrying multiple wavelengths.
As in the MAN, the wavelengths will be used to transport optical packets of fixed
time duration, which was set at 1 μs (for both MAN and WAN). The bit rate in
the WAN will typically be higher, carrying aggregated traffic [78].
Header transportation in the metro ring network is out-of-band; that is, the
headers are transmitted using a separate wavelength. In the DAVID backbone
WAN, headers will be transported in-signal. Within DAVID, electronics are still
used for header processing and the payload is switched transparently in the
optical domain [78].
The metro network (Fig. 3.16) comprises one or more unidirectional optical
physical rings interconnected by an optical packet hub (OPH), whose structure
and operation is described in the following. A ring within the MAN will consist
of one or more fibers, each containing multiple wavelengths that can be used to
transport packets. A packet is composed of a header and a payload. The packet
duration is equal to one timeslot. All nodes can potentially access all wavelengths
at each timeslot. Therefore, the adopted ring architecture uses both wavelength
division multiple access (WDMA) and time-division multiple access (TDMA).
Ring nodes place optical packets (containing client layer traffic, such as Internet
protocol traffic) on the ring using a MAC protocol to decide when to transmit and
which wavelength to use. Thus, contentions are avoided and the optical path
within the MAN is kept bufferless.
The use of the hub node that controls the network resources differentiates
the DAVID network from other optical ring networks. The hub node is used to
forward optical packets between ring networks, as well as to interconnect the
metro area to the backbone through an electronic gateway. The hub is an SOA-based
optical packet switch capable of coping with traffic at very high rates.
The structure of the optical packet hub resembles that of a backbone node
(optical packet router) except that the target capacity for the hub is smaller.
Naturally, there are differences between the hub and an optical packet router at
the control level. The hub comprises synchronization stages, a space switching
stage, a wavelength switching stage, and regeneration stages if required (depending
on the power budget).
By enforcing proper constraints via the MAC protocol, contention on the optical
packet level is avoided, and the need for buffering on the optical path within the
MAN is eliminated: all buffering is done electronically in the add/drop interfaces.
At the optical level, each node transceiver, even if tunable, is capable of transmitting
and receiving on only one channel at a time (i.e., its bandwidth is equal to the
capacity of one channel). Thus, a good compromise is achieved between optical
and electronic technologies, keeping the high-speed electronic data path at an acceptable
level of complexity.
The hub, which is also bufferless, forms the interconnection point of multiple
rings and provides access toward the WAN through a gateway. This WAN connection,
from a logical point of view, can be seen as an extra ring to and from which to
switch traffic. The gateway will be responsible for solving contention between
packet flows between MAN and WAN. The latter consists of optical packet
routers (OPRs) interconnected in a meshed topology. In contrast to the MAN, an
OPR in the WAN may exploit optical buffers in the form of fiber delay lines
(FDLs) to aid in contention resolution. The hub performs switching of the entire
metro network’s capacity, transferring packets from any incoming to any outgoing
fiber and performing wavelength conversion if required to achieve spectral adaptation/
matching for a source/destination node pair.
For the MAN ring nodes, two alternative architectures have been studied in
DAVID. The first is a very simple passive architecture, relying on commercially
mature low-cost technology, using only passive optical components. At the input
of an add/drop node, the control channel is first extracted and converted to electrical
form for processing. On the data path, a simple 2 × 2 coupler is used to add packets
by coupling light coming from a burst-mode transmitter and to drop packets
by guiding light to a burst-mode receiver. The WDM channels from multiple receivers
and transmitters are separated and combined by demultiplexers
and multiplexers, respectively. To allow simultaneous add and drop operations
within the same timeslot, upstream and downstream traffic channels are spectrally
separated (i.e., they use different wavelengths), which in addition obviates crosstalk
between add and drop channels. Clearly, in such a design, the hub will need
to perform wavelength conversion from the “send” to the “receive” spectrum to
allow communication.
The main drawback of the passive architecture is that packets cannot be physically
removed from the wavelength comb and therefore propagate past their final
destination (prohibiting reuse of the same slots for transmission). Consequently,
the hub needs to take care of packet erasure from the ring. In addition, the so-called
space reuse is impossible with a passive structure; because all traffic necessarily
needs to cross the hub, the same slot cannot be reused for nonoverlapping connections
on the same ring. Moreover, spectral separation of upstream and downstream
traffic doubles the amount of required wavelengths.
More advanced components are used in the active node structure, which, in
addition, employs a waveband concept. Instead of a passive coupler, a waveband
demultiplexer is used to isolate light in groups of B wavelengths per band (e.g.,
for B = 4). Each node is equipped with a single receiver and a transmitter that is
tunable over the B wavelengths in a particular waveband. Furthermore, selective
erasure of packets by the gates in the through path is supported. Apparent advantages
of this active structure are that it allows for slot reuse (i.e., the dropped slot can be
reused for transmission of new data), it does not require separation of upstream
and downstream traffic, and it facilitates flexible utilization of the WDM domain.
The main drawback of the active structure clearly is its higher initial cost, yet its
modular structure may allow for longer term savings.
3.9.2 The DAVID Project
The DAVID (Data and Voice Integration over DWDM) project [75–78] is part of
the Information Society Technology (IST) Program, a research program of the
European Union. DAVID is pursued by a fellowship of major operators, manufacturers,
as well as leading universities and research organizations from all over
Europe. The project aim is to study and demonstrate an agile, dynamically reconfigurable,
and intelligent optical layer based on a packet-over-DWDM solution,
that offers capabilities of traffic engineering and network management, covering
the entire area form MAN to WAN. The proposed solution seeks the optimum
balance between the “optical” and the “electronic” layers. The long term design
goals of DAVID include the formulation of a multi-QoS, service-transparent,
flexible optical layer.
The proposed network architecture covers both metropolitan area networks and
wide area networks. In both domains, fixed-length packets are used in a synchronous
mode of operation [78]. In order to reduce the optical complexity, the optical part of
the MAN ring is bufferless, something that requires (1) aggressive exploitation of
electronic buffering and advanced traffic engineering in the network periphery
and (2) the deployment of a MAC protocol for collision-free accessing of the
optical layer.
The WAN is...
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