Xcell Journal: The authoritative journal for programmable ... - Xilinx
Xcell Journal: The authoritative journal for programmable ... - Xilinx
Xcell Journal: The authoritative journal for programmable ... - Xilinx
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EMBEDDED SYSTEMS<br />
<strong>The</strong> MCNC-RDI has developed a<br />
NASA-funded OBS protocol implementation,<br />
called JIT (Just-In-Time), which<br />
recently achieved successful testing in an<br />
ATDnet (Advanced Technology<br />
Demonstration network) testbed.<br />
Established by the Defense Advanced<br />
Research Projects Agency (DARPA) <strong>for</strong><br />
demonstrating advanced networking technology,<br />
the all-optical ATDnet runs at 2.5<br />
Gbps through six sites using eight wavelengths<br />
and wavelength division multiplexing<br />
(WDM) switches. <strong>The</strong> testbed<br />
included applications in multiple areas<br />
like optical networking, network security,<br />
and networked in<strong>for</strong>mation systems.<br />
Technology Overview<br />
WDM is a method of transmitting data<br />
from different sources over the same<br />
fiber-optic link at the same time; each<br />
data channel is carried on its own unique<br />
wavelength. <strong>The</strong> result is a link with an<br />
aggregate bandwidth that increases with<br />
the number of wavelengths employed. In<br />
this way, WDM technology can maximize<br />
the use of the available fiber-optic infrastructure<br />
– what would normally require<br />
two or more fiber links will now require<br />
only one.<br />
WDM technologies primarily differ in<br />
the number of available channels. Coarse<br />
wave division multiplexing (CWDM)<br />
combines as many as 16 wavelengths onto<br />
a single fiber; dense wave division multiplexing<br />
(DWDM) combines as many as 64<br />
wavelengths onto a single fiber.<br />
With DWDM technology, the wavelengths<br />
are closer together than CWDM,<br />
meaning that transponders are generally<br />
more complex and expensive than<br />
CWDM. However, with DWDM, the<br />
advantage is a much higher density of<br />
wavelengths, and also longer distance.<br />
DWDM is emerging as a preferred solution<br />
<strong>for</strong> providing scalable and efficient optical<br />
networking technologies of the future.<br />
<strong>The</strong> key objective of the hardware-based<br />
OBS protocol implementation is to<br />
dynamically manage commercially available<br />
WDM switches. An OBS network<br />
comprises OBS network controllers and<br />
clients with OBS network interface cards<br />
CALLING HOST CALLING SWITCH CALLED SWITCH CALLED HOST<br />
SETUP<br />
ACK<br />
SETUP<br />
RELEASE<br />
CROSSCONNECT<br />
CONFIGURED<br />
FOR EXPLICIT<br />
RELEASE<br />
CONNECT<br />
(NICs). OBS network controllers direct<br />
the optical data bursts received from a<br />
source-client OBS NIC to a destinationclient<br />
OBS NIC.<br />
Advances in <strong>Xilinx</strong> FPGA technology<br />
have made it possible <strong>for</strong> the MCNC-RDI<br />
to build a NIC that implements the JIT<br />
signaling protocol <strong>for</strong> an OBS network.<br />
<strong>The</strong> OBS NIC uses DWDM technology to<br />
transmit and receive data optically on specific<br />
wavelengths and is capable of handling<br />
data rates as high as 1.25 Gbps. <strong>The</strong> NIC<br />
card can be tuned dynamically to as many<br />
as eight different DWDM wavelengths.<br />
In the JIT protocol, a control packet<br />
reserves a wavelength channel in the network<br />
<strong>for</strong> a period of time L equal to the<br />
burst length, starting at the expected arrival<br />
time R (this can be adjusted by the number<br />
of hops that a burst needs to travel and the<br />
processing time at each intermediate node).<br />
If the reservation is successful, the control<br />
packet adjusts the offset time <strong>for</strong> the<br />
next hop and <strong>for</strong>wards it on. If the reservation<br />
is not successful, the burst will be<br />
blocked and the packet will be discarded.<br />
SETUP<br />
OPTICAL<br />
BURST<br />
CONNECT<br />
Figure 1 – JIT signaling scheme<br />
SETUP<br />
CONNECT<br />
RELEASE<br />
Because JIT is a one-way reservation protocol,<br />
buffering does not occur at the node<br />
level, thus reducing any latency.<br />
Implementation of JIT with an efficient<br />
scheduling algorithm can further decrease<br />
the probability of burst loss.<br />
<strong>The</strong> JIT protocol uses a SETUP message<br />
to announce a burst in the OBS network.<br />
Each optical burst of data,<br />
comprising some number of contiguous<br />
packets destined <strong>for</strong> a specific destination,<br />
is sent immediately after the node receives<br />
a SETUP ACK from the ingress OBS<br />
node. An out-of-band SETUP message is<br />
sent across all switches be<strong>for</strong>e this step to<br />
prepare all path switches <strong>for</strong> the burst data.<br />
OBS does not use any optical buffering or<br />
packet parsing. For a long burst, a<br />
KEEPALIVE message may be required to<br />
keep all switches in active state. <strong>The</strong> JIT<br />
signaling scheme is shown in Figure 1.<br />
<strong>The</strong> Role of the FPGA<br />
<strong>The</strong> development of the OBS NIC was<br />
enabled by the availability of integrated<br />
high-speed multi-gigabit RocketIO<br />
30 <strong>Xcell</strong> <strong>Journal</strong> Winter 2004