Optical packet routers blaze at 100 Gbits/sec
DAVE WILSON
Researchers are evaluating the future of optical packet routers at British Telecom, or BT, Laboratories in Martlesham Heath, UK. They claim to have demonstrated an optical packet router that sorts information by its destination address at 100-gigabit-per-second speeds.
The researchers contend that this router--a key building block of a futuristic 100- Gbit/sec optical data packet network--could lead the way to a communications network with the capacity and flexibility necessary to meet the future growth demands of computers and fiber-optic multimedia photonics. BT expects these routers to be available within 10 years.
The ability to design a fast optical router has implications for both local and wide area networks. If the address size were increased, such technology could be used to design routers to meet the high-speed needs of emerging local area standards.
Each data packet switched in the router comprises a burst of optical pulses encoded with a destination address and a data payload. The optical packet router reads the address of each incoming packet and switches it onto the output path toward its destination.
Reading the address optically
In describing the technology, David Cotter, a researcher at BT Labs, says that the system differs from other approaches because the router reads the packet address optically, rather than electronically. This approach simplifies the design of the router and allows BT to use a packet bit-rate of 100 Gbits/sec in the demonstration. According to Cotter, this rate is 100 times faster than that of existing packet networks.
The ability to read the address optically also lets the packets pass through the router without being delayed by an electronic processor. A network based on these routers would, therefore, have a low end-to-end delay. A packet could be sent around the world in less time than it would take to pass through a present-day electronic router.
A packet sent through the system comprises a marker pulse, an address field and a data field. The router has two output ports and detects a particular address: All incoming packets with the address are sent to one port, while all other packets are sent to the other port.
Keyword generator
The router extracts the marker pulse from the incoming packet and passes it through a planar silica integrated optical circuit known as a keyword generator. This circuit generates a pattern of optical pulses that is the logical complement or keyword of the address that the router is programmed to recognize. In the experiment, the router is programmed to look for the address 101001, so the generator produces the keyword 010110 from the marker pulse of each incoming packet.
"The keyword generator consists of several optical waveguides with precisely controlled lengths fabricated on a silicon device," says Cotter. "The input pulse is coupled into the device, then split into the waveguides, which operate at different lengths. Finally, the outputs from the waveguides are recombined. A marker pulse is split, delayed and recombined, thus giving a word at the output of the device."
Two types of planar silica devices have been developed to generate the keyword: In the demonstration device, the recognition address is fixed, but in another device that BT has developed, the address is reprogrammable.
"In the experimental demonstration, the keyword generation was built on a silicon wafer where the configuration of the wave guides formed a fixed pattern. The reconfigurable device will be a hybrid integration of the silicon waveguides and some active devices as well--in the current experimental devices, these are semiconductor laser amplifiers," explains Cotter. "Once generated, the keyword is passed into an optical AND gate so that it overlaps with the address field of the incoming packet."
If the router does not recognize the address of the incoming packet, the coding scheme chosen for the address set ensures that at least one pair of pulses coincides, and one or more pulses are generated by the AND gate. The number of pulses and their positions are irrelevant. They collectively generate an electrical pulse at the photodetector. This electrical pulse triggers an electro-optic switch so that the packet emerges from port 1 of the switch.
If the router recognizes the address of the incoming packet, no pulses are generated by the AND gate. Consequently, no electrical pulse is generated at the photodetector, and the packet emerges from port 2 of the switch.
"The optical AND gate is based on a semiconductor laser amplifier," says Cotter. "The two optical inputs mix with a third continuous optical input to generate an output pulse if the two signal inputs are simultaneous. We are using an optical nonlinear effect in a semiconductor device that has a fast response time, so you can distinguish the individual bits in the 100-Gbit/sec stream."
In the demonstration, 6-bit addresses were used, and the router examined the entire address. The principle, however, is valid for addresses of any length and, depending on their location within the network, routers could be programmed to examine the entire address of incoming packets or just part of the address.
Also in the demonstration, the router forwarded 500 million packets per second; each packet had a peak line rate of 100 Gbits/sec. Both rates are at least 100 times greater than those achievable in today`s packet networks.
In the future, in addition to offering a low end-to-end delay throughout the network, optical packet routers could possibly allow data sources to send packets on-demand. This connectionless approach leads to flexible networking and suits computer-to-computer communications. q
Dave Wilson writes from London.