June 19, 2006 Atlanta -- Wave7 Optics today introduced its patent-pending "idle-code mitigation" technology, which enables the delivery of RF video services via EPON and GPON systems. Without idle-code mitigation, the effects of Stimulated Raman Scattering on the 1490-nm wavelength cause interference on the 1550-nm video transmission wavelength, explains Jim Farmer, CTO of Wave7.
One of the key characteristics of data transmission, the use of idle codes can cause significant interference to RF video signals through a phenomenon known as Stimulated Raman Scattering (SRS). Specifically, the downstream data wavelength (1490 nm) is close enough to the required video wavelength (1550 nm) to cause SRS interference. The phenomenon works much like an Erbium-doped fiber amplifier (EDFA), says Farmer.
"Because of the Raman effects in the fiber itself, the 1490 [-nm data] signal will slightly amplify the 1550 [-nm video] signal," he explains. "It's a very inefficient amplifier; you'd never use it for that purpose. But it does amplify it a little bit. And what happens is because the data signal goes on and off, on and off--depending on the transmission of 1s and 0s--amplification only occurs when you're transmitting a 1, not a 0. Since I'm amplifying and not, amplifying and not, I am impressing interference onto the 1550-nm wavelength."
This interference affects video transmission in both EPON and GPON systems, degrading the carrier-to-noise ratio and effectively introducing snow to the video picture. While the interference can be found primarily on channels two through six, Farmer says he has seen examples of interference measurable up to channel 24.
Idle periods significantly exacerbate this problem because the repetitive idle pattern has strong frequency components that overlap those key television channels. The idle codes cause peaks in the data spectrum, which, in turn, create tremendous interference peaks in the video. In EPON systems, for example, one of those peaks occurs at "a particularly dastardly frequency for creating interference on channel 3," notes Farmer.
But the folks at Wave7 Optics say they have developed a technology that solves the problem. Back in 2000, company scientists began looking at the issues inherent in the transmission of three wavelengths. They ran some tests, crunched some numbers, and found that the math predicted interference between the 1490- and 1550-nm wavelengths. (Interference is not a problem between the 1310- and 1550-nm wavelengths.) When they determined that the transmission of the idle code was to blame, the answer seemed simple: Don't transmit the idle code.
"But that's part of the standard, so the question then is how do you get around transmitting the idle code and yet stay within the standard?" muses Farmer. "That really is what this invention is all about. We recognized early on that if we could somehow detect when no idle code was being transmitted, then we could insert random data that was intended for no one in particular. We would stay within the standard, and yet we would be able to avoid transmitting that random data that has a peak in the noise at a particularly bad space."
Wave7's idle-code mitigation technology does not violate either the IEEE 802.3ah EPON standard nor ITU-T G.984 GPON standard. It is completely compatible with existing standards-compliant equipment, is software-based, and enabled via an optional software key. Wave7 is in the process of incorporating the technology within its Trident7 EPON/GPON system and also intends to license the technology on a fair and equitable basis.
Farmer reports that several people in the Far East already have encountered the problem and have contacted Wave7 for advice. He expects other inquiries to follow as EPON and GPON deployments gain momentum.
For his part, Farmer believes that RF video overlay will remain a significant component of FTTP deployments in the near term, thus rendering Wave7's idle-code mitigation technology a necessity in both the EPON and GPON worlds. "When it comes to video and PONs, carriers deploying Trident7 based EPON or GPON can choose between RF and IPTV or both without experiencing any degradation of the video quality," he says. "We believe, in fact, that IP and RF will continue to co-exist in FTTP networks for the foreseeable future."
--Meghan Fuller