Q: What is the maximum reach of your DWDM solution?
A: With the use of optical amplifiers, Optelian solutions can reach distances up to a few thousand kilometers when designed to do so. The actual reach achievable for a given solution, however, depends on many factors, including fiber type and loss, where and how optical amplifiers and/or dispersion compensators are placed, the type of transceivers used, the type and strength of forward error correction (FEC), etc. A properly designed system must account for the many impairments present in an optical system, including noise from optical amplifiers, the effects of fiber nonlinearities, the cascaded effect of optical filtering from inline components, how optical power levels are controlled and managed, etc. Optelian takes such factors into account when designing a solution to ensure the resulting design is cost-effective and reliable, while meeting the functional requirements of the network.
Q: How does dispersion affect the maximum data rate?
A: It depends on the type of transceiver being used. For coherent transceivers, such as DP-QPSK used for 100G and above, the use of a coherent receiver with advanced DSP allows signals of essentially any data rate to be transmitted over hundreds, or even thousands, of kilometers in the presence of fiber dispersion, without the need for, or use of, any optical dispersion compensation. Hence, only optical amplification is generally needed for such transceivers to reach very long distances. For such coherent transceivers, the dispersion compensation is done in the electrical domain by the receivers’ DSP. For commoditized DWDM 10G pluggable transceivers, including (T)SFP+ and/or (T)XFP, such transceivers can typically accommodate dispersion up to ~1400 ps/nm, corresponding to approximately 80km of standard fiber. Note, however, that such commoditized transceivers can provide all-optical transmission over many hundreds of kilometers if optical amplifiers are used in conjunction with the appropriate placement of optical dispersion compensators. One of Optelian’s specialties is designing optically-compensated systems that can support optical circuits over very long distances using a mix of commodity 10G transceivers and coherent transceivers on the same network.
Q: How many DWDM channels do you support and what is the limitation?
A: Optelian supports two standard channel plans, including a 40 channel plan on a 100 GHz ITU grid, and an 80 channel plan on a 50 GHz ITU grid. Both channel plans have the first channel wavelength at 1530.33 nm, with higher channel numbers at increasing wavelengths within the C band. For a system to fully support a given channel plan, all components within the system must support the plan: The bandwidth of all optical amplifiers must be sufficiently wide to accommodate all channels, such that all amplifier specifications are met for all channels, and the power rating for all amplifiers must be able to accommodate a fully-loaded system when all channels are present; All ROADMs and optical channels monitors must accommodate and function properly for the full channel plan when all channels are present; If periodic DCMs are used, then they must accommodate all channels in the plan; All optical multiplexers and de-multiplexers used in the system much accommodate all channels and provide connectors to access all channels; All tunable transmitters must be fully tunable over the entire channel plan; The network management system and its user and control interfaces must accommodate all channels. These are many of the requirements to truly and fully support a specified channel plan, which Optelian does with its networking products and solutions; hence, there are no limitations regarding which channel, or combination of channels, are used in an Optelian system. Note that some equipment vendors claim to support channel plans with more channels, but do not actually fully support the plan according to all the requirements stated here.
Q: What is the difference between a transponder and a regenerator?
A: As a signal is transmitted over a medium such as fiber, it accumulates distortion and noise from various impairments within the transmission medium. The purpose of a regenerator is purely to regenerate the bits in a data stream, in order to restore the signal integrity for further transmission. Regeneration is usually so called ‘3R’, which retimes, reshapes and then retransmits a signal, i.e., the three ‘Rs’. In some rare cases a regenerator may also be ‘2R’, which reshapes and retransmits the signal without retiming. Retiming involves recovering the clock from the signal to be regenerated, and using the recovered clock to drive the transmitter and regeneration circuitry. This process is usually referred to as clock-and-data recovery (CDR), and the circuit or ASIC used to do this function is usually simply called a ‘CDR’. The purpose of a transponder is to convert from one media type to another, such as from a 1310 nm “Grey” wavelength or 850 nm multimode wavelength to a CWDM or DWDM wavelength for further transmission and multiplexing with other wavelengths over a common fiber strand. By definition a transponder must ‘regenerate’ the signal, but it also has the task of converting the signal between media types. Hence, regenerators are used within a given network simply to extend reach, whereas transponders are used to allow signals to pass from one network domain to another. For example, it is common for a router to have a short-reach interface, such as 1310 nm Grey, which would then connect to the client port, also with a 1310 nm Grey interface, of a transponder in a DWDM transport network.
Q: Can you use third-party pluggable transceivers in Optelian equipment?
A: Potentially, but with some caveats. Although Optelian does not prohibit the use of third-party devices, it has no control or influence over the quality or nature of third-party transceiver modules. Such devices cannot be guaranteed to work in Optelian equipment, and any damage to Optelian equipment resulting from their use is not covered under warranty. Although there are multisource agreements (MSAs) for SFPs, XFPs, etc., it is very common for there to be differences in optical specifications, or how certain internal registers are programmed, or variations in the precise timing required for the electrical interface pins, etc. Many things can go wrong when using third-party devices that can range from complete non-functional response to more subtle issues and behaviors that can be difficult to detect and isolate. For this reason, Optelian provides a complete and comprehensive portfolio of pluggable transceivers, all of which are fully qualified and certified to work with Optelian equipment, and it is highly recommended that Optelian devices be used with Optelian equipment.
Q: What is the difference between WDM, CWDM and DWDM?
A: First, they all involve wavelength-division multiplexing (WDM), which means to combine (multiplex) two or more wavelengths onto a common fiber, with each wavelength carrying an independent information signal. In some contexts the term WDM may be used to specifically imply the multiplexing of a 1310 nm wavelength with a 1550 nm wavelength (two wavelengths), but in its more general context it means multiplexing any number of wavelengths. Coarse WDM (CWDM) involves the multiplexing of wavelengths on a 20 nm spaced grid, whereas dense WDM (DWDM) involves the multiplexing of wavelengths on a much finer grid, usually at 100 GHz or 50 GHz spacing, corresponding to a fine wavelength spacing of 0.8 nm and 0.4 nm, respectively. The ITU defines the exact wavelengths or frequencies used for CWDM and DWDM systems. Although CWDM can support up to 18 wavelengths from 1271 to 1611 nm, it is most common for such systems to be deployed with up to 8 wavelengths from 1471 to 1611 nm, largely due to the higher loss of standard fiber at the lower wavelengths. DWDM systems are usually deployed in the C band from 1525 to 1565 nm, with approximately 40 wavelengths on a 100 GHz grid, or 80 wavelengths on a 50 GHz grid. Hence, DWDM systems can provide extremely high transmission capacity, especially when each wavelength transports a coherent signal operating at 100 Gb/s or higher. DWDM systems also operate in a wavelength range compatible with Erbium-doped fiber amplifiers (EDFAs), allowing signals to be transmitted over very long distances of hundreds or even thousands of kilometers without the need for optical-electrical-optical regeneration.
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