International Standardization Activities on Optical Interfaces Global Standardization Activities

This article reviews international standardization activities on optical interfaces over the last four years. Standardization is the key to the cost-effective manufacture, purchase, and installation of largecapacity high-speed optical network elements. This article mainly focuses on the activities related to wavelength division multiplexing (WDM) interfaces in ITU-T (International Telecommunication UnionTelecommunication sector) and also mentions very short reach (VSR) interfaces in OIF (Optical Internetworking Forum) and ITU-T. International Standardization Activities on Optical Interfaces Global Standardization Activities † NTT Network Innovation Laboratories Yokosuka-shi, 239-0847 Japan E-mail: [email protected] Global Standardization Activities 86 NTT Technical Review to 1460 nm), the S-band (short wavelength band: 1460 to 1530 nm), the C-band (conventional band: 1530 to 1565 nm), the L-band (long wavelength band: 1565 to 1625 nm), and the U-band (ultra-long wavelength band: 1625 to 1675 nm). ITU-T has also defined, in G.671 [2], WDM categories according to the frequency/wavelength spacing of the channels multiplexed onto a single optical fiber: WWDM (Wide WDM) has a channel spacing larger than 50 nm, CWDM (Coarse WDM) smaller than 50 nm and larger than 1000 GHz, and DWDM (Dense WDM) smaller than 1000 GHz. 2.2 Two types of compatibility on optical interfaces ITU-T has specified two types of compatibility in terms of optical interfaces (Fig. 1). The first is called “transverse compatibility”, which is defined as multivendor interoperability along the optical fiber. Compatible interfaces of this type allow a transmitter from one manufacturer to communicate with a receiver from another manufacturer. To get this compatibility, we need to specify a full set of optical parameters, such as output power, channel central frequency, central frequency deviation, line-coding of each channel, maximum attenuation of the fiber, chromatic dispersion, maximum bit error ratio, and optical path penalty. The second type of compatibility is called “longitudinal compatibility”, where the manufacturers produce transmitter and receiver sets that offer identical transmission distances. The benefit of longitudinal compatibility is that the network operator can design NE locations (buildings) without being locked into one manufacturer. Longitudinal compatibility does not demand a full set of specifications, only maximum values of attenuation, chromatic dispersion, and frequency/wavelength “grid” (described below). 2.3 Frequency/wavelength grid Longitudinal compatibility was the first step to DWDM interface standardization, partly because the world market for DWDM grew very rapidly before ITU-T could specify a full set of parameters for transverse compatibility. The main discussion point was the specification of the frequency grid [3]. The frequency grid is a set of candidates for channel frequencies. Which frequencies are used is up to the operator or manufacturer; however, they should be selected from the frequency candidates on the “ITU grid”. This specification drastically reduced the possible number of laser frequencies to a finite number and contributed to the cost reduction of laser/filter products. The DWDM frequency grid is specified with reference to its anchor frequency of 193.1 THz (basically the center of optical amplifier bandwidth) according to proposals from NTT and other organizations. Channel spacing is specified as 200, 100, 50, 25, and 12.5 GHz, in ITU-T Recommendation G.694.1 [4]. On the other hand, CWDM for metropolitan or access networks is a recent hot topic in standardization. ITU-T also specifies (in G.694.2) that the CWDM wavelength grid has 20-nm spacing, such as 1290, 1310, 1550, 1570 nm, and so on [5]. The rationTransverse compatibility Longitudinal compatibility