CWDM transceiver standard analysis and introduction
According to different standards, CWDM transceivers include CWDM SFF, CWDM GBIC, CWDM SFP, CWDM SFP+, CWDM XFP, CWDM X2, CWDM XENPAK, and CWDM LX-4.
CWDM SFF (Small Form Factor): The SFF was one of the first commercially available small form transceivers that used only half the space of the popular conventional SC types. CWDM SFF transceivers found their way into applications ranging from 100 Mbps all the way up to 2.5 Gbps.
CWDM GBIC (GigaBit Interface Converter): GBIC was the first pluggable MSA (Multi-Source Agreement) transceiver with applications primarily in switch and router blades for Gigabit Ethernet. An easy migration from the standard LH part was to have specific wavelength DFB lasers; this enabled the development of CWDM GBICs and eventually DWDM GBICs. The GBIC transceivers have been used primarily in Gigabit Ethernet applications, but in some cases both lower speed, multi-rate parts and higher speeds of around 2.5 Gbps have been developed. The GBIC was one of the first transceivers to have an APD receiver rather than a PIN type for higher sensitivity. Tips: There were three basic types of Ethernet transceivers: the short reach, an 800-nm VCSEL-laser-based part called the SX with a span of 500 m over MMF; the medium reach, 1310-nm FP laser part called the LX with a span of 2 km over SMF and a 1550-nm DFB part used for long haul called the LH with a span of 80 km, also over SMF.
CWDM SFP (Small Form Pluggable): SFP is the transceiver combining the space savings of the SFF and the flexibility of the GBIC. It has approximately the same size of the SFF but with the functionality of a GBIC. Besides the size advantages over the GBIC, the SFP had lower power dissipation, since it operates at 3.3 V while the GBICs worked with 5V supply voltage. In addition, the transceivers provide monitoring capabilities for power, temperature, and voltage levels. With technology being pushed to meet the demands of both function and size, CWDM and DWDM SFP transceivers have been developed for use in Gigabit Ethernet as well as 2.5 Gbps SONET applications. Moreover, SFPs have also been developed for use in 1, 2, and 4 Gbps Fibre Channel applications.
CWDM 10G (CWDM SFP+, CWDM XFP, CWDM X2, CWDM XENPAK, and CWDM LX-4): There are multiple MSAs for the 10-Gbps transceivers, typically denoted by an X in the name, X standing for the Roman numeral 10. XFP, XENPAK, X2 and LX-4 are the names of some of these devices. To enable longer spans at 10 Gbps, the institute of IEEE has developed a standard for use of CWDM in the design of these high bit-rate devices, IEEE 802.3 ae-2002. Instead of having a serial link running at 10 Gbps line rate, the LX-4 concept optically transmits and receives four signals at different wavelengths spaced at 1275, 1330, 1325, and 1350 nm. The 25-nm grid is slightly different than the ITU 20-nm grid. The operating bit-rate per channel is 3.125 Gbps, the aggregate bandwidth of all four channels is 12.5 Gbps, which includes the associated protocol overhead. The advantage of the LX-4’s parallel approach is that each of the four tributary channels has a lower bit-rate and better dispersion tolerance. At the same time, lower-cost lasers and detectors can be used.