This part of IEC 62149 covers the performance specification for 850-nm discrete vertical cavity surface emitting laser (VCSEL) devices of transverse multimode types used for the fibre optic telecommunication and optical data transmission application. The performance standard contains a definition of the product performance requirements together with a series of sets of tests and measurements with clearly defined conditions, severities, and pass/fail criteria. The tests are intended to be run on a "once-off" basis to prove any product's ability to satisfy the performance standard's requirements. A product that has been shown to meet all the requirements of a performance standard can be declared as complying with the performance standard, but should then be controlled by a quality assurance/quality conformance program. Depending on the modulation speeds, subcategorized specifications are defined. Types A1, A2 and A3 correspond to 1,25 Gbit/s, 2,5 Gbit/s and 4,25 Gbit/s VCSELs, respectively. Each subcategorized specification is also defined by separate details depending on the device types, such as specifications for a VCSEL device without a monitor photodiode (Case a) and for a VCSEL device with a monitor photodiode (Case b).