SDI is a family of digital video interfaces (SDI, HD-SDI, 3G SDI, 6G SDI and 12G SDI), that was first standardized in 1989 by SMPTE (The Society of Motion Picture and Television Engineers).  

SDI is mostly used in professional areas where an uncompressed, unencrypted digital video signals (optionally including embedded audio) must be distributed over a long distance, for instance in Broadcast where the SDI cables allow to connect cameras to studio equipment like video mixers, recorders, monitors, PCs, etc. 

It can be also found in the Live events industry, Conferencing, etc.

PROS

•Very low latency

•No compression, no conversion

•Maximum length of 300m with coaxial cables but limited to 100m with HD bitrates; almost illimited distance with Fiber optic cables and/or repeaters.

•Lockable and rugged BNC connector the most common used connector for SDI coaxial cables (see picture)

•Many professional equipment use SDI (especially in Broadcast)

CONS

•Contrary to inexpensive consumer equipment, professional SDI equipment can be relatively expensive

•SDI cables can be more expensive compared to other types of cable

Make sure that the SMPTE  standards match across the installation :

Source: Wikipedia (http://en.wikipedia.org/wiki/Serial_digital_interface)

* Note: the 3G-SDI connector on our CM7x-IP cameras support SMPTE 425M level A. 424M and 425M, as defined by SMPTE, define different aspects of the same 3G-SDI standard : SMPTE 424m means basically that the device as a 3G-SDI port, capable of handling sufficient data and nothing more than that. SMPTE 425m specificities how this bandwidth is being utilized: in the case of Level A, this is a direct mapping of uncompressed images into a serial digital interface operating at a nominal rate of 3Gb/s