Multiple Integrated Laser Engagement System (M.I.L.E.S.) 

The inspiration for MilesTag.

The green box on the end of the M-16A2 is the M.I.L.E.S. laser transmitter unit.  The small yellow knob on the transmitter unit is a key that is used to enable the transmitter or silence the harness (very loud) "I am dead" tone.  When you are "killed", you remove the key from the transmitter (disabling it) and insert it into the harness to shut off the tone.

Here you can see the MILES Harness and Halo.  These are linked by a low-power RF signal.  The RF link is also used for "grenades" in the MILES system, so that they can accurately simulate the blast radius of a real grenade.