Sidecar - This type of set-top box provides an additional transport stream of data from the content provider (network operator) to compliment the original stream going to the set-top box.  With Charter Communications’ interactive TV rollout, the BMC-8000 (Broadband Media Center) is/was a sidecar box that works/worked in tandem with the Motorola DCT-2000 set-top box. A fully integrated set-top box would not require a sidecar. (This is a 2003 definition.)