In the simplest case, a single connection is established to an upstream ISP and is used to transmit data to or from areas of the Internet beyond the home network; this mode of interconnection is often cascaded multiple times until reaching a tier 1 carrier. In reality, the situation is often more complex. ISPs with more than one point of presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and may have connections to each one of them at one or more point of presence. Transit ISPs provide large amounts of bandwidth for connecting hosting ISPs and access ISPs.
Try to answer this question if possible (otherwise reply "unanswerable"): What is the simplest case of an ISP using an upstream ISP?
transmit data to or from areas of the Internet beyond the home network