Zenith On the International Space Station, "zenith" and "nadir" are used instead of "up" and "down", referring to directions within and around the station, relative to the earth. Zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir. The zenith is the "highest" point on the celestial sphere. The word "zenith" derives from an inaccurate reading of the Arabic expression (), meaning "direction of the
Point-to-Point Protocol over Ethernet (Ethernet MAC, variable) + 10 (RFC 2684 LLC, variable) + 8 (AAL5 CPCS). This overhead is that obtained when using the LLC header option described in RFC 2684 for PPPoEoA. Compare this with a vastly more header-efficient protocol, PPP + PPPoA RFC 2364 VC-MUX over ATM+DSL, which has a mere 10-byte overhead within the ATM payload. (In fact, just simply 10 bytes = 2 bytes for PPP + zero for RFC 2364 + 8 (AAL5 CPCS).) This figure of 44 bytes AAL5 payload overhead can be reduced in two ways: (i) by choosing the RFC 2684 option of discarding the