pport interface requirements to permit the addition of high-speed data transfer to an existing cable TV system. There are several generations of DOCSIS standards: DOCSIS 1.0, 1.1, 2.0, and 3.0. The ITU has adopted DOCSIS 1.1 and 2.0 as international standards. DOD (direct outward dialing) A system in which trunks are used specifically for outgoing calls (e.g., when you dial an access code such as the number 9 or the number 8 to get an outside- line dial tone before you can dial the actual number that you want to reach). downlink The portion of a satellite circuit that extends from the satellite to the earth station. download To receive data from a remote system. downstream The direction of transmission flow from the source toward the user. downtime The total time a system is out of service due to equipment failure. DPNSS (Digital Private Network Signaling System) The European standard for com- mon channel signaling between PBXs. DPSK (Differential Phase-Shift Keying) A form of phase-shift keying used for digital transmission in which the phase of the carrier is discretely varied in relation to the phase of the immediately preceding signal element and in accordance with the data being transmitted. DQDB (distributed queue dual bus) The media access method of the IEEE 802.6 stan- dard for MANs. DQPSK (Differential Quadrature PSK) A digital modulation scheme used with extended-rate 2Mbps 802.11 WLANs. drop A connection point between a communicating device and a communications network. DRR (Deficit Round Robin) A queuing mechanism in which a maximum packet size num- ber is subtracted from the packet length, and packets that exceed that number are held back until the next visit of the scheduler. DS (Digital Signal) level The increments of the PDH hierarchy (North American standard). DS-0 is a single channel with a capacity of 64Kbps; DS-1 is 24 DS-0 channels multiplexed into one 1.544Mbps T-1 digital trunk; DS-1C is a 3.152Mbps digital signal carried on a T-1 facility; DS-2 is a 6.312Mbps digital signal carried over 96