two-frequency (2VF) signaling, 151-152 (TCP/IP), 319-327. See also internet protocol two-way circuits, 9-10 (IP); IP entries; TCP entries; voice over IP two-wire/four-wire conversion, 187, 188 entries two-wire telephone transmission, 75 data-link layers and, 320-322 transmission convergence sublayer, ATM, 522 transmission link capacity, traffic intensity and, UDP/IP stack, 556 55-67 UGS (unsolicited grant service), 355 transmission loss distributions, digital loop carrier UIUCs (uplink interval usage codes), 354 design, 102 ULP modes, 326-327 transmission-loss engineering, 191-192 ULP synchronization, 326 transmission loss radio propagation model, 463 unacknowledged connectionless service, logical transmission media, 1 link control, 297 LAN, 291 unbalanced configuration, HDLC, 284 performance characteristics of, 246 types of, 31 uncompressed video, transmitting on CATV trunks, 447-448 transmission parameters uncontrolled access mode, 518 message transfer part, 376 video, 409-410 uniform numbering, 161-162 transmission reference point, 632-634 UNI management entities (UMEs), 569 transmission system impairments, 45-51 unintelligible crosstalk, 50 amplitude distortion, 45, 46 unique field, 261 noise, 45, 48-51 unit call (UC), 57 phase distortion, 45, 46-48 United States. See also American entries; North transmission systems, survivability of, 174. See America also coaxial cable transmission systems development of telegraph system in, 20, 23 transmission transport, 195-249 digital network loss plan for, 193 coaxial cable transmission systems, 244-245 PSTN organization in, 16-17 fiber-optic communication links, 231-244 transcontinental communications in, 173, 174 radio systems, 196-217 universal coordinated time (UTC), 140 satellite communications, 217-231 Universal Mobile Telecommunication System transmission media summary, 246 (UMTS), 458 transparency, frame, 332 unknown quantities, 603-604 transparent bridge, 309 U-plane, ATM layering at, 522, 523 transport layer, OSI model, 283-284 uplinks, satellite, 228 trap, 55