Referential integrity is a relational database concept, which states that table relationships must always be consistent. In other words, any foreign key field must agree with the primary key that is referenced by the foreign key. Thus, any primary key field changes must be applied to all foreign keys, or not at all. The same restriction also...
V.92 is an ITU Telecommunication Standardization Sector standard for modems that is an enhancements to recommendation V.90. V.92 emerged in 1999 and allows 56 Kbps downloads and 48 Kbps uploads. It also uses the V.44 compression method for adaptive data compression. V.92 was the last dial-up standard.
V.92 allowed pulse-code modulation (PCM) for both upstream and downstream connections; V.90 only allowed this for downstream connections. V.92 still required one analog/digital conversion, and even by 2003 most modems did not support PCM upstream. Two modems that did support PCM upstream, 3Com and Patton, only allowed a maximum upstream rate of 33.3 Kbps, which was even less than the maximum V.34 rate.The V.44 compression method allowed an average of 15 percent greater throughput than the previous V.42bis standard. In some cases, depending on the actual compression ratio, noise on the line and already compressed data, the transmission rate could be as high as 320 Kbps for pure text files and 160 Kbps for uncompressed files.Other enhancements to the V.90 standard included a reduced connection time, a modem on hold (MOH) feature and the storage of previous connection data in a buffer. The latter allowed modems to maintain a connection while the user answered an incoming call-waiting call or placed an outgoing voice call. However, MOH only worked if the server modem was configured to allow this feature. In addition, Internet service providers had the ability to limit waiting times from between zero and 16 minutes. Some modems even included software to warn the user as the set time limit was approaching. V.92 modems also remembered connect rates of past connections, called a quick connect feature, and compared line quality variables stored in a buffer. When a match was found with a previous connection, the handshake - an automated process that dynamically sets parameters before actual data transmission takes place - occurred at the previous rate. This “quick connect feature” also worked after a connection using the MOH feature.One source remarked that the upstream transmission rate of 48 Kbps was as elusive as V.90's 56 Kbps downstream rate. This may have been due to a similar limitation to maximum transmission rates imposed by the U.S. Federal Communications Commission and North American public switched telephone networks, although there is little data to verify this.
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