39/10 Modems of applications and a relentless downward trend in pricing. Features STREAM can be categorised into three groups. Some enhance the basic group's modem performance itself. Some actually restrict perfor- mance in order to better accomplish a specialised task, whilst others aid user operation independent of performance. The features em- ENCODED phasise dial up modems since they incorporate such a wide range of SIGNAL functionality. 39.5.1 Transmission The most common standards are those of the CCITT. The CCITT transmission standards are covered by the V series, the code desig- nation for data transmission over the telephone network. Digital signal processors now deliver multiple transmission standard capa- bility in a single chipset. Two of the most popular combinations today are the low speed Figure 39.11 Baseband signalling (Manchester encoding) 'quad' combination covering V.22bis, V.22, V.23 and V.21 and the high speed 'quin', V.32. V.22bis, V.22, V.23 and V.21. Although demanding ever more powerful processing to be able to provide and from carriers like BT. Unloaded circuits are restricted to local negotiate the inter modem communication for this automatic multi exchange areas which means a maximum distance of approximately standard capability, the user benefits of universal connectivity are a 7 miles. Short haul modems and local data sets are two popular powerful selling point. terms for devices using baseband techniques over short distance unloaded lines, providing both synchronous and asychronous oper- 39.5.2 Operating modes ation up to 19.2kbit/s, and up to 64kbit/s in special applications. The maximum operating speed is usually distance dependent. A modem will typically offer a range of operating modes over a Limited distance modems are hybrid devices. Some offer the same variety of lines, as described in following sections. performance as CCITT standard modems over shorter distances by using less sophisticated, non standard modulation and equalisation techniques. Indeed some U.S. developed mod