ynchronous data transmission at 9.6kbit/s over mobile land channels and is set to foster a new generation of wireless local area networks. 39.5.2.2 Dial up vs leased lines Calls over the PSTN are routed by tone or pulse signals generated Typical modem features by the modem which duplicate those of an ordinary telephone. The advantages are that one modem can connect to any other modem on The years have seen a tremendous change in modem features and the same network no matter where it is and charges are based on functionality. Although quite adequate at the time, the 300 baud distance and connection time, not the amount of data transmitted. modem of the early 70s is a far cry from the sophisticated and Disadvantages are that the line quality varies because the circuits powerful, multi transmission standard, error correcting and data are switched through exchanges which introduce transmission loss compressing products of today. Techniques like multi layer printed and noise. Even if the same destination is dialled, the route may be circuit boards, surface mount technology and DSP (digital signal different, SO the line cannot be "tuned" for better quality. processing) have combined increasing functionality with decreas- Leased lines or private lines obtained from telephone companies ing physical size. are permanent connections between two points. Because the quality The range of options, once available only in hardware, can now of the line is constant and the route is fixed, it can be fine tuned or be programmed by software, and each year brings a new diversity conditioned to carry high volumes of data, at higher speeds than dial Typical modem features 39/11 up lines and with lower error rates. Most of today's sophisticated 39.5.2.6 Data compression dial up modems incorporate 2 or 4 leased line operation as standard. Data compression schemes are a way of increasing throughput as it gets increasingly difficult to improve the modem's basic operating 39.5.2.3 Synchronous or asychronous speed as upper limits of performanc