1.  Field of the Invention
The present invention is directed to a method for transmitting information signals in a subscriber terminal network connected between a local exchange and individual subscribers wherein measurements of the transmission parameters of the subscriber terminal lines are carried out and, according to the standard provided by these measurements, a maximum bit rate is determined for each subscriber terminal line. 
2.  Description of the Prior Art
As a rule, telecommunication networks in the subscriber terminal region are provided with a subscriber terminal network that connects one, or several, local exchanges with a plurality of subscribers or, respectively, the terminal apparatuses thereof.  The two ends of each respective subscriber terminal line are terminated via a terminal installation in the local exchange at the switching side and via a network terminal means at the subscriber side. 
For some time, telecommunication networks have been constructed worldwide using digital technology.  It therefore follows that every attempt is made to use the already-existing subscriber terminal networks for the transmission of digital information signals Indeed, given that the subscriber terminal networks are already widely present as two-wire or coaxial terminal lines and, thus, no new lines (such as glass fiber, for example) need to be installed, a considerable economic advantage may be had in practice. 
It is true that these already-existing subscriber networks were originally designed for the transmission of low-frequency information signals such as, for example, in analog telephony or in the use of narrow-band services.  Despite these original designs, however, these networks are also suited for the transmission of broader-band services.  Thus, in the last few years methods have come into use with which the existing copper lines in the subscriber terminal networks are used for the transmission of digital information signals.  These transmission methods include, for example, the ADSL (Asymmetrical Digital Subscriber Line) and HDSL (High Bit Rate Digital Subscriber Line) methods. 
The bit rate transmitted in the subscriber terminal network is typically determined with 2.times.64 kbit/s in such a way (2 B channels) that the longest existing terminal lines can still be used.  However, under this scenario, a large part of the maximum possible bit rate in the subscriber terminal network remains unused.  This is due to the fact that the attenuation constant of the symmetrical wire pairs of copper terminal lines increases as the line length increases between the local exchange and the subscriber.  This attenuation constant thus limits the range of the digital signal transmission. 
In general, the shorter the length of a subscriber terminal line, the higher the bit rate can be which is to be transmitted by it.  As an illustration, assume, for example, a subscriber terminal line of 8 km which is to be the longest line in the subscriber terminal network.  In prior art, the transmission capacity (2B+D) is determined, and the cables are used, in such a way that problem-free operation is possible in this longest line (8 km) in the subscriber terminal network.  This also holds for the shorter subscriber terminal lines.  These lines can therefore transmit a much higher bit rate since, among other things, they are considerably shorter than 8 km. 
These fixed bit rates, available to all subscribers in the subscriber terminal region, have the advantage in that they are determined in dependence on the longest subscriber terminal line.  Thus, the shorter subscriber terminal lines in the subscriber terminal network, which are suited for the transmission of information signals of broader-band services, cannot be fully exploited. 
From the German OS 43 43 982, a terminal line network with subscriber terminal lines for the transmission of digital information signals is known.  It is disclosed therein how the maximum possible bit rate per transmission line can be determined.  For this purpose, bit patterns are transmitted by a bit sample generator in the local exchange along the subscriber terminal line to the subscriber wherein they are, in turn, reflected back to the local exchange.  An evaluation of the bit patterns received then ensues in the exchange.  The maximum possible bit rate is subsequently determined in dependence on the results.  However, a disadvantage of this type of procedure is that a solution of this sort is very specific as to the individual subscriber terminal line and, thus, cannot be used everywhere in the subscriber terminal network.