s no guarantee of any (3) and to some extent (2) could have been solved by extensions and management information. The usual example here is that of the modifications to the existing PDH standards. In the future, though, 2Mbit/s private circuit, leased to the customer as a clear channel it is likely that the sheer quantity of management information i.e. the PTO imposes no G.704 frame structure and the cus- needing to be transported around a network, will eventually pose an tomer has full control over the entire 2.048Mbit/s bandwidth. insoluble problem for even a modified/extended PDH, hence any In this case, there is no room for the PTO to insert any monitor- solution to this problem would always be viewed as a stopgap measure. ing or control information, hence it cannot tell when the per- formance of such a circuit drops out of specification. On the The problem of traffic path inflexibility is much more difficult to other hand, the customer has almost certainly imposed his own overcome by modifying the PDH. It is tempting to suggest syn- frame structure, and therefore can monitor the delivered per- chronising an entire PDH transmission network, as many PTOs formance with great accuracy. have already done at the 2Mbit/s level. Unfortunately, as well as 3. Lack of 'Mid-Fibre Meet'. Although the PDH specifies the being progressively more difficult to do at the higher bit rates in the exact format of the bit stream at the aggregate port of any PDH PDH, synchronisation alone does nothing to preserve the byte multiplexer, it puts no such constraints on the bit stream on the boundaries within each bit stream. Without any easy method of line side of a line transmission terminal. (See Figure 42.2.) identifying these byte boundaries, the task of extracting both low Consequently, every manufacturer has used his own proprietary order traffic and management channels has not been significantly line code and optical interface specification, making it im- simplified. As a final nail in the coffin, strict network synchronisa- pos