 the ratio of SES to total seconds in available time during a fixed measurement interval. Background Block Error Ratio (BBER) This is the ratio of background block errors (BBE) to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs. Block Sizes Some examples of block sizes are: For 1.544 kbps PDH path, the PDH block size is 4632 bits. For 2.048 kbps PDH path, the PDH block size is 2048 bits. For 1664 kbit/s VC-11 path, the SDH block size is 832 bits. For 2240 kbit/s VC-12 path, the SDH block size is 1120 bits. Availability and Unavailability A period of unavailable time begins at the onset of 10 consecutive SES events. These 10 seconds are considered to be part of unavailable time. A new period of available time begins at the onset of 10 consecutive non-SES events. These 10 seconds are considered to be part of available time. A bidirectional path is in the unavailable state if either one or both directions are in the unavailable state. Quality degradation in an access transmission network may be due to capacity problems, microwave link performance related issues including interference, synchronisation failures or even those due to the parameter setting (e.g. addressing scheme) in the network management system. The process can start with monitoring the PCM failures from the network management system (as shown in Figure 3.78). In this figure the two peaks on the right-hand side are a cause for concern as they degrade the quality of the system. The reason for the outage could be human error, power failures or performance degradation in terms of aspects mentioned above. The impact of the DAP on the access transmission system and how it can be resolved was discussed in the section above. However, sometimes the transmission network is simply not capable of taking the load of the subscriber growth. This situation takes place when the sites are added and without considering the impact it will have on the transmission system. In networks that have h