n several ways due to three main limitation factors. Capacity Link capacity brings a limitation in terms of traffic channels that can be handled by a given site. For example, if a site with a 12 TRX configuration presents high TCH blocking rates it needs a TRX extension. Since one PCM (which refers to a 2 Mbps link) can handle only 12 TRXs, more transmission capacity (another PCM) is needed. Quality/Interference Quality is a relevant factor, especially for microwave media. Performance errors, availability and interfer- ence are the main aspects of quality. Bad frequency planning of hoppers, for example, negatively impacts on the quality of the network. Synchronisation Synchronisation is crucial for a good performing network. Transmission slips may not affect speech quality but will definitely affect the handover success rate (HSR). Most operators have a multivendor transmission network. In such a situation, a unique procedure to monitor the entire network is important. Two different configuration modes exist for a transmission link. The DBLF (double frame mode) is a configuration mode, which reports a few indicators on link quality. It is suitable only for short distances, as in the MSC-Transcoder links. When the CRC4 (4 bit cyclic redundancy check) mode is enabled, multiframes are transmitted and received. These frames are monitored for CRC4 errors and errors are reported in the CRC4 error counter. This mode therefore allows better monitoring of the transmission media by providing more indicators than the first one. It is recommended for long distances, as in the BTS-BSC and BSC-Transcoder links. When this mode is active, the BSS can report statistics on exchange terminals downtime, availability and synchronisation. Transmission experts then analyse the statistics, extract bad performing links and operate corrective actions. NSS Performance The NSS part plays a crucial role in interfacing mobile and fixed networks. The centralised architecture of GSM networks emphasises this point more (one MSC may handle severa