The "network-wide seamless handover" function requires an exact phase relationship between the basic stations, especially in connection with data applications. The deviation of the DECT frame at the air interface must not be more than +/- 2 µsec between the basic stations for "seamless handover". Frequency economics also require this in-phase condition. Due to their synchronization over TDM connections, OpenScape 4000 systems are frequency- and phase-synchronous in relation to one another. However, every system has its own frame clock phase relationship because the different propagation times of the digital interfaces are compensated by frame adjustments on the receiver side. The PCM frame clock (8 kHz) of various OpenScape 4000 systems can therefore have any phase relationship of module 125 µsec in relation to one another and cannot be used for network-wide synchronization of the DECT basic stations. Instead, GPS receivers are used which deliver a 12-sec pulse from 1 or 2 measuring boards to the master SLMC board of each system. The pulse phase relationship must be absolutely exact with a deviation of +/- 500 ns. This exact phase time information is then evaluated and measured in relation to the system clock. The CMI basic stations receive setting information from the derived value so that they can keep their frame clock at the air interface in relation to one another with a deviation of +/- 2 µsec. This procedure provides the hardware for network-wide "seamless handover".

In the case of VoIP, please note that TDM connections are no longer needed to connect the systems, but that IP trunking can be used exclusively. It has been demonstrated that in the case of Cordless with ISS, direct external system clocking via the 2.048 MHz clock from the GPS receiver increases stability more than synchronization over tie trunks. It is therefore strongly recommended to always use the "frontref" supplied by the GPS receiver.

A Datum GPS receiver has been certified for ISS V1.0 for this. The GPS receiver also has a V.24 output to which a cable to a OpenScape 4000 input can be connected (thus making the OpenScape 4000 UTC compliant).

If a receiver of this kind or an SLCSM fails in 2008 or later and cannot be repaired or replaced, the relevant system will have to be upgraded to ISS V2.0. The network supports combinations of V1.0 and V2.0. When replacing a GPS receiver, the aerials must also be replaced but the aerial cable can continue to be used. When exchanging SLCSM for SLCSS, both master and standby have to be replaced. The cables can still be used.

A Meinberg GPS receiver (available from Unverdross-Technik) has been certified for ISS V2.0. The time output here is suitable for SLCSS measurement modules, the clock output is suitable for "frontref" (special cable). A suitable NTP time normal for this receiver can also be ordered so that time is supplied to the systems, servers and terminals via the LAN.