User's Manual

Configuring the traffic interfaces | 115
HSS clocking types
HSS internal clocking
Internal clocking relies on the (highly accurate) terminal system clock, that is, it does not allow for any
independent clocks coming in from client equipment.
For this mode, all incoming clocks must be slaved to a clock emanating from the HSS card.
HSS pass-through clocking
The HSS card is capable in hardware of passing two clocks from one side of a link to the other.
Passing a clock means that the difference between the client clock(s) and the terminal clock is
transferred across the link continuously. Passing a single clock in each direction requires 40 kbit/s
additional link overhead, passing two clocks from DTE to DCE requires 56 kbit/s overhead, whereas
relying on internal clocking requires no overhead.
Network topology determines if passing a clock makes sense. Passing a clock is used where a client's
incoming clock must be kept independent of the clock sourced by the HSS card. The only time it
makes sense to pass two clocks is when a client DCE in one of the HSS modes provides two
independent clocks, that is, the HSS is set to Clock Mode 2.
Pass-through clocking does not require using the HSS incoming clock as a Primary or Secondary
master clock for the link, but does not preclude it either.
HSS primary / secondary master clocking
When implementing an external clock master, all other interfaces in the terminal and internal system
timings are slaved to this external clock. The remote terminal is also slaved to this master clock. This
master clock must be within 100 ppm of the accuracy of the terminal system clock, otherwise the
terminal will revert to using its internal clock. Ideally, the external clock should be much better than 100
ppm.
Mode 6 is offered for those network topologies that require RxC and TxC to be locked. For example,
this is useful when interworking with an Aprisa SE HSS interface.