[nsp] clock extraction

[Carlson Per]

> The question i have is,what's the difference between a clocked and an
> unclocked circuit.

On a "clocked circuit", the (layer 2) service provider has network equipment
which controls the clocking of the circuit. This is normal for a fractional
E1/T1 and OC-n/STM-n circuits. Here, both terminals can use the incoming 
bitstream for clocking (clock source line).

Conversely, an "unclocked circuit" is transfered transparently through the
SP network. 
No SP equipment changes the clocking of the circuit. This is normally true
for
all non-fractional PDH-circuits (E1/E3/T1/T3). One of the terminals must
clock the
line (clock source internal) and the other can use the bitstream (clock
source line).

The clocking terminal *must* ensure the clock is within the limits defined
by the
PDH-standard (ITU-T G.703). For a E1-circuit, this is +/- 50ppm. If the
clock is 
without the limit, a LOF will be generated. 

Using line clocking on both terminals on an "unclocked circuit", is *not* a
good idea!

> If this is the case,how the network elements detect the 
> absence or existence of clock in a circuit.

They can't. This has to be manually provisioned. 

Pelle