[nsp] clock extraction

[Michael Loftis]

It works something like this...

On a clocked line you are given so many channels.  You must make sure that 
your data goes out inside of that envelope, thus your equipment listens to 
the incoming data stream, and makes the outgoing data stream match that for 
clocking.

In a non-frac or unchannellized (CCT1 or full DS1, or other nomenclature, 
depending on who you talk to) one end must clock the line.  The whole T1 is 
passed inside of the SP network unmolested as it were, so clocking inside 
the T1 is the responsibility of the endpoints.  If it's fractional the SP 
network MUST be able to derive the individual DS0's (56/64k circuits...) so 
they must clock the line, and for legacy reasons this really means that 
both ends will be line clocked and the SP provides the incoming clock 
signal.

DS0s are bundled into DS1s, DS1->DS2->DS3->....  If you have a full DS1 the 
network only cares about the DS1 as a whole.  If you've a bundle of DS0s (a 
frac DS1) then the network must care about the timing of individual DS0s so 
it can agg them up to the DS1 level.  Likewise if you have a clear DS3 the 
network cares not how the individual DS1s and DS0s inside are timed and 
stuffed since it doesn't need access to them.

I know it doesn't make much sense, and yes it's a complicated thing.

--On Tuesday, December 02, 2003 13:42 +0200 Kostas Anagnopoulos 
<kostas.anagnopoulos at oteglobe.net> wrote:

> Hi Pelle,
>
> Thank you very much for your reply.However,still it is not very clear to
> me how the receiver derives a clock from the bitstream of a fractional
> e1/t1 etc.
> while it cannot do the same from the bitstream of an unclocked circuit,
> and requires a terminal to send the clock.
>
> Is there a difference in the clock recovery method of the two.
>
>
> Many thanks,
>
> Kostas
>
>
>
>
> -----Original Message-----
> From: Carlson Per [mailto:per.carlson at banetele.com]
> Sent: Tuesday, December 02, 2003 11:35 PM
> To: cisco-nsp at puck.nether.net
> Cc: 'Kostas Anagnopoulos'
> Subject: RE: [nsp] clock extraction
>
>
>> 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
>
>
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/



--
GPG/PGP --> 0xE736BD7E 5144 6A2D 977A 6651 DFBE 1462 E351 88B9 E736 BD7E