Clarify L1 and L2 clocking on voice interfaces

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Oct 16th, 2010
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I want to clarify the following questions:

1. What is L1 clocking on E1/T1 interfaces? How to setup?

2. What is L2 clocking on T1/E1 interfaces? How to setup?

3. What does network-clock-participate and network-clock-select do?

4. What does clock source line and internal do?

5. What does  isdn protocol-emulate network and  isdn protocol-emulate network?

6. What are the possible mix of those commands?

Please describe but not provide us links. I have some experience and know most of the info on the Cisco site. I need a good summary of the things.

Thanks in advance.

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paolo bevilacqua Sat, 10/16/2010 - 14:58
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Please describe but not provide us links.

Excuse me, that would mean that to answer your post, someone is supposed to write a new description for quite simple things that are explained in full detail exactly in the links you are not interested ?

BTW: there is no such thing as L2 clock.

z.zdravkov Sat, 10/16/2010 - 21:44
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Hi Paolo,

Sometimes we need to read the same thing on a few places to understand correctly what is it. If you know useful thing on on clocking subject please post them. I will try to do the description by myself.

Thanks in advance.

pamistry Sun, 10/17/2010 - 00:34
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there is no difference between L1 and L2 clocking. The reason clocking is important is to have the transmit (Tx) and receive (Rx) operations be clocked in a consistent manner. If they are not synchronized line errors will occur in form of timing slips. These will result in bad audio and clipping and loss of data for faxes.

ISRs have a TDM backplane and a mainboard TDM switch. NMs also have the their own TDM switch and maybe own DSPs. Every device will have its own clocking domain if a DSP is present on it. For eg, a NM with DSP on board will have its own clocking domain. and so will the motherboard. These are defined as clocking domains.

The T1 cards must be clocked synchronously with these DSPs.

The clocking domain is driven via the Phase locked loop (PLL).

The PLL can be driven either by the clock derived from the line (Telco) or by another source which clocks the TDM backplane.

So, if an ISR has 2 NM in it with DSPs residing on each one of them. There will be 3 clocking domains - Motherboard Domain, & 2 NM domain.

network-clock-participate : command whill thus instruct that NM to join the TDM backplane of the motherboard and drive the clocking off it. The T1 ports will propogate the backplane clock to teh devices they are connected to (unless you have network-clock-select command configured)

network-clock-select : this command instrcuts the TDM backplane to use the clock derived on that T1/E1 port as the clock reference to drive the mainboard PLL

clock source line : instructs the T1/E1 to derive clocking reference off the external device (Telco)

clock source internal : instructs the T1/E1 to use the local PLL to drive the external device (in case you have a PBX connected to it)

Note : You must configure clock source as line, if you plan on configuring network-clock-select as the clocking must be derived from an external source. Also, the NM on which the T1/E1 port resides must be network clock participated.

Sample configuration :

network-clock-participate wic 0    <<<< Wic 0 in slot 0 has joined the TDM backplane clocking domain

network-clock-select 1 t1 0/0/0     <<<< This TDM port is being used to drive the clock of the TDM backplane

the controller is set to drive clocking from the line. Hence now, if we do show controller t1 0/0/0 we will not see any slips under it.

HQ#sh controller t1
T1 0/0/0 is up.
  Applique type is Channelized T1
  Cablelength is long gain36 0db
  No alarms detected.
  alarm-trigger is not set
  Soaking time: 3, Clearance time: 10
  AIS State:Clear  LOS State:Clear  LOF State:Clear
  Version info Firmware: 20071129, FPGA: 20, spm_count = 0
  Framing is ESF, Line Code is B8ZS, Clock Source is Line.
  CRC Threshold is 320. Reported from firmware  is 320.
  Data in current interval (541 seconds elapsed):
     0 Line Code Violations, 0 Path Code Violations
     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
  Total Data (last 24 hours)
     0 Line Code Violations, 0 Path Code Violations,
     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,
     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Hope this helps..

Thanks and Best Regards,

Pashmeen Mistry

P.S : Please always rate documents and mark it answered if it has

pamistry Sun, 10/17/2010 - 00:42
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Forgot to answer the isdn protocol-emulate network command question.

That is configured based on whether you want to conifgure the port to behave as slave (user) or master (network). User is the default configuration and is used when connecting to the Telco in all cases.

If you are configuring 2 T1 back to back and intend to use as voice PRI, then you should configure one side as user and the other side as network, for the L2 state to be in "Multiple_Frame_Established"

Hope this helps.

Thanks and Best Regards,

Pashmeen Mistry

z.zdravkov Sun, 11/14/2010 - 06:19
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Hello Pashmeen,

Thank you very much for your comprehensive answer.

I readed your post few times. It becomes more clear.

Now I wondering the following:

1. What exacly clocking does on physical and datalink layer on ISDN?

2. Lets assume that we have clock source line, network clock participate and network clock select on E1 connected to telco. This means that we are getting clock from PSTN. Does it mean that we use the telco clock to synchronize the frames of isdn with DSP proccesors? I will appreciate furher details for that scenario.

3. Does "isdn protocol emulate ...." command work for datalink master / slave determination?

Thnaks in Advance.


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