two routers connected by T1

Answered Question
Mar 30th, 2009
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HI every body!

I want to set up this scenario( though only on paper) to learn pertinent configurations to T1 circuit

r1s0----T1---------r2so

Both routers have integrated csu/dsu

r1 config


Interface serial 0/0

service-module t1 clock source internal

service-module t1 timeslots 1-24 speed 64

service-module t1 framing esf

service-module t1 linecode b8zs

ip address 10.1.1.1 255.255.255.0

encapsulation ppp

fair-que

no shut


===============


r2 config:


Interface serial 0/0

service-module t1 clock source internal

service-module t1 timeslots 1-24 speed 64

service-module t1 framing esf

service-module t1 linecode b8zs

ip address 10.1.1.2 255.255.255.0

encapsulation ppp

fair-que

no shut


===============================


Is this all we need to connect two routers with integrated dsu/csu by T1?


My book says one of the csu/dsu is master while the other is slave. slave adjust its clock to match the master clock. What determines which csu/dsu is master and which one is slave? Is there specific configuration to make one csu/dsu master and the other slave?



Thanks a lot!

Correct Answer by viyuan700 about 8 years 4 weeks ago

There are 3-1 DACS systems, 3-1-0 DACS systems, M13 multiplexers, statistical multiplexers, channel banks,


Lowest speed SONET mux Oc-3 can only provide T1, to provide subrate T1 Telco use DACS,Statistical mux etc.


Can a Telco provide a WAN circuit using only DACS etc if customer office are hundreds of miles away ?


If not then these DACS are riding on SONET boxes. So now the question is, SONET is deriving clock from DACS or DACS is deriving clock from SONET.


As far as i know DACS will take clock from SONET box which in turn deriving clock from better source or BOTH DAcs & SONET derving clock from better source. If i am wrong i have no problem in learning new stufff.


I posted my stuff as one of your post mentions "Stratum 4 standard, which boasts a very high rate of accuracy and extremely low frequency drift" whereas for SONET it is the lowest standard clock.


If you already know this then maybe for guys who dont know. As i dig some topics on this forum which i dont know.

Correct Answer by lamav about 8 years 4 weeks ago

Sarah:


What that clip from the book is trying to say is that Frame Relay service providers allow clients to burst to port speed/access speed if network congestion levels permit. That is what you should be walking away with.


If the access speed is a T-1, then the carrier can allow you to burst past the CIR and up to port speed if possible. That also is limited by contractual obligations.


THE FECN and BECN bits are used to report congestion to the end nodes so that they can throttle back their send rates.


There's volumes that can be written on this, but as I suggest, the thing to walk away with is the fact that you can burst to T1 access speed if the carrier allows, and the T1, of course, uses a data clock that is derived from the T1 signal. I think that's what the book was trying to say when they mention the clock -- just another way of emphasizing that the access speed is a T1.


HTH


Victor


Correct Answer by lamav about 8 years 4 weeks ago

"but Internal Clock of Telco SONET/SDH equipments is Stratum 3 clock"


SONET uses stratum 3 clocks, yes. SONET is not the only method and system used in a T-carrier system. There are 3-1 DACS systems, 3-1-0 DACS systems, M13 multiplexers, statistical multiplexers, channel banks, etc. The timing standard used by a particular T-carrier sub-system depends on where the NE (network element) sits in the TELCOs network architecture with reference to the source clock.


"Telco equipments are NOT working on their internal clock."


I never said they were. I didn't mention at all where the TELCO gets its source clock from. But since you are mentioning it, yes, the TELCOs get their clocks from a stratum-1 timing source.


HTH


Victor

Correct Answer by lamav about 8 years 4 weeks ago

Sarah:


The master/slave relationship is applicable in both models that we have discussed. Either one CSU/DSU is the master and the other at the distant end is the slave. Or both are slaves to the TELCO master.


In either case, the paradigm survives.


In the case of the TELCO providing the clock, you can look at it as though there are two separate components to the circuit with regard to timing. CSU/DSU 1 to TELCO and CSU/DSU 2 to TELCO.


Why use the TELCO? Because the TELCOs use a Stratum 4 standard, which boasts a very high rate of accuracy and extremely low frequency drift. To maintain that stability, some TELCOs use the so-called BITS system, the Building Integrated Timing Supply.


Maintaining synchronized timing is of the essence in a synchronous circuit, of course. Timing drifts can cause errors on the T1 facility known as "slips."


Since the data clock is a derivative of the received data signal, and uses the voltage shifts from low to high of the data stream to maintain synchronization, it is important that a minimum number of "ones" (high voltage) bits exist in the data stream. This is called ones-density.


If more than 15 consecutive zeroes are received, a loss of synchronization can occur and you will have a timing slip.


To maintain ones density on AMI circuits, the 7th bit of every 8-bit BYTE is forced to be a one (high). This is called bit-7 stuffing. That means only 7 bits out of 8 actually represent data. At a sampling rate of 8,000 frames per second, this leaves a modified DS0 data rate of only 56kbps of data, and the other 8,000 are just timing bits.


To overcome the loss of 8,000 bits, a B8ZS circuit will replace a data pattern of 8 consecutive zeroes with a code that purposefully includes bi-polar violations (BPVs). When the receiving end receives this altered data signal with the BPVs, it will know that it was inserted at the transmitting end and that the code should be replaced with the original 8 zeroes.


In this way, all 64,000 bits per second are used for data transfer and none are wasted on timing. This is called a clear channel.



2.) Im still trying to understand this question. Can you elaborate further?


HTH


Victor


Correct Answer by Richard Burts about 8 years 4 weeks ago

Telco usually provides clocking to both ends. When your routers are connected to a telco you normally do not need to configure clocking at either end.


HTH


Rick

Correct Answer by lamav about 8 years 4 weeks ago

My apologies, Sarah. I read your question wrong. Dyslexic today. :-)


Edison is right, of course.


Internal timing means that the CSU/DSU will use an internal circuit known as an oscillator to generate the timing source. That is considered to be a stratum-4 clock, the lowest in terms of accuracy.


The CSU/DSU which will time off of it will be set for "line," of course, meaning that it is taking its timing from the signal received from the line (the SP). The receiving CSU will extract the clock from the received data and use it as its transmit clock.


If you want to know more about clocking and its relationship to ones density, B8ZS coding, etc, let me know.


Thanks


Victor

Correct Answer by Edison Ortiz about 8 years 4 weeks ago

Sarah,


The other way around. The master will have the clock source internal while the slave will have the clock source line.


You will find most ISPs will provide clocking so the default configuration on the T1 modules is actually line, not internal.


__


Edison.

Correct Answer by Edison Ortiz about 8 years 4 weeks ago

The clock source command:


http://www.cisco.com/en/US/docs/ios/interface/command/reference/ir_s1.html#wp1015411


When set to internal, it's not relying on the remote for clocking.


When set to line, it's relying on the remote for clocking.


HTH,


__


Edison.

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sarahr202 Mon, 03/30/2009 - 09:52
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Thanks Edison!

In leased line such as T1, one of the csu/dsu is configured as master while the other configured as slave.

So if i want to configure one csu/dsu as master. should i use this command:


Router(config-if)# service-module t1 clock source line ?


If i want to configure the other csu/dsu as slave should i configure the command;



Router(config)# interface serial 0


Router(config-if)# service-module t1 clock source internal ?


thanks a lot!



lamav Mon, 03/30/2009 - 10:13
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Correct, Sarah. That is how you would configure it.


Victor

Correct Answer
Edison Ortiz Mon, 03/30/2009 - 10:26
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Sarah,


The other way around. The master will have the clock source internal while the slave will have the clock source line.


You will find most ISPs will provide clocking so the default configuration on the T1 modules is actually line, not internal.


__


Edison.

Correct Answer
lamav Mon, 03/30/2009 - 10:31
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My apologies, Sarah. I read your question wrong. Dyslexic today. :-)


Edison is right, of course.


Internal timing means that the CSU/DSU will use an internal circuit known as an oscillator to generate the timing source. That is considered to be a stratum-4 clock, the lowest in terms of accuracy.


The CSU/DSU which will time off of it will be set for "line," of course, meaning that it is taking its timing from the signal received from the line (the SP). The receiving CSU will extract the clock from the received data and use it as its transmit clock.


If you want to know more about clocking and its relationship to ones density, B8ZS coding, etc, let me know.


Thanks


Victor

sarahr202 Mon, 03/30/2009 - 16:32
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Thanks Edison and Victor



Just want to know who provides this clocking.


for example

r1---dsu/csu---teleco---------csu/dsu--r2


Both r1 and r2 are connected by T1.

Now Who is providing the clock? Based on your post, Telco is providing the clock to dsu/csu. But to which one? Because if telco is providing the clock to csu/dsu connected to r2, then i can not configure r1 dsu/csu with " internal " option. Does telco tell us at which end it will provide clock?


Thanks a lot!


Correct Answer
Richard Burts Mon, 03/30/2009 - 17:39
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Telco usually provides clocking to both ends. When your routers are connected to a telco you normally do not need to configure clocking at either end.


HTH


Rick

sarahr202 Mon, 03/30/2009 - 19:00
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thanks Rick.

My book says One csu/dsu is master while the other csu/dsu is slave On T1 either ends. slave csu/dsu try to keep up with master csu/dsu clock.


Since telco is providing clock on either ends, then what is the whole deal about slave csu/dsu and master csu/dsu?


2) How does the clock rate on access link connected to frame relay switch affects the data rate from DTE ( router)?


This is what my book says:

" if a router has a T1 frame relay access link,but only a 128-kbps CIR on a VC that goes over that link,the router can send a lot more data into frame relay network than bussiness contract with frame relay provider allows."


thanks a lot!

Correct Answer
lamav Mon, 03/30/2009 - 20:02
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Sarah:


The master/slave relationship is applicable in both models that we have discussed. Either one CSU/DSU is the master and the other at the distant end is the slave. Or both are slaves to the TELCO master.


In either case, the paradigm survives.


In the case of the TELCO providing the clock, you can look at it as though there are two separate components to the circuit with regard to timing. CSU/DSU 1 to TELCO and CSU/DSU 2 to TELCO.


Why use the TELCO? Because the TELCOs use a Stratum 4 standard, which boasts a very high rate of accuracy and extremely low frequency drift. To maintain that stability, some TELCOs use the so-called BITS system, the Building Integrated Timing Supply.


Maintaining synchronized timing is of the essence in a synchronous circuit, of course. Timing drifts can cause errors on the T1 facility known as "slips."


Since the data clock is a derivative of the received data signal, and uses the voltage shifts from low to high of the data stream to maintain synchronization, it is important that a minimum number of "ones" (high voltage) bits exist in the data stream. This is called ones-density.


If more than 15 consecutive zeroes are received, a loss of synchronization can occur and you will have a timing slip.


To maintain ones density on AMI circuits, the 7th bit of every 8-bit BYTE is forced to be a one (high). This is called bit-7 stuffing. That means only 7 bits out of 8 actually represent data. At a sampling rate of 8,000 frames per second, this leaves a modified DS0 data rate of only 56kbps of data, and the other 8,000 are just timing bits.


To overcome the loss of 8,000 bits, a B8ZS circuit will replace a data pattern of 8 consecutive zeroes with a code that purposefully includes bi-polar violations (BPVs). When the receiving end receives this altered data signal with the BPVs, it will know that it was inserted at the transmitting end and that the code should be replaced with the original 8 zeroes.


In this way, all 64,000 bits per second are used for data transfer and none are wasted on timing. This is called a clear channel.



2.) Im still trying to understand this question. Can you elaborate further?


HTH


Victor


sarahr202 Mon, 03/30/2009 - 20:23
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Thanks Victor!

Let me give you little context about my 2nd question.

The author explains the purpose of FECN and BECN bits in frame relay header.

" The frame relay header includes three single-bit flags that frame relay can use to help control what occurs inside the frame relay cloud. these bits can be particularly useful when one or more sites use an access rate--the clock rate of the access link--- that far exceeds the CIR of a VC. For example, if a router has a T1 frame relay access link,but only a 128-kbps CIR on a VC goes over thst link,the router can send a lot more data into frame relay network than bussiness contract with the frame relay provider allows.These bits FECN,BECN, help mitigate such situations."


Based on the above, there is direct link between data rate ( from the router on access link) and clock rate on access link.

I was just wondering what is the relation between the two? is there any mathmatical expression showing the relation between data rate and clock rate on access link?

In our case, the root cause of heavy traffic is clock rate on access link which causes the router to send more data into frame relay cloud than CIR on that VC.


Thanks a lot!

Correct Answer
lamav Tue, 03/31/2009 - 05:15
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Sarah:


What that clip from the book is trying to say is that Frame Relay service providers allow clients to burst to port speed/access speed if network congestion levels permit. That is what you should be walking away with.


If the access speed is a T-1, then the carrier can allow you to burst past the CIR and up to port speed if possible. That also is limited by contractual obligations.


THE FECN and BECN bits are used to report congestion to the end nodes so that they can throttle back their send rates.


There's volumes that can be written on this, but as I suggest, the thing to walk away with is the fact that you can burst to T1 access speed if the carrier allows, and the T1, of course, uses a data clock that is derived from the T1 signal. I think that's what the book was trying to say when they mention the clock -- just another way of emphasizing that the access speed is a T1.


HTH


Victor


viyuan700 Mon, 03/30/2009 - 21:08
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Hi Victor,


"Because the TELCOs use a Stratum 4 standard, which boasts a very high rate of accuracy and extremely low frequency drift. "


Agree with most of the things mentioned in your post but Internal Clock of Telco SONET/SDH equipments is Stratum 3 clock and Telco equipments are NOT working on their internal clock. They use their internal clock if they lost the line timing from better source.


Telco equipments derive their clock from a PRS (Primary Reference Source) Stratum Level 1 (Highest Level) if not than Startum 2.


The quality of clock deteriorate as it passes through several Telco equipments and qulaity of CSU/DSU clcok are considred as Startum 4 clock.


"To maintain that stability, some TELCOs use the so-called BITS system, the Building Integrated Timing Supply"


All SONET/SDH boxex have BITS (Building Intergrated Timing Supply ), so its not that some Telco are using.



Correct Answer
lamav Tue, 03/31/2009 - 04:56
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"but Internal Clock of Telco SONET/SDH equipments is Stratum 3 clock"


SONET uses stratum 3 clocks, yes. SONET is not the only method and system used in a T-carrier system. There are 3-1 DACS systems, 3-1-0 DACS systems, M13 multiplexers, statistical multiplexers, channel banks, etc. The timing standard used by a particular T-carrier sub-system depends on where the NE (network element) sits in the TELCOs network architecture with reference to the source clock.


"Telco equipments are NOT working on their internal clock."


I never said they were. I didn't mention at all where the TELCO gets its source clock from. But since you are mentioning it, yes, the TELCOs get their clocks from a stratum-1 timing source.


HTH


Victor

Correct Answer
viyuan700 Tue, 03/31/2009 - 07:11
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There are 3-1 DACS systems, 3-1-0 DACS systems, M13 multiplexers, statistical multiplexers, channel banks,


Lowest speed SONET mux Oc-3 can only provide T1, to provide subrate T1 Telco use DACS,Statistical mux etc.


Can a Telco provide a WAN circuit using only DACS etc if customer office are hundreds of miles away ?


If not then these DACS are riding on SONET boxes. So now the question is, SONET is deriving clock from DACS or DACS is deriving clock from SONET.


As far as i know DACS will take clock from SONET box which in turn deriving clock from better source or BOTH DAcs & SONET derving clock from better source. If i am wrong i have no problem in learning new stufff.


I posted my stuff as one of your post mentions "Stratum 4 standard, which boasts a very high rate of accuracy and extremely low frequency drift" whereas for SONET it is the lowest standard clock.


If you already know this then maybe for guys who dont know. As i dig some topics on this forum which i dont know.

viyuan700 Mon, 03/30/2009 - 21:08
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In our case, the root cause of heavy traffic is clock rate on access link which causes the router to send more data into frame relay cloud than CIR on that VC.


You have T1 port ( works at 1.544Mbps)since the price of T1 is too much, you buy a CIR of let say 128Kbps. So 128Kbps is guaranted but anything above it will be delivered if network is not congested(because other are not using). Otherwise dropped.


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