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New Member

3810 with 512k leased line

I want to tie two hicom 150 PBXs together with 3810s

The customer wants 4 uncompressed channels nailed up for full feature transparency on the voice side with the other 256k for data.

How do I map the channels across for signalling.

Any exactly similar configs out there.

Thanks in advance


Cisco Employee

Re: 3810 with 512k leased line

You would need to use connection trunk to do this. A configuration example of this can be found at:

The Hicom PBXs need to be using a combination of either FXO <--> FXS or E&M <--> E&M. If they are doing digital signaling then T1 CAS (e.g. wink start) will also work. If they are doing ISDN based signaling then this will also work but you would need to use the Transparent CCS feature which is a different configuration.

New Member

Re: 3810 with 512k leased line

The PBX installer has requested CCS.

A DVM-E1 and DSP compression module have been specified for both 3810s.

The interface to the PBX will be via 120 ohm twisted pair E1.

The serial interface between the two sites will be presented as X21.

I have not installed a 3810 before.

I assume that I will be using frame relay encapsulation on the serial line.

How do I map the 4 uncompressed channels for voice across the serial interface. ( Frame forwarding )

Does the data automatically use the remaining bandwidth or do I have to configure this also.



Cisco Employee
New Member

Re: 3810 with 512k leased line

Mr. Hayes,

Are you using Frame Relay on the 3810’s serial ports and if so what is the bandwidth? Where are the 4 uncompressed voice channels? Are you talking about logical voice streams across the Frame or are you referring to channels on a fractional E-1 to the PBX (on the POTS side)?

If the 256K to which you are referring is on the Frame Relay link two things are true, first unless you take steps to reserve a portion of the bandwidth for voice all of the link bandwidth is up for grabs, and second if you do reserve 256K for voice the remaining will be available for data.

Using CCS on an E-1 requires PRI or QSig.


New Member

Re: 3810 with 512k leased line

Yes, I will be using Frame Relay on the serial side.

The leased line is 512k.

The PBX installer wants 4 uncompressed channels from the the E1 at either PBX to be carried across the serial link to allow for full feature transparency so that for example, calls can be handled at either site when the other site is unmanned.

I want to nail up the 256k for the voice.


New Member

Re: 3810 with 512k leased line

General guidelines for what you need on the Frame side.

1. Use LLQ (Low Latency Queuing) to allocate bandwidth to voice through a priority queue.

2. You will have to identify voice packets for LLQ through either IP/UDP port number or packet precedence.

3. You will need approximately 83K per voice call at G.711. This means that you will have to allocate more than 256K for voice. You might try cRTP (RTP header compression) to pickup some bandwidth. This should reduce bandwidth requirements 40K per call but some Cisco bandwidth charts show 68K as being required (I don’t know why).

4. In any event you should not allocate more than 75% of the physical speed of the link in LLQ (a Cisco guideline) to voice and data combined.

5. You will need to specify codecs of G.711 on the dial peers at each side of the Frame connection.

6. The remaining bandwidth on the Frame connection should default to WFQ. You can insure this by specifying it in your LLQ configuration.

I recommend trying G.729. You might want to do a test asking the users which they prefer and not telling them which test uses which codec.


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