Understanding the relation among T1, isdn, voice-port

[eigrpy]

Hi I read some papers on some terms in voip, but I am still confused with some of them. I hope you can help me to understand the relation among T1, isdn and voice-port etc.. I am talking about the relation below. If I am wrong, please correct me. Thank you

T1 config is bottom layer(first layer), isdn or pri is set up on the T1. Voice-port, which is logical layer, is on the isdn or pri.

If we can see interface s0/0/0:23 which is also called logical layer, that means that isdn is setup

[Ajay Viswanath · Accepted solution]

Hello,

T1 config is to specify it the link is going to be used for voice or data and how many channels we are going to configure.  The serial interface is the signalling link which directs the calls to the b channels. Voice-port comes into picture only if the isdn link is used for voice traffic.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/isdn/configuration/15-mt/vi-15-mt-book/vi-isdn-vi-cfg.html

"Under most circumstances, default voice-port command values are adequate to configure voice ports to transport voice data over your existing IP network. However, because of the inherent complexities of PBX networks, you might need to configure specific voice-port values, depending on the specifications of the devices in your network."

AJ

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[eigrpy]

Thank you all for your reply. In the below output, why the subinterface are down except s0/1/0:23? 

R1#sh ip int bri

Serial0/1/0:0 unassigned YES unset down down
Serial0/1/0:1 unassigned YES unset down down
Serial0/1/0:2 unassigned YES unset down down
Serial0/1/0:3 unassigned YES unset down down
Serial0/1/0:23 unassigned YES NVRAM up up

[Ajay Viswanath]

23rd channel is the signalling(D Channel) channel for T1, it should always be up. The other channels are Bearer channels and they are up only when you have a call going through any of those interface.

AJ

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[Ryan Huff]

In actuality, the 24th channel is used for signaling; in IOS syntax it is referenced as 23 because the first bearer channel starts at 0. I draw distinction not to be picky but to offer clarification as it appears the O.P is in the process of learning.

Thanks,

Ryan

[Carlo Poggiarelli · Accepted solution]

Hi.

23 is the D channel and if up means that t1 circuit is up.

Other ar B channels and you should see them up once there are active calls.

HTH

Regards

Carlo

[Ryan Huff]

Hello there and Happy New Year!

To give you a quick answer;

• L1 = Physical connections
• L2 = Data bits, encapsulation on the wire (LAPD - Link Access Protocol/Procedure Data)
• L3 = ISDN Protocol stack

We could get really, really deep on this topic and explore things like how time division multiplexing works as it applies to T1, how the fundamentals of the PSTN are structured and the ISDN stack because when you start explaining WHAT a T1 is, you almost have to explain HOW a T1 works.

However, that goes way beyond what a forum reply can (and should) do. As T1 is (and has been for some time) an aging technology (although still in substantial use), you would be best served research the depth of the topics in books (referenced below) and through other sources on the Internet.

T1:

T1 (sometimes can be called a DS1) is a standard more than anything. It refers to a standard for transmission of digital bits over phone lines. A T1 is split into 24 channels that can support 64Kbps of data or voice transmission per channel. Back in the day (and still with some carriers), the signaling (not the voice media) was carried inband which is more popularly known as robbed bit signaling. Over the span of a T1 this would sacrifice the capacity of one whole channel resulting in  a "D" channel and 23 "B" Channels.

• D Channel: Data Channel (where the control signaling will occur)
• B Channel: Bearer Channel (Bearer is used in the sense that the channel is able to bear or carrier a particular capability such as voice media)

Although, as T1 carriers have enhanced their technology (in part to stay relevant in the industry as it shifts more and more to SIP), most carriers offer Clear Channel service where it frees up the robbed bits for the bearer (i.e 24 channels for voice versus 23).

ISDN:

The Integrated Service Digital Network service has two modes of operation; Basic Rate Interface (BRI) and Primary Rate Interface (PRI). In voice implementations you will almost exclusively see T1's represented with a Primary Rate Interface. ISDN takes place at L3 of the OSI model. At L2 a protocol called LAPD (Link Access Protocol/Procedure and it is used on the "D" channel of the PRI - LAPD) exists to move and encapsulate the bits. At L3 the ISDN protocol exists.

If you are interested in knowing more about the ISDN stack, Cisco Press has an older book (Circa 2002) that explains the subject in great detail. http://www.ciscopress.com/store/telecommunications-technologies-reference-9781587050367?w_ptgrevartcl=Integrated+Services+Digital+Network+Primer_29737

Thanks,

Ryan

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