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Dedicated circuits - T1, leadsed line framing.

Normal settings to get sync serial interface working are NO SHUT, give encap(PPP/HDLC for dedicated circuits), provide ip address, give authentication (if required in case of PPP). These things at list required for both T1 and any other dedicated circuit (leased line)and link will be established.

In dedicated circuits, for T1 1.5 mbps we need to specify the framming type SF/ESF.

But in leased lines circuits ( 64, 128, 256, 512 kbps etc. or less than 1.5 mbps) link we don't need to specify framing type. Why it is so?

Or am I misunderstanding dedicated circuits?


Re: Dedicated circuits - T1, leadsed line framing.

I think you're confusing the PHYSICAL line parameters with higher-level protocols (PPP, HDLC, FR, etc).

For any sync serial line (T1, Fractional T1, Frame-Relay, etc) you have to define the lowest-level line parameters; line encoding (AMI, B8ZS) and framing (SF/D4, ESF). If the interface is also a DCE, a clock rate may/must also be defined.

Above that, you have the protocol that will communicate at link-level ("encapsulation") ; Frame-Relay, HDLC, PPP, etc ...

In the case of a channelized media, you may also need to further define how many of the channels will participate in the data transport as an intermediate step.

Hope this helps ....


Re: Dedicated circuits - T1, leadsed line framing.

On serial link line coading and framing are physical parameters.

On T1 link I need to specify them to get link up, these are must.

But on fractional T1(leased line 64, 128 kbps etc) I don't specify line coading and framming. Though fractional T1 line also channelized media.

why it is so or are there some default settings for that in IOS? I never learn any such thing in books.


Re: Dedicated circuits - T1, leadsed line framing.

Uyless Black write an awesome book "Sonet T1" which talks all about this stuff if you interested in learning more. To answer your question, in short, Fractional services do no specify framing/coding because this data (time slot) is being put into a frame of the whole (i.e. 64Kbps service will not specify framing due to it's T1 carrier providing this function). Also if there is no CSU/DSU internal to the serial connection you are doing you'll not provice Framing / Coding either. This is a function of the CSU/DSU.

Hope this helps you,



Re: Dedicated circuits - T1, leadsed line framing.

For built-in-CSU/DSU serial interfaces, ESF is the default framing format. ESF is the most common T1 framing format these days. All external CSU/DSUs have options to manually set framing formats, line encoding etc.

T1s (full/fractional) have to have some framing eg. D4/SF, ESF. There has to be a way to identify each of the 24 DS0, whether you use all of them or few.


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