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Community Member

## TI/E1 and ISDN

From the Ciscopress CCIE book:

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BRI = 2B + 1D

2x64 + 16 = 144 Kbit/sec

I'm cool with this

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PRI T1= 23B + 1D

23x64 + 64 = 1536 Kbit/sec

If it's true that the D channel is 64, from my calculation the the total bandwidth required is: 1536 which is more that 1522 (standard T1)

confused :-?

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PRI e1= 30B + 1D

30x64 + 64 = 1985 Kbit/sec

Is in this case is the E1 underutilized as the bandwidth available is 2048?

And if so is there any tech reason for this?

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2 REPLIES
Purple

## Re: TI/E1 and ISDN

All the numbers indicated 2B+1D, 23B+1D and 30B + 1D are all properl. And for B and D channels in PRI (T1 and E1) is 64 kbps.

So when you sum it up, the bandwidth is only 1536 for T1. Where is the other 8 kbps (True T1 is 1544kbps and not 1522kbps). 1536 kbps is the payload bandwidth. 8kbps is used for framing and synchronization.

A base T1 frame has 193 bits, 24 time slots. The first bit of each frame forms the synchronization channel which creates the 8 kbps channel.

A E1 frame has 256 bits, 32 time slots and one 64 bit channel is dedicated for frame synchronization, alarms etc just like the 8 kbps channel for T1. Another 64 kbps channel is used for signalling (the D channel of E1) and the rest 30 channels are used for payload (each 64kbps).

HTH

Sankar.

Community Member

## Re: TI/E1 and ISDN

I think now it's quite clear, but I miss to understand the function of the 8Kbit in T1 and 64Kbit in E1.

I thought the D channel was meant for this function, but according with what you say there are 2 things to take into account:

1) The low lever framing synchronization/alarms fro T1 and E1 (8 Kbps and 64 Kbps)

2) the D channel used at higher lever by ISDN for signaling

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