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Why are there 2 EA field in Frame Relay and ISDN addressing?

Hi,
If you look at the addresing format of ISDN and also Frame Realy, you would be amazed at seeing 2 fields of Extended Address bits in the address field. I know that they use only one bit to state which would use more bits (extended address). The bit is set to 0 or else with short address it is set to 1. Now why the 2ndEA bit in the address?
Any takers here?
Regards,
Amit
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amonty_99
Asked:
amonty_99
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1 Solution
 
nhuanvnCommented:
Note: This answer is dedicated for the Frame Relay addressing format only.
As you know, the addressing format of the Frame Relay
contains 16 bits with the following usage:
    10 DLCI bits +
    2 EA bits +
    1 C/R bit +
    3 congestion control bits

And notice that when you want to extend the address,
that means you want to extend the DLCI field.
The DLCI is extend PER BYTE. Each EA bit (8th bit in the
byte) marks whether that byte is the last byte in the
address. So when you don't use extended address, you'll see
two EA bits and it seems to be clumsy.
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amonty_99Author Commented:
You are correct so far, but I want to know whats there further on. I know there are 2 EA bits, 6+4 bits of DLCI. Forget the rest of the bits in the address frame format.
If I set the 1 EA bit to 0, I can use the rest of the 4 bits of DLCI, in the 2nd byte.
Thus I already have a extended address.
Why the other EA bit (8th bit on the 2nd byte) for? I dont require it. For a extended address, I am not using this bit. I may very well use this last bit of the 2nd byte fo some other purpose.
Right Sir?
Rgds,
Amit
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amonty_99Author Commented:
You are correct so far, but I want to know whats there further on. I know there are 2 EA bits, 6+4 bits of DLCI. Forget the rest of the bits in the address frame format.
If I set the 1 EA bit to 0, I can use the rest of the 4 bits of DLCI, in the 2nd byte.
Thus I already have a extended address.
Why the other EA bit (8th bit on the 2nd byte) for? I dont require it. For a extended address, I am not using this bit. I may very well use this last bit of the 2nd byte fo some other purpose.
Right Sir?
Rgds,
Amit
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nhuanvnCommented:
Currently the DLCI is defined to be 6 + 4 bits, so you don't see the dedicated use of the 2nd EA bit. But if you
use the DLCI with more than 10 bits, you'll see the role
of that EA bit. For example, if you mark the 2nd EA bit
as 0, you can use 1 more byte for the DLCI (In fact, only 7 bits, as the 8th bit again is the EA bit).   Then you can
have 17 bits in the DLCI.
If you look at the HDLC frame format, you will see the similiar way of extending the address field. Or refering to
the Frame Relay Forum for particular FR addressing.
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amonty_99Author Commented:
You are correct. Infact this last bit (EA) would be used when we have more than 10bits DLCI addressing. Absolutely correct in that. HDLC addressing is also similar and same is the case for ISDN.
17 bis of DLCI addressing seems too big for a dynamic addressing scheme like DLCI. I just cant understand why we need to use a 17 bit address!!
Do let me knwo if you have a answer.
Thanks for the previous answer.
Bye.
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nhuanvnCommented:
Currently the DLCI is defined to be 6 + 4 bits, so you don't see the dedicated use of the 2nd EA bit. But if you
use the DLCI with more than 10 bits, you'll see the role
of that EA bit. For example, if you mark the 2nd EA bit
as 0, you can use 1 more byte for the DLCI (In fact, only 7 bits, as the 8th bit again is the EA bit).   Then you can
have 17 bits in the DLCI.
If you look at the HDLC frame format, you will see the similiar way of extending the address field. Or refering to
the Frame Relay Forum for particular FR addressing.
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nhuanvnCommented:
Yes, up to now it seems too big to use 17 bits in the DLCI.
But when they design the frame format for Frame Relay,
they must put a safe way to overcome the problem of
extending addresses there. In fact, more than 17 bits in
the DLCI WILL NOT be the trend. We can see that the
IT community agrees [implicitly] the DLCI length to be
up to 20 bits at most. This can be seen in the view of
those who are developing MPLS (Multiple Protocol Label
Switching). In their design, they have 20 bits LABELS.
And they fit DLCI there if the packets are Frame Relay!!!
So if the DLCI size is longer than 17 bits (then it can
be 24 bits, right ?) MPLS is not good in its Labels idea
anymore.
Regards,
nhuanvn
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amonty_99Author Commented:
Correct. But after 16 bits it can be 20 bits. As you can see, we have 10 bit DLCI addressing (6+4). Thus it can be 6, 10, 16, 20 bits long, nothing different.
Yes MPLS will be using DLCI when Frame Realy is being used as the technology. And I think it will still carry a extended address bit of DLCI. Payload still is maximum of 20 bit address.
Thus I think it will work. But as per MPLS, they use a diffrent addressing scheme and the whole FR frmae is put as payload and dont have anything to do with the DLCI address. Basically MPLS encapsulates the Frame Relay frame and adds it own address bits or better known as tags and sends it through the WAN.
Right?
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amonty_99Author Commented:
Correct. But after 16 bits it can be 20 bits. As you can see, we have 10 bit DLCI addressing (6+4). Thus it can be 6, 10, 16, 20 bits long, nothing different.
Yes MPLS will be using DLCI when Frame Realy is being used as the technology. And I think it will still carry a extended address bit of DLCI. Payload still is maximum of 20 bit address.
Thus I think it will work. But as per MPLS, they use a diffrent addressing scheme and the whole FR frmae is put as payload and dont have anything to do with the DLCI address. Basically MPLS encapsulates the Frame Relay frame and adds it own address bits or better known as tags and sends it through the WAN.
Right?
0

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