>Dear Guru's,
>Have seen that in the frame format of ISDN, Frame Relay and HDLC, there are
>two bits of Extended Address field. I would like to know why two fields when
>one can suffice?
>With my limited knowledge, I can understand that may be when (in case of FR)
>the DLCI bits increase beyond 10 bits, then might require another frame to
>take along the extra bits. This is purely my understanding, nothing to do
>with any whitepaper or site. Have I hit the taget?
>Moreover I was thinking why do we need more than 10 bits of DLCI? When will
>we need it?
>Do throw some light here, please.
>Amit
You ask a question here that digs deeply into why protocols are
designed the way they are. First, it's worth remembering that
protocol standards developed by ISO, ITU and CCITT usually are
developed on paper long before there are any prototypes or products.
In contrast, IETF standards require at least two running
implementations before you can move to the second of three levels of
standardization.
So, without real-world modifiers about how the technology will be
used, ISO/ITU standards (including ISDN, Frame, and HDLC), are rather
consciously designed so there can be extensions later. You have
correctly interpreted why the EA bit is there -- it's for extending
the field length.
And you are also quite correct that no one has found a real reason to
use more than 10 bits. But the capability is there if it's needed.
We have a long history of running out of space in protocol fields --
witness IPv4 addressess.
Another good comparison is the difference in detailed protocol
message design in OSPF and ISIS. OSPF is designed for processing
efficiency. You will find that most important fields are aligned on
32-bit boundaries. There's a lot of bit-flag level encoding.
ISIS, however, was designed for extensibility. Its optional fields
are in variable-length Type-Length-Value constructs, so it's much
easier to add features than it is to OSPF.
Again, rememeber when these protocol were designed, conditions
weren't the same as they are today. Processors were much slower.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=11247&t=11035
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