RE: 64b/66b scrambler
From: Eric Ng [mailto:eng@xxxxxxxxxxx]
Sent: Thursday, October 12, 2000 2:00 PM
Subject: 64b/66b scrambler
I have some questions on the 64b/66b scrambler.
In sec 49.2.6 of draft D1.0 stated that there is no
reqiurement on the initial value. Please correct me
if I'm wrong. The descrambler can recover 58th bit
by doing S0^S39^S58 and so do the subsequent bits.
<PAT> The descrambler can start recovering data after
it has received 58 bits so the first bit it can descramble
after Frame Lock has been achieved is the 59th bit.
What happen at power up or the system is reset? Are we
assuming that the first few frames will be used for
synchronization? That we will be well over the first
58bits by the time, the first valid payload is recevied.
<PAT> At start up, nothing can be received until frame
sync has been achieved. Frame lock requires getting
64 consecutive code blocks with valid sync headers.
Getting the descrambler sync'ed just requires receiving
58 bits which is less than a code block. Therefore,
the descrambler should be in sync by the time frame
lock has been achieved.
Also, the PMD may require some time to adjust to the
incoming signal, the PLL must lock and for the WAN
PHY sync to the Sonet frames must be acquired before
the 64B/66B PCS can acquire Frame lock.
100's of bits must be received before payloads can be
received. The scrambler does not have to extend that
time because it can initialize while block sync is being
Also, once the scrambler/descarmbler are initialized at
power up or system reset, the shift register will not
reset at packet boundary and/or SPE boundary.
<PAT> The scrambler only needs to reinitialize if lock
to the code blocks is lost. That is, when the No_Frame_Lock
state has been entered.
In the bluebook, the sample test vector, the scrambler
is initialized to all 1's. Is this the default value
at power up reset?
<PAT> There is no default value for the scrambler. The
meaning of "There is no requirement on the initial value"
is that there is no required starting value. An example
has to use some starting state but there is no significance
to the particular starting value that was chosen.
Thanks in advance.
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