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Pat's second paragraph highlights an important item to remember. Since many in HSSG may not have been through specification of a complete PHY, I'll risk restating what may be obvious to HSSG participants that have been through this before. The working group has, since its beginning, paid serious attention to the undetected error characteristics of proposed link technology. The base error rate of the PMD and medium are a major factor in the mean time to false packet acceptance; and the characteristics of the PCS running over that PMD are also important. How data is stripped over lanes (error multiplication and CRC coverage of any error multiplication) and other factors will have to be considerations for each PHY type. As Pat's explanation below alludes to, delimiter robustness is important because of its effect on the undetected error rate.
It is still early in the process, but when it comes down to selection of baseline proposals, I expect I'll not be the only 802.3 member asking about the undetected error rate of proposed baseline PHYs.
From: Pat Thaler [mailto:pthaler@BROADCOM.COM]
Sent: Monday, September 17, 2007 3:32 PM
Subject: Re: [HSSG] Clause-49 (Physical Coding Sublayer (PCS) for 64B/66B, type 10GBASE-R), query
There are two reasons.
One is in the name "sync header." One always has a transition between the two bits of the sync header. The rest of the 66 bit block is scrambled so at any other position between two bits of the 66 bit block one has a 50% chance of having a transition. This is used to obtain block sync.
The other reason is error detection robustness. If there was a single bit difference between a data block and a control block, one could create a false end of packet or start of packet with two bit errors. For example, one might have two data blocks in sequence in a packet where the block payload was the same as the block payload in a valid end of packet followed by idle. If bit errors in the sync header turned both those into control blocks, then a false end of packet could be created with two bit errors and if the last 4 bytes of the false packet happened to match the CRC we would deliver a false packet. With a 2 bit sync header, it takes a minimum of 4 bit errors to create a false start or end of packet giving us a 4-bit Hamming distance.
From: Hari S. patel [mailto:firstname.lastname@example.org]
Sent: Monday, September 17, 2007 6:28 AM
Subject: [HSSG] Clause-49 (Physical Coding Sublayer (PCS) for 64B/66B, type 10GBASE-R), query
I have a question on Clause-49 (Physical Coding Sublayer (PCS) for 64B/66B, type 10GBASE-R).
We have two bits for sync header having following meaning,
00 - Invalid block
01 - Data block
10 - Control block
11 - Invalid block
I get confused,why we have taken two bits, even if we can indicate data/control block using one bit only.
Can I know the reason,why is it so?
Hari S. Patel