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RE: [802.3ae] Questions on section 49.2.5




Thanks Pat for the answers, in the first question,
I confused data-rate with transfer-rate.

Thanks again.

Burt S, Christian.
Ceyba Inc.,
Systems Specialist
Tel : (613)599-5797
Fax : (613)599-1247
burt@xxxxxxxxx               www.ceyba.com


> -----Original Message-----
> From: THALER,PAT (A-Roseville,ex1) [mailto:pat_thaler@xxxxxxxxxxx]
> Sent: Monday, November 19, 2001 7:29 PM
> To: Burt Christian; HSSG Reflector (E-mail)
> Subject: RE: [802.3ae] Questions on section 49.2.5
> 
> 
> 
> My responses are bracketed by my initials. Pat
> 
> -----Original Message-----
> From: Burt Christian [mailto:burt@xxxxxxxxx]
> Sent: Monday, November 19, 2001 1:53 PM
> To: HSSG Reflector (E-mail)
> Subject: [802.3ae] Questions on section 49.2.5
> 
> 
> 
> Hi,
> 
> I have the following questions about section 49.2.5
> 
> 1. In the first paragraph, the ratio between the data-rates
>    of XGMII and the PMA service interface is mentioned to be
>    16:33. Is this correct or is 32:33, the correct ratio ?
> 
> <PAT begin>
> 16:33 is the correct ratio. Here is the calculation:
> The XGMII sends 32 unencoded bits each cycle. 
> The PMA sends 16 encoded bits each cycle.
> 
> Let Ud be the unencoded data rate;
> Cxgmii be the tranfer rate of the XGMII; and
> Cpma be the PMA service interface transfer rate.
> 
> Cxgmii = Ud/32 transfers/second
> 
> Cpma = Ud unencoded bits *  66 encoded bits   * 1 transfer
>        -----------------   ------------------   ----------
>               s            64 unencode bits     16 bits
>    
>      = Ud *33/512 transfers/second
> 
> Therefore
> Cxgmii    Ud/32          16
> ------ = ----------- = ------
> Cpma     Ud *33/512      32
> 
> <PAT end>
> 
> 2. With respect to the following sentence in the same paragraph:
>     "Where the XGMII and PMA sublayer data rates are not
>      synchronized to that ratio, the transmit process will
>      need to insert idles, delete idles, or delete sequence
>      ordered sets to adapt between the rates."
> 
>    The above rate adaptation is intended for adapting for the
>    minute frequency offset that can be present if the XGMII
>    transmit clock (TX_CLK) and the PMA_TXCLK are not frequency
>    locked - correct ? What I am trying to clarify is that this
>    rate-adaptation is completely independent of the rate
>    adaptation required for interfacing to WIS.
> 
> <PAT begin>
> The ability to avoid rate adaptation by synchronizing PMA clock
> to the XGMII clock on transmit and vice versa on receive applies
> only to the LAN PHY. Such synchronization only works for the LAN
> PHY where the XGMII and PMA are running at the same nominal 
> data rate. With the WAN PHY, they are running at different nominal
> data rates and rate adaptation must be performed.
> 
> When the clocks have not been synchronized, the same rate 
> adaptation mechanism (inserting idles and deleting idles or 
> ordered sets) applies to adjusting for the small difference 
> when due a PMA and XGMII running at nominal the same data 
> rate as they do for the LAN PHY and to rate adaptation for 
> the somewhat larger rate mismatch of the WAN PHY. 
> 
> In the case of the WAN Phy, one knows that one will be deleting 
> idles at the transmitter and adding them at the receiver
> (and that the MAC will be inserting extra idles as necessary). This
> contrasts with the LAN PHY case where one doesn't know the direction
> of the mismatch. One will also need a bit bigger elastic buffer for 
> the WAN PHY case. Therefore, the constraints on 
> implementation differ  
> slightly for the two cases but the general mechanism is the same.
> <PAT end>
> 
> Thanks.
> 
> Burt S, Christian.
> Ceyba Inc.,
> Systems Specialist
> Tel : (613)599-5797
> Fax : (613)599-1247
> burt@xxxxxxxxx               www.ceyba.com
>