Subject: [Hubmib] Support of LAN FCS in Ethernet packet over ATM AAL5.

From: Nurit Sprecher <[email-masked]>
Date: Thu, 27 Jun 2002 10:18:33 +0200

In RFC 2684 on Multiprotocol Encapsulation over ATM Adaptation Layer 5, the
Payload Format for Bridged Ethernet/802.3 PDUs is defined as follows:

                +-------------------------------+
                |    MAC destination address    |
                +-------------------------------+
                |                               |
                |   (remainder of MAC frame)    |
                |                               |
                +-------------------------------+
                |  LAN FCS (if PID is 0x00-01)  |
                +-------------------------------+
 
The LAN FCS option is VC dependent. 
In case that the LAN FCS is supported, duplicate checksum calculations are
performed: one for the AAL5 frame and one for the Ethernet frame.

In what cases is it required/suggested/worth to support the LAN FCS when
transmitting a packet within an AAL5 packet over an ATM network? 

Can a bridge that resides at the end of the ATM, de-capsulate the ATM packet
and transmit the Ethernet packet over a LAN, calculate the LAN FCS and set
it in the Ethernet packet (if required)?

Thanks in advance, 
Nurit Sprecher

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