padding to ethernet using fpga

[arena_yang]

In my design, I have to insert some payload in the PAYLOAD of ATM AAL5 CPCS PDU,so I will have to modify the length field and the PAD field of CPCS PDU tailer.

But I have to recalculate the number of the PADding. So I want to know how to add pad ?

If Anybody have experience with Ethernet MAC PAD or ATM pad, please help me!!!!!

ThX in Advance!!!!!!!!!!!!!!

 

[arena_yang]

Nobody knows?
Or Nobody Will answer>!!!???

 

[radix]

padding

arena_yang,

Maybe it's not clear to everyone what the problem is. What do you mean when you say "recalculate the number of the PADding"?

If I understand what you are trying to do, you want to overwrite some of the padding with new data. This means you will need to update the packet data length field as well as the CRC. What's the problem?

Radix

 

[juggernaut]

The rule is that pdu_length+pad+trailer = a multiple of 48 (the number of payload bytes in an ATM cell. Since, for any given PDU, the length is fixed, and the trailer size is fixed (8 bytyes from memory) it is an easy exercise to determine what the pad length should be (it will always be between 0 and 47). DO NOT recalculate the "length" field in the trailer, as it indicates the length of the real PDU. I believe that the CRC does include the padding though...

J

 

[juggernaut]

err, sorry. I just re-read your post - you are adding more data to the existing PDU, so the length field will have to change to reflect this new data. Depending on the amount of data you add, the amount of required padding may decrease or increase to satisfy the rule pdu_length+pad+trailer=multiple of 48.

J

 

[tom_hanks]

I am not clear about your question, but about Ethernet padding it works like this…

In case of Ethernet the minimum packet length is 72 octets up-to maximum 1526.
In that 26 octets are always Ethernet frame information and while other is data field.

For 72 octets, required data field is 46 (minimum) data + Pad = 46 octets.
There is possibility that no data is there and only pad (all zeros) is sent. The Mac generates pad bytes to extend the frame to a minimum 46 octets.
Now if some data have to insert in that packet, then that many octets of pad will get reduce from pad generation.

PM me if some quiry is there…..