R
Appendix E: Rx Elastic Buffer Specifications
Note that this analysis assumes that the buffer is approximately at the
Since there is a
5000 x 12 = 60000 bytes
This translates into a maximum frame size of 60000 bytes.
Clock Correction
The calculations in all previous sections assumes that the Rx Elastic Buffers are restored to approximately half occupancy at the start of each frame. This is achieved by the elastic buffer performing clock correction during the interframe gaps either by inserting or removing Idle characters as required.
•If the Rx Elastic Buffer is emptying during frame reception, there are no restrictions on the number of Idle characters that can be inserted due to clock correction. The occupancy will be restored to half full and the assumption holds true.
•If the Rx Elastic Buffer is filling during frame reception, Idle characters need to be removed. Restrictions that need to be considered are described in the following sections.
Idle Character Removal at 1Gbps (1000BASE-X and SGMII)
The minimum number of clock cycles that may be presented to an Ethernet receiver, according to the IEEE 802.3 specification, is
In a minimum interframe gap at 1 Gbps, we can only assume that two /I2/ characters are available for removal. This corresponds to 4 bytes of data.
Looking at this from another perspective, 4 bytes of data will need to be removed in an elastic buffer (which is filling during frame reception) for a frame which is 5000 x 4 = 20000 bytes in length. So if the frame being received is 20000 bytes in length or shorter, at 1 Gbps, we can assume that the occupancy of the elastic buffer will always self correct to half full before the start of the subsequent frame.
For frames which are longer than 20000 bytes, the assumption that the elastic buffer will be restored to half full occupancy does not hold true. For example, for a long stream of 250000 byte frames, each separated by a minimum interframe gap, the Rx Elastic Buffer will eventually fill and overflow. This is despite the 250000 byte frame length being less than the maximum frame size calculated in the “Rx Elastic Buffers: Depths and Maximum Frame Sizes” section.
However, since the legal maximum frame size for Ethernet frames is 1522 bytes (for a
VLAN frame), idle character removal restrictions are not usually an issue.
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| UG155 March 24, 2008 |