J-cell
From Juniper Clue
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J-Cells and Bandwidth
On Martini architecture routers, per-FPC bandwidth limitation comes from the B-Chip to A1-Chip link, which has a raw bandwidth of 4 Gigabits/sec in each direction. Each J-Cell has an overhead of 16 bytes per 64 bytes of payload, leaving only 3.2 Gigabits/sec for J-Cell payload. Thus each FPC can switch at most in each direction:
It is a common misconception that the limit comes from the SDRAM speed on the FPC, which is also 4 Gigabits/sec (64-bit data path at 125MHz). However, the packets sprayed across the FPCs SDRAM buffer memory are addresses by the B-Chip as 64-byte words, and do not have any J-Cell overhead.
