Deep dive

OSI vs silicon layering — what carries over

Networking OSI layers and silicon protocol layers rhyme but don't map 1:1. Silicon 'transaction layer' is closest to OSI session/presentation: it defines what the transfer means. Channel layer is transport: reliable delivery mechanics. Physical is physical. The useful habit from OSI: never debug a high-layer symptom by tweaking a low layer without evidence.

Request/response vs streaming models

Two fundamental transaction shapes appear everywhere. Request/response (AXI, PCIe mem rd/wr): address + completion. Streaming (AXI-Stream, PCIe streaming, video pipelines): ordered data without per-beat addresses. Layering still applies — streams have packet boundaries (TLAST), flow control, and sideband (TUSER) at the transaction layer even without addresses.

REQUEST/RESPONSE vs STREAMING

Req/Rsp:  AR -> R beats -> done     (identity = ID + addr)
Stream:   TVALID beats -> TLAST     (identity = TID + packet #)

Both need: flow control, ordering rules, reset behavior

Reset and clock domains as layer boundaries

Clock-domain crossings and reset sequencing are integration boundaries. A multi-bit protocol transaction normally crosses through a protocol-aware asynchronous structure; synchronised Gray-coded pointers may be part of an asynchronous FIFO, but are not themselves a general transaction transport. If a master admits work before the destination path is available, the design needs an explicit hold, retry, error or recovery contract.

• CDC: use protocol-aware FIFOs (AXI CDC), not ad-hoc valid/ready sync.

• Reset order: document which blocks must be alive before masters issue.

• Power transition: define quiesce, rejection, retention, isolation and recovery behaviour for in-flight and newly offered transactions.