The protocol stack — every sublayer above the radio
The PDSCH note draws five boxes above the physical layer and says what each hands down. This note opens the boxes. Each section defines one sublayer: what it is, the one problem it owns, the functions its specification lists, and why the layer below could not have done the job.
In one line. Every sublayer exists because it owns exactly one problem the others must not touch. SDAP owns which flow is this, PDCP owns secrecy and order, RLC owns did it arrive, MAC owns what goes out this millisecond, and RRC owns what shape is everything in. Reading the four function lists side by side is the fastest way to see the seams.
The concepts this note covers
Each concept now has a page of its own. This note is the narrative — the group read as one argument, with the connective tissue that a page-per-concept cannot carry. The pages are the definitions, and each one shows what to learn before it.
| Concept | What it is | Needs | Primary clause |
|---|---|---|---|
| MAC | Medium access control — multiplexing, HARQ, the scheduler | 2 | TS 38.321 |
| PDCP | Ciphering, integrity, ordering, duplicate removal | 1 | TS 38.323 |
| Physical layer | Layer 1 — what turns a transport block into radio | 1 | TS 38.202 §4 |
| RLC | Radio link control — segmentation and ARQ | 1 | TS 38.322 |
| RRC | Radio resource control — the configuration protocol | 1 | TS 38.331 |
| SDAP | Maps a QoS flow to a radio bearer | 1 | TS 37.324 |
Which to read. Come here to see how these ideas hang together; go to a term page when you want one definition, its prerequisites, and everything that depends on it. The concept map indexes all 118, and its hierarchy puts them in learning order.
The seams, side by side
Reading the five function lists together shows the design rule: no two sublayers own the same problem, and each hides its problem completely from the one above.
| Sublayer | Spec | Owns | Hands down | Why not the layer below |
|---|---|---|---|---|
| SDAP | TS 37.324 | Which QoS flow this is | An SDAP PDU on a chosen bearer | Nothing below knows what a QoS flow is — below SDAP there are only bearers |
| PDCP | TS 38.323 | Secrecy, integrity, order, header size | A PDCP PDU with a sequence number | Ciphering must sit above retransmission, or a retransmitted block would need re-encrypting |
| RLC | TS 38.322 | Segmentation, and did it arrive | An RLC PDU sized for the grant | MAC's HARQ feedback is one fallible bit; RLC's status reports are unambiguous |
| MAC | TS 38.321 | What goes out now, and HARQ | A transport block | Layer 1 has no notion of priority, of queues, or of several logical channels |
| PHY | TS 38.202 | Turning bits into radio | Resource elements on antenna ports | — |
The one seam to remember is the bottom one. Everything above MAC deals in packets, flows and bearers; everything below deals in bits with a length. That is the transport block, and it is why PDSCH §2.2 calls it the handover point.
Sources
| Document | Clause | What it gives |
|---|---|---|
| TS 38.202 v19.0.0 | 4.1, 4.2 | The physical layer as a service; the eleven L1 functions |
| TS 38.321 v19.3.0 | 4.4, 5.1, 5.3.2 | MAC functions; random access; the HARQ entity |
| TS 38.322 v19.3.0 | 4.2.1.1–.3 | The TM, UM and AM RLC entities |
| TS 38.323 v19.1.0 | 4.4 | The PDCP function list — ciphering, integrity, reordering, duplication |
| TS 37.324 v19.0.0 | 4.4, 5.3 | SDAP functions; QoS flow to DRB mapping, including reflective mapping |
| TS 38.331 v19.3.0 | — | Where every higher-layer parameter used in these notes is actually defined |
| TS 38.300 v19.3.0 | 6.2, 7.2 | Channel mappings; the three RRC states |
cd ~/Documents/md-engine/"Courses (RAW)/5G Topics"
pdftotext -layout ts_138321v190300p.pdf - | grep -n -A10 "^4.4 *Functions"
pdftotext -layout ts_138323v190100p.pdf - | grep -n -A16 "The PDCP layer supports the following"
pdftotext -layout ts_137324v190000p.pdf - | grep -n -A8 "The SDAP sublayer supports"