This page shows the protocol stack of a GSM/GPRS MS in one diagram, and it names the specification behind each layer. The diagram focuses on the packet switched side and its signalling. So I'll first walk through the boxes, then follow the three kinds of arrows, and finally list what the diagram leaves out. The layer descriptions are checked against 44.006, 44.060, 44.064 and 44.065 v19.0.0.

The topics on this page are listed below.

• Which layers make up the GSM/GPRS protocol stack ?
• How do user data and signalling move through the stack ?
• What does the stack diagram leave out ?
• Reference

Which layers make up the GSM/GPRS protocol stack ?

Let's start with the layers, from the top of the diagram to the bottom. Each box carries the name of a protocol and the number of the 3GPP specification that defines it. So the diagram also works as a map of the specifications.

Following is overal Protcol Stack for PS and Signaling (CS is much simpler).

The diagram draws MM, RR, LLC, RLC, MAC and PHY as one column on the left. SNDCP sits to the right of RR and connects down to LLC. A dark blue box without a label sits above SNDCP and exchanges user plane data with it. From its position, it stands for the network layer that uses the GPRS service, such as IP. The legend on the right explains the three kinds of arrows.

Figure 1. GSM/GPRS protocol stack of the MS. User data enters through SNDCP and LLC, signalling runs down the left column, and RR controls the lower layers through internal paths.

• MM, 24.008 : mobility management is the top of the signalling column. The diagram shows only MM, although 24.008 also defines GMM, SM and CC.
• RR, 44.018 : radio resource management. In 44.060, the RR sublayer provides services to the MM and LLC sublayers, which is the order the column shows. The red dashed lines from RR reach SNDCP, LLC, RLC, MAC and PHY.
• SNDCP, 44.065 : it multiplexes several PDPs, compresses user data and protocol control information, and segments each NPDU into LLC PDUs.
• LLC, 44.064 : it runs a logical link between the MS and the SGSN. A SAPI in each LLC frame tells which layer 3 entity the frame belongs to.
• RLC and MAC, 44.060 and 44.160 : RLC segments LLC PDUs into RLC/MAC blocks and retransmits failed blocks in acknowledged mode. MAC lets several MSs share the same physical channels. 44.160 is the RLC/MAC specification for Iu mode.
• PHY, 45.002 : the physical layer. 45.002 defines how logical channels are multiplexed onto bursts, frames and timeslots.

How do user data and signalling move through the stack ?

The legend of Figure 1 separates three kinds of arrows. Each kind answers a different question: where user data goes, where signalling goes, and which layer configures which. Let's follow them one at a time.

The black U Plane Path starts at the dark blue box and runs through SNDCP and LLC down to RLC, MAC and PHY. SNDCP turns each NPDU into SN-PDUs, and LLC carries them in LLC frames. RLC then cuts each LLC PDU into RLC/MAC blocks for the radio interface. In acknowledged mode, RLC also retransmits the blocks that the receiver did not get.

The blue C Plane Path runs from MM through RR and LLC down to RLC, MAC and PHY. On the packet side, signalling also travels inside LLC frames. The SAPI in the LLC header decides which entity receives each frame, and 44.064 Table 2 lists the values below.

SNDCP uses the four user data SAPIs, 3, 5, 9 and 11. For UI frames on these SAPIs, LLC can ask RLC/MAC for reliable transfer with GRR-DATA or for unreliable transfer with GRR-UNITDATA. All other LLC frames use GRR-DATA. GMM messages use SAPI 1, so they reach the SGSN inside LLC frames rather than as 44.018 RR messages.

The red dashed Internal Control Path starts at RR and ends at SNDCP, LLC, RLC, MAC and PHY. It does not carry protocol messages. It shows that the radio resource function configures the other layers, for example when the network assigns a channel or a TBF.

• User data takes the right side of the diagram : it goes from SNDCP to LLC, and then down through RLC, MAC and PHY.
• Packet signalling shares LLC with user data : the SAPI keeps them apart. GMM uses SAPI 1, SMS uses SAPI 7, and user data uses SAPIs 3, 5, 9 and 11.
• RLC gives the reliability : acknowledged mode retransmits failed RLC/MAC blocks, so LLC UI frames can still be delivered reliably.
• The red lines are not messages : they mark local control from RR to the other layers inside the MS.

What does the stack diagram leave out ?

The page says the CS stack is much simpler, and the diagram does not draw it. A few other layers and peers are also missing. A reader who knows the LTE or UMTS stack will probably look for them, so this section lists them.

The first missing layer is the data link layer for CS signalling. LAPDm, which 44.006 defines, carries the information on the BCCH, AGCH, NCH, PCH, FACCH, SACCH and SDCCH. LAPDm SAPI 0 carries call control, mobility management and radio resource signalling, and SAPI 3 carries short message service. So the CS signalling column is MM over RR over LAPDm over PHY, without LLC, RLC or MAC.

The second gap is at the top. The diagram shows only MM, but 24.008 also defines CC for CS calls, GMM for GPRS mobility and SM for PDP contexts. CC runs on top of MM, while GMM uses LLC SAPI 1 and SM uses the services of GMM.

The third gap is the network side. The peers of the MS layers sit in different nodes. RLC and MAC end in the BSS, because 44.060 is the MS to BSS interface. LLC and SNDCP end in the SGSN, so the BSS relays each LLC frame toward the SGSN without reading its content.

• CS signalling uses LAPDm, not LLC : RR, MM and CC messages on the dedicated and common control channels travel over LAPDm SAPI 0.
• SMS has two paths : it uses LAPDm SAPI 3 on the CS side and LLC SAPI 7 on the GPRS side.
• Peers are split between BSS and SGSN : RLC/MAC ends in the BSS, and LLC and SNDCP end in the SGSN.

Reference

• 44.006 Mobile Station - Base Station System interface; Data Link layer specification - v19.0.0, subclause 4 General and Table 2 Allocation of SAPI values
• 44.060 GPRS; MS - BSS interface; Radio Link Control / Medium Access Control protocol - v19.0.0, subclauses 4, 4.1 and 4.2
• 44.064 MS - SGSN; Logical Link Control layer specification - v19.0.0, subclauses 1, 6.2.3 and 7.2.3 with Table 2 Allocation of SAPI values
• 44.065 MS - SGSN; Subnetwork Dependent Convergence Protocol - v19.0.0, subclause 1 Scope