DcPdcpLegSplitter

simple module

TX-side leg dispatcher of a dual-connectivity split bearer (two legs), steering each PDCP PDU to a leg from the split policy RRC pushes off the bearer's definition (TS 38.331 PDCP-Config, TS 38.323 5.2.1). A splitter serves exactly one direction: a UE's TX side is uplink, a base station's is downlink.

Uplink (at the UE, which has both legs' RLC queues locally): below the ul-DataSplitThreshold the bearer stays on its primary path; at or above it, either leg may be used, and the spec leaves that choice to the implementation. The splitter fills it with ulLegSelection if the definition carries one, else by offloading to the least-occupied leg.

Downlink (at the DC master): the secondary leg's RLC queue is at another node, across X2, and cannot be weighed here, so there is no threshold or load-balancing -- dlLegSelection decides each PDU's leg, or the bearer stays on its primary leg. (Real downlink split flow control is X2-feedback-driven and is not modeled.)

The leg-selection expression (RRC passes the direction's -- ulLegSelection or dlLegSelection) is over the per-bearer PDU counter "packetOrdinal" and returns the leg index directly, e.g. packetOrdinal % 2 for round robin. A single-leg (MCG or SCG) bearer has nothing to steer, and the module serves its second role alone.

Legs whose peer is not attached are not offered to either policy: with one leg live the choice is made already. That leg's DC-specific id mapping on the way out is selected by its cell group (the legs parameter):

an MCG leg (the anchor stack): ids already correct; emits pdcpSduSent.
an SCG leg on a UE (the local secondary stack): rewrites the flow's source/destination to that stack's node ids (serving node looked up per packet, so handover is honored); emits pdcpSduSentNr.
an SCG leg on a DC master (remote leg): rewrites the ids to (secondary node, the UE's secondary-stack id) and adds the X2 addressing tag; the PDU leaves via the DcMux.

If no leg is live at all, the PDU falls back to leg 0 with a warning.

Inheritance diagram

The following diagram shows inheritance relationships for this type. Unresolved types are missing from the diagram.

Parameters

Name	Type	Default value	Description
numLegs	int		Number of legs (set by the enclosing PdcpEntityBase)
legs	string		Cell group of each leg, space-separated "MCG"/"SCG" (set by the enclosing PdcpEntityBase)
binderModule	string	"binder"

Properties

Name	Value	Description
display	i=block/dispatch	The split policy (primary path, split threshold) and any legSelection override are pushed by RRC from the bearer's definition; see banner comment.

Gates

Name	Direction	Size	Description
in	input		PDUs from the TX entity
out [ ]	output		To leg k (sized by the enclosing PdcpEntityBase's connections)

Signals

Name	Type	Unit	Description
sentPacketToLowerLayer
pdcpSduSent
pdcpSduSentNr

Source code

//
// TX-side leg dispatcher of a dual-connectivity split bearer (two legs), steering
// each PDCP PDU to a leg from the split policy RRC pushes off the bearer's definition
// (TS 38.331 PDCP-Config, TS 38.323 5.2.1). A splitter serves exactly one direction:
// a UE's TX side is uplink, a base station's is downlink.
//
// Uplink (at the UE, which has both legs' RLC queues locally): below the
// ul-DataSplitThreshold the bearer stays on its primary path; at or above it, either
// leg may be used, and the spec leaves that choice to the implementation. The splitter
// fills it with ulLegSelection if the definition carries one, else by offloading to
// the least-occupied leg.
//
// Downlink (at the DC master): the secondary leg's RLC queue is at another node,
// across X2, and cannot be weighed here, so there is no threshold or load-balancing --
// dlLegSelection decides each PDU's leg, or the bearer stays on its primary leg. (Real
// downlink split flow control is X2-feedback-driven and is not modeled.)
//
// The leg-selection expression (RRC passes the direction's -- ulLegSelection or
// dlLegSelection) is over the per-bearer PDU counter "packetOrdinal" and returns the
// leg index directly, e.g. packetOrdinal % 2 for round robin. A single-leg (MCG or
// SCG) bearer has nothing to steer, and the module serves its second role alone.
//
// Legs whose peer is not attached are not offered to either policy: with one leg
// live the choice is made already. That leg's DC-specific id mapping on the way out
// is selected by its cell group (the legs parameter):
//  - an MCG leg (the anchor stack): ids already correct; emits pdcpSduSent.
//  - an SCG leg on a UE (the local secondary stack): rewrites the flow's
//    source/destination to that stack's node ids (serving node looked up per
//    packet, so handover is honored); emits pdcpSduSentNr.
//  - an SCG leg on a DC master (remote leg): rewrites the ids to (secondary
//    node, the UE's secondary-stack id) and adds the X2 addressing tag; the
//    PDU leaves via the DcMux.
//
// If no leg is live at all, the PDU falls back to leg 0 with a warning.
//
simple DcPdcpLegSplitter like IPdcpLegSplitter
{
    parameters:
        int numLegs;                        // Number of legs (set by the enclosing PdcpEntityBase)
        string legs;                        // Cell group of each leg, space-separated "MCG"/"SCG" (set by the enclosing PdcpEntityBase)
        string binderModule = default("binder");
        // The split policy (primary path, split threshold) and any legSelection override
        // are pushed by RRC from the bearer's definition; see banner comment.
        @display("i=block/dispatch");
        @signal[sentPacketToLowerLayer];
        @signal[pdcpSduSent];
        @signal[pdcpSduSentNr];
    gates:
        input in;               // PDUs from the TX entity
        output out[];           // To leg k (sized by the enclosing PdcpEntityBase's connections)
}

File: src/simu5g/stack/pdcp/DcPdcpLegSplitter.ned