MAC-level throughput

[Manuel Cano]

Hello everyone,

I’m configuring a onoff traffic source with a 5 Mbps constant rate in downlink, 20MHz (PRB = 100), MCS = 28, TBSize = 9422 bytes and 60 sec. We use UDP sockets:

// Create the On-Off application in the remote node

// This one represents the Video server in the internet

OnOffHelper onoffremoteVideo ("ns3::UdpSocketFactory",

                        Address (InetSocketAddress (ueAddr[0], dlPort)));

onoffremoteVideo.SetAttribute ("OffTime", StringValue ("ns3::ConstantRandomVariable[Constant=0]"));

onoffremoteVideo.SetConstantRate(DataRate ("5Mbps"),500);  //. PacketSize: 2500-> If we want to avoid IP fragmentation (MTU<1472). Back off IP(20)+UDP(8) header.

serverVideoApps.Add(onoffremoteVideo.Install(remote));

 

  I’m running  LTE LENA simulator on ns 3.19 and according to FlowMon  the resulting throughput is 5.03473 Mbps which matches the transmission rate defined. When I calculate throughput using PDCP traces I get 662500 Bps = 5,3 Mbps, as it can be seen in the following trace fragment:

start	end		nTxPDUs			TxBytes		nRxPDUs		RxBytes
2		2.25		312	165360		313		165890
2.25		2.5		313	165890		312		165360
2.5		2.75		312	165360		313		165890
2.75		3		313	165890		312		165360

                                       DlPdcpStats

But when I calculate throughput at MAC level I have 1000 packets/second * 9422 Bytes = 9,422 MBps = 75,376 Mbps, as extracted from the DlMac.

% time            cellId              IMSI               frame              sframe             RNTI               mcsTb1           sizeTb1            mcsTb2           sizeTb2

1.1                   1                      1                      111                  1                      1                      28                    9422                0                      0

1.101               1                      1                      111                  2                      1                      28                    9422                0                      0

1.102               1                      1                      111                  3                      1                      28                    9422                0                      0

1.103               1                      1                      111                  4                      1                      28                    9422                0                      0

1.104               1                      1                      111                  5                      1                      28                    9422                0                      0

…

1.198               1                      1                      120                  9                      1                      28                    9422                0                      0

1.199               1                      1                      120                  10                    1                      28                    9422                0                      0

1.2                   1                      1                      121                  1                      1                      28                    9422                0                      0

                                                                                      DlMacStats

If I run ns3 with a throughput higher than 4 Mbps I always get 75,376 Mbps because up to 4 Mbps I have 1 transmission per second in MAC level.

Why the achieved throughput at MAC-level (DlMacStats) is so high with respect to IP (FlowMon) and PDCP-level (DlPdcpStats)?

Maybe there is something that we are missing out, any explanation for this?

[Nicola Baldo]

with the application setting that you have, you'll see approx one new IP packet per 0.8 milliseconds, right? So this explains why the LTE MAC sends one TB per TTI.
The size of the TB depends on the LTE MAC scheduler implementation, but it could be that it's large just because of the very strong signal and the absence of other users, i.e., the TB could be partially empty.