Adding delay in core network

[praghur]

Hello all. 

In my profile (Profile Repository) for srsLTE with 3 eNBs and 1 EPC, UEs connected to eNB1 and eNB2 communicate with UEs to eNB3. 

I wanted to add a certain delay in the EPC. Since, I did not find this option in epc.conf, I added the delay in the destination eNB (i.e. eNB3) as shown below. 

# delay.enable:      Enable/disable delay simulator
# delay.period_s:    Delay period in seconds.
# delay.init_time_s: Delay initial time in seconds.
# delay.maximum_us:  Maximum delay in microseconds
# delay.minumum_us:  Minimum delay in microseconds

[channel.dl.delay]
enable        = true
period_s      = 1
#init_time_s   = 0
maximum_us    = 5000
minimum_us    = 3000

I added similar option for uplink delay as well. However, when I ping eNB3, the latency is obtained much less than 3 ms. Please see attached the results from the ping. My thinking was since the minimum delay added to the channel is 3 ms, the overall latency must be higher than 3 ms. 

Am I missing something? Does ping bypass this added delay? Is there any other way to measure latency that will also include this added delay? Lastly, is there a more efficient way to add delay in the core network? 

Thanks in advance for any insights. 

[Dustin Maas]

Hi Priya,

I don’t think you want to add delays at the PHY layer for your use case, which is what the options in the srsLTE eNB are doing.
Rather, have a look at using something like tc (part of the iproute2 package) to add latency to the interfaces/links between your EPC and eNBs.

-Dustin

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[praghur]

Thank you, Dustin. This was really helpful. I looked into adding the delay to the interfaces between eNB and EPC. For the EPC node, I see that there are several interfaces as shown below with their corresponding IP addresses. Based on this, I added a 5ms delay to the vlan135 that is connected to eNB.

lo 127.0.0.1/8
lo ::1/128
eno1 155.98.36.26/22
eno1 fe80::1618:77ff:fe26:68ed/64
enp4s0f1 fe80::3efd:feff:fe04:8182/64
vlan135 10.10.1.1/24
vlan135 fe80::3efd:feff:fe04:8182/64
srs_spgw_sgi 172.16.0.1/24
srs_spgw_sgi fe80::1b29:5fc2:b2b4:2b29/64

Yet, the ping results is still giving me a latency less than 5 ms as shown below.

3 packets transmitted, 3 received, 0% packet loss, time 2045ms
rtt min/avg/max/mdev = 0.039/0.060/0.097/0.026 ms

Am I still missing something? Sorry for my rudimentary question. Please see my profile and the running experiment below.

Profile -  praghur/srsLTE_PR at Reduced-nodes_S1-adapted (github.com)
Experiment - POWDER Experiment - 3 eNBs and 1 EPC

Thanks again in advance.