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Congestion control for 4G/5G networks
Ericsson Research - Ingemar Johansson
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 2
› Don’t rely too much on RTT measurements
› Inter-arrival estimates may be noisy
› Packet pacing improves stability
› Delay based CC’s can be good fallback to combat bufferbloat
› High peak throughput or low latency (under load) ?, pick one
› Warm up your device before ping measurements
Congestion control for 4G/5GSome considerations
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 3
Segmentation, ARQ
Ciphering
Header Compr.
Hybrid ARQHybrid ARQ
MAC multiplexing
Antenna and resrouce mapping
Coding + RM
Data modulation
Antenna and resource mapping
Coding
Modulation
Antenna and resource
assignment
Modulation
scheme
MA
C s
ch
ed
ule
r
Retransmission control
Priority handling, payload
selection
Payload selection
RLC
PHY
PDCP
User #i User #j
Reassembly, ARQ
Deciphering
Header Compr.
Hybrid ARQHybrid ARQ
MAC demultiplexing
Antenna and resrouce mapping
Coding + RM
Data modulation
Antenna and resource
demapping
Decoding
Demodulation
RLC
PHY
PDCP
MAC
Red
un
da
ncy
ve
rsio
n
Radio Bearers
Logical Channels
Transport Channel
MAC
MAC
The LTE Protocol stack
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 4
› Parallel stop-and-wait processes
– 8 processes 8 ms roundtrip time
› ~10% probability of retransmission on MAC layer
– Gives 8ms extra latency
– RLC implements in sequence delivery
MAC LaYER HARQHybrid-ARQ with Soft Combining
Process transport block 3 Process transport block 5
Process #0
Process #1
Process #2
12 3 5 14
Process transport block 2 Process transport block 4
Process transport block 1 Process transport block 1 Process transport block 1
Hybrid-ARQ
protocol
To RLC for in-sequence delivery
Block 2
Process #7
Block 3 Block 4 Block 5 Block 1
1
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 5
› Scheduler behavior is not standardized, implementations are vendor
specific
› Different kinds of schedulers
–Delay : VoLTE (voice)
–Proportional fair, Round Robin : Default (=best effort) bearers
–Max CQI..
› Scheduling has impact on IP packet delay characteristics
DL and UL schedulinG
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 6
› Downlink queuing delay show
little tendency to increase
› Uplink delay is however relatively
large, why ?
Example 1
0
0.5
1
1.5
2x 10
5 CWND
0 0.05 0.1 0.15 0.20
0.05
0.1
T[s]
IP packet delay [s]
Downlink
Uplink
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 7
› Problem free packet
transmission in downlink
› TCP bursts are transmitted as
quickly as resource allocation
allows
Example 1, Sequence numbers DL
0
1
2
3x 10
5 TCP sequence numbers
0
100
PDCP sequence numbers (DL)
0 0.05 0.1 0.15 0.20
50
100
T[s]
RLC sequence numbers (DL)
TX DL
RX DL
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 8
› Uplink ACKs are bundled RTT
affected by UL scheduling
› An uncongested downlink may
thus be considered as congested
by a too sensitive RTT based
congestion trigger
– Example : Cubic HyStart
Example 1, Sequence numbers UL
0
1
2x 10
5 TCP ACKed sequence numbers
0
50
PDCP sequence numbers (UL)
0 0.05 0.1 0.15 0.20
10
T[s]
RLC sequence numbers (UL)
TX UL
RX UL
SC Req
SC Grant
Don’t rely too much on RTT estimates
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 9
› Gaps in IP packet delay, why ?
Example 2
0
5
10
x 104 CWND
0 0.05 0.1 0.15 0.20
0.02
0.04
0.06
0.08
T[s]
IP packet delay [s]
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 10
› RLC AM bearer (in-sequence
delivery)
› HARQ retransmissions delay
packet forwarding to higher layers
Example 2, Sequence numbers DL
0
1
2x 10
5 TCP sequence numbers
0
50
100
150PDCP sequence numbers (DL)
0 0.05 0.1 0.15 0.20
50
100
T[s]
RLC sequence numbers (DL)
TX DL
RX DL
Inter packet arrival measurements can become noisy
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 11
› LTE transport block sizes are limited
– Example : 10MHz BW, max TB size ~36kByte
› Large bursts are queued up, queuing delay can trigger unnecessary AQM drops
or ECN marks
› Lower AQM/ECN thresholds Higher risk of false congestion indication
– Consider L4S
Packet pacing
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 12
Bursty traffic example
› Video streaming example
› Bursty on/off pattern generates
large amounts of immediate
data in RLC queue
› Leads to queuing delay spikes
when new burst starts AQM
packet discard Video rate
reduction
› Solution : Enable packet pacing
44 45 46 47 48 49 50 51 520
0.1
0.2
0.3
0.4Queuing delay [s]
44 45 46 47 48 49 50 51 520
200
400
600
800
1000Congestion window [MSS]
T [s]
New burst,
600packets ~880kB
gives delay spike
AQM discard
CWND is reduced
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 13
Smaller risk of unnecessary AQM discards or ECN marks
with packet pacing
Packet pacing example
0
2
4
6
8x 10
4 CWND
0 0.05 0.1 0.15 0.20
0.05
0.1
T[s]
IP packet delay [s]
Downlink
Uplink
0
2
4
6
8x 10
4 CWND
0 0.05 0.1 0.15 0.20
0.05
0.1
T[s]
IP packet delay [s]
Downlink
Uplink
Packet pacing OFF Packet pacing ON
More consistent
queue buildup
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 14
› Throughput variations
– Larger variations expected
› Carrier Aggregation
› Dual Connectivity
› Multi-beam technology
– Solutions :
› Fast increase options
- SIAD
- Cubic with fast increase option
› Other : PCC, Verus ?
› Bufferbloat
– Expected to be solved over time
› AQM, ECN, L4S
– But fallback to a delay/loss based CC good
Congestion control for 4G/5G
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 15
5 10 15 20 25 300
0.5
1
TCP segment delay [s]
5 10 15 20 25 300
1000
2000
Congestion window [kbyte]
5 10 15 20 25 300
20
40
Throughput [Mbps]
T [s]
5 10 15 20 25 300
0.5
1
TCP segment delay [s]
5 10 15 20 25 300
1000
2000
Congestion window [kbyte]
5 10 15 20 25 300
20
40
Throughput [Mbps]
T [s]
5 10 15 20 25 300
0.5
1
TCP segment delay [s]
5 10 15 20 25 300
1000
2000
Congestion window [kbyte]
5 10 15 20 25 300
20
40
Throughput [Mbps]
T [s]
Example : Cubic modificationRTTmin = 100ms , Variable BW = 5505Mbps
Cubic +fast increase +delay target 100ms
Modified congestion controls can improve performance in
typical 4G/5G scenarios
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 16
Peak throughput vs low delay10MB file transfer
0
0.05
0.1
TCP segment delay [s]
0
500Congestion window [kB]
0
10
20
Throughput [Mbps]
0 5 10 15 20 25 30 350
500
1000RLC queue [bytes]
T [s]
0
0.05
0.1
TCP segment delay [s]
0
100
200
Congestion window [kB]
0
10
20
Throughput [Mbps]
0 5 10 15 20 25 30 350
500
1000RLC queue [bytes]
T [s]
ECN th. 100ms ECN th. 20ms
A too shallow
RLC queue
gives degraded
resource utilization
A low target queue delay can give poor link utilization
Reason : Too little data in RLC queue
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 17
Latency in 5G ?
› Numerology
– Short subframe duration, 0.25-0.5 ms or less low latency (together with tighter UE/eNB processing)
› HARQ RTT reduction from ~8ms to ~1ms possible
– More robust HARQ
– Multiple numerologies may be needed
› Depending on deployment scenario, use case and frequency band
– Many alternatives
› Option for contention based scheduling in UL
15 kHz20 MHz 1 ms
Phase-noise robustness,
reduced CP overhead
Reduced latency
Increased bandwidth Increased subcarrier spacing Reduced subframe duration
Very high data rates
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 18
Questions/comments?
65.359466N, 22.478387E
Extra slides
More on latency in LTE
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 20
› Two ECM states
– ECM-IDLE: No physical resources assigned
– ECN-CONNECTED: Physical resources assigned
› UE goes into ECM-IDLE if inactive for a longer time
– (vendor specific, 10s, 60s...)
LTE states
Physical resource : Radio resource (SRB/DRB) and
Network Resource (S1 bearer/S1 signalling connection)
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 21
› C-DRX = Connected Mode DRX (ECM-CONNECTED)
› UE radio is turned off if nothing transmitted or received for an inactive timer
period (e.g 10ms)
– Radio turned on immediately if UL data available in UE
› UE wakes up and listens at intervals given by a short DRX timer (e.g 40ms)
› If no data transmitted or received for a given number of short DRX cycles, go
into long DRX
– UE wakes up and listens at intervals given by a long DRX timer (e.g 320ms)
› If UE inactive for a longer time (10s-60s) go into idle state (ECM-IDLE)
– UE wakes up and listens for paging at standardized intervals (e.g 1024ms)
DRX Discontinuous Reception
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 22
› LTE– Connection establishment
– Synchronization
– Air transport
– Processing
– Paging/C-DRX (DL)
– Handover/Mobility
› EPC– Connection establishment
– Handover/Mobility
– Packet processing (very small)
› Transport network– Propagation delay
– Processing
› Application– Processing delay
› Internet transport– Propagation delay
– Processing
LTE Latency breakdown
EPC eNB App UE
Conditional:
Nothing/Synchronization/
Connection establishment
Trigger
Air transport +
processing
Propagation +
processing Internet
Propagation +
processing
Application
processing
Application
processing
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 23
LTE UL air interface latenciesexample 1
SR
Grant
Data + BSR
Data
Data is created
and packetized
Data
Data
Grant
Data
Data
Grant
Data
Data
Data
T1
T2
T3a
T3b
T4
T5
T6
T7
T3
T0
Modem eNB SGWMME PGWApp
Server
UE L2 Processing
+ SR periodicity
SR transmission
+ eNB processing
Grant
transmission +
UE L2
processingData (incl BSR
reporting) + eNB
processing
eNB
processing
and grants
(conditional
due to BSR)
Data (incl BSR
reporting) + UE
and eNB
processing
T0
T1 0.5-5 ms
T2 1 ms
T3a 2 ms
T3b 0 ms
T4 1 ms
T5 3 ms
T6 1 ms
T7 2 ms
Total: 10.5-15 ms
From ECM-CONNECTED
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 24
LTE UL air interface latenciesexample 2
UE in ECM-IDLE
Data
Initial UE Message
Data packet
Modify Bearer Resp
Modify Bearer Req
Modify Bearer Resp
Modify Bearer Req
T1RRC Connection
Request
RRC Connection
Set-up
RRC Connection
Set-up Complete
Security Mode Comm.Initial Context
Setup Req.Security Mode Comp.
Initial Context
Setup Resp.
UE in ECM-CONNECTED
Conditional: UE Capability Retrieval (not included)
RRC Connection
Reconf.RRC Connection
Reconf. Complete
Optional: Authentication and NAS Security Activation (not included)
RACH preamble
RACH Response
Modem eNB SGWMME PGW HSS
T2
T3
T4
Data
T0
T5
From ECM-IDLE: 130 ms – Don’t mix states in testing
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 25
LTE DL AIR Interface latenciesexample 1
T1a 2 ms
T1b 0 ms
T1c 0.5 ms
T2 1 ms
T3
Total: 3.5 msData packet
to higher
layers
T1a
T2
Data
Data packet
Data
T1b
Data
T3
T1
T1c
Modem eNB SGWMME PGW
eNB processing
including frame
alignment
scheduling and C-
DRX cycle
Data transport
From ECM-CONNECTED
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 26
Data packet
to higher
layers
Preamble on
PDCCH
Data packet
Data
Data
RACH
Preamble
RA
Response
Data
T1a
T1b
T2
T3
T4
T5
T6
T7
T8
T9
Modem eNB SGWMME PGW
From ECM-CONNECTED but out of synch: 23 ms – Don’t mix states in testing
LTE DL AIR Interface latenciesexample 2
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 27
0
Pre-scheduling Pre-scheduling OFF
Delay Breakdown
Time [ms]
SR
Grant
PING
PINGPING
PING
PINGPING
5 SR encod. and alignment
1
eNB processing3
1
11
11
3GPP specified delay4
Core network delay3
2
23 ms Total round trip delay *
5
10
15
20
*Note: Numbers are example only
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 28
10
0
Pre-schedulingPre-scheduling ON
Delay Breakdown
Time [ms]
GrantPING
PINGPING
PING
PINGPING
11
11
3GPP specified delay4
Core network delay3
2
13 ms Total round trip delay *
5
15
20
*Note: Numbers are example only
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 29
› The gain of the prescheduling on short file
transfers is clear and directly proportional to
the period of the prescheduling.
› The delta between best and worst case
scenario gains is around 100 ms
› Cost : More interference and resource usage
Pre-scheduling Short file download
No Prescheduling
2 ms period
No Pre-scheduling
2ms
period
5ms
10ms20ms
ICCRG - Congestion control for 4G/5G networks | 2016-07-01 | Page 30
› EPS : Evolved Packet System
› EPC : Evolved Packet Core
› EMM : EPS Mobility Management
› ESM : EPS Session Management
› ECM : EPC Connection Management
› SRB : Serving Radio Bearer
› DRB : Data Radio Bearer
› DRX : Discontinuous Reception
› UE : User Equipment ~ Terminal
Glossary
Recommended