Slide title In CAPITALS 50 pt Slide subtitle 32 pt Active Queue Management for LTE uplink in eNodeB Yifeng Tan Supervisor: Professor Riku Jäntti Instructor:

Slide titleIn CAPITALS

50 pt

Slide subtitle 32 pt

Active Queue Management for LTE uplink in eNodeB

Yifeng Tan

Supervisor: Professor Riku Jäntti Instructor: D.Sc. Riikka Susitaival

Top right corner for field-mark, customer or partner logotypes. See Best practice for example.

Slide title 40 pt


Text 24 pt

Bullets level 2-520 pt

Ericsson Confidential 2009-02-202

Long-Term Evolution (LTE)

Evolved 3G Radio Access Network Provides high data rate

– Downlink: up to 300 Mbps– Uplink: 50 Mbps

”Mobile Internet”– TCP/IP over wireless

R99 Rel4 Rel5 Rel64

Rel7 Rel8

WCDMA HSDPA HSPA HSPA evolution

LTE


Slide title 40 pt


Text 24 pt



TCP basics

One of the core protocols of the Internet Provides reliable transmission

– Automatic Repeat reQuest (ARQ): Receiver sends ACKs to indicate the reception of segments.

TCP sender’s window: How many packets are ”in flight”– Ideally: Sender’s window = Pipe Capacity (Bandwidth ∙ RTT)– Smaller: under-utilization of the link– Bigger: queuing of packets. When queue exceeds buffer limit,

some packets have to be discard → congestion

ACKs received Not transmitted

Sender’s window


Slide title 40 pt


Text 24 pt



TCP Congestion Control

TCP sender tries to ”probe” optimal send-window– ACK received →Increase send-window

”More bandwidth available”– Packet loss → Decrease send-window

”Light congestion”– Time-outs → Start probing from scratch

”Serious congestion”


Slide title 40 pt


Text 24 pt



TCP Congestion Control example

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1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35Time (k * RTT)

Co

ng

est

ion

Win

do

w (

n *

MS

S)

Pipe Capacity

Slow StartThresholdreached

3rd DUPACK=> Congestion

Avoidance Phase

Timeout=> Slow Start

Phase

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1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35Time (k * RTT)

Co

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Win

do

w (

n *

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Pipe Capacity



3rd DUPACK=> Congestion

Avoidance Phase

Timeout=> Slow Start

Phase


Slide title 40 pt


Text 24 pt



Queue Management

Passive Queue Management– Drop packets when the buffer is full– We can only decide ”which” but not ”when”– Drop-tail, drop-front, random-drop– PQM may cause some problems

Significant end-to-end delay Unfair sharing Viscous web browsing

Active Queue Management– Drop packets even there is space– We can decide both ”which” and ”when”


Slide title 40 pt


Text 24 pt



Advantages of AQM

Maintains lower average queue size as well as end-to-end delay than PQM

– Important when e.g. Web browsing

Prevents lock-out– Avoid the situation where one or a few TCP flows dominate

the link

Reduce the number of packet drops– There is enough space to accommodate the packets in

burst


Slide title 40 pt


Text 24 pt



Data transmission in LTE uplink

Buffer Status Report (BSR): created by UE and sent to eNodeB to indicate the queue size in UE’s buffer

eNB

PHY

UE

PHY

MAC

RLC

MAC

PDCPPDCP

RLC

UE

BSR

Data


Slide title 40 pt


Text 24 pt



Example: PDCP discard

Specified in 3GPP TS 36.323 V8.2.1

for each outgoing packet (PDCP SDU)if (delay > delayThreshold ) discard packetelse

transmit packet

Suffer throughput degradationeNB

PHY

UE

PHY

MAC

RLC

MAC

PDCPPDCP

RLC

UE

BSR

Data

PDCP discard


Slide title 40 pt


Text 24 pt



Transmitter-AQM Algorithm

for each outgoing packetif(size <= lowerDropThreshold)

transmit packetelse if (delay > minAgeThreshold

AND (now-previousDropTime > minInterDropTime))

OR (delay >maxAgeThreshold)) discard packet previousDropTime=now

else transmit packet

UE

channel

RLCRLC

eNodeB

Drop triggered?

No

Yes

PDCP PDUs

PDCP PDUs

RLC SDU

RLC PDU

Improve the throughput of PDCP discard Maintains congestion window bigger than PC –> minAgeThreshold Prevent from consecutive packet drops -> minInterDropTime Prevent from drop when queue size is small -> lowerDropThreshold Drain buffer fast -> maxAgeThreshold


Slide title 40 pt


Text 24 pt



Implement in eNodeB?

Why?– Transmitter-AQM was suggested

to be standardized but not accepted

– UE with or without AQM may have quite different performance

– We may control the UE buffer from eNodeB by Receiver-AQM so that all the UEs can have improved throughputs

We need to know– queue size (BSR)– packet delay -> How?

UE

BSR

Data

R-AQM


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Text 24 pt



A method to estimate queuing delay The method estimate the delay of headmost data in a queue

some assumptions

1. We can monitor the queue length (Qi) discontinuously with small time

intervals

2. The amount of data left the queue in each time interval is known (Li)

3. The length of time interval is known (∆ti)

The amount of new incoming data (Ri) in each time interval

can be calculated by Ri = Qi + Li – Qi-1


Slide title 40 pt


Text 24 pt



Waiting time

Queue size

∆t1/2

0 Q1

Time t0

0

Time t1Time t2

Q1 + R2R2

∆t1/4+∆t2∆t2

Q2

∆t1/2+∆t2

At the beginning...


Slide title 40 pt


Text 24 pt



Waiting time

Queue size

0

0

Rn

∆tn

1

n

ii n

R

1

n

ii n

t

∙∙∙∙∙∙

At Time tn

1

2n n

i ii r i r

t t

2

n

ii r

R

2

n

ii r

t

1

n

ii r

t

1

n

ii r

R

n

ir

R

n

ii r

t

Qn-1

+Rs

Qn

1

2n n

i ii r i r

t t


Slide title 40 pt


Text 24 pt



How to implement in eNodeB

Queue length (Qi) -> BSR

∆ti -> time interval of receiving BSRs

Leaving data (Li) -> the data delivered to higher layer

in eNodeB during each ∆ti


Slide title 40 pt


Text 24 pt



Estimated delay vs. Real delay


Slide title 40 pt


Text 24 pt



Receiver-AQM Algorithm

for each outgoing packetif(BsrSize <= lowerDropThreshold)

transmit packetelse if (estimatedDelay > minAgeThreshold

AND (now - previousDropTime> minInterDropTime))

discard packet previousDropTime=now

else transmit packet

Drop triggered?

RLC

RLC

UEeNodeB

PDCP PDUs

RLC SDU

No

Yes

channel

UE

BSR

Data

R-AQM

Drop packets in eNodeB

The algorithm is similar to T-AQM

– Queue size reported by BSR -> real queue size

– Estimated delay -> real delay


Slide title 40 pt


Text 24 pt



Performance comparison with one TCP flow

Throughput and end-to-end delay (UE speed = 0.83 m/s)

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R-AQM T-AQM PDCP discard drop-front

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Mean end-to-end delay

Max end-to-end delay

Throughput


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Text 24 pt



Performance comparison with multiple TCP flows

Average throughput of new incoming files

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20 Mbps 3 Mbps 384 kbps

Th

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R-AQM

T-AQM

PDCP

Drop-from-front


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Text 24 pt



Performance if both AQM implemented (Bandwidth = 3 Mbps)

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R-AQM T-AQM Both AQM

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Mean end-to-end delay

Max end-to-end delay

Throughput


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Some conclusions

Using AQM can efficiently reduce the end-to-end delay in LTE uplink.

PDCP discard may cause considerable throughput reduction in some situations.

R-AQM and T-AQM can maintain small delay as well as high throughput in most situations.

R-AQM is a good option to enhance the performance of UE from network side, no matter if the UE has implemented any AQM itself.


Slide title 40 pt


Text 24 pt



Documents

Slide title In CAPITALS 50 pt Slide subtitle 32 pt Active Queue Management for LTE uplink in eNodeB Yifeng Tan Supervisor: Professor Riku Jäntti Instructor: