Implementing & evaluating the RLC/AM protocolpages.cs.aueb.gr/~makidis/rlc-am.pdf · The RLC...

Makidis [email protected]

mailto:[email protected]

RLC protocol overview RLC/AM procedures Simulation setup Performance of applications without

contention Performance of applications with contention Performance of CBR Media Distribution

application SDU discard policy impact Conclusion

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The RLC protocol

RLC/AM Functions

Frame types

SUFIs for the Status PDU

Windows

Protocol States

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Radio Link Control protocol: link layer protocol for the UMTS◦ Transparent Mode: No services provided by the RLC e.g. voice calls

◦ Unacknowledged Mode: Duplicate avoidance & reordering only Low delay and higher reliability than the raw link

e.g. video streaming using UDP

◦ Acknowledged Mode: Reliable data transfer (mostly) e.g. web browsing, file transfer, …

Sliding window protocol (like Selective Repeat, GBN)

Uses sequence numbers, performs retransmissions

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All logos and trademarks are property of their respective owners.

Not implemented

Transfer of user data◦ Mostly reliable data transfer◦ Discards data after time period or number of

(re)transmissions

Error correction

Sequence number check, duplicate detection, in-sequence delivery

Protocol error detection & recovery

Flow control

Segmentation & reassembly, concatenation, padding

Ciphering

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Reset &

Reset

AC

K P

DU

Sta

tus P

DU

Data

PD

U

Data (AMD) PDU◦ Transfers the user’s data◦ Has a sequence number

in [0, 4096)◦ Contains segments of SDUs from upper

layers◦ Has a polling bit◦ Can have a piggybacked Status PDU in the

padding space

Status PDU◦ Has one or more SUFIs (Super Fields)◦ Used for reporting ACKs/NACKs◦ Used by the SDU Discard function◦ Used by the Flow Control function

RESET & RESET ACK PDU◦ Used by the RLC Reset procedure

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DATA

Sequence number

Length Indicator (LI) E

D/C

Length Indicator (LI) E

…

Padding orPiggybacked Status PDU

Byte 1

Byte 2

Byte 3 (opt)

Byte N

Sequence number P HE

SUFI 1D/C

Padding

Byte 1

Byte 2

Byte N

PDU type

…

SUFI k

SUFI 1

R1D/C

Padding

Byte 1

Byte N

PDU type RSN

HFNI

HFNI

HFNI

ACK/NACK SUFIs◦ ACK: Acknowledges all PDUs up to the ACKed SN◦ BITMAP: Contains bitmap indicating ACKed/NACKed PDUs of the

Receiver’s window◦ LIST: List of SN/length pairs indicating NACKed PDUs◦ RLIST: List of NACKed PDUs in encoded, relative form◦ The spec does not specify which SUFIs to use (other than ACK). We

used ACK & BITMAP.

Move Receiving Window SUFIs◦ MRW: Request from the Sender to discard SDUs and Move the

Receiver’s Window◦ MRW_ACK: ACK of an MRW; indicates the Receiver’s new window

position

Other SUFIs◦ WINDOW: Request from the Receiver to change the Sender’s

window size (used for Flow Control)◦ NO_MORE: Indicates the end of the Status PDU

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ReceiverSender

Flow Control: The Receiver can change the Sender’s window size (by using the WINDOW SUFI). The Flow Control algorithm is not specified.

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Acknowledged PDU

Send but unacknowledged PDU

Usable slot, PDU not sent

PDU outside the window (will be rejected)

VT(S)VT(A) VT(MS)

Transmission window [VT(WS)]

Received PDU(in-sequence)

Received PDU(out-of-sequence)

Acceptable (not received) PDU

PDU outside the window (will be rejected)

VR(H)VR(R) VR(MR)

Receive window

NULL: The RLC entity is not created (or is terminated)

DATA_TRANSFER_READY

RESET_PENDING: RLC Reset procedure initiated

LOCAL SUSPEND: RLC entity suspended (cannot send PDUs higher than a SN)

RESET_AND_SUSPEND: RLC entity suspended and RLC Reset procedure initiated

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NULLDATA_

TRANSFER_READY

RESET_PENDING

LOCAL_SUSPEND

RESET_AND_SUSPEND

Rq. from upper layers



Reset triggerRESET

RESET ACK

RESET ACKRESETRESET ACK

Rq. from upper layersConf. to upper layers

RESET ACKRESET



Rq. from upper layersConf. to upper layersRq. from

upper layers

RESETRESET ACK


Data transfer

Polling

Status Report transmission

Limited reliability of the RLC/AM

RLC Reset procedure

Other procedures

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Sender◦ Segmentation, Concatenation, Padding◦ Increase sequence number (ensure it’s within

window)◦ Start discard timer/Count (re)transmission◦ Set polling bit, if poll triggered◦ Attach piggybacked Status PDU (optional)

Receiver◦ Advance window◦ Reassembly, Deliver SDUs in sequence◦ Send a Status report, if polled

Abnormal cases◦ Receive PDUs outside the reception window◦ Duplicate PDUs◦ Invalid PDU format/size◦ Full buffer (for the UE)


Sender Receiver

AMD PDU

Poll: Request from the Sender to the Receiver for a Status Report (i.e. ACKs & NACKs)

Poll triggers (any combination may be used)◦ Every last PDU in buffer (not impl.)

◦ Every last PDU in Retransmission buffer◦ Poll Timer◦ Every x PDUs◦ Every x SDUs◦ Window Based (poll triggered if a percentage of

Sender’s window is used)◦ Timer Based (periodic polling)

Poll Prohibit (optional)◦ After the transmission of a Poll, a timer is

started◦ Polling is prohibited (delayed) while the timer is

active


Sender Receiver

AMD PDU with polling bit

set

Status PDU with ACKs/NACKs

Status Report: Status PDU from the Receiver to the Sender with ACKs/NACKs◦ Uses the ACK SUFI and one or more of the

BITMAP, LIST, RLIST SUFIs

Status Report triggers◦ Polling◦ Detection of missing PDU(s) (optional)◦ Timer based (periodic transmission)

(optional)◦ Request from lower layers (not impl.)

Status Prohibit (optional)◦ After the transmission of a Status Report, a

timer is started◦ Status Report transmission is prohibited

(delayed) while the timer is active


Sender Receiver

AMD PDU with polling bit

set

Status PDU with ACKs/NACKs

The Sender discards PDUs if they have not been successfully sent◦ Within a certain time period, or◦ Within a number of (re)transmissions

SDU Discard Function operation modes◦ Discard SDUs (and PDUs) after a period of

time, or◦ Discard PDUs (and SDUs) after x

transmissions, or◦ Reset the RLC entities after x transmissions

The Receiver is informed of the discarded SDUs in order to Move (advance) its Reception Window (MRW)◦ The MRW request is periodically send until it

is ACKed◦ If the MRW request is not ACKed within a

number of attempts, the RLC Reset procedure is initiated


Sender Receiver

Status PDU with MRW SUFI

Status PDU with MRW_ACK SUFI

Reset triggers◦ Erroneous sequence numbers in a Status PDU◦ Too many attempts to send a MRW command◦ Using the “Reset after x transmissions” SDU Discard

function and the limit is reached

RLC Reset: clear buffers, stop timers, reset variables

Procedure◦ Sender sends a RESET PDU (periodically)◦ Receiver gets the RESET PDU, resets itself, responds

with a RESET ACK◦ Sender receives RESET ACK, resets itself

Abnormal cases◦ RESET PDU sent too many times without ACK:

unrecoverable error signaled; protocol stalled◦ Duplicate RESET PDU is received: the Receiver

responds with a RESET ACK without resetting itself◦ RESET PDU received by the Sender: the Sender acts

as the Receiver


Sender Receiver

RESET PDU

RESET ACK PDU

Local Suspend & Resume◦ The Sender allowed to send only N more PDUs◦ PDUs after the Nth are delayed until the entity is resumed

Stop & Continue◦ All transmissions & receptions are delayed until the upper layers initiate the Continue

procedure◦ Timers are not affected

Re-establishment◦ The RLC entity is re-established by upper layers◦ All buffers cleared◦ All parameters & variables reset

Reconfiguration of RLC parameters by upper layers◦ The upper layers change one or more parameters of the RLC entity while it is active◦ Variable values may be changed because of the parameters◦ Window sizes can be changed as well◦ A number of buffered PDUs may be dropped as a result (especially in the UE).


Applications & topology

Selective Repeat variants & RLC parameters


64 Kbps data rate, 50 ms delay, 250 bytes frame payload, Uniform or Two State error model

LAN: 10 Mb data rate, 1 ms delayWAN: 2 Mb data rate, 50 ms delay

ns-2 simulator with extra link layer protocols, wireless links & error models

Web Browsing◦ Metric: Server-to-client data throughput◦ Measured completed transactions only

File transfer (FTP)◦ Transfer of a 10 MB file (unidirectional)◦ Metric: Application data throughput

CBR Media Distribution◦ Audio or Video distribution (56 kbps CBR)◦ Alternates between talking and silent state◦ Uses UDP (bypasses reliable LL protocol)◦ Metric: CBR stream delay◦ Contention with TCP application (CBR

stream consumes 37,5% of bandwidth)

Each test performed 30 times Tested frame loss at rates 0%, 1.5%,

2.5%, 5.4% and 9.8%


TCP/UDP

IP

LL

PHY

TCP/UDP

IP

LL

PHY

TCP/UDP

IP

LL

PHY

Wired server Wired host Wireless client

Simulation topology

Uniform error model Loss probability same for each pkt

Two state error model Good state of the channel

(10-6 error rate)

Bad state of the channel(10-2 error rate)

Selective Repeat◦ Sliding window protocol◦ Receiver uses ACK and NACK frames

to inform the Sender of its status◦ Sender retransmits NACKed frames◦ Delayed, piggybacked ACKs in data

frames◦ Timer for each not ACKed data frame

(in case its ACK/NACK gets lost)◦ Supports multireject (a frame can be

NACKed multiple times)◦ Offers good performance

SR variants◦ Simple SR with fixed timeout value

1.1s◦ Adaptive SR with dynamic timeout

value (self-cloacking, like TCP)

Uniform model: 3×srtt+2×srttvar Two State model: 4×srtt+0×srttvar


RLC/AM parametersParameter Uniform model

(“Cellular” link)

Two State model

(“PCS” link)

Window size

(Sender & Receiver)

128 frames 128 frames

Poll triggers Timer_Poll, Window Based Timer_Poll, Window Based,

Every last PDU in

retransmission buffer

Status report triggers

(other than polling)

Detection of missing PDU,

Periodic status reporting

Detection of missing PDU,

Periodic status reporting

Poll Prohibit enabled Yes Yes

Status Prohibit enabled No Yes

RESET enabled No No

Poll Window 70% 80%

SDU Discard Mode SDU discard after x number

of transmissions

SDU discard after x number

of transmissions

MaxDAT 3 2

Timer_Status_Prohibit

timeout

- 90 ms

Timer_Status_Periodic

timeout

400 ms 500 ms

Timer_Poll timeout 200 ms 200 ms

Timer_Poll_Prohibit timeout 100 ms 100 ms

Timer_MRW timeout 500 ms 110 ms

Status report piggybacking

enabled

No No

Application impact (Uniform mdl.)

Application impact (Two State mdl.)

Topology impact


RLC/AM performance depends on application◦ Poor performance for web browsing

Adaptive SR has high throughput Almost identical results for WAN topology


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0 1 2 3 4 5 6 7 8 9 10

Th

rou

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bp

s)

Frame loss rate (%)

HTTP w/o contention, LAN/Cellular links

Raw LinkSelective Repeat

Adapt. Selective RepeatRLC/AM

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40

50

60

70

0 1 2 3 4 5 6 7 8 9 10

Th

rou

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bp

s)

Frame loss rate (%)

FTP w/o contention, LAN/Cellular links



Web browsing File transfer

Lower throughput in Two State model

RLC/AM performance depends on application◦ Poor performance for web browsing, OK for FTP but not for high frame loss rates

Adaptive SR has high throughput

Almost identical results for WAN topology

It has been shown that TCP performs worse in the Two State model


25

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50

0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)

HTTP w/o contention, LAN/PCS links



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40

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65

0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)

FTP w/o contention, LAN/PCS links



Web browsing File transfer

Lower throughput in

WAN topology due

to higher delay in

interactive application

File t

ransfe

rW

eb b

row

sin

g Almost the same performance

for both topologies


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Frame loss rate (%)




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Th

rou

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Frame loss rate (%)

HTTP w/o contention, WAN/Cellular links



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0 1 2 3 4 5 6 7 8 9 10

Th

rou

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Frame loss rate (%)




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0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)

FTP w/o contention, WAN/Cellular links



Similar results for the Two

State model as well.

LAN

LAN

WAN

WAN

CBR cont. impact (HTTP) (Un. mdl.)

CBR cont. impact (HTTP) (TS mdl.)

CBR cont. impact (FTP) (Un. mdl.)

CBR cont. impact (FTP) (TS mdl.)


Similar relative performance for all protocols Identical results for the WAN topology as well Adaptive SR has the highest throughput


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Th

rou

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Frame loss rate (%)




5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8 9 10T

hro

ug

hp

ut

(Kb

ps

)

Frame loss rate (%)

HTTP with contention, LAN/Cellular links



Without CBR contention With CBR contention

Similar relative performance for RLC/AM and Adaptive SR Fixed SR performance drops slightly in the Two State model Similar results for the WAN topology Adaptive SR has the highest throughput


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Th

rou

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t (K

bp

s)

Frame loss rate (%)

HTTP w/o contention, LAN/PCS links



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0 1 2 3 4 5 6 7 8 9 10T

hro

ug

hp

ut

(Kb

ps

)Frame loss rate (%)

HTTP with contention, LAN/PCS links




Similar relative performance for all protocols Identical results for the WAN topology as well


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20

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0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)




10

15

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30

35

40

45

0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)

FTP with contention, LAN/Cellular links




Similar relative performance for RLC/AM and Adaptive SR Fixed SR performance drops significantly in the Two State

model Similar results with the WAN topology


35

40

45

50

55

60

65

0 1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (K

bp

s)

Frame loss rate (%)




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30

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42

0 1 2 3 4 5 6 7 8 9 10T

hro

ug

hp

ut

(Kb

ps


FTP with contention, LAN/PCS links




CBR with HTTP contention

CBR with FTP contention


RLC/AM causes the lowest delay (but has low throughput as well); fixed SR causes the highest (esp. in the Two State model)

Adaptive SR causes low delay SR variants perform more retransmissions as the frame loss rate increases, thus

causing more delay Similar results for WAN topology


0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7 8 9 10

Pa

ck

et

de

lay

(s

ec

on

ds

)

Frame loss rate (%)

CBR with HTTP contention, LAN/Cellular links



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1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6 7 8 9 10

Pa

ck

et

de

lay

(s

ec

on

ds

)

Frame loss rate (%)

CBR with HTTP contention, LAN/PCS links



Uniform error model Two State error model

RLC/AM causes the lowest delay; fixed SR the highest (esp. in the Two State model)

Adaptive SR causes low delay, almost as low as RLC/AM Similar results for WAN topology


0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6 7 8 9 10

Pa

ck

et

de

lay

(s

ec

on

ds

)

Frame loss rate (%)

CBR with FTP contention, LAN/Cellular links



1.5

2

2.5

3

3.5

4

4.5

5

5.5

0 1 2 3 4 5 6 7 8 9 10P

ac

ke

t d

ela

y (

se

co

nd

s)

Frame loss rate (%)

CBR with FTP contention, LAN/PCS links



Uniform error model Two State error model

FTP throughput, Uniform model

FTP throughput, Two State model

Web Browsing throughput


Discard after x transm.: Higher throughput◦ Highest throughput for MaxDAT=3 for the Uniform error model

Timer-based discard: Lower throughput◦ Higher than raw link only in high frame loss rates◦ Timeout values 300-700 ms have almost the same throughput


10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10

Th

rou

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t (K

bp

s)

Frame loss rate (%)


Raw LinkRLC/AM MaxDAT=2RLC/AM MaxDAT=3RLC/AM MaxDAT=4RLC/AM MaxDAT=5RLC/AM MaxDAT=6RLC/AM MaxDAT=7

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20

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Th

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bp

s)

Frame loss rate (%)


Raw LinkRLC/AM disc. to=0.1sRLC/AM disc. to=0.2sRLC/AM disc. to=0.3sRLC/AM disc. to=0.4sRLC/AM disc. to=0.5sRLC/AM disc. to=0.6sRLC/AM disc. to=0.7s

Discard after x transmissions Discard after x time

Discard after x transm.: Higher throughput◦ Highest throughput for MaxDAT=2 for the Two State error model

Timer-based discard: Lower throughput◦ Almost the same as the raw link◦ Timeout values 300-700 ms have almost the same throughput


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60

65

0 1 2 3 4 5 6 7 8 9 10

Th

rou

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pu

t (K

bp

s)

Frame loss rate (%)



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Th

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bp

s)

Frame loss rate (%)




Discard after x transm.: Higher throughput Timer-based discard: Lower throughput

◦ Higher than raw link only in high frame loss rates◦ Timeout values 300-700 ms have almost the same throughput

Similar results for the Two State model


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Frame loss rate (%)



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hro

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(Kb

ps





RLC/AM throughput is dependent on the application RLC/AM offers better throughput than the raw link in all

cases Relative throughput is generally not affected by the

topology (LAN/WAN) Contention from the CBR stream has minimal impact on

relative RLC/AM and Adaptive SR throughput RLC/AM causes the lowest delay to the CBR stream Adaptive SR generally has the highest throughput in TCP

applications and causes low delay on the CBR stream Fixed SR has higher throughput in some apps/models and

lower throughput in others compared to the RLC/AM; it causes the highest delay on the CBR stream

In RLC/AM, “SDU Discard after x retransmissions” policy offers better performance than the timer-based one


Makidis [email protected]

mailto:[email protected]

Implementing & evaluating the RLC/AM protocolpages.cs.aueb.gr/~makidis/rlc-am.pdf · The RLC...

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