ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO

ENHANCE TCP OVER 3G LINKS

Juan J. Alcaraz, Fernando Cerdán

Department of Information Technologies and Communications, Polytechnic University of Cartagena, Plaza del Hospital 1,

30202 Cartagena, Spain

Keywords: TCP over 3G, Radio Link Control (RLC).

Abstract: When TCP is carried over 3G links, overbuffering and buffer overflow at the RLC layer degrades its

performance. In order to prevent these undesired cross-layer interactions, we propose an algorithm to

control the rate of the acknowledgements (ACK) traversing the RLC entity at the network side. Since the

sending rate of a TCP source is determined by the arrival rate of the ACKs, the basic idea of this scheme is

to adjust the inder-departure time of the ACKs in the uplink direction, according to the congestion level of

the downlink buffer. By means of extensive simulation experiments, we disclose the influence of each

parameter on TCP performance. We provide several configurations that improve end-to-end goodput while

reducing the delay. The operation of the algorithm is deterministic; it does not require changes in 3G

specifications nor in TCP itself, and preserves TCP end-to-end semantics.

1 INTRODUCTION

Third generation cellular networks (3G) are

expected to be an important part of the Internet.

Many Internet applications like e-mail, web surfing

and file transfer, rely on TCP for the end-to-end

transport. In 3G radio access networks, the link layer

is managed by the Radio Link Control (RLC)

protocol (3GPP, 2005). For packet switched

services, RLC is usually configured to provide a

reliable service, recovering from propagation errors.

A reliable RLC layer reduces packet losses

perceived at TCP layer, avoiding the triggering of

unnecessary congestion control measures

(Inamura, 2002).

Although, in general, RLC improves TCP transfer

rate over a radio bearer (RB), several characteristics

of 3G links like high and variable latency and buffer

overflow of the RLC downlink buffers (Alcaraz,

2006), have undesired effects on TCP performance.

One of the strategies to overcome these effects is

to improve the RLC layer. In contrast to other

proposals, like split-connection proxies, this

approach does not require changes in the TCP itself

and does not break the end-to-end semantics of TCP.

Several works propose the use of Active Queue

Management (AQM) techniques in the RLC

downlink buffer. It was shown in (Alcaraz, 2006)

that these mechanisms can enhance TCP goodput

while reducing end-to-end latency. A slightly

different approach is the use of ACK Delay Control

at RLC. This mechanism was presented in (Wu,

1999) as an algorithm to improve the performance of

TCP over satellite links. The underlying idea is to

delay acknowledgements travelling through a node

where its forward connection is congested. Since the

ACK arrival rate at a TCP source determines the

sending rate of new packets, this is a simple

mechanism to reduce packet drops due to buffer

overflow. The congestion is detected when the

buffer occupancy exceeds a fixed threshold (minth).

When this happens a delay is applied to the ACK

packets in the reverse flow to slow down its

departure rate. This mechanism takes advantage of

the TCP self-clocking principle, as the reduction in

the ACK flow rate will reduce the sending rate of

the source. In the original proposal the delay applied

to ACKs is a constant value.

In this paper we propose an adaptation of this

algorithm for 3G links, where ACK rate is adapted

according to the downlink queue length. We

consider a general delay computing function, in

which the fixed delay and the linear relation are

particular cases. This generalization gives us a wider

point of view to find optimum configurations. We

J. Alcaraz J. and Cerd

an F. (2006).

ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO ENHANCE TCP OVER 3G LINKS.

In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 34-42

 SciTePress

provide a feasible implementation of the algorithm,

and by means of extensive simulation tests, we

discuss the influence of each parameter and propose

configurations that clearly improve end-to-end

performance compared to the conventional RLC

operation.

The rest of the paper is organized as follows.

Section 2 describes the characteristics of the 3G

radio bearer that degrade TCP performance.

Section 3 explains our ACK rate control algorithm

in detail. Section 4 provides a brief description of

the simulation environment. Section 5 discusses the

influence of each parameter in end-to-end

performance based on extensive simulation results.

The paper concludes in section 6.

2 MOTIVATION

Previous experimental (Chakaravorty, 2005) and

simulation (Rossi, 2003) results provide a clear view

of the characteristics of 3G wireless links. The

behaviour of the link buffer occupancy has shown a

great impact on TCP performance.

3G links employs per-user buffering. Flows going

to a single mobile terminal share a single buffer.

Therefore, radio bearers are expected to multiplex a

number of simultaneous connections ranging form 1

to 4 TCP flows (Gurtov, 2004). At a reliable RLC

layer, the upper layer packets will be stored in the

downlink buffer until they are fully acknowledged

by the receiver side. The consequence is that, as

described in (Bestak, 2002) frame losses in the

downlink channel result in higher RLC buffer

occupancy at the network side. Considering that the

current RLC specification propose a drop-tail

scheme, the buffer may overflow causing

consecutive packet losses. This situation is

especially harmful in the first stages of a TCP

connection (slow start) and has a higher impact in

TCP Reno, which can only recover from consecutive

losses with a Retransmission TimeOut (RTO). An

RTO reduces TCP transmission window to one,

causing the highest reduction of the source rate.

The buffer should be large enough to avoid

frequent overflow. However, excessive queuing

causes some additional problems (Chakaravorty,

2005) like Round Trip Time (RTT) inflation,

unfairness between competing flows and viscous

web surfing.

Fig. 1 illustrates the end-to-end goodput and

delay of TCP over an RB for different RLC buffer

sizes and a number of flows ranging from 1 to 4.

The goodput accounts for the successfully delivered

packets at the receiver and the delay is the transfer

time of a packet in the downlink direction, at the

TCP layer. The buffer size is given in RLC Service

Data Units (SDU) of 1500 bytes. Table 1 shows the

parameter configuration for the RLC and TCP

protocols. The RLC parameters were set according

to the optimizing considerations described in

(Alcaraz, 2006) and (Rossi, 2003). Further details on

the simulator are provided in section 4. As expected,

Fig. 1 reveals that a larger buffer benefits the

goodput performance but the overbuffering increases

the latency.

Fig. 2 shows the trace of a TCP connection over a

384 kbit/s radio bearer multiplexing one TCP flow.

The curve at the top shows the RLC buffer

occupancy (BO). The buffer size is limited to 50

SDUs. The TCP congestion window (cwnd) is

depicted under this curve, and the curve at the

bottom shows the sequence number of the packets

when they are sent, received and dropped.

100

150

200

250

300

350

30 40 50 60 70

0.5

1.5

2.5

Goodput (Kbit/s)

End-to-end delay (s)

Buffer Size (SDUs)

goodput 1 flow

goodput 2 flows

goodput 3 flows

goodput 4 flows

delay 1 flow

delay 2 flows

delay 3 flows

delay 4 flows

Figure 1: TCP performance over a 384 kbit/s RB.

Table 1: Simulation Parameters

3G link parameters Setting

PDU payload size 320 bits

TTI (Transm. Time Interval) 10 ms

Transmission window 1024 PDUs

maxDAT 10

In-order-delivery true

Status Prohibit Timer 60 ms

Missing PDU detection true

Poll Timer 60 ms

Wireless Round Trip Delay 50 ms

Normalized doppler frequency 0,01

Poll window 50 %

Last PDU in buffer Poll yes

Last retransmitted PDU Poll yes

Frame Error Ratio (FER) 10%

TCP parameters Setting

Maximum TCP/IP packet size 1500 bytes

Maximum allowed window 64 kbytes

Initial window 1

Wired Network Round Trip Delay 200 ms

ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO ENHANCE TCP OVER 3G LINKS

At the first stages of the connection, multiple

packets are dropped due to buffer overflow, causing

an RTO. The buffer is drained because the

connection reduces its rate, with the consequent

underutilization of the resources. After that, the rate

is recovered slowly and the overbuffering appears

again, causing high delay and additional

packet losses.

3 PROPOSED ALGORITHM

TCP is basically a sliding window algorithm. The

transmission window limits the total amount of data

that the source can transmit without receiving any

acknowledgement from the receiver side. The

window moves forward and its length increases with

the arrival of ACKs, thus the source can inject new

segments in the network. This is usually called TCP

self-clocking mechanism.

In this paper we propose a scheme for controlling

the rate at which TCP ACKs cross the RLC layer.

Since the throughput of a TCP source is basically

determined by the arrival rate of ACKs, it is possible

to avoid overbuffering and buffer overflow in the

RLC layer if this rate is conveniently adapted to the

link situation. The buffer occupancy (BO) is the

variable used to perceive the link quality, and the

rate control measures are applied only when certain

BO threshold is reached (minth). In our algorithm,

the inter-departure time, t

, of ACKs traversing the

RLC layer is computed with a non-linear function of

BO, t

= f(BO), which is obviously a continuous and

only-increasing function because the rate has to be

reduced when the BO increases.

The motivation of the variable delay strategy is to

gradually adapt the source’s perception of the

100

0 10 20 30 40 50 60 70 80 90 100

Sequence Number

Time (s)

data sent data rx drop

cwnd

Figure 2: TCP trace over a 384 kbit/s RB.

for each event

if (event == ack_arrival)

insert ack in uplink buffer

if (event == id_timer_expiration)

send first ack in buffer

LADT Åcurrent_time

if (uplink buffer nonempty)

and(id_timer off)

delay Å 0

if (BO>=

minth)

= f(BO)

actual_t_id Åcurrent_time – LADT

if (actual_t_id <

)

delay Å

− actual_t_id

if (delay == 0)

send ack

LADT Åcurrent_time

else

id_timer Å delay

activate id_timer

Events:

ack_arrival: arrival of an upcoming ACK

id_timer_expiration: expiration of the

id_timer

Saved Variables:

LADT: Last Ack Departure Time

Other:

current_time: system clock

delay: artificial delay applied to

outgoing ACKs

Figure 4: Proposed ACK rate control algorithm.

ACK’s

arrival

time

ACK’s

departure

time

1id

2id

3id

)(

nidn

BOft =

nthmi

Figure 3: TCP performance over a 384 kbit/s RB.

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available bandwidth and the round trip delay. Higher

buffer occupancies require higher reductions in the

sending rate of the source, while for a short error

burst an excessive delay on the ACKs could be

unnecessary. This should also help to avoid

excessive delay spikes causing spurious TCP

timeouts.

It may be argued that a drawback of the ACK

delaying strategy is the Retransmission Time Out

(RTO) inflation caused by the increment on the

perceived RTT. The RTO inflation slows down the

reaction of the source upon the eventual loss of

consecutive packets. However, the higher delay of

the reverse path is compensated by a lower buffer

occupancy, which reduces the downlink packet

latency. In addition, a main goal of this algorithm is

the avoidance of buffer overflow, thus TCP timeouts

are expected to happen rarely.

The proposed ACK rate control algorithm is

detailed in Fig. 3. The algorithm relies in the use of a

single timer (id_timer), which delays ACKs assuring

that they are spaced by t

. The pseudocode identifies

two different events, the arrival of an ACK packet

(ack_arrival) and the expiration of the id_timer

(id_timer_expiration). Figure 4 illustrates the

operation of the algorithm.

3.1 Calculating the Inter-departure

Time

The original proposal for controlling the rate of the

reverse ACK flow relies on a fixed delay (Wu,

1999). This strategy may be enhanced using a

variable delay, e.g. setting a linear relation between

the delay and the downlink queue length.

In this paper we consider a more general function

(1) to estimate t

with BO, where maxd and mind are

the maximum and minimum values for t

, and maxth

is the buffer size.

ndmi

nthmixthma

nthmiBO

ndmixdmat

⎟

⎠

⎞

⎜

⎝

⎛

−

−=

)(

This function let us evaluate the effect of the

variation rate of t

respect BO. This variation rate,

which is related to the “aggressiveness” of the

algorithm, is determined by

(≥0). Lower values of

are tied to more aggressive delaying policies.

Figure 5 shows the shape of (1) for different values

. From now on, to reduce complexity, we

consider mind = 0 s. The shape of f(BO) with

mind > 0, can be approximated using f(BO) with

mind = 0 and a lower

. In consequence, in out tests,

the value of mind showed very little influence on

performance.

In (1) it is easy to see that, if

Æ 0, f(BO) tends

to the step function, i.e. t

is constant when

BO ≥ minth. On the other hand, if

= 1, f(BO) is a

linear function. Therefore, the step and the linear

functions are particular cases of (1). Moreover, the

absence of delay is also a particular case of this

function, when

Æ ∞. These cases are discussed

in Section 5.

3.2 Implementation Issues

An important advantage of the ACK delay control

algorithm compared to random or RED-like

mechanisms is that it does not need to generate

random numbers to compute the discarding

probability because of its deterministic operation.

This reduces the computational cost of the

algorithm, and makes it more feasible for its

implementation at the RLC level where the buffering

is done in a per-user basis.

The maintenance of a new timer, id_timer, does

not add too much complexity to the RLC operation,

which already handles several timers, e.g. Poll

Timer and Status Prohibit Timer (3GPP, 2005). The

RLC can synchronize id_timer to the Transmission

Time Interval (TTI), which is equivalent to a clock

signal with a granularity of 10 ms (similar to that of

other RLC timers).

The protocol could be further simplified in terms

of implementation with the use of pre-calculated t

values. Therefore, the buffer could be considered as

divided in gaps, each one having a corresponding t

value. In our implementation, the number of t

values equals the number of SDUs fitting in the

buffer space above minth.

Our implementation does not deal with ACK

identification, because the services considered to

benefit from our algorithm are assumed to be

strongly asymmetric, e.g. file downloading and web

browsing. Therefore, in our simulations, every

upcoming packet is an ACK. However, a simple

way to implement IP packets monitoring and ACK

filtering is described in (Wu 1999). It is a subject of

further research if, given the traffic profile of a 3G

user, an ACK filtering mechanism is justified.

4 SIMULATION ENVIRONMENT

The simulation environment for this research has

been developed in OMNeT++ (Varga, 2001) and

comprises a complete implementation of TCP and

RLC protocols. Similar simulators were described in

(Rossi, 2003) and (Bestak, 2002). The simulation

topology, shown in Fig. 6 consists of one or several

TCP sources connected to their respective receivers

in the user’s equipment (UE). The end-to-end

connection consists of two sections, the wired

ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO ENHANCE TCP OVER 3G LINKS

network and the radio bearer. The wired network

comprises the Internet and the 3G core network. The

radio bearer has a round trip time (RTTw) of 50 ms

(Holma, 2004) and a nominal transmission rate

equal to 384 kbit/s in both directions, representing

the bottleneck link, which is the situation expected

in most cases (Gurtov, 2004). The wired network is

modeled with a 1 Mb/s link with a round trip delay

(RTTf) of 200 ms.

The wireless channel generates error bursts

according to the model described in (Chockalingam,

1999) where the Doppler frequency, f

, of the UE

determines the average burst length. Lower f

causes

longer bursts of errors. It is usual to employ the

normalized Doppler frequency, equal to the product

of f

and the radio frame duration (10 ms).

In order to obtain more realistic results, the error

probability is the same in the uplink and in the

downlink direction. The frame loss ratio is 10%, a

typical UMTS design value (Meyer, 2003).

The simulation results exposed in this paper are

obtained averaging 20 runs per sample. Each run is a

60 second download session. The confidence

intervals are obtained with a confidence degree of

90% according to a t-student distribution.

The TCP flavour employed is TCP Reno, one of

the most extended in the Internet (Gurtov, 2004).

RLC and TCP parameter setting is shown in Table 1.

5 PERFORMANCE EVALUATION

In order to disclose the effect of each parameter in

end-to-end performance, multiple parameter

combinations were tested: The values of

ranged

from 0 to 40; for minth, the following values were

evaluated: 0, 10, 20, 30 and 40 SDUs; maxd ranged

from 50 ms to 2000 ms. The value of maxth equals

the size of the RLC buffer, 50 SDUs, enough to

prevent buffer overflow in most configurations

achieving low packet latency. Fig. 7 shows the

average goodput and delay of some selected results

(

= 0.5,

= 1,

= 5), considering a single TCP

flow over the RLC layer.

Fig. 8 shows the same measures in a scenario

with four TCP flows. In these figures, the goodput

and delay performance for the conventional RLC

drop-tail (DT) operation is also shown as a reference

value.

It should be stated that our objective is to

improve the goodput while reducing the delay,

which, as can be seen on first inspection of figures 7

and 8, is achieved for the single-flow scenario in

many configurations. In the multiple-flow scenario,

the goodput improvement is negligible but the delay

reduction is up to 50%.

The aggressiveness of the algorithm is

determined by the combined values of minth, maxd

and

. Given a certain BO value, the algorithm is

more aggressive if the ACK rate reduction is higher

(higher t

). For an optimum performance, the

relation between BO and t

should balance two

opposed objectives: avoiding overbuffering and

achieving full use of the radio bearer bandwidth.

Figures 7 and 8 show that the aggressiveness

increases for lower values of minth and

, and for

higher values of maxth. As expected, if the

configuration is too aggressive, the goodput decays.

Wireless

Channel

RLC

Frame Error Rate: 10%

RTT: 50 ms, Nominal Rate: 384 kbit/s

Networ

Side

User

Side

RLC

P sources

TCP receivers

#1 #3

RTT Internet: 200 ms

minth

Downlink

buffer

Uplink

buffer

(reverse

ACK flow)

rtificial

delay

Figure 6: Simulator topology.

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In most TCP implementations, one ACK packet

is sent when two TCP segments are received in

sequence. In our environment it implies that, in

normal operation, two consecutive ACKs are spaced

approximately 140 ms. Therefore, if t

is around this

value, the effect of the algorithm is imperceptible,

and if t

reaches e.g. 240 ms, the ACK rate (and thus

the source’s rate) is halved. Our simulations show

that optimum configurations follow roughly a

similar pattern: t

is lower than 240 ms for BO

values below 10 SDUs, and for BO values around 20

SDUs, the rate is divided by 3.

Table 2 shows the performance figures of four

possible optimum configurations, contrasted with

that of RLC without ACK rate control algorithm.

Fig. 9 shows the shape of the function t

= f(BO) for

these configurations. It can be seen that the rate

reduction is similar for certain BO levels, as stated

before.

It is clear that optimum configurations require

< 1. The effect of

is shown in Fig. 10 (single

flow) where minth = 10. The performance is worse

for

> 1 because the rate reduction is concentrated

on higher BO values (see Fig. 5). The algorithm

tends to react only when the congestion is too high,

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

Figure 7 (a): α = 0.5

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

Figure 7 (b): α = 1

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

minth: 40

minth: 30

minth: 20

minth: 10

minth: 0

delay DT

delay minth: 40

delay minth: 30

delay minth: 20

delay minth: 10

delay minth: 0

Figure 7 (c). α = 5

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

Figure 8 (a): α = 0.5

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

Figure 8 (b): α = 1

100

150

200

250

300

350

200 400 600 800 1000 1200 1400 1600 1800 2000

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd

minth: 40

minth: 30

minth: 20

minth: 10

minth: 0

delay DT

delay minth: 40

delay minth: 30

delay minth: 20

delay minth: 10

delay minth: 0

Figure 8 (c). α = 5

Figure 7: Performance of the single-flow scenario. Figure 8: Performance of the 4-flows scenario

ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO ENHANCE TCP OVER 3G LINKS

which is less effective preventing buffer overflow.

In fact, for

>> 1, the algorithm effect vanishes

= 0).

Finally, Fig. 11 shows traces of three of the

selected configurations (

= 0,

= 0.2 and

= 1).

The traces provide further insight on the algorithm

performance. In general, buffer overflow is avoided,

and the overall behaviour is better than that shown in

Fig. 2. For

= 0.2, we can see a single packet loss,

because minth is higher in this scenario. The

= 0

and

= 1 cases show a noticeable oscillation in the

buffer occupancy process. For

= 0 it is caused by

the abrupt increment on t

. For

= 1, the increment

100

0 10

20 30 40 50 60 70 80

Sequence Number

Time (s)

data sent data rx drop

cwnd

(a):

= 0, maxd = 0.5 s, minth = 20

100

0 10 20 30 40 50 60 70 80

Sequence Number

Time (s)

data sent data rx

cwnd

(b):

= 0.2, maxd = 0.5 s, minth = 10

100

0 10 20 30 40 50 60 70 80

Sequence Number

Time (s)

data sent data rx

cwnd

(c):

= 1, maxd = 1 s, minth = 0

Figure 11: TCP traces with ACK rate control.

Table 2: Performance Figures.

TCP

flows

α minth

(SDUs)

maxd

(ms)

Goodput

(kbit/s)

Delay

(s)

1 Conventional RLC 281.9 ± 8.9 0.85±0.05

1 0 20 500 310.4±5.9 0.84±0.02

1 0.2 10 500 308.2 ± 6.2 0.78±0.03

1 0.5 10 700 305.9±5.6 0.78±0.03

1 1 0 1000 311.2±2.5 0.76±0.01

4 Conventional RLC 299.9 ± 3.9 1.09±0.02

4 0 20 500 302.7±7.9 0.72±0.02

4 0.2 10 500 301.4. ± 7.9 0.59±0.03

4 0.5 10 700 309±5.6 0.65±0.02

4 1 0 1000 306.3±6.9 0.56±0.03

Figure 9: f(BO) of the selected configurations.

100

150

200

250

300

350

0.001 0.01 0.1 1 10

0.25

0.5

0.75

1.25

1.5

1.75

Goodput (Kbit/s)

End-to-end delay (s)

maxd: 500

maxd: 700

maxd: 1000

maxd: 1200

delay DT

delay maxd: 500

delay maxd: 700

delay maxd: 1000

delay maxd: 1200

Figure 10: Effect of α. Single-flow scenario.

0,1

0,2

0,3

0,4

0,5

0,6

0 2 4 6 8 1012141618202224262830

(SDUs)

(4)

(3)

(2)

(1)

nominal rate

nominal rate / 2

nominal rate / 3

(1)

= 0, maxd = 0.5 s, minth = 20

(2)

= 0.2, maxd = 0.5 s, minth = 10

(3)

= 0.5, maxd = 0.7 s, minth = 10

(4)

= 1, maxd = 1 s, minth = 0

)(st

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is gradual but relatively slow given the transmission

delay of the control loop. The source reacts “a bit

late” and “a bit too stark” causing an apparent

instability. The stability of the algorithm from a

control system point of view is an issue for further

research. For 0 <

< 1, the algorithm shows lower

BO oscillations, which justifies the generalized

delay computation approach proposed in this paper.

6 CONCLUSIONS

Our proposed ACK rate control algorithm improves

end-to-end goodput and delay performance when

RLC supports a single or multiple TCP flows. The

algorithm performs equally well in the slow start and

in the congestion avoidance phase of TCP

connections. In contrast to similar proposals done in

the field of satellite links, we consider a more

general way of calculating the inter-departure time

of ACKs. Moreover, we provide a detailed algorithm

that clarifies the operation and the implementation.

By means of extensive simulation tests we

provide insight about the influence of each

parameter, which let us find multiple possible

configurations improving overall performance. The

key issue in selecting an accurate parameter setting

is to tune the rate reduction of the algorithm

according to the congestion level of the link,

reflected in the buffer occupancy. Configurations

showing similar performance figures (goodput and

delay) presented a similar ACK rate reduction

pattern for low and high BO levels. However, it was

found that the value of

has a noticeable impact on

the oscillatory behavior of the BO process.

An important advantage of our algorithm

compared to random or RED-like mechanisms is its

deterministic operation, which reduces the

computational cost of the algorithm, and makes it

more feasible for its implementation at the RLC

level where the buffering is done in a per-user basis.

Some additional advantages of our algorithm are

that it does not require changes in 3G specifications

nor in TCP itself, and preserves TCP end-to-end

semantics.

REFERENCES

3GPP, 2005. “Radio Link Control (RLC) protocol

specification”, v. 6.4.0., TS 25.322.

H. Inamura et al., 2003. “TCP over Second (2.5G) and

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3481.

J. J. Alcaraz, F. Cerdan and J. García-Haro, 2006.

“Optimizing TCP and RLC Interaction in the UMTS

Radio Access Network”, IEEE Network, vol 20, no.

2, pp. 56 - 64.

Jing Wu et al., 1999. “ACK Delay Control for Improving

TCP Throughput over Satellite Links”, Proc. IEEE

ICON’99, pp. 303- 312.

M. Rossi, L. Scaranari and M. Zorzi, 2003, “On the

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ACK RATE CONTROL STRATEGY AT THE LINK LAYER TO ENHANCE TCP OVER 3G LINKS