On the use of interwoven order of oncoming packets

for reliable underwater acoustic data transfer

Oleksiy Kebkal

EvoLogics GmbH

Berlin, Germany

Email: lesha@evologics.de

Abstract—Physical properties of hydroacoustic communicationchannels differ essentially from those of conventional terrestrialradio channels, videlicet the channel is characterized by thelong propagation delays, limited bandwidth, extremely complexand quickly varying reverberation, and half duplex natureof underwater acoustic modems. This motivates a substantialredesign of the algorithms and techniques of the underwaterdata transfer concerning both physical and data link layers,especially concerning the development of underwater acousticsensor networks. In this paper, the task of point-to-point datatransfer is addressed. Since the task seems to be simple, itcaptures the most important characteristics of the hydroacousticchannel and is the most practical case for the use of underwateracoustic modems nowadays. Conventional protocols of reliabledata transfer suffer from the long propagation delays of acousticsignals. The protocols spend the major amount of time waiting forpackets with acknowledgments. This results in a poor efficiencyof the channel utilization. The use of more complex solutions,namely the packet train transmission or rate-less coding, allowsone to improve the efficiency of the data transfer and to reducethe energy consumption significantly. But, in the case of packettrain transmission, the optimal number of packets in a trainshould have quite a high value. Moreover, in the case of thetransmission of a packet sequence coded with rate-less code, thefeedback channel is either excluded or used to send a “stop” mes-sage from the receiver side after the transmission of a big numberof packets. This results in a partial or complete elimination ofthe feedback channel, blocking the remote side from oncomingdata transfer. The elimination of the feedback channel excludesa possibility to send urgent data from the receiver side, as wellas a possibility for the combination of an underwater acousticmodem with some other devices using the same bandwidth,for instance, position tracking devices. Moreover, this excludesa possibility of the parameter adaptation on a physical layerduring the data transmission, which is necessary for the efficientdata transmission for given channel parameters. The describedrestriction can be lifted with the use of the interwoven orderof oncoming packets. The idea is to measure the propagationdelay between communicating devices during handshaking andthen to send the data packets continuously one after another,by interrupting only to accept the control packet from theopposite side at the estimated time of arrival. The propagationdelay change tracking during the data transfer is also possibleand highly important for the communication between movingstations. This makes the data transfer protocol fully propagation-delay-tolerant and returns the feedback channel, which can beused to deliver acknowledgments and other urgent control datafrom the receiver side. In the present paper, the efficiency ofconventional methods and advanced methods of data transferusing the interwoven order of packets are compared. In thisanalysis, the task of data transfer is divided into two subtasks.The first one is to send a file, namely the a priori specified amount

of data, and the second is to send a byte stream, where the amountof data to transfer is not known by the modem initially.

I. INTRODUCTION

Underwater technologies are rapidly improved during the

last decades and induce, in turn, a further advance in underwa-

ter communication. Challenges of hydroacoustic communica-

tion and, particularly, those of underwater acoustic sensor net-

working technologies attract a growing number of researchers.

Nevertheless, the underwater acoustic networks are still in

its infancy, and its usability in practice remains considerably

restricted. On the other hand, for the major number of practical

applications, including the data exchange with autonomous

underwater vehicles and the uploading of sensor data from

ocean bottom observatories, the primarily reliable efficient

point-to-point communication is needed. Since the task seems

to be simple, it captures the most important characteristics

of the hydroacoustic channel and is the most practical case

for the use of underwater acoustic modems nowadays. The

conventional protocols of reliable data transfer suffer from

the long propagation delays of acoustic signals. The protocols

spend the major amount of time waiting for packets with

acknowledgments. This results in a poor efficiency of the

channel utilization.

In [1], the ability to compensate the long propagation delays

by means of the packet train transmission with the subsequent

acknowledging delivery of packets in a train over a feedback

channel has been studied. Work [3] introduced the idea of

the interwoven order of oncoming packets. This technique

makes the data transfer fully propagation-delay-tolerant. In

[2], several protocols of reliable point-to-point data transfer are

reviewed, wherein protocols based on the use of rate-less codes

are favored. However, the review doesn’t include the technique

of interwoven order of packets and contains some inaccuracies.

This paper contains a more comprehensive review of the

protocols of reliable data exchange for hydroacoustic channels,

analysis of their efficiency, and applicability to practical tasks.

Protocol efficiency is usually estimated in relation to the

time required to deliver some fixed data volume. The less the

delivery time, the more effective is the protocol. This seem-

ingly simple definition hides serious methodological issues

related to the efficiency intercomparison for different underwa-

ter communication techniques. On the one hand, the optimal

bitrate depends on a structure of the intersymbol interference,

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Doppler shift, signal-to-noise ratio, and other characteristics

of a hydroacoustic channel and may rapidly fluctuate even in

the channels with stationary fixed nodes. Thus, the physical

layer implementation, including its modulation/demodulation

and synchronization techniques, has decisive impact on the

time required to deliver data. On the other hand, the reliability

of data exchange and the channel utilization efficiency with

given bitrate and bit error rate are determined by the data link

layer implementation, what is the subject of investigation in

this paper.

Let’s define the channel utilization efficiency, similarly to

[2], [3], as the ratio of the data volume per given bitrate

to the average duration of a data delivery. The probability

of a synchronization error is assumed to be negligible. This

parameter determines the influence of a propagation delay on

the data link layer protocol. In practice, the channel utilization

efficiency is not so important for the better part of real

applications as bit delivery costs that vary depending on the

energy consumption in the transmission/receiving and idle

listening modes, bitrate, protocol overhead, and the channel

utilization efficiency. The bit delivery costs can be defined, as

ǫ =TT NTX + (TS − TT )NRX

F, (1)

where NTX , NRX - consumed power in transmission and

receiving modes, respectively, and TT - period of time spent

for the signal transmission needed to deliver F bits of data

with a data exchange session duration TS .

The rest of the paper is organized as follows. Section II

describes a packet train acknowledgment protocol, the most

common means of channel utilization efficiency maximization.

Section III describes a protocol based on rate-less codes, where

a feedback channel is not required. Section IV analyzes the ef-

ficiency increase of the previously considered protocols using

the interwoven-order technique. Finally, Section V concludes

the paper with consideration of practical issues concerning the

choice of a protocol for real applications.

II. PACKET TRAIN ACKNOWLEDGMENT PROTOCOL

The simplest protocol of reliable data transfer is an indi-

vidual packet acknowledgment protocol, where the data are

split into packets and transmitted singly. An acknowledgement

is expected after every sequent packet transmission. If the

acknowledgment doesn’t arrive during the estimated round trip

time, the packet considered to be lost, and its transmission is

repeated. The receiving side transmits acknowledgments on

every received packet and drops all duplicated and erroneous

packets. This is the most ineffective data transmission schema,

because the long propagation delay of acoustic signals implies

long periods of idle listening before the acknowledge arrival

and results, respectively, in a poor efficiency of the channel

utilization. The efficiency of this protocol can be considerably

improved by means of the packet train transmission, rather

than by sending a single packet. Receiver, using the identifi-

cator from a packet header, schedules the acknowledgment

transmission time after the successful reception of at least

one packet from the train. As the number of packets in the

train increases, the channel utilization efficiency increases

arbitrarily close to its limit [1]. The packet train acknowl-

edgment protocol has been taken as the starting point of

the investigation, since the individual packet acknowledgment

protocol is its special case.

Let H be the length of a packet header, L be the length of

a data payload in a packet, and M be the number of packets

in a train. As in [2], let’s assume that a packet consists of a

header and a data payload, and an acknowledgment consists of

a header and a bit array of length M , where each bit denotes

the result of a particular data packet reception. In the header,

every packet includes its number in the train. Let R be the

bitrate, D be the distance between communicating nodes, cbe the average sound speed in the channel, and F be the size

of a data volume to be delivered. The data volume is split into

the packets such that

N = ⌈F/L⌉ ≈ F/L, where F ≫ L. (2)

Let p be the probability of the bit error in a packet. Then

the probability of the correct packet reception for a packet of

length H + L is

PD = (1 − p)H+L, (3)

and that for an acknowledgment packet is

PTA = (1 − p)H+M . (4)

The probability of the event that at least one packet from the

train and the acknowledgment packet are successfully received

is

PT = (1 − (1 − PD)M )PTA. (5)

Assuming a packet processing time is negligible, the single

cycle duration of a packet train and the acknowledgment

exchange is

T1 = MH + L

R+

H + M

R+

2D

c. (6)

The acknowledgment reception failure means that either a

receiver was unable to receive at least one packet from the

train or a transmitter failed to receive the acknowledgment

packet. Relation 5 implies that, when the train length in-

creases, the probability of the failure on the transmitter side

increases, and that on the receiver side decreases. We assume

in the both cases that the train transmission is repeated. The

receiver aggregates the repeated and originally transmitted

trains and sends a joint acknowledgment of both packet trains.

For the transmission of the same packet train k times, the

average number of successfully received packets is mk and

the acknowledgment reception probability exactly after the k-th transmission of the packet train are

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mk=M(1 − (1 − PD)k), (7)

pk=PT (1 − PT )k−1. (8)

Then the mean number of successfully received packets

and the average train transmission time for the above-defined

transmission schema are

MT =

∞∑

k=1

mkpk, (9)

TT =∞∑

k=1

kT1pk. (10)

Disregarding the fact that the last train may be incomplete,

we obtain the average data delivery session duration

TS ≈ TT

N

MT

(11)

and the channel utilization efficiency

ηT =F/R

TS

≈LMT

RTT

. (12)

The bit delivery cost for this protocol is

ǫT =TTXNTX + (TS − TTX)NRX

F, (13)

where

TTX =

∞∑

k=1

kNH + L

Rpk (14)

is the time spent for the signal transmission.

With M fixed, the efficiency and the bit delivery cost are

dominated by the header size and the propagation delay, when

L is small. Increasing L helps to improve the efficiency and the

bit delivery cost till the packet reception probability remains

high. At large values of L, the packet reception probability

is a decisive factor for the efficiency to decrease and for the

bit delivery cost to increase. The optimal packet length value

corresponds to some intermediate value, where the efficiency

is maximal and the bit delivery cost is minimal. However, the

optimal packet lengths for the efficiency and the cost don’t

coincide.

Figure 1 shows the efficiency and the bit delivery costs

for various values of D, p, and M with H = 16bits, R =10 kbps, c = 1500 m/s. Higher efficiency values are achiev-

able with a longer train, when the propagation delay influence

can be considerably decreased. Thus, ηT value decreases only

by 5% after changing the distance from 1 km to 5 km with

p = 10−4,M = 1000, and by 18% with a shorter train,

M = 10, even with lower bit error rate p = 10−5. The

efficiency ηT reaches 93.5% and 95.6% with p = 10−5,M =1000, D = 5 km and D = 1 km, respectively. With p = 10−4,

the high efficiency values are also reachable at the expense

of the essential train lengthening and the packet shrinking.

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Bit d

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p = 10−4

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p = 10−5

, M = 10, D = 1 km

p = 10−4

, M = 103, D = 1 km

p = 10−5

, M = 103, D = 1 km

p = 10−4

, M = 10, D = 5 km

p = 10−5

, M = 10, D = 5 km

p = 10−4

, M = 103, D = 5 km

p = 10−5

, M = 103, D = 5 km

p = 10−3

, M = 10, D = 1 km

p = 10−3

, M = 103, D = 5 km

p = 10−4

, M = 10, D = 1 km

p = 10−5

, M = 10, D = 1 km

Fig. 1. Efficiency of packet train acknowledgement protocol

For bit delivery costs, the bit error rate is the dominating

factor, whereas the propagation delay and the train length are

of secondary importance.

The packet train acknowledgement protocol proposed in

[2] assumes implicitly that, in the case of acknowledgement

reception failure, all unacknowledged packets are dropped.

Let’s name this version as oblivious protocol. In this paper,

we assume that a receiver combines all retransmissions of a

packet train. Let’s name it as stingy protocol. It is possible

to prove that a stingy protocol doesn’t increase hardware

requirements and is preferable for implementations. In Fig. 2,

we plot the efficiencies for these two versions of the protocol

with p = 10−3, p = 10−4, p = 10−5. The stingy version

achieves a higher efficiency and lower bit delivery costs. The

efficiency difference increases with the bit error rate.

III. A PROTOCOL BASED ON RATE-LESS CODES

An alternative to the above-described protocol is a protocol

based on rate-less codes [4]–[6]. This is a new class of error

control codes for erasure channels. The sender may generate

a virtually infinite sequence of encoded packets based on the

data volume consisting of N packets. The data on the receiver

side may be reconstructed using an arbitrary subset of size N ′

of this sequence, where N ′ by a few percent greater than Nfor large N (depending on the particular rate-less code). Using

this code, the sender may stop the data transmission if he/she

has decided that the reception probability of the data volume

is high enough, or after receiving some kind of a “stop” packet

from the receiver side. In the second schema taking the half-

duplex nature of an underwater link into account, the feedback

channel has to be reserved in the time division mode.

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p = 10−3

, oblivious

p = 10−3

, stingy

p = 10−4

, oblivious

p = 10−4

, stingy

p = 10−5

, oblivious

p = 10−5

, stingy

Fig. 2. Efficiency comparison of the oblivious and stingy versions of thepacket train acknowledgement protocol, D = 5 km, M = 10

3

As will be shown below, the first schema requires higher

delivery costs for data transmission, which is rather undesir-

able for practical applications. The efficiency of the second

schema is somewhat higher in comparison to the packet train

acknowledgement protocol.

In what follow, we consider a protocol without feedback

channel reservation based on the online rate-less code [4].

Using this code, the original data can be decoded from any set

of N(1 + 3ε) packets with the asymptotic success probability

(ε/2)q+1, where ε and q are parameters of the code. The values

ε = 0.01 and q = 3 have been recommended in the literature

and correspond to the failure probability 10−9 for N > 103.

The receiver must have successfully received Nε = N(1 +3ε) packets in order to decode the transmitted data volume.

An analysis of this protocol is presented in [2] and based on

the assumption that the average number of packets needed to

be sent is

NR =Nε

PD

. (15)

But this assumption is correct only in a case where the feed-

back channel is given, which contradicts the initial assumption.

Otherwise, the probability of successfull data volume recep-

tion is 0.5. Using the de Moivre–Laplace integral theorem

to approximate the binomial distribution of the number of

successfully received packets, let’s find a value of NR such

that the data delivery failure probability be close to zero.

Pn(Nε ≤ m)≈1

2− Φ(x) (16)

x=Nε − NRPD

√

NRPD(1 − PD). (17)

With x = −5 [the argument of a normal cumulative

distribution function Φ(x)], the failure probability Pn(m <Nε) is under 10−7 after sending NR packets, where NR is

NR = ⌈Nε + 5

√

Nε(1 − PD)

PD

⌉. (18)

The average data transmission duration is

TR = NR

H + L

R+

D

c, (19)

and the channel utilization efficiency

ηR=F/R

TR

≈ (20)

≈

(

1 + 3ε + 5√

(1+3ε)(1−PD)N

)

(1 + HL

)

PD

+DR

Fc

−1

.

The asymptotic estimate of the efficiency for large N is

ηRlim =

(1 − p)H+L

(1 + HL

)(1 + 3ε), (21)

and, respectively, the delivery cost ǫR and its asymptotic

estimation ǫRlim are

ǫR=NR

H+LR

NTX + DcNRX

F, (22)

ǫRlim=

1 + 3ε

PD

H + L

RNTX . (23)

Aa an important result, we indicate that the asymptotic

estimations of the efficiency and the cost are independent of

the propagation delay, which makes the protocol attractive for

practical applications. However, it is worth noting the fact that

these estimations are suitable for the size of data volume of

107 bits and more. For the data sizes of 10-100 kb typical

of the practice, the estimations are unacceptable. In particular,

the errors of the efficiency estimation are 3% and 26% for the

data sizes of 106 and 105 bits, respectively, (see Fig. 3) and

increase with decrease in the data size.

In Fig. 3, we present also the comparison of bit delivery

costs for this protocol and the packet train acknowledgement

protocol with bit error rates p = 10−4 and p = 10−5 and

the transmission range D = 5 km. The delivery costs with

p = 10−5 are higher for the protocol based on rate-less codes

by virtue of the requirement to send the excessive number

of packets to ensure a low failure probability (equation 18).

With p = 10−4, the delivery costs are higher for the packet

train acknowledgement protocol, which is determined by a

rather high probability of train retransmissions caused by the

acknowledgement reception failure.

IV. AN INTERWOVEN ORDER OF ONGOING PACKETS

As shown in the previous section, the packet train acknowl-

edgement protocol is inferior to the protocol based on rate-

less codes in efficiency due to the requirement to reserve a

feedback channel for the acknowledgement delivery, which

implies the idle listening during the round trip time and, hence,

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(εR

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p = 10−3

, F=100Kb

p = 10−3

, limit

p = 10−4

, F=100Kb

p = 10−4

, limit

p = 10−5

, F=100Kb

p = 10−5

, limit

p = 10−4

, F=100Kb

p = 10−4

, 1000−packet ack

p = 10−5

, F=100Kb

p = 10−5

, 1000−packet ack

Fig. 3. Efficiency of the rate-less code based protocol

decreasing the efficiency. On the other hand, the protocol

based on rate-less codes doesn’t cover the application range

of an underwater modem, as a general-purpose communication

device. In particular, this protocol becomes ineffective in the

case of small data sizes.

In order to increase the efficiency of the packet train

acknowledgement protocol, in [3] was proposed the idea of

interwoven order of ongoing packets. The essence of the idea

is to split the train into two portions, send the first portion of

the train, interrupt for the duration of the packet expecting

to be delivered over the feedback channel, and then send

the second portion without waiting for the feedback packet,

so that the transmission stops nearly before the expected

feedback packet arrival time moment. This technique provides

a means for the essential efficiency increase for the packet train

acknowledgement protocol, by weakening the dependence of

the efficiency on the propagation time. The technique of

interwoven order of packets proves to be useful to increase

the efficiency of the protocol based on rate-less codes, where

a feedback channel is used to send the control packet, by

turning-off the data transmission (hereinafter, referred to as

a stop-packet).

In the following subsections, we consider both the inter-

vowen packet train protocol based on a packet train acknowl-

edgement protocol and the protocol based on rate-less codes

with the use of the technique of interwoven order of ongoing

packets.

A. Interwoven Packet Train

Before the data transmission starts, a connection establish-

ing is necessary, for example, by means of the exchange

by RTS/CTS packets. In the case of point-to-point com-

munication, the RTS/CTS handshake is required by several

reasons [7]. The handshaking allows addressing, source level

management, channel parameter estimation for the adjustment

of the optimal modulation parameter, and, finally, propagation

delay estimation. Therefore, at the data transmission start, the

propagation time is known on a transmitter.

Let us split a packet train into two parts, D1 of length MD1

and D2 of length MD2, as follows:

MD2=

⌊

2DRc

− GRc

− (M + H)

L + H

⌋

, (24)

MD1=

{

M − MD2, M < MD2

0, M ≥ MD2.(25)

The transmitter sends MD1packets enclosing the trans-

mission with the acknowledgement request packet of length

H , holds a pause corresponding to the acknowledgement

length and then sends the second part of the train of the

length MD2. The guard time duration G prevents from the

desynchronization of the communicating nodes due to relative

movements. The quantity G depends on both the time elapsed

after the last propagation delay measurement and the maximal

relative velocity of the nodes. Analysis of the influence of this

parameter on the efficiency is beyond the scope of the present

paper. Let’s make a simplifying assumption that the relative

velocity is zero, and the guard time duration is negligible.

An acknowledgement packet contains the reception result

of the D2 part of the previous train and D1 part of the last

train. The transmitter repeats the acknowledgement request in

the case of the acknowledgement reception failure.

The probability of the event that both the acknowledgement

and its request are successfully received is

PIT = (1 − p)2H+M . (26)

The average number NIT of trains to be transmitted and

the average number of acknowledgement requests NACK are

NIT =⌈N

PDM⌉, (27)

NACK=⌈NIT

PIT

⌉. (28)

The average data transmission duration for this protocol is

TIT = NACK

(

2H + M

R+

2D

c

)

+ NIT MD1

H + L

R. (29)

The efficiency can be written as

ηIT =F/R

TIT

(30)

≈LPIT PD

1 + 2M

(

H + DRc

)

+ PITMD1

M(H + L)

.

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10−3

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p = 10−4

, M = 50

p = 10−5

, M = 50

p = 10−4

, M = 200

p = 10−5

, M = 200

p = 10−4

, rateless

p = 10−4

, M=100

p = 10−5

, rateless

p = 10−5

, M=100

Fig. 4. Efficiency of the packet train acknowledgement protocol withinterwoved order of packets

For M ≥ MD2, by assuming that the probability 1 − PIT

is low (0.1% with p = 10−4, M = 100), the efficiency can

be approximately evaluated as

ηITappr ≈

PD

1 + 1L

(

2 + 3HM

+ H) . (31)

Bit delivery costs for the protocol and its approximation are

TTXIT =NACK

H

R+ NIT M

H + L

R(32)

ǫIT =TTX

IT NTX + (TIT − TTXIT )NRX

F(33)

ǫITappr≈

(H + M(H + L))NTX + 2(H + M)NRX

PDRML. (34)

Thus, we have obtained an important result that the ef-

ficiency and costs approximations are independent of the

propagation delay, and the efficiency approximation doesn’t

depend on a bitrate. The approximations are suitable in the

cases where the probability PIT is high. This requirement can

be satisfied in practice at the expense of a bitrate decrease for

the acknowledgement and request packets.

A further important result is that it is not necessary to send

a very long packet train in order to achieve a high efficiency of

the protocol. The difference of maximal efficiency values for

M = 50 and M = 200 is under 1% (Fig. 4). The efficiency

is 97% for M = 50, p = 10−5 and the optimal packet length

L = 1300 bits. For p = 10−4, the efficiency decreases to the

value of about 90%.

Bit delivery costs for the optimal packet length turned out to

be 12− 13% lower for this protocol than that for the protocol

based on rate-less codes (described in the previous section)

for p = 10−4 and p = 10−5. The result is determined by two

factors: the idle listening time is essentially decreased, and the

number of packets required to be transmitted becomes less.

B. Protocol Based on Rate-less Codes with Interweaving

The interwoven order of ongoing packets can be exploited to

increase the efficiency of a protocol based on rate-less codes.

The transmitter chooses the time to send the request for a stop-

packet after sending the next number of packets. This request

may be sent either with some constant period M or with some

decreasing period starting with the period O(N). A relatively

frequent request with constant period has some advantages in

practice and doesn’t pull down the efficiency, as shown in the

previous subsection above. Thus, lets assume that the requests

have constant period of M packets.

The probability of the event that both a request and a stop-

packet are successfully received is

PIR = (1 − p)2H . (35)

In the case of the stop-packet reception failure, it is possible

to send next M packets instead of repeating the request. Then

the average number of packets NIR to be transmitted for the

successful data volume reception is

NIR =N(1 + 3ε)

PD

, (36)

and the average number of transmission periods is

NIRS =NIR

M+

1

PIR

. (37)

The average data transmission duration for this protocol is

TIR = NIRS

(

2H

R+

2D

c+ MD1

H + L

R

)

, (38)

where

MD2=

⌊

2DRc

− GRc

− H

L + H,

⌋

(39)

MD1=

{

M − MD2, M < MD2

0, M ≥ MD2.(40)

For N ≫ 1, the channel utilization efficiency is

ηIR ≈(1 − p)H+L

1+3εLM

(

MD1(H + L) + 2H + 2DRc

) , (41)

and an approximation of the channel utilization efficiency

can be written as

ηIRapprox ≈

(1 − p)H+L

(

1 + 3ε)(1 + 1L

(

H + 3HM

)) . (42)

Bit delivery costs for the protocol and its approximation are

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iency (

ηIR

)

102

103

10−3

Packet Length (L, bits)

Bit d

eliv

ery

costs

(εIR

, J/b

it)

p = 10−4

, iw ackN

p = 10−4

, iw rateless

p = 10−3

, iw ackN

p = 10−3

, iw rateless

p = 10−4

, iw ackN

p = 10−4

, iw rateless

p = 10−3

, iw ackN

p = 10−3

, iw rateless

Fig. 5. Efficiency of the rate-less based protocol with interwoven order ofpackets

TTXIR =NIRS

H

R+ NIR

H + L

R(43)

ǫIR=TTX

IR NTX + (TIR − TTXIR )NRX

F(44)

ǫITappr≈

(H + M(H + L))NTX + 2HNRX

PDRML(45)

The efficiency of the protocol in comparison with the inter-

woven packet train protocol for bit error rates p = 10−4−10−5

is slightly below due to the factor (1+3ε). In the case of a highbit error rate, this protocol outruns the others. For example,

for p = 10−3andM = 100, the maximal efficiency of the

protocol is 66% versus 54% for the packet train based protocol.

Therefore, in the case of big downloads, the protocol based

on rate-less codes with interweaving has the highest efficiency

under poor conditions (see Fig. 5). The same is true for bit

error costs.

V. CONCLUSIONS

In [2], it was shown that the protocol based on rate-

less codes doesn’t suffer from long propagation delays in a

hydroacoustic channel and achieves a high channel utilization

efficiency in the case of big downloads. However, this protocol

is ineffective for the transmission of small data volumes and in

applications, where it is required to deliver data in the online

mode, as well as in applications, where channel conditions

are highly variable and the estimation of optimal modulation

parameters is questionable. Hence, the practical application of

the protocol is restricted and fits poorly for general purpose

communication devices.

This paper proposes the technique with interwoven order

of ongoing packets as a means to essentially increase the

efficiency for both the packet train protocol and the protocol

based on rate-less codes. Another powerful feature of this

technique cobsists in that it liberalizes the requirement to send

very long packet trains in order to achieve a high efficiency

of the protocol. This feature is important for underwater

communication protocols, since it allows one to smoothly

respond on variations of the channel parameters.

Alternative to the above-described protocol is a protocol

based on rate-less codes [4]–[6]. This is a new class of error

control codes for erasure channels. The sender may generate

virtually an infinite sequence of encoded packets based on the

data volume consisting of N packets. The data on the receiver

side may be reconstructed using an arbitrary subset of size N ′

of this sequence, where N ′ by a few percent greater than N for

large N (depending on the particular rate-less code). Using this

code, the sender may stop the data transmission after he/she

has decided that the reception probability of the data volume

is high enough or after receiving some kind of a “stop” packet

from the receiver side. In the second schema, taking the half-

duplex nature of the underwater link into account, the feedback

channel has to be reservered in a time division manner.

Rate-less codes are a rather new result of the development

of information theory which will have significant impact on

data transmission protocols in the nearest future. In the present

paper, we have demonstrated the combination of this approach

with the interweaving technique, by giving a possibility to ef-

ficiently deliver data over challenging channels. Nevertheless,

the interwoven packet train protocol fits better to practical

requirements, in particular, this protocol is highly efficient and

energy-conserving for the transmission of both small and big

data volumes over hydroacoustic channels.

ACKNOWLEDGMENT

The author would like to thank Dr. Konstanin Kebkal for

valuable remarks during preparation of the paper.

REFERENCES

[1] M. Stojanovich, “Optimization of a Data Link Protocol for an UnderwaterAcoustic Channel’,” in IEEE OCEANS’05, Brest, France, 2005.

[2] M. Chitre, M. Motani, “On the use of rate-less codes in underwateracoustic files transfers,” in OCEANS’07 Europe, Aberdeen, UK, 2007.

[3] A. Kebkal, K. Kebkal, and M. Komar “Data-link protocol for underwateracoustic networks,” in IEEE OCEANS’05, Brest, France, 2005.

[4] P. Maymounkov, D. Mazieres, “Rateless Codes and Big Downloads,” in2nd International Workshop on Peer-to-peer Systems, Berkley, 2003.

[5] M. Luby, “LT Codes” in IEEE Symposium on the Foundations of

Computer Science (FOCS), 2002, pp. 271-280.[6] A. Shokrollahi, “Raptor Codes,” IEEE Transactions on Information

Theory, vol. 52, pp. 2551-2567, 2006.[7] J. Rice at al. “Evolution pf Seeweb Underwater Acoustic Networking,”

in Proc. IEEE Oceans 2000, vol. 3, pp. 2007-2017.

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