Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed◆招待論文◆

Performance Evaluation of
RObust Header Compression(ROHC) over Unidirectional Links
using DVB-S Testbed
DVB-S を使用した片方向通信路における
RObust Header Compression (ROHC) の性能評価
Way-Chuang Ang
Research Officer, School of Computer Sciences, Universiti Sains Malaysia
アン ウェイ チャン
マレーシア科学大学計算機科学部研究員

Tat-Chee Wan
Senior Lecturer, School of Computer Sciences, Universiti Sains Malaysia
ワン タ チー
マレーシア科学大学計算機科学部専任講師

Kotaro Kataoka
Doctoral Program, Graduate School of Media and Governance, Keio University
片岡 広太郎慶應義塾大学大学院政策・メディア研究科後期博士課程

Chee-Hong Teh
Research Officer, School of Computer Sciences, Universiti Sains Malaysia
テ チー ホンマレーシア科学大学計算機科学部研究員

Unidirectional Lightweight Encapsulation (ULE) was introduced to carry IP data overUnidirectional Link (UDL) using Digital Video Broadcasting via Satellite (DVB-S) system toovercome efficiency problems in satellite data communication. This paper introduces a method tofurther improve the efficiency of IP packets transmission over satellite communication system using
RObust Header Compression (ROHC). ROHC is a framework to compress headers of IP packets. This
paper presents a study of performance characteristics of ROHC over DVB-S via an actual satellite link.
The analysis of results showed that header compression yields significant improvement in terms of data
throughput when the payload sizes of IP packets are less than 512 bytes, typical of VOIP and otherrealtime traffic.
Unidirectional Lightweight Encapsulation (ULE) は、衛星通信回線の片方向通信路（Unidirectional
Link, UDL）上で Digital Video Broadcasting via Satellite (DVB-S) を用いて効率的な IP データ通信を行うための仕組みである。本稿では、RObust Header Compression (ROHC) を用いて、より効率的に衛星通信回線上で IP データ通信を行う手法を提案した。ROHC は IP ヘッダの圧縮手法である。
実験により衛星通信回線上で DVB-S と ROHC を利用した際の通信効率を測定し、解析した。その結果、VOIP 等のリアルタイム通信に多く使われる、512 バイト未満の IP パケットに適用すること
で、通信効率が大幅に改善することが判明した。
Keywords: DVB-S, ROHC, ULE, MPEG2-TS, UDL Mesh

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1 Introduction
A technique to setup a mesh network overUnidirectional Links (UDL) was proposed in
our work [1] to achieve better spectral efficiency,
whereby each peer node participating in the network
broadcasts its traffic to its peers. This topology
allows for lower latency as the communication path
between peers is only one hop. The star topology
networks as seen in Figure 1 incur an additional hop
because traffic between leaf nodes has to go througha central hub. In addition, star topology networks
are not as spectrum efficient since their resultant
spectrum usage is greater than that of UDL meshbased topology[1]Figure 1 Star configuration satellite network. The UDL mesh based topology

is logically represented in Figure 2. Each site listens
to the transmissions from its counterparts andbroadcasts its data to every partner sites, whereas ina star topology, each site only communicates with thecentral hub. The single point of failure at the central
hub in the star topology will bring down the wholenetwork.
Digital Video Broadcasting via Satellite (DVB-S),
a standard within the family of DVB standards,
is commonly used to deliver audio/video content
over satellite links, but it can be used to deliver IP
packets as well. DVB-S system uses Moving Picture
Experts Group 2 Transport Stream (MPEG2-TS)
frame to carry data. MPEG2-TS is a stream of fixed

Figure 2 UDL mesh satellite network

size frames consisting of 4 bytes of header and 184
bytes of payload. Traditionally, IP packets wereencapsulated using Multi-Protocol Encapsulation

communications technology.

(MPE) before they are encapsulated within payload

This paper proposes an approach to further

section of MPEG2-TS frames. In addition to a more

reduce the overhead of headers in a DVB-S link using

efficient UDL mesh network topology, Unidirectional

ROHC. The performance characteristics of ROHC

Lightweight Encapsulation (ULE)[2], a lightweight

are evaluated over real satellite links. The second

encapsulation format proposed by IETF, was usedsection of this paper discusses the related works onas the replacement for MPE. A study by Teh, et.

header compression schemes proposed by other

al.

[3]

showed that ULE was more efficient than MPE.

researchers. Then it is followed by an overview ofEfficient utilization of bandwidth is very importantthe software implementation of ROHC compressor

when its cost is expensive as is the case for satellite

and decompressor. The fourth section of this22Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

paper discusses software parameters and hardware

RTP headers to a minimum of 4 bytes when UDP

configuration used in the experiment. Subsequently,

checksum is enabled. If the UDP checksum wasresults of the experiment and conclusions are

not enabled, CRTP could compress the header to apresented in the last two sections.

minimum of 2 bytes. CRTP used explicit signaling
messages from the decompressor to the compressor

2 Related Worksfor error recovery. Similar to IPHC, CRTP also did

Over the past 15 years, there are many proposed

not perform well on lossy links with long round trip

header compression schemes. The original Van
Jacobson compression scheme (VJHC)

[4]

time.

was

To address the high BER and long RTTs on

developed to reduce the TCP/IP packet overheadsatellite links, an efficient and robust compression

to an average of 4 - 5 bytes per packet. It does not

scheme is needed. The header compression schemes

support compression of IP/UDP headers because

mentioned above were not able to satisfy thisat that time UDP traffic was very low. This scheme

criteria. RObust Header Compression (ROHC)[7] wasemploys delta compression, where the value ofdeveloped and standardized by the ROHC Working

changing fields will be sent, helping to minimize theGroup within IETF to address these issues. RObust

number of bits sent. This scheme relies on the TCP

Header Compression (ROHC) is a framework torecovery mechanism to recover from errors in thecompress headers of IP packets over error prone

compressor context due to bit errors and residual

links using suitable feedback mechanism. It iserror arising from packet losses on the link. Thisa flexible framework that can compress various

scheme is only suitable for less error-prone links

types of headers. Each ROHC profile defines the

such as terrestrial links. It is because one corrupted

required methods to handle the compression anddelta header can cause all subsequent packets to be

decompression of a specific type of header chains. A

misinterpreted.

stream of related packets is tracked using a context.

Subsequently, a general IP header compression

When encountering a fresh stream of packets, a[5]

ROHC compressor creates and initializes a new

was proposed to improve on VJHC. Unlike VJHC,

context with static and dynamic context information

IPHC supports compression of UDP packets.

using the information derived from the header

This header compression scheme essentiallyfields of these packets. At the initial stage, packets

uses a similar mechanism of delta compression

belonging to the same context are sent with theas found in VJHC. The difference between theseleast efficient header format. When the compressor

two header compression schemes is that IPHC

is confident that the decompressor has enoughuses its own feedback mechanism to recover frominformation to construct full header with only

error conditions rather than depending on the TCP

dynamic fields of header, the compressor will switch

recovery mechanism. IPHC is suitable for links withto a higher state sending only dynamic fields tostrong link error checksums but it is not robust

update a context.

scheme named IP header compression (IPHC)

enough to support links with high bit error rates,

The ROHC framework defines 3 operational

high losses, and long round trip times.
Compressed Real Time Protocol (CRTP)

mode, namely unidirectional, bidirectional optimistic[6]

isand bidirectional reliable. Unidirectional modea header compression scheme for IPv4/UDP/RTP

is best used when no return path is available toheaders. CRTP can compress 40 -byte IPv4/UDP/

send feedback. In bidirectional optimistic mode,

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a ROHC compressor may switch to higher

3 ROHC over ULE Stream

state of compression when it is confident that

3.1 Implementation of the ROHC Framework over
ULE Stream

the decompressor has sufficient information todecompress a packet using only dynamic fields

Encapsulation overheads in MPEG2-TS andor when it receives a positive feedback from the

ULE are significant for IP packets with small payload

ROHC decompressor. This mode has the potential

size. This paper presents an approach to reduce theof realizing the most efficient bandwidth utilization

overhead of carrying IP packets by applying RObustespecially when the round trip time of the link is

Header Compression (ROHC) [7] over the ULE

high. When ROHC compressor is operating instream.

bidirectional reliable mode, the compressor can only

Implementation of the ROHC framework overswitch to a higher state when it is informed via a

ULE stream discussed in this paper is an extension topositive feedback message from the decompressor.

our earlier work [8]. From the software point of view,

Each context updating packet is protected by CRC in

the bidirectional ULE encapsulator was extended

this mode, thus this mode is the most error-resilient

to include the capability to compress packets using

among the three modes.

ROHC as well as to decompress ROHC compressed
packets by means of an external library librohc.
Figure 3 depicts the software architecture of thesystem. Decapsulation of MPEG2-TS frames andFigure 3 Interaction of bidirectional with other components of the system

24Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

ULE SubNetwork Data Unit (SNDU) are done by theis depicted in Figure 4. When a compressor receives

linux kernel dvb_net subsystem. Each logical channela packet, it first looks for the most suitable ROHCof the MPEG2-TS stream is indicated by PID in the

profile to compress the packet. The only criterion

MPEG2-TS frame header and represented as a DVB

for the most suitable ROHC profile is the efficiency

virtual network interface. The bidirectional daemonof compression for the given packet. For example,

performs decompression on packets received fromgiven a TCP packet and that the compressor supports

these virtual network interfaces before sending them

TCP compression profile and IP compression profile,to the tun/tap virtual network interface. Figure 3

both profiles can be selected to compress the TCP

shows a simplified version of the overall process

packet. However, the TCP compression profile isand does not take uncompressed streams into

more suitable since it can compress the TCP header

consideration.

as well as the IP header of the packet, whereas the IP

From then on, the network stack of Linux kernel

compression profile can only compress the IP header

performs the necessary processing and will deliver

but not the TCP header. If no suitable compression

the packet to network applications as appropriate.

profile can be found, the packet will be returned

For the transmission of IP packets, the compression

without compression. This is not to be confused withof the packets and encapsulation of packets using

the uncompressed profile specified in RFC 3095[7].

ULE header and MPEG2-TS frames are done within

Once the most suitable compression profile isthe bidirectional daemon. These MPEG2-TS frames

found, a search is conducted among existing contexts

are then sent to ASI transmitter card. The next

with similar profiles to look for a context that thesubsections will explain the process involved in thepacket belongs to. If no matching context is found, acompression of IP header and decompression of

new context is created using the information derived

ROHC compressed packets.

from this packet with the help of compression profile.
The compressor then encodes the Context Identifier(CID) using the information from the context. The

3.1.1 Implementation of ROHC Compressor
Framework

compressor then looks for any ROHC feedback that

The pseudocode of ROHC compressor framework

can be piggybacked. ROHC feedback will be inserted

Figure 4 Pseudocode of ROHC compressor framework

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into packet if found.

decompressor receives a compressed packet, it strips

Armed with the right context and compression

all padding bytes if present. If it encounters an Add-

profile, the compressor will perform profile specific

CID octet, then it needs to decode CID of the packet.

compression on the packet. In the end, the portion

If the Add-CID octet is encountered when the ROHC

of packet that cannot be compressed will bechannel uses LARGE-CID, the packet is considered

concatenated to the compressed header to form afaulty and will be discarded. Then decompressor may

compressed packet.

encounter ROHC feedback. ROHC feedback can only
be decoded if the decompressor is associated with acompressor. Otherwise the whole packet will have

3.1.2 Implementation of ROHC Decompressor
Framework

to be discarded because the decompressor cannot

The ROHC decompressor framework is moredetermine the length of the ROHC feedback.

complicated than the ROHC compressor framework.

At this stage, the decompressor will encounter

Its pseudocode is depicted in Figure 5. When a

LARGE-CID octets if it is using LARGE-CID and will

Figure 5 Pseudocode of ROHC decompressor framework26Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

have to decode the CID of the packet. Or else thelook for existing context failed. If the packet type isdecompressor will have to assume CID 0 is being

Initialization and Refresh-Dynamic (IR-DYN), CRC

used if SMALL-CID is used and no Add-CID octet

check will be conducted. Decoding of the header will

was found previously. A search for existing context

then be performed based on the context and profile

will be conducted based on the decoded CID. Theof the packet. The decompressed header and payload

packet type is determined at this stage. If the packetof the compressed packet will be concatenated totype is Initialization and Refresh (IR), the profile thatform the uncompressed packet.

is used to decode this packet is searched for. If thedecompressor does not support the profile, then the

4 Setup of Experiment

packet will be discarded. Otherwise a CRC check4.1 Physical Environment

will be performed and if the CRC check passes, theThe experiment was conducted throughcompressed packet will be compared against existing

C-Band satellite link provided by JSAT’s JCSAT-

context for the CID, if there is one, to determine if

3A transponder. The participant sites were Shonan

it matches existing context. If it does not match the

Fujisawa Campus (SFC) at Keio University andexisting context, the compressor must have reused

Universiti Sains Malaysia (USM) in Penang. The

the CID for a new context, thus the old context

satellite link from USM to SFC had a link capacity

will have to be deleted and a new context based onof 3.507Mbps, while the satellite link from SFC tocurrent IR packet will be created. The decompressor

USM used a shared link over an ASI multiplexer asthen performs profile specific decompression of the

depicted in Figure 6. The UDL mesh bridge machine

IR packet.

at SFC was configured to transmit data at 3Mbps overIf, however, the packet is not an IR packet, thenthe 12Mbps link.

the packet will be discarded if the earlier effort to

Figure 6 Configuration of UDL mesh gateway at SFC campus

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experiment was conducted using packing mode with

4.2 Parameters of ULE Encapsulator
There are two mechanisms to encapsulate ULE

a 3 milliseconds packing threshold as it was the best

SubNetwork Data Unit (SNDU) within MPEG2-TS

tradeoff in terms of throughput and latency. Padding

frames known as padding and packing. In padding

mode was not used as the default setup because ofmode, after the ULE SNDU is encapsulated within a

its inefficiency. The results in Figure 7 were obtained

MPEG2-TS frame, the unused portion of the MPEG2-

by running the experiment using padding mode with

TS frame is padded with padding bytes and sent

various packet sizes. Even though the compressed

immediately. Whereas when packing mode is used,stream achieved better results when compared

the unused portion of MPEG2-TS frame will be filled

to uncompressed stream in padding mode, it stillwith the subsequent ULE SNDU if a ULE SNDU isyielded worse results than an uncompressed stream

received before packing threshold expires. Basedin packing mode whenever compressed packet sizeson the results from previous experiments, setting

slightly exceed a multiple of 184 bytes (one MPEG2-

packing threshold of 1 millisecond packing threshold

TS frame).

was not ideal for throughput in certain cases. Whileusing 5, 10 and 20 milliseconds packing threshold

4.3 Parameters of the ROHC Compressor

were ideal for throughput, higher latencies were

The ROHC compressor used in this experiment

incurred on these packing thresholds, by default, theonly supports UDP profile operating in unidirectional

Figure 7 Data throughput of compressed and uncompressed streams in padding mode28

Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

mode. The compressor was configured to send0
1

3 Initialization and Refresh (IR) packets before
moving to a higher state. The compressor was
set to perform periodical down-transition to the
IR state after transmitting 50 Second Order (SO)packets consecutively to ensure that the context

1
1

2
3
4
5
1
1
1
1
Profile ＝ 0 × 2 (1 octet)
CRC (1 octet)
Version ＝ 6
Flow Label (20 bits)
Next Header (1 octet)

6
0

7
D

Source Address (128 bits)

is synchronized properly. To achieve highest
efficiency, the ROHC channel was configured touse SMALL_CID to reduce the overhead of Context

Destination Address (128 bits)

Identifier (CID). A CID of zero is sent implicitly andnever shows up as part of header.

Source Port (2 octets)

When a ROHC compressor is operating at
the highest efficiency, it is capable of reducing the

Destination Port (2 octets)headers of IP packet down to 1 byte. However, forTraffic Class (1octet)
Hop Limit (1 octet)
Generic extension Header list (1 octet)

IPv6 UDP datagrams, the UDP packet must be
coupled with a 2 octets UDP checksum as requiredby IPv6. Thus, the smallest compressed header for

UDP Checksum (2 octets)

IPv6/UDP stream will always occupy 3 bytes. Figure
8 shows the format of the most compact compressed

SN (2 octets)header.
Figure 9 Packet format of UDP/IPv6 IR header
0
0

1

2

3

4

5

SN

6
CRC

7

UDP Checksum(2 octets)

By making several assumptions, a theoreticalmodel of achievable data throughput for ROHC

Figure 8 Packet format of UDP/IPv6 UO-0 header

compressed UDP/IPv6 stream and uncompressed
UDP/IPv6 stream was developed. Let us denote
When the ROHC compressor is in the IR state,

the bandwidth of the link in bps as Bandwidth, thus

it has to initialize the context with static and dynamic

the number of MPEG2-TS frames transmitted per

information using the IR packet. The IR packet issecond, denoted by TS, can be calculated using theslightly bigger than the original uncompressedfollowing formula:

packet. Figure 9 shows the structure of the IR packet
for a UDP/IPv6 stream. The notation of UDP/IPv6
follows the style proposed in the RFC for ROHC						
Bandwidth
TS ＝−		
188 × 8[7]

and it denotes UDP over IPv6. This type of notation(1)

is applied throughout this paper. Generic extension
header list may span more than 1 byte depending on

Each MPEG2-TS frame is 188 bytes and out of

the UDP/IPv6 header. The IR packet is at least 50

these 188 bytes, 184 bytes were used to carry its

bytes.

payload, the ULE SNDU. Assuming that the overhead

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of the Payload Pointer (PP) of MPEG2-TS frame iswhere:

negligible, the number of bytes that can be used byan ULE SNDU, denoted by ULE, can be deduced

IRcount is the number of IR packets sent by thefrom the following formula:compressor before moving to the SO state.
SOcount is the number of SO packets sent by the

ULE ＝ TS × 184(2)compressor before down-transiting to the IR state.
IR size is the size of the IR header in bytes. It

It should be noted that the overhead of PP is not

is assumed to be 50 bytes for a typical UDP/IPv6

negligible when the size of payload gets very small.

stream as depicted in Figure 9.

Each ULE packet adds 8 bytes of overhead with four
bytes taken up by the ULE header and another 4

SOsize is the size of the SO header in bytes. It isbytes used for 32-bit CRC. For a typical UDP/IPv6

assumed to be 3 bytes for a typical UDP/IPv6 stream

datagram, 40 bytes are taken up by IPv6 header andas depicted in Figure 8.

8 bytes are used for UDP header. Let us denote thepayload of UDP packet in bytes as Payload, hence the

The number of compressed UDP/IPv6 packets that

number of uncompressed UDP/IPv6 packets that can

can be sent in 1 second, denoted by compressedcountIPv6,

be sent in 1 second, denoted by uncompressedcountIPv6,

and achievable data throughput in bps, denoted by

and achievable data throughput in bps, denoted bycompressed bps, can be derived using the following

uncompressedbpsIPv6, can be derived as follows:

formulae:						

			ULE
uncompressedcountIPv6 ＝−(Payload ＋ 48 ＋ 8 )					uncompressedbpsIPv6 ＝ uncompressedcountIPv6
× Payload × 8

ULEcompressedcountIPv6 ＝−− (6)(Payload ＋ averageROHC ＋ 8 )						
compressedbpsIPv6 ＝ compressedcountIPv6(7)
× Payload × 8

(3)

(4)
Using the assumptions stated above, Figure 10

Assuming that the compressor directly transits

shows a comparison of maximum theoretical datafrom the Initialization and Refresh (IR) state to thethroughput between compressed and uncompressed

Second Order (SO) state without going through

UDP/IPv6 streams over ULE/MPEG2-TS using

First Order (FO) state, the average size of ROHC

packing mode over the 3.507Mbps DVB-S link.

compressed headers, averageROHC, can be calculated

Figure 11 shows the percentage of data throughput

using the following formula:gain for a ROHC compressed stream over an
uncompressed stream.

						
IRcount × IRsize ＋ SOcount × SOsize
averageROHC ＝−− (5)IRcount ＋ SOcount30Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

Figure 10 Comparison of maximum theoretical data throughput between uncompressedand compressed UDP/IPv6 streams

Figure 11 Data throughput gain of ROHC compressed UDP/IPv6 stream

measurement. The iperf client ran in the gateway

4.4 Experiment Details
The results of the experiment were gatheredlocated at USM, while the iperf server ran in theusing iperf, a client-server software for throughput

gateway located at SFC. The ULE encapsulator

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operated in packing mode. Tests were conductedthat saturation point will lead to packet losses.

with UDP payload sizes of 40 bytes, 80 bytes, 120

For payload sizes of 40 bytes, 80 bytes and 120

bytes, 160 bytes, 200 bytes, 300 bytes, 512 bytes andbytes, compressed data throughput saturated at1024 bytes. More samples were taken for smaller

828 kbps (23.7% ), 1656 kbps (47.3 % ) and 2315 kbps

payload sizes since significant changes occur in this(66.1% ) respectively. For these payload sizes, un-region of the graph as depicted in Figure 10 and 11.

compressed data throughput saturated at 746 kbps
(21.3 % ), 1547 kbps (44.2 % ) and 2238 kbps (63.9 % )5 Results

respectively. The benefit of ROHC for these packet

For Figure 12 - 14, tests were conducted

sizes was minimal especially for payload size of 120

incrementally by instructing the iperf client tobytes where compressed traffic yielded only 2.2

send data stream at 100kbps up to 3500kbps withincrease in data throughput. The measured data

an increment of 100kbps for consecutive tests tothroughput was significantly lower compared todetermine the point where data throughput peaked

the available bandwidth of the link when the UDP

for both compressed and uncompressed stream. The

payload sizes were small. This was caused by thedata stream generation rate and the measured data

overheads introduced by MPEG2-TS frame, ULE

throughput were normalized against the bandwidth

SNDU, IPv6, and UDP headers, which took up aof the link. Packet loss rates (PLR) became non-significant portion of the available link bandwidth

zero when measured maximum data throughput wascompared to the actual data payload size.

obtained. Increasing packet generation rates beyond

Figure 12 Normalized data throughput of compressed and uncompressed UDP/IPv6 streams with
40, 80 and 120 bytes of UDP payload

32%

Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

Figure 13 Normalized data throughput of compressed and uncompressed UDP/IPv6 streams with
160 and 200 bytes of UDP payload

Results for payload size of 160 bytes were

The reason for this is unknown although several runs

marginally better. The highest data throughputof the experiment gave consistent results.

recorded for uncompressed traffic approximatedFigure 15 is a summary of Figure 12 - 14.

the result from the theoretical model as shown in

It shows that the measured data throughput of

Figure 15. The highest data throughput recorded forcompressed traffic and uncompressed traffic fromcompressed traffic was about 19% better than highest

DVB-S link were comparable to the results fromdata throughput of uncompressed traffic. Likewise,

theoretical model for UDP payload of 200 bytes orpacket loss rates were marginally lower. Unlike thegreater.

results in Figure 12, compressed data throughput

Contrary to the results from the theoreticalwith 160 bytes payload was better than uncompressed

model, the measured results from DVB-S link did

data throughput with 200 bytes payload.

not exhibit higher gains when payload sizes were

For payload sizes of 300 bytes, 512 bytes and 1024

small (< 200 bytes), possibly due to interactions

bytes, the highest data throughput for compressed

between the hardware device and software drivers.

traffic was greater than 90% of link capacity.

Interestingly, Figure 16 shows that the gain achieved

Percentage of data throughput gains decreasedfor UDP payload of 1024 bytes was higher than that

gradually as the payload size increased. However,of theoretical model. The measured data throughput

for a payload size of 1024 bytes, data throughput ofof uncompressed traffic for that payload size was

uncompressed traffic was less than that of 512 bytes.

slightly lower than expected, while the measured

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Figure 14 Normalized data throughput of compressed and uncompressed UDP/IPv6 streams
with 300, 512 and 1024 bytes of UDP payload

Figure 15 Comparison of maximum data throughput between theoretical model and DVB-S link34

Performance Evaluation of RObust Header Compression (ROHC) over Unidirectional Links using DVB-S Testbed

Figure 16 Comparison of data throughput gains between theoretical model and DVB-S link

data throughput for compressed traffic was closeshould also improve the correlation between theto the expected result from the theoretical model.

theoretical and experimental results. Nonetheless,This combination led to a higher than expected data

the experiments show that the ROHC over ULE

throughput gain for the UDP payload size of 1024

performance is very close to the predicted behavior

bytes.

for UDP payloads of 200 bytes or greater. An earlier
study to predict the performance of ROHC over ULE

6 Conclusionshows that efficient bandwidth utilization can be

The implementation of UDL mesh network, ULE

achieved for various types of RTP traffic [9]. Futureand ROHC were intended to improve the utilization

work include the implementation and testing of moreof satellite bandwidths. However, the implementation

advanced profiles (e.g., RTP, TCP) which is expectedof ULE encapsulator and ROHC framework deviatedto improve the achievable data throughput of ROHC

from the theoretical model when payload sizes wereover ULE even more for such real time audio andsmall (< 200 bytes). Due to intricacies of interaction

video streams.

between hardware and software, additional fine7 Acknowledgment

tuning must be done to achieve results closer tothe theoretical model when payload sizes are small.

The authors wish to thank the AI3 Project for

Modifications to the theoretical model to accountthe satellite link bandwidth used in the experiments.

for overhead introduced by the Payload PointerImplementation of the ROHC framework by Lulea

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University of Technology was used as reference

References

for the implementation of librohc used in the[1] T.C. Wan, “Interactive wide area collaboration via unidirectional
links,” Proceedings APAN Conference, Beijing, P.R. China,
2000.
[2] G.Fairhurst and B.Collini-Nocker, “Unidirectional Lightweight
Encapsulation (ULE) for Transmission of IP Datagrams overan Mpeg-2 Transport Stream (TS),” RFC 4326, 2005. [Online].
Available: http://www.ietf.org/rfc/rfc4326.txt[3] C.H. Teh, T.C. Wan, and R.Budiarto, “A comparison of IP
datagrams transmission using MPE and ULE over MPEG-2/
DVB networks,” In Proceedings Fifth Int’l Conference on
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[4] 4V.Jacobson, “Compressing TCP/IP headers for low-speed
serial links,” RFC 1144, 1990. [Online]. Available: http://tools.
ietf.org/html/rfc1144[5] M.Deger mark, B.Nordg ren, and S.Pink, “IP Header
Compression,” RFC 2507, 1999. [Online]. Available: http://
tools.ietf.org/html/rfc2507[6] T.Koren, S.Casner, J.Geevarghese, B.Thompson, and P.Ruddy,“Enhanced Compressed RTP (CRTP) for links with high delay,
packet loss and reordering”, RFC 3545, 2003
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H.Hannu, L.-E. Jonsson, R.Hakenberg, T.Koren, K.Le,
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T.Yoshimura, and H.Zheng, “RObust Header Compression(ROHC): Framework and four profiles: RTP, UDP, ESP, anduncompressed”, RFC 3095, 2001
[8] W.C. Ang, N.A. binti Yusri, C.W. Tan, and T.C. Wan,
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DVB-S links,” Proceedings Int’l Conference on Computing andInformatics, ICOCI 2006, Kuala Lumpur, Malaysia, Jun. 2006.
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e5pheghwjeuburqa/

experiments. This research was partially fundedby Ministry of Science, Technology and Innovation(MOSTI) Malaysia via e-Science grant titled
Improving Performance of Unidirectional Link (UDL)
Mesh Network via Header Compression (Project
01-01-05-SF0036).

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