Proceedings of the 2007 IEEE lnternational Conference on Telecommunications and Malaysia Intemational conference on communications, l4-17 May 2007, penang, Malaysia

Perfonnance Evaluation of Unidirectional Lightweight Encapsulation usin g ns-2 and DVB-S TestBed

Way-Chuang Angr, Chee-Hong Teh2, Tat-Chee Wan3, Rahmat Budiartoa Network Research Group School of Computer Science Universiti Sains Malaysia 11800, Minden, Penang. Malavsia {wcangr, chteh2, tcwan:, t at trrttn y g* g. cs.usm.my

used to define the standard for high qualrty video and audio Absttact-Unidirectional Ughtweight Encapoulation (ULE) is a compression which is applicable to high quality real-time recently published standard to overcome the elliciency problems conferences. The MPEG-2 standard defines a method for of Multi-Protocol Encapsulation for satellite data transmission. multiplexing different elunentary stream t5pes to form a This paper presents a comparative study of Unidirectional Transport Steam (TS) used in bnoadcasting systerns. This paper Lightweight Encapsulation performance using ns-2 simulation [ll studied the performance and a DVB-S testbed. Performance metrics such link ufitizatisa, of IP traffic over DVB Transport packet loss and latency were taken. It was found that the results of Streams. MPEG-2 TS was desigrred for tanqporting MpEG-2 both approaches were comparable over unreliable media where errms usually occur. The TS is a stream that consists of consecutive and relatively short Index Termy-I)VB, IP over MpEG-2 TS, MpE. ULE fixed-length packets. The packet length for MPEG-2 TS is 188 bytes. Each TS packet structure starts with a4-bytefixed length I. INrRooucnoN TS header, followed by a 184-byte optional adaptation field and pgyload atellite communications play a vital role in providing as shown in Figgt?le6 communication links to geographically remote sites. flead€r I Ada&Uon Ftetd (not used In ULE) Peycd Traditionally we use satellites to receive Standard Definition Television (SDTV), High Definition Television (IDTV) programs, and for voice telephony. However, due to the rapid convergenc€ of data and multimedia applications, the Internet has become an important service for a vast majority of people today. The use of satellite links has changed from not only supporting ftaditional media applications to packet data tansmissions. Now, satellites that were initially deployed for military commrmications, voice telephony and TV broadcasts tE nspdl_erTor_indcator are being used to offer Int€rnet access globally. Many Internet Service Providers (ISPs) offer global Intemet access using Fig. I Transport Stream and Header stucture satellite links. Broadcasters can dedicate a portion ofa satelliti broadcast channel that was fiaditionally used for audio and video streams to include unicast or multicast Ip taffic [21. II. IPovenMPEG-2TS Data is sent directly to satellite from a single sourc€, and then received by a large downsteam population [3] using packet The European Telecommunication Standards Instihrte (ETSI) lata seryces over (Digital Video Broadcasting) OVe, e.g., specified several methods to transmitnetwork data over InterneVWorld Wide Web (WWW) access with high speed MPEG-2 TS. The most widely used method intoduced in [5] is tansmission via a DVB satellite hub station to tansmit packet called Multi Protocol Encapsulation (lvPE).Later, the Internet data to users' satellite dishes used for TV distribution. This data Engineering Task Force (IETF) proposed an alternative b'roadcasting technology enables broadcasters to broadcast large encapsulation method named Unidirectional Lightweight amount of data to personal computers using MpEG-2 Transport Encapsulation (ULE) 16l that is simpler than MPE. Streams (MPEG-2 TS) I4l. MPEG-2 is the standard for generic coding of moving Unidirectional Lightweight Encapsulation is a recently published pictures and associated audio information. It has been widelv standard. A ULE packet is layered directly into the

81 payload field of MPEG-2 TS fiame. Similar to MPE, a ULE packet may span several MPEG-2 TS frames if its size exceeds the size of a MPEG-2 TS payload field. The 2 mechanisms used to encapsulate ULE into MPEG-2 TS frame are packing and padding. These mechanisms will be discussed in detail in later section. HEEEEHHE! mandatory fields and is [-"*-*_l True to its name, ULE only carries PES I llDsrr_ccs€dixl lightweight by design. The benefits of the simplicity of ULE HH l*l packet format are two-folds. Firstly, having fewer mandatory fields translates to greatsr saving ia bandwidth. This fact is MPEG2 Tc$sl St"dfr greatly amplified in situation whae bandwidth resource is prohibitively as in satellite networks ernployed by expensive 0d!. PhFaal DVB-S and DVB-S2 standards. Secondly, simple packet header makes ULE easier to be processed and thus demands less EEEFig.3 Architecture of DVB protocol processing poliler on the participating nodes than that of MPE. In order to meet the criteria of optimized transrnission Before getting into more details about the efficieacy offered wiihout sacrificing flexibility, ULE intoduces extension by MPE and ULE, the two encapsulation mechanisms - Padding headers to carry additional header information. Fig. 3 illustrates and Packing mode will first be b,riefly discussed. the packet format of a ULE packet with only mandatory fields. In MPEG-2 TS Padding modg a large SN)U need to be The type field ofULE packet is used to identify the existence of fragmented and encapsulated independently into TS packets. extension header in a packet and the type ofpayload carried in Since a TS packet payload is a fixed size cell with 184 bytes, a PDU field. For packets with type field that contain values less SNDU, generally, does not fit into an integral number of TS than 1536, extension header is present and the values are packets. Thereforg the unutilized part of the payload will be assigrred by IANA. For t1p field that contai:rs values greater padded with stufrng bytes (OxFF also knows as NULL bytes). 0ran 1535, the values correspond to the type code specified by Padding is reqr:ired to align the end ofthe data with the final TS the IEEE/DIX type assignment for Ethernet. Thus, both IPv4 packet so that the TS packet maintails 188 bytes. and IPv6 are supported directly using the type field without In MPEG-2 TS packing mechanism, each new SNDU does requiring any modification. not have to start in a new TS packet. Like padding mode, a large SNDU will be fragmented and encapsulated independently ir*o Srbrlsfi ot u.* (3.aoul TS packets. All filled TS packet will be fransmiued immediately but for the last TS packet with some unutilized space, the unutilized pmt of the payload will not be padded with padding bytes. In the same TS pcket, if there are leftover spaces, a new SNDU can start immediately following the srd of l- srou tt-o.t the prwious SNDU payload. Therefore this mechanism has Fig. 2 Unidirectional Lighlweight Encapsulatior packet fomat removed the need for padding. In MPEG-2 TS datatansmissiot, thetransmissior efrciency can be defined as the ratio of total number of information byta II1. IP Dnra EncapsulattoN PRocsss -EFrrcmNcv over the total number of bytes in tansmission [81. By using ANALYSIS At.lD CoMPARISoN MPEG-2 TS Padding mode, fhe tansmission efficiency offered tn MPEG-2 TS, there me two mechanisms to encapsulate IP by this mode can be calculated based on equation below. packets into MPEG-2 TS using MPE and ULE. These two Assuming that the sizes of MPE or ULE header is,[I, Z is the mechanisms are known as padding mode and packingmode [7]. size ofthe SNDU, ,S is the I byte Payload Pointer (S is inserted To encapsulate an IP packet into MPEG-2 TS, multiple into first byte ofMPEG-2 TS payload field whenever there is a overheads are intoduced during the ancapsulation process as new SNDII) andlis the length ofIP datagram. The TS payload shown in Fig. 3. During this process, IP packets need to go utilization, U, v/hich is the total number of TS packets required through multiple protocol stacks, as a result, various overheads to ftansmit the I, can be denoted as: will be added, and how much overheads are inserted into the L=H+I packet during this process depends on the PDU size and also the encapsulation method behg used. This amount of overhead will Iz*sl give a direct impactto MPEG-2 TS sptem tansporting IP data- '=l;l where Uhere is always the smallest integer greatc than or equal to the real number U. To calculate the Transmission Efficiency Tpaaog for MPEG-2 TS padding from (2), Teaaos can be defined as:

82 TPtddng =axtooz Uxl88 s Similarty, when the MPEG-2 TS packing mode is enabled, the TS payload utilization is: From (2), A u=L*s /I UE 184

where Uis an integer. To calculate the Transmission Efficiency keIib sb T*gw for MPEG-2 TS packing, from (4), Tpncktne can be defined as: Fig. 5 Simulation model of bidirectional satellite link for Ip over MpEG-2 TS TPaeang = Lx100o/o Uxl88 B. TestBed Environment The layout of the testbed is depicted in Fig. 6 below. The With these equationg the Transmission Efficiencygained using testbed is composed of 2 Unidirectional Link ([IDL) gateways MPE and ULE can be theoretically analyzed, as shown in Fig. 4. Calolarior of Tmsmision Efiory e r Fmction of pmtet Sia with each gateway having a DVB-S modulatm. Each UDL tu* gatewayhas an Asynchronous Serial Interface (ASI) cardardat 9(m Ieast one DVB-S receiver card. Asynchronous Serial Interface wb inside UDL gateway sends MPEG-2 TS fiames to DV&S

70% modulatm. DVB-S modulator then converts diglhl data from ASI card into intermediate frequency (IF) in ttre range of Fffi x 50-l80MHz. Since DVB-S receiver card only accepB input dsw within the range of 950 Mllz -zls0tfrIz,an IF up.converter is n% needed to convert IF to L-band &equency. In the en4 radio ggh frequency is then dernodulated into digital data by the DVB-S 2W receiver card of the recipient UDL gateway.

10%

M t* * m looo rrm :m t4oo 16m ,#""iff, lffi MPEPeking-.."* MPOPaddiry- Umpeti4--.. Fig.4 Transmission Efliciency ofMPE vs. ULE tSl

l4cnld r]_ lF spllfl.r IV. IP DareENcApsuLATroN- STMTJLATToN AND TEsrBro l:F SYSTEM rF combrmr d+LT Performance of the newly proposed encapsulation method Oulgolng R€ 6Gout lC.bL - lncomlng RG O CoxLl CabL ULE, has been evaluated using the ns-2 simulation tool as well Fig.6 DVB-S testbed as a testbed environment. The aim of this resemch was to make The decapsulation ofULE packets a comparative study on the performance and the efficiency of occurs in the kernel space of the operating systern using the dvbnet the self implemented ULE encapsulator and the simulation software stack, whereas the encapsulation process ocsurs in user space using model which was provided by European Space Agency (ESA). software developed by the authu. Herg a simple bidirectional A. ns-2 Sirmlotion Iink between 2 sites was assumed; whereas the UDL gateway is capable of operating in mesh TIte ns-2 simulation tool was used for evaluating the a environment with several sites performance of ULE encapsulation. The ULE Ip packet tel. ancapsulation module from ESA was used as the basis of the simulations described in this paper. The simulation model of Ip over MPEG-2 TS is shown in Fig.5. V. ULEPERToRMANCE EvaluarloN The simulation results from ns-2 were compared to the testbed results obtained usng iperf version 2.0.2 and UDp utility programs on the testbed. Delay, packet loss and link utilization metics were measured" Tests were conducted on 2Mbit/s, 4Mbit/s and 8Mbils links for all simulations and

83 experiments. Both padding and packing modes for ULE were DVB-S testbed was dismal. This was largely due to the tested. ?acking mode tests were carried out using a packing constraint of the TCP protocol because UDP traffic did not thresholdof 40 ms. exhibit the same symptom for these packet sizes. Through In addition, a delay of 250ms was added to each link in the observation, it is noted for these packet sizes, most of the time testbed system via the Network Emulator (netem) module in was spent on waiting for TCP acknowledgments because the Linux. The netem module is configwed to emulate the 250ms advertised TCP windows from the receiver were very small. propagation delay found in typical geostationary satellite links. U* Udlizatiq tor Uridirectiooal Lighiwaight Encapsulatim - Padding mode (UDP Tatlic) 100

A. Link utilizqtion tests 9{) Link utilization tests were conducted for both TCP and UDP -E) tanspo* protocols. Due to the fact that TCP doesn't respect .E zo record boundary of data being sent, data fragments of smaller ;€o sizes may be accumulated into a big TCP packet before the ;"^ actual tansmission occurs. Thus, for testbed envirqrment, the 840 packet size of TCP is contolled by the MTU size of the Eso tansmitting network device. The packet sizes indicated in Fig' zo 7 wtil Fig. 12 are inclusive of the IP header, UDP header as well as UDP paylmd. Adjustments were made to the results 10 does not take the size of IP o obtahed from rpey'because iperf 0 2€o ltoo €00 €00 l00O and tanqport protocol headers into account when calculating Pen€t Sizos (byls) siimhti*2Mb -f simulatioG$tb + T€€rb€d-ar'b -f,- results. Sknulato4Mb + Tertb€d2srb + T4.{b€d€ift *- It should be noted ttrat the rezults used to derive the graph for Fig. 8 Cornparison of UDP link using ULE padding mode link utilizatiqr of TCP fiaffic were the average values achiwed after the TCP shearn enters steady state. TCP protocol The UDP packets loss rate shovm in Fig. 9 is just the performance for initial window growth was not considered. inversion of its UDP li* utilization counterparts. The Bit Error A comparison behveen simulated and actual link utilization Rate (BER) of these links was zero. The links were fed wittt for ULE paddingmode is given in Fig. 7 and 8 for both TCP and constant stream of UDP traffic according to the bandwidth of UDP tranqport protocols. The saw tooth straped result obtained the link. For an 8 Mbit/s link, the stream of UDP data is fiom the simulator is due to the packet sizes. Valleys are formed generated at a rate of 8 MbiUs regmdless of the packet s2es whenever the packet sizes slightly exceed multiple of 188 bytes. used. However, these IJDP streams only take the payload of For these packets, efficiency is reduced because much of the UDP packets into accoult. Thus, the actual generated UDP MPEG-2 TS frame was wasted on padding byes. However, as haffic would exceed the capacity of these links. This was more the size of UDP packets increases, the saw tooth shape becomes obvious at small packet sizes because the ratio of the less pronounced. This is because the number ofpadding bytes is combination of IP and UDP headers over UDP payload are overshadowed by the size of the actual packet. A close higher for smaller packets. As such, some of the UDP packets resemblance can be found between Fig. 4 and 8. were dropped due to buffer

Unk Utiliration lq Udd6dml Ughtw€iglt EEpaulstio. - Paddhg modo (ICP T'a'nc) r00 ""J,ffi,***TYtsrfi"*,#*

so 90 00 g) +7O em E E€eo f=w Eso !so i40 -Src E €() d30 z n m 10

0 o 2& * 8oo 1d!o o 2oo * 8d) lqxl *o.r*"&t Sirdstlon-2tlib -t*- tiimu|at*8lirb""*",u.offi*, -O'- Tesb€d-'$fb + Siruldc2iib -l+- Slmulation€l,b -+ I€€6€44Mb +- Slrulatlon-4lib +- Y€srb6+2lrb + T€€lbq!8tdb -lF- atia|dd4fr + Ts6d,.2rr,tb {- T€€b€d-8lrb -tF Fig. 9 Comparison of UDP packets drop rale using ULE padding mode Fig. 7 Comparison ofTCP link utilization using ULE padding mode The second set of tests shown in Figs. 10, ll and 12 were Surprisingly, results obtained from the testbed were better conducted using ULE packing mode. The measr:red LIDP link than that from simulatior, especially for TCP fraffic. However, utilization from the simulator and fie DVB-S testbed were results meastred for TCP packet sizes less than 200 byles on the

u pretty even and is close to theoretical limit [Bl. Again, the same B. Latency tests problem occurred for TCP packet sizes less than 200 byes in For DVB-S testbed, two simple UDP applications were size on the DVB-S testbed. specifically written to measure one-way latency of UDp packet Packet loss rates were much betta using ULE packing mode. over DVB-S link However, kcause ICMP can only measure round trip the same buffer ovemrn problem still occurred for time normally. *tt t-'trt"::r.ll The clocks on the 2 UDL gateways were synchronized through the Nefwork Time Protocol (NTp). The time difference between 2 gateways ,- Ljshtueishl Encapsurarion - packins (rep was measured using the -"-., "".r r€de rrarric) 100 same set of UDP applications over an Ethernet link. The results s0 obtained through the DVB-S link were thor adjusted using the

80 previously measured time difference. By comparing Fig 13 and 14, it is obvious that LILE padding 6,0 .I* mode offers lower average latency over ULE packing mode. 5 Sampled data from DVB-S testbed used to derive the average Euo were inconsistent and have a surprisingly high tatency. This can .& 40 T be attributed to the behaviour of DVB-S receiver cards and ASI zbg) card used on the testbed. In general, lower average latency can 20 be obtained by using a higher symbol rate on the DVB-S 't0 modulator. ln addition, the ASI link has a constant baud rate on its physical link. Thus, to achieve a lower data rate m an ASI 0 200 4oo 600 8oo 1000 link, more stuffing characters (K2S.5 character) have be Packet Siz€ (by{$) to Simuladon-zMb +e Shutstjon.SMb -q T€etbed-4Mb +_ inserted into the symbol steam. Therefore, real data have to Sirulation-4Mb --+- T6dr6&2Mb +- foUidAUO *- compete with more stuffing characters contributing Fig..l0 Comparison of TCP link utilization using ULE packing mode with to the packing threshold of 40 milliseconds variation in the reported delay because the nunber ofqueued stuffing characters varies based on when data arrive at ttre ASI Ltnk Utitiatio, for Unidtrsctionst Lightweiglit EGrFltatio, - packing mode (UDp T€ttic) card. 100 There were no surprises in the ns-2 simulatm results in Fig. *) f-t't-7*-r----r-i 13 and 14. Unlike the DVB-S testbed, one way latancy 80 '(iii i i ; slliiiiF/t f; measured onthe ns-2 simulator increased slowly as packet size I tl """"'1" increases. Naturally, to accommodate 5'o tlti; *'i"""' larger UDP pckel more $* MPEG-2 TS frames are required. Thus, a longer f duratior is iso required to send larger UDP packets. .&o Figs 13 and 14 show that link wittr higher bandwidth has E lower latency. This effect is more obvious on the DVB-S zE30 .i r i i. '{ testbed and can be explained by the characteristics of the ASI 20 link mentioned above. 10

0 Ono Way Latffiy tor Unitiwtonat Ltghtwdgfl Emp€uhid - psfihg modquop TE ffc] 0 200 4OO 600 8OO 1OOO Peckot Slus (bytes) Strutauo.2Mb *i+ SlTutadoftgMb +_ Tstwt b _ts Slrulaion-4Mb --{- T6ibed"2Mb 4-- rrib€i-SMb -.L Fig, I I Comparison of UDP link utilization 440 using ULE packing mode with ii:il packing threshold of 40 milliseconds 4n

"00 Pad€i Los Rat€ lor Untdtr*{onat Ltghtwotght Ercapsuhils - paddng rpde (UDp TEffic) €..g* cq 440 'm 300 ao ?60 240 m 200 0 2@ 400 600 8d, loo PEck€t Siz€ (b!i6) Simtail*2Mb + Siquta0onSf,D

u m 400 600 8OO 10( Packet Stzs (byt6e) S:mtati*zMb -rC- Sir$tation-OMb _O- Tstbed,4Mb {: Stmutatton-4Mb + tsibqt_2Mb _{r_ resOeO+iUO _*_ Fig. 12 Comparison of UDP packets drop rate using ULE packing mode

85 - rnde(UDP TBtiic) IP One way Latency lor Unidirelional Lightweight En€psulation Packing t71 Collini-Nocker, B & Fairhurst, G., "ULE versus MPE as an over DVB Encapsulation. HET-NETs 04 Second Intemational Working Conference, Performance Modeling and Evaluation of Heterogeneous Networks. July 2004. pp Y70/l-P7019. 440 *lnternet t8t Montpetit, M. J. & Schmidt, M., Protocols over Digital Video 1N Broadcasting Media (IP over DVB) Study of Encapsulation and Protocol ,loo Performance-Final Report. European Space Agency Technical Report. a 3so March 2004 Isoo goo Ang, W. C. et al. Implementation of IP Mesh Networks using ULE Eq tel -w Protocol over DVB-S Links," &oceedings Int'l Conference on Computing 300 and Informatics, ICOCI 2006, Kuala Lumpur, Malaysi4 Jun 6-8, 2006 2&] 2€O 240 m m o 2oo loo 6m 800 1m0 Pqck6{ Sh@ (br,{s) Slml€tik2i/b -+ Slmladoc€l& + Isb€d4Ml -F Stui.dd-4\,b +- T6b€d-2ilb +' Tss€d-8llb + Fig. 14 Comparison oftatency ofUDP packets using ULE padding mode

VL CoNct-ustou This paper discussed the characteristics of the new ULE encapsulation methd. The results from both simulation flrd implunentation w€re compaxable. Diferences arise due to the behaviour of tle ASI card and DVB-S receiver car4 w?rereas the simutatm simplified the hardware characteristics and therefore could not caphre the nqr-ideal behaviour of the actual equipm€nts. TCP models need refinem€nt to better reflect such isiues. Nonetheless, the UDP behaviour ofthens-2 models was quite close to actual measured results.

ACIG..IOWLEDGMENT The authors wish to thank the Etnopean Space Agency for the zs-2 models for MPE and ULE used in the experiments. The work desffibed in this pape is frmded by tre Background Study on Ih/6 over Adhoc Wireless project (304/PKOMP/636074\.

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