performance evaluation of a tcp proxy in wcdma networks

IEEE Wireless Communications • October 200370 1536-1284/03/$17.00 © 2003 IEEE

Availa

ble n

et d

ata r

ate:

10

ACCEPTED FROM OP E N CALL

INTRODUCTIONUbiquitous Internet access is regarded as a keysuccess factor for third-generation mobile com-munication systems. Wideband code-divisionmultiple access (WCDMA) systems reflect thistrend by providing efficient support for packet-switched data services with data rates up to 384kb/s for wide area coverage and even 2 Mb/s inhot spot areas. Thus, with WCDMA Internetaccess will be possible with data rates one to twoorders of magnitude higher than with previouslydeployed cellular systems. However, it is impor-tant to understand how Internet applications incombination with their underlying transport pro-tocols make use of such “bit pipes.”

The dominating transport protocol in theInternet is the Transmission Control Protocol(TCP). Many important applications like filetransfer, the Web, and e-mail use its services.Since TCP is a generic protocol designed for sta-bility in heterogeneous environments, it is notspecifically tailored to wireless networks (andthis would contradict to the underlyingparadigms of the Internet). However, it is knownthat TCP sometimes does not perform well inspecific scenarios.

For example, inadequately designed wirelesslinks could introduce non-congestion-relatedpacket losses due to transmission errors over theair, which would heavily interfere with TCP’scongestion control. The performance woulddegrade dramatically [1].

Another example for a scenario where TCPdoes not perform well is satellite communica-tions. In this case, large round-trip times due topropagation delays typically result in large band-width delay products. Especially in these envi-ronments, the TCP slow start leads to significantunderutilization of the link provided. Link asym-metry is another characteristic of satellite com-munications that might cause problems.Therefore, a lot of research effort was dedicatedto this scenario during the past years, summa-rized in [2, 3].

For both above-mentioned scenarios, TCPproxies have been considered to improve per-formance (e.g., [1, 4, 5]). Typically, a proxy islocated at a suitable node along the connectionpath to isolate the problematic link. For exam-ple, for a cellular network this could be in thecore network of the system to separate the wire-less link.

Previous studies [6] have identified thatWCDMA radio bearers supporting high datarates are characterized by large bandwidth delayproducts of up to around 20 kbytes. Althoughthis is not as large as for many satellite links, itmotivates our study of how a TCP proxy wouldimprove performance for WCDMA.

The main contribution of this article con-sists of detailed analysis of the specific perfor-mance-related effects caused by theintroduction of a TCP split connection proxy ina WCDMA network. Although many effects

MICHAEL MEYER AND JOACHIM SACHS, ERICSSON RESEARCHMARKUS HOLZKE, T-MOBILE

ABSTRACTThis article addresses the end-to-end perfor-

mance of TCP in a scenario where WCDMA isused as the access link. In particular, the per-formance gain that can be achieved by placing aTCP split connection proxy in the WCDMAcore network is examined. It is well known thatperformance enhancing proxies are able toimprove the performance of TCP over wirelesslinks that suffer from impairments. However,while previous work on TCP proxies for wire-less systems either focused on other wirelesssystems like wireless LAN or satellites, or pro-vided a more generic framework, this articleaddresses in detail the characteristics of aWCDMA access scenario supported by a TCPproxy. The characteristics of WCDMA as per-ceived by TCP are thoroughly discussed in thisarticle. We argue that the motivation for intro-ducing a proxy is only to overcome problemsstemming from a large bandwidth delay productand not to assist local transport layer errorrecovery at the wireless link. Based on simula-tions that consider both link layer protocols andTCP, the end-to-end performance for file down-loads is investigated. Simulation results showthat a proxy can significantly improve perfor-mance in the case of high data rates like 384kb/s. For lower data rates like 64 and 128 kb/s,it is sufficient to use a well configured TCPimplementation.

PERFORMANCE EVALUATION OF ATCP PROXY IN WCDMA NETWORKS

Previous studies haveidentified that WCDMAradio bearers thatsupport high datarates are characterizedby large bandwidthdelay products of upto 20 kbytes.Though this is not aslarge as for manysatellite links, itmotivates a study ofhow a TCP proxywould impact theperformance forWCDMA.

Markus Holzke was withEricsson Research at thetime the work described inthis article was done.

IEEE Wireless Communications • October 2003 71

are in principle already known [6, 7], this arti-cle goes beyond previously published work inproviding a very detailed description of howWCDMA protocols and TCP interact. Also, forthe first time quantitative results on the impactof a split connection TCP proxy in theWCDMA context are presented, giving guid-ance to when the use of such a TCP proxy isrecommended. Finally, the article highlightsthat the motivation for the use of a TCP proxyfor WCDMA is primarily a large bandwidthdelay product of such links, not transmissionerrors over the wireless link. This conclusion isopposed to previous results (e.g., [1, 5]), wherethe main motivation for a TCP proxy has beento speed up error recovery from transmissionerrors over the wireless link.

The article is organized as follows. The nextsection presents the considered scenario ofmobile Internet access via WCDMA anddescribes its specific characteristics. It starts witha summary of the relevant details for WCDMAand TCP followed by considerations of TCP in aWCDMA context. This section is intentionallywritten in tutorial style, to assist those readerswho are familiar with either WCDMA or TCP/IPprotocols, but not necessarily with the interac-tion between them. More details of TCP proxiesare given. We devote a section to the perfor-mance analysis of a TCP proxy in the consideredenvironment. Simulation results are presentedthat demonstrate the effects which occur withand without a TCP proxy. Different parametersets are studied to identify those parameters thatinfluence performance most. Finally, our conclu-sions are summarized.

MOBILE INTERNET ACCESS VIA WCDMAINTRODUCTION TO WCDMA

The subjective performance impression of Inter-net access in a wired environment differs in gen-eral significantly from wireless access. The onlyexceptions are probably wireless LAN (WLAN)systems, because those trade coverage and sup-port of mobility for throughput and delay. In a

broad sense differences are due to either largepropagation delays (e.g., for satellites) or meth-ods that achieve fairly reliable transmission butintroduce delays (e.g., for cellular systems).These methods are necessary to ensure reason-able reliability over difficult radio channels,which might suffer from fading and shadowing.Examples of such techniques are diversity con-cepts, forward error correction (FEC) in combi-nation with interleaving, or link layer automaticrepeat request (ARQ). In particular, interleavingand ARQ cause additional delays on top of thepropagation delay.

In the following the main fundamental princi-ples for data transmission using dedicatedWCDMA radio bearers shall be described as faras necessary for the subsequent sections. Thisoverview on the complex mesh of techniquesused at the physical and link layer will show howWCDMA supports wireless Internet access. Fora more detailed description see [8].

Figure 1 shows the WCDMA network archi-tecture, which is separated into the radio accessnetwork (RAN) and the core network (CN). TheRAN comprises radio network controllers(RNCs) and Node Bs, which transmit/receive theradio signal to/from the mobile terminals (oruser equipment, UE). The CN provides thebackbone for the radio network and the connec-tivity toward external networks (e.g., the publicInternet or corporate intranets).

Figure 2 depicts the protocol stack for the

� Figure 1. WCDMA network architecture.

RAN

UE

Core network

Node B RNC3G

servingnode

3Ggateway

node

InternetPSTNISDN

� Figure 2. WCDMA protocol stack for the user plane.

UE RAN

PHY

MAC

RLC

PDCP

PHY

MAC

RLC

PDCP

L1

IPUDP

ATMAAL5

GTP-U

IP

TCP

Application

Applicationserver

L1

IP

TCP

Application

Relay

3G serving node

L1

GTP-U

L1

L2

IPLDP

IPUDP

ATMAAL5

GTP-URelay

3G gateway node

L1

GTP-U

L1

L2 L2

L2

IPUDP

IP IP

IEEE Wireless Communications • October 200372

user plane in WCDMA plus the stack in theapplication server in the Internet whileneglecting intermediate routers in the Inter-net. As is shown later, the radio interfacedominates the traffic characteristics. Thus, thesubsequent description focuses on this part ofthe network.

Basically, WCDMA provides two types ofchannels [9]: dedicated channels exclusively usedby one user and common channels sharedbetween users. Throughout this article, only ded-icated channels are considered.

Four different protocols are involved in trans-mission over the air interface (Fig. 2):• Packet data convergence protocol (PDCP)• Radio link control protocol (RLC)• Medium access control (MAC)• Physical layer (PHY)

On the RAN side, the PHY functionality ismainly located in a Node B; only soft handovercombining is done in the RNC. The layer 2 pro-tocols MAC, RLC, and PDCP are all terminatedin the RNC.

The lowest layer, PHY, contains, besides allradio frequency (RF) functionality, the signalprocessing including Rake receiver, channel esti-mation, power control, FEC, interleaving, andrate matching. The important components forthis study are the interleaver and deinterleaver,since they contribute to the delay. The WCDMAstandard foresees interleaving depths of 10, 20,40, and 80 ms, allowing a trade-off betweendelay and error correction capability. Intuitivelya smaller interleaving depth leads to shorterround-trip time over the radio link and provideslower latency.

The MAC layer is responsible for control-ling the amount of data sent according to nego-tiated minimum and maximum data ratesduring radio bearer setup. The instantaneousdata rate is determined for every transmissiontime interval (TTI), where the TTI correspondsto the interleaving depths mentioned above.The MAC layer requests the amount of databuffered at the RLC layer, which is ready fortransmission. Based on available radioresources and the amount of data to transmit,the MAC layer submits the appropriate num-ber of RLC blocks to the PHY layer. If a useruses only one service (as is assumed through-out this article) no scheduling is required atthe MAC layer.

More important for this study is the RLClayer, because of its link layer ARQ functionali-ty, which is typically required in a cellular sys-tem. Besides the acknowledged mode (AM),the protocol also supports a transparent modeand an unacknowledged mode, which do notinclude ARQ functionality. The appropriateRLC mode is chosen based on the require-ments of the application. Since TCP perfor-mance is sensitive to packet loss due totransmission errors, it is natural to use RLCAM for TCP traffic [10, 11].

RLC segments incoming packets into smallerunits of, say, 320 bits, which is a reasonableblock size for typically achieved bit error rates.The appropriate block size results from a trade-off between header overhead (16 b/block) andretransmission overhead, since a radio block is

the RLC retransmission unit. Larger radio blockswould result in a higher block error rate for agiven bit error rate, while smaller blocks wouldimply an increased header overhead. A blocksize of 320 bits allows operation in a block errorrate (BLER) range between almost zero and afew tens percent, which can be regarded as agood trade-off between FEC and ARQ, andmaximizes the throughput from a link layer per-spective. Depending on the data rate, one ormore RLC blocks can be transmitted within oneTTI. For example, for 384 kb/s 12 blocks aretransmitted every 10 ms.

The RLC protocol in WCDMA consists of acomplex toolbox, which provides the means tooptimize the protocol for either delay orthroughput, or a trade-off between them. Basi-cally, the protocol uses a selective repeat requeststrategy, where the acknowledgments (called sta-tus messages) are triggered by either poll flagssent by the data sender or the detection of erro-neous blocks at the receiver, or based on period-ic events. A status report contains the blocksequence number up to which all blocks arereceived correctly and a list of those that havenot been received correctly. When the RLCsender receives a status message it responds withretransmission of the indicated blocks. Theseretransmissions have higher priority than blocksthat have not yet been transmitted. This ensurestimely delivery.

Since the standard foresees that on recep-tion of each status message the requestedretransmissions should be sent, it is very impor-tant for protocol performance to control thenumber of status messages. One way of doingthis is to use the so-called status prohibit timeron the receiver side and to configure this timerwith a value slightly larger than the RLC round-trip time (RTT). This ensures that only one sta-tus report is sent during one RLC RTT, so aretransmission of a particular block is triggeredonly once. If the status prohibit timer were notused or the timer value was smaller than theRLC RTT, unnecessary retransmissions mightbe triggered. This could be appropriate fordelay-critical applications that still require reli-able service. However, in general this wouldhave a negative impact on throughput andresource efficiency.

In summary, we can state that the WCDMARLC protocol is a configurable link layer proto-col that can be tuned to the needs of transportprotocols and applications. Attention needs tobe paid to the RLC mode, the configuration ofradio block sizes and block error rates, and thestrategy on when to report transmission errors.From a transport layer perspective, it is impor-tant to note that the RLC can be configured toprovide in-order delivery of data and reasonablereliability due to link layer ARQ.

The fourth protocol involved in the transmis-sion of IP packets is the PDCP. This protocol ismainly responsible for header compression. It isnot considered further.

TRANSMISSION CONTROL PROTOCOL (TCP)TCP is the most commonly used transport proto-col in the TCP/IP protocol suite for non-real-time data transmission. In this section the main

The WCDMA RLCprotocol is aconfigurable linklayer protocol, whichcan be tuned to theneeds of transportprotocols andapplications. Attentionneeds to be paid tothe RLC mode, theconfiguration of radioblock sizes and blockerror rates and thestrategy on when toreport transmissionerrors.


concepts of TCP shall be introduced as requiredfor our study.

TCP sends data segments according to asend window, and the receiver acknowledgesthe receipt of each segment based on a cumu-lative acknowledgment strategy. Thus,although data might be sent only in one direc-tion, it is important for TCP to also have rea-sonable performance for the acknowledgment(ACK) path. The amount of sent data is con-trolled by the size of the send window, whichlimits the amount of data the sender may haveoutstanding at any point in time. Effectively,this means that during one TCP RTT one win-dow of data can be sent. To ensure reliabletransport, TCP comprises ARQ functionality.Lost packets are retransmitted by TCP, whenthe loss is detected.

Beyond providing a reliable transport on topof an unreliable IP network, TCP includes func-tionality to react to congestion [12] in the Inter-net. Hereby, congestion control and TCP’s ARQare heavily intertwined, since TCP uses packetlosses as congestion signals. TCP is able to detectpacket losses with two mechanisms:

First, on receipt of so-called duplicate ACKs,which acknowledge the same packet as before,the sender can derive that something wentwrong. These duplicate ACKs are generated atthe receiver when not the expected segment butanother segment arrived out of order. This canbe the case when a packet got dropped or whenpackets were reordered. After the receipt of thethird duplicate ACK the sender retransmits thesegment indicated as lost. This procedure iscalled fast retransmit. In addition, after the errorrecovery is completed, the congestion window isdecreased to 50 percent of the value before thepacket loss has been detected. TCP concludesthat the packet loss occurred due to congestionand slows down.

The second possibility to detect a packet lossis based on a retransmission timer. This timer isstarted with an adaptive timeout value that iscalculated based on previously measured RTTs.The timer expires if no ACK has been receivedin the meantime. After a timeout the oldest stillunacknowledged packet is retransmitted. Evenmore drastic than in fast retransmit is the adap-tation of the congestion window. It is reducedto one segment and TCP slows down signifi-cantly.

TCP also comprises two mechanisms toincrease the window in congestion-free periods.The first is called slow start, which is used afterthe connection setup (initial slow start) or aftera timeout (reduction of the window to one seg-ment). During slow start, TCP is allowed toincrease the window by one segment for eachreceived ACK. This leads to an exponentialincrease until the first packet loss is detectedor the maximum negotiated window size isreached. The initial slow start and the slowstart after a timeout differ in their window size.Although for the initial slow start TCP may usetwo segments [12] (and it is proposed to useeven up to four [13]), after a timeout TCPalways begins the slow start with one segment.The second algorithm to increase the windowis called congestion avoidance. During this

phase, occurring after a fast retransmit, thewindow is increased by roughly one segmentper RTT. In congestion avoidance state TCP iscautiously probing for a higher data rate untilultimately the next packet is lost (or the filetransfer is over).

TCP IN THE CONTEXT OF WCDMAThe optimal link from a TCP perspective oper-ates at high speed and low delay while not intro-ducing non-congestion-related packet losses orpacket reordering. In the following we reviewhow WCDMA meets these requirements.

WCDMA provides for data rates of up to 384kb/s for wide area coverage. Compared to ananalog modem or ISDN dialup, this data rate ishigh, while it is still low compared to digital sub-scriber line (xDSL). In addition, 384 kb/s is a bigstep forward from data rates used in second-gen-eration cellular systems.

The requirement of avoiding packet losses[14] over the air has been considered thoroughlyduring the standardization of the RLC protocol.In AM this protocol can be configured in such away that a single radio block can be retransmit-ted up to N times. Dimensioning N properlyensures that in most cases RLC provides a reli-able link and the ARQ mechanism of TCP is nottriggered. If, after all, a packet is lost, TCP hasto recover from this situation.

Another important issue in this context is theavoidance of spurious TCP time-outs. Thismeans that pre-mature expiration of the TCPretransmission timer needs to be avoided. Thus,RLC retransmissions should be performed muchfaster compared to the retransmission timeoutvalue of TCP. Typically, this is the case while itis hard to avoid pathological cases completely.

Packet reordering is also a critical issue forTCP, since out-of-order segments trigger dupli-cate acknowledgments. If reordering of threesegments occurs, a retransmission is initiated,which first wastes network resources and secondreduces TCP’s send rate by decreasing the TCPcongestion window by 50 percent. The RLC con-figuration does allow in-order and out-of-orderdelivery, but obviously only the in-order deliveryis a reasonable choice for TCP traffic.

Finally, the delay introduced by WCDMA isconsidered. Cellular systems are known for theirsignificant share in the overall RTT, since thearchitecture (e.g., soft handover support) mayintroduce delays and complex mechanisms, andalgorithms must be used to achieve high datarates like 384 kb/s in wide area networks. Assum-ing that the RLC RTT is 80 ms, this can resultin a TCP RTT of 400–500 ms. This is partly dueto RLC retransmissions, because the AM is arequirement of TCP, as discussed above. Still,the amount of required retransmissions is adesign choice, but an air interface configurationwith a very low BLER (e.g., 0.1 percent) wouldbe too costly from a radio resource perspective.Realistic BLERs for general-purpose traffic arebetween a few percent and a few tens percent.

For a TCP performance evaluation, the TCPRTT is an important measure. It can be shownthat the TCP RTT can be written as [10]

RTTTCP = n* RTTRLC + C,

The amountof required

retransmissionsis a design choice,but an air interfaceconfiguration witha very low BLER

(e.g., 0.1 percent)would be too costly

from a radio resourceperspective. Realistic

BLERs for general-purpose traffic are

between a fewpercent and a few

tens percent.


where n depends mostly on the average BLER,and C gives the delay contribution of the CNand the Internet. With a BLER of 10 percent, nis roughly 3, assuming independently and identi-cally distributed block errors, which representsthe worst case. Correlated block errors wouldgive lower values of n.

To summarize, WCDMA meets the require-ments of TCP regarding high data rates, packetlosses, and packet reordering. However, physicalconstraints of cellular networks result in a signif-icant delay of a few hundred milliseconds, whichis not favorable for TCP but very hard toimprove.

The next aspect under consideration is thebandwidth delay product of a TCP connectionusing WCDMA, because this describes the gen-eral TCP behavior. If the bandwidth delay prod-uct is small, this typically means that TCP is ableto fully utilize the pipe, and the slow start behav-ior causes no problems. For high bandwidthdelay links, the slow start might take a fewround-trips, during which the link is underuti-lized, resulting in poor performance [15].

Depending on the assigned data rate,WCDMA offers links with small to large band-width delay products. For simplicity, the workingassumption of a TCP RTT of 500 ms is used forthe following considerations. For 64 kb/s thiswould result in a bandwidth delay product of 4kbytes, but for 384 kb/s it would be 24 kbytes.The latter value is large enough to cause a linkunderutilization for three to six round-tripsdepending on the maximum segment size (MSS)and the initial window size of TCP used for aparticular TCP connection.

Figure 3 visualizes the effect of an increasingbandwidth delay product. It shows the cumula-tive distribution for the resulting bit rate for filedownloads of 50 kbytes for different configura-tions. As the performance measure the packetbit rate (PBR) is used, which is defined by thefile size divided by the time required to transferthe file. The results were achieved with a simula-tor that includes a detailed implementation of

the RAN protocols and TCP Reno. More detailsabout the simulator can be found later.

The results show that the link utilization ishigh (>95 percent) for a 64 kb/s1 radio bearer.When the link speed increases the utilizationdecreases. For 384 kb/s the median data rate isonly 180 kb/s, resulting in link utilization of 54percent. The low link utilization is due to thefact that it takes several round-trips until fulllink utilization is reached. In addition, small filesare already downloaded before full utilizationhas been achieved.

In addition, Fig. 3 confirms the well-knownfact that a large MSS improves throughput [2,6]. A small maximum segment size of 512 bytesresults in a median data rate of 73 kb/s for 128kb/s. An MSS of 1460 bytes gives a median datarate of 97 kb/s. Thus, a large MSS is highly rec-ommended for WCDMA.

The discussion in this section has shown thatthere is significant room for improvement con-cerning the TCP performance over WCDMA.As mentioned above, TCP proxies are well-known means to overcome performance prob-lems.

TCP PERFORMANCE ENHANCING PROXY

INTRODUCTION

Recently, [7] has been published by the Inter-net Engineering Task Force (IETF). The docu-ment discusses various aspects aboutperformance enhancing proxies (PEPs). Thefollowing discussion uses [7] as the referencedocument and reviews the main points for aWCDMA scenario.

PEPs are introduced when the performancesuffers due to characteristics of a link or subnet-work. Basically they are placed at the problem-atic link and attempt to mitigate theperformance problems, which could be due totransmission errors, link outages, or large band-width delay products. While application layerPEPs are also widely used, this article focusessolely on transport layer PEPs. Different typesof TCP PEPs exist. One has to distinguishbetween integrated and distributed PEPs. Anintegrated PEP is implemented in a single node,while a distributed PEP is typically located atboth ends of the problematic link. An importantsubclass of PEPs are split connection proxies.This means a TCP connection from one end sys-tem is terminated at the proxy and another con-nection originates there. For WCDMA (Fig. 4),a TCP connection from an Internet server is ter-minated in the proxy located in the CN, and asecond TCP connection is used toward themobile client.

This is the configuration considered in theremainder of this article. In other words, weinvestigate an integrated split connection PEPlocated in the CN. Obviously, the aim of a PEPis to improve TCP performance. Local knowl-edge in the proxy about the state of a TCP con-nection can be used to enhance the performanceby shortcutting the transmission of ACKs orretransmissions. For example, in wireless net-works, which suffer from a large number ofpacket losses due to transmission errors (e.g.,

� Figure 3. Cumulative distribution of data rates for 50 kbyte file downloads,MSS = 1460 bytes for 64, 128, and 384 kb/s, and MSS = 512 bytes for 128kb/s.

Packet bit rate (kb/s)

200

P (≤

PBR

)

0.25

0

0.5

0.75

1

15010050

64 kb/s

128 kb/s

384 kb/s

512 bytes

1460 bytes

0 250

1 Note that the net datarate available for theapplication is lower thanthe nominal link data ratedue to TCP/IP headeroverhead and link layerretransmissions. Linkrates of 64 kb/s and 384kb/s are fully utilized atPBRs of 56 kb/s and 336kb/s, respectively.


without appropriate link layer ARQ), retrans-missions can be performed locally from the TCPproxy instead of relying on the end host. Such amechanism can speed up error recovery signifi-cantly and avoids interactions with TCP’s con-gestion control.

Especially for connections with large band-width delay products, a technique called localTCP acknowledgments is very useful. How localACKs are applied is depicted in the schematicpresentation below.

Figure 5 shows the difference in the informa-tion flow for the two cases with and without PEPsupporting local ACKs. While in case a, withoutPEP the server has to wait for the response fromthe client resulting in a poor slow start perfor-mance, the PEP speeds up the data transfer incase b. The PEP effectively implements pipelin-ing for the two TCP connections. On the con-nection toward the client the PEP sends the firstsegment, and in parallel requests already newdata from the server.

Finally, an important issue shall be highlight-ed. The end-to-end use of the security mecha-nism IPSec is mutually exclusive to the use of aproxy [7]. Thus, one has to decide between bet-ter performance and end-to-end security. Analternative would be to use security mechanismson top of transport protocols.

TCP PROXY FOR WCDMABesides general proxy issues, [7] also discussesthe case for wireless wide area networks(WWANs). There, cellular networks are char-acterized by very low bandwidth having, say,GSM in mind. As shown in the previous sec-tion, WCDMA is different in this sense, sincedata rates could be at least an order of magni-tude higher. Large bandwidth delay productsare not mentioned as a problem for WWANin [7]. Otherwise, the network characteriza-tion in [7] applies as well for WCDMA. Itintroduces significant latencies and due totransmission errors and ARQ, delays are largeand variable.

Since the characterization of the networksdiffers partly from [7], here also the motiva-tion for a PEP is different. In the present sce-nario a split connection proxy divides thebandwidth delay product into two parts, result-ing in two TCPs with smaller bandwidth delayproducts and thus operating at higher speeds.Therefore, such a proxy is investigated in thisarticle.

In addition, most previously publishedpapers (e.g. [1], [5]) are aiming at improvingthe performance over wireless links by tacklingproblems introduced by packet losses due toair interface transmission errors. Here, i tneeds to be clearly stated that this problemdoes exist for WCDMA only to a very smallextent, since the link layer has been carefullydesigned. Packet losses due to transmissionerrors occur mostly in such situations wherethe coverage is so poor that it is hard to main-tain the service anyway. Otherwise, the RLCtakes care of a reliable delivery of the data.Note that this is not the case for wireless sys-tems where the link layer ARQ is to a smallerextent reliable [16].

PERFORMANCE EVALUATION

THE SIMULATION ENVIRONMENTThe end-to-end performance of a TCP proxyhas been evaluated by means of an event-drivensimulator. The model assumes that an applica-tion transfers files on request from a server inthe Internet to a mobile client. A TCP Renomodel is used as transport protocol, which isconfigured for a mobile environment accordingto the recommendations in [6]. The TCP win-dow buffer space is dimensioned to 32 kbyteswhich is sufficient to not limit the performance.All radio protocols have been implemented indetail as described earlier, because the air inter-face mainly determines the performance. The

� Figure 4. WCDMA network architecture with proxy.

RAN

UE

Core network

Node B ProxyRNC3G

servingnode

3Ggateway

node

InternetPSTNISDN

� Figure 5. Schematic TCP transfer with and without PEP.

a)

Client

Data

ACK

Server

b)

Client

DataData

ACKLocal

LocalACK

ServerPEP

� Table 1. Simulation parameters.

Parameter Setting

Transferred file size 50 kbytes

TCP maximum segment size 1460 bytes

TCP initial window Three segments

TCP initial window in proxy 3, 5, 10 segments

Packet loss rate (Internet) 1%

Delay of Internet and UMTS CN 70 ms (one way)

RLC payload size 320 bits

RLC RTT 80 ms

RLC status prohibit timer 90 ms

Transmission time interval (TTI) 10 ms

Data rates on air link 64, 128, 384 kb/s

BLER on radio link 10%


Internet and CN have been simplified to add aconstant delay to transmitted IP packets. Inaddition, independent packet losses are consid-ered in the Internet. For the radio link it isassumed that a dedicated physical channel isused. It is characterized by applying fast powercontrol to compensate channel variations causedby, for example, shadowing and multipath fad-ing. The power control is assumed to work per-fectly, resulting in a constant block error rate onthe radio link. Block errors are assumed to beindependently distributed. Corresponding to thediscussion above, for the applied configurationnot a single packet loss occurs due to wirelesstransmission errors. Unless mentioned other-wise, the simulation parameters are as listed inTable 1.

AN EXAMPLE OF IMPROVEMENTSACHIEVED BY A PROXY

The gain of a TCP proxy shall be shown bymeans of a TCP trace for a 128 kb/s radio bear-er. In order to demonstrate the effect of a proxy,a nonoptimal TCP configuration has been cho-

sen which emphasizes the gain of the proxy. ATCP MSS of 512 bytes is used, and TCP in boththe server and the proxy apply an initial windowsize of only one segment. For the following per-formance analysis an optimized TCP is used.

In Fig. 6 the sequence numbers of TCP seg-ments received at the client and proxy are depict-ed over time. This is compared to a trace whereno proxy is used. In addition, the available linkrate offered by WCDMA indicates the upperlimit that can be achieved. The link rate can besimply approximated by reducing the radio bear-er rate by the overhead introduced by TCP/IPheaders2 and block errors on the radio link,leading to 128 kb/s * (512 bytes/552 bytes) * 0.9≈ 107 kb/s.

In this example, the transfer time of a 50kbyte file is reduced by 21 percent when using aproxy. Two effects cause this gain. First, theeffects of the TCP slow start, which lasts up tothe transition from the exponential increase ofreceived data to a linear increase, is decreasedfrom around 1.8 s to approximately 1 s (Fig. 6).During this time, the WCDMA radio bearer isnot fully utilized. Second, the recovery from twosegments lost in the Internet (sequence numbers24576 and 28160) slows down the transfer forroughly 500 ms. When a proxy is used, the lossrecovery is initiated by the proxy after 1.2 s and1.5 s, which means that by the time these seg-ments are to be transmitted over the radio linkthey have already been retransmitted over theInternet to the proxy. Both effects, slow startand loss recovery, result in situations where thetransmission rate of the TCP sender is lowerthan the link rate. Since a TCP proxy reducesthe RTT perceived by the sender, high link uti-lization can be achieved more quickly with asplit connection.

PROXY CONFIGURATIONIt shall now be analyzed how a TCP proxyshould be configured. In the previous example,it has been demonstrated that a proxy can accel-erate the slow start, but still the radio link is notalways fully utilized. To improve this, the TCPproxy is configured with an initial window sizelarger than the two segments allowed by [4] orthe three segments proposed by [13]. Thisapproach is motivated by the observation thatthe RTT for the TCP connection traversing theInternet is typically smaller than the RTT forthe connection over the air interface. Thismeans that during a conventional slow start dataarrive faster at the proxy than they can be senttoward the client, because the congestion win-dow of the proxy is limiting. An increased initialwindow enables the proxy to forward segmentswithout waiting for ACKs from the client (com-pare Fig. 5).

In shared networks an initial window largerthan three would make the network more vul-nerable to congestion collapse and should not beused. This consideration can be neglected for aTCP proxy facing a radio bearer for whichresources have already been allocated at setup;in this case more aggressive configurations of theinitial window are acceptable than for sharednetworks.

Figure 7 compares, for different radio bear-

� Figure 6. Received TCP segments at proxy and client.

Time (s)

5

7300

Seq

uenc

e nu

mb

er (

byt

es)

0

14,600

21,900

29,200

36,500

43,800

643

Availa

ble n

et d

ata r

ate:

107

kb/s

21%

21

Received TCP segments at:Client (without proxy)Client (with proxy)Proxy

� Figure 7. Median PBR for different proxy configurations.

Without proxy

64 kb/s

128 kb/s

384 kb/s

Initial window at proxy

10

50

Med

ian

PBR

(kb

/s)

100

150

200

250

300

053

2 TCP/IP header com-pression is not considered,although its use is recom-mended.


er rates, the median PBR achieved with andwithout proxy. For the proxy different initialwindows have been investigated. For a TCPconfiguration according to Table 1, a proxyprovides a small gain for link rates of 64 and128 kb/s. Also, a larger initial window of up to10 segments in the proxy only marginallyimproves the result for these data rates. How-ever, at a rate of 384 kb/s, a proxy with an ini-tial window of three and 10 segments achievesa gain of 28 percent and 52 percent, respective-ly. This demonstrates that a proxy is particular-ly useful for radio links with high data rates,since link utilization is especially improved forthose because of their large bandwidth delayproduct.

INTERNET LOSS AND DELAYThe quality of Internet connections is generallynot constant, since it reflects the dynamics ofnetwork traffic, and the conditions of routersand transmission links. Therefore, the qualityof an Internet connection, expressed in termsof delay and loss rate, depends on the time ofday, as well as on the geographic path acrossthe network. Monitoring of many Internet con-nections showed that typical loss rates in theInternet are in the order of 0–3 percent withworst cases around 10 percent [17]. Similarly,the one-way delay perceived by packets travers-ing the Internet is on the order of 25 ms forgood links and 1 s in very poor conditions.Correspondingly, it is worthwhile to study theimpact of a TCP proxy for different packet lossand delay scenarios. Accordingly, these param-eters have been varied in the simulations pre-sented below.

Figure 8 shows cumulative distributionfunctions for a scenario with and withoutproxy, respectively, describing the probabilityfor a 50 kbyte file transfer to perceive a PBRlower than or equal to the one given on thehorizontal axis. This is depicted for differentloss rates of the Internet link, where an accessvia a radio link of 128 kb/s is assumed. With-out any loss in the Internet, the largest part offile transfers experience a PBR in the range of95–108 kb/s with proxy and 90–104 kb/s with-out proxy. The variance depends on the blockerror events and the error recovery of theRLC protocol.

Comparing Fig. 8a and b, two positive effectsof the proxy can be identified. First, the proxyimproves robustness against packet losses in gen-eral. This can be seen by comparing the medianvalues. With a proxy the median PBR is above102 kb/s for losses up to 3 percent, compared to90 kb/s without proxy. For a loss rate of 10 per-cent, the proxy increases the median PBR from36 kb/s to almost 80 kb/s. Second, the proxyreduces the variance in the distributions, as canbe seen by the steeper slope for the distributionswith proxy. Both effects show that the proxy iscapable of shielding the TCP connection overthe WCDMA link from packet losses in theInternet.

A different result can be found when theInternet delay between the server and the 3Gcore network is significantly larger. Figure 9shows the median PBR for different link rates

dependent on varying Internet delays. Althougha proxy can achieve a performance gain, perfor-mance degrades similarly with and without proxyif the delay in the Internet increases. It is inter-esting to note that at very large delays of 1 s, file

� Figure 8. PBR for downloads at a data rate of 128 kb/s and different packetloss rates in the Internet: a) without proxy; b) with proxy.


100

0.1

P (≤

PBR

)

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

a)

110 1209080706050

10%

3%

1%

0%

403020100


100

0.1

P (≤

PBR

)

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

b)

110 1209080706050

10%

3%

1% 0%

403020100

� Figure 9. Varying delay in the Internet.

Packet delay in the Internet (ms)

1000

50

Med

ian

PBR

(kb

/s)

100

150

200

250

300

08006004002000 1200

384 kb/s (without proxy)384 kb/s (with proxy)128kb/s (without proxy)128 kb/s (with proxy)64 kb/s (without proxy)64 kb/s (with proxy)


transfer performance is almost independent ofthe data rate of the radio link. In other words, inthis case the Internet is the limiting part of theend-to-end path.

FILE SIZESSo far the proxy has only been discussed in thecontext of a file transfer, where a file of 50kbytes has been transmitted via a TCP connec-tion. Now the influence of file size is investi-gated. At the same time this gives anindication of how a proxy would influenceother types of applications (e.g., Web surfing).Compared to file transfer, in Web surfing usu-ally small objects are transmitted from theserver to a client.

In Fig. 10 the median PBR is presented fordifferent sizes of files being transmitted. It isquite obvious that for very small fi le sizes(smaller than the initial window of TCP), noperformance gain can be achieved with a proxy.For larger file sizes, a proxy gives a perfor-mance gain. The major benefit of a proxy atrates of 64 and 128 kb/s is that it reduces thetime spent in slow start. Since the proportion of

slow start as part of the total transmission timedecreases for increasing file sizes, so does thegain achieved by a proxy. But for a data rate of384 kb/s, the gain of the proxy increases furtherfor larger file sizes. In this case, the link is notonly underutilized during slow start but alsoafter packet losses that are recovered by a fastretransmit. This is due to the reduction of thecongestion window by 50 percent for each lossevent. Since for large files fast retransmit andcongestion avoidance become more and moreimportant, a proxy provides a constant gain incase of large bandwidth delay products. Inother words, a proxy helps for smaller datarates mainly during slow start, but for higherrates also after packet losses.

RADIO BLOCK ERROR RATESFor the model of the radio link, it has beenassumed that perfect power control allows con-tinuous provision of the desired BLER. The tar-get BLER is generally set in order to optimizethe radio capacity of the cellular network. Thiscan be achieved for values on the order of 10percent [18]. However, with realistic power con-trol the achieved BLER value will fluctuatearound the target value.

In Fig. 11 the median PBR is depicted fordifferent BLER values of 1, 5, and 10 percent,and a data rate of 128 kb/s. Naturally, the PBRincreases for lower values of BLER, since asmaller part of the radio link rate is required forretransmissions. At the same time, a lowerBLER has to be achieved at the price ofincreased transmission power. It can be seenthat the gain of a proxy remains on the sameorder for different BLER values. Therefore,realistic power control would add some addition-al fluctuations on top of the results obtainedwith constant BLER without changing the con-clusions.

CONCLUSIONS

In this article we investigate how a TCP splitconnection proxy affects the performance of filedownloads over WCDMA access links. Byreviewing the characteristics of the WCDMAradio protocols it is shown that such a systemmeets the requirements of TCP: no non-conges-tion-related packet losses, no packet reordering,and high data rates. However, it introduces unfa-vorable delays. These delays are due to a rela-tively large link layer RTT and link layerretransmissions required for radio resource effi-ciency reasons. It is shown that the specificdelays and high data rates result in a large band-width delay product of up to 20 kbytes. This hasmotivated our study of a split connectionapproach to improve the TCP performance.

Different from previous work, the introduc-tion of the TCP proxy does not aim to improveperformance due to packet losses induced bywireless transmission errors, which are rare inWCDMA due to proper design of the link layer.The proxy focuses solely on tackling the largebandwidth delay products of such links and aimsto improve wireless link utilization. In addition,it is proposed to use an extra large initial win-dow for TCP.

� Figure 10. Median PBR at different rates for varying file sizes.

File size (bytes)

384 kb/s

128 kb/s

64 kb/s

100,000

50

Med

ian

PBR

(kb

/s)

100

150

200

250

300

350

501,000,00010,0000

: with proxy: without proxy

� Figure 11. Performance at different radio block error rates for a 128 kb/sbearer.

BLER on the radio link (%)

10

85

Med

ian

PBR

(kb

/s)

80

90

95

100

105

110

115

120

125

130

5

With proxy

Without proxy

1


The presented results were achieved by usinga simulator that includes a detailed WCDMAradio protocol and TCP Reno implementation.

The simulation results show that for mediumdata rates such as 64 kb/s and 128 kb/s, a splitconnection proxy provides only small perfor-mance improvements, but for 384 kb/s significantgains are possible. Investigating medium-sizedfiles (50 kbytes) highlights that the proxyimproves slow start performance. Especially anextra large (> 3 segments) initial window in theTCP proxy toward the wireless link provides asignificant gain over default window sizes.

In addition, simulation results also revealedthe positive impact of the PEP in downloadinglarge files in congested Internet, because theproxy shields Internet packet losses from theTCP control loop running over the wireless link.

REFERENCES[1] H. Balakrishnan et al., “Improving TCP/IP Performance

over Wireless Networks,” 1st ACM Conf. Mobile Com-mun. and Networking, Berkeley, CA, Nov. 1995.

[2] M. Allman, D. Glover, an d L. Sanchez, “Enhancing TCPOver Satellite Channels Using Standard Mechanisms,”RFC 2488, Jan. 1999.

[3] M. Allman et al., “Ongoing TCP Research Related toSatellites,” RFC 2760, Feb. 2000.

[4] T.R. Henderson and R. H. Katz, “Transport Protocols forInternet-Compatible Satellite Networks,” IEEE JSAC, vol.17, no. 2, Feb. 1999, pp. 326–44.

[5] A. Bakre and B. R. Badrinath, “I-TCP: Indirect TCP forMobile Hosts,” 15th Int’l. Conf. Distrib. Comp. Sys.,May 1995.

[6] H. Inamura et al., “TCP over 2.5G and 3G Wireless Net-works,” RFC 3481, Feb. 2003.

[7] J. Border et al., “Performance Enhancing Proxies Intend-ed to Mitigate Link-Related Degradations,” RFC 3135,June 2001.

[8] E. Dahlman et al., “WCDMA - The Radio Interface forFuture Mobile Multimedia Communications,” IEEE Trans.Vehic. Tech., vol. 47, no. 4, Nov. 1998, pp. 1105–18.

[9] 3GPP Tech. Spec. 25.301 v. 4.0.0, “Radio Interface Pro-tocol Architecture (Rel. 4),” Mar. 2001.

[10] J. Peisa and M. Meyer, “Analytical Model for TCP FileTransfer over UMTS,” 3G Wireless 2001, San Francisco,CA, May 30–June 2, 2001, pp. 42–47.

[11] R. Ludwig, “Eliminating Inefficient Cross-Layer Interac-tions in Wireless Networking,” Dissertation, RWTHAachen, Germany, 2000.

[12] M. Allman, V. Paxson, and W. Stevens, “TCP Conges-tion Control,” RFC 2581, Apr. 1999.

[13] M. Allman, S. Floyd, and C. Partridge, “IncreasingTCP’s Initial Window,” RFC 2414, Sept. 1998.

[14] P. Karn et al., “Advice for Internet Subnetwork Design-ers,” Internet draft, draft-ietf-pilc-link-design-13.txt,work in progress, Feb. 2003.

[15] J. Sachs and M. Meyer, “Mobile Internet — Perfor-mance Issues Beyond the Radio Interface,” 10th AachenSymp. Signal Theory, Sept. 2001, pp. 163–68.

[16] F. Khan et al., “TCP Performance over CDMA2000RLP,” VTC 2000-Spring, May 2000, pp. 41–45.

[17] Matrix.net, “Internet Average” and “Internet Ratings,”http://www.matrix.net, Jan.–May 2001.

[18] C. Johansson, “Packet Data Capacity in a WidebandCDMA System,” VTC98, Ottawa, 1998, pp. 1878–83.

BIOGRAPHIESMICHAEL MEYER ([email protected]) received hisdiploma and doctoral degree in electrical engineering fromthe University of Paderborn in 1991 and 1996, respectively.In 1996 he joined Ericsson Research in Aachen. Currentlyhe holds the position of a senior specialist for link andtransport layer interactions and is leading a performanceanalysis team. His research interests include the perfor-mance analysis of radio link layer protocols for WCDMAand GPRS/EGPRS, and their interactions with Internet pro-tocols.

JOACHIM SACHS [M] ([email protected]) studiedelectrical and electronics engineering at Aachen University(RWTH), Germany, at ENSEEIHT, France, and at the Univer-sity of Strathclyde, Scotland. He received his Diplom-Ing.degree from Aachen University in 1997. In 1997 he joinedthe Mobile Multimedia Networks group of EricssonResearch in Aachen. He has led a project to evaluate theWCDMA radio protocol performance and is currently lead-ing a project on the evolution of wireless networks. Hisresearch interests include performance analysis of wirelesssystems for Internet services.

MARKUS HOLZKE ([email protected]) studied elec-trical and electronics engineering at Aachen University,Germany. He received his Diploma degree from AachenUniversity in 2001. In 2001 he joined the Department forNetwork Fault and Service Management Systems of T-Mobile Deutschland. Currently, he is in the position ofexpert in network fault management and alarm correlation.

The simulation resultsshow that for

medium data ratessuch as 64 kb/s and

128 kb/s, a splitconnection proxy

provides only smallperformance

improvements,but for 384 kb/s

significant gainsare possible.

performance evaluation of a tcp proxy in wcdma networks

Documents