SPECTROSCOPY OF THE PACKET RADIO INTERFACESA CASE STUDY WITH GSM/GPRS

Jorma KilpiVTT Information Technology

P.O. Box 1202, FIN 02044 VTT, FinlandE-mail: Jorma.Kilpi@vtt.fi

ABSTRACT

In this paper we apply some ideas presented in [1] of network spectroscopy to the analysis of a packetradio interface. We analyze the IP packet interarrival time distribution of upstream traffic, taking intoaccount also the packet sizes in the case where the IP packet trace is measured after the Time DivisionMultiple Access (TDMA) based radio interface of General Packet Radio Service (GPRS). When theLogical Link Control (LLC) layer works in unacknowledged mode and the Radio Link Control (RLC)layer works in acknowledged mode, we show that using the ideas of spectroscopy it is possible to someextent to detect the behaviour of the TDMA based radio interface of GPRS from the IP level packet data.More precisely, it is possible to extract the number of Packet Data Channels (PDCHs) and ChannelCoding Scheme used for uplink traffic of a single mobile station (MS). Physical connections betweenthe MS and the Base Station Subsystem (BSS), called Temporary Block Flows (TBFs), can be extractedalso, and it is possible to indirectly detect some retransmissions of radio blocks within TBF. The generalmethod is not restricted to TDMA and should give good results also for other packet radio interfaceslike Code Division Multiple Access (CDMA) based packet radio interfaces or Carrier Sense MultipleAccess (CSMA) based WLANs.

1 Introduction

This study was motivated by the ideas of Broido & al.presented in paper [1]. They defined network spec-troscopy as a branch of Internet science that deals withobject identification on the basis of delay, period andfrequency spectra. Packet delay analysis is one of theapplication areas of network spectroscopy. In this pa-per we shall apply the ideas of [1], especially the use ofthe Radon transform, to the analysis of the TDMA-basedpacket radio interface of GPRS.

As a case study we will analyze the IP packet interar-rival time distribution taking into account also the packetsizes in case when the IP packet trace was measured afterthe GSM/GPRS radio interface. The LLC layer workedin unacknowledged mode and the RLC layer worked inacknowledged mode. In that case, the interarrival timedistribution contains information about the performanceof the radio interface. We combine the technical knowl-edge of how the corresponding radio interface and thoseinterfaces closely related to it work with the statisticalanalysis of the data trace. In this way we get a better un-derstanding of those factors that are important in analysisand modelling of the radio interface. We show examplesfrom upstream traffic of GPRS sessions. We also presentthe main ideas of the data analysis procedure along withthe case study. However, the general idea and methodis not in any sense limited to TDMA and should be use-

ful also for analyses of WCDMA or CSMA based packetradio interfaces.

As one concrete application of our approach onecan consider a system which continuously monitors up-stream traffic inside a GPRS backbone network. Theuser IP packet is tunneled inside the GPRS backbone. Itshould be possible to trace backwards the mobile routingof the tunnel packet, which carries the user IP packet aspayload, and identify the cell where the Mobile Station(MS) was when the packet was sent. This requires onlycombining information from the Mobility Management(MM). Such a system could, for example, at given timeinstances select, according to some criteria or just ran-domly, some sessions or upstream flows, search for thesigns or indicators of problems in the radio interface, orinterfaces just after it, and report or even alarm in somecases.

Hence, using only a few monitoring positions, itshould be possible to see some performance character-istics of all active uplink radio interfaces of the wholenetwork. It is also reasonable to expect that the perfor-mance problems of uplink and downlink are often cor-related, even in the cases when the uplink and downlinkshould work independently, as with GPRS. Of course,problems of the radio interface are more likely due to ex-ternal factors or the MS than to the network. But, forexample, if one cell continuously shows worse perfor-mance than others, this would eventually show up. As

such it could be a complementary tool for a radio inter-face protocol analyzer, a physical device, which has to beclose to the base station in order to capture and monitorthe traffic of the radio interface.

2 Case studyThis case study is based on examples from the data froma GPRS traffic measurement trace which was capturedfrom the monitoring port of a firewall router between theGPRS backbone network and the Internet access point.The measurement was made in May 2002. See [2] forfurther details of the measurement and a general descrip-tion of the data. See also [3] for a similar but much moredetailed GPRS traffic measurement and [4] for a discus-sion of some measurement based performance issues re-lated with GPRS.

The study is done per session, or sessionwise. In thedata analysis, an Internet session over GPRS was definedby the temporary IP address given to the user in the acti-vation phase of an IP context. These temporary addresseswere delivered in a round robin manner and, since the in-tensity of session arrivals was rather low during the mea-surement time, we can be certain that all traffic from thesame temporary IP address came from the same GPRSMS. Unlike in [3] we were not able to measure the PacketData Protocol (PDP) contexts since, due to technical andprivacy issues, this would have been too demanding withthe resources available. Moreover, since in our case thePDP is always IP, we will talk about IP context insteadof PDP context.

The 3rd Generation Partnership Project (3GPP) isresponsible for the technical specifications of GPRS,downloadable at ETSIs homepage [5]. At the time of themeasurement only the Release 97 version of the specifi-cations can be expected to have been filled.

The emphasis is on the uplink direction, which is thesame as the upstream direction for an Internet connec-tion. The phrase mobile-originated is often used in the3GPP specifications. The upstream data was thus mea-sured after the radio interface, but not immediately after.

2.1 The Um, Abis and Gb interfaces ofGSM/GPRS

The basic idea of our data analysis procedure is not verycomplicated. However, it requires some quite detailedknowledge of the Um, Abis and Gb interfaces and forthis reason we recall and point out briefly those proper-ties of GPRS that are important to know in this contextand/or are needed later in this paper, so that readers notvery familiar with GPRS could more easily understandour data analysis procedure in Section 2.2. Readers al-ready very familiar with GPRS can just check the formu-las (1), (2) and (3) of this section before going directly tothe section 2.2.

Table 1 recalls the definitions and durations of the time

frame structure used in TDMA with GPRS, see the tech-nical specification GSM 05.01 from [5].

Definition Duration(ms)

Time Slot (TS) Basic unit

TDMA Frame 8 TSs

Multiframe 52 TDMA Frames 240

Superframe 8 multiframes 1920

Table 1: The TDMA time frame structure.

Every 13th TDMA frame inside a multiframe is re-served for other purposes than data transfer; they are ei-ther used for synchronizing the MS with the base station,or for a MS to monitoring for a better base station withbetter signal to interference ratio. The time slots of theTDMA frame available for GPRS use are called packetdata channels (PDCHs). The multislot functionality ofGPRS allows a MS to be allocated more than one PDCHsimultaneously. PDCHs are dynamically and temporar-ily allocated to a MS for the transmission or reception ofdata.

Figure 1 shows a part of the transmission plane ofGSM/GPRS. The network elements of the transmis-sion plane that a packet sees on the way up are calledthe Mobile Station (MS), the Base Station Subsystem(BSS), the Serving GPRS Support Node (SGSN) andthe Gateway GPRS Support Node (GGSN). The proto-col layers are Subnetwork Dependent Convergence Pro-tocol (SNDCP), Logical Link Control (LLC), Radio LinkControl (RLC), Media Access Control (MAC), and theTDMA based GSM Radio Frequency (RF). As the nameindicates, one role of SNDCP is to make the underlyingprotocols independent of the higher layer protocols: TCP(or UDP) and IP are not part of GPRS protocol stack.They are present in figure 1 since they are the applica-tions that are used in the data that we have.

The transmission plane in figure 1 is not complete,since the BSS consists of one Base Station Controller(BSC) and several Base Transceiver Stations (BTS). Theinterface between a BTS and the BSC is called Abis, seefigure 3.

At the MS, more precisely at the users GPRS handset,an IP packet sent upstream is divided into SNDCP seg-ments, LLC frames and RLC radio blocks. When thereshould not be any danger of confusion, we will simplytalk about segments, frames and blocks. See figure 1 asmnemonic. One block is transmitted by the RF layer asfour bursts of bits where one burst of bits is the physicalcontent of one time slot. One block is always transmit-ted within one PDCH and the delay of one block at theUm interface is ! #" . After the uplinkUm and Gb interfaces the IP packet is reassembled from

Network Elements, Protocol Layers and Interfaces of the Transmission Plane

MS BSS SGSN GGSN

Um Gb Gn Gi

RF RF

MAC

RLC

MAC

RLC BSSGP BSSGP

FR FR

LLC LLC

SNDCP SNDCP GTP GTP

IP IP

TCP, UDP

Frames

Blocks

Packets

Bursts

Segments

Figure 1: Transmission plane of GPRS. The up- anddownlink directions work independently.

bursts, blocks, frames and possible segments. At the net-work side this reassembly is not done at a single device;each block is reassembled from four bursts of bits at theBTS and forwarded to the BSC. There, frames are re-assembled from blocks and relayed to the SGSN, wheresegments are reassembled from frames. A segment mayconsist of one or more IP packets of the same MS. TheSNDCP can optimize channel efficiency by multiplexingseveral small packets into one segment. It also dividestoo large packets into several segments with the aim thatone segment fits within one frame. If the segment con-sists of one packet only, then the packet is in practice re-assembled already at the BSC, but the SGSN is the placewhere the packet is completely reassembled after the Umand Gb interfaces. The GGSN is the first place whereany action due to the IP packet header is done.

During the time of our measurement there was at leastone GPRS mobile phone available in the Finnish mar-ket that could use 2 PDCHs in the uplink radio inter-face. Moreover, this particular model had been availableat least a few months. On the other hand, it is unlikelythat any phone could have used more than 2 PDCHs inthe uplink.

A physical connection between a MS and a BSS iscalled a Temporary Block Flow (TBF). The TBF is a uni-directional concept and consists of the allocation of oneor more PDCHs and the number of blocks to be sent orreceived. The allocation can be fixed or dynamic; by dy-namic allocation, the number of blocks to be transmittedis not fixed beforehand. The TBFs are the main object ofour study.

The high Bit Error Rate (BER) of radio inter-faces makes it necessary to use error correcting codes.Technical specifications introduce four channel codingschemes, but only two of them, called CS-1 and CS-2,were implemented at the network side during the time ofour measurement. CS-1 has a better error correcting ca-pability and thus a larger coverage, but less user payloadbits at each block, and thus it offers a lower bit rate to theuser. Channel codings are used at the Um interface.

Given a packet of size bytes, the exact number ofbits of the packet at one block depends on the channel

coding scheme, the number of framing bits of the LLClayer, and whether the framed segment consists of one ormore packets. It may vary a little. Let be a parameterwith value in case of CS-1 and incase of CS-2. These are approximate numbers of userbits within one block. We need to calculate the numberof blocks that one packet of size bytes most typically isassumed to generate, , and use the followingsimple formula: (see also figure 2)

Size of one LLC frame in bitsBits in one block

(1)

Here "!#%$'&)(+*,(-.0/ and is just the sizeof the packet, the payload of an LLC frame, in bits. Thevalue 40 is explained next.

Even the use of error correcting codes does not guar-antee that the blocks (and frames) are correctly receivedover the Um interface. To improve the reliability of datatransfer, both the RLC and/or LLC layers may work inacknowledged mode; blocks (frames) are sent within awindow and are periodically acknowledged using a se-lective acknowledgement method allowing retransmis-sions of erroneously received blocks (frames). When thedata used in this paper were measured, the RLC layerworked in acknowledged mode but the LLC layer didnot. It means that no reordering mechanism of the LLClayer Packet Data Units (PDUs) was provided. Onlytransmission and format errors of PDUs were detected,and duplicate Unconfirmed Information (UI) frames thatcarried PDUs were discarded. The LLC layer PDU isthe same thing as a SNDCP segment, hence typically thesame thing as one IP packet or, sometimes, two smallpackets multiplexed into one segment.

The smallest size of the UI frame without payload is5 octets, 40 bits, see 3GPP TS 04.64 from [5]. We haveused this value as framing bits in formula (1). Luckily,the formula (1) and our analysis are not very sensitiveto the exact value of framing bits; an error of 2-3 octetswould not be even visible in figure 2.

0 200 400 600 800 1000 1200 1400Packet Size (B)

0102030405060

Nbl

ock

Nblock(B,CS)

CS-1

CS-2

Figure 2: Approximate number of blocks given thepacket size.

Let the parameter be the number of PDCHstemporarily allocated for the TBF; in the upstream casewe assume it takes only the values 1 or 2. The packetdelay of the Um interface is then approximately

"

(2)

The SGSN needs to know the location of the MS be-fore any radio resource allocation can exist and data betransmitted or received. The network functions that takecare of the locating of the MS are called Mobility Man-agement (MM). The MM states are called idle, stand-byand ready states. After IP context activation, and if nodata is transmitted, the MS is in stand-by state. When theMS needs to transmit (or receive) data, it first changesfrom stand-by state to ready state, and only then sometime slots from the Um interface are reserved. After re-leasing the radio resources both in the uplink and down-link there is a timer that changes the MS back to thestand-by state unless a new allocation is requested. In[3] it was observed that the MS usually stayed in readystate several seconds after the data transfer was ended.This information is important in our study since it givesus some realistic upper bound to the packet interarrivaldelay in the case when we have to decide whether twosuccessive packets have been transmitted within the sameTBF or not.

During the time of the measurement, GSM voice callshad strict priority over GPRS when the time slots fromthe TDMA frame were allocated. If there was contentionof available GPRS resources between several MSs, traf-fic from different MSs was multiplexed into availablePDCHs. This resource sharing, if it has occured, hasprobably been fair between users and not based on dif-ferent QoS profiles.

The BTS contains the GSM RF layer and the Chan-nel Codec Unit (CCU) which, in the case of an up-link block, decodes the channel coding scheme. Theblock is then transmitted over the Abis interface to theBSC, where the Packet Control Unit (PCU) differenti-ates GPRS blocks from GSM voice blocks; look at Fig-ure 3. The RLC/MAC protocol layer is implemented inthe PCU. It turned out to be important in this study thatthe transfer rate of a single block at the Abis interface hasalways been 16 kb/s and the PCU frame, which carriesone decoded block, is always of size 320 bits, makingthe transfer delay 20 ms. PCU frames are also sent withperiods of 20 ms. In this way, all 12 blocks that can betransmitted within one multiframe of one PDCH get theirown PCU frame, and the total time, 240 ms, remains thesame. These values do not depend on the number of userbits at the block, i.e. on the channel coding scheme used.

The formula we use to calculate the packet delay ofthe Abis interface is

! "

(3)

This is because one block is always transmitted within

Base Station Subsystem (BSS)

AbisUm Gb

BTS BSC

RF

CCU

PCU

RLCMAC

PCUFrame

320 b

Block

456 b

FRFrame

Figure 3: The Abis interface between BTS and BSC.

one PDCH, and each of the PDCHs has its own 16 kb/schannel at the Abis interface. Compare with formula (2).

The transfer protocol of the Gb interface of figure 1can be Frame Relay (FR), ATM, or even FR over ATM.FR offers the bit rates 64 kb/s, 128 kb/s, 256 kb/s, 384kb/s, 512 kb/s and 1984 kb/s. We do not know a pri-ori the upstream (or downstream) transfer rate in the Gbinterface. Since it depends on how many BTSs are con-nected to one BSC, it need not be the same for differentphysical Gb interfaces. We will see that the upstreamtransfer rate can sometimes be inferred from the session-wise data. In general we assume that one LLC frame istransmitted within one FR frame, see figure 3.

2.2 Data analysis procedureOur measurement was thus done from a fast interfacewhich, however, was not placed right after the slow Um,Abis and Gb interfaces, see figure 1. After the Gi in-terface and before the true Internet access there may be asmall operators service network, which in the case of ourmeasurement contained at least the firewall router and aNetwork Address Translation (NAT) box.

The accuracy of the time stamps is not as crucial asin [1]; if they are accurate enough in the fast interface,then they are accurate also for the modified idea that wewill present. The drift of the time stamping clock wasafterwards estimated to be approximately 19 ppm, andbefore the analysis all time stamps were corrected bymultiplying them with the factor (1-19 ppm). In addi-tion to this, there were other drifts in the time stampsthat were constant for different sessions. The origin ofthese other drifts is uncertain but, if not due to the mea-surement setup, they may be due to different SGSNs orthe distance between the BSS and the SGSN, which canin Finland probably be hundreds of kilometers.

Figure 4 below describes our basic idea. In Figure 4the time refers to the time at the SGSN when the IPpacket is completely reassembled and the time refersto the measured time stamp at the fast interface. Themeasured time stamp is also associated at the end ofthe packet since the Berkeley Packet Filter [6] on whichtcpdump is based does packet time stamping at the end

di* = di*s + di*r

ti* ti+1*ti ti+1di

di*

di*r di*s

Time

Bi Bi+1

Figure 4: The grey boxes refer to the length of thepacket at the slow Um interface.

of the packet.We define and . One

of the basic assumptions is that the size dependent dis-tortion defined by is small. Distortioncan be written as , whichshows that it depends on the sizes of the packets and

. The maximum increase occurs, when and bytes. The maximum decrease occurs,when ! and " bytes. Figure 5 showsthis maximum distortion as a function of the bit rate. Weassume that the interface Gn between SGSN and GGSNhas to be at least of multiplexing hierachy E3, and theconclusion is that the size dependent distortion shouldbe less than ms. We assume now that anddo not distinguish and any more in the notation.

10 20 30 40 50Rate (Mb/s)

-10

-5

0

5

10

Dm

ax

(ms)

Maximum distortion

Bi > Bi+1

Bi < Bi+1

Figure 5: Maximum distortions as a function of constantbit rate.

A more serious problem than distortion is the buffer-ing that occurs in the packets path before and at the mea-surement interface. However, at the BTS, there shouldnot be any buffering of blocks: erroneous blocks neednot be stored, and correctly received blocks are relayedimmediately to the BSC. At the BSC, there must be somebuffering capability of frames and blocks since, due toRLC layer retransmissions, the blocks may not come inthe right order to the BSC. Hence the BSC must also havereordering capabilities of blocks which may also induce

some packet size dependent delays. The GGSN acts asa gateway between mobile packet routing and fixed IProuting of the Internet. The SGSN and the GGSN are as-sumed to have buffering capabilities like a router. Buffer-ing of packets at the measurement equipment before thetime stamp is attached to the packet may also be a prob-lem, see for example [7].

Luckily, during the time when our data was measured,the intensity of GPRS traffic was so low that the bufferingof upstream traffic was about minimal at the network de-vices. Also, the buffering at the measurement equipmentwas about minimal, even though this item was not opti-mized when the measurement was planned and made. Insection 3 we will discuss about planning of the optimalmeasurement scenarios in the case of GSM/GPRS.

The data that we will use for analysis consist of pairs

(

(4)made from the measured values , ( , ofupstream traffic of a single session. See figure 4 for theexplanation why is associated to and figure 7 fora data example of the -plane. We will analyze theempirical probabilities

(

"! #%$

(5)

Writing the packet interarrival delay as &' (") infigure 4 we interpret*

+' as the size dependent or deterministic part, and*

") as the residual or random partof the interarrival delay . Our interest is on the caseswhere is small, otherwise ' could be ignored. Then is typically quantized to some discrete set of possiblevalues.

Examples of single sessions are most interesting ifthey contain both a large number of upstream packetsand packets of many different sizes. Table 2 showsglobal characteristics of a few chosen examples. Someof them are used in figures hereafter.

Figure 6 shows the packet interarrival time histogramof Example 2. The 20 millisecond quantization due tothe Abis interface is rather exact. While one packet hasalways required at least one TBF, our interest is in longTBFs where at least two packets have been transmittedwithin one TBF. We restrict our main interest to values of smaller than 480 ms, the duration of two multiframes,since then we can be almost certain that and have been transmitted within the same TBF. Figure 7shows the ,, -plane (4) of Example 2.

The analysis continues like in [1] by taking a suitableRadon transform of the empirical probability ,, ,defined in (5), for the simultaneous detection of the trans-fer rate of the Gb interface and of the parameters and

. (See formulae (1), (2) and (3)). The Radontransform in our case is defined as.-

#/ 10

32

1/

"

054

Packet Duration Volume Mean Bit % of TCPCount (kB) Rate (kb/s) Packets

Example 1 1155 19 min. 1 sec. 111.5 0.8 94.8%Example 2 2259 65 min. 3 sec. 290.7 0.6 9.5%Example 3 3360 10 min. 0 sec. 270.1 3.7 99.1%Example 4 2525 29 min. 7 sec. 156.2 0.7 92.4%Example 5 5529 29 min. 2 sec. 215.6 1.0 99.2%Example 6 2808 11 min. 5 sec. 152.9 1.8 98.6%

Table 2: Upstream traffic characteristics of some sessions.

0 480 960 1440 1920Interarrival Delay (ms)

1

10

100

1000

Num

bero

fPac

kets

Example 2, Bin Size 1 ms

Multiframe

Superframe

Figure 6: Histogram of packet interarrival delay of Ex-ample 2.

40 200 576 1024 1500Packet Size (B)

480

960

1440

1920

Inte

rarri

valD

elay

(ms)

Example 2

Figure 7: The ,, -plane of example 2.

The term / of above stands for the residual delay. Theterm 0 is our simplified model of the packet delayof the Gb interface assuming that it has transfer rate 0b/ms, i.e. kb/s. Note that the use of a formula like 0assumes a flow of bits with constant bit rate 0 , which ingeneral is far from reality. It works in our case sincethe physical Gb interface was not congested due to othersimultaneous sessions at the time. Since we do not knowthe effect of distortion accurately enough we round thesize dependent term

#"

0

of - #/ 10 to whole milliseconds (ms).Like in [1] we detect the value of the (deterministic)

parameter 0 by looking at the minimum of Shannonsentropy

0

)

-

/ 0

-

/ 0

(6)

Here the sum is taken over all those values of / suchthat - / 0 . The heuristics of the minimum en-tropy is that the best explanation reduces the randomnessmost. Figure 8 shows that, from all four realistic alterna-tives for the values of the parameters and ,the one corresponding to CS-2 and 2 PDCHs is the mostpromising one. Indeed, figure 9 shows that with CS-2and 2 PDCHs the minimum is achieved with 385 kb/s,which is 384 kb/s plus some error due to inaccuracies ofour formulas, distortion and measurement setup. Notealso that the value 384 kb/s is not contradictory to theassumption that the distortion can be neglected since thedelay of the Gb interface was not included when we ar-gued that due to figure 5.

0 500 1000 1500 2000v (kb/s)

4.4

4.6

4.8

5

5.2

H(v)

Four Cases

CS-2 and 2 PDCHs

Figure 8: From all 4 different alternatives, CS-2 with 2PDCHs is the most promising one for Example 2.

Next we describe what could be done if the accuracyof the time stamps would be high enough and the effectof distortion could be either ignored or corrected. As-sume now that we have already identified the values of and . Namely, had there been congestion atthe BSC or at the Gb interface, we could have used theRadon transform more like in [1] and, instead of lookingat the transfer rate 0 of Gb interface, look for the time that one LLC frame requires at the Gb interface:

.-

#/

32

/

! #"

4

300 350 400 450 500v (kb/s)

3.9

4

4.1

4.2

4.3

H(v)

Example 2

Figure 9: The value of 0 that minimizes 0 is 385kb/s.

and choose which minimizes . This entropy-minimizing could then be interpreted as the equilib-rium time of one LLC frame at the Gb interface. Recallfrom figure 3 that, at the BSC, one LLC frame is put in-side the FR frame for the Gb interface. If we know thetransfer rate of the physical Gb interface and the size ofthe FR frame, then could be used to detect whether theBSC or the Gb interface was congested or not. This is be-cause we could compare to the ideal case with no con-gestion. But, as already mentioned, this would requiremore accurate time stamps and more precise knowledgeof the possible distortion so that we could control the er-rors.

After removing ' from we get the residual values")

"' . Figures 10 and 11 show two data examplesof the distribution of the residual delay +) . Compare fig-ure 10 with figure 6. The first block of each multiframeof one of the downlink PDCHs is reserved to signallingby default. If no other downlink signalling is used, itexplains partially the gap between 240 ms and 480 ms.

0 480 960 1440 1920Residual Delay (ms)

1

10

100

1000

Num

bero

fPac

kets

Example 2, Bin Size 1 ms

Figure 10: Histogram of residual delays.

2.3 Application: statistical inference aboutlong TBFs

Figures 12 and 13 show two examples of the residual -plane, i.e. the plot ") , ( ,

0 480 960 1440 1920Residual Delay (ms)

1

10

100

1000

Num

bero

fPac

kets

Example 1, Bin Size 1 ms

Figure 11: The overall shape is similar with Examples 1and 2.

and raises some natural questions: How do we explainnegative values? How do we explain horizontal posi-tioning? We already know that the 20 ms vertical shiftbetween horizontal layers is due to the Abis interface.

200 400 600 800 100012001400Packet Size (B)

-200

-100

0

100

200

Res

idua

lDel

ay(m

s)

Example 2, Residual (B,d)-Plane

Figure 12: The residual -plane of Example 2.

200 400 600 800 100012001400Packet Size (B)

-200

-100

0

100

200

Res

idua

lDel

ay(m

s)

Example 3, Residual (B,d)-Plane

Figure 13: The residual plane of Example 3.

One block can carry bits of two different IP packetswhen the SNDCP segment consists of more than onepacket. In this case none of these packets can be for-warded before all the blocks that carry the segment ar-rive to the BSC and the corresponding SNDCP layer seg-ment arrives to the SGSN, and when they are forwarded,

the measured packet interarrival time can be almost zero.Assume, for example, that two packets are multiplexedinto one segment. In this case the residual delay of thefirst packet is the same as its size dependent delay, butnegative: ") +' . The size dependent delay of thelatter packet is the sum of the size dependent delays ofboth packets. Figures 6 and 7 contain also examples ofthis phenomenon. The definition of the residual delay isthus actually more complicated than in the previous sec-tion. In some other sessions there are interarrival times assmall as 1 " since the nominal precision of time stampswas six decimals. This seems to be quite a common phe-nomenon. In the upstream case it is a property of thecorresponding MS.

Due to block retransmissions, and when the LLC layeris not in acknowledged mode, the rationale of multi-plexing many packets into one segment is more thanquestionable since it may delay several packets simul-taneously and significantly. The performance of GPRSwould probably be improved if blocks of such a multi-plexed segment would be sent always using CS-1, i.e.maximizing reliability, like in the case of signallingblocks.

Anyhow, several IP packets within one LLC frame donot explain all of the negative values of the residual de-lays. Consider the case where three packets, , and ,are sent from an MS in this order and within one TBF inone PDCH. For simplicity we assume first that one blockcarries bits of one IP packet only. The packet is car-ried by three blocks whereas packets and are eachcarried by two blocks. Assume that the second blockcarrying bits of user packet , denoted by in figure14, is not received correctly at the BSS and a retransmis-sion is done as soon as possible when the MS receives anegative acknowledgement. At the BSC the LLC frames,

Block Retransmission

TimeA1 A2 A3 B1 B2 A2C1 C2

ErroneousBlock

SuccessfulRetransmission

Temporary Block Flow (TBF)t1 t2 t3

Figure 14: A theoretical example of a single block re-transmission that occurs within the same TBF that usesone PDCH.

which in this case can be thought of as the IP packets, arereassembled and relayed to the SGSN and the GGSN inthe order B, A and C. The data that would be measuredin this case, , and , give two pairs

and . The interarrivaldelay is the time of two blocks, and is the time of

one block. In both cases, the value of the residual delayis the time of one block but negative!

Hence, by looking at the RLC retransmissions that oc-cur in the same TBF we see that the value of the inter-arrival delay can be as small as the time of 1 blockwhich, due to the Abis interface, is always 20 ms, andhence the residual delay +) can be negative. A negativevalue may thus indicate that there has been at least oneerroneously received block which carried bits of someearlier IP packet and thus required at least one retrans-mission. Positive values may in general be due to sig-nalling blocks, block retransmissions, multiplexing ofother TBFs into the same PDCHs, or other more or lesstractable reasons.

Since the LLC layer was not in acknowledged modethe order of packets may change. Unfortunately, the IPidentifier field was not saved during the measurement ofthe data. With that information we could have checkedthe order in which packets were originally transmittedfrom the MS.

In our theoretical example we can infer that the lengthof the TBF that carried packets , and has beenat least the duration of 8 blocks (8 20 ms = 160 ms)instead of the optimal duration of 7 blocks (140 ms). Inreal data we can also make some statistical conclusionsof how the bit rate that, for example, a TCP connectionsees is fluctuating. Note that we can recognize only thoseTBFs that have successfully carried packets.

Moreover, we can reconstruct the TBF flow of our the-oretical example up to some level: we know the positionsof blocks , and and we know that and were before , was before and was before . If we could know the original order in which thepackets were transmitted from the MS, we could recon-struct our theoretical TBF to even higher level. The ob-served long TBFs could also be reconstructed up to asurprisingly high level. If the QoS profiles are taken inuse this might make it easier for the operator to detectthe possible QoS level that the user gets for the upstreamtraffic.

3 Optimal measurementsThe data we used in this paper was not optimal in thesense that it was originally measured for a different pur-pose, for traffic characterization. The quality of timestamps was good enough to show examples of how thedata analysis procedure should be done. Unfortunately,this quality was not good enough to go further on. Thenonavailable IP identifier field is one example of infor-mation which would have been extremely useful in ourapproach. After having done some preliminary studieswith the available data it is easy to plan optimal measure-ments in the case of GSM/GPRS, when only RLC/MAClayer works in acknowledged mode. It is clear that ifthe LLC layer works in acknowledged mode, the perfor-mance of the RLC layer cannot be visible any more.

3.1 Passive measurementIn order to see what kind of service the user IP packetsget we must measure at the point where the IP packetis completely reassembled. The optimal measurementfor the GPRS analysis shown here should be made fromthe SGSN or from the Gn interface between SGSN andGGSN, see figure 1 at Section 2.1. At the Gn inter-face the traffic is tunneled by GPRS Tunneling Protocol(GTP), 3GPP TS 09.60. Some freely accessible proto-col analyzing programs like Ethereal, www.ethereal.org,can dump the GTP. The trace should have accurate timestamps and, at least, the user IP packet size, GTP packetsize and IP Identification field from the user IP packetheader. The transfer rate and the transfer media of themonitoring interface, the accuracy of time stamps andthe drift of the time stamping clock should be known orverified beforehand.

The GTP header contains a tunnel identifier (TID)which points out the MM and IP contexts. The inter-esting result of [3] about user mobility during the datatransfer was based on the MM information which is, andwhich has to be, quite detailed when the MS is in readystate. Hence this type of a passive measurement togetherwith some simple and fast TBF analysis procedure couldbe extended to a monitoring system described in the in-troduction. The main advantage of this approach is thatfrom only a few monitoring positions it should be possi-ble to see some upstream performance characteristics ofall active BSSs in the whole network.

3.2 Active measurementInstead of choosing randomly large sessions or flows atthe monitoring system described in the previous subsec-tion, one could capture traffic from a fixed MS with self-generated traffic. In this way one could easily arrangevariability in packet sizes by either going through allpacket sizes systematically or selecting the packet sizerandomly at the MS. Then one could also control a lit-tle the time gap between two successively transmittedpackets. Such a measurement should give enough infor-mation for calibrating and testing the passive measure-ment or monitoring system scenarios. It could also givesome evidence whether the assumption that the bad per-formance periods in uplink and downlink are often cor-related is true or not.

This type of an active measurement is also one of thefirst steps in extending the ideas of the present paper tothe study of WCDMA and WLANs.

4 Conclusions and further workIn this paper we have analyzed sessionwise IP packet in-terarrival time distributions taking into account packetsizes in the case where the IP packet trace was measuredafter the GSM/GPRS packet radio interface. In our anal-ysis it was crucial that only the RLC layer worked in ac-

knowledged mode. In this specific case we have shownthat it is possible to some extent to detect the behavior ofthe Gb, Abis and TDMA based Um interface of GPRSfrom the MS originated IP level packet data with accu-rate time stamps. More precisely, it is possible to detectthe number of PDCHs and channel coding scheme used,extract long TBFs and detect retransmissions of blockswithin a long TBF. We do not know yet the best wayto make informative statistics about TBFs, which is thusone item for further study. We also described a methodwhich could be used to see whether the upstream Gb in-terface was congested or not. Although we do not yethave any examples, it should also be possible to detectwhether multiplexing of users at the Um interface hasbeen the case. Of course, these are statistical results andvalid only with some probability but we believe that thisprobability is rather high.

Moreover, increasing knowledge of the correspond-ing packet radio interface and further development of thestatistical tools will certainly increase the probability ofcorrect identification of the above mentioned phenom-ena. They will also allow to extend the scope of spec-troscopy to other packet radio interfaces. A natural con-cept for further work is to study whether a similar ap-proach, namely the analysis of the empirical distribution

,, , yields interesting results on the WCDMA basedUMTS and the CSMA based WLANs.

Acknowledgement. The author is grateful to AndreBroido from CAIDA for his advice and support all theway from the very beginning, when the first ideas hadjust came up to the authors mind, up to careful readingand commenting of the draft. Thanks to Research Pro-fessor Pertti Raatikainen from VTT for patiently answer-ing all my very elementary questions about multiplex-ing hierarchies and teaching me how to make intelligentguesstimates about them. Thanks to Research ProfessorIlkka Norros from VTT for a careful reading of the finaldraft.

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