sms over gprs

SMS over GPRS

A Comparison Between GSM and GPRS Architectures as

Carriers for SMS and Between SMS and Other Protocols

as Carriers of Short Messages over GPRS 1

Charlotta Baath, Joanna Kuhn,

23rd April 2003

1Report for the course 2G1330Wireless and Mobile Network Architectures givenby professor Gerald Q. Maguire Jr. at the Department of Microelectronics andInformation Technology (IMIT), Royal Institute of Technology (KTH), Sweden.

Abstract

The short message service (SMS) has gained great popularity over the lastseveral years. SMS is a protocol and architecture integrated in the GSMnetwork. In this report we make comparisons between the GSM and GPRSarchitectures as carriers of SMS. SMS is also compared with other protocolsfor sending short text messages. We show that SMS is a useful protocoleven as we switch to GPRS, since it does not exchange as many packetsas other protocols when delivering a message, thereby being considerablyfaster over a radio network such as GSM/GPRS. Other ways of sendingshort text messages, such as email and instant messaging, are on the otherhand cheaper (despite the bigger load on the network). Therefore we predictthat the prices for SMS will drop. We also foretell that new protocols willbe developed, suitable not only for the Internet, but for networks with ahigh packet cost (measured in time).

Contents

1 Introduction 11.1 Problem Statement . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 GSM 32.1 GSM Architecture . . . . . . . . . . . . . . . . . . . . . . . . 32.2 GSM Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2.1 Physical Channels . . . . . . . . . . . . . . . . . . . . 42.2.2 Logical Channels . . . . . . . . . . . . . . . . . . . . . 42.2.3 Data Rate . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 SMS 63.1 SMS Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 SMS Features and Usage . . . . . . . . . . . . . . . . . . . . . 63.3 SMS Delivery Procedure . . . . . . . . . . . . . . . . . . . . . 73.4 SMS Protocol Hierarchy . . . . . . . . . . . . . . . . . . . . . 73.5 SMS over GSM . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.5.1 Data Rate . . . . . . . . . . . . . . . . . . . . . . . . . 123.5.2 Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.5.3 Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 GPRS 134.1 GPRS Architecture . . . . . . . . . . . . . . . . . . . . . . . . 134.2 GPRS Channels . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2.1 Data Rate . . . . . . . . . . . . . . . . . . . . . . . . . 144.3 SMS over GPRS . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3.1 Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3.2 Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.4 Email over GPRS . . . . . . . . . . . . . . . . . . . . . . . . . 154.4.1 Sending Email Using SMTP . . . . . . . . . . . . . . . 164.4.2 Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.4.3 Price and Cost . . . . . . . . . . . . . . . . . . . . . . 18

4.5 Instant Messaging over GPRS . . . . . . . . . . . . . . . . . . 18

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5 Conclusions 205.1 SMS is Here To Stay . . . . . . . . . . . . . . . . . . . . . . . 205.2 Prices Will Drop For SMS . . . . . . . . . . . . . . . . . . . . 205.3 Operator Independent Alternatives To SMS . . . . . . . . . . 205.4 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . 21

A Mail Sessions over GPRS 25A.1 A SMTP Session . . . . . . . . . . . . . . . . . . . . . . . . . 25

A.1.1 Tcpdump output . . . . . . . . . . . . . . . . . . . . . 25A.2 A POP3 Session . . . . . . . . . . . . . . . . . . . . . . . . . 26

A.2.1 Receiving email using POP3 . . . . . . . . . . . . . . 26A.2.2 Tcpdump output . . . . . . . . . . . . . . . . . . . . . 27

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Chapter 1

Introduction

This report investigates SMS over GPRS from both technical and economicpoints of view. We begin in chapter 2 with a background on the GSMsystem, including a thorough description of the GSM channels and the datarate obtained over these channels.

Then we continue, in chapter 3, with a description of the SMS protocolsand architecture, including the data rate and price for sending a SMS mes-sage over GSM. In chapter 4 we show how the new GPRS channels work anduse that to calculate the time to send a SMS message over the air interface.After that we compare SMS with email, making measurements on sendingand receiving an email over GPRS. Finally we compare it with a third case,instant messaging.

In chapter 5 we make some conclusions and predictions for the future.

1.1 Problem Statement

The main question we try to answer in this report is: Are we going to needSMS over GPRS?

Is there something actually useful about SMS itself or does it lose itsmeaning with the introduction of GPRS do you need an applicationwhich only allows you to send short messages when you have the possibility,through GPRS, to do much more (e.g. send email)? Is SMS part of the pastor of the future?

And what about the price; SMS has up till now been a good way for theoperators to make money using something originally intended for controlmessaging only. Will that picture change in the future?

1.2 Method

To evaluate SMS over GPRS we estimate the efficiency of sending a textmessage of 140 octets (the original SMS message limit) in three different

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cases. We calculate the data rate, or time to send one message, and pricefor three different cases; sending a text message the common way, with SMSover GSM, sending the message with SMS over GPRS and finally, sendingit using ordinary email (over GPRS). Sending the message using an instantmessaging protocol is also considered.

The first thing we look at is the sending of the actual message. Not thepackets sent before or after, to setup or release either a SMS or email session.We choose to look at this packet when calculating the data rate and timeover the air interface. This because the packet containing the actual textmessage will naturally be larger than the packet containing, for example, anacknowledgment.

After that we measure or estimate the total time, including the setupand release phase.

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Chapter 2

GSM

The global system for mobile communications (GSM) is a digital cellularcommunications system. GSM was originally developed in Europe (fromthe beginning it denoted groupe speciale mobile) is now spread around theworld.

GSM was designed for circuit switched voice calls, but also includes theshort message service (SMS), which makes it possible to send and receiveshort text messages via a GSM mobile phone.

2.1 GSM Architecture

The architecture of a GSM system can be divided into the mobile station(MS), the base station system (BSS), and the network and switching subsys-tem (NSS). The MS is carried by the user, the BS subsystem controls theradio link to the MS and the NSS performs the switching of calls betweenthe MS and other fixed or mobile network users. It also handles mobilitymanagement. The MS and the BSS communicate across the Um interface 1

also known as the radio link.The MS consists of two different entities which are needed in order to

access the services that GSM network provides; the SIM card, that containsinformation about the user, and the mobile equipment, that is used to ac-cess the radio resources in the GSM network. Without a SIM card onlyemergency calls can be made.

A GSM network normally consists of several BSSs that connect MSs indifferent geographical regions to the NSS. The BSS controls the transmit-ting and receiving of calls and data. The NSS manages the communicationbetween mobile users and other users and contains databases with subscriberinformation needed to handle routing, authentication, and mobility.

1The Um interface is the GSM network interface for providing circuit and packet dataservices over the radio interface to the MS.

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One important unit in the NSS is the mobile service switching center(MSC). It is the entity that handles the switching within the network andsets up, supervises, and releases calls. It can connect phone calls betweenMSs within one particular GSM network, and it can connect calls betweenMSs in different GSM networks.

Another unit in the NSS is the gateway mobile services switching center(GMSC). It is the gateway between the GSM network and other networks(GSM or others).

2.2 GSM Channels

The GSM radio link uses both frequency-division multiple access (FDMA)and time-division multiple access (TDMA) to share the bandwidth amongthe users.

2.2.1 Physical Channels

FDMA divides the frequency bands of a link into a number of separate fre-quency channels [3]. The GSM-900 system has two frequency bands avail-able: 890 - 915 MHz for the uplink (direction MS to BS) and 935 - 960 MHzfor the downlink (direction BS to MS). These frequency bands are dividedinto 124 pairs of channels with FDMA.

TDMA divides the channel into time slots, such that each user gets aspecified time slot to send in. The frequency channels are split into 8 timeslots. The 8 time slots constitutes a TDMA frame of length 4.615 ms. Thuseach time slot has the length of 4.615/8 = 0.577 ms. The recurrence of oneparticular time slot makes up a physical channel for the user to transmitdata (voice or signaling). Hence a physical channel is defined by both thefrequency and the TDMA frame time slot number. Each physical channeltransmits in a series of short bursts; hence a GSM terminal using a singletime slot is only transmitting 1/8 of the time [14].

2.2.2 Logical Channels

The physical channels can be used to send different types of data. Con-sequently two kinds of logical channels are defined; traffic channels (TCHs)are physical channels that carry user traffic (voice or data) and control chan-nels (CCHs) that carry signaling information[1].

2.2.3 Data Rate

A TCH is defined by a multiframe that consists of 26 TDMA frames together(see Fig. 2.1). Since the length of a TDMA frame is 4.615 ms the lengthof this 26-frame multiframe is 26 4.615 = 120 ms. Out of these 26 frames

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6 70 1 2 3 4 5

0 1 2 3 4 25242322

1 TDMA frame = 8 time slots 4.615 ms

1 (26frame) multiframe = 26 TDMA frames

Frames 011, 1324: TCHFrames 12, 25: SACCH and unused

Figure 2.1: 26 TDMA frames

flag 1

flag 157 encrypted bits

Data57 encrypted bitsData

3TB Training sequence

26 3TB GP

8.25

Time slot 4.615/8 = 0.577 ms

Burst 0.546 ms

Figure 2.2: Time slot structure. TB is tail bits and GP guard band.

24 are used for traffic, one is used for the slow associated control channel(SACCH), and one is currently unused. A logical channel can use differentphysical channels at different times, but a full-rate TCH (TCH/F) uses onespecific time slot per TDMA frame for traffic data.

The 0.577 ms time slot contains a burst of 148 bits (0.546 ms) followedby 0.031 ms guard time (see Fig. 2.2). Out of these 148 bits, 2 57 = 114bits contains data. That gives a transmission rate of 114 bits/4.615 ms =24.7 kbit/s. Since only 24 frames of a multiframe are used for TCH, thefull-rate TCH (TCH/F) actually has the data rate of 24 24.7/26 = 22.8kbit/s. But this is just the gross bit rate; because you apply channel codingto the original data, to protect against errors, the maximum net bit rate isonly 14.4 kbit/s (TCH/F14.4 defined in GSM Phase 2+).

For half-rate TCH (TCH/H) the gross bit rate is only 22.8/2 = 11.4kbit/s, since two half-rate channels are using the same time slot in altern-ating frames.

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Chapter 3

SMS

Using the short message service (SMS) the user has the ability to send andreceive text messages to and from mobile phones. SMS was created as partof the GSM Phase 1 standard. The text of an short message can consist ofwords or numbers or an alphanumeric combination. A short message cancarry up to 140 octets of information 1. This makes it possible to send upto 160 characters if the standard GSM alphabet, i.e. Latin, is used. Whenanother character coding scheme is used, such as Arabic and Chinese, fewercharacters (70 using 16 bits per character) will fit into the 140 octets [18].

3.1 SMS Architecture

In order to provide their users with SMS, the GSM network operator hasto extend their fixed network with a new entity, the short message servicecenter (SM-SC). In addition to adding the SM-SC, the GMSC, the MSC andthe MS have to be modified to be able to handle short messages. Since SMSis a store and forward service, the short messages are sent via the SM-SCand not directly from sender to recipient. The SM-SC acts as a relay stationfor short messages, by first storing and later forwarding them to the rightdestination. Each network that supports SMS has one or more SM-SCs tohandle and manage short messages.

3.2 SMS Features and Usage

SMS features include confirmation of message delivery (as shown in Fig.3.1), which lets the sender of the short message choose to receive a returnmessage notifying him whether the short message was delivered or not. An-

1The message size is constrained by the maximum frame length of 272 octets on the Ainterface (the interface between MSC and BSC). The overhead of higher protocol layersleaves 140 octets for the text [26].

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other feature is that a mobile phone should be able to send and receive theshort message independently of ongoing calls.

When a short message is sent from the mobile phone it is called a mobileoriginated short message (SM MO) and when it is sent to a mobile phoneit is called mobile terminated (SM MT). There is yet another type, calledcell broadcast short messages (SM CB), which is used by the GSM networkoperator to send short messages to all users within a certain service area.

3.3 SMS Delivery Procedure

To send a single SMS message, a number of primitives are exchanged (seeFig. 3.1). The MS (1) sends a service request to the MSC, the MSC forwardsa (2) service acceptance, and then (3) the SMS message is transmitted usingthe connection management sublayer, which is the lowest layer in the SMSprotocol hierarchy. After the SMS message is transmitted the MS waits for(4) an acknowledgment as well as (5) a delivery report as mentioned above.As a final step the MS will (6) acknowledge that the status report has beenreceived and then, by sending a couple of more packets, the connection isreleased.

Obviously this transmission procedure takes both time and bandwidth,but in this report we will concentrate on step (3), measuring the resultingtime and cost of the actual SMS transmission as argued in chapter 1.

3.4 SMS Protocol Hierarchy

The SMS protocol hierarchy includes the application layer (SM-AL), thetransfer layer (SM-TL), the relay layer (SM-RL), and the connection man-agement sublayer (CM).

If we look at the packet sent in step (3), when the actual message isbeing delivered (see Fig. 3.1), it is comprised of several different protocoldata units (PDUs). Figure 3.2 shows the packet sent by the short messagecontrol protocol (SM-CP) which handles the connection management withinthe CM sublayer. This PDU is called CP-DATA and all the octets followingthe message type are the user data (i.e. payload) [7].

The user data of CP-DATA is the PDU of a higher-layer protocol, herethe short message relay protocol (SM-RP). Table 3.1 shows the contents ofthat PDU, called RP-DATA-MO (for mobile originated).

Fields 4 to 12 represents the next protocol in the SMS protocol hierarchy,the short message transfer protocol (SM-TP).

As you can see from Figure 3.2 and Table 3.1 the total size of the CP-DATA packet, when all fields have their maximal length, is 2 + 175 = 177octets. We shall use this value later when calculating the transmission speedover GSM and GPRS respectively.

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5. CPDATA with status report

1. Service request

2. Service accept

3. CPDATA with message

4. CPACK

6. CPACK

Connectionsetup

Short message

Connection release (not shown)

MS MSC SMSC

Delivery report

Figure 3.1: SMS delivery procedure (based on the description in [1]). Thisfocuses on the messages exchanged between the MS and the MSC, but alsoshows two of the messages exchanged between the MSC and the SM-SC.

Upto175 octets

Transaction

Message type

Protocol Discriminator

4

Payload

Bits 0 8

2 octets

Figure 3.2: SMS packet structure for control messages [24]. Protocoldiscriminator 10012 identifies the SMS protocol. Message types may be000000012 CP-DATA or 000001002 CP-ACK (there is also a CP-ERRORtype).

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Field Octet(s) Description1 1 Length of the SMSC information (i.e. fields 13).2 1 Type-of-address of the SMSC. Indicates the format of a

phone number. The most common value of this octet is0x91, which represents the international format thatstarts with the country code.

3 10 Service center number. The number of octets of thisfield is derived from field 1. Since 12 is the maximumnumber of octets for fields 1-3 together [7], this field islimited to a maximum of 10 octets. (If the internationalformat is used though, this field would be of length 7.)

4 1 First octet of this SMS-DELIVER message.5 1 Address-Length. Length of the sender number-fields.6 1 Type-of-address of the sender number. Same as above.7 10 Sender number. (See service center number.)8 1 Protocol identifier (TP-PID) defined in [7]. Identifies

the above layer protocol, if any.9 1 Data coding scheme (TP-DCS). The coding scheme

used within the user data (TP-UD, see below).10 7 Service center time stamp (TP-SCTS). The local time

represented in semi-octets.11 1 User data length (TP-UDL). The length of message.12 140 The message (TP-UD). Max 140 octets.

Sum: 175 The number of octets in the SMS PDU.Table 3.1: RP-DATA-MO. SMS PDU used to send the short message fromthe MS. This table is based on [23] and [7]. Fields 13 and 57 are addressfields and can consist of a maximum of 12 octets each [7].

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3.5 SMS over GSM

SACCH, the control channel that is assigned to one of the 26 frames in aTCH multiframe, is one of the two channels that can be used to send SMSs.The other channel is the standalone dedicated control channel (SDCCH).During a phone call SMSs are sent through SACCH, and otherwise, whenthe MS is idle, messages are sent through SDCCH.

SDCCH is mainly used for signaling during the call set-up phase andhas a 51-frame multiframe structure 2 . The logical channel combinationsfor such a multiframe containing SDCCH are

SDCCH/8+SACCH/C8 FCCH+SCH+BCCH+CCCH+SDCCH/4+SACCH/C4

FCCH, SCH, BCCH and CCCH are other control channels (see Fig. 3.3for full names). SDCCH/4 and SACCH/C4 means that it is possible toassign up to four SDCCHs with their required SACCHs. These are referredto as subchannels and they are transmitted once per 51-frame multiframe[16], where each of the subchannels occupies 4 time slots (see Fig. 3.3).

2The multiframes makes up superframes of 6.12 s. One superframe consists either of 2651-frame multiframes or 51 26-frame multiframes. By sending control information usinga 51-frame structure MSs in dedicated mode (sending on a dedicated channel, e.g. TCH)can still monitor the common control channels [15].

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3.5.1 Data Rate

If we choose to look at SDCCH/4 3 there are 4 SDCCH bursts in each 51-frame multiframe, and each SDCCH burst consists of 4 time slots (shownin Fig. 2.2). Hence every multiframe carries 4 4 2 57 = 1824 SDCCHbits. These are coded bits, coded with the GSM channel coding used forerror correction and detection. In SDCCH 456 coded bits are obtainedfrom originally 184 information bits (through adding parity bits, tail bitsand then coding with a convolutional encoder). Hence a total of 184/456 1824 = 736 information bits are being sent per multiframe. Given that a51-frame multiframe takes 51 4.615 ms = 235.365 ms we get a net bit rateof 736/235.365 = 3.12 kbit/s for SDCCH/4.

3.5.2 Time

Since a SMS of 177 octets is 1778 = 1416 bits, it will take two multiframes(736 2 = 1472 bits) to send it, i.e. it will take 235.365 2 = 470.730ms or roughly 0.5 seconds. That yields a bit rate of 1416/470.730 = 3.00kbit/s. This later value is perhaps a more appropriate estimate than 3.12kbit/s since the SMS data is sent in SDCCH bursts which are not evenlydistributed over the multiframe (see Fig. 3.3).

The total time however, including the setup and release phase, of sendinga SMS message of 160 characters is roughly 4.6 seconds (using a Nokia 6510MS and Telia as operator). While sending a empty message takes around4.15 seconds.

3.5.3 Price

Today the price for sending one SMS in Sweden is between 1.25 kr and 1.50kr (roughly between e0.1335 and e0.1869). Around Europe the price variesbetween e0.05 and e0.3.

3For SDCCH/8 the data rate will be doubled since twice as much information is sentper multiframe.

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Chapter 4

GPRS

GPRS is a new technology for packet data transmission based on the existingGSM network. This packet switched technology is used as a complement tothe circuit switched services and SMS. It provides a connection to Internetat rates from 56 upto 115 kbit/s [19].

The advantage of GPRS, besides reusing GSM-based technology, is thatapplications based on standard data protocols are supported, such as IP andX.25.

By sending the information as packets over unused voice channels GPRSmakes more efficient use of the network resources. Still the use of traditionalGSM services is not affected by the GPRS communication since voice callsare given priority for using the the radio resources.

With GPRS the user seems to always be on-line, but is only chargedfor the amount of data sent and received, not for the time connected tothe network. Since GPRS mobile phones are constantly online (within theGPRS coverage area) they are able to send and receive data at any time.GPRS is designed for fast reservation in order to quickly begin transmissionof packets, between 0.5 and 1 second [20].

4.1 GPRS Architecture

The GPRS Architecture introduces two new network nodes, the servingGPRS support node (SGSN) and the gateway GPRS support node (GGSN).SGSN is at the same hierarchical level as the MSCs; it switches packets tothe correct BSS. GGSN acts as the gateway between the GPRS network andexternal IP networks.

4.2 GPRS Channels

GPRS radio technology is, as stated earlier, based on the GSM radio ar-chitecture. The physical channel dedicated to packet data traffic is called

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B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11T X T X

Multiframe with 52 TDMA frames

T: Reserved for PTCCH B: Block which a logical channel is mapped ontoX: Idle frame

Figure 4.1: 52-frame multiframe

the packet data channel (PDCH). In a PDCH there can be different logicalchannels. One such channel is the packet data traffic channel (PDTCH)which is used for the transfer of user data. The main difference betweenPDTCH and the GSM TCH is that one user can occupy several PDTCHssimultaneously (i.e. get several time slots). Furthermore multiple users canshare one single PDTCH (i.e. share one time slot).

GPRS was designed to support bursty data traffic such as web browsing.Thus, a single mobile phone can, theoretically, transmit on up to all eightchannels of the same TDMA frame. However, in practice it is highly unlikelythat an operator would hand out all the time slots to one single user, sincethe allocated PDCHs are taken from a common pool of physical channelsthat would otherwise be used as traffic channels [4].

4.2.1 Data Rate

The multiframe that the logical GPRS channels use has a 52-frame structure(see Fig. 4.1). The 52 TDMA frames are divided into blocks of 4 TDMAframes each, making a total of 12 blocks. Each block contains 456 data bits,so on average there are 45612/52 = 105 data bits per TDMA frame. Sinceeach TDMA frame is 4.615 ms this gives a gross data rate of 105 bits/4.615ms= 22.8 kbit/s, which is the same as for GSM TCH.

Depending of the quality of the channel, four different coding schemesare used. The channel coding prevents errors by adding some redundancyto the data. The highest data rate of 21.4 kbit/s is achieved with the CS-4coding scheme [5]. In that scheme only parity bits and flag bits are added,no encoding is done. That leaves 428 bits of information, which explainsthe data rate of 428/456 22.8 kbit/s = 21.4 kbit/s. If we would get alleight time slots to ourselves, we would obtain the maximum data rate of21.4 8 = 171.2 kbit/s.

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A more realistic assumption perhaps, is a bit rate of 40 kbit/s, whichis obtained with coding scheme CS-3 and three users sharing the time slots[5].

4.3 SMS over GPRS

When using GPRS as a bearer for SMS, PDTCH can be used to transferthe message [21, 25].

4.3.1 Time

As stated above (Sect. 3.5.1) the data rate when sending a SMS over GSM(using SDCCH/4) is 3 kbit/s. This is dramatically slower than even 40kbit/s (see above). With 40 kbit/s you will send a SMS in 177/40 = 4.425ms. This is almost 100 times faster than SMS over GSM. Thus, as T. Kunzsstates, GPRS is a fast carrier of SMSs [22].

On the other hand, even here a lot of time is spent in the setup phase.But since SMS over GPRS is not available in Sweden today we have notbeen able to make measurements on this. All we can say is that it should beat least as fast as sending an empty message over GSM, 4.15 seconds (seeSect. 3.5.2).

4.3.2 Price

A quick overview of the prices for GPRS traffic set by operators in Swedenshows that they charge from 0.12 kr per kbyte for a subscription with 0 krmonthly fee. In Denmark the cost is considerably lower; 20 Dkr per Mbyte(in Danish crowns) and they charge 0.40 Dkr per SMS - using GPRS ascarrier: SMS Hele dgnet 0,40 [35]. One can note though, that the pricefor sending SMS over GPRS is the same as for sending SMS over GSM.

As mentioned above none of the big Swedish GSM operators offer SMSover GPRS today1.

4.4 Email over GPRS

To be able to compare SMS in itself with other types of messaging we choosedto look first and foremost at email. This is because it has a very widespreaduse and because many people already have mobile phones capable of send-ing and receiving email. Email is also used by many people to send shortmessages when we are sitting in front of the computer.

1To be able to offer SMS over GPRS changes has to be made to the SMSC connectingit to the GPRS SGSN as well as to the GSM MSC [21].

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When sending or receiving email over GPRS the process is similar tothe SMS process shown in Fig. 3.1; a number of messages are exchangedbefore the actual text message can be delivered. When sending an emailmessage from a mobile terminal the simple mail transfer protocol (SMTP)[10] is used, and when fetching an email typically the post office protocol(POP) [11] is used [31].

Since we earlier described the mobile originated SMS we continue thatapproach and look at SMTP, i.e. the sending portion of an email deliv-ery. (Fetching or receiving email is analogous and described briefly in theappendix, section A.2.)

4.4.1 Sending Email Using SMTP

An email message consists of three pieces [2].

1. The envelope specified with two SMTP commands: MAIL From:and RCPT To:. The envelope is exchanged before the email messageitself.

2. Headers lines that start with for example Received:, Message-ID:or Date:.

3. The body the text itself.

The packet that carries the actual email consists of the headers and thebody. The headers are optional, but ordinary email programs usually adda number of them, e.g. Subject: and From:. If we assume that we send aminimal message containing only 160 characters of text (as in SMS) usingno headers, the final message size including the TCP and IP headers willbe 180 bytes (140 + 20 + 20 bytes), essentially the same size as the SMSmessage. Hence looking only at the packet containing the text message willreveal no difference between SMS over GPRS and email over GPRS.

4.4.2 Time

If we look closer at the total sending process it is clear, however, that theSMTP protocol together with the TCP protocol uses several more packagesin the setup and release phase than the SMS protocol does. Figure 4.2shows the packets exchanged between an email client and a SMTP serverover GPRS. The client is a Windows PC running Outlook Express andthe server is a Debian GNU/Linux computer running Sendmail [27] version8.12.3-6.3. The client is connected to Internet using Nokia 6510 as modemand the Nokia MS is connected to the Telia network. The timestamps showsthe time the packets were sent or received at the server side, i.e. on theDebian computer. The whole session (including the plain TCP packets) isshown the appendix in section A.1.

16

Client Server

4.

5.

7.

8.

9.

10.

11.

12.

13.

14.

16.

18.

19.

21.

SYN

sendmail

HELO

...pleased to meet you

MAIL From:

Sender OK

RCPT To:

Recipient OK

DATA

Enter mail...

Message body

EOM

Message accepted

QUIT

...closing connection

SYN, ACK

ACK

ACK

ACK

ACK

ACK

FIN, ACK

FIN, ACK

ACK

ACK

08:44:58.272015

08:44:55.511740

08:44:58.301312

08:44:59.122128

08:44:59.132027

08:44:59.954002

08:44:59.964150

08:45:01.071970

08:45:01.081284

08:45:02.292588

08:45:03.239825

08:45:03.343266

08:45:04.390085

08:45:04.444422

1.211304 s.

0.930162 s.

Timestamp

Pack

et n

umbe

rs

SMTP packets

TCP packets

Figure 4.2: A SMTP-session sending an email over GPRS. The total timeof this session is about 16 seconds. This is an example session, in anothersession more or less TCP acknowledgments could have been piggy-backed onthe SMTP packets resulting in less or more packets transmitted respectively.

17

The total time of the session is around 16 seconds 2. Packet number 14,the message body, is, in this case, 564 bytes (160 characters are sent, butOutlook adds a number of headers). The time from the previous acknow-ledgment is sent (packet 13) till packet 14 is received is 1.21 seconds. Ascan be seen in Figure 4.2 the time between packet 15 (an acknowledgmentagain) and packet 16 is also around 1 second, 0.93 seconds to be exact. Thisdespite that packet 16 is the smallest SMTP packet sent; it contains onlythe end of message (5 bytes). Hence the size of the packet does not seem tohave a big impact on time. The number of packets clearly does though.

Since the SMS protocol uses only 8 packets to send a message (comparedto 25 in this case) it is faster than SMTP, at least over the GPRS radionetwork.

4.4.3 Price and Cost

Yet if we look at the price the picture is quite different. Continuing tolook at the session shown in Figure 4.2 and section A.1 a total of 2118bytes are being sent and received. (Without the extra Outlook-headers1714 bytes would have been transmitted.) Using the Swedish price forGPRS traffic the charge for sending these 2 kbyte would be 0.24 kr. Thisis five times cheaper than the 1.25 kr that is charged for SMS messages.Using the Danish example the price would be 0.04 Dkr instead of 0.40 Dkr.Consequently email is a better option than SMS for a user that has moretime than money. Anyway, text messages are not used for urgent messages;if we want to be sure that our message is received instantly we make a phonecall.

Still for the GSM operators sending an email is more expensive thansending a SMS message. More resources are occupied for a longer time. Sofrom their point of view SMS is preferred, at least as long as they can chargemore for it.

4.5 Instant Messaging over GPRS

If the number of packets exchanged has such a big impact on the total timeof the message sending, what about instant messaging (IM) then? Nokia7650 and 3650 supports all the big IM protocols using two kinds of software.ICQ, AOL, MSN, and the open IM protocol Jabber is supported by IM+[32], and IRC is supported by WirelessIRC [33]. Nokia is also involvedWireless Village [34] together with Ericsson and Motorola. It was formed to

define and promote a set of universal specifications for mobileinstant messaging and presence services

2Sending an email over the Internet without using GPRS normally takes a couple ofseconds.

18

and is supported by the IEEE industry standards and technology organ-ization (IEEE-ISTO). Today most of the mobile phones also supports Java,making them able to run Java IM clients.

Instant messaging is good for sending short messages, especially over awireless link such as GPRS, since in principle only two messages are sent(the message itself and the acknowledgment). But this is once you havea session. Using Gabber, the GNOME Jabber client [30], around 24 TCPpackages are sent during the setup phase. This is almost as many messagesas when sending an email. But if you have a phone with IM support andyou know you are going to send more than one message, definitely IM is abetter choice than email. On the other hand, there are more phones withthe possibility to send and receive email than phones with the possibility todownload a IM client. At least today.

19

Chapter 5

Conclusions

There is a main difference between SMS and other protocols developed forthe Internet; it takes care not to send to many packages back and forth.

5.1 SMS is Here To Stay

Hence we conclude that SMS is here to stay. Mainly because it is a protocoldeveloped for a radio network such as GSM and therefore is optimal for thatuse. When we adopt GPRS and start using SMS over GPRS it will takeless time to send a SMS message, but obviously that does not change theusefulness of the protocol.

5.2 Prices Will Drop For SMS

At the same time we believe that prices will drop for SMS. This as a resultof the fact that with GPRS the user anyway gets different alternatives onhow to send text messages. The alternatives may not be exactly the samething as SMS, but there is a difference between only being able to take thecar and being able to choose between taking the car, the subway, and thebike.

As more and more users will be able to choose something other thanSMS, and discover that it is, as we have seen, cheaper, it will probably bedifficult for the GSM operators to keep up the prices. The operators wouldalso want to encourage the users to use SMS instead of something else, sinceit gives a smaller load to the network (at least with the protocols we havetoday).

5.3 Operator Independent Alternatives To SMS

This is perhaps the most exciting conclusion. With GPRS it will be, asmentioned, easier to use a message service that doesnt depend on the oper-

20

ator. So perhaps we will see protocols developed that are economical whenit comes to the number of messages exchanged, protocols that are designedwith the GPRS network in mind, but for use in the Internet. Perhaps theInternet will start to adjust to GSM and GPRS and not only the other wayaround.

Possibly more protocols will use UDP instead of TCP in the transportlayer. (The reliable TCP is, as we have seen, not very economical withthe number of messages it sends.) An email server such as Sendmail couldsupport a smaller version of SMTP running over UDP for the use of mobilephones and other hand-held devices. We have the trivial file transfer protocol(TFTP) [13] used for devices that do not have space for the whole TCP stack(TFTP is implemented on UDP). Perhaps we will get TMTP , the trivialmail transfer protocol too?

5.4 Acknowledgment

At last, our own acknowledgment. We want to thank Gerald Q. Maguireand Maxim Teslenko for their valuable input when writing this report.

21

Bibliography

[1] Yi-Bing Lin and Imrich Chlamtac, Wireless and Mobile Network Ar-chitectures, John Wiley & Sons, 2001.

[2] W. Richard Stevens, TCP/IP Illustrated, Volume 1, The Protocols,Addison Wesley, 1994.

[3] James F. Kurose and Keith W. Ross, Computer Networking: A Top-Down Approach Featuring the Internet, 2nd ed., Addison Wesley, 2003.

[4] Jian Cai and David J. Goodman, General Packet Radio Service in GSM,Rutgers University, IEEE Communications Magazine, October 1997.

[5] Christian Bettstetter et al., GSM Phase 2+ General Packet Radio Ser-vice GPRS: Architecture, Protocols, and Air Interface, IEEE Commu-nications Surveys, http://www.comsoc.org/pubs/surveys, vol. 2 no.3, 1999.

[6] ETSI, Digital cellular telecommunications system (Phase 2+); MobileStation - Base Station System (MS - BSS) Interface Channel Structuresand Access Capabilities (3GPP TS 44.003 version 5.0.0 Release 5),http://www.etsi.org/, 2002.

[7] ETSI, Digital cellular telecommunications system (Phase 2+); Tech-nical realization of the Short Message Service (SMS) Point-to-Point(PP) (3GPP TS 03.40 version 7.5.0 Release 1998), http://www.etsi.org/, 2001.

[8] ETSI, Digital cellular telecommunications system (Phase 2+); MobileStation - Base Station System (MS - BSS) Interface Channel Structuresand Access Capabilities (3GPP TS 44.003 version 5.0.0 Release 5),http://www.etsi.org/, 2002.

[9] ETSI, Digital cellular telecommunications system (Phase 2+); Channelcoding (3GPP TS 45.003 version 5.6.0 Release 5), http://www.etsi.org/, 2002.

[10] Jonathan B. Postel, Simple Mail Transfer Protocol, RFC 788, November1981.

22

[11] J. Myers and M. Rose, Post Office Protocol - Version 3, RFC 1939,May 1996.

[12] C. Kalt, Internet Relay Chat: Client Protocol, RFC 2812, April 2000.

[13] K. Sollins, The TFTP Protocol (Revision 2) , RFC 1350, July 1992.

[14] Thierry Turletti, A brief Overview of the GSM Radio Interface, http://tns-www.lcs.mit.edu/~turletti/gsm-overview/, Laboratory forComputer Science, Massachussets Institute of Technology, 1996.

[15] GSM airlink information, http://www.bryte.net/gsm/airlink.asp.Accessed 20th April 2003.

[16] Nortel Networks, A Comparison Between GERAN Packet-SwitchedSupplementary Services Using SIP and GSM Circuit-Switched Supple-mentary Services Using RIL3-CC, RIL3-MM, RIL3-RR, and DTAP,2000.

[17] Juan Li, S-72.260 Laboratory Works in Radiocommunications, Laborat-ory Exercise 3, BSS Radio Parameters, www.comlab.hut.fi/opetus/260/3v053.pdf, Communications Laboratory, Helsinki University ofTechnology, 2001.

[18] Hay Systems Ltd, SMS Connectivity, www.haysystems.com/documents/HSL.SMPP.Interface.Brochure.pdf, 2002.

[19] Silvan Mayer, Impact of GPRS on the Signalling of a GSM-based Network, Institute of Communication Networks and ComputerEngineering, University of Stuttgart.

[20] Stephane Piot, Security over GPRS, Master of Science in Tele-communications, www.ee.ucl.ac.uk/~lsacks/tcomsmsc/projects/pastproj/s_piot.pdf, University Collage London, 1998.

[21] Logica,Why SMS if we have GPRS, www.totaltele.com/whitepaper/docs/Logica_WhyhaveSMSifwehaveGPRS.pdf, 1999.

[22] Thomas Kunz, Course notes GSM and GPRS, Systems and ComputerEngineering, Carleton University.

[23] Lars Pettersson, SMS and the PDU format, http://www.dreamfabric.com/sms/.

[24] Protocols.com, Protocols directory, telephony, http://www.protocols.com/pbook/cellular.htm#SMS.

[25] Hannu H. Kari, Short Message Service (SMS) over GPRS radio asdefined 04.11, http://www.cs.hut.fi/~hhk/GPRS/lect/sms/index.htm, 1998.

23

[26] Thomas Kunz, GSM and GPRS Course material course 94.536Mobile Computing Systems, http://kunz-pc.sce.carleton.ca/sce536/GSMandGPRS.pdf, Systems and Computer Engineering,Carleton University.

[27] Sendmail, http://www.sendmail.org/

[28] Tcpdump, http://www.tcpdump.org.

[29] Popa3d, http://www.openwall.com/popa3d/.

[30] Gabber, http://gabber.sourceforge.net/.

[31] Nokia, http://www.nokia.se/support/phones/ota.php.

[32] IM+, Shape Services, http://www.shapeservices.de/eng/im/.

[33] WirelessIRC, Mobileways, http://www.mobileways.de/.

[34] Wireless Village, http://www.openmobilealliance.org/wirelessvillage/.

[35] TDC Mobil, http://www.tdcmobil.dk/.

24

Appendix A

Mail Sessions over GPRS

In this chapter the output of Tcpdump [28] in two example sessions is shownfor reference. The first session, the SMTP session, was described in detailin section 4.4.2, while the POP3 session is described more briefly here.

A.1 A SMTP Session

This is the SMTP session shown in Figure 4.2.

A.1.1 Tcpdump output

tcpdump: listening on eth0

08:44:49.681231 host-248-33.1459 > c-aea072d5.smtp: S 3507904700:3507904700(0) win ...

08:44:49.681305 c-aea072d5.smtp > host-248-33.1459: S 1707044676:1707044676(0) ack ...

08:44:50.453740 host-248-33.1459 > c-aea072d5.smtp: . ack 1 win 17520 (DF)

08:44:55.511740 c-aea072d5.smtp > host-248-33.1459: P 1:208(207) ack 1 win 16060 (DF)

08:44:58.272015 host-248-33.1459 > c-aea072d5.smtp: P 1:10(9) ack 208 win 17313 (DF)

08:44:58.272093 c-aea072d5.smtp > host-248-33.1459: . ack 10 win 16060 (DF)
















08:45:04.469974 c-aea072d5.smtp > host-248-33.1459: F 509:509(0) ack 635 win 16060 (DF)

08:45:05.203487 host-248-33.1459 > c-aea072d5.smtp: F 635:635(0) ack 509 win 17012 (DF)


08:45:05.289819 host-248-33.1459 > c-aea072d5.smtp: . ack 510 win 17012 (DF)

25 packets received by filter

0 packets dropped by kernel

25

A.2 A POP3 Session

A.2.1 Receiving email using POP3

The commands used by Outlook Express (the POP3 client in this case) tocommunicate with the server (Popa3d [29] version 0.5.1-2) are in order:

1. USER to provide a username

2. PASS to provide a (plaintext) password

3. STAT to ask for the number of messages

4. LIST to ask for a listing of the messages (showing the sizes of eachmessage)

5. RETR to retrieve a certain message

6. DELE to delete a certain message

7. QUIT to end the session

Not all of these commands are necessary though, if we know for examplewe have at least one message waiting for us, we can issue RETR 1 directlyafter the password is provided, and retrieve message 1. The answer fromthe POP3 server starts either with a +OK for a positive response or a -ERRif it, for example, doesnt have the message we ask for [11].

Time and Amount of Data

This session takes around 14 seconds. It is roughly the same time as thesending took so the conclusions drawn from the SMTP-example are stillvalid.

The total amount of data is 2131 bytes, also that almost the same as inthe SMTP-case.

26

A.2.2 Tcpdump output

tcpdump: listening on eth0

08:45:49.753272 host-248-33.1460 > c-aea072d5.pop3: S 3522969293:3522969293(0) win ...

08:45:49.753347 c-aea072d5.pop3 > host-248-33.1460: S 1764157835:1764157835(0) ack ...

08:45:51.003082 host-248-33.1460 > c-aea072d5.pop3: . ack 1 win 17520 (DF)

08:45:51.104331 c-aea072d5.pop3 > host-248-33.1460: P 1:6(5) ack 1 win

16060 (DF)

08:45:52.184347 host-248-33.1460 > c-aea072d5.pop3: P 1:11(10) ack 6 win 17515 (DF)

08:45:52.184425 c-aea072d5.pop3 > host-248-33.1460: . ack 11 win 16060 (DF)

08:45:52.334146 c-aea072d5.pop3 > host-248-33.1460: P 6:11(5) ack 11 win 16060 (DF)





















08:46:02.162437 c-aea072d5.pop3 > host-248-33.1460: F 835:835(0) ack 58 win 16060 (DF)

08:46:03.434301 host-248-33.1460 > c-aea072d5.pop3: F 58:58(0) ack 835 win 16686 (DF)



31 packets received by filter

0 packets dropped by kernel

27

sms over gprs

Documents

sms architecture

sms message

carriers of sms

sms features

sms protocolsand architecture

sms protocol hierarchy

sms message

sms delivery procedure