-
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
ii
-
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
iii
-
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
1
-
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.
2
-
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.
3
-
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
4
-
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.
5
-
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].
6
-
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.
7
-
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).
8
-
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].
9
-
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].
10
-
FS
FS
FS
FS
FS
B
CC
C
FS
FS
FS
FS
FS
B
CC
C
SD0
SD1
SD0
SD1
SD2
SD3
SD2
SD3
SA0
SA1
SA2
SA3
F: T
DM
A fr
ame
for f
requ
ency
cor
rect
ion
burs
tS:
TD
MA
fram
e fo
r syn
chro
niza
tion
burs
tB:
TD
MA
fram
e fo
r BCC
HC:
TDM
A fra
me
for C
CCH
51 fr
ames
SAn:
TDM
A fra
me
for S
ACCH
SDn:
TDM
A fra
me
for S
DCCH
Figure3.3:
51-framemultifram
eused
bythecontrolchannelSD
CCH[17][6].Notethat
thefour
SACCH-sub
channels(SA0-
SA3)
arespread
over
two51-framemultifram
es.
11
-
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.
12
-
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
13
-
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.
14
-
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].
15
-
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)
08:44:58.301312 c-aea072d5.smtp > host-248-33.1459: P
208:296(88) ack 10 win 16060 (DF)
08:44:59.122128 host-248-33.1459 > c-aea072d5.smtp: P
10:34(24) ack 296 win 17225 (DF)
08:44:59.132027 c-aea072d5.smtp > host-248-33.1459: P
296:332(36) ack 34 win 16060 (DF)
08:44:59.954002 host-248-33.1459 > c-aea072d5.smtp: P
34:54(20) ack 332 win 17189 (DF)
08:44:59.964150 c-aea072d5.smtp > host-248-33.1459: P
332:369(37) ack 54 win 16060 (DF)
08:45:01.071970 host-248-33.1459 > c-aea072d5.smtp: P
54:60(6) ack 369 win 17152 (DF)
08:45:01.081284 c-aea072d5.smtp > host-248-33.1459: P
369:419(50) ack 60 win 16060 (DF)
08:45:02.292588 host-248-33.1459 > c-aea072d5.smtp: P
60:624(564) ack 419 win 17102 (DF)
08:45:02.309663 c-aea072d5.smtp > host-248-33.1459: . ack 624
win 16060 (DF)
08:45:03.239825 host-248-33.1459 > c-aea072d5.smtp: P
624:629(5) ack 419 win 17102 (DF)
08:45:03.259776 c-aea072d5.smtp > host-248-33.1459: . ack 629
win 16060 (DF)
08:45:03.343266 c-aea072d5.smtp > host-248-33.1459: P
419:475(56) ack 629 win 16060 (DF)
08:45:04.390085 host-248-33.1459 > c-aea072d5.smtp: P
629:635(6) ack 475 win 17046 (DF)
08:45:04.409660 c-aea072d5.smtp > host-248-33.1459: . ack 635
win 16060 (DF)
08:45:04.444422 c-aea072d5.smtp > host-248-33.1459: P
475:509(34) ack 635 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)
08:45:05.203565 c-aea072d5.smtp > host-248-33.1459: . ack 636
win 16060 (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)
08:45:53.034619 host-248-33.1460 > c-aea072d5.pop3: P
11:24(13) ack 11 win 17510 (DF)
08:45:53.059672 c-aea072d5.pop3 > host-248-33.1460: . ack 24
win 16060 (DF)
08:45:53.211165 c-aea072d5.pop3 > host-248-33.1460: P
11:16(5) ack 24 win 16060 (DF)
08:45:54.334610 host-248-33.1460 > c-aea072d5.pop3: P
24:30(6) ack 16 win 17505 (DF)
08:45:54.341781 c-aea072d5.pop3 > host-248-33.1460: P
16:27(11) ack 30 win 16060 (DF)
08:45:55.621289 host-248-33.1460 > c-aea072d5.pop3: P
30:36(6) ack 27 win 17494 (DF)
08:45:55.639644 c-aea072d5.pop3 > host-248-33.1460: . ack 36
win 16060 (DF)
08:45:55.691119 c-aea072d5.pop3 > host-248-33.1460: P
27:32(5) ack 36 win 16060 (DF)
08:45:57.071072 host-248-33.1460 > c-aea072d5.pop3: . ack 32
win 17489 (DF)
08:45:57.071141 c-aea072d5.pop3 > host-248-33.1460: P
32:42(10) ack 36 win 16060 (DF)
08:45:58.321131 host-248-33.1460 > c-aea072d5.pop3: P
36:44(8) ack 42 win 17479 (DF)
08:45:58.339642 c-aea072d5.pop3 > host-248-33.1460: . ack 44
win 16060 (DF)
08:45:58.393744 c-aea072d5.pop3 > host-248-33.1460: P
42:47(5) ack 44 win 16060 (DF)
08:45:58.394094 c-aea072d5.pop3 > host-248-33.1460: P
47:822(775) ack 44 win 16060 (DF)
08:45:59.639768 host-248-33.1460 > c-aea072d5.pop3: . ack 822
win 16699 (DF)
08:45:59.639841 c-aea072d5.pop3 > host-248-33.1460: P
822:825(3) ack 44 win 16060 (DF)
08:46:00.884500 host-248-33.1460 > c-aea072d5.pop3: P
44:52(8) ack 825 win 16696 (DF)
08:46:00.891861 c-aea072d5.pop3 > host-248-33.1460: P
825:830(5) ack 52 win 16060 (DF)
08:46:02.153141 host-248-33.1460 > c-aea072d5.pop3: P
52:58(6) ack 830 win 16691 (DF)
08:46:02.161907 c-aea072d5.pop3 > host-248-33.1460: P
830:835(5) ack 58 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)
08:46:03.434370 c-aea072d5.pop3 > host-248-33.1460: . ack 59
win 16060 (DF)
08:46:03.521080 host-248-33.1460 > c-aea072d5.pop3: . ack 836
win 16686 (DF)
31 packets received by filter
0 packets dropped by kernel
27