Page 318 - Mechatronics for Safety, Security and Dependability in a New Era

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MOBILE CONTROLLED RFID SYSTEM

MCRS enables the remote tracking of RFID tags through the GPRS network. The system architecture
consists of RFID tags, one or more RFID readers, and a Mobile Gateway Server (MGS) that controls
readers and communicates with the user mobile. MCRS operation may be briefly as follows. A reader
lists every tag detected in its reading range. This list is updated on tag arrivals in or departures from the
reading range; all changes are sent to the MGS. The MGS automatically informs the user about the
movements of those tags that are set to follow-up. The user can also check all the tags that are within
range of a certain reader at a particular time.

GENERAL PACKET RADIO SYSTEM AND QOS ISSUES

GSM is the world's most widespread digital mobile phone standard for cellular circuit-switched
communications. GPRS is a technology that utilizes the upgraded GSM networks radio interface
offering packet-switched networks and always-on connections for the user [3]. Modern mobile phones
and developed networks make remote solutions possible. Still, high-level performance in GPRS
communication systems is not guaranteed. The ETSI standard [4] defines five different QoS attribute
classes: precedence, delay, reliability, and mean and peak throughput. Combinations of these attributes
can define different GPRS QoS profiles.
However, there are some limits affecting QoS in GPRS such as 1) only one QoS profile can be used
for a given PDP (Packet Data Protocol) address, 2) QoS profiles are vaguely specified, and 3) GPRS
radio only supports best-effort traffic [5]. The first of these indicates that the above mentioned ETSI
standard determines a specific QoS profile to an end-to-end connection for a subscriber, not for an
application. Thus if different levels of QoS are needed a new PDP context or logical network
connection [6] must be activated for every application using the limited address space of the network.
Secondly, loosely specified GPRS QoS standards lead to compatibility problems between different
manufacturers' devices. Thirdly, the radio access network is designed for best-effort traffic. Thus
handovers, IP-address changes, signal strength weakening, limited bandwidth, and contention for
resources are properties that decrease the quality of GPRS.

QOS SIMULATIONS

The paper presents functionality of three different applications in variable GPRS conditions. These
applications are Internet Explorer v6.0 (IE), WS_FTP Pro 7.61 (FTP), and MCRS. In simulations the
quality of the GPRS connection is varied by different measures (delay, jitter, packet loss) and the goal
is to study how the variations in these parameters and the security level (VPN) affect the data transfer
time and the amount of transferred packets. The reference for the simulations is the operation of GPRS
class 10 (typical for mobile phones) in practical environments [2], The reference values are: Capacity
Downlink 26kbps, Capacity Uplink 15kbps, Packet Loss 3%, Mean Delay 690ms, and Jitter 350ms.
The functionality of the IE was tested by downloading a test page (lOOkB, 79 jpeg pictures) through
the simulated network. The functionality of the FTP was studied by transmitting a test packet (lOOkB,
zip file) through the simulated network. Access to an MGS that controls the RFID system is through a
certification page where the user is identified and authenticated as a legitimate user. The size of the
certification page used in these simulations is 449 bytes.
Delay effect. Figure 1 shows how delay affects applications. MCRS tolerates very long delays. VPN
tunnelling increases the amount of transferred data by 30 to 80 %. The access time to the MGS
increases linearly as a function of delay. VPN does not affect access times. IE tolerates even extremely
long delays very well and the connection stays open but data transfer capacity that is successfully