Mobile client-server
communications
CSE 390 Fall 2010
Cellular connection types
• Time Division Multiple Access (TDMA) – assigns calls a time slot on a
given frequency. Used by GSM.
• Code Division Multiple Access (CDMA) – assigns each call a unique code.
Used by UMTS.
• Global System for Mobile Communication (GSM) – most popular
standard. 80% of the global market, 1.5 billion users, 212 countries.
Extensive roaming arrangements with international mobile operators.
Second Generation (2G) technology – meaning both signaling and speech
channels are digital. GSM introduced Short Message Service (SMS).
Different frequencies are used in many countries (USA and Canada use
850MHz and 1900MHz). The iPhone has a tri-band frequency chipset to
accommodate the most common frequencies.
Cellular connection types (cont.)
• Enhanced Data Rates for GSM Evolution (EDGE) – a “bolt-on”
enhancement for GSM networks to provide higher bit rates.
Does not require installation of new transmitter towers.
• Universal Mobile Telecommunications System (UMTS) –
Third Generation (3G) technology based on many of the GSM
protocols (and was originally called 3GSM), but uses CDMA as
the underlying air interface. Uses same core network standard
as GSM – Mobile Application Port (MAP). Uses Universal
Subscriber Identity Module (USIM) – compatible with GSM’s
SIM – for user authentication. Requires installation of new
transmission towers – expensive to migrate.
Wireless connection types
• IEEE 802.11a/b/g – Wireless Local Area Network (WLAN) technology.
More generally called WiFi. Network access points are called hotspots.
Typically operates in the range of 2.4 GHz, with a range of about 120 feet
indoors and 300 feet outdoors. Signal can be affected by radio reflection
off buildings and trees. Interference can also be caused by devices
operating on the same frequency (such as microwave ovens). Uses Wired
Equivalent Privacy (WEP) for security.
• Bluetooth 2.1 – Wireless Personal Area Network (WPAN) technology. Uses
frequency hopping spread spectrum radio technology, also in the 2.4GHz
range. Data rate of 1Mbit/s is possible. Range is about 33 feet (10 meters).
The most common applications for mobile devices are connecting wireless
headsets and printing to bluetooth-enabled printers. A master bluetooth
device can connect with up to seven devices in a wireless user group.
Thin clients
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Less device and OS specific than fat clients, but require a persistent network
connection. No custom application code is needed on the device, instead an
industry-standard web or WAP (Wireless Application Protocol) browser provides
the functionality. The browser may run some client-side scripting, such as
Javascript. But typically the server handles all the logic and data functions.
Still, devices vary in many ways, such as screen size, so a renderer running on the
server is often used to customize the content for the device. The application data
is stored in XML files and separate XSL style sheets are created for each supported
device. At run time the server checks the device’s browser type, transforms the
XML, and renders the the content to the device.
The downside of this approach is the need to develop multiple XSL files for each
web page you wish to display. The upside of this approach is that application code
can be modified and updated in one place – the server – and not on potentially
thousands (or millions) of devices.
Fat clients
• Advantages of this approach are when devices are only connected to a
network infrequently, or when you want utilize a specific peripheral of the
device, such as the camera, phone, or GPS.
• Use cases, such as can be modeled using the Unified Modeling Language
(UML), include the following:
– Log in (for many reasons, such as the device may be lost or stolen, or
that you will allow the user to access and edit data on a server)
– Task selection, by presenting a menu of options for the user
– Data download, such as doctor searching for patient records, or a
salesperson looking for account information
– Edit data locally, to modify the patient or account records
– Upload changes
Fat clients (continued)
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Object reuse – for many business applications code may have already been written that would have to be
rewritten for each device you want to support. One way around this is to expose these business objects on
the server by creating an XML web service, which would forward method calls to those objects.
Object design – object oriented code design allows code to be more easily ported. Common object
properties and methods include:
– Constructor (often called New) creates a new object by allocating memory for it and initializing it
– Destructor releases the object and frees up the memory allocated to it
– Save allows data and preferences to persist between uses of the application
– Delete
– Get back-end data
Data Access Object – common methods include:
– Create database
– Check that database exists
– Delete database
– Execute SQL command
– Execute dataset (executes a SQL command and returns a dataset)
Client-server data transfer
• HTTP and HTML – A web browser on the client uses an HTTP GET request
for information of the server, typically through port 80. The server
retrieves the information and sends it, after which the sever connection
closes. A client may also send information to the server using the HTTP
POST method.
• WAP and WML – When a client with a WAP browser accesses a web page
the request is initially sent to a WAP gateway, which then forwards the
request to the HTTP server. The WAP gateway then encodes the response
and returns it to the client as WML (Wireless Markup Language). The
structure of WML is a deck of cards. A deck has a URL like a web page and
contains one or more cards. Each card represents a small screen of
information, like a menu or a data entry screen. Generally each card only
displays a few lines of data. In Japan, i-Mode is a competing technology.
Client-server data transfer (cont.)
• Remote Database Access (RDA) – is a protocol standard for database
access. RDA describes the connection of a database client to a database
server. It includes features for:
– communicating database operations and parameters from the client
to the server,
– transporting result data from the server to the client,
– database transaction management.
• RDA is an application-level protocol, meaning that it builds on an existing
network connection. It allows the developer to push and pull data from
the mobile database and the server side database. It also allows the
application to send SQL statements to be executed on the remote server.
If there is an error pushing data to the server, the records that were not
uploaded are placed into an error table.
Client-server data transfer (cont.)
• Simple Object Access Protocol (SOAP) - is a protocol for sending
structured data between remote computers over a network. The SOAP
spec was defined by the W3C, and was designed as a mechanism for
performing remote procedure calls using XML over HTTP. Because SOAP is
an open standards-based protocol, it can be used on any OS, and provides
interoperability among devices running different operating systems.
• SOAP’s messaging framework defines rules for constructing and
processing SOAP messages. A SOAP message is an XML document
consisting of a SOAP envelope. The envelope has a Body element, which
contains the message payload and or error information. The envelope may
also have a Header element which can pass information not part of the
main method call, such as message priority or authentication information.
• SOAP messages can be quite large.
Client-server data transfer (cont.)
• Web Services Description Language (WSDL) –
provides a spec for describing a web service in
XML. The description contains information about
the messages, data types, and protocols the web
service supports. It contains the following items:
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Service Name
Port (the URL of the web service)
Binding (transport and encoding protocols of the port)
Port Type (operations supported by the web service)
Messages