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EventHelix Update Q3 2004
A perfect design is an enemy of a good design. Often, designers striving
for a perfect design may end-up with no design at all due to schedule and
cost overruns. A simple design may not provide best solution to a given
problem but it would probably have the best chance of meeting the schedule
and cost constraints with acceptable quality. Also, a simple design is
easier to implement, maintain and enhance.
It is better to start with a simple design, making reasonable and rational
compromises during the design phase. This will save you from making
unreasonable compromises in quality when faced with looming product
delivery dates.
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How does a GSM mobile phone receive a call?
We have already seen that GSM mobiles keep the GSM network informed about
their current location area. The knowledge of the location areais not
adequate for setting up a terminating call as the location area might
spawn several cells.
In this call flow we will look at how a terminating call is handled in GSM.
Setting up a terminating call is a two step process. (1) Interrogation
procedure to locate the subscriber (2) Actual call setup after the
subscriber has been located.
GSM Network Example
This example shows a few cells in a GSM network. We will be referring to this image in several sequence diagrams and call flows. GSM Location Update Sequence Diagram
Have you ever wondered how your cellular provider is able to route calls
to you virtually anywhere? How does the cellular provider know where you
are?
The short answer to these questions is that your cell phone keeps the
cellular operator informed about your location. This sequence diagram we
will examine how a mobile phones keep selecting the best cell to service
your call and also keeps the cellular provider informed about your
location.
This scenario describes the call setup for a GSM originating call. A
mobile user calling a land line subscriber is covered here.
SSL Sequence Diagram
This sequence diagram describes the SSL processing and the basic cryptography concepts need to understand SSL operation. SMTP E-Mail Sequence Diagram
Simple Mail Transfer Protocol (SMTP) is the most widely used e-mail
sending protocol. This sequence diagram describes the steps involved in
sending an e-mail. The DNS queries involved in this process have also been
covered.
In this example luke@sender.com is
trying to send an e-mail to aibo, c3po and r2d2 at receiver.com.
Protocol Stack Design Pattern
We have already seen that Protocol Layer and Protocol Packet provide a standardized interface between different layers of a protocol. The Protocol Stack design pattern takes advantage of the layer decoupling and provides a mechanism for dynamic insertion and removal of protocol layers from a stack. Protocol Layer Design Pattern A typical protocol layer interfaces with an upper layer and a lower layer of the protocol. In most designs there is a lot of dependency between different layers of the protocol. This makes the protocol stack implementation rigid and inflexible. Changes in one layer often percolate to other layers. The Protocol Layer Design Pattern addresses these limitations by decoupling the individual protocol layers. - EventHelix.com Team |
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