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<p>As can be seen in Figure 4, the SAFE4CS SA Forum middleware and the Asterisk PBX application are combined together on the IBM BladeCenter T. Utilizing two HS20 blades one in active mode the other as a hot-standby the highly available environment is created. The proof of concept modifies the standard open Asterisk source base to use the SA Forum middleware to enable failover capability. This can be demonstrated as follows: SIP phone A initiates a call to phone B through the VoIP PBX. During the call, a fault is injected into the blade supporting the active node. The middleware detection mechanism is signaled and seamlessly facilitates the failover to the hot-standby blade. The callers are able to continue their conversation and have no perception that anything has changed. Thus we are able to see a failure in an active IP stream without any user detectable disruption of service. Figure 5 below shows the two logical nodes that are running the Asterisk PBX high-availability setup inside the BladeCenter T.</p>

About the author Alban d'Halluin earned a Master in Applied Mathematics from Harvard University, Boston, USA and From Ecole Polytechnique, Paris, France. Mr d'Halluin worked on the development of super-resolution at Let It Wave and is now VP Marketing. He can be reached at .

There are still three blocks left and task 2 could complete, and then task1 with the two resources freed by task 2 and finally task 3 could complete with the resources freed up from task 1.

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Shared Resource Integrity A second hazard of concurrent programming is resource corruption. When two (or more) interleaved threads of execution both try to update the same variable (or otherwise modify some shared resource), the two threads can interfere with each other, corrupting the state of the variable or shared resource.

As an example, consider a DSP system with the following characteristics:

There are two possible interleavings for these tasks on the system, as shown below:

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Solution—Mutual Exclusion Interleaving 1 does not work correctly because the preemption occurred in the middle of a read/modify/write cycle. The variable X was shared between the tasks. For correct operation, any read/modify/write operation on a shared resource must be executed atomically ; that is, in such a way that it cannot be interrupted or pre-empted until complete. Sections of code that must execute atomically are critical sections . A critical section is a sequence of statements that must appear to be executed indivisibly.

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Another way of viewing this problem is to say that while the shared variable can be shared, it can only be manipulated or modified by one thread at a time; that is, any time one thread is modifying the variable, all other threads are excluded from access. This access discipline (or type of synchronization) is called mutual exclusion .

Synchronizing Tasks for Mutual Exclusion There are a number of potential solutions to the synchronization requirement. Some approaches are dependent on the DSP hardware. These approaches can be very efficient but they are not always useable.

These semiconductor processor developments will make sure that displays will match the characteristics of the human eye and therewith give a better sensory experience.

Standards for anywhere/anytime access in the home There is still an additional set of technologies that must be included if the reinvention of the STB is to be properly realized.

The third consumer benefit cited above—anytime/anywhere media—can only be achieved through robust wireless RF such as Wi-Fi (more specifically IEEE 802.11n), WiMAX, advanced 3G cellular and 4G cellular networks, and undoubtedly other technologies as well such as powerline, HomePNA, Moca, ethernet, etc.

An option that is receiving a great deal of technical interest is the introduction of peer-to-peer networks in the home because it reduces Internet bandwidth requirements—a worthy goal in a telecommunications infrastructure stressed by sharply rising video content. Today's paradigm calls for material that is requested from any part of the home to be routed through the home network.

The IP-STB is likely to play a key role in this, as it is already connected to a (home) IP network. Therefore access to content in other places in the home over the same IP network is enabled. Current WiFi technology (based on IEEE 802.11g) lacks bandwidth for HDTV video streaming. The new 802.11n standard operating on 5Ghz will be essential for IP-STB enabling fast and reliable streaming of video content through the home. New silicon available for 802.11n is all in the draft certification phase and currently used on a small scale for gateways. As this WiFi standard is the ideal fit with an IP-STB, a strong market penetration is expected starting in 2008.


Synchronizing Tasks for Mutual Exclusion There are a number of potential solutions to the synchronization requirement. Some approaches are dependent on the DSP hardware. These approaches can be very efficient but they are not always useable.

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