3/22/14

Wireless Links Network Characteristics.

We can find number of important differencies between a wired link and a wireless link:

* Decreasing signal strength - electromagnetic radiation attenuates as it passes through matter. Even in free space, the signal will disperse, resulting in decreased signal strength (sometimes referred to as path loss) as the distance between sender and receiver increases.
* Interference from other sources - radio sources transmitting in the same frequency band will interfere with each other.
* Multipath propagation - occurs when portions of the electromagnetic wave reflect off objects and the ground, taking paths of different lengths between a sender and receiver. This results in the blurring of the received signal at the receiver. Moving objects between the sender and receiver can cause multipath propagation to change over time.

Errors, signal loss and correction.

The discussion above suggests that bit errors will be more common in wireless links than in wired links. For this reason, it's perhaps not suprising that wireless link protocols employ not only powerful CRC (Cyclic Redundancy Check) error detection codes, but also link-level reliable-data transfer protocols that retransmits corrupted frames.

Signal to noise ratio.

Host receives a signal from the sender, degraded by attenuation and multipath propagation and also receives background noise in the enironment. The signal-to-noise-ratio (SNR) is a relative measure of the strength of the received (i.e., the information being transmitted) and this noise. SNR is measured in decibels (dB), a unit of measure which confuses computer scientists.

Better SNR, the less errors and faster transmission - sometimes it's better to walk around a place to find a perfect communication spot.

Hidden Terminal Problem.

If an internet device A communicates with an internet device B, and internet device C communicates with the same device B there can be signal collisions, signal strength detoriating that way. Communication that way (via the internet device B) can be caused by situations such as:

* an obstacle preventing direct communication between internet device A and internet device C,
* distance between internet device A and internet device C and signal fading effect.

Source: [3].

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