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Wireless communications for mobile devices

·468 words·3 mins· ·
Table of Contents
Mobile systems - This article is part of a series.
Part 1: This Article
Part 2: Wifi
Part 3: Wimax
Part 4: Cellular networking
Part 5: Bluetooth

To understand better the problem that mobile device’s software have to face we need to analyze the physical medium that allows this devices to communicate, the wireless medium.

Wireless medium
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The physical layer over wireless medium has a lot of problems due to the signal nature:

  • shadowing
  • frequency-selective fading
  • Rayleigh fading

Signal can also bounce over reflective surfaces generating reflections

There are also meteorological conditions that can influence the signal strength.

In real scenarios with mobile nodes, mathematical models cannot be used to estimates the power loss due to the nature of the environment been to variable.

In general it can be said that power fluctuations depends on the distance from the transmitter.

ℹ️ Important

layer 2 protocols need to take into account this physical layer problems

Medium access control
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IEEE defines a section of standards for wireless connectivity technologies at data link layer under the section 802.xx

flowchart TD subgraph higher_layer A[802.2] B[802.3] C[802.5] D[802.11] end subgraph medium_access_control E[CSMA/CD] F[Token-passing] G[CSMA] end higher_layer ~~~ medium_access_control

Wirelles medium access control protocols
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To manage the medium access control there are different protocols in the litterature

Csma/cd
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The CSMA/CD relies on sensing the channel and transmit only when the channel il free

flowchart TD A{channel is\n free} B[send message] C{collision\n detected} D[wait a random time] A-->|yes|B B-->C C-->|yes|D A-->|no|D D-->A

In wireless environment this approach have some issues due to the nature of the nodes not being able to listen the entire channel and the node mobility

Hidden and exposed node issues
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This are particular problems that can manifest in a wireless environment where the communication is obstructed from the presence of one or more nodes in the range of the transmitter and receiver

Csma/ca variant
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A way to limit the hidden/expose node issue is to send short acknowledgment frames before transmit:

flowchart TD A[A] B[B] A --sends rts--> B B --sends cts--> A

So the node A sends a request to send packet to B and wait for a clear to send reply before start transmitting the data

This solution limits collisions only to the rts and cts packets reducing the cost of re transmission but increase the overhead because every communication requires an rtc/cts exchange

Wireless architectures
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There are two possible configuration for a wireless infrastructure:

Base station mode
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The nodes are connected to a base station and communication can only append trough the base station itself

flowchart LR subgraph net1 A[ap1] C[node1] end subgraph net2 B[ap2] D[node2] end C --> A D --> B E((internet)) A & B --> E

Ad hoc mode
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All nodes are potentially mobile and communicate directly

flowchart LR subgraph net A[node1] B[node2] C[node3] D[node4] E[node5] A <--> B & C B <--> D C <--> D D <--> E E <--> B end
Matteo Longhi
Author
Matteo Longhi
I’m a software engineer with a passion for Music, food, dogs, videogames and open source software, i’m currently working as a devops engineer
Mobile systems - This article is part of a series.
Part 1: This Article
Part 2: Wifi
Part 3: Wimax
Part 4: Cellular networking
Part 5: Bluetooth