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Cross-Layer Design in Wireless Networks

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Cross-Layer Design in Wireless Networks Powered By Docstoc
					                                       2.4.2008




Cross-Layer Design
in Wireless Networks

Seminar Rechnernetze und Verteilte Systeme (RVS)
Philipp Hurni
Universität Bern




                                                   1
   Cross-Layer Design



> What is Cross­Layer Design ?

> Cross Layer Design Advantages and Disadvantages

> Cross­Layer Design Examples

> Cross­Layer Design Architectures

> Skepticism towards Cross­Layer Design

> A small Case Study 




                                                    2
“Traditional” Layered Architecture
OSI Reference Model (Open Systems Interconnection)




                ­ Communication Systems are organized and divided 
                into layers. Each layer is built on top of the one below it.


                ­ Reduced Complexity: each layer should fulfill a limited and 
                well­defined purpose.


                ­ Each layer offers services to the respective higher layer. It 
                encapsulates the implementation­specific details and provides 
                an abstract interface for its service.




                                                                                   3
“Traditional” Layered Architecture
OSI Reference Model (Open Systems Interconnection)




              ­ Interface should provide only a limited set of primitives 

              ­ no assumptions / dependencies with the layer above. 
                ­> only rely on primitive operations of the adjacent lower layer

              ­ Each layer adds header and/or trailer information. This 
              information is only intended for the peer layer in the destination 
              stations and not for lower layers. (i.e. reading the header/trailer 
              information of layer 4 in layer 3)

              ­ Information exchange and coupling between layers should be 
              kept as low as possible.



                                                                                     4
“Traditional” Layered Architecture
OSI Reference Model (Open Systems Interconnection)

Advantages

­ Modularity/Simplicity
  ­>  implementation details are hidden behind abstract
    interfaces – facilitates programming tasks

­ “Divide and Conquer” ­ complex problem is broken  
into smaller manageable problems

­ Layer interfaces can be easily standardized
  ­> facilitates interoperability among different software, 
      network hardware, and operating systems

­ Flexibility / interchangeability / least impact on changes 
  ­> changes/updates in one layer do not affect the upper and lower layers
  ­> layers can be replaced, new layers can be introduced

                                                                             5
“Traditional” Layered Architecture:
TCP/IP(v4)

          ­ The TCP/IP protocol stack followed the OSI reference model 
          guidelines to a large degree. Since its standardization in 1981, 
          it has been by far the most successful implementation of the 
          OSI reference model


          ­ TCP/IP has been designed and fits very well for point­to­point 
          communication in wireline communication systems.


          ­ The success of the TCP/IP based Internet and the fast 
          progress is based on its modular layer architecture. The 
          approach has been successful in providing modularity, 
          flexibility, interchangeability and standardization.




                                                                              6
From Wireline to Wireless Networks


               ­ With the emergence of wireless networks and other new
               networking technologies in the past decade, environments
               and circumstances have changed.

               ­ Wireless networks characteristics are quite different 
               from wireline systems. Systems developers and 
               researchers face different problems and challenges than 
               in wireline networks.

               ­ Wireless channel characteristics generally affect all 
               traditional OSI­layers 
                 ­> fixing problems locally inside the layers and optimizing   
                     layers independently leads to unsatisfactory results



                                                                             7
Wireless Networks: new Challenges

 ­ noise (receiver & background)
   ­> higher Bit­Error­Rates
   ­> channel quality dynamically changes 

 ­ path loss / inaccuracies ( what exactly is a “link” ? )

 ­ multipath signal propagation 
   ­> fading effects, signal dispersion

 ­ interference problems
   ­> from other stations within range (co­channel) 
   ­> from other devices/technologies 
        (i.e. UMTS, GSM, bluetooth, 802.11)
   ­> electromagnetic interference 
        (microwaves, electric engines, vacuums)
 ­ mobility
 ­ limited (energy) resources
                                                             8
Why Cross-Layer Design?


- Exploiting the dependencies and interactions between layers has been
shown to increase performance in certain scenarios of wireless networking


- “Although layered architectures have served well for wired networks, they
are not suitable for wireless networks”


- sharing knowledge about layer state and conditions proved to be a promising
paradigm for performance optimization in wireless systems.

- providing knowledge about channel conditions (PHY and MAC) to routing,
transport and application layers allows to design more sophisticated allocation
and optimization algorithms


                                                                                  9
common misconceptions of
Cross-Layer Design


  - “Layered approach must be completely eliminated”


  - “All layers must be integrated and jointly optimized”

   ->    clearly impractical

   ->    disastrous consequences in regard of implementation,
         changes, upgrading, standardization




                                                                10
  An attempt to define Cross-Layer Design



Main idea: holistic view of wireless networking

Account for special conditions of wireless networks to fulfill
QoS-demands of applications by exploiting dependencies
of protocol layers. Share knowledge between layers to
achieve highest adaptivity and stability in situations of
instable channel conditions.

-> maintain the layered approach
-> allow interactions between various protocols of
   nonadjacent layers through introduction of new interfaces
-> keep the impact of design violations as small as possible




                                                                 11
Cross-Layer Design: Examples


- There have been countless proposals and research papers applying
some sort of Cross Layer Design. They address various problems and
issues of different kinds of wireless networks.

 Examples:
 - Optimal Channel Use

 - Energy-Conservancy issues

- Transport over Wireless Links (i.e. IP setting Explicit Congestion Notification – Bit)
- Interference and Congestion-aware Routing
- Power Control and Power Management

  et cetera et cetera

                                                                                           12
Cross-Layer Design: Architectures

 - Cross-Layer approaches violate the traditional layer architecture
   -> Creation of new interfaces
   -> Merging of adjacent layers
   -> Sharing of variables and parameters among multiple layers




 M. Srivastava “Cross-Layer Design: A Survey and the Road Ahead”, IEEE Communications Magazine, Dec. 2005   13
Cross-Layer Design: Controversy


Critics argue that CLD is a misleading trend for the following reasons:

- Modular Architecture has proven itself time over time. Modularity and
standardization of interfaces has accelerated research and development.


- Cross-Layer Designs with tight coupling between the layers become hard
to review and redesign. Changing one subsystem implies changes in other parts,
as everything is interconnected.

- Cross-Layer Designs without solid architectural guidelines inevitable leads to
“spaghetti-design” and “spaghetti-implementations”

  -> reduced flexibility, interoperability and maintainability.
  -> systems become unpredictable. It is hard to foresee the impact of modifications

                                                                                       14
 A Case Study in a
 Wireless Sensor Network Environment

- Given a Wireless Sensor Network with the power saving protocol WiseMAC
- Find hop-count-optimal AND delay-optimal routes by exploiting MAC layer
knowledge for routing decisions.
-> Need MAC-layer specific information on the routing layer!
-> 1-hop MAC-layer information does not suffice -> need more!




                                                                            15
A Case Study in a
Wireless Sensor Network Environment

- need MAC-specific information of n-hop nodes on the routing layer
- typical Cross-Layer Design: exploit dependency between the power saving MAC
and the routing layer to improve delay.
- unorthodox “Multi-Hop Cross-Layer Design”
  -> pass MAC information of nodes in the n-hop neighborhood to routing layers




                                                                                 16
A Case Study in a
Wireless Sensor Network Environment

Questions topiced:
>   Can we decrease delay by exploiting MAC schedules?
>   How much? 
>   What parameter of n makes sense? How many hops are needed?

     Source




                                   Sink
                                                                 17
A Case Study in a
Wireless Sensor Network Environment




                      -21%
                               -30%




- The parameter of n = 2 leads to a decrease in the end-to-end delay by roughly 30%
- A further increase of n has no beneficial impact
                                                                                      18
A Case Study in a
Wireless Sensor Network Environment




- WiseMAC could be redesigned to consider the wake-up schedules all
nodes in the 2-hop neighborhood to make nearly delay-optimal routing decisions.

                                                                                  19
Conclusions


 >   Can exploit dependency between layers of the 
     WSN protocol stack to achieve some 
     performance gains

 >   Cross Layer Design will decrease Interchangeability, 
     Flexibility, Modularity

 >   Trade off : how probable are changes in the WSN 
     network stack?
 >   How frequent are changes?
 >   What are the impacts of the changes? 




                                                             20

				
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