1 ISO/OSI reference model: end ISO/OSI reference model: end- -to - - PDF document

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Mesh Network Cross Layer Design : Seminar 3

Jonas Karlsson Karlstad Universitet

Email: jonas.karlsson@kau.se

Topics in Computer Networks 2010

Slides adopted from ”Cross-layer Air Interface Design for Wireless Systems” - Dr. Giovanni Giambene, “Cross Layer Overview” - Murad Khalid and “Cross-Layer Design in Wireless Networks” - Philipp Hurni

Summary Summary

Introduction, What is CrossLayer Design ? Taxonomy of cross-layer methods Detailed overview on cross-layer

interactions

Introduction

ISO/OSI reference model: end ISO/OSI reference model: end-

• to

to-

• end dialogue 1/2

end dialogue 1/2

• 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 well defined

purpose.

• Each layer offers services to the respective higher layer. It encapsulates the

implementation specific details and provides an abstract interface for its service

Application Presentation Session Transport Network Link Physical level Physical medium End System, A Network Application Presentation Session Transport Network Link Physical level End System, B Intermediate System (network) User-to-network Interface Source Destination Link Link

• Phy. Lev.
• Phy. Lev.

Network Relaying

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ISO/OSI reference model: end ISO/OSI reference model: end-

• to

to-

• end dialogue 2/2

end dialogue 2/2

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. ISO/OSI reference model: Advantages ISO/OSI reference model: Advantages

Modularity/Simplicity

• implementation details are hidden behind abstract

Interfaces – facilitates programming tasks

Layer interfaces can be easily standardized

• facilitates interoperability among different software,

network hardware, and operating systems

“Divide and Conquer”

• complex problem is broken into smaller manageable

problems

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

Current view of the Internet protocol stack

Blu eTo

• th

WiFi, WiM ax

3G, Sate llite

Eth ern et

IPv6/MIPv6 TCP UDP RTP Applications AD SL The TCP/IP protocol stack follows the OSI reference model guidelines to a large degree. The success of the TCP/IP based Internet and the fast progress is based on its modular layer architecture. TCP/ IP has been designed and fits very well for point-to-point communication in wireline communication systems.

Challenging characteristics of wireless communications Challenging characteristics of wireless communications Lack of channel reliability (need of

countermeasures: coding, retransmissions, modulation techniques, diversity, etc.)

Dynamically varying channel characteristics Bandwidth shortage:

• Necessity of managing the bandwidth in an

efficient way to support broadband applications

QoS support for multimedia traffic classes Pathloss / inaccuracies ( what exactly is a

“link” ? )

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Disadvantages of the OSI model

The needs of a service provided by the communication system to its users are defined at the top-level. The hierarchy and the overall performance of the system is however build upon the bottom-level. Each layer can be considered as a black-box, with a simple send/receive API. Such information hiding is a simple approach, but can lead to poor performance. The bottom level does not communicate directly, but through all higher layers with the top-level. Information is lost during this layer- by-layer conversion. Layers are independently optimized.

Layered design and wireless communications Layered design and wireless communications

The OSI layered approach has some ‘limits’ in the case of

wireless communications:

• Lack of flexibility: It is not flexible enough to cope with the

dynamics of mobile networks.

• Redundancy & inefficiency: it is inadequate to optimize each

layer independently in the wireless scenario. There exists tight interdependence between layers.

• Restrictive: layer N may need access to lower layers than N-1.

Multiradio/multichannel, directional antenna, MIMO Multi-hop:

• Accumulated effect

MAC, routing, and transport protocols have to work

together with the physical layer.

Available approaches for cross Available approaches for cross-

• layering

layering

• Information coupling between layers
• Defines additional interface between non-adjacent layers to exchange parametric info. for

performance optimization

• Merging layers
• PHY and MAC layer exchange information more frequently and therefore can be merged as

“MAPHY”; e.g, link adaptation for link QoS

• Design coupling between layers
• A new protocol at a layer is designed to accommodate new features of another layer; e.g.,

multi-packet capture at Phy layer needs modification at MAC layer

Application Transport Network Physical Data Link Application Transport Network

DATA + PHY

Info. Coupling Layer Merging Application Transport Network Data Link

(new protocol for

multipacket Phy)

Physical

(Multipacket capability)

Design Coupling

Vineet Srivastava, Mehul Motani, "Cross-Layer Design: A Survey and the Road Ahead", IEEE Communcations Magazine, December 2005, pp. 112-119

Available approaches for cross Available approaches for cross-

• layering

layering

Vertical Coupling

• The QoS metric at application layer is used

to fine tune all the lower layer parameters at the application layer; e.g., delay requirement metric is used to tune the MAC level ARQ timer and modulations and at network layer must use routing protocol to route packets through congestion free routes

• Static Vertical Coupling

Parameters are defined based on some metric at design time

• Dynamic Vertical Coupling

Parameters are exchanged in runtime to tweak the values for some pre-defined metric Application Transport Network Physical Data Link Vertical Coupling

Vineet Srivastava, Mehul Motani, "Cross-Layer Design: A Survey and the Road Ahead", IEEE Communcations Magazine, December 2005, pp. 112-119

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Cross-layer exchange of information Although interfaces between adjacent layers are in general preferable, there can be the need for efficient and direct interaction between non-adjacent layers; in general, a layer should be aware of the other layers of the protocol stack. Cross-layer information can be exchanged from higher to lower layers (top-down approach) or from lower to higher layers (bottom-up approach). Coordination of both exchanges is necessary to avoid loops in the system and

• scillating behaviors.

Detailed view of possible cross Detailed view of possible cross-

• layer interactions

layer interactions Network Application

QoS requirements, priority level, service class, etc.

Transport Link Physical

Selected ACM mode TCP version, cwnd & ssthresh values Explicit congestion notification (network congestion) Link capacity saturation (link congestion) Receiver congestion indication Scaling of multimedia flow Need of a cell handover and related resource allocation with re-routing in the network

End host

AQM

Network Application

QoS requirements, priority level, service class, etc.

Transport Link Physical

Selected ACM mode TCP version, cwnd & ssthresh values Explicit congestion notification (network congestion) Link capacity saturation (link congestion) Receiver congestion indication Scaling of multimedia flow Need of a cell handover and related resource allocation with re-routing in the network

End host

AQM

Taxonomy of cross-layer methods

Examined papers Examined papers

The following papers provide a good

description on cross-layer methods in different scenarios.

• Q. Wang, M.-A. Abu-Rgheff, “Cross-layer Signalling for Next-Generation

Wireless Systems”, IEEE Wireless Communications and Networking Conference (WCNC), 16-20 March 2003, New Orleans, USA.

• M. Conti, J. Crowcroft, G. Maselli, G. Turi, “A Modular Cross-Layer

Architecture for Ad Hoc Networks”, Chapter 1 in Handbook on Theoretical and Algorithmic Aspects of Sensor, Ad Hoc Wireless, and Peer-to-Peer Networks, Jie Wu (editor), CRC Press, New York, 2005.

• V. Vardhan, D. G. Sachs, W. Yuan, A. F. Harris, S. V. Adve, D. L. Jones,
• R. H. Kravets, K. Nahrstedt,“GRACE: A Hierarchical Adaptation

Framework for Saving Energy”, Computer Science, University of Illinois Technical Report UIUCDCS-R-2004-2409, February 2004.

5 Cross Cross-

• layer method taxonomy

layer method taxonomy

Implicit cross-layer design:

• There is no exchange of information among different layers

during operation, but in the design phase cross-layer interactions are taken into consideration for a joint

• ptimization.

Explicit cross-layer design:

• Signaling interactions among (non-)adjacent protocol levels

are employed so that dynamic adaptation on the basis of different protocol layer behaviors is possible.

Explicit cross Explicit cross-

• layering

layering

The coordination of signaling could be made by a

protocol layer (horizontal approach) or an external element that is common to all the layers (vertical approach).

• In the first case, the coordinating protocol layer can have

interfaces only with adjacent layers; note that the application layer or the Medium Access Control (MAC) layer could trigger the signaling, thus respectively having an Application-centric approach or a MAC-centric one.

• In the second case, a global coordinator of different layers

could be considered having interfaces with all the protocol layers.

Explicit cross Explicit cross-

• layering: vertical approach

layering: vertical approach

A 'Shared Database' is adopted that is able

to get the internal state information from the different protocols (data stored in a shared memory);

The 'Shared Database' can set the internal

state of the protocols at different layers on the basis of suitable external events.

The 'Shared Database' realizes the new

interfaces beyond the stacked classical protocol architecture.

Explicit cross Explicit cross-

• layering: vertical approach (cont’d)

layering: vertical approach (cont’d)

Events (e.g., ACK timeout,

handoff start, outage, etc.) are notifications sent to the coordinator and used to trigger the management algorithms.

State variables (e.g.,

congestion window, routing table, BER, etc.) represent entry points to get/set

• perations that allow the

coordinator to query or modify the internal state of a protocol.

IPv6/MIPv6 TCP UDP RTP Applications

Bluetooth

WiFi, WiMAX

3G, 3G+, Satellite

Ethernet

ADSL

Cross-layer manager

Events State variables Events State variables Events State variables Events State variables MAC/PHY Network Transport Application

IPv6/MIPv6 TCP UDP RTP TCP UDP RTP UDP RTP Applications

Bluetooth

WiFi, WiMAX

3G, 3G+, Satellite

Ethernet

ADSL

Cross-layer manager

Events State variables Events State variables Events State variables Events State variables MAC/PHY Network Transport Application

Application Transport Network Data Link Vertica l layer Physical Vertical Interface

6 Interactions some examples MAC/PHY Cross layer design MAC/PHY Cross layer design

Physical layer can provide:

• OFDM, UWB, multiradio/multichannel,
• MIMO, etc

MAC layer can provide:

• The function to tune the parameters to

achieve optimal performance

Resource management scheme (i.e.,

access protocols and scheduling techniques) aware of the physical layer behavior and of the related use of Adaptive Coding & Modulation (ACM).

Routing/MAC Cross layer design Routing/MAC Cross layer design

The routing performance is poor if

the MAC layer doesn’t provide satisfying performance.

The information in MAC layer, such

as link quality, interference level, traffic load information and channel allocation, is also important to determine the best routing path.

It is important that the resource

allocation scheme (layer 2) manages traffic in a way that is coherent with the for e.g. QoS support for IP traffic (layer 3)

Transport/PHY/Application Cross layer design Transport/PHY/Application Cross layer design

TCP can use physical information to

determine real congestion or poor quality in a link.

The adaptation of several modulation

and coding levels at the PHY level should be feed back to the application layer to change dynamically the source generation bit-rate (bottom-up approach);

• Adaptation in the source coding is

important in order to avoid buffer overflow and loss of information when conservative transmission modes are employed during fading intervals.

7 Cross-layer signaling methods

Cross Cross-

• layer signaling methods

layer signaling methods

In general, different methods can be used to

support the exchange of signaling information of the explicit approach. In particular, two relevant and complementary methods are:

• In-band signaling with the use of enriched packet

headers to notify internal state variables to either other layers within a given host or in a peer-to-peer case with another host or gateway (this method needs the redefinition of packets headers or the use of spare bits);

• Out-of-band signaling via the control plane and the use
• f new primitives to allow the dialogue between protocol

layers.

• Q. Wang, M.-A. Abu-Rgheff, “Cross-layer Signalling for Next-Generation Wireless Systems”, IEEE WCNC, 16-20 March 2003, USA.

Packet headers Packet headers

This method makes use of

packet headers as in-band message carriers; there is no need of a dedicated internal message protocol.

This approach can be

visualized as a “signaling pipe”.

This method is well suited for

a top-down approach.

ICMP messages ICMP messages

• ICMP is a widely-deployed

signaling protocol in IP-based

• networks. This method tries to “punch holes in the

protocol stack” and to propagate information across layers by using ICMP messages.

• A new ICMP message is generated only when a parameter

changes beyond a given threshold.

• Since cross-layer communications are carried out through

selected “holes” (not a general “pipe”), this method seems more flexible and efficient.

• This method is more mature since it has been

implemented

• n Linux operating system with suitably

deployed APIs.

• An ICMP message is always encapsulated in an IP packet,

and this indicates that the message has to pass by the network layer even if the signaling is only between link layer and application layer .

• This method is well suited for the bottom-up cross-layer

approach.

8 Network service Network service

• In this scheme, a specific access network

service called Wireless Channel Information (WCI) is used.

• Channel and link states from physical layer

and link layer are gathered, abstracted and managed by third parties, i.e., distributed servers.

• Interested applications then access to the

servers for their required parameters from the lowest two layers.

• Although there is not a cross-layer

signaling scheme within a terminal, we can deem it complementary to the two previous schemes.

• Any intensive use of such method would

introduce considerable signaling overhead and delay across a radio access network.

Local profiles Local profiles

In this method, local

profiles are used to store periodically update information coming from nodes.

Cross-layer information

is abstracted from each necessary layer and stored in separate profiles within the nodes.

Other interested

layer(s) can then select the profile(s) to fetch the desired information.

An example A cross-layer technique with signaling scheme and performance

Example #1: cross Example #1: cross-

• layer techniques for WiFi systems

layer techniques for WiFi systems

Interest: combining MAC and TCP layers for vertical

• ptimization across the protocol stack.

Strategies to sense the channel quality and adjusting the

transmission parameters accordingly are of great importance.

• Specific possibilities are: use of a more error-resistant modulation

scheme, increased Forward Error Correction (FEC), retransmission

• f lost packets, division of packets into smaller fragments, and

channel probing. Proposal of an “intelligent link layer":

• Adoption of adaptive link layer strategies as a means to improve

transmission over a time-varying faulty wireless channel.

• A vertical interaction between the application and the lower

protocol layers is required to notify the QoS needs.

• M. Methfessel, K. F. Dombrowski, P. Langendörfer, H. Frankenfeldt, I. Babanskaja, I. Matthaei, R. Kraemer, “Vertical
• ptimization of data transmission for mobile wireless terminals”, IEEE Wireless Communications, pp. 36-43, 2002.

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Example #2: TCP-PHY interactions with ACM (cont’d)

Increasing the code-rate implies reducing the BER but also reduces the information bit-rate available for higher layers. These phenomena have contrasting effects at the TCP level: there is an optimal code rate that can be seen in different cases depending on the Eb/N0 values.

GEO-bent-pipe satellite, 2 MHz BW Classical TCP version AWGN channel BPSK Different conv. codes available

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• 4

10

• 3

10

• 2

10

• 1

100 10

1

TCP goodput [Mbit/s] Eb/N0 = 4dB Eb/N0 = 5dB Eb/N0 = 6dB Eb/N0 = 7dB Eb/N0 = 8dB 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 10

• 5

Coding rate

Example #3: MobileMan

Ad hoc Networks

Vertical layer architecture with five horizontal layers Network status layer stores variables related to energy management, security and cooperation Middleware Transport Network MAC and Physical Application

Network Status (Energy, security, cooperation management ) Cross layer Interaction

Cross layer limitations Cross layer limitations

No standard reference architecture Cross layer optimization may vary from one technology to

another?

Cross layer interaction could lead to “spaghetti code” which is not

easy to debug

Cross layer interaction can lead to conflicting optimization

• For example, at one level (NET-TRANSP) trying to find a route that
• ffers less delay based on minimum hops may result in conflict with

another level (PHY-MAC) that tries to increase throughput by going to higher level modulations when the SNIR at the receiver is good

Cross layer interactions may lead to instability Cross layer optimization increases complexity and hence may

consume more power

Conclusions Conclusions

The design of new IP-based wireless

communication systems calls for system

• ptimization.

The potentialities of the novel cross-layer approach

in the air interface design have to be investigated, considering interactions among lower layers, network, and transport layers according to a more comprehensive framework.

The cross-layer design is an interdisciplinary

research field that allows new optimization methods and could require new signaling schemes

10 Any questions ?

Thank you!