Cross Layering in MANETs’ Design
Marco Conti
CNR-IIT, Pisa, Italy, marco.conti@iit.cnr.it
The IETF MANET WG proposes a view of mobile ad hoc networks as an evolution of the
Internet. This mainly implies an IP-centric view of the network, and the use of a layered
architecture. This paradigm has greatly simplified network design and led to the robust scalable
protocols in the Internet. The use of the IP protocol has two main advantages: it simplifies
MANET interconnection to the Internet, and guarantees the independence from wireless
technologies [MC03].
The layered approach leads the research efforts mainly to target isolated components of the
overall network design (e.g., routing, MAC, power control). Each layer in the protocol stack is
designed and operated independently, with interfaces between layers that are static and
independent of the individual network constraints and applications. However, in a MANET some
functions cannot be assigned to a single layer. Energy management, security and cooperation,
quality of service, among the others cannot be completely implemented in a single layer but they
are implemented by combining and exploiting mechanisms implemented in all layers. An
efficient implementation of these functionalities can thus be achieved by avoiding a strict
layering approach in which the protocols at each layer are developed in isolation, but rather
within an integrated and hierarchical framework to take advantage of the interdependencies
between them. In addition, a cross-layering design, by emphasizing inter-layer dependencies, is a
promising direction for the overall design of an ad hoc network. For example, by exploiting a
cross-layering approach, the physical layer can adapt rate, power, and coding to meet the
requirements of the application given current channel and network conditions; the MAC layer
can adapt based on underlying link and interference conditions. Adaptive routing protocols can
be developed based on current link, network, and traffic conditions. Finally, the application layer
can adapt to the underlying network conditions to deliver the highest possible application quality.
Relaxing the Internet layered architecture, by removing strict layer boundaries, is therefore an
open issue in the MANET evolution. An open question is whether clear protocol layer separation
is something that must guide our work in MANET, or should take advantage of the benefits
provided by cross layer interactions to improve performance.
We believe that, without any doubt, efficient solutions for ad hoc networks require a more strict
cooperation among protocols belonging to different layers. The question is to what extent the
pure layered approach needs to be modified?
At one end, we have solutions based on layer triggers that are still compatible with the principle
of separation among layers. Layer triggers are pre-defined signals to notify some events to the
higher layer, e.g., failure in data delivery. For example, to minimize the impact of mobility and
link disconnection on TCP performance, it was proposed to introduce explicit signaling (Route
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Failure and Route Re-establishment notifications) from intermediate nodes to notify the sender
TCP of the disruption of the current route, and construction of a new one [CRVP01].
A full cross-layer design is a more extreme solution that violates the separation principle by
exploiting layers’ interdependencies in the protocols design. In this case control information is
continuously flowing both ways through the protocol stack, and a protocol behavior adapts both
to higher and lower protocols’ status. For example, the physical layer can adapt rate, power, and
coding to meet the requirements of the application given current channel and network conditions;
the MAC layer can adapt based on underlying link and interference conditions as well as delay
constraints and bit priorities. Adaptive routing protocols can be developed based on current link,
network, and traffic conditions. Finally, the application layer can utilize a notion of soft QoS that
adapts to the underlying network conditions to deliver the highest possible application quality
[GW02].
Several examples exist in the literature applying the cross-layer design principle to different
layers of the protocol stack: from the MAC and physical layers to middleware and routing layers
[YLA02], [FZX03], [CZN02]. The idea that cross-layer design is a valuable tool for an efficient
design of the overall architecture of an ad hoc network is currently discussed in the framework of
the MOBILEMAN project. MOBILEMAN (http://cnd.iit.cnr.it/mobileMAN) is a project funded by
the European Commission under the Future and Emerging Technologies arm of the “Information
Society Technologies” Programme. The aim of the project is to investigate the potentialities of
the MANET’s paradigm from several perspectives (technical, social and economic).
Figure 1: MobileMAN reference architecture
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Figure 1 shows the reference architecture of an ad hoc network organized according the cross
layer principle. As shown in the figure, the kernel of the architecture is a shared memory
(“Network Status” in the figure) that is a repository of all the network status information
collected by the network protocols. All protocols can access this memory to write the collected
information, and to read information produced/collected from the other protocols. This avoids
duplicating the layers’ efforts for collecting network-status information, thus leading to a more
efficient system design. In additions, inter-layer co-operations can be easily implemented by
variables sharing. In detail, the cross-layer architecture shown in Figure 1 guarantees:
•
a full context awareness at all layers of the protocol stack;
•
a reduced overhead for collecting the network status information avoiding duplicate date at
different layers;
•
joint optimization in the protocols’ design;
•
both global and local adaptation to system’s conditions.
On the other hand, to achieve these advantages we have to pay some costs: i) all protocols need
to be re-designed, ii) the independence from underlying wireless technologies is not guaranteed,
and iii) the interconnection to the Internet is not straightforward.
The use of layers’ triggers could be a compromise between maintaining a layered approach and
exploiting inter-layer cooperation. In the layer-trigger case, to guarantee the independence
among layers (e.g., the network protocol from the underlying wireless technologies), the
information to be exchanged across layers must be standardized
Acknowledgements. This was partially funded by the Information Society Technologies programme of
the European Commission, Future and Emerging Technologies under the IST-2001-38113 MobileMAN
project.
References
[CZN02] K. Chen, S.H. Shah, K. Nahrstedt, “Cross-Layer Design for Data Accessibility in Mobile Ad Hoc
Networks”, Wireless Personal Communications 21: 49–76, 2002
[CRVP01] K. Chandran, S. Raghunathan, S. Venkatesan, R. Prakash, “A Feedback Based Scheme for Improving
TCP Performance in Ad Hoc Wireless Networks”, IEEE Personal Communication Magazine, Special Issue on
Ad Hoc Networks, Vol. 8, N. 1, pp. 34-39, February 2001.
[FZX03] Zhenghua Fu, Petros Zerfos, Kaixin Xu, Haiyun Luo, Songwu Lu, Lixia Zhang, Mario Gerla, “The Impact of
Multihop Wireless Channel on TCP Throughput and Loss”, Proc. Infocom 2003, San Francisco, April 2003.
[GW02] A.J. Goldsmith, S.B. Wicker, “Design Challenges for Energy-Constrained Ad Hoc Wireless Networks”,
IEEE Wireless Communications, Volume 9, Number 4, August 2002. pp. 8- 27.
[MC03] J.P. Macker, S. Corson, “Mobile Ad hoc Networks (MANET): Routing technology for dynamic,
wirelessnetworking”, in Mobile Ad hoc networking, S. Basagni, M. Conti, S. Giordano, I. Stojmenovic
(Editors), IEEE Press and John Wiley and Sons, Inc., New York, 2003.
[YLA02] Wing Ho Yuen, Heung-no Lee, Timothy D. Andersen, “A Simple and Effective Cross Layer Networking
System for Mobile Ad Hoc Networks”, Proc. of IEEE PIMRC, 2002.
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