Issues in ad-hoc networks

Miguel Sanchez

Nov-2000

Contents

• Introduction to ad-hoc networks

• Media access control in wireless networks

• Routing in ad-hoc networks

• Wireless Local Area Network standards

• Research project

What an ad-hoc networks is?

• It is a network made of wireless nodes

• No network infrastructure is present

• Transmission range is limited, not all the destinations can be reached in one hop

• Multi-hop forwarding is needed to provide any-to-any communication

• Nodes act as both, end-points and routers

What an ad-hoc networks is for?

• Cellular networks are infrastructure-based networks

• When there is no network available, ad-hoc networks can be of help:– Disaster recovery (floods, earthquakes, …)– Space exploration (multiple probes)– Network of sensors (NOS)

• Even if a network is available:– Meeting rooms (education or business)– Military applications (armies communication)

Why not to have a longer transmission range?

• A long transmission range can avoid the multi-hop routing

• By using low power transmissions a double benefit is obtained:– Reduced power consumption (batteries lasting

longer)– Increased spectrum reuse (more network

throughput)

Contents

• Introduction to ad-hoc networks

• Media access control in wireless networks

• Routing in ad-hoc networks

• Wireless Local Area Network standards

• Research project

Media access in wireless networks

• Dashed circle is a transmitting node

• Arrows show receiving nodes

• Dashed-line big circle is the transmission range

• Nodes than can hear a given transmitter are called neighbors of it

New MAC problems

• Carrier sense algorithms suffer the hidden terminal problem

• Some protocols establish a reservation handshake before actually sending the data to avoid this problem

RTS/CTS reservation dialog

• Unicast only• Transmitter sends a RTS packet to the

destination node• Destination node acknowledges by sending

back a CTS packet• If CTS is successfully received, then

transmitter sends a data packet. If not, backs off and it retries later

Leading idea of our research at the MAC level

• We question the current approach of letting the hardware features to decide

• We study both, static and dynamic settings for the transmission range:

1. Optimal transmission range

2. Adaptive power control

Transmission range adjustment

• We have proposed an heuristic to adjust the transmission range

• With this method, we can estimate a transmission range for best throughput results

• In this work we obtain a static value for the transmission range

Our power control algorithm

• RTS/CTS dialog is a reservation mechanism• This dialog always reserve the same area for

each transmission• Our algorithm uses a variable transmission

range and to adapt it depending on the distance to the destination node

• Each node can use a different power level for each transmission

Contents

• Introduction to ad-hoc networks

• Media access control in wireless networks

• Routing in ad-hoc networks

• Wireless Local Area Network standards

• Research project

IETF MANET working group

• The primary focus of the working group is to develop and evolve MANET routing specification(s) and introduce them to the Internet Standards track

• Several proposals are available for end-to-end and multicast traffic

Ad-hoc routing 101

• Topology changes can happen anytime and at a high pace

• Two basic approaches:– Proactive (i.e. table-driven routing)– Reactive (i.e. source routing)

• Some hybrid algorithms, too• Mobility scenario not defined• Multicast protocols also proposed

More ad-hoc routing problems

• Scalability, or lack of. Some protocols work ok for tens of nodes but they degrade for hundreds or thousands of nodes

• Providing QoS in such an scenario seems to be quite a challenge (some proposals exist)

• Power-conserving routing is also under study (there are some proposals too)

Contents

• Introduction to ad-hoc networks

• Media access control in wireless networks

• Routing in ad-hoc networks

• Wireless Local Area Network standards

• Research project

IEEE 802.11

• This working group developed the 1 and 2 Mbps specification

• Now a new revision, called 802.11b, is supported in some new devices, raising the speed to 11 Mbps

• Two modes of operation (alternative):– Access point based (infrastructure)– Ad-hoc (infrastructure-less)

Wireless LANs

• It is a commercial reality• It is being used as an installation-free setup

for some offices• Access points provide the necessary

connectivity to the wired backbone• Ad-hoc mode is supported but not very

often used (user has to choose the operating mode when installing the driver)

Contents

• Introduction to ad-hoc networks

• Media access control in wireless networks

• Routing in ad-hoc networks

• Wireless Local Area Network standards

• Research project

Research project

• Operational mode is alternative. Or it is access point based or it is ad-hoc

• A combined mode will allow better coverage with less access points

• A mobile node could act as a “helper node” to nearby nodes unable to reach the access point

Some extensions

• We envision an scenario where a convergence of cellular and ad-hoc networks can bring some benefits for the user

• This new scenario also brings the necessity of rethink billing systems

• Multiple access technologies can be present in smarter, new mobile devices, that could use some of these mobile extensions