Heterogeneity-aware Ad hoc Networking[Routing Perspective]

Dr. Young-Bae Ko

(youngko@ajou.ac.kr)

Dept. of Information and Computer Engineering

Ajou University

Contents PART 1: Mobile Ad hoc Networks in General– Brief overview

– Main issue: “Routing”

– Different approaches in routing protocol design

– Key observations

PART 2: Heterogeneity-aware Ad hoc Networks– Motivation

– Heterogeneity-aware ad hoc “routing” protocols• AODV-BlackListing protocol

• Our proposal

• Performance Evaluation

– Conclusions

Contents

Infrastructure-less wireless networks– Neither pre-existing, wired backbone nor centralized administration– Peer-to-Peer and self-organizing networks

Characteristics– Easy and rapid deployment– Dynamic topology change– Limited resources

Applications– Military environments (battlefield network)– Emergency services (disaster relief, search-and-rescue sites)– Spontaneous networking (unplanned meetings or conferences)– Embedded computing and networking applications (ubiquitous computers

with short-range interactions, vehicle networks)– Wireless sensor networks

Mobile Ad Hoc Networks

Main Issue – “Routing”

Each mobile serves as routers, not just an end point– If there is NO direct link between a source and a destination, multi-hop

routing is needed to discover their routes.

Routing is a very challenging task.– Mobility and link failure/repair may cause frequent

route changes.

– Routing protocol must be distributed, with a minimal overhead.

Significant research has been conducted.

– Mostly designed for IP based, homogeneous, mobile ad hoc networks.

– Focuses on fast route establishment and maintenance with minimal overhead

Different Approaches in Protocol Design

Proactive, Table-driven Approach– Based on traditional link-state and distance-vector routing protocols.– Continuously update the topological view of the network by periodically

exchanging appropriate information among the nodes.– Determine routes independent of traffic pattern– Examples: DSDV, OLSR, TBRPF etc.

Reactive, On-demand Approach– Discover and maintain routes only if needed– Do not continuously maintain the overall network topology – The network is flooded with “route request” control packets when a new

route is required.– Examples: DSR, AODV, LAR, etc.

Hybrid Approach– Combine the two approaches above: locally proactive, globally reactive !– Example: ZRP

Routing problems are quite well-understood.– At least, when using common assumptions about the network,

Designing reactive protocols: “Solved” problem & Designing proactive protocols: “Solved” problem

– However, interesting issues still exist when other missing parameters are considered.

• Energy efficiency, QoS support, Heterogeneity, physical layer properties (e.g., directional antennas),

Most of proposed ad hoc routing protocols are NOT heterogeneity-aware. – They assume a physically flat network architecture with bidirectional, sy

mmetric routes. – The performance of these protocols may degrade in networks with hetero

geneous nodes.

Key Observations

PART II

Heterogeneity-aware Ad Hoc Routing

Motivation In real-world multi-hop wireless networks, a physically

hierarchical network architecture is quite natural.

Reasons:– Node Heterogeneity: There exist various types of mobile hosts with

different characteristics, roles, capacities, and mobility patterns.• e.g.) In the military scenarios, the troop leader usually have more capable

networking equipment than the private soldiers.

– Link Heterogeneity: There are difference in radio transceiver capabilities of nodes (i.e., different propagation range) and difference in wireless channel interference experienced by each node.

• Unidirectional links can be quite common in ad hoc networks.

“Heterogeneity” in Ad Hoc Networks

Motivation Such an heterogeneity may create challenges to current ad hoc networking protocols, most of which simply assume a physically flat architecture.

– Most ad hoc unicast routing protocols do not take heterogeneity into account when making routing decisions

• Each node is considered “identical” in capabilities.• All network links are considered “bidirectional”.

– Most of the proposed ad hoc multicasting protocols such as ODMRP and ADMR also assume a flat network architecture.

• There are schemes that propose to utilize clustering to build hierarchical networks, but these hierarchies are merely logical, as opposed to physical.

– Current IEEE 802.11 MAC protocols with RTS/CTS handshake mechanism simply assume that links are symmetric.

• when node A can send RTS to node B, A will also be able to receive CTS from B.

Motivation

Heterogeneity-aware Ad Hoc Routing

Routing performance can be suffered from the existence of unidirectional links and routes.

The link between B and C is a unidirectional link. The packets transmitted by node B reach node C, but not vice versa.

S A B

E

C D

Motivation There has been recent attention on routing in ad hoc networks with unidirectional links [1, 2, 3].– However, these protocols rely on proactive routing mechanisms where

each node periodically exchange link information for route maintenance.

Very few reactive, on-demand protocols give attention to unidirectional links [4].

– DSR requires two separate route discoveries to find unidirectional paths: one from the source and the other from the destination, as opposed to a single route discovery to find bidirectional paths.

– In the latest AODV draft, some mechanisms are newly added to handle the problem of unidirectional links

• We refer to this version of AODV as the “AODV-BL (Blacklist)” scheme.

Related Works

Basic AODV protocol

Two important assumptions of the basic AODV protocol– All links between neighboring nodes are symmetric.

– Only one RREQ arrives at a final destination via the shortest path.

Example scenario with the previous figure:– A RREQ packet generated by a source node S traverses the path,

S-A-B-C-D.

– When node D receives the RREQ, it sends a RREP back to node S via the reverse path, D-C-B-A-S.

– However, the RREP is NOT able to reach to node B from C because node B is not located within node C’s transmission range.

– As a result of a RREP transmission failure by node C, the source S cannot receive the corresponding RREP packet and hence it experiences a route discovery failure in its first trial.

• Such a failure will cause a number of route discoveries repeatedly with no benefit !

AODV-BL (BlackListing) protocol

The newly proposed scheme for AODV for efficient operation in presence of unidirectional links– Whenever a node detects a unidirectional link to its neighbor, it blacklists

that neighbor from which a link is unidirectional.

– Later when the node receives a RREQ from one of the nodes in its blacklist set, it silently discards the RREQ to avoid forming a reverse path with unidirectional link.

The example scenario continued:– Node C cannot be acknowledged by node B and therefore it will put node B in its

blacklist set.

– Later when node S re-broadcasts a new RREQ packet, node C will ignore this RREQ received from B but forward another copy of the RREQ from node E.

– Finally, a destination node D will receive the RREQ through a longer route at this time, and return a RREP back to the source S via a reverse path; D-C-E-B-A-S, having no unidirectional links.

Problems of AODV-BL

This modified AODV scheme is simple and may be efficient when there are few unidirectional links

However, as a number of asymmetric links increase, its routing overhead is likely to become higher since a source node will always suffer from a failure in its first trial of route discovery.– It needs to flood RREQ messages more than once to find a symmetric

route with all bidirectional links.

– Thus, more number of route discoveries may be needed before a fully symmetric route is found.

It also results in an increase of a route discovery delay.

Our Proposal

We propose the improved scheme, in which a node detects a unidirectional link immediately in its a RREQ forwarding process.– All nodes receiving RREQ packet are required to decide whether to

forward it or not.

– This decision is based on the comparison of transmission range and distance

Our Simple Idea – When a node X receives a RREQ from node Y, node X compares its

transmission range using the highest power level to an estimated distance between them.

• Question: How to calculate an estimated distance between X and Y ?

– If the estimated distance from X to Y is larger than the transmission range of node X, node X considers its link towards Y to be unidirectional, and therefore drops the receiving RREQ packet without any further broadcast.

Illustration

S R

RSRR

d

S

RS

d

R

RR

(a) Bidirectional link (d<RR) (b) Unidirectional link (d>RR)

RX: Transmission range of node X d: Estimated distance between S and R)

How To Calculate the Distance (1)

Question: How to calculate an estimated distance between two communicating nodes for a RREQ message, especially at a receiver side ?– This can be done in two different ways as described below.

Network Layer Solution with location information– Assumption: Each node can know its position.

– How to work:• A transmitter node X includes its own location information with a RREQ to be

flooded.

• When some node Y receives the RREQ from X, Y can calculate the estimated distance, based on its own physical location.

– This method can be useful and easy to implement, but requires that information about physical location of the participating nodes be available.

How To Calculate the Distance (2)

A Cross Layer Solution– Utilize a path loss model for the two-ray ground reflection model.

– In wireless networks, if we know the transmitted power of a packet at the transmitter and a separation distance of the receiver, the received power is given by the following equation.

– We can then derive the following equation from the equation above.

d4 * L

Pt * Gt * Gr * (ht2 * hr2)Pr =

Pr * L

Pt * Gt * Gr * (ht2 * hr2)d =

4

(1)

(2)

Appendix

The variables used in the previous equations are listed below.

Parameters Meaning Default Value

Pt

Pr

Gt

Gr

ht

hr

L

d

Transmitter power

Received power

Transmitter antenna gain

Receiver antenna gain

Transmitter antenna height

Receiver antenna height

System loss factor

Transmitter-Receiver separation

distance in meters

1.0

1.0

1.5

1.5

1.0 (No loss)

How To Calculate the Distance (3)

A Cross Layer Solution (cont’d)

– Eq. (2) shows that a distance between two communicating nodes can be estimated at a receiver side, if the transmitted power level (Pt) of the packet transmitter and the power received at the receiver (Pr) are known.

– To implement this method, the transmitter should make the transmitted power information available for the receiver, by putting the power information either to a RREQ or to a MAC frame.

• The latter approach may cause a compatibility problem to IEEE 802.11, because it has to modify the format of the MAC header

• Instead, we use the former approach of modifying a RREQ packet format for our simulation.

– In order for these lower layer information (such as values of both transmitted and received power levels) visible to the networking layer, we can borrow a Cross Layer Design Concept from [5].

A Cross Layer Design Concept

Lower layer parameters affect the parameters in the higher layer.

Illustration of the Cross Layer communication– At the physical layer, channel estimation is per

formed to obtain the instantaneous SNRi of a link.

– A transmitter then chooses a transmission rate Ri based on the estimated SNRi. This in turn affects the packet delay Di of the link.

– At the network layer, routing decisions are made based on the packet delay associated with each link.

– The routing decisions in turn affect the offered load distribution in the network and impact the Parameters Di, Ri and SNRi on individual links. Thus the communication of various layers are inter-related.

Network

MAC

PHY

Channel

SNR_i

R_i

D_i MAC packet delay

Link capacity

SNR information

(make routing decision)

Performance Evaluation

Simulation Model (ns2 w/ CMU extensions)

– 100 nodes in a rectangular region of 1000×1000 unit area.

– Two different values for a node’s transmission range: 250 and 125 units• Equal probability

– Traffic pattern: 10 CBR connections running on UDP. • Each CBR source generates four 512-byte data packets every second.

– Mobility model: Random Waypoint model.• Each node moves with a maximum speed of 20 units per second (i.e., average

speeds of 10 units per second).

Performance Evaluation – cont’d.

Performance metrics:

– Data Packet Delivery Ratio:• Packet Delivery Ratio is defined as the ratio of the number of data

packets received by the CBR sink at a final destination, and the number of data packets originated by the application layer CBR source.

– Normalized Routing Overhead:• The Normalized Routing Overhead is defined as the ratio between the

total number of routing control packets transmitted by all nodes and the total number of data packets received by all destinations

– Route Discovery Delay:• The Route Discovery Delay is the average delay that takes to find a

route from the source to the destination.

Simulation Results (1)

Our proposed scheme’s delivery ratio is consistently higher than others.– Due to more efficient and fast detection method of unidirectional links.

Packet delivery ratio as a function of pause time

Simulation Results (2)

Our proposed scheme provides a lower rate of increase than the other two.– This is because, with the proposed scheme, number of route requests is

significantly reduced by preventing route discovery failures due to a route reply packet loss caused by the existence of unidirectional links.

Normalized routing overheadas a function of pause time

Simulation Results (3)

We see that the proposed routing scheme yields a better performance (i.e., smaller latency) than others, due to the similar reasons before.

Route Discovery Delayas a function of pause time

Conclusions

We motivated and presented heterogeneity-aware approaches for protocol design at the network layer in physically hierarchical ad hoc networks.– Simulation results indicate that our protocols perform well in

heterogeneous ad hoc networking environments.

Topics for Future Research– More complete cross-layer system solution ?

– More powerful heterogeneity-aware routing protocols ?

– What about multicasting and MAC ? • I have a short paper [6] for a multicast case though.

– And more…

References

1. L. Bao and J.J. Garcia-Luna-Aceves, “Link state routing in networks with unidirectional links,” in Proc. of IEEE ICCCN’99

2. R. Prakash, “A routing algorithm for wireless ad hoc networks with unidirectional links,” ACM/Kluwer Wireless Networks, Vol. 7(6), pp.617-625, Nov. 2001.

3. V. Ramasubramanian, R. Chandra, and D. mosse, “Providing a bidirectional abstraction for unidirectional ad hoc networks,” in Proc. of INFOCOM’02.

4. IETF mobile ad hoc networks (MANET) working group charter. [Online]. Available: http://www.ietf.org/html.charters/manet-charter.html

5. W. Yuen, H.-N. Lee, and T. Anderson, “Simple but effective cross-layer networking system for mobile ad hoc networks,” in Proc. of IEEE PIMRC’02.

6. Young-bae Ko and S.-J. Lee, “A Multicast protocol for physically hierarchical ad hoc networks,” in Proc. of VTC’03 Spring.

THANK YOU !

For more information on whatever (:-), please visit my homepage at

http://dmc.ajou.ac.kr/~youngko

(Distributed and Mobile Computing Lab)