CHAPTER 1 Introduction
1.4 Organization
modules are very popular. A pedestrian carries a cell phone is perfectly match to a node in DTNs with high mobility, short radio range, limited buffer size, and limited battery energy. In order to realize DTNs on cell phones, we try to design a grouping scheme and messages relay policy by considering the moving pattern and the limited battery.
1.3 Our Goal
In traditional routing protocols of DTNs, they can divide into two categories [1][2]:
flooding-base routing protocol and forwarding-based routing protocol. Since there are some energy management schemes to enhance those protocol for energy limitation problem,
redundancy messages transmission and information computing are the main wastes of limited energy.
Considering the moving pattern above, we are using GPS information to design a scheme with lower redundancy messages transmission and lower information computing but good delivery ratio for DTNs.
1.4 Organization
The remainder of this thesis is structured as follows. Chapter 2 discusses related work about routing protocols and energy management schemes of DTNs. Chapter 3 describes our design in detail. Performance evaluation results are discussed in Chapter 4. Chapter 5 in final summarizes our works and discusses future work.
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CHAPTER 2 Related Work
We will discuss other researches about DTNs routing protocols and energy control scheme in multi-hop network . Without energy control, DTNs routing protocols can divide into two categories: flooding-based routing protocol and forwarding-based routing protocol [1][2].
With energy control in multi-hop wireless networks, T.Armstrong was summarized into three categories: scheduled rendezvous, asynchronous, and on-demand [3].
We will briefly describe the operation of each protocol and make a simple summary at the end of the section.
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2.1 Flooding-based routing Protocol
In this category, it uses redundancy messages to make the delivery success. In order to make sure that there will at least one copy of message reach the destination, this kind of protocol is using a great number of redundancy copies. The advantage of flooding-based routing protocol is easy to implement and needs no information of the networks. On the other side, it needs a bigger buffer to cache more copies to get a good delivery ratio.
2.1.1 Epidemic Routing Protocol
Epidemic is the earliest routing protocol over DTNs. In this protocol, each message has an unique identity. When two nodes contact to each other, they will exchange their buffered messages’ identities. If there is any identity it doesn’t have, it will request to the other node for the message. With this rule, nodes do not need any information about the networks and all of them will get all messages if resources are unlimited [4].
When resources are infinite, Epidemic routing will get the minimal delay latency and maximal delivered message counts. Epidemic is also as a benchmark to evaluate the performance in DTNs.
2.1.2 Direct Contact Routing Protocol
In this protocol, nodes will not transmit any message until the destination is covered by its radio range. Since it transmits nothing but the last one to goal, it needs the minimal resource of a node. On the other side, it might take the longest delay latency because of only one transmission node for a message. Between mobile nodes and fixed gateways, direct
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contact has been purposed that will increase throughput and decrease resource usages [5].
2.2 Forwarding-based Protocol
This kind of protocol is using the information about networks to make the relay selection.
Network topology is the major property they are used. They collect or calculate information, such as location information and historical contact recorders, to choose the best relay node from contact nodes. With its relay policy, it reduces the consumption of resource. But the information is difficult to get in DTNs.
2.2.1 Location-based Routing Protocol
The Location-based routing protocol uses position information, such as GPS (Global Positioning System) information, to make the decision of relay node selection. Before transmit a message, it will calculate if the contact node has higher probability for delivery.
Lebrun et al. proved that the location-based routing protocol has better delivery ratio and lower overhead than Epidemic routing protocol [6].
2.2.2 Gradient Routing Protocol
The gradient routing is using a weighted value to recognize if the contact node is a good relay to the message. Each node has a metric table about all of possible destinations.
Messages are only transmitted when the contact node has higher probability to the given destination of the message. This kind of protocol needs more information about networks than
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a location-based one and has to maintain the metric table of each node for probability
computing. Lindgren et al. propose a probability routing protocol called PROPHET [7] using history of encounters and transitivity information.
2.2.3 Clustering and Cluster-Based Routing Protocol for Delay-Tolerant Mobile Networks
The basic idea is to let each mobile node to learn unknown and possibly random mobility parameters and join together with other mobile nodes that have similar mobility pattern into a cluster. The nodes in a cluster can then interchangeably share their resources for overhead reduction and load balancing in order to improve overall network performance [8].
In this paper, they are using some fixed hot spots and moving model in real-life [9]-[15]
to form a distributed cluster by contact probability (CP) with a threshold β (e.g. classmates at home) . And all nodes have their fixed partial path during a fixed duration (e.g. a path from home to school in the morning). Our design is extended from the idea of this paper but without fixed hot spots.
2.3 Wakeup scheduling in multi-hop wireless networks
Many wakeup scheduling schemes were proposed for efficient power management in multi-hop wireless networks. T.Armstrong [3] was summarized into three categories:
scheduled rendezvous, on-demand, and asynchronous.
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In scheduled rendezvous schemes [16] [17], there is generally assumed that clock synchronization is exist. The predefined schedule can wake nodes up at the same moment. In the distributed environment as DTNs, it is hard to synchronize all nodes without inaccuracy.
2.3.2 On-demand
In this kind of schemes [18][19], a node is usually equipped with two radio devices. The high power radio is used to transmit data and the lower power one is to explore neighbors.
The low power radio will activate the high power one when nodes need it. However, the low power radio often has short transmission range to detect contact in the environment of nodes are sparsely distributed such as DTNs. Here is a simple scenario shows on figure 1.
Figure 1：On-demand wakeup scheduling
A. Each node contains with both high power radio and lower power radio architectures.
(e.g.WiFi and BlueTooth )
B. All nodes are using lower power radio to explore neighbors.
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C. When a contact occurs, nodes are switching to high power radio to transmit and receive data.
2.3.3 Asynchronous
Asynchronous mechanisms do not require time synchronization [20] [21]. The trade-off is that system has to control the wakeup scheme carefully to make sure that nodes will be awake while a contact occurs. Yong Xi et al. [22] present the Context-Aware Power Management scheme, CAPM, to improve the PROPHET routing protocol for power management. The main idea of CAPM is to calculate the best combination of parameters shows on figure 2:
W: wake-up period of a node C: total wake-up cycle period
K: a full wake-up cycle consists of K wake-up cycles.
Figure 2：The tuple (W,C,K) as the sleep pattern in CAPM
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Figure 3：Neighbor discovery
Figure 3 shows the situation of neighbor discovery in CAPM. Although all nodes are waked up at different time in the part A of figure 3, their regular beacon messages will help them to know the existence of neighbors. In the part B, node 3 notices its requirement with a special message. Other nodes will feedback the acknowledgement to node 3. Their CAPM scheme has an adaptive on period sleeping feature that allows it to achieve high delivery ratio when used with Prophet.
We use the idea of CAPM to modify the routing protocol: Epdemic and PROPHET, to improve their ability of energy control as comparisons in the simulator with our design.
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In this section, we will introduce our design in detail. At first, we give an overview of the protocol and describe details in the subsections below. We purpose a location-assisted routing with Energy-aware grouping design by considering moving pattern within two grouping methods to extend system lifetime.
Most wireless devices can operate in different modes to control the power consumption, such as idle listening, sleeping, transmitting, and receiving. The power consumption of idle listening is the main target to reduce in wireless ad hoc networks for efficiency energy usage [23]. Our main idea is to form groups where each group has only one node awake, called
“leader node”, to charge with contacts and messages’ transmission in listening mode or transmitting/receiving mode. And all the others in the same group, called “member nodes”, will turn the communication device to sleeping mode to save energy. The isolated node is also a leader node which is keeping awake. We also set a timed-out parameter for sleeping
duration to prevent a node been dead-lock in the sleeping mode without role changing.
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We model a DTN as a set of mobile nodes in a pedestrian environment. A node is a pedestrian who carries a mobile device equipped with GPS and 802.11 communication modules. A “contact” is defined as two nodes are within their 802.11 radio range so that they can exchange messages for the duration. Since member nodes turn to sleeping mode most time, we limit a contact will only happen between two leader nodes.
A navigation system is preinstalled with map data in each node. Base on it, nodes can get the GPS information of both its moving route and messages’ destinations to packet in beacon message for contact broadcast. Each message has a fixed destination location such as a portal to internet.
3.2 Moving Pattern
We assume that all nodes can only move on the roads. Each node has its own group which has been predefined such like school, job…etc. And we consider three kinds of nodes’
moving scenarios:
1. Nodes never depart.
2. Nodes may depart or join.
3. Nodes move randomly.
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3.2.1 Scenario 1：Nodes Never Depart
In this scenario, a group looks like a touring party in a tourist attraction. People have same moving path, speed, and stop duration. They will always move around their guide until the tour ended. We assume each node has an unique node identity and a group identity (GID) it belongs to. Each group is formed by default with a group table and an unique identity which is different to all other groups. Because of all nodes are moving with similar parameters, they will never depart from its group.
3.2.2 Scenario 2：Nodes May Depart or Join
Same as the first scenario, groups are formed by default with group tables, but some nodes might depart from the group because of different moving speed or path. When a node leaves away, it won’t be found on leader selecting duration. After that, the group will keep working with rest nodes and the isolated node will change to be a leader node itself.
“Join” event will only happen between two leader nodes. When two leader nodes contact, they will try to negotiate if one of them join to another or not. If the join event happens, all other member nodes will turn to be leader nodes to charge with everything themselves.
3.2.3 Scenario 3：Nodes Move Randomly
In the last scenario, nodes are randomly moving with a group identity (GID). Like people
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work in same company or students study in same school, they will have a same GID. Similar to scenario 2, “Join” event will only happen between two leader nodes. If both of them are single node, the event can be also called “form a group” event. The different between 2 and 3 is nodes are along in the beginning. And “depart” and “join” events occur more often in this scenario.
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3.3 Energy-aware Grouping (EG) Design
Figure 4 is an overview of our design: Energy-aware Grouping (EG). Since nodes are equipped GPS device, a leader node will record the waypoints of its moving path which are predefined in the preinstall navigation system. A beacon message contains with its group identity (GID), the record of historical waypoints, the next waypoint, and the destinations of messages. Once a leader node gets it, it will check the path information first to recognize if they are suit to form a group or not. If the “join” event won’t happen, they will go to the step of “message relay”.
Figure 4：Overview of Energy-aware Grouping (EG)
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When the sleeping duration timed-out, all nodes are checking their status and transmit a message of it to their leader node to make the decision of leader selection for next round. After the selection, the original leader node will alert all other nodes who will be the new leader node in next round. A member node goes to sleeping mode after received the alert and the new leader node charges with group’s contact and messages’ transmission.
3.3.1 Methods choose when contact between method 1 (M1) and method 2 (M2)
Our design contains with two methods for different scenarios. The first method, called M1, is designed for less changing situation like scenario 1 and the other method, called M2, is for others.
A single node uses M2 as its default grouping policy for more opportunities to form a group. When a contact occurs, leader nodes will check the other one’s group identity (GID) and the record of historical and next waypoints.
If they have a same GID and same waypoints in the last three (or more) waypoints, they will use M1 to form a group. This is also the only chance for a node to change its grouping method from M2 to M1. Once a node chooses M1, it will not change the group method until it becomes a group contains with only single node, such as the situation of can’t find its leader node after sleeping duration timed-out. Choosing M1 is the highest priority if the conditions are satisfied.
When the conditions for M1 are dissatisfied, they will choose M2 as their grouping method if they have same records of next waypoints. Otherwise, they will go to message relay policy checking.
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3.3.2 Method 1 (M1) for less variation
When a group formed by M1 method, it will only check the group identity (GID) as the main condition any time. Even if a node has same moving path in the future approaches, it is not allowed to join the M1 group if its GID is not match. Each GID is an unique code with a group table in the system. Every node has its group table to realize which node can be grouping with and which one is the leader node of the group. The first leader node is set by default with randomly chosen. Once the M1 group is formed, nodes will only check GID and group table to maintain it. The flow shows on Figure 5 and the detail shows below.
Figure 5：The flow of method 1 (M1)
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At the beginning of the round, nodes check its group table to recognize its GID and the role of the group. If it is a member node, it broadcasts a beacon message contains with its GID and energy level. If it gets leader’s acknowledgement, it will wait the message contains with next leader node information until time-out. If it gets nothing but time-out, it will turn to be a leader node itself and change the grouping method to M2. On the other side, a leader node tries to collect all information of its group members until time-out. When it gets all
information, it chooses a new leader which has the highest power in the whole group. Then it transmits a message to notify which one is the new leader node at next round to all member messages but the new leader node. After all, member nodes turn to sleeping mode and the old leader exchange the buffered messages to new leader and then change its role to member node and turn to sleeping mode.
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3.3.3 Method 2 (M2) for the environment with groups will change
In order to be more flexible for nodes’ mobility, we purpose another method, called M2, to improve M1. A group contains with only leader node is setting to use M2 as its grouping method. Figure 6 is the flow of M2 and the detail shows below.
Figure 6：The flow of method 2 (M2)
The main concept of M2 which is different to M1 is the grouping policy. M1 uses GID to make the grouping decision. In the M2 method, nodes will check their future routing path.
When they have same path in nearly future, they are allowed to form a M2 group. If they don’t, they will check only the message relay policy. The leader node’s selection of the group is still considering the energy level of nodes. The higher one will be the leader node and the
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other one will be the member node and then turns to sleeping mode after buffered messages have been exchanged. The sleeping interval of a node is calculating by itself. Since it can recognize the same path when grouping, it can estimate how much time will take to reach the location that is the end of the same path. When a member node wakes up and cannot find its leader node, it will change its role to be a leader node itself.
The rest part of the design M2 is similar to the design in M1. A group has only one awake node, called leader node, to charge with messages’ transmission and others turn to sleeping mode.
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3.4 Message Relay Policy
In the contact duration, when and which message to transmit for relay nodes is an issue.
Similar to forwarding-based routing protocol, we are using location information to make the decision of relay. Although the GPS module consumes more energy than updating a
probability table of forwarding-based protocol at once, but the table should be update in a short periodic to support high delivery ratio so that the total energy consumption of GPS are smaller than table updating computing.
As the Figure 7 shows below,
Figure 7：Message Relay Policy
→ ： The direction from node A to its message m(a)’s destination.
→ ： The direction from node B to its destination.
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θ ：The angle between → and → .
When a leader node A contacts with a leader node B, their beacon message contains with their destination’s GPS information. Node A which has a message m(a) will calculate the angle θ between m(a)’s destination and node B’s destination. If the angle θ is less than the threshold, it means that node B is a good receiver for relay the message m(a) so that node A will transmit m(a) to node B.
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In this section, we describe parameters of simulation and analyze results between three protocols:
1. Energy-aware Grouping Design, EG 2. Epidemic with CAPM [22], EpC 3. PROPHET with CAPM [22], ProC
We insert the idea of CAPM [22] as the energy saving scheme to both Epidemic and PROPHET routing protocols for comparison. Both of them work in the energy limitation environment are more efficiency and the system live longer than their original version in it.
We insert the idea of CAPM [22] as the energy saving scheme to both Epidemic and PROPHET routing protocols for comparison. Both of them work in the energy limitation environment are more efficiency and the system live longer than their original version in it.