Difference between revisions of "S15: Wireless Mesh Network"

Revision as of 21:21, 10 May 2015

Grading Criteria

Project Title

Wireless Mesh Network

Abstract

A mesh network topology is a design in which each node in the network is connected to at least two other nodes in the network. Communication between two nodes is established without a internet connection. Each node in the network can talk to every other node in the network by exchanging a payload data. An important feature of mesh network is that there is no single point of failure(SPoF), i.e. data is transmitted via an alternative route even if a node in the network fails. In this project we are implementing a wireless mesh network using Nordic wireless on SJOne boards. Each SJOne board acts as a node in the wireless mesh network. Features such as self-healing, shortest path routing, address nodes with auto-replies, and node participation are implemented to as to make the communication more reliable.

Objectives & Introduction

Fig 1. Full mesh topology

Fig 2. Partial mesh topology

Fig 3. Self-healing demonstration

The wireless mesh network is achieved through the built-in Nordic Wireless interface. Each board acts as a node in the wireless mesh network which sends and receives information. The following objectives are to be implemented:

• Self-healing
• Shortest path routes
• Address nodes with auto-retries
• Each node participation

Mesh networks play an important part in the Internet of Things(IoT). Mesh networks can use a full mesh topology or a partial mesh topology. In full mesh topology, each node is connected to every other node in the network and in a partial mesh topology at least one node is connected to every other node and the rest of the nodes are connected to the nodes that they frequently exchange data with. Wired mesh network pose several constraints. Each node in the network has to be physically connected to every other node. The advancement of wireless mesh networks simplifies this approach.

A Wireless Mesh Network(WMN) is a communication network made up of radio nodes organized in a mesh topology. It was originally developed for the military communication. A typical wireless mesh network consists of mesh clients, mesh routers, and gateways. In this project the SJOne boards are the mesh clients. Mesh routers route traffic from the gateways, which can happen over the internet. A mesh cloud comprises of the area covered by the radio nodes working as a single network. A mesh network is highly reliable, during transmission if a node fails, the data is sent to the other node through a redirected path. The data is transmitted from one node to other in hops through the intermediate nodes. Each intermediate nodes maintains a routing table which includes information about the network topology, destination node, source node and next hop. A routing table is a database which keeps track of the path and assists the intermediate node in delivering the information from source to the destination.

Self-healing is a technique in which a new connection is formed between the nodes in case of a failure such as a node moving out of scope due to external or internal disturbances. This ensures that the data is transmitted to the destination node through a new path even if there is any conflict in the established path. This technique is demonstrated through a gif in figure 3. The node in red is the node which is destroyed, a communication link is established between the other two nodes ensuring reliable transmission of data.

Shortest path routes is a technique in which the message from node A to node B is delivered through the shortest possible path in the network. Suppose in the mesh network shown in figure 1, if node A wants to send the data to node C, it will directly send it to C instead of routing it through B or D.

Address nodes with auto-retries is a technique in which a node re-sends the packet if it doesn't get an acknowledgement from the destination node. The number maximum number of re-transmits, delay for a single transmit and delay from one transmission to the other are fully configurable.

Each node participation is a technique through which the nodes in the network actively participate in the network to deliver the packets. This is essential for self-healing.

Team Members & Responsibilities

1. Akhil Bhargav Josyabhatla

• Software implementation

2. Dheeraj Dake

• Software implementation

3. Manideep Kurumella

• Software implementation

4. Vishnu Vardhana Reddy Mandalapu

• Software implementation

Schedule

Show a simple table or figures that show your scheduled as planned before you started working on the project. Then in another table column, write down the actual schedule so that readers can see the planned vs. actual goals. The point of the schedule is for readers to assess how to pace themselves if they are doing a similar project.

Parts List & Cost

Design & Implementation

The design section can go over your hardware and software design. Organize this section using sub-sections that go over your design and implementation.

Hardware Design

Wireless mesh network is achieved using Nordic nRF24L01 transceiver. The hardware schematic of the chip is taken from the datasheet. The pins CE, SCN, SCK, MOSI, MISO and IRQ are connected to the CPU pins. The electrical connections between the CPU and wireless transceiver are taken from SJOne_lpc1758_rev4.pdf. The detailed hardware schematic is shown in figure 4. The block diagram of nRF24L01 from the datasheet is shown in figure 5.

Fig 4. Hardware Schematic of nRF24L01

Fig 5. Block diagram of nRF24L01

Hardware Interface

The hardware interface consists of 4 SJOne boards that are powered via laptops through a USB. Antennas are attached for increasing the wireless range. The nordic wireless communicates with the LPC 1769 CPU via SPI bus. The pins that are used for establishing the communication between the nordic wireless and CPU are shown in the table below.

An air data rate of 2Mbps is chosen for sending and receiving the data between the boards. Nordic wireless nRF24lo1 uses GFSK which is Gaussian Frequency Shift Keying which is a technique in which the positive and negative frequency deviations are smoothed by a Gaussian filter. This technique reduces the spectral width, making the effective bandwidth smaller which improves the quality of the transmitted signal. This ensures that the channels intersymbol interference is under control. In RF its sole purpose is to make the signal fit in its frequency band.

Software Design

Fig 6. Software Hierarchy for wireless mesh network

The wireless mesh network we are designing follows the software hierarchy mentioned in figure 6. It has 4 layers. The upper level layer being the mesh layer and the lower level being the hardware layer.

1. Mesh Layer This is a high level layer. The routing tables needed to maintain the source address and hop count, macros and other initialization's are defined in this layer. The network discovery is done in this layer. The packet is formed and sent from this layer to the lower layers. This layer takes care of the packet collision and implements self-healing, shortest path techniques for reliable communication between the nodes. It features automatic re-transmission of the packets in case of NACK. The number of auto retries are user configurable.

2. Packet Layer This layer defines the interface between Nordic and mesh layer to receive and send packets. it receives 32 bytes data from lower layer and passes to upper layer and vice versa. It is also responsible to Nordic RF module. The nordic chip is initialized in this layer. The send and receive functions are implemented in this layer. The send function takes data and length as arguments and passes them to the Nordic. The receive function receives data and length from Nordic.

3. Nordic Layer This layer is responsible for exchanging the data packets through the SPI bus with the hardware layer for transmission. This layer sets the air data rate, channel frequency and payload. The operating modes of the Nordic wireless are implemented in this layer.

4. Hardware Layer This is the lower most layer in the hierarchy and initializes the nRF24L01+ transreceiver. The respective board I/O for the Nordic wireless are set and the SPI bus interface is initialized. The hardware layer is responsible for the transmission of the packets over the air.

Implementation

This section includes implementation, but again, not the details, just the high level. For example, you can list the steps it takes to communicate over a sensor, or the steps needed to write a page of memory onto SPI Flash. You can include sub-sections for each of your component implementation.

Testing & Technical Challenges

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Include sub-sections that list out a problem and solution, such as:

My Issue #1

1. Huge packet loss while transmitting packets using Nordic wireless

Conclusion

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Project Video

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Project Source Code

• Sourceforge Source Code Link

References

Acknowledgement

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References Used

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Appendix

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