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tags: Master, Simulation
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# GeRaF Paper Summary

[TOC]


## Intro

- :green_heart:**Energy efficiency** is one of the main research challenges in the design of protocols for sensor networks
- ♻ **cyclic node sleeping** for energy saving $\implies$ `dynamic environment`
- evaluate the resulting **performance** using energy savings as one of the key metrics

## Routing algorithms

- :x:**SPAN** requires extensive topology knowledge
- `costly in a dynamic environment` where sleep modes are used
- **MFR:** ❔
- **DIR:** ❔
- **GEDIR:** ❔
- **GPSR:** solves the problem of connectivity holes by routing around the perimeter of these holes
- **FACE–1/-2:** similar to GPSR, guaranteed delivery as long as the network graph is connected
- **GEAR:** extends the concept of directed diffusion by forwarding queries to sensors that are located in a restricted geographic area
- **MACRO**
- shows interestingly good performance, but lacks a precise physical layer model to take into account additional radio effects, e.g., fixed costs due to radio hardware, that could in some cases over-weigh power control benefits
- :heavy_check_mark:**GeRaF (**`Geographic Random Forwarding`**):** an integrated MAC/routing converge-casting protocol
- shows promising performance in terms of `energy efficiency` and `latency`
- how is its `multi–hop` performance ❔
- **GeRaF+:** proposed improvement in the paper, can greatly increase the **MAC performance**

## GeRaF Protocol Description

👁‍🗨**Assumption:** `sensor nodes` are aware of the sink and their own position.

- `converge-casting` protocol
- integrates a `geographic routing`
- integrates an **awake/sleep scheduling** scheme:
- integrates a **CSMA/CA–like MAC protocol** (relay selection and data packet forwarding):
1. **a node has no packets to send** $\implies$ periodic sleeping, according to the duty cycle, and channel listening
- no activity detected in the channel during awake time $\implies$ back to sleep
- otherwise, if RTS is heard $\implies$ the node enters the `relay contention phase`
2. **a node has a packet to send** $\implies$ it senses the channel to avoid collisions
- channel heard idle for enough time $\implies$ the node broadcasts a **RTS (**`Request To Send`**)** message, that awake neighbors can hear
3. **relay contention phase:** the best relay is elected from the highest–priority region (closer to the sink) that is not empty
- after sending the RTS, the transmit node waits for a **CTS (**`Clear To Send`**)** from nodes in the highest–priority region
- 👁‍🗨 all nodes who lost the contention go to sleep
- the elected relay receives the data packet and replies with an **ACK**
- if `a single CTS is received` $\implies$ packet transmission can start
- if `multiple CTSs are received` $\implies$ a special RTS message triggers some **collision resolution** algorithm, and notifies all nodes in lower–priority regions to go to sleep
- if `no reply is received` $\implies$ the highest–priority region is empty, send another RTS that polls nodes in the second highest–priority region, and so on...
- if `the whole forwarding area is empty` $\implies$ the sending node **backs off** and retries later, when a different distribution of awake neighbors will be seen
- repeated until either a relay is found or the maximum allowed number of attempts is reached, after which the packet is discarded
4. **repeat:** this relay in turn becomes the sending node, **until a node within coverage of the destination is reached**, and the process terminates in one more hop

**More about relay contention:**

👁‍🗨 Each node is able to determine its own **priority region** based on:
- its own coordinates (that the node knows)
- the coordinates of the sending node and of the final destination (included in the received RTS packet)

If the node is in the $i\text{–th}$ priority region, it only answers after hearing the $i\text{–th}$ RTS.
A node can only participate in a contention phase for which it receives the $1\text{–th}$ RTS:

- an extension of the GeRaF protocol where this limitation is removed performs significantly better

## Performance Evaluation

...