vault backup: 2024-10-12 13:15:08

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Autonomous Networking/notes/4 WSN pt. 2.md

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+### Communication patterns
+- **Broadcast**
+	- a broadcast pattern is generally used by a base station (sink) to transmit information to all sensor nodes of the network
+- **Convergecast or data gathering (all/many to 1)**
+	- all or a group of sensors comunicate to the sink
+	- typically used to collect sensed data
+
+We need to define a good MAC protocol for wireless sensor networks, the following attrivutes must be considered:
+- energy efficiency
+- scatability and adaptability to changes
+Latency, throughput and bandwidth utilization may be secondary, but desirable.
+
+### Reasons of energy waste
+- Collision: they need to be discarded and retransmitted
+- Overhearing: node receiving packet destined to other nodes
+- Control-packet overhead
+- Idle listening
+- Overemitting: destination not ready
+
+## S-MAC protocol
+S-MAC: Sleep MAC
+
+As idle listening consumes significant energy (50-100% of the energy required for receiving), the solution is to periodic listen and sleep, with a listen duty cycle of about 10% (e.g. listen 200ms and sleep 2s).
+
+![[Pasted image 20241011175026.png]]
+
+- Duration of sleep and listen cycles are the same for all nodes
+- All nodes are free to choose their own listen/sleep schedules
+- to reduce control overhead, **neighbor nodes are syncronized together**
+- neighboring nodes form **virtual clusters** so as to set up a common sleep schedule
+	- each node maintaines a table with neighbors’ schedule
+	- table entries are filled when the node receives sync packets
+- SYNC packets are exchanged periodically to maintain schedule synchronization
+	- they are sent every SINCHRONYZATION PERIOD
+- Receivers will adjust their timer counters immediately after they receive the SYNC packet
+
+- If there are no neighbors, the node will chose a random schedule. These nodes will be called **synchronizers**, nodes who receive a schedule are called **followers**.
+
+- In a large network we cannot guarantee that all nodes follow the same schedule
+- node on the border will follow both schedules
+	- they need to broadcast packet twice, for schedule 1 and 2
+	![[Pasted image 20241011180413.png]]
+
+
+#### Collision avoidance
+- RTS/CTS with duration is used (so NAV is used for backoff)
+- carrier sense before initiating a transmission
+- neighbor nodes of both sender and receiver sleeps during transmission to save power
+
+- listen time is divided into minislots
+	- sender selects a minislot to end carrier sense
+	- if channel is free it transmits SYNC in the next minislot
+
+### S-MAC Performance evaluation
+- Topology: Two-hop network with two sources and two sinks
+- Sources periodically generate a sensing message which is divided into fragments
+- Traffic load is changed by varying the inter-arrival period of the messages: (for inter-arrival period of 5s, a message is generated every 5s by each source. Here it varies between 1-10s)
+![[Pasted image 20241011182036.png]]
+
+- In each test, there are 10 messages generated on each source node
+- Each message has 10 fragments, and each fragment has 40 bytes (200 data packets to be passed from sources to sinks)
+- The total energy consumption of each node is measured for sending this fixed amount of data
+![[Pasted image 20241011182343.png]]
+
+- S-MAC consumes much less energy than 802.11-like protocol without sleeping
+- At heavy load, idle listening rarely happens, energy savings from sleeping is very limited
+- At light load, periodic sleeping plays a key role
+
+Conclusions:
+- A mainly static network is assumed
+- Trades off latency for reduced energy consumption
+- Redundant data is still sent with increased latency
+
+### Routing
+- routing technique is needed to establish multi-hop communication
+- the routing strategy should ensure
+	- mminimun energy consumption
+	- maximization of the network lifetime
+
+#### Ad Hoc Routing Protocols – Classification
+- **network topology**
+	- flat
+	- hierarchical
+- **which data is used to identify nodes**
+	- arbitrary identifier
+	- the position of a node
+		- can be used to assist in geographical routing problems to decide next hop
+		- scalable and suitable for sensor networks
+
+##### Flat routing protocols
+Three main categories
+
+- Proactive protocols (table driven)
+	- always tries to keep routing data up-to-date
+	- active before tables are actually needed
+		- routes are always already known
+		- more bandwidth and energy usage
+- Reactive protocols
+	- route determined only when needed
+	- operates on demand
+		- when a route is needed, a kind of global search is started
+			- causes delays if routes are not already cached
+- Hybrid protocols
+	- combination of these behaviors
+
+### Destination Sequence Distance Vector (DSDV)
+- based on bellman-ford algorithm
+- proactive protocol
+- add aging information to avoid routing loops
+- on topology change, send incremental route updates
+- unstable route updates are delayed
+
+![[Pasted image 20241011191033.png]]
+
+- to avoid loops, DSDV adds a **sequence number** to each routing table entry which is periodically updated. Routes with higher sequence number are preferred
+
+##### Reactive protocols
+
+### Flooding
+- copies of incoming packets are sent by every link except the one by which the packet is arrived
+- generates a lot of superfluous traffic
+- flooding is a reactive technique, and does not require costly topology maintenance and complex route discovery algorithms
+
+Characteristics:
+- derivery is guaranteed (e grazie al cazzo)
+- one copy will arrive by the quickest possible route (wow)
+
+Drawbacks:
+- implosion: duplicated messages are broadcasted to the same node
+- overlap: if two nodes share the same under observation region, both of them may sense the same stimuli at the same time. As a result, neighbor nodes receive duplicated messages
+- resource blindness (no knowledge about the available resources)
+- does not take into consideration all the available energy resources
+- consumes a lot of energy
+
+### Gossiping
+- nodes send the incoming packages to a randomly selected neighbor
+- avoids implosion, but it takes long to propagate the message
+
+### Dynamic Source Routing (DSR)
+- Source routing: Each data packet sent carries in its header the complete, ordered list of nodes through which the packet will pass
+- The sender can select and control the routes used for its own packets and supports the use of multiple routes to any destination
+
+
+not finished yet