RAP and SPEED Presented by Octav Chipara

Realtime Systems Concerned with two aspects Concerned with two aspects Control Control RAP RAP Predictability Predictability SPEED SPEED

Modeling the sensor networks A sensor: A sensor: Limited memory Limited memory MAC layer may provide QoS MAC layer may provide QoS Communication: Communication: Scarce bandwidth Scarce bandwidth Voids exists Voids exists Energy intensive Energy intensive Communication generates congestion hot-spots Communication generates congestion hot-spots end-to-end communication time depends on single-hop delay and the distance it has to travel end-to-end communication time depends on single-hop delay and the distance it has to travel

Modeling the sensor networks Tight integration with the physical world Tight integration with the physical world Location aware Location aware Communication patterns: Communication patterns: Local coordination Local coordination Sensors coordinate with one another [usually by defining a group] in order to aggregate data Sensors coordinate with one another [usually by defining a group] in order to aggregate data Usually involves a small number of hops Usually involves a small number of hops Sensor-base communication Sensor-base communication Sends data from the local group to the base station Sends data from the local group to the base station Requires multiple hops Requires multiple hops

RAP

Contributions A high level architecture for large wireless networks A high level architecture for large wireless networks Ability to define queries Ability to define queries Use event services Use event services Velocity Monotonic Scheduling (VMS) Velocity Monotonic Scheduling (VMS) A policy for scheduling packets in a sensor network A policy for scheduling packets in a sensor network

Design Goals Provide APIs for micro-sensing and control Provide APIs for micro-sensing and control Maximize the number of packets meeting their e2e deadlines Maximize the number of packets meeting their e2e deadlines Scale well to large number of nodes and hops Scale well to large number of nodes and hops Introduce minimum communication overhead Introduce minimum communication overhead

RAP Stack

Query/Event Service API Query {attribute list, area, time constraints, querier location} Query {attribute list, area, time constraints, querier location} Register {event, area, query} Register {event, area, query} Periodically the results will be send back to the querier Periodically the results will be send back to the querier

Location-Addressed Protocol Connectionless transport layer Connectionless transport layer Address is based on geographic location Address is based on geographic location Services: Services: Unicast Unicast Area multicast Area multicast Area anycast Area anycast

Geographic forwarding Greedy algorithm Greedy algorithm Selects the node with the shortest geographic distance to the packet’s destination Selects the node with the shortest geographic distance to the packet’s destination At every step the packet gets closer to the destination At every step the packet gets closer to the destination Works really good for high density network Works really good for high density network

Velocity Monotonic Scheduling FCFS policy is generally used in sensor networks FCFS policy is generally used in sensor networks Works poorly for realtime systems Works poorly for realtime systems VMS VMS It is both deadline and distance aware It is both deadline and distance aware Assigns priority based on the requested velocity Assigns priority based on the requested velocity A higher velocity denotes higher priority A higher velocity denotes higher priority

Velocity Monotonic Scheduling(2) Two flavors Two flavors Static monotonic velocity (SMV) Static monotonic velocity (SMV) V=dist(x 0,y 0,x d,y d )/D V=dist(x 0,y 0,x d,y d )/D Dynamic monotonic velocity (DMV) Dynamic monotonic velocity (DMV) V=dist(x i,y i,x d,y d )/(D-T j ) V=dist(x i,y i,x d,y d )/(D-T j )

MAC –layer prioritization When communicating multiple host compete for the shared medium When communicating multiple host compete for the shared medium In b all messages have the same priority In b all messages have the same priority To enforce packet prioritization MAC protocols should provide distributed prioritization on packets from different nodes To enforce packet prioritization MAC protocols should provide distributed prioritization on packets from different nodes We can changed two parameters We can changed two parameters The time you can wait after idle The time you can wait after idle DIFS = BASE_DIFS * PRIORITY DIFS = BASE_DIFS * PRIORITY Backoff increase function Backoff increase function CW=CW*(2 + (PRIORITY-1)/MAX_PRIORITY) CW=CW*(2 + (PRIORITY-1)/MAX_PRIORITY)

Experiments Routing protocols Routing protocols Dynamic Source Routing (DSR) Dynamic Source Routing (DSR) Originally designed for ID networks Originally designed for ID networks GF GF Location based routing Location based routing Scheduling Scheduling FCFS FCFS DS – fixed priority based on their e2e deadline DS – fixed priority based on their e2e deadline Velocity Scheduling Velocity Scheduling DVS DVS SVS SVS

Overall Miss Ratio of GF/DSR

Impact of scheduling on deadline misses

SPEED

State of the art Only a few real-time algorithms exist for sensor networks Only a few real-time algorithms exist for sensor networks Routing based on sensor’s position Routing based on sensor’s position GPSR GPSR GEDIR GEDIR LAR LAR

Design Goals State State Information regarding the immediate neighbors Information regarding the immediate neighbors Soft Real Time Soft Real Time Provides uniform speed delivery across the network Provides uniform speed delivery across the network Q: is this uniform speed guaranteed globally or piece- wise Q: is this uniform speed guaranteed globally or piece- wise Q: why would the application want something like this Q: why would the application want something like this Minimum MAC Layer support Minimum MAC Layer support

Design Goals Traffic load-balancing Traffic load-balancing Localized behavior Localized behavior Void avoidance Void avoidance

Speed Protocol API API Neighbor beacon exchange scheme Neighbor beacon exchange scheme Delay estimation algorithm Delay estimation algorithm Stateless nondeterministic geographic forwarding algorithm Stateless nondeterministic geographic forwarding algorithm Neighbor feedback loop Neighbor feedback loop Backbone rerouting Backbone rerouting Void avoidance Void avoidance Last mile processing Last mile processing

API AreaMulticastSend(position,radius, packet) AreaMulticastSend(position,radius, packet) Send a message to all nodes in the specified area Send a message to all nodes in the specified area AreaAnyCastSend(position,radius,packet) AreaAnyCastSend(position,radius,packet) Sends a message to at least one node inside the specified area Sends a message to at least one node inside the specified area UnicastSend(Global_ID, packet) UnicastSend(Global_ID, packet) Send a message to the specified ID (id is given by its geographic position) Send a message to the specified ID (id is given by its geographic position) SpeedReceive() SpeedReceive()

API (2) Where is the time constraint? Where is the time constraint? “SPEED aims at providing a uniform packet delivery speed across the sensor network, so that the end-to-end delay of a packet is proportional to the distance between the source and destination. With this service, real-time applications can estimate end-to-end delay before making admission decisions. Delay differentiation for different classes of packets is left as future work.” “SPEED aims at providing a uniform packet delivery speed across the sensor network, so that the end-to-end delay of a packet is proportional to the distance between the source and destination. With this service, real-time applications can estimate end-to-end delay before making admission decisions. Delay differentiation for different classes of packets is left as future work.” Q: if the velocity is imposed piece wise how can we give end-to end guarantees? Q: if the velocity is imposed piece wise how can we give end-to end guarantees? Q: By distance we mean Euclidian distance or the Euclidian distance of the path? Q: By distance we mean Euclidian distance or the Euclidian distance of the path?

Neighbor beacon exchange scheme Periodically broadcasts a beacon to neighbors to exchange local information Periodically broadcasts a beacon to neighbors to exchange local information In order to reduce traffic we can piggyback the information In order to reduce traffic we can piggyback the information Possible enhancement: Possible enhancement: Advertising state changes may reduce number of beacons Advertising state changes may reduce number of beacons On-demand beacons On-demand beacons Delay estimation Delay estimation Back pressure pressure Back pressure pressure Fields: Fields: Neighbor ID Neighbor ID Position Position Send To Delay Send To Delay

Delay estimation algorithm Due to scarce bandwidth cannot use probe packets Due to scarce bandwidth cannot use probe packets Delay is measured at the sender as the difference between when the packet was queued and its ACK and the processing time on the receiver time Delay is measured at the sender as the difference between when the packet was queued and its ACK and the processing time on the receiver time Keeps track of multiple data points to compute the current delay using (EWMA): Keeps track of multiple data points to compute the current delay using (EWMA): intervalavg = intervalavg + w * err intervalavg = intervalavg + w * err Where: Where: I is the refresh time sample from this packet I is the refresh time sample from this packet err = I - intervalavg err = I - intervalavg w is the smoothing constant w is the smoothing constant

Delay estimation algorithm (2) Delay estimation beacon is used to communicate to other neighbors the estimated delay Delay estimation beacon is used to communicate to other neighbors the estimated delay Hope to make a set of neighboring nodes react to changes in traffic patters [avoid congestion] Hope to make a set of neighboring nodes react to changes in traffic patters [avoid congestion]

SNGF Neighbor set of node i Neighbor set of node i Forwarding candidate set Forwarding candidate set Where Where L = d(i, destination) L = d(i, destination) Lnext = d(next, destination) Lnext = d(next, destination) Relay speed Relay speed

SNFG(2) If (|FSi| > 0) { if (|Viable| > 0) { candidate=choose(Viable(FSi)) send to candidate } else { compute relay ratio if no nodes to support Ssetpoint downstream, drop packet if a random chosen between (0,1) is bigger than the relay ratio } } else { drop packets send pressure beacon upstream }

SNFG(3) Delay Bound = L e2e / S setpoint Delay Bound = L e2e / S setpoint Where: Where: L e2e is the end-to-end Euclidian distance measured L e2e is the end-to-end Euclidian distance measured S setpoint the speed maintained across the network S setpoint the speed maintained across the network Drawbacks: Drawbacks: All messages have the same speed All messages have the same speed Does this really mean we can enforce a deadline? Does this really mean we can enforce a deadline?

Neighbor feedback loop Goal: Goal: Maintain a single hop speed above a desired Ssetpoint Maintain a single hop speed above a desired Ssetpoint Ssetpoint is a network wide parameter that tunes how harsh the realtime requirements are Ssetpoint is a network wide parameter that tunes how harsh the realtime requirements are

Neighbor feedback loop

Back pressure rerouting Rerouting due to pressure Rerouting due to pressure The congested area is detected and the probability of sending to that node is limited The congested area is detected and the probability of sending to that node is limited Issue: Issue: Maybe reinforcement should refer to a geographic area rather than a node! Maybe reinforcement should refer to a geographic area rather than a node!

Void avoidance Voids occur if the density is not high enough Voids occur if the density is not high enough Deals with voids similarly to hotspots by applying backbone pressure Deals with voids similarly to hotspots by applying backbone pressure Several packets may be dropped when trying to avoid a void Several packets may be dropped when trying to avoid a void

Last mile processing Processing close to the destination area Processing close to the destination area Area anycast Area anycast Area multicast Area multicast

E2E Miss Ratio Ssetpoint = 1km/s Ssetpoint = 1km/s e2e deadline = 200 ms e2e deadline = 200 ms

Protocol Evaluation

Improving Speed

Possible Improvements Allow the application to specify different speeds Allow the application to specify different speeds Combine speed and rap Combine speed and rap Change the MAC layer to support priority sending messages with different priorities Change the MAC layer to support priority sending messages with different priorities What scheduling policy to use? What scheduling policy to use? Why are messages dropped? Why are messages dropped?

Questions?