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Multihop Wireless Networks

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Multihop Wireless Networks. Multihop. The wireless networks we have discussed so far, i.e., wireless LANs, cellular phone networks, are all single-hop in wireless. Multi-hop wireless network has attracted lots attentions, because it may offer interesting solutions to a number of problems - PowerPoint PPT Presentation
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Multihop Wireless Networks
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Page 1: Multihop Wireless Networks

Multihop Wireless Networks

Page 2: Multihop Wireless Networks

Multihop• The wireless networks we have discussed so far, i.e., wireless LANs,

cellular phone networks, are all single-hop in wireless.• Multi-hop wireless network has attracted lots attentions, because it may

offer interesting solutions to a number of problems– Wireless sensor networks: a large number of sensors are deployed in an area.

The sensors may collect data and then send data to the outside observer for processing. The transmission power of sensor is limited and cannot reach very far, therefore sensors have to reply other’s message. Applications include: scientific data gathering, military monitoring. Power is a main issue in wireless sensor networks because sensor nodes are battery-powered.

– Wireless Ad Hoc networks: wireless nodes organize themselves into a network with no infrastructure. Applications scenarios: emergency response.

– Wireless Mesh Networks: install mesh wireless routers to extend the one-hop to multi-hop. The routers forward the message from the nodes to the next mesh router in wireless. Comparing to installing the wired network such as Ethernet, this may offer faster, cheaper solutions for covering an area.

Page 3: Multihop Wireless Networks

Multihop

• Multihop networks are far from as widely deployed as the wireless LANs and the cellular phone networks. Still pretty much in research phase.

• Lots of problems to be solved.

Page 4: Multihop Wireless Networks

Routing• One of the key problems in multihop networks is routing.

– Provide connectivity• General approach: Link State (LS) and Distance Vector (DV)

– LS: nodes broadcast the link state information to all nodes in the network, and everyone computes the routing paths based on the same information.

– DV: a node only tells its neighbor the hop count to a certain destination node. A node will pick the shortest path from all neighbors.

• Challenges in wireless networks– Nodes may be mobile– Link is not binary but more a delivery ratio– Some networks, such as wireless sensor networks, can be very limited in

resources, e.g., computation, storage, battery, and periodical update of link state is a burden

– Other nodes can sometime overhear…

Page 5: Multihop Wireless Networks

Proposed Protocols

• Dynamic Source Routing (DSR)– Source routing. The entire routing path specified in the

packet.– http://www.ietf.org/rfc/rfc4728.txt

• Ad-hoc On-demand Distance Vector Routing (AODV)– Hop-by-hop” protocol– http://www.ietf.org/rfc/rfc3561.txt

• Both on demand. Routing information discovered only when needed– Does not require periodic route updates– Suitable for mobile networks

Page 6: Multihop Wireless Networks

DSR

• Route discovery. Invoked when the source needs to send packet to the destination but has no path. Basic idea:– Source broadcast Route Request message, with the

destination node’s ID and a unique packet ID– If a node overheard the request, it checks

• If it is the destination, it sends a packet back to the source. The routing path from the source to the destination is already in the packet.

• If it is not the destination, if it has not received the packet before, it appends its own ID to the packet and broadcast it. It does nothing if it received the packet before.

Page 7: Multihop Wireless Networks

Routing Cache in DSR

• A node may keep the learned routing paths in a cache.

• If a node receives a routing request to a destination with a path it knows of, it does not need to forward the request, and may reply with the path directly.

• Other nodes may overhear some routing paths and will also store the overheard information

Page 8: Multihop Wireless Networks

DSR

• Routing maintenance– When a link stops functioning, e.g., cannot receive

ACK, send a Rout Error message to the source of the packet.

– Intermediate nodes may also update their routing cache

– Source detects the error, deletes the path, try another one if there is, or start a new route discovery

Page 9: Multihop Wireless Networks

AODV

• Intermediate nodes stores the routing information in a lookup table. Needs only the destination address.

• Route discovery initiated when a node needs next hop information to a destination node

Page 10: Multihop Wireless Networks

AODV

• Route Request– Source broadcast Route Request (RREQ) message

to a destination– Intermediate node broadcast the received RREQ

message. Creates a path entry for a reverse path to the source. Assumes bidirectional links. • S sends the RREQ message. A forwarded this message,

A knows the next hop to S is S. B forwards the message from A, B knows the next hop to S is A.

Page 11: Multihop Wireless Networks

AODV

• Route Reply– Destination should reply the Route Reply message

(RREP).• This message will be forwarded along the reverse path

created when forwarding the RREQ message– An intermediate node creates next-hop entry for

destination as RREP is received. Also forwards along “reverse path” hop.

Page 12: Multihop Wireless Networks

AODV• Each node will maintain a non-decreasing sequence number sent in RREQ and

RREP messages and increments it by one in every message.• The RREQ message contains the source node's ID, current sequence number,

broadcast ID. Nodes keep track of the RREQ's source ID and broadcast ID. If they receive a RREQ which they have already processed, they discard the RREQ and do not forward it.

• RREQ also contains the most recent sequence number for the destination of which the source node is aware. A node receiving the RREQ may send a route reply (RREP) if it is either the destination or if it has a route to the destination with corresponding sequence number greater than or equal to that contained in the RREQ. If this is the case, it unicasts a RREP back to the source. Otherwise, it rebroadcasts the RREQ.

• As the RREP propagates back to the source, nodes set up forward pointers to the destination. Once the source node receives the RREP, it may begin to forward data packets to the destination. If the source later receives a RREP containing a greater sequence number it may update its routing information for that destination and begin using the better route. If the sequence numbers are the same in two replies, pick the one with a smaller hop count.

Page 13: Multihop Wireless Networks

Route Maintenance

• A link may break, detected by losing ACKs.• The detecting node may – try a local repair. Send RREQ for destination– Send the Route Error (RERR) message, which

Contains list of unreachable destinations to neighbors who recently sent packet which was forwarded over broken link. Propagated recursively.

Page 14: Multihop Wireless Networks

A problem• Wireless links are lossy.– Good links may have a delivery ratio close 1. But some

links may have a delivery ratio between 0 and 1. – This is because the wireless channel is constantly

fluctuating.– Compared to wired networks, in which links are either

there or not there.• Questions: – Should you consider the lossy paths?– given two paths, how do you measure the quality of

the paths to pick the better one?

Page 15: Multihop Wireless Networks

Path Quality

• Lossy paths. If there are two paths from S to T:– Path 1: S->T. Link pass ratio 0.5.– Path 2: S->A->B->T. Perfect links.– Which path is better? Assume a lost packet is

retransmitted.

Page 16: Multihop Wireless Networks

Path Quality

• Sending a packet on path 1 results in 2 transmissions on average. Sending a packet on path 2 results in 3 transmissions. In wireless networks, less transmission is good because it is– Faster– Uses less power– Creates less interference, therefore the network

throughput is higher

Page 17: Multihop Wireless Networks

Path Quality

• The path quality should be measured by the Expected Transmission Count (ETX). It is summation of the expected transmissions of all links on a path.

• Implemented LQSR on top of DSR, basically using ETX to select paths.

• http://www.cse.cuhk.edu.hk/~cslui/STUDY_GROUP/metrics.pdf

Page 18: Multihop Wireless Networks

Yet another thing can do• http://pdos.csail.mit.edu/papers/roofnet:exor-sigcomm05/roofnet_exor-sigcomm05.pdf

• Again, wireless links are random.• Consider the situation below. S wishes to send to T through the 4

intermediate links. The links from S to the intermediate nodes have pass ratio 0.25. The links from the intermediate nodes to T have pass ratio 1.

• If S only selects one node as the next hop, the average number of transmissions is 4+1.

• However, wireless links are usually independent. So, if one of the intermediate node didn’t get it, chances are some got it. So, why not ask all nodes to forward?

S

1

2

3

4

T

Page 19: Multihop Wireless Networks

The Idea of MORE

• So, the idea is to not to select only one next hop, but to select a number of nodes as next hop to enjoy the “spatial diversity.”

• Challenges:– How to specify the set of next hop neighbors that

should receive the packet?– How to determine which node in the subset of the

nodes to send?– How to minimize the overhead?

Page 20: Multihop Wireless Networks

The Basic Idea of the Solution

• Source send packets in batches – to reduce the percentage of overhead

• Source lists the potential forwarders in the packet, prioritized by the expected cost for a node to forward the packet

• After the source finishes sending the batch, the highest priority nodes sends first the packets it received correctly. The rest of the nodes in the list sends their received packets in turn.

Page 21: Multihop Wireless Networks

Hierarchical Routing• AODV and DSR is suitable for

– small and flat networks, such as wireless ad hoc networks, where all nodes are the same, i.e., have similar processing power, similar battery.

– relatively small, such that the number of hop count is reasonable• In some other networks, nodes can be different.

– In wireless sensor networks, there may be cluster head nodes and the sensor nodes

– In wireless mesh networks, there are mesh routers and mesh clients– Forwarding message is costly, better let the nodes with more power

supply do it• In some networks, the size of the network can be very large

– A wireless sensors network for some applications should have thousands of nodes

Page 22: Multihop Wireless Networks

Hierarchical Routing

• Hierarchical routing is more efficient in these scenarios.

• The basic idea is – partition the entire network into regions or clusters. – Select one or more nodes as the cluster head – The routing is nodes -> cluster head A -> cluster head

B -> cluster head C -> nodes• Achieves– Better scalability – Removes the load to less powerful nodes

Page 23: Multihop Wireless Networks

A Case Study in Wireless Sensor networks in Intra-cluster Routing

• The routing between cluster heads and the routing within a cluster may follow different protocols.

• Focusing on intra-cluster routing.• Part of this work is in

Page 24: Multihop Wireless Networks

Wireless Sensor Networks

• Challenges– the network must

be scalable– to reduce the

cost, sensors should be made as simple as possible

– must be energy efficient

Page 25: Multihop Wireless Networks

Wireless Sensor Networks

• To make network scalable, introduce hierarchies.

• Sensors only communicate within a cluster. The inter-cluster communication is taken care of by the cluster head.

Page 26: Multihop Wireless Networks

Wireless Sensor Networks

• Existing works such as LEACH, HEED, for cluster head selection.

• Disadvantage: Sensors become more complicated because every sensor could be elected as cluster head.

Page 27: Multihop Wireless Networks

Wireless Sensor Networks

• Our first idea: Instead of selecting cluster head among the sensors, why not simply add some more powerful sensors to the network as cluster head?

Page 28: Multihop Wireless Networks

Wireless Sensor Networks

• Advantage of such a two-layer heterogeneous network:– Better scalability – Majority of the sensors

can be made very simple

• Question: How does the cluster head collect data from the sensors in an energy efficient way?

Page 29: Multihop Wireless Networks

Wireless Sensor Networks

• Data collecting can be done by letting sensors make their own decisions when to send packets.

• Problem – energy wasted in collision, collision avoidance, idle-listening, etc.

Page 30: Multihop Wireless Networks

Wireless Sensor Networks• Our second idea: Since we already have a powerful cluster

head, why not use it as a master and use polling to get data from sensors?

• Polling – Cluster head sends out polling messages, only those are polled will send packets.

• It is energy efficient because no energy will be wasted in collision and collision avoidance, and by designing a good polling schedule, the time needed in data collection is reduced so the idle listening time is reduced.

• The key issue is to find a good (fast and collision free) polling schedule.

Page 31: Multihop Wireless Networks

Polling Schedule

• Packet relaying paths are assumed to have been found by the Ford-Fulkerson Algorithm.

S1

S2

S4

S3

S5

S6

t

S1 t

S2 S4

S3 S5 S6

t

t

Page 32: Multihop Wireless Networks

Polling Schedule

• A poor schedule could take six time slots.

S1

S2

S3

S5

S4

S6

t

S1 t

time slot 1

S2

time slot 2

S4 S4

time slot 3

t S3

time slot 4

S5 S5

time slot 5

S6 S6

time slot 6

t

Page 33: Multihop Wireless Networks

Polling Schedule

• To get a faster schedule, it is crucial to encourage spatial reuse: Allow simultaneous transmissions if they do not cause destructive interferences.

• Such transmission are called compatible.

S1

S2

S3

S5

S6

S4

t

S1 t

S2 S4S3 S5

Page 34: Multihop Wireless Networks

Polling Schedule

S1

S2

S3

• A good schedule needs only three time slots.

S6

S5

S4

t

S1 t

time slot 1

S2 S4S3 S5

time slot 2

S4 t

S5 S6

time slot 3

S6 t

Page 35: Multihop Wireless Networks

Optimal Polling Schedule

• The compatibilities of transmissions can be found by measuring the received signal strength.

• The Multi Hop Polling Problem: Given relaying paths and the compatibilities, find the minimum time schedule (optimal schedule).

Page 36: Multihop Wireless Networks

Optimal Polling Schedule

• Use the Hamilton Path Problem. Given any instance of HP Problem, construct an MHP instance, called the two-level star.

• Each node in the graph corresponds to a branch.• Each sensor in the second level has exactly one packet to

send. Sensors in the first level have no packet to send.

Page 37: Multihop Wireless Networks

Optimal Polling Schedule

• The connection pattern in the HP instance determines the compatibility in the two level star: If there is edge (v1,v2), S1–>t and S’

2–>S2 are compatible, so do S2–>t and S’

1–>S1.

v1

v2

v3

v4

v5

t

S2S1 S4S3 S5

S’1 S’5S’4S’3S’2

Page 38: Multihop Wireless Networks

Optimal Polling Schedule

As a result of this construction, a schedule with k=n+1 will determine a Hamiltonian Path in the original graph.

Page 39: Multihop Wireless Networks

Optimal Polling Schedule

• Theorem 1. The MHP problem is NP-hard.• Theorem 2. The MHP problem is NP-hard

when each sensor has exactly one packet to send.

• Theorem 3. The MHP problem is NP-hard when packets can be queued.

Page 40: Multihop Wireless Networks

Online Polling Algorithm

• Another challenge is that the algorithm must be able to run on-line.– Packet can be lost in some links, and the cluster

head must ask the sensor to send it again – new requests may arrive while data gathering is going on

Page 41: Multihop Wireless Networks

Online Polling Algorithm

• The algorithm is simple: Check the requests in an arbitrary order, add a request to the schedule if it is compatible the current schedule.

• Time complexity: linear to input size.

Page 42: Multihop Wireless Networks

Online Polling Algorithm

Page 43: Multihop Wireless Networks

Simulation Set-up

• We have implemented the proposed algorithm on NS-2 simulator.

• All sensor nodes are uniformly deployed within a 200m * 200m square. The cluster head is at the center of the square. Each node can communicate with other nodes as far as 40m away.

Page 44: Multihop Wireless Networks

The Percentage of Active Time to Ensure 100% Throughput

• For a modest size cluster (30 sensors), when the data is generated at 60 Bps, less than 30%.

• Will grow with cluster size – choose a suitable size.

Page 45: Multihop Wireless Networks

Comparison with SMAC+AODV in Terms of Throughput

• Polling achieves 100% throughput with less than 30% of active time.

• SMAC+AODV cannot achieve 100% if sleeping is allowed.

• A lot of packets are generated for routing.

Page 46: Multihop Wireless Networks

Reading• http://www.ietf.org/rfc/rfc4728.txt • http://www.ietf.org/rfc/rfc3561.txt • http://www.cse.cuhk.edu.hk/~cslui/STUDY_GROUP/metrics.pdf • http://pdos.csail.mit.edu/papers/roofnet:exor-sigcomm05/roofnet_exor-sigcomm05.pdf • http://www.ece.gatech.edu/research/labs/bwn/mesh.pdf


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