Part I. Wireless Communication

Transcription

1 1 Part I. Wireless Communication 1.5 Topologies of cellular and ad-hoc networks

2 2 Introduction Cellular telephony has forever changed the way people communicate with one another. Cellular networks enable people to stay connected with the world from almost anywhere and everywhere, even while on the move. A typical cellular network connects different mobile users to one another via a fixed, i.e. stationary Base Station (BS). In this sense, all present day cellular radio links are mobile at one end only - the user end, while the service provider end is stationary. Wireless communication & mobile computing AAU

3 3 Cont In contrast, ad-hoc networks are envisioned to eliminate the need for central BSs by directly connecting mobile users to one another. they are also referred to as \mobile-to-mobile" or \doubly mobile" networks. In such networks, all the nodes are mobile and data is routed by relaying from one node to another. Wireless communication & mobile computing AAU

4 4 Basic Wireless Infrastructure and Topologies

5 5 Basic Network Topologies 4/18/2018

6 6 Topologies Relevant for Wireless Networking Star Yes, standard wireless topology Tree Yes (a combination of star and Bus) Line Yes, with two or more elements (PtP) Mesh Yes, mainly partial mesh Ring Possible, but rarely found Bus Not applicable. Why? 4/18/2018

7 7 Wireless components Access Point Wireless transmitter/receiver that bridges between wireless nodes and a wired network IEEE Wired Ethernet connection Wireless clients Any computer with a wireless network adapter card that transmits and receives RF signals Laptop, PDA, surveillance equipment, VoIP phone

8 8 Some General Remarks Wireless communication needs no medium EM waves travel through nothing The line in a network diagram is the connection that is being made Wireless communication is always 2way Except for passive sniffing Applies to transmitters/receivers, clients/masters

9 9 Wireless Network Operates in 2 different modes: Infrastructure mode Associates with an access point All communication goes through the access point Used for wireless access at a company or campus Peer-to-Peer Ad Hoc Mode If two nodes are within range of each other they can communicate directly with no access point A few users in a room could quickly exchange files with no access point required

10 10 Infrastructure Access Access Points: Provide infrastructure access to mobile users Cover a fixed area Wired into LAN a wireless access point(ap) is a device that allows wireless devices to connect to a wired network using Wi-Fi, or related standards. The AP usually connects to a router (via a wired network) as a standalone device, but it can also be an integral component of the router itself.

11 11 Infrastructure Access

12 12

13 13 Problems Access Point placement depends on wired network availability. Obstructions make it difficult to provide total coverage of an area. Site surveys are performed to determine coverage areas. Security Concerns: rogue access points in companies etc.. Each Access Point has limited range.

14 14 Peer to Peer Ad Hoc Mode 4/18/2018

15 15 Peer to Peer Ad Hoc Mode X 4/18/2018

16 16 Problems Communication is only possible between nodes which are directly in range of each other.

17 17 Problems for both Infrastructure and Ad hoc Mode If nodes move out of range of the access point (Infrastructure Mode) OR nodes are not in direct range of each other (Ad Hoc Mode) Then communication is not possible!!

18 18 What if?? Multi-hop Infrastructure Access Multi-hop Ad Hoc Network OR 4/18/2018

19 19 Multi-hop Infrastructure Access Nodes might be out of range of the access point, BUT in range of other nodes. The nodes in range of the access point could relay packets to allow out of range nodes to communicate.

20 20 Multi-hop Ad Hoc Network If communication is required between two nodes which are out of range of each other, intermediary nodes can forward the packets. Source Destination 4/18/2018

21 21 Mobile Ad Hoc Networks (MANET) Formed by wireless hosts which may be mobile. Without (necessarily) using a pre-existing infrastructure Routes between nodes may potentially contain multiple hops. 4/18/2018

22 22 Cont. May need to traverse multiple links to reach a destination

23 23 Cont. Mobility causes route changes

24 24 Why Ad Hoc Networks? Ease of deployment Speed of deployment Decreased dependence on infrastructure 4/18/2018

25 25 Many Applications Personal area networking cell phone, laptop, ear phone, wrist watch Military environments soldiers, tanks, planes Civilian environments taxi cab network

26 26 Cont meeting rooms sports stadiums boats, small aircraft Emergency operations search-and-rescue policing and fire fighting

27 27 Challenges Limited wireless transmission range Broadcast nature of the wireless medium Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security hazard)

28 Unicast Routing in Mobile Ad Hoc Networks 28 4/18/2018

29 29 Why is Routing in MANET different? Host mobility link failure/repair due to mobility may have different characteristics than those due to other causes. Rate of link failure/repair may be high when nodes move fast New performance criteria may be used route stability despite mobility energy consumption

30 30 Unicast Routing Protocols Many protocols have been proposed Some have been invented specifically for MANET Others are adapted from previously proposed protocols for wired networks No single protocol works well in all environments some attempts made to develop adaptive protocols

31 31 Routing Protocols Proactive protocols Determine routes independent of traffic pattern Traditional link-state and distance-vector routing protocols are proactive Reactive protocols Maintain routes only if needed Hybrid protocols More

32 32 Trade-Off Latency of route discovery Proactive protocols may have lower latency since routes are maintained at all times Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y Overhead of route discovery/maintenance Reactive protocols may have lower overhead since routes are determined only if needed

33 33 Cont Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating. Which approach achieves a better trade-off depends on the traffic and mobility patterns.

34 Overview of Unicast Routing Protocols 34

35 35 Flooding for Data Delivery Sender S broadcasts data packet P to all its neighbors Each node receiving P forwards P to its neighbors Sequence numbers used to avoid the possibility of forwarding the same packet more than once Packet P reaches destination D provided that D is reachable from sender S Node D does not forward the packet

36 36 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Represents a node that broadcast packet P Represents that connected nodes are within each other s transmission range

37 37 Flooding for Data Delivery Broadcast transmission Y Z A B H S C I E G F K J D M N L Represents a node that receives packet P for the first time Represents transmission of packet P

38 38 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Node H receives packet P from two neighbors: potential for collision

39 39 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P once

40 40 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Nodes J and K both broadcast packet P to node D Since nodes J and K are hidden from each other, their transmissions may collide Wireless => communication Packet & mobile P may Computing not be Addis delivered Ababa University to node D at all, despite the use of flooding

41 41 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Node D does not forward packet P, because node D is the intended destination of packet P

42 42 Flooding for Data Delivery Y Z B A H Flooding completed S C I E G F K J D M N L Nodes unreachable from S do not receive packet P (e.g., node Z) Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N)

43 43 Flooding for Data Delivery Y Z A B H S C I E G F K J D M N L Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet)

44 44 Flooding for Data Delivery: Advantages Simplicity May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions Potentially higher reliability of data delivery Because packets may be delivered to the destination on multiple paths

45 45 Flooding for Data Delivery: Disadvantages Potentially, very high overhead Data packets may be delivered to too many nodes who do not need to receive them Potentially lower reliability of data delivery Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead Broadcasting in IEEE MAC is unreliable In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet in this case, destination would not receive the packet at all

46 46 Wireless Sensor Networks/WSN Wireless Sensor Networks (WSNs): Highly distributed networks of small, lightweight wireless nodes, Deployed in large numbers, Monitors the environment or system by measuring physical parameters such as temperature, pressure, humidity. Node: sensing + processing + communication

47 47 WSN: A Wireless Sensor Network (WSN) consists of base stations and a number of wireless sensors (nodes).

48 48 Cont Large number of heterogeneous sensor devices Ad Hoc Network Sophisticated sensor devices communication, processing, memory capabilities

49 49 Applications of WSNs Constant monitoring & detection of specific events Military, battlefield surveillance Forest fire & flood detection Habitat exploration of animals Patient monitoring Home appliances

50 50 Comparison with Ad Hoc Wireless Networks Both consist of wireless nodes but they are different. In WSN: The number of nodes is very large Being more prone to failure, energy drain Not having unique global IDs Resource limitations: memory, power, processing

51 51 Design Issues & Challenges Random deployment autonomous setup & maintenance Infrastructure-less networks distributed routing Energy, the major constraint trading off network lifetime for fault tolerance or accuracy of results Hardware energy efficiency Distributed synchronization Adapting to changes in connectivity Real-time communication Security

52 52 Design Factors Scalability Fault tolerance Power consumption Sensor network architectures: Layered Clustered