1-1Introduction to Ad hoc NetworksCS-647: Advanced Topics in Wireless NetworksDrs. Baruch Awerbuch & Amitabh MishraDepartment of Computer ScienceJohns Hopkins University© Amitabh Mishra & Baruch Awerbuch 20081-2Outline❒ What is an ad hoc network?❒ Challenges facing ad hoc networks❒ History of Ad hoc Networks❒ General Concepts❒ Introduction to IEEE 802.11❒ Physical Layers of 802.111-3Reading❒ C. K. Toh, Chapter 3, “Ad Hoc Wireless Networks”, Prentice Hall, 2002❒ D. P. Agrawal and Qing-An Zeng, Chapter 13, “Wireless & Mobile Systems”, Thompson/Brooks Cole, 2003❒ Refer one of the suggested textbooks1-4Types of Wireless Networks: infrastructure vs. ad-hoc networksinfrastructurenetworkad-hoc networkAPAPAPwired networkAP: Access Point• Infrastructure Networks•Fixed, wired backbone• Mobile communicates directly with access points• Suitable for locations where access points can be placed• Cellular networks1-5Cellular Networks – UMTS (3G)UMTS Core NetworkCircuit Switched NetworkUTRANMAPGiIuISUPGSM NetworkSS7 NetworkPacket Data Network1-6Why Ad Hoc Networks ?❒ Ease of deployment❒ Speed of deployment❒ Decreased dependence on infrastructure1-7What is an Ad hoc Network?❒ A network without any base stations “infrastructure-less” or multi-hop ❒ A collection of two or more devices equipped with wireless communications and networking capability❒ Supports anytime and anywhere computing❒ Two topologies:❍ Heterogeneous (left)• Differences in capabilities❍ Homogeneous or fully symmetric (Right)• all nodes have identical capabilities and responsibilitiesHomogeneous network infrastructurenetworkad-hoc networkAPAPAPwired networkAP: Access Pointinfrastructurenetworkad-hoc networkAPAPAPwired networkAP: Access Point1-8Mobile Ad Hoc Networks?1-9Mobile Ad Hoc Networks?❒ Mobility causes route changes1-10What is an Ad hoc Network?❒ Self-organizing and adaptive –Allows spontaneous formation and deformation of mobile networks❒ Each mobile host acts as a router❒ Supports peer-to-peer communications ❒ Supports peer-to-remote communications ❒ Reduced administrative cost ❒ Ease of deployment infrastructurenetworkad-hoc networkAPAPAPwired networkAP: Access Pointinfrastructurenetworkad-hoc networkAPAPAPwired networkAP: Access Point1-11Ad Hoc Networks – Operating Principle❒ Fig. depicts a peer-to-peer multihop ad hoc network❒ Mobile node A communicates directly with B (single hop) when a channel is available❒ If Channel is not available, then multi-hop communication is necessary e.g. A->D->B❒ For multi-hop communication to work, the intermediate nodes should route the packet i.e. they should act as a router ❒ Example: For communication between A-C, B, or D & E, should act as routersExample of an Ad Hoc Network ABCDEABCDE1-12Bringing up an Ad hoc Network1. Ad hoc network begins with at least two nodes broadcasting their presence (beaconing) with their respective address information2. They may also include their location info if GPS equipped3. Beaconing messages are control messages. If node A is able to establish a direct communication with node B verified by appropriate control messages between them, they both update their routing tablesABCDEABCDE1-13Bringing up an Ad hoc Network4. Third node C joins the network with its beacon signal. Two scenarios are possible:(i) A & B both try to determine if single hop communication is feasible (ii) Only one of the nodes e.g. B tries to determine if single hop communication is feasible and establishes a connectionABCDEABCDE1-14Bringing up an Ad hoc Network5. The distinct topology updates consisting of both address and the route updates are made in three nodes immediately.5. In first scenario, all routes are direct i.e. A->B, B->C, and A->C (Lets assume bi-directional links)ABCDEABCDE1-15Bringing up an Ad hoc Network❒ In the second scenario, the routes are updated 1. First between B & C, 2. then between B & A, 3. Then between B & C again confirming that A and C both can reach each other via BCBA[topologyupdate][topologyupdate][topologyupdate][topologyupdate]CBA1-16Topology Update Due to a Link Failure❒ Mobility of nodes may cause link breakage requiring route updates❒ Assume link between B & C breaks because of some reason❒ Nodes A & C are still reachable via D and E❒ So old route between A &C was A->B->C is to be replaced by A->D->E->C ❒ All five nodes are required to incorporate this change in their routing table❍ This change will happen first in nodes B & C ❍ Then A & E ❍ Then DABCDEABCDE1-17Outline❒ What is an ad hoc network?❒ Challenges facing ad hoc networks❒ History of Ad hoc Networks❒ General Concepts❒ Introduction to IEEE 802.11❒ Physical Layers of 802.111-18Traffic Characteristics❒ Traffic characteristics may differ in different ad hoc networks❍ bit rate❍ timeliness constraints❍ reliability requirements❍ unicast / multicast / geocast❍ host-based addressing / content-based addressing / capability-based addressing❒ May co-exist (and co-operate) with an infrastructure-based network1-19Traffic Profiles❒ Three distinct types of traffic patterns observed in ad hoc networks❒ Peer-to-peer between two entities (Fig. a) – Bursty❒ Two or more devices in a group communication while moving as a group (correlated traffic) -> remote to remote communication❒ Hybrid non-coherent communication among nodes -> uncorrelated traffic1-20Challenges in Ad hoc Mobile Networks (1)❒ Host is no longer an end system - can also be an acting intermediate system❒ Changing the network topology over time❒ Potentially frequent network partitions❒ Every node can be mobile❒ Limited power capacity ❒ Limited wireless bandwidth ❒ Presence of varying channel quality1-21Challenges in Ad hoc Mobile Networks (2)❒ No centralized entity – distributed❒ How to support routing?❒ How to support channel access?❒ How to deal with mobility?❒ How to conserve power?❒ How to use bandwidth efficiently?1-22Problems Facing Routing in Ad hoc Networks ❒ Routers are now moving❒ Link changes are happening quite often❍ Packet losses due to transmission errors❒ Event updates are sent often – a lot of control traffic❒ Routing table may not be able to, converge❒ Routing loop may exist❒ Current wired routing uses shortest path metric1-23Problems facing channel access in Ad hoc Networks❒ Distributed channel access, i.e. no fixed base station