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Lecture 3 4. prnet


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Lecture 3 4. prnet

  1. 1. Chandra Prakash LPU Origins of Ad Hoc: Packet Radio Networks 1
  2. 2. Objectives of the Chapter Introduction PRNETs Architecture of PRNETs Components of Packet Radios Routing in PRNETs Route Calculation PacingTechniques MediaAccess in PRNETs FlowAcknowledgments in PRNETs Technical Challenges 2
  3. 3. Ad hoc networks  Temporary network composed of mobile nodes without preexisting communication infrastructure, such as Access Point (AP) and Base Station (BS).  Each node plays the role of router for multi-hop routing.  Self-organizing network without infrastructure networks  Started from DARPA PRNet in 1970  Cooperative nodes (wireless)  Each node decode-and-forward packets for other nodes  Multi-hop packet forwarding through wireless links  Proactive/reactive/hybrid routing protocols  Most works based on CSMA/CA to solve the interference problem  IEEE 802.11 MAC 3 Chandra Prakash, LPU
  4. 4. History  Principle behind ad hc networking ::: Multi hop relaying  (In 522-486 B.C) King of Persia –use line of shouting men positioned on tall structures or heights.  Video 4
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  6. 6. Origin of Ad-Hoc Networks Generic View of mobile ad hoc networks  First generation  They were used for different military scenarios.  Packet radio networks was the first ad-hoc network system  Second generation from 1980s’ to the mid 1990’s  Main aim were the same as for the first generation ad-hoc networks system i.e.Aiding combat/Battle operations.  Second generation developments focused on the further advancement of the previously build ad-hoc network structure.  Some important developments: Global mobile information Systems, Near term Digital Radio (NTDR)  Third generation ad-hoc network systems  known as commercial ad-hoc network systems.  developments , Bluetooth – ad-hoc sensor networks etc6
  7. 7. First generation ad-hoc network systems  The merits of having an infrastructure-less network were discovered in the 1970s.  At that time, computers were bulky and so were radio transceivers.  Defence Advanced Research Projects Agency (DARPA) had a project known as packet radio, where several wireless terminals could communicate with one another on a battlefield.  DARPA initiated research on the feasibility of using packet-switched radio communications to provide reliable computer communications.  Came up with packet radio network 1973-1987  Packet radio was a technology that extended the concept of packet switching (which evolved from point-to-point communication networks) to the domain of broadcast radio networks. 7
  8. 8. Cont…  During the 1970s, the ALOHA project at the University of Hawaii demonstrated the feasibility of using the broadcasting property of radios to send/receive data packets in a single radio hop system.  ALOHA net utilized single-hop wireless packet switching and a multiple access solution for sharing a single channel.  TheALOHA project later led to the development of a multi-hop multiple-access packet radio network (PRNET).  Initial attempt had a centralized control.  UnlikeALOHA, PRNET permits multi-hop communications over a wide geographical area.  The DARPA PRNET has evolved through the years (1973-1987) to be a robust, reliable, operational experimental network.  The DARPA PRNET projects includes network devices, routing protocols and protocols for automatic distributed network management. 8
  9. 9. Features of PRNET  One of the most attractive features of PRNET is rapid deployment.  Once installed, the system is self-initializing and self-organizing.  Network nodes should be able to discover radio connectivity among neighboring nodes and organize routing strategies based on this connectivity.  PRNETs are expected to require no system administration and can be left unattended. 9
  10. 10. First generation ad-hoc network systems Packet radio network components  Firmware  Firmware can be loaded into a Packet Radio(PR) either locally (via serial interface) or from the PRNET.  The firmware in each PR gathers information about bidirectional link quality, nodal capacity and route characteristics and provides this knowledge to debugging and monitoring  Communication  Use radio frequency technology to transmit and receive data  The implemented packet radios support omni-directional, spread spectrum, half-duplex transmission and reception at 400kbit/s and 100kbit/s rates  They implement the physical, data link and network layer (OSI model). 10
  11. 11. Second generations ad-hoc network systems Started in 1980’s -1993  Main Aim  Providing packet switched networking to the mobile battlefield elements in infrastructure- less environments.  Beneficial in improving  Radios performance by making them smaller, cheaper and power-thrifty.  Scalability of algorithms  Resilience to the electronic attacks. Survivable radio network (SURAN)– aimed at providing adhoc n/w with small, low cost, low power, devices with efficient protocols. Internet EngineeringTask Force (IETF) – introduce MANET term -Work to standardized the protocol and functional specifications of ad hoc network.11
  12. 12. Second generations ad-hoc network systems  GloMo (Global Mobile Information Systems) project  Aim:to make the mobile environment user friendly connectivity and access to services for wireless mobile users.  Includes self organizing/self healing networks;both flat and hierarchical multihop routing algorithms 12
  13. 13. Second generations ad-hoc network systems  NearTerm Digital Radio Systems  The NTDR system is an experimental, mobile packet data radio network.  The NTDR provides a self-organizing, self-healing, network capability. Radio network management is provided by a Network ManagementTerminal.  The primary purpose of the NTDR is to provide data transport for the Army Battle Command System automated systems to units at brigade and below  Lessons learned from this experimental fielding provide a portion of the technical baseline for radios being designed for future fielding13
  14. 14. Third generation mobile ad hoc network system  1990’s- onwards  known as commercial ad-hoc network systems.  Invention of notebook computers and viable communication devices based on radio waves concept of commercial ad-hoc networks has arrived.  Idea of collection of mobile nodes were purposed in research conferences  two main and important applications of mobile ad-hoc networks  Bluetooth – proposed by Ericsson in 1994  Ad-hoc sensors 14
  15. 15. Network Architecture of PRNETs 15 PRNET consists of several  Mobile radio repeaters:  The role of a repeater is to relay packets from one repeater to another, until the packets eventually reach the destination host.  Wireless terminals  Dedicated Mobile stations :  The mobile station is present to derive routes from one host to another.  has knowledge of the overall network connectivity,that is,the network topology.  As network conditions change (terminal movement, repeater failures or recovery, changes in hop reliability, and network congestion state), routes are dynamically reassigned by the station to satisfy minimum delay criteria.  Hosts and terminals attached to the PRNET are unaware of the station's assignment and reassignment of communication routes.
  16. 16. Network Architecture of PRNETs 16  Mobile radio repeaters:  Wireless terminals Mobile stations
  17. 17. Components of Packet Radios  User computer -> mobile device/terminal  User computer is interfaced to a radio via the terminal-network controller (TNC).  Radio andTNC logic ->packet radio 17
  18. 18. Components of Packet Radios 18  The packet radio, therefore, implements functions related to protocol layers 1, 2, and 3.  It is an intermediate system (IS) in the ISO context. A packet radio network (PRN) is a collection of packet radios, with some packet radios connected to user devices while others are not.
  19. 19. Routing in PRNETs 1. Point-to-Point Routing  PRNETs support point-to-point communications through point-to-point routing.  A packet originating at one part of the network moves through a series of one or more repeaters until it eventually reaches the final destination.  Point-to-point route is an ordered set of repeater addresses  Specific routes are derived prior to data transmission  determined by the mobile station:  Mobile station is the only element in the network that has knowledge of the overall network connectivity (the network topology).  Mobile station computes the best point-to-point route and distributes this information to all repeaters in the route or directly to the source packet radio.  This scheme was found to be suitable for slow moving user terminals. 19
  20. 20. Routing in PRNETs 2. Broadcast Routing  Radio technology provides very good broadcasting properties.  Broadcasting information to all radios in a network is equivalent to flooding.  To ensure that each packet radio only forwards a packet once, each repeater has to maintain a list of packet identifiers for previously broadcast packets that it recently had received and forwarded.  In broadcast routing, a packet radiates away from the source packet radio in a wave-like fashion, i.e the packet ripples away from the source. 20
  21. 21. Routing in PRNETs 2. Broadcast Routing (cont…)  Broadcasting is very robust  All other nodes in the network must participate in the transmission and reception of packets even that is not intended for them.  When broadcast routing is used for point-to-point communication, the destination host address is included in each data packet.  Routing decisions are not centralized:  No specific routes are derived prior to data transmission  Packets will eventually reach the destination host if the network is not partitioned. For fast moving user terminals, broadcast routing was found to be useful as it avoids the need to process rapidly changing routes. 21
  22. 22. Packet Forwarding 1. Connectionless approach :  Requires some background operation to maintain up-to-date network topology and link information in each node.  As network topology changes, the background routing traffic can be substantial.  Associated with broadcast routing, where each packet carries sufficient routing information for it to arrive at the destination. 2. Connection-oriented packet :  An Explicit route establishment phase is required before data traffic can be transported.  Associated with point-to-point routing, where each node in a route has a lookup table for forwarding incoming packets to the respective outgoing links.  If a topology changes, a route re-establishment phase is needed.22
  23. 23. Impact of Mobility  In a PRNET, all elements of the network can be mobile.  Assumption : User terminals normally move slowly enough such that the assigned point-to-point routes are valid for at least a few seconds before another route must be chosen.  If User rate of mobility increased point-to-point routing may not be practical .Why ???  Because most of the time will be spent in computing alternate point-to-point routes instead of forwarding the packets to their intended destinations. Solution ???  Broadcast routing is less affected by user mobility  the packets do not follow a specific point-to-point route.  Every node is supposed to relay the packets to the destination host.  No need to cope with rapidly changing routes in broadcast routing under conditions of rapid host mobility.  broadcasting is power inefficient.23
  24. 24. Route Calculation Each packet radio gathers and maintains information about current network topology so that it can make independent decisions about how to route packets toward their destinations. Each node maintains the following tables: 1. Neighbor table 2. Tier table 3. Device table 24
  25. 25. Neighbor Table  Broadcast a Packet Radio Organization Packet (PROP) every 7.5 seconds  Announcing its existence and information about the network topology from its own perspective at time.  Neighbors that hear a PROP make entry in their neighbor tables  When nodes hears a PROP, it updates its neighbor table  Transmitted data packets also used to build neighbor table  Tracks bidirectional quality of links with neighbors (retransmission counts) 25
  26. 26. Neighbor Table Neighboring PR Link Quality Node 1 3/9 Node 5 4/5 Node 7 6/9 Node 9 5/8 26 Link Quality = number of packets correctly received from the transmitting packet radio during a PROP number of packets that the transmitting packet radio actually transmitted at that same interval
  27. 27. Tier Table  Routing in PRNETs relies on each packet radio maintaining adequate knowledge of the best packet radio to forward packets to for every prospective destination.  The tier information ripples outward from each packet radio at an average rate of 3.75 seconds per hop and eventually reaches all packet radios.  Every packet radio knows its distance in tiers (or radio hops) from itself to every prospective destination and the next-hop packet radio.  Similar to the earlyARPANET routing algorithm which is based on the classical Bellman-Ford routing. 27
  28. 28. Tier Table  Every packet radio knows the “best” next node on the route from it to a given destination node  Tier 1 = 1 hop neighbors  These neighbors send out their PROPs indicating that they are one hop from the originator  At next step, receivers of these PROPs know that they are 2 hops away from the originator  Process continues until every radio knows its distance in tiers from every other radio  “Best”: shortest route with “good” connectivity on each hope  To change table, must discover a new node with better link quality and lower tier number than currently recorded next node  Also disseminate information about bad links in PROP messages 28
  29. 29. Tier Table Destination PR Next-Hop PR Tier Count Node 1 Node 7 2 Node 4 Node 4 0 Node 5 Node 8 1 Node 6 Node 3 1 29
  30. 30. Device Table 30  Each mobile device/terminal periodically sends a control packet across the wired interface to its attached packet radio.  Packet radio keeps track of affiliated devices and propagates this mapping information via a PROP to other packet radios in the network at an average rate of 3.75 seconds per hop.  when a packet radio receives a packet addressed to a specific mobile device, the device currently attached to the packet radio is known and the appropriate next hop-packet radio is chosen to forward the packet.
  31. 31. Device Table  Logical addressing: maps device to a packet radio  Information about the radio’s attached device is included in PROP messages  This allows new radios to be attached to devices and vice versa  Such correspondences are maintained in the device table at each packet radio P NML Q 1 2 Device PR Node Device 31
  32. 32. Principles of Forwarding Protocols Forwarding is accomplished via information read from the device and tier tables and from the packet headers.  Packet Headers:  End-to-end header  The end-to-end header (ETE) is created by the source mobile device/terminal, not the packet radio. <Src Device ID, Dest Device ID,Type of Service Flag>  Source device ID/address: used to update the packet radio's device-to- packet radio mapping information  Destination device ID/address: used in packet forwarding.  ToS: indicate low latency/low reliability, e.g., speech  The ETE header remains intact as the packet transits toward the destination device.32
  33. 33. Principles of Forwarding Protocols Routing header  In contrast to the ETE header, the routing header is created by the source packet radio.  The routing header encapsulates the ETE header, since it is the routing header that the packet radio will use to forward the packets  <Src PR ID, Seq No, SpeechToS flag, Prev PR ID (for acks), Prev PR transmission count,Transmitting PR ID,Transmitting PR transmit count (for pacing), Next PR ID, Lateral alternative routing flag,Alternative routing request flag,Tier, Dest PR ID> 33
  34. 34. Routing Header 34
  35. 35. Forwarding Protocol  Device 1 --> Device 2 via PRs L, M, N  Device 1 --> PR L  Device sends packet PR L via its wired connection; Prepare packet to forward on to PR N via PR M:  Dest PR ID =N  Prev PR ID =null  Trans PR ID = L  Next PR ID =M (known from tier table)  Tier =2 (from tier table) P NML Q 1 2 Device PR Node Device 35
  36. 36. Forwarding Protocol  PR L --> PR M  PR M receives packet over the air  Next PR ID = M, this PR should process the packet  Prepare to forward packet on to PR N:  Prev PR ID = L  Transmitting PR ID = M  Next PR ID = N (known from tier table)  Tier = 1 (from tier table)  Transmit packet to PR N … and any other PR within range, including L!This is an example of the passive acknowledgement. P NML Q 1 2 Device PR Node Device 36
  37. 37. Forwarding Protocol  PR M --> PR N  N receives packet, determines it should process it based on Next PR ID  Determines that packet should be delivered to the attached Device 2 (from ETE header and device table)  Wire-line transmits the packet to Device 2  Sets in header, for the ack message:  Prev PR ID = M  Trans PR ID = N  Next PR ID = null  Tier = null  Ack message is sent, consisting only of header  Note that end PR can’t use passive acknowledgement, so is forced to transmit ack message to PR M P NML Q 1 2 Device PR Node Device 37
  38. 38. Forwarding Protocol  Criteria for recognizing an Ack  Source PR ID and Seq No match the original packet  AND must have arrived from further downstream:  Transmitting PR ID in ack packet is same as next PR ID in original packet  Previous PR ID is same as receiving PR’s ID--the forward packet came from this packet radio  Ack packet contains a smaller tier number, indicating it got closer to the destination PR 38
  39. 39. Forwarding Protocol  Retransmissions  If a packet is forwarded, and no ack is received, the packet will be retransmitted after a time out  Will do this six times before giving up  Interval between retransmission based on pacing protocol, and grows with each successive unsuccessful retransmission  At some point, sending PR assumes that it can no longer reach the next radio on the hop and sets its connectivity to that radio to 0 39
  40. 40. Transmission Protocols  Pacing protocol  Provide Flow and congestion control mechanisms  Transmission parameters are chosen based on measure link quality and the type of service desired by the user.  Also promotes fair use of the radio spectrum  The time at which a packet is selected for transmission is determined by a three-component packing protocol.  SingleThreading  Forwarding Delay Measurement  Measurement of Retransmissions 40
  41. 41. Transmission Protocols  SingleThreading  Last packet sent to PR must be ack’d or discarded before next packet is sent to the same PR  Passive acks imply that next hop PR now ready to accept a new packet  Deflects congestion bottleneck away from source PR  Forwarding Delay packet radio records the time at which its transmission completes and when it receives the acknowledgment from the next packet radio.This difference is known as forwarding delay  Affects the setting of retransmission intervals  Includes processing, queuing, carrier sense/random access, transmission delay from neighboring PR 41
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  43. 43. Media Access in PRNET’s  PRNETs employ the Carrier Sense MultipleAccess (CSMA) protocol to coordinate communications among mobile hosts.  CSMA prevents a packet radio from transmitting at the same time when a neighboring packet radio is using the medium.  A packet radio is aware if a neighbor is transmitting by reading its hardware indication bit-synchronization-in-the- lock.  Basically, this bit, when set, implies that the channel is busy and a carrier is being sensed.  Whenever a carrier is being sensed, a packet radio will refrain from transmitting. 43
  44. 44. CSMA  Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum.  "Carrier Sense" describes the fact that a transmitter uses feedback from a receiver that detects a carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission. In other words, CSMA is based on the principle "sense before transmit" or "listen before talk".  "Multiple Access" describes the fact that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium. 44
  45. 45. Media Access in PRNET’s  While carrier sensing reduces the probability of channel contention, it cannot eliminate hidden terminal and exposed nodes problems.  The former is a result of a node lying within the radio range of the receiver, but not another transmitter that is two hops away.  The latter results in neighboring nodes of a transmitter being blocked from transmission. 45
  46. 46. Flow Acknowledgments in PRNETs  Packets are forwarded via a single communication route through a PRNET.  Each packet radio must  examines the information contained in the packet headers and in its own device and tier tables.  decide if it should be the one to transmit the packet, if it should update the routing header before transmitting, and if it should update its own tables. 46
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  48. 48.  Other packet radios within the radio range will also receive the transmitted packet.  If these neigbors are not part of the route, they will discard the overheard packets.  The downstream node that receives the packet will process the packet and proceed with issuing a passive acknowledgment.  The single transmission, not only forwards the packet on to the next packet radio but also acknowledges the previous packet radio that the packet was successfully received and is being forwarded. This principle of passive acknowledgment will proceed until the packet reaches the destination node. reception of the packet. 48
  49. 49.  Since the destination node does not have a downstream node and it is the terminating point, an active acknowledgment is sent by the destination node to its upstream node to confirm successful 49
  50. 50. Technical Challenges  PRNETs are different from wired networks in many aspects.  They have an infrastructure-less backbone and network nodes that act as routers or packet switches to forward packets from one node to another.  Routers are connected without wires and routers themselves can be mobile.  The introduction of wireless connectivity and the presence of mobility result in great technical challenges in the field of computer communications.  PRNET is the network that attempts to merge computer communications with telecommunications.  It allows networks to be formed and de-formed on-the-fly, through a set of innovative and adaptive communication protocols. 50
  51. 51. Technical challenges for PRNET The technical challenges for PRNET can be summarized as:  Flow control over a wireless multi-hop communication route  Error control over wireless links  Deriving and maintaining network topology information  Deriving accurate routing information  Mechanisms to handle router mobility  Shared channel access by multiple users  Processing capability of terminals  Size and power requirements 51