The Basics of Wireless Communication Octav Chipara

Transcription

1 The asics of Wireless ommunication Octav hipara

2 genda hannel model: the protocol model High-level media access TM, SM hidden/exposed terminal problems WLN Fundamentals of routing proactive on-demand 2

3 hannel models hannel models - document assumptions of wireless properties the basis upon which we build and analyze network protocols good model is one that is simple - reason effectively about the properties of protocols accurate - capture prevalent properties of wireless channels these requirements are often conflicting Must provide insight into fundamental problems media access routing congestion Today, simple channel model..., next lecture more realistic models 3

4 Protocol model Network is modeled as a graph vertices - all nodes in a graph edges - connect nodes that may communicate Properties: captures connectivity information packet collisions (collisions happen only at the receiver) radios are half-duplex 4

5 Protocol model Network is modeled as a graph vertices - all nodes in a graph edges - connect nodes that may communicate Properties: captures connectivity information packet collisions (collisions happen only at the receiver) radios are half-duplex 4

6 Protocol model Network is modeled as a graph vertices - all nodes in a graph edges - connect nodes that may communicate Properties: captures connectivity information packet collisions (collisions happen only at the receiver) radios are half-duplex 4

7 Media ccess and ontrol (M) Problem: multiple nodes want to transmit concurrently nodes transmitting concurrently packet collisions Metrics for characterizing M performance throughput - number of packets delivered per second latency - time to deliver a packet energy efficiency - energy consumed for tx and rx fairness - each node gets its fair share of the channel flexibility - how does the M handle changes in workload pproaches SM - arrier Sense Multiple ccess TM - Time ivision Multiple ccess 5

8 SM SM - arrier Sense Multiple ccess pproach: 1: node will attempt to transmit after a random delay t W 2: check if channel is available free perform packet transmission busy W = W * 2, go to step 1 Notes: nodes operate independently! the underlying performance is highly dependent on selecting W W - reflects the expected number of contenders for the channel W increases exponentially [the rate depends on protocol] assumption: the sender can accurately check if channel is free/busy usually holds because: receiver sensibility << channel quality required for communication 6

9 SM SM - arrier Sense Multiple ccess pproach: 1: node will attempt to transmit after a random delay t W 2: check if channel is available free perform packet transmission busy W = W * 2, go to step 1 Notes: nodes operate independently! the underlying performance is highly dependent on selecting W W - reflects the expected number of contenders for the channel W increases exponentially [the rate depends on protocol] assumption: the sender can accurately check if channel is free/busy usually holds because: receiver sensibility << channel quality required for communication 6

10 Signal propagation ranges Transmission range communication possible low error rate etection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission detection interference distance 7

11 TM TM - Time ivision Multiple ccess pproach: 1: construct a frame in which each node gets a slot to transmit F - frame size, fn - slot in which node n is assigned to transmit 2: a node n will transmit at time (t mod F) = fn Notes: time synchronization is required frame construction requires a global agreement among nodes underlying performance depends on matching a node s workload demand with its slot allocations hard to do due to dynamic workloads and channel properties assumption: only one successful transmission per slot 8

12 TM TM - Time ivision Multiple ccess pproach: 1: construct a frame in which each node gets a slot to transmit F - frame size, fn - slot in which node n is assigned to transmit 2: a node n will transmit at time (t mod F) = fn Notes: time synchronization is required frame construction requires a global agreement among nodes underlying performance depends on matching a node s workload demand with its slot allocations hard to do due to dynamic workloads and channel properties assumption: only one successful transmission per slot 8

13 Single-hop vs. multiple hops Single-hop networks both SM and TM protocols are easy to implement Multi-hop networks important challenges arise due to asymmetrical views of the networks hidden-terminal problem exposed-terminal problem 9

14 Hidden terminal problem Node and are hidden (edge () is not in the graph) they cannot sense their packet transmissions onsequences for M protocols SM protocols will never increase W TM protocols will have to agree on a frame over multiple hops 10

15 Hidden terminal problem Node and are hidden (edge () is not in the graph) they cannot sense their packet transmissions onsequences for M protocols SM protocols will never increase W TM protocols will have to agree on a frame over multiple hops 10

16 Hidden terminal problem undetected collisions Node and are hidden (edge () is not in the graph) they cannot sense their packet transmissions onsequences for M protocols SM protocols will never increase W TM protocols will have to agree on a frame over multiple hops 10

17 Exposed terminal problem Node and can communicate () and () can occur currently (collisions at receivers) onsequences for M protocols SM will increase W unnecessarily 11

18 Exposed terminal problem false positives Node and can communicate () and () can occur currently (collisions at receivers) onsequences for M protocols SM will increase W unnecessarily 11

19 RTS/TS a solution for SM protocols dd two additional messages to the TM protocol RTS - request to send TS - clear to send lgorithm node n wants to send packet to m transmit RTS(n, m) node a1, a2,..., an, m receive RTS(n, m) node m replies with TS(n, m) if its channel is free node b1, b2,..., bn, n receives TS(n, m) node n transmits the data packet The algorithm signals access requests over 2-hops 12

20 Hidden terminal problem 13

21 Hidden terminal problem send RTS(,) 13

22 Hidden terminal problem recv RTS(,) send RTS(,) 13

23 Hidden terminal problem recv RTS(,) send RTS(,) send TS(,) 13

24 Hidden terminal problem recv RTS(,) send RTS(,) recv TS(,) send TS(,) recv TS(,) 13

25 Exposed terminal problem send RTS(,) 14

26 Exposed terminal problem send RTS(,) send TS 14

27 Exposed terminal problem send RTS(,) ready to tx send TS 14

28 Exposed terminal problem send RTS(,) ready to tx send TS send RTS(, ) ready to tx 14

29 Exposed terminal problem send RTS(,) ready to tx send TS send RTS(, ) ready to tx send TS ready to tx 14

30 Exposed terminal problem send RTS(,) ready to tx send TS send RTS(, ) ready to tx send TS ready to tx ready to tx 14

31 WLN technology Protocol soup: Local wireless networks WLN WiFi a h i/e/ /w b g Goals: seamless operation leverage on existing wired infrastructure low-power operation on stations Two architectures: infrastructure + ad-hoc 15

32 802.11: rchitecture of an infrastructure network ST 1 ESS SS LN ccess Point istribution System SS 2 ccess Point 802.x LN Portal ST LN ST 3 Station (ST) terminal with wireless access mechanisms to contact the access point asic Service Set (SS) group of stations using the same radio frequency ccess Point station integrated into the wireless LN and the distribution system Portal bridge to other (wired) networks istribution System interconnection network to/form one logical network 16

33 802.11: rchitecture of an ad-hoc network SS 1 ST LN ST 3 irect communication within a limited range Station (ST): terminal with access mechanisms to the wireless medium Independent asic Service Set (ISS): group of stations using the same radio frequency When no direct link is feasible between two station, a third station may act as a relay (multihop communications) ST LN 17

34 802.11b - istributed oordination Function Exponential back-off hosen for uniformly from (0, W-1), W increase exponentially with the number of failed attempts Wmin minimum contention window Wmax = 2 m Wmin maximum contention window 18

35 802.11b - istributed oordination Function Message resent when the backoff counter reaches zero ackoff counter decremented only when the channel is idle ackoff counter is reset to zero after a successful transmission 19

36 Routing Routing consists of two fundamental steps Forwarding packets to the next hop (from an input interface to an output interface in a traditional wired network) etermining how to forward packets (building a routing table or specifying a route) Forwarding packets is easy, but knowing where to forward packets (especially efficiently) is hard Reach the destination Minimize the number of hops (path length) Minimize delay Minimize packet loss Minimize cost 20

37 Routing ecision Point Source routing Sender determines a route and specifies it in the packet header Hop-by-hop (datagram) routing routing decision is made at each forwarding point (at each router) Standard routing scheme for IP Virtual circuit routing etermine and configure a path prior to sending first packet Used in TM (and analogous to voice telephone system) 21

38 Routing Table routing table contains information to determine how to forward packets Source routing: Routing table is used to determine route to the destination to be specified in the packet Hop-by-hop routing: Routing table is used to determine the next hop for a given destination Virtual circuit routing: Routing table used to determine path to configure through the network 22

39 Routing pproaches Reactive (On-demand) protocols discover routes when needed source-initiated route discovery Proactive protocols traditional distributed shortest-path protocols based on periodic updates. High routing overhead Tradeoff state maintenance traffic vs. route discovery traffic route via maintained route vs. delay for route discovery

40 istance Vector lgorithms (1) istance of each link in the network is a metric that is to be minimized each link may have distance 1 to minimize hop count algorithm attempts to minimize distance The routing table at each node specifies the next hop for each destination specifies the distance to that destination Neighbors can exchange routing table information to find a route (or a better route) to a destination 24

41 istance Vector lgorithms (2) est Next Metric est Next Metric est Next Metric est Next Metric

42 istance Vector lgorithms (3) Node will learn of Node s shorter path to Node and update its routing table est Next Metric est Next Metric

43 Reactive Routing Source initiated Source floods the network with a route request packet when a route is required to a destination flood is propagated outwards from the source pure flooding = every node transmits the request only once estination replies to request reply uses reversed path of route request sets up the forward path 27

44 Route iscovery RREQ FORMT G Initiator Initiator seq # estination I Partial Route E H -- Route Request (RREQ) F -- Route Reply (RREP) 28

45 Route iscovery RREQ FORMT G Initiator Initiator seq # estination I Partial Route -- E H Route Request (RREQ) F -- Route Reply (RREP) 28

46 Route iscovery RREQ FORMT - G Initiator Initiator seq # estination I Partial Route -- E H Route Request (RREQ) F -- Route Reply (RREP) 28

47 Route iscovery RREQ FORMT - G Initiator Initiator seq # estination I Partial Route -- E H Route Request (RREQ) - -- F Route Reply (RREP) 28

48 Route iscovery RREQ FORMT - G Initiator Initiator seq # estination I Partial Route --E -- - E --E --E H Route Request (RREQ) -- F Route Reply (RREP) 28

49 Route iscovery RREQ FORMT - -- G Initiator Initiator seq # estination I Partial Route --E -- - E --E --E H Route Request (RREQ) -- F Route Reply (RREP) 28

50 Route iscovery RREQ FORMT - --E -- G --E-H Initiator Initiator seq # estination I Partial Route -- - E --E --E H Route Request (RREQ) -- F Route Reply (RREP) 28

51 Route iscovery RREQ FORMT ---G - ---G ---G --E -- G --E-H Initiator Initiator seq # estination I Partial Route -- - E --E --E H Route Request (RREQ) -- F Route Reply (RREP) 28

52 Route iscovery: at source need to send to G Lookup ache for route to G Start Route iscovery Protocol wait uffer packet ontinue normal processing yes no Route found? yes Write route in packet header Route iscovery finished Packet in buffer? no done Send packet to next-hop 29

53 Route iscovery: t an intermediate node ccept route request packet <src,id> in recent requests list? yes iscard route request no ppend myddr to partial route Store <src,id> in list no Host already in partial route no myddr =target Send route reply packet yes iscard route request roadcast packet done 30

54 Route iscovery Route Reply message containing path information is sent back to the source either by the destination, or intermediate nodes that have a route to the destination reverse the order of the route record, and include it in Route Reply. unicast, source routing Each node maintains a Route ache which records routes it has learned and overheard over time 31

55 Route Maintenance Route maintenance performed only while route is in use Error detection: monitors the validity of existing routes by passively listening to data packets transmitted at neighboring nodes When problem detected, send Route Error packet to original sender to perform new route discovery Host detects the error and the host it was attempting; Route Error is sent back to the sender the packet original src 32