Wireless Communication

Transcription

1 Wreless Communcaton Dr. Ganluca Franchno Scuola Superore Sant Anna Psa, Italy Emal:

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3 ReTS Lab Real-Tme Systems Laboratory Outlne Graduate Course on Embedded Control Systems: Theory and Practce Wreless Communcaton n Real-Tme Embedded Systems Ganluca Franchno g.franchno@sssup.t Introducton to: Dstrbuted Embedded Systems QoS and Real-Tme Communcaton Wreless Communcaton: MAC protocol technques: CSMA/CA, TDMA (schedulng approach) and IEEE /ZgBee Real-tme MAC protocols Capacty of Wreless Networks: Capacty of adhoc networks Real-Tme Capacty of Fxed-Prorty schemes Capacty of Implct EDF Dstrbuted System Dstrbuted Systems Dstrbuted System: s an applcaton that executes a collecton of protocols to coordnate the actons of multple processes on a network, such that all components cooperate together to perform a sngle or small set of related tasks. Why s a dstrbuted archtecture desrable? Composablty: the system s bult by composng/ntegratng sub-systems Scalablty: a new system functon can be obtaned addng a new node. A system functon can be replcated n the same way Informaton Processng close to data sources/snks: n-node data elaboraton: ntellgent sensors and actuators. Dependablty: Robustness: a node falure does not jeopardze the system operaton (no sngle pont of falure); Mantanablty: tanks to system modularty, a node can be replaced easly Dstrbuted Systems Applcatons The network s a fundamental part of a Dstrbuted System In general, a node falure does not compromse system servces In general, loss of network operaton jeopardzes system servces THE NETWORK IS THE KERNEL OF A DISTRIBUTED SYSTEM Wreless Sensor/Actuator Networks Home automaton (domotc systems) Wearable WSN for health-care Envronment montorng Mult-robot systems Control Systems for Cars, Arcrafts, Trans etc. Factory Automaton Etc

4 Wreless Dstrbuted Systems Communcaton Stack (ISO/OSI Model) Wreless Dstrbuted System: Dstrbuted System where the network s composed by wreless nodes Wth respect to a wred channel, the management of a wreless channel s more dffcult A wreless channel s characterzed by: Hgh bt error rate e.g > 10-3 Asymmetrc lnks: NODE1 NODE2 Varable Channel Capacty (Bandwdth), both over tme and node by node Network systems desgned by a modular methodology-> layered stack Each layer s delegated to specfc functonaltes EachE h layer mplements: Protocols to manage the communcaton wth the correspondng layer n other nodes Servces provded to adjacent layer through servce nterfaces NODE APPLICATION PRESENTATION SESSION TRANSPORT NETWORK DATA LINK PHYSICAL Communcaton Stack Communcaton Stack PHYSICAL LINK LOGICAL LINK NODE APPLICATION PRESENTATION Servces (Protocols) for Applcatons Data semantcs NODE A APPLICATION PRESENTATION NODE B APPLICATION PRESENTATION SESSION TRANSPORT Remote Actons End-to-End Communcaton SESSION TRANSPORT NETWORK SESSION TRANSPORT NETWORK NETWORK DATA LINK Routng and Logcal Addressng LLC, MAC, Physcal addressng DATA LINK PHYSICAL DATA LINK PHYSICAL PHYSICAL Bt Encodng, Modulaton, Physcal Channel, Transcever Control COMMUNICATION CHANNEL Node Archtecture (Embedded Systems) Real-Tme Communcaton Real-tme Communcaton: Effcent communcaton of Short Data: Sensor Data (few bytes) Perodc transmsson wth low Jtter: Control, Sensor and Montorng Data: Tme Trggered Transmsson Fast transmsson of Event Data (Asynchronous Data, e.g. alarms): Event Trggered Transmsson Mxed traffc Communcaton: Coexstence of best-effort traffc (non real-tme traffc as log data, confguraton data ) and Real- Tme traffc

5 Real-Tme Communcaton Communcaton Effcency End-to-End communcaton delay must be bounded Each layer ntroduces computatonal and communcaton overheads (header bytes) All layer servces must be tme-bounded Thus not all stack layers are mplemented: Short Messages: message fragmentaton/reassembly s not needed (no Transport Layer) When there s only a sngle-hop doman, the network layer s not mplemented (no routng) Applcaton Layer nterfaces the Data Lnk Layer drectly (when there s no need of Network layer) CEff: Communcaton Effcency Data_length (payload): s the length (tme unts) of data generated by the applcaton runnng nto the node Comm_length:s the tme length of the message transacton (end-to-end delay). It comprses layer servces overhead plus transmsson overhead due to the control characters (packet headers) Data_ length CEff = Comm_ length Communcaton Overhead Collapsed Model Ex. Sngle-Hop Doman: Factory Automaton (Feld Bus). Ex. Mult-Hop Doman: Wreless Sensor Networks. Sngle-Hop Doman NODE APPLICATION DATA LINK PHYSICAL Mult-Hop Doman NODE APPLICATION NETWORK DATA LINK PHYSICAL ZgBee Stack QoS defnton QoS requrements are applcaton dependent Man QoS metrcs: Throughput: (AvalBand - OverheadBand)/ AvalBand Maxmum Delay: tme-bounded transmsson (real-tme) Jtter: varablty on message transmsson/recevng tme Relablty: Integrty of messages. Guarantee that all messages wll be delvered correctly Other Performance Metrcs: : Energy Dsspaton: Energy wasted should be lmted, ether to acheve a predefned system lfetme or to maxmze the system lfetme Farness: assgnment of network resources n a balanced fashon among the nodes Stablty: the network s a dynamc system. The protocols performance should be stable under any workng condton Robustness: normal network (protocols) operaton should be guaranteed even under some control packet losses or node falure (e.g. coordnator node falure)

6 Examples on QoS requrements Multmeda streamng: Hgh packet delvery rato Low Delay Low Jtter Control Applcatons: Low Jtter Perodc Message Delvery Dstrbuted Informaton Systems (data base): Integrty of exchanged messages -> the system should guarantee the ntegrty of retreved data (data wthout errors) Data Lnk Layer Data Lnk Layer (It s of paramount mportance for Real-tme Communcaton) Logc Lnk Control (LLC) Medum Access Mechansm (MAC) LLC: Formaton and mantenance of lnks between one-hop neghbors nodes Lnk dscovery, setup, mantenance, lnk qualty estmaton Relable and Effcent nformaton (packets) transmsson over the establshed lnks Addressng and Flow Control Admsson Control Error Control: ACKnowledgement (ACK) Automatc Repeat request (ARQ) Forward Error Correcton (FEC) (Preferred for Real-Tme Comm.) MAC: Management of medum (channel/lnk) access MAC protocols Wreless MAC protocols The task of a MAC protocol s to manage the channel access by network nodes. From the standpont of a MAC sub-layer, the network s composed by n nodes sharng a common channel (medum). It determnes the order of the channel access by contendng nodes. Hence t determnes the network access delay The MAC protocol s fundamental for the real-tme performance of a network that shares a common medum NODE 1 NODE 2 NODE 3 NODE n CHANNEL NODE 4 NODE 7 NODE 6 NODE 5 Aloha Aloha Desgned by Norman Abramson at Unversty of Hawa n 1970s The base algorthm s smple: Whenever you have a packet to transmt, send the packet If the packet colldes wth an other transmsson, wat for a random nterval and then try to send the packet It s assumed that a node can be aware of a collson ether by lstenng to the channel whle transmttng, or by some feedback mechansm (e.g. ACK)

7 Slotted-Aloha Slotted-Aloha mproves the basc verson The tme s dvded n tme-slots Anodecantrytosend a packet only at the begnnng of each slot. Anodecannot try to send a packet n the mddle of a slot The number of collsons s reduced Nodes must be synchronzed. 0.5 Aloha 1 Maxmum Throughput of Aloha: S = 0.5e Maxmum Throughput of Slotted-Aloha: S Slotted Aloha Aloha max = S 1 max = e = G = Offered Load. G s the average number of transmsson attempts per packet (slot) tme Wth Aloha S max s for G =0.5 Wth Slotted-Aloha S max s acheved for G= G 6 8 CSMA Carrer Sense Multple Access (CSMA): Sense the channel every tme you have a packet to send. If the channel s free (dle) then send your packet If the channel s busy then retry CSMA non persstent CSMA x-persstent CSMA 1-persstent Step 1: f the channel s free, transmt the packet Step 2: f the channel s busy, contnue to lsten the channel untl t s free then transmt the packet If two nodes are lstenng the channel when a thrd node s transmttng, when ths last fnshed the two nodes start transmttng causng a collson CSMA non persstent Step 1: f the channel s free, transmt the packet Step 2: f the channel s busy, wat for a random tme and repeat Step1 Random backoff reduces collsons probablty Too long backoff reduces the throughput CSMA p-persstent Ths algorthm s usually used when the tme s dvded n slots Step 1: f the channel s free, transmt wth probablty p and defer to next tme slot wth probablty 1-p Step 2: f the channel s busy, contnue to sense the channel. When the channel s free repeat Step 1 Step 3: f transmsson s deferred by a tme slot repeat Step 1 A tradeoff between non-persstent and 1-persstent Throughput (S) vs Offered Load (G) CSMA/CA S persstent CSMA non-persstent CSMA 0.1-persstent CSMA persstent 0.5-persstent CSMA persstent CSMA Slotted Aloha Aloha G To reduce the wasted tme due to collsons, f two or more nodes transmt at the same tme, hence there s a collson, t would be better that the nodes stop transmttng Possble wth wred networks, because a node can transmt and lsten the channel at the same tme (e.g CSMA/CD-Ethernet) t) Wth a wreless channel, to transmt and to lsten at the same tme s dffcult or even mpossble Soluton: CSMA/CA (Collson Avodance) The worst stuaton: when the medum s busy and two or more nodes are sensng the medum watng to transmt CSMA/CA tres to reduce the collson probablty by a random backoff procedure: f the channel s free then backoff for a random tme, after that, f the channel s stll free transmt

8 IEEE CSMA/CA Nodes ready to transmt sense the medum If the channel s busy, wat untl the end of current transmsson Then wat for an addtonal predetermned tme perod DIFS (Dstrbuted Inter Frame Spacng) Then pck up a random number of slots (the ntal value of backoff counter) wthn a Contenton Wndow to wat before transmttng the frame (packet) Contenton Wndow s defned by [0,CW], where CW mn CW CW max If there are transmssons by other nodes durng ths tme perod (backoff tme), the node stops ts counter It resumes count down after nodes fnsh transmsson plus DIFS. The node can start ts transmsson when the counter value s zero If the channel access fals (e.g. there s a collson), then ncrement the CW value. (CW = 2*CW) The ntal backoff makes CSMA/CA smlar to p-persstent CSMA end of last packet transmsson IEEE CSMA/CA DIFS = Dstrbuted Inter-Frame Spacng IEEE /ZgBee un-slotted CSMA/CA IEEE /ZgBee slotted CSMA/CA NB: number of performed tres BE: backoff exponent Backoff nterval: (0, 2 BE -1) The ntal backoff makes CSMA/CA smlar to p-persstent CSMA NB: number of tres performed BE: backoff exponent Backoff nterval: (0, 2 BE -1) CW (Contenton Wndow). CW=2 The ntal backoff makes CSMA/CA smlar to p-persstent CSMA Hdden Node problems Exposed Node problem CSMA protocols suffer the Hdden Node problem: Node 1 wants to transmt to Node 2, t fnds the channel free and starts transmttng Node 3 wants to transmt to Node 2, snce Node 3 s out of range wth respect to Node 1, t fnds the channel free and start transmttng to Node 2 There s a collson between the transmssons of Node 1 and Node 3 CSMA protocols suffers the Exposed Node problem: Node 1 wants to transmt to Node 4, t fnds the channel free and starts transmttng Node 2 wants to transmt to Node 3, f fnds the channel busy, then t blocks watng for the channel to be free Transmsson from Node 1 cannot reach Node 3, transmsson from Node 2 cannot reach Node 4, therefore Node 1 and Node 2 could transmt smultaneously!

9 Mtgatng Hdden/Exposed Node Problem The Hdden/Exposed Node Problem can be mtgated by a handshakng mechansm: A node that wants to transmt sends a Request To Send (RTS) packet to recever node The recever reples wth a Clear To Send packet (CTS) A node that ears a CTS packet keeps slent for duraton of ncomng transmsson A node that ears a RTS packet but not a CTS, assumes to be an Exposed node, then t can transmt also whether t fnds the channel busy for the duraton of the ncomng transmsson Both RTS and CTS report the length of the packet beng to be transmtted Ths mechansm s used, for nstance, n IEEE , MACA, MACAW protocols Mtgatng Hdden/Exposed Node Problem Both Node 1 and Node 3 want to send a packet to Node 2 Node 1 senses the channel free and sends a RTS packet Node 2 receves the RTS and responds wth a CTS packet Node 3 receves the CTS then keeps slent Node 4 receves a RTS but not the CTS, then t assumes to be an exposed node Node 1 transmt ts packet Node 4 beng an exposed node mght transmt a packet even f t senses the channel busy Schedulng approaches The tme s dvded n slots Each tme slot s reserved/dedcated to a node Each node has an exclusve access to ts tme slots: no collsons Dfferent schedulng polcy can be used to assgn the tme-slots: Round Robn (RR) Weghted Round Robn (WRR) Rate Monotonc (RM) Earlest Deadlne Frst (EDF) Etc. Any algorthm from resource schedulng theory mght be appled Tme slot dmenson s an mportant parameter: All packets have the same dmenson: tme-slot=packet tme Packets have dfferent dmenson: an mportant porton of bandwdth can be lost: a bandwdth reclamng mechansm s desrable Nodes must be synchronzed: synchronzaton mechansms are needed Fully dstrbuted Schedulng Approaches In case of fully dstrbuted approaches: Each node must know/buld the schedule In order to buld a common schedule ether each node must know the traffc parameters of other nodes, or at least some common nformaton should be shared by the nodes Example of such approaches: Implct EDF WewllseesomedetalsofImplct EDF later Round Robn Schedulng Coordnated (Centralzed) Schedulng Approaches There s a central Coordnator node (a.k.a. Master) The Coordnator decdes when a node can access the channel Pollng (Master/Slave): Coordnator polls the nodes for packet transmsson usng a schedulng gpolcy y( (Bluetooth) Access Wndow approach: Coordnator defnes a channel Access Wndow by means of a perodc beacon transmsson. The Access Wndow s defned by two consecutve beacons (Ex: IEEE beaconed-mode) The Access Wndow s dvded n tme slots Coordnator communcates the Access Wndow schedulng n the beacon packet (for nstance) Both poll and beacon mechansms synchronze the nodes

10 Pollng approach (MASTER SLAVE): Token Passng There s a token travelng among the nodes Each node has a tme budget Every tme a node receves the token, t can transmt ts traffc for a tme no greater then ts budget It needs a polcy to exchange the token among the nodes (e.g. crcular fashon) It needs a polcy to assgn the budgets (e.g. Weghted RR) RR RR WRR WRR Mxed Approaches (Hybrd) DCF and PCF coexstence Mxed approaches explot both CSMA technques and schedulng based (collson-free) technques Several standard protocols use a mxed approach: e.g. IEEE and IEEE IEEE (W-F) Dstrbuted Coordnaton Functon (DCF) RTS/CTS + CSMA/CA + NAV (Network Allocaton Vector ->Vrtual l Carrer Sensng) Vrtual Carrer Sensng: a node extracts the length of the ncomng transmsson from ether RTS or CTS, then keeps slent for the entre packet length A postve ACK s used to confrm the packet has been receved correctly; Pont Coordnaton Functon (PCF) (Pollng approach) Central Coordnator (Access Pont) IEEE : Access Wndow wth CSMA/CA and reserved tme slots (more detals later) Inter Frame Spacng (IFS): mnmum space between two consecutve packets Dstrbuted IFS (DIFS): between consecutve packets under DCF Pont IFS (PIFS): PCF traffc Short IFS: ACK or CTS Real-Tme MAC protocols Contenton Based Protocols Dfferentaton Mechansms for IEEE CSMA/CA Black Burst Schedulng Based Protocols Implct EDF WBuST Mxed Contenton and Schedulng Protocols IEEE /ZgBee Dfferentaton Mechansm IEEE IEEE DCF farness: each node has the same probablty to access the channel For a tmely communcaton (QoS n general), a node (network traffc source) should receve: precedence (probablty) on channel access based on ts traffc prorty a porton of bandwdth proportonal to ts prorty/traffc parameters Prorty traffc dfferentaton mechansms: Scalng Contenton Wndow (CW) accordng to the prorty of each traffc source (node) Assgnng dfferent DIFSs based on the prorty of traffc sources

11 Contenton Wndow Scalng DIFS dfferentaton prorty 1 CW = CW 2 + max_ prorty CW s expressed n tme slots, e.g. CW=4 backoff slots CW s the base value for the Contenton Wndow CW s the Contenton Wndow of node Example: Network composed by n nodes Each node has a perodc stream S =(m,t,d =T ) Node prorty assgned by Rate Monotonc prorty proportonal to T (RM) max_prorty proportonal to max(t ) (RM) The hgher the prorty number, the lower the prorty The hgher the prorty, the lower CW DIFS = BASE _ DIFS * prorty Example: Network composed by n nodes Each node has a perodc stream S =(m,t,d =T ) Node prorty assgned by Rate Monotonc prorty proportonal to T (RM) The hgher the prorty number, the lower the prorty The hgher the prorty the lower DIFS IEEE e Black Burst IEEE e s the standard verson supportng QoS requrements It defnes Enhanced DCF (EDCF) whch provdes servce dfferentaton mechansms It defnes also a new pollng based access mechansm called Hybrd Coordnaton Functon (HCF) Controlled Access Channel (HCCA) (an enhanced PCF). EDCF defnes four classes of channel Access Categores (ACs) Each AC has a dfferent prorty Servce dfferentaton s acheved by: Conteton Wndow dfferentaton: t assgns to each AC a dfferent CW mn, CW max DIFS dfferentaton: Instead of usng an unque DIFS, EDCF uses a dfferent Arbtraton IFS (AIFS) value for each AC. The hgher the AC prorty the shorter the AIFS. Black Burst s a technque to guarantee a better performance for real-tme traffc under IEEE A Real-Tme (RT) nodesonethathasreal-tme traffc to delver RT nodes contend to access the channel after a Medum IFS (MIFS<DIFS) RT nodes sort the access rght by jammng the channel sendng pulses of energy (BB) The node that sends the longest BB wns the contenton and t can transmt ts real-tme packet Black Burst Black Burst t + BB ( t ) = (1 + t t ) t rt = ttx tsch rt rt bbslot t Current tme nstant t rt ttx tsch tbbslot Tme nstant at whch node attempts to access the channel for transmttng Tme nstant at whch node transmts ts real-tme packet Mnmum nterval between two consecutve real-tme packet transmsson attempts (equal for all nodes) BB slot dmenson

12 Implct EDF (IEDF) [Caccamo et al. 2002] Implct EDF (IEDF) Nodes are grouped nto hexagonal cells: cellular structure Each cell contans a router and a set of nodes Each router has two rados A rado channel s assgned to each cell Intra-cell and Infra-cell communcatons It uses a schedulng base channel access mechansm It uses the Earlest Deadlne Frst (EDF) algorthm to compute the transmsson schedule Each node must know the traffc parameters of each other node: S =(m,tt,dd =T ) Traffc Parameters T message perod D message relatve deadlne m message length (#packets) Each node computes the schedule. The schedule s replcated at each node: Each node wll know whch one has the shortest deadlne hence the rght to access the channel to transmt. Each nodes has an exclusve access to the channel IEDF Intra-cell communcaton IEDF A C B Nodes must by synchronzed Unused bandwdth problem: a reclamng bandwdth mechansm s necessary: FRASH Dynamc schedule update mechansm s needed when a node wants to jon the network or a node leaves the network It s possble to manage both perodc traffc and sporadc traffc (through Aperodc Servers) Consder havng a message stream set M =(S 1,S 2,..S n ), a set of s servers wth: Q j Server Capacty Ts j Server Perod Stream set Feasblty Test (classc EDF+Servers test): n m + T = 1 j= 1 s Q j 1 Ts j IEDF Infra-cell communcaton IEDF T block = 2 Schedulablty Test, =1,..., n n mk B + 1 k = 1 Tk T T block B = T A nter-cell frame s nserted every T block 2

13 WBuST Network structure The Wreless Budget Sharng Token (WBuST) protocol s a MAC layer protocol desgned for real-tme communcaton n wreless networks WBuST supports both sngle-hop and mult-hop communcaton WBuST manages both real-tme and best-effort (non real-tme) traffc Coordnator node Cluster node Cluster Network nodes are grouped nto clusters A cluster s formed by a coordnator node and a set of cluster nodes Each cluster has a dfferent communcaton channel Communcatons nsde dfferent clusters can proceed n parallel wthout nterfere Intra-cluster communcaton example Traffc model Beacon perod T b Communcaton wndow Coordnator node Cluster node B M B B SLEEP Beacon Budget for cluster management Budget node B M B 1 B 2 B 3 B 4 B SLEEP B M B 1 B 2 B 3 B 4 B SLEEP T b Sleep Budget t Each cluster node can have a perodc stream of messages S (m, T, D ): m : maxmum message length T : message perod D : relatve deadlne It s also possble to represent sporadc streams wth h the same model Each node can also have non real-tme traffc to delver A node uses ts tme budget B to transmt both realtme (perodc) and non real-tme traffc Node budget s shared between real-tme and non real-tme traffc Real-tme traffc has precedence over non real-tme traffc Implct bandwdth reclamng Energy savng Bandwdth reclamng under WBuST: If node does not use ts tme budget entrely, t sends a TX- END packet to node +1 Node +1 can so start usng ts budget before than expected When node +1 fnshes ts transmsson sends a TX-END packet to node +2, and so on untl the last node If the last node (node 4 n the example) sends a TX-END packet, then the coordnator transmts the new beacon T b <T b T b Energy model: P = TX TX RX RX SL [ P U + P U + P U SL ] t T b TX B U = T b RX b U = + Tb BSL U SL = Tb P s the average power wasted after t tme unts Gven a desred network lfe tme, by means of the energy model, both node budgets and sleep budget are calculated to guarantee both the desred network lfetme and the message deadlnes T b T b n j= 1, j B T j b B M B 1 B 2 B 4 B 3 B M B 1 B 4 B M B 1 B 2 B 3 B 4 t b B M B 1 B 2 B 3 B 4 B SLEEP B M B 1 B 2 B 3 B 4 B SLEEP t

14 Budget Allocaton Tme budgets B are assgned by Budget Allocaton Schemes (BAS) Example. Modfed Local Allocaton (MLA): B = Stream node : S (C, T, D ) D T C D D mn D = mn( D ) mn Message deadlnes guarantee Consder a cluster of n nodes that generate a perodc traffc descrbed by a stream set M ={S 1, S 2,,S n } Message deadlnes guarantee s based on U * U * s the least upper bound on the stream set utlzaton U RT, whch guarantees the message deadlnes n RT C U = T Suffcent Condton for stream set feasblty: gven a stream set M ={S 1, S 2,,S n },fu RT (M) U * then M s feasble. A stream set M s sad to be feasble when all message deadlnes are met. = 1 Message deadlnes guarantee Hardware/Software tools Under WBuS, U * depends on both the choce of beacon perod T b and D mn. Example Example. Modfed Local Allocaton (MLA): 1 α Avalable bandwdth D mn * Tb U = 1 D mn + 1 T b ( α ) Hardware: FLEX Base board featurng Mcrochp dspic33fj256mc710 Daughter board CC2420 IEEE complant transcever H M + tbe α = T b Bandwdth lost due to protocol overhead Software: RTOS Erka Enterprse Expermental Analyss 6 nodes, only RT traffc U Network Utlzaton U RT Real-Tme load U=U RT U RT

15 6 nodes, RT and Best Effort traffc ReTS Lab Real-Tme Systems Laboratory U Network Utlzaton U RT Real-Tme load U NRT Non Real-Tme load U = U RT + U NRT = 1-α uwreless and IEEE Ganluca Franchno U RT g.franchno@sssup.t IEEE /ZgBee IEEE topologes IEEE defnes MAC and PHY layers ZgBee defnes the Network layer and the Applcaton layer Nodes are grouped nto Personal Area Network (PAN): A PAN s a cluster of nodes managed by a Coordnator node PAN Topologes: Star and Mesh IEEE defnes two workng modes: Non-beaconed mode operaton Beaconed mode operaton Non-beaconed mode (used by ZgBee): whenever anodenthepan wants to transmt, t uses the unslotted CSMA/CA algorthm The task of the Coordnator s to manage the PAN: Es. Node assocaton/dsassocaton Two knds of devce: 1. Full Functon Devce (FFD); 2. Reduced Functon Devces (RFD). FFD can communcate to both FFD and RFD devces; RFD can communcate only to FFD devces. IEEE /ZgBee IEEE /ZgBee Beaconed mode: Coordnator defnes a superframe structure by sendng a perodc beacon The superframe contans 16 tme slots The superframe contans a Contenton Access Perod (CAP) and a Contenton Free Perod (CFP) (optonal) The superframe can contan also an Incatve porton to save energy (optonal) Durng the CAP nodes use slotted-csma/ca to: Send data packets Send GTS allocaton requests To jon the PAN Etc. Nodes can have Guaranteed Tme Slots (GTS) allocated n the CFP GTS s composed by one or more superframe slots Max 7 GTSs Durng the CFP, there are no collsons: Exclusve channel access by nodes holdng a GTS Schedulng of GTSs s contaned n the beacon

16 μwreless μwreless archtecture μwreless: an mplementaton of the IEEE standard for WPAN Fast prototypng of IEEE based protocols To decrease the tme for developng new protocols Example: BACCARAT a protocol for bandwdth management n WSN μwreless s developed by a software archtecture that makes code wrtng ndependent from both the hardware and the RTOS IEEE ReTS Lab Real-Tme Systems Laboratory Real-Tme Capacty of Multhop Wreless Networks Introducton Assumptons: Nodes communcate wthout any centralzed control: Ad Hoc networks Multhop networks A proper schedulng scheme s used Transport Capacty of a Wreless Network: sum of products of bts and the dstances over whch they are carred n an unt of tme ([bt-meters/sec]) A capacty of k bt-meters: k bts have been transported over a dstance of one meter toward ts destnaton Ganluca Franchno g.franchno@sssup.t Arbtrary networks: Arbtrary Networks Each node can transmt at W bps (bts per second) over a wreless channel n nodes located arbtrarly n a dsk of area A Nodes send traffc at an arbtrary rate to an arbtrary destnaton Arbtrary Traffc Pattern A Network Transport Capacty Per node Transport Capacty Per node Troughput Arbtrary Networks [Gupta and Kumar 2000] C = Θ W C = Θ ( W An ) W C = Θ n A n [bt-meters/sec] [bt-meters/sec] [bt/sec] Knuth s notaton: f=θ(g(n)) ff f(n) = O(g(n)) and g(n) =O(f(n)) f s bounded both above and below by g asymptotcally

17 Real-Tme Capacty Real-Tme Capacty Capacty bounds gve an nsde on the network throughput as a functon of network parameters: Bandwdth (W), sze(a) and average densty (# nodes n) etc. Real-Tme Capacty: t concerns capacty lmts on real-tme nformaton transfer n mutlhop wreless networks Schedulablty n dstrbuted system s NP-Hard, hence, no closed-formula l for Real-Tme Capacty Real-Tme Capacty depends on the packet schedulng protocol Suffcent (not necessary) closed-formula for fxed-prorty packet schedulng protocols [Abdelzaher et al. 2004]. Real-tme capacty C RT of a network (suffcent bound) The capacty requrement U of a wreless multhop network s gven by the bt-meters product of messages normalzed by ther relatve deadlnes Adoptng the usual notaton: Consderng a set M(S 1,, S, S n ) of message streams S (C, D ), wth =1,..,n C s the product of the message lenght (bts) and the destnaton dstance (meters) D s the relatve deadlne Capacty Requrement C U = D Suffcent Condton for stream set feasblty: gven a multhop wreless network wth a capacty requrement U, fu C RT then all message deadlnes wll be met Capacty Requrement Load-balanced Networks All nodes have on average m neghbour nodes Dstances measured n hops m s called node densty ([nodes/hops]) Ne(j): neghbourhood of node j, whch s the set of nodes whose transmssons can be heard by node j H j s the traffc generated nto Ne(j) j Ne(j) W U = m H j = j Ne( j) H j C D N s the communcaton dameter ([hops]) N s the longest communcaton path a node can be part of α s the mnmum relatve deadlne rato across all prortysorted packet It descrbes the degree of urgency nverson: a packet wth a shorted deadlne receve a prorty lower than a packet wth a larger deadlne Deadlne Monotonc (DM) algorthm: the lower the deadlnedl the greater the prorty Wth Deadlne Monotonc α=1 (t maxmzes C RT ) It happens beacuse for any possble couple of packets D lo =D h Assgnng prortes randomly (RAND) RAND D D DM lo α = mn α = mn 1 α = 1 lo pck h, pck Dh D max C RT for load-balanced networks Realstc MAC protocols C RT n α = 1+ m N For large networks, N s large n = Ο( n) N 2 α 1+ N 2 α <<11 N If N s bounded and does not depends on n If the routng paths are randomly chosen: C RT W C RT C RT α = Ο( n ) W m nα = W mn α = Ο( n ) W m Before we consdered deal MAC protocols wth no tme overhead (manly due to channel access arbtraton) Consderng the overhead, let d the maxmum delay that a packet can experence for channel arbtraton at each hop C RT n α ' = 1+ m N 2 α ' 1+ N d α ' = α 1 N D α mn D = mn( D ) mn W

18 Un-balanced networks: WSN example How to use Real-Tme Capacty Realstc load patterns are dffcult to characterze WSN (Wreless Sensors Network) example: K snk nodes N k maxmm number of hops between a sensor and a snk C RT αknk = ln N k W Snk C RT can be used to: Gven a set of message streams, verfy the schedulablty (U C RT ) Fnd network parameters thatt guarantee packets schedulablty WSN dmensonng Pseudo Prorty Inverson Consder a WSN composed by: n = 1000 nodes, K= 5 snk nodes N k =5s the average hops between sensors and snks W = 250 kbps n perodc streams S (C, T, D ): for any, C =C=25 bytes, D =1 sec, T =T C RT Deadlne Monotonc α =1 Real-Tme Capacty αkn k = W = ln N k Schedulablty Condton byte hops sec Capacty Requrement D N K D byte hops U = nc = T D T sec # messages generated be each source when the system s schedulabe U CRT T 330 [ msec] So far we assumed that a packet transmsson n a neghbourhood Ne(j) contends only wth other transmssons n Ne(j) Ths assumpton s not always true Ne(R1) j Ne(j) Ne(R2) Hgh prorty S3 Low prorty S1 S2 R2 R1 Medum prorty Real-Tme Capacty and Pseudo- Prorty nverson Consderng both Pseudo-Prorty Inverson and channel arbtraton delay: Real-Tme Capacty of IEDF Caccamo and Zhang (2003) derved the Real-Tme Capacty bound (throughput) of Implct EDF Real-Tme Capacty for load balanced networks nα' C RT = W 2mN Fully connected network C RT W = Θ N N s the number of nodes Real-Tme Capacty for networks wth K snk nodes C RT ' KNk = α 2 + ln N d α ' = α 1 N D mn D = mn( D ) mn k W Cellular structure C RT W = Θ nc n s the number of nodes per cell c number of channels

19 Lterature Lterature Dstrbuted Systems Paulo Verssmo, Luìs Rodrgues. Dstrbuted Systems for System Archtects. Kluwer Academc Publsher A general overvew on networkng Andrew Tanembaum. Computer Networks. Prentce Hall Wreless Sensor Networks (protocols and energy aware ssues) Holger Carl, Andreas Wllng. Protocols and Archtecture for Wreless Sensor Networks. Wley- Interscence Real-tme, QoS and resource management on Wreless Communcaton Mhaela Carde, Ionut Carde, Dng-Zhu Du. Resource Management n Wreless Networkng. Sprnger Bulent Tavl, Wend Henzelman. Moble Ad Hoc Networks, Energy-Effcent Real-Tme Data Communcatons. Sprnger CSMA mechansms L. Klenrock and F. Tobag. Packet swtchng n rado channels: Part I carrrer sense multple-access and modes and ther throughput delay characterstcs, IEEE Transacton on Communcaton, vol 23, no.12, Dec 1975 G.Banch. Performance analyss of the of the IEEE dstrbuted coordnaton functon CSMA traffc Dfferentaton Mechansms Imad Aad, Claude Castellucca. Dfferentaton Mechansms for IEEE , INFOCOM 2001 Yang Xao. Performance Analyss of Prorty Schemes for IEEE and IEEE e Wreless LANs, IEEE Transacton on wreless communcatons, vol. 4, no. 4, July 2005 Black Burst Joào L. Sobrnho, A. S. Krshnakumar. Qualty-of-Servce n Ad Hoc Carrer Sense Multple Access Wreless Networks. IEEE Journal on selected areas n communcaton, vol. 17, NO. 8, August 1999 Schedulng Protocols (I-EDF) M. Caccamo, L. Y. Zhang, L. Sha, and G. Buttazzo. An Implct Prortzed Access Protocol for Wreless Sensor Networks..Proceedngs of the IEEE Real-Tme Systems Symposum, Dec T. L. Crenshaw, A. Trumala, S. Hoke, and M. Caccamo, "A Robust Implct Access Protocol for Real-Tme Wreless Collaboraton", Proceedngs of the IEEE Euromcro Conference on Real-Tme Systems, Palma de Mallorca, Span, July 2005 IEEE analyss J. Mšc, S. Shaf, and V. B. Mšc, The Impact of MAC Parameters on the Performance of PAN, Elsever Ad hoc Networks Journal, 3(5): , Ans Koubaa, Máro Alves, Eduardo Tovar. A Comprehensve Smulaton Study of Slotted CSMA/CA for IEEE Wreless Sensor Networks Proceedngs of the 5th IEEE Internatonal Workshop on Factory Communcaton Systems (WFCS 06), Torno, Italy, JUN, 2006 Ans Koubaa, Máro Alves, Eduardo Tovar. An mplct GTS allocaton mechansm n IEEE for tme-senstve wreless sensor networks: theory and practce Real-Tme Systems Journal, Volume 39, Numbers 1-3, pp , Sprnger, August At the followng lnk you can fnd several papers on real-tme communcaton over IEEE , and other smlar topcs Lterature Real-Tme Capacty P. Gupta, P. R. Kumar, The Capacty of Wreless Networks, IEEE Trans. on Informaton Theory, Vol. 46, No. 2, March 2000 T. F. Abdelzaher, S. Prabh, R. Kran, On Real-Tme Capacty Lmts of Multhop Wreless Sensor Networks, Proc. Of the 25th IEEE Real-Tme Systems Symposum (RTSS2004), Lsbon portugal, Dec M. Caccamo and L. Y. Zhang, "The Capacty of Implct EDF n Wreless Sensor Networks", IEEE Proceedngs of the 15th Euromcro Conference on Real-Tme Systems, Porto, Portugal, July 2003

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