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Slides for Chapter 3: Networking and Internetworking From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 4, © Pearson.

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Presentation on theme: "Slides for Chapter 3: Networking and Internetworking From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 4, © Pearson."— Presentation transcript:

1 Slides for Chapter 3: Networking and Internetworking From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 4, © Pearson Education 2005

2 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Networking Issues (1) z Performance: yLatency (time between send and start to receive) yData transfer rate (bits per second) [max] yTransmission time = latency + length / transfer rate ySystem bandwidth, throughput [actual]: total volume of traffic in a given amount of time yUsing different channels concurrently can make bandwidth > data transfer rate ytraffic load can make bandwidth < data transfer rate ynetwork speed < memory speed (about 1000 times) yAccess to local disk is usually faster than remote disk yFast (expensive) remote disk + fast network xcan beat slow (cheap) local disks

3 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Networking Issues (2) z scalability z reliability y corruption is rare ymechanisms in higher-layers to recover errors yerrors are usually timing failures, the receiver doesn't have resources to handle the messages z security yfirewall on gateways (entry point to org's intranet) yencryption is usually in higher-layers zmobility--communication is more challenging: locating, routing,... zquality of service--real-time services zmulticasting--one-to-many communication

4 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (1) zLocal Area Networks (LAN) yfloor/building-wide ysingle communication medium yno routing, broadcast ysegments connected by switches or hubs yhigh bandwidth, low latency yEthernet - 10Mbps, 100Mbps, 1Gbps yno latency guarantees (what could be the consequences?) yPersonal area networks (PAN) [ad-hoc networks]: blue tooth, infra-red for PDAs, cell phones, …

5 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (2) zMetropolitan Area Networks (MAN) ycity-wide, up to 50 km yDigital Subscriber Line (DSL): Mbps, 5.5km from switch xBellSouth:.8 to 6 Mbps yCable modem: 1.5 Mbps, longer range than DSL xBright house w/ Road Runner:.5 to 10Mbps

6 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (3) zWide Area Networks (WAN) yworld-wide yDifferent organizations yLarge distances yrouted, latency seconds y1-10 Mbps (upto 600 Mbps)

7 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (4) zWireless local area networks (WLAN) yIEEE (WiFi) y Mbps, 1.5km x (1997): upto 2 Mbps, 2.4 GHz x a (1999): upto 54 Mbps, 5 GHz, ~75 feet outdoor x b (1999): upto 11 Mbps, 2.4 GHz, ~150 feet [most popular] x g (2003): upto 54 Mbps, 2.4 GHz, ~150 feet [backward compatible with b, becoming more popular] zWireless metropolitan area networks (WMAN) yIEEE (WiMax) y Mbps, 5-50km

8 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (5) zWireless wide area networks (WWAN) yworldwide yGSM (Global System for Mobile communications) y9.6 – 33 kbps y3G (“third generation”): kbps to 2Mbps

9 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Types of Networks (6) zInternetworks yconnecting different kinds of networks yrouters, gateways

10 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network performance ExampleRangeBandwidth (Mbps) Latency (ms) Wired: LANEthernet1-2 km MANATM250 km WANIP routingworldwide InternetworkInternetworldwide Wireless: WPANBluetooth ( ) m WLANWiFi (IEEE ) km WMANWiMAX (802.16)550 km WWAN GSM, 3G phone netsworldwide

11 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (1) zPacket transmission ymessage: logical unit of informatio ypacket: transmission unit yrestricted length: sufficient buffer storage, reduce hogging

12 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (2) zData Streaming yaudio/video yNeed 120 Mbps (1.5 Mbps compressed) yplay time: the time when a frame need to be displayed yfor example, 24 frames per second, frame 48 must be display after two seconds yIP protocol provides no guaranteesIPv6 (new) includes features for real-time streams, stream data are treated separately yResource Reservation Protocol (RSVP), Real-time Transport Protocol (RTP)

13 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (3) zSwitching schemes (transmission between aribitrary nodes) yBroadcast: ethernet, token ring, wireless yCircuit switching: wires are connected yPacket switching: xstore-and-forward xdifferent routes x“store-and-forward” needs to buffer the entire packet before forwarding yFrame relay xSmall packets xLooks only at the first few bits xDon’t buffer/store the entire frame

14 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (4) zProtocols yKey components xSequence of messages xFormat of messages

15 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (5) zProtocol layers, why? Layer n Layer 2 Layer 1 Message sent Message received Communication medium SenderRecipient

16 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (6) zEncapsulation in layered protocols

17 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (7) zISO Open Systems Interconnection (OSI) model

18 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (8) yInternet layers xApplication = application + presentation xTransport = transport + session

19 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (9) zPacket assembly yheader and data ymaximum transfer unit (MTU): 1500 for Ethernet y64K for IP (8K is common because of node storage) zports: destination abstraction (application/service protocol) zaddressing: transport address = network address + port yWell-known ports (below 1023) yRegistered ports ( ) yPrivate (up to 65535)

20 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (10) zPacket delivery (at the network layer) y Datagram packet xone-shot, no initial set up xdifferent routes, out of order xEthernet, IP y Virtual circuit packet xinitial set up for resources xvirtual circuit # for addressing xATM zSimilar but different pairs of protocols at the transport layer (connection-oriented and connectionless)

21 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (11) zRouting yLAN? yRouting Algorithm xdecide which out-going link to forward the packet for circuit switching, the route is determined during the circuit setup time for packet switching, each packet is routed independently xupdate state of the out-going links yRouting Table xa record for each destination xfields: outgoing link, cost (e.g. hop count)

22 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (12) zRouter example Hosts Links or local networks A DE B C Routers

23 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (13): Routing tables Routings from DRoutings from E ToLinkCostToLinkCost A B C D E local A B C D E Routings from ARoutings from BRoutings from C ToLinkCostToLinkCostToLinkCost A B C D E local A B C D E A B C D E

24 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (14) zRouter information protocol (RIP) y"Bellman-Ford distance vector" algorithm ySender: send table summary periodically (30s) or changes to neighbors yReceiver: Consider A receives a table from B, A updates 1.A -> B -> … -> X: A updates--B has more up-to-date (authoritative) info 2.A -> not B -> … -> X: Does routing via B have a lower cost? 3.B -> … -> X: A does not know X 4.[B -> A -> … -> X]: A doesn’t update--A has more up-to-date info 5.Faulty link, cost is infinity yRIP-1 (RFC 1058) yMore recent algorithms xmore information, not just neighbors xlink-state algorithms, each node responsible for finding the optimum routes

25 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (15): Pseudocode for RIP routing algorithm zTl is the table local table; Tr is the received remote table Send: Each t seconds or when Tl changes, send Tl on each non-faulty outgoing link. Receive: Whenever a routing table Tr is received on link n: for all rows Rr in Tr { if (Rr.link != n) { // destination not routed via the receiver Rr.cost = Rr.cost + 1; Rr.link = n; if (Rr.destination is not in Tl) add Rr to Tl; // add new destination to Tl else for all rows Rl in Tl { if (Rr.destination = Rl.destination and (Rr.cost < Rl.cost or Rl.link = n)) Rl = Rr; // Rr.cost < Rl.cost : remote node has better route // Rl.link = n : remote node is more authoritative }

26 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network principles (16) zCongestion control yhigh traffic load, packets dropped due to limited resources yreducing transmission rate: "choke packets" from sender to receiver

27 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Networking principles (17) zNetwork connecting devices yHubs: extending a segment of LAN (broadcast) ySwitches: switching traffic at data-link level (different segments of a LAN), making temporary hardware connections between two ports (or store and forward) [switches do not exchange info with each other] yRouters: routing traffic at IP level yBridges: linking networks of different types, could be routers as well

28 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Networking principles (18) zTunneling ycommunicate through an "alien" protocol y“Hide” in the payload yIPv6 traffic using IPv4 protocols AB IPv6 IPv6 encapsulated in IPv4 packets Encapsulators IPv4 network

29 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (1) zIP (Internet Protocol) y"network" layer protocol yIP addresses z TCP (Transmission Control Protocol) ytransport layer yconnection-oriented z UDP (User Datagram Protocol) ytransport layer y connection-less

30 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (2): TCP/IP layers Messages (UDP) or Streams (TCP) Application Transport Internet UDP or TCP packets IP datagrams Network-specific frames Message Layers Underlying network Network interface

31 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (3): layer encapsulation Application message TCP header IP header Ethernet header Ethernet frame port TCP IP

32 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (4): Programmer’s view

33 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (5): Internet address structure z32-bit

34 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (6): Decimal representation z (www.fit.edu) octet 1octet 2octet 3 Class A: 1 to to to 254 Class B: 128 to 191 Class C: 192 to to 239 Class D (multicast): Network ID Host ID Multicast address 0 to to to 255 Multicast address 0 to to to 255 Class E (reserved): to to to to to Range of addresses

35 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (7) zClassless interdomain routing (CIDR) yshortage of Class B networks yadd a mask field to indicate bits for network portion y /22 [subnet: first 22 bits; host: 10 bits]

36 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (8)

37 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (9): Network Address Translation zSharing one “global” IP address at home zRouters with NAT yRouter has a “global” IP address from ISP yEach machine has a “local” IP address via DHCP yMachine -> router xRouter stores the local IP addr and source port # xTable entry indexed by a virtual port # yRouter -> outside xput the router IP addr and virtual port # in the packet yOutside -> router xReply to the router IP addr and virtual port # yRouter -> machine xUse the virtual port # to find table entry xForward to the local IP address and port # zWhat happens if we want the device to be a server, not a client?

38 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (10)

39 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (11) zServer with NAT yFixed internal addr and port # yFixed entry in the table yAll packets to the port on the router are forwarded to the internal addr and port # in the entry zWhat if more than one internal machines want to offer the same service (port)?

40 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (12) zIP Protocol yunreliable or best-effort y lost, duplicated, delayed, out of order y header checksum, no data checksum y IP packet longer than MTU of the underlying network, break into fragments y before sending and reassemble after receiving y Address resolution (on LANs) xmapping IP address to lower level address xARP: address resolution protocol xethernet: cache; not in cache, broadcast IP addr, receive Ethernet addr y IP spoofing: address can be stolen (not authenticated)

41 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (13) zRIP-1: discussed previously zRIP-2: CIDR, better multicast routing, authentication of RIP packets zlink-state algorithms: e.g., open shortest path first (OSPF) zObserved: average latency of IP packets peaks at 30- seconds intervals [RIP updates are processed before IP] y because 30-second RIP update intervals, locked steps y random interval between seconds for RIP update z large table size y all destinations!! y map ip to geographical location y default route: store a subset, default to a single link for unlisted destinations

42 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet Protocols (14): IPv6 zIP addresses:128 bits (16 bytes) y 3 x addresses (7 x addresses per square meter!) zrouting speed y no data checksum as before y no fragmentation – need to know the smallest MTU in data-link layer zreal-time and special services y traffic class: priority, time-dependent (expired data are useless) y flow label: timing requirements for streams (reserving resources in advance) z“next” header field y extension header types for IPv6 y routing information, authentication, encryption... zAnycast: at least one nodes gets it zsecurity ycurrently handled above the IP layer yextension header types zMigration from IPv4 ybackward compatibility: IPv6 addresses include IPv4 addresses yIslands of IPv6 networks, traffic tunnels though other IPv4 networks

43 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (15): Source address (128 bits) Destination address (128 bits) Version (4 bits)Traffic class (8 bits)Flow label (20 bits) Payload length (16 bits)Hop limit (8 bits)Next header (8 bits)

44 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet Protocols (10): Mobile IP zDynamic Host Configuration Protocol (DHCP) y assign temporary IP address y provide addresses of local resources like DNS zRouting to maintain continuous access y IP routing is subnet-based, fixed relative locations y Home agent (HA) and Foreign agent (FA) y HA - current location (IP addr) of the mobile host x is informed by the mobile host when it moves x proxy for the host after it moves x inform local routers to remove cached records of the host xresponds to ARP requests yFA - informed by the host when it arrives x new temp IP addr x contacts HA what the new IP address is y HA - receives the new IP address and may tell the sender the new IP addr

45 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (11): MobileIP routing mechanism Sender Home Mobile host MH Foreign agent FA Internet agent First IP packet addressed to MH Address of FA returned to sender First IP packet tunnelled to FA Subsequent IP packets tunnelled to FA

46 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (12) zTransport protocols: TCP and UDP ynetwork protocol: host to host ytransport protocol: process to process yPort #’s to indicate processes zUDP yno guarantee of delivery ychecksum is optional ymax of 64 bytes, same as IP yno setup costs, no segments

47 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (13) zTCP yarbitrarily long sequence yconnection-oriented ysequencing of segments yflow control: acknowledgement includes "window size" (amount of data) for sender to send before next ack yinteractive service: higher frequency of buffer flush, send when deadline reached or buffer reaches MTU yretransmission of lost packets ybuffering of incoming packets to preserve order and flow ychecksum on header and data

48 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (14) zDomain names zDNS y distributed data y each DNS server keeps track of part of the hierarchy y unresolved requests are sent to servers higher in the hierarchy

49 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (15) zFirewalls y monitor and filter communication y controlling what services are available to the outside y controlling the use of services y controlling internal users access to the outside z Filtering at different protocol levels y IP packet filtering: addresses, ports.. y TCP gateway: check for correctness in TCP connections x e.g., are they partially opened and never used (why?) y Application-level gateway: proxy for applications x no direct communication between the inside and outside x e.g., smtp proxy can check addresses, content...

50 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (16) zBastion (tcp/ application filter) zC): two router filters yAccess to web/ftp server, but not LAN yHide internal IP addresses xBastion has the mapping xSecond router is the second IP filter (invisible to the outside)

51 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Internet protocols (17) zVirtual Private Network (VPN) y extending a secured internal network to an external unsecured host y e.g. IPSec tunneling through IP

52 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (1): Ethernet and WiFi IEEE No.NameTitleReference 802.3EthernetCSMA/CD Networks (Ethernet)[IEEE 1985a] 802.4Token Bus Networks[IEEE 1985b] 802.5Token Ring Networks[IEEE 1985c] 802.6Metropolitan Area Networks[IEEE 1994] WiFiWireless Local Area Networks[IEEE 1999] BluetoothWireless Personal Area Networks[IEEE 2002] ZigBeeWireless Sensor Networks[IEEE 2003] WiMAXWireless Metropolitan Area Networks[IEEE 2004a]

53 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (2): Ethernet zEthernet, CSMA/CD, IEEE y Xerox Palo Alto Research Center (PARC), 1973, 3Mbps y 10,100,1000 Mbps y extending a segment: hubs and repeaters y connecting segments: switches and bridges y Contention bus y Packet/frame format x preamble (7 bytes): hardware timing x start frame delimiter (1) x dest addr (6) x src addr (6) x length (2) x data ( ): min total becomes 64 bytes, max total is 1518 x checksum (4): dropped if incorrect

54 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (3) zCarrier Sensing Multiple Access / Collision Detection (CSMA/CD) y CS: listen before transmitting, transmit only when no traffic y MA: more than one can transmit y CD: collision detected when signals transmitted are not the same as those received (listen to its own transmission) x After detection of a collision send jamming signal wait for a random period before retransmitting z T (Tau): time to reach the farthest station z When is the collision detected? yA and B send at the same time yA sends, B sends within T seconds yA sends, B sends between T and 2T seconds yA sends, B sends after 2T seconds zMinimum length of packet for collision detection: ypacket length > 2T, between T and 2T, and < T ?

55 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (4) zPhysical implementation: y y R: data rate in Mbps y B: medium signaling type: baseband [one channel] or broadband [multiple channels] y L: max segment length in 100meters or T (twisted pair cable, hierarchy of hubs)

56 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (5): Ranges and speeds 10Base510BaseT100BaseT1000BaseT Data rate10 Mbps 100 Mbps1000 Mbps Max. segment lengths: Twisted wire (UTP)100 m 25 m Coaxial cable (STP)500 m 25 m Multi-mode fibre2000 m 500 m Mono-mode fibre25000 m m2000 m

57 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (6): WiFi zIEEE wireless LAN y up to 150m and 54Mbps y access point (base station) to land wires y Ad hoc network--no specific access points, "on the fly" network among machines in the neighborhood y Radio Frequency (2.4, 5GHz band) or infra-red

58 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (7): Problems with wireless CSMA/CD yHidden station: not able to detect another station is transmitting xA can’t see D, or vice versa y Fading: signals weaken, out of range xA and C are out of range from each other y Collision masking: stronger signals could hide others x A and C are out of range from each other, both transmits, collide, can't detect collision, Access point gets garbage

59 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (8) z Carrier sensing multiple access with collision avoidance (CSMA/CA) y reserving slots to transmit y if no carrier signal x medium is available, x out-of-range station requesting a slot, or x out-of-range station using a slot

60 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Network Case Studies (9) zSteps 1.Request to send (RTS) from sender to receiver, specify duration 2.Clear to send (CTS) in reply 3.in-range stations see the RTS and/or CTS and its duration 4.in-range stations stop transmitting 5.acknowledgement from the receiver zHidden station & Fading: CTS, need permission to transmit zRTS and CTS are short, don't usually collide; random back off if collision detected zShould have no collisions, send only when a slot is reserved


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