Presentation on theme: "Chapter 10 – Wireless LANs"— Presentation transcript:
1 Chapter 10 – Wireless LANs Local Area NetworksChapter 10 – Wireless LANs
2 Wireless Communication The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.Companies jumped in, quickly developing incompatible wireless products in the 1990’s.Industry decided to entrust standardization to the IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!!Wireless communications compellingEasy, low-cost deploymentMobility & roaming: Access information anywhereSupports personal devicesPDAs, laptops, data-cell-phonesSupports communicating devicesCameras, location devices, wireless identificationSignal strength varies in space & timeSignal can be captured by snoopersSpectrum is limited & usually regulated
3 Wireless Links Many end systems use wireless links: Portable PCs within a wireless LANPDAs that connect to the Internet through wireless telephony infrastructureCameras, automobiles, etc.Two standards for wireless networking:IEEE b standard for wireless LANs (aka Wi-Fi)Bluetooth standard that allows devices to communicate without being in line of sightWireless devices classified wrt power, range, and data rateIEEE high power, medium range, and high rate “access” technologyBluetooth low power, short range, low rate, “cable replacement” technology
4 IEEE 802.11 Wireless LAN Wireless LANs: mobile networking IEEE standard:MAC protocolUnlicensed frequency spectrum: 2.4Ghz (802.11b) or 5-6 Ghz (802.11a)Provides wireless Ethernet access at 11 Mbps or 54 Mbps (802.11a)Basic Service Set (BSS) (a.k.a. “cell”) contains:wireless hostsaccess point (AP): base stationBSS’s combined to form distribution system (DS)
9 Wireless Standards Frequency, Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (FHSS) HR: High RateOrthogonal Frequency Division Multiplexing (OFDM, VOFDM, COFDM)
10 Wireless Physical Layer Physical layer conforms to OSI (five options)1997: infrared, FHSS, DHSS1999: a OFDM and b HR-DSSS2001: g OFDMInfraredTwo capacities 1 Mbps or 2 Mbps.Range is 10 to 20 meters and cannot penetrate walls.Does not work outdoors.FHSS (Frequency Hopping Spread Spectrum)The main issue is multipath fading.79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.Same pseudo-random number generator used by all stations.Dwell time: min. time on channel before hopping (400msec).
11 Wireless Physical Layer Frequency Hopping Spread Spectrum
12 Wireless Physical Layer DSSS (Direct Sequence Spread Spectrum)Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA see Tanenbaum sec ).Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.1 or 2 Mbps.802.11a OFDM (Orthogonal Frequency Divisional Multiplexing)Compatible with European HiperLan2.54Mbps in wider 5.5 GHz band transmission range is limited.Uses 52 FDM channels (48 for data; 4 for synchronization).Encoding is complex ( PSM (Power saving mode) up to 18 Mbps and QAM above this capacity).E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.More difficulty penetrating walls.
13 Wireless Physical Layer Direct Sequence Spread Spectrum
14 Wireless Physical Layer 802.11b HR-DSSS (High Rate Direct Sequence Spread Spectrum)11a and 11b shows a split in the standards committee.11b approved and hit the market before 11a.Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec.Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers.Range is 7 times greater than 11a.11b and 11a are incompatible!!802.11g OFDM(Orthogonal Frequency Division Multiplexing)An attempt to combine the best of both a and b.Supports bandwidths up to 54 Mbps.Uses 2.4 GHz frequency for greater range.Is backward compatible with b.
15 Infrastructure Network B2B1A1AP1AP2Distribution SystemServerGateway tothe InternetPortalBSS ABSS BPermanent Access Points provide access to Internet
17 802.11 Definitions Basic Service Set (BSS) Extended Service Set (ESS) Group of stations that coordinate their access using a given instance of MACLocated in a Basic Service Area (BSA)Stations in BSS can communicate with each otherDistinct collocated BSS’s can coexistExtended Service Set (ESS)Multiple BSSs interconnected by Distribution System (DS)Each BSS is like a cell and stations in BSS communicate with an Access Point (AP)Portals attached to DS provide access to Internet
18 Ad Hoc NetworksAd hoc network: IEEE stations can dynamically form network without APFormed “on the fly” when mobile devices are in proximityApplications:“Laptop” meeting in conference room, carInterconnection of “personal” devicesBattlefieldIETF MANET (Mobile Ad hoc Networks) working group
20 Hidden Terminal Problem CAData FrameA transmits data frameC senses medium,station A is hidden from CData FrameABCC transmits data frame & collides with A at B(b)New MAC: CSMA with Collision Avoidance
21 IEEE 802.11 MAC Protocol: CSMA/CA (collision avoidance) CSMA: senderif sense channel idle for Distributed Inter Frame Space (DIFS) sec.then transmit entire frame (no collision detection)if sense channel busy then binary backoffCSMA receiver:if received OKreturn ACK after Short Inter Frame Spacing (SIFS)(DIFS = SIFS + 2 × slot time)Time slot= 20 micro s, SIFS=10 micro s, DIFS=50 micro s.
22 IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others Other stations wait for a random backoff period after DIFS after current transmissionAvoids collisionsCollisions uses exponentially increasing backoff periodCollisions detection is difficult:Hidden terminal problemFadingNAV: Network Allocation Vector:frame has transmission duration fieldOthers (hearing stations) defer access (to save power) for NAV time units
24 Hidden Terminal effect Hidden terminals: A, C cannot hear each otherObstacles, signal attenuationCollisions at BGoal: avoid collisions at BCSMA/CA: CSMA with Collision AvoidanceFading can also result in collisions
25 Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit channel reservationsender: send short RTS: request to sendreceiver: reply with short CTS: clear to sendCTS reserves channel for sender, notifying (possibly hidden) stationsBenefit: RTC-CTS avoids hidden station collisions
26 Collision Avoidance: RTS-CTS exchange CA with RTS-CTS:Collisions less likely, of shorter durationEnd result similar to collision detectionIEEE allows:CSMACSMA/CA: reservationspolling from AP
27 CSMA with Collision Avoidance RTSA requests to sendBC(a)CTSABCB announces A ok to send(b)Data FrameA sendsBC remains quiet(c)
28 IEEE Wireless LANStimulated by availability of unlicensed spectrumU.S. Industrial, Scientific, Medical (ISM) bandsMHz, GHz, GHzTargeted wireless 20 MbpsMAC for high speed wireless LANAd Hoc & Infrastructure networksVariety of physical layers
29 Infrastructure Network B2B1A1AP1AP2Distribution SystemServerGateway tothe InternetPortalBSS ABSS B
30 Distribution Services Stations within BSS can communicate directly with each otherDS provides distribution services:Transfer MAC SDUs between APs in ESSTransfer MSDUs between portals & BSSs in ESSTransfer MSDUs between stations in same BSSMulticast, broadcast, or stations’s preferenceESS looks like single BSS to LLC layer
31 Infrastructure Services Select AP and establish association with APThen can send/receive frames via AP & DSReassociation service to move from one AP to another APDissociation service to terminate associationAuthentication service to establish identity of other stationsPrivacy service to keep contents secret
32 IEEE 802.11 MAC MAC sublayer responsibilities Channel accessPDU addressing, formatting, error checkingFragmentation & reassembly of MAC SDUsMAC security service optionsAuthentication & privacyMAC management servicesRoaming within ESSPower management
33 MAC Services Physical Distribution coordination function (DCF) Contention Service: Best effortContention-Free Service: time-bounded transferMAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs). MAC Service Data Unit (MSDU)PhysicalDistribution coordination function (DCF)(CSMA-CA)Point coordinationfunctionContention-free serviceContention serviceMACMSDUs
34 Distributed Coordination Function (DCF) DIFSPIFSSIFSContentionwindowNext frameDefer accessWait forreattempt timeTimeBusy mediumDCF provides basic access serviceAsynchronous best-effort data transferAll stations contend for access to mediumCSMA-CAReady stations wait for completion of transmissionAll stations must wait Interframe Space (IFS)
35 Priorities through Interframe Spacing DIFSPIFSSIFSContentionwindowNext frameDefer accessWait forreattempt timeTimeBusy mediumHigh-Priority frames wait Short IFS (SIFS)Typically to complete exchange in progressACKs, CTS, data frames of segmented MSDU, etc.PCF IFS (PIFS) to initiate Contention-Free PeriodsDCF IFS (DIFS) to transmit data & MPDUs
36 Contention & Backoff Behavior If channel is still idle after DIFS period, ready station can transmit an initial MPDUIf channel becomes busy before DIFS, then station must schedule backoff time for reattemptBackoff period is integer # of idle contention time slotsWaiting station monitors medium & decrements backoff timer each time an idle contention slot transpiresStation can contend when backoff timer expiresA station that completes a frame transmission is not allowed to transmit immediatelyMust first perform a backoff procedure
37 RTSCTSData FrameA requests to sendBCAA sendsC remains quietB announces A ok to send(a)(b)(c)ACK(d)B sends ACK
38 Carrier Sensing in 802.11 Physical Carrier Sensing Analyze all detected framesMonitor relative signal strength from other sourcesVirtual Carrier Sensing at MAC sublayerSource stations informs other stations of transmission time (in msec) for an MPDUCarried in Duration field of RTS & CTSStations adjust Network Allocation Vector to indicate when channel will become idleChannel busy if either sensing is busy
39 Transmission of MPDU without RTS/CTS DataDIFSSIFSDefer AccessWait for Reattempt TimeACKNAVSourceDestinationOther
40 Transmission of MPDU with RTS/CTS DataSIFSDefer accessAckDIFSNAV (RTS)SourceDestinationOtherRTSCTSNAV (CTS)NAV (Data)
41 Collisions, Losses & Errors Collision AvoidanceWhen station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots)Station transmits frame when backoff timer expiresIf collision occurs, recompute backoff over interval that is twice as longReceiving stations of error-free frames send ACKSending station interprets non-arrival of ACK as lossExecutes backoff and then retransmitsReceiving stations use sequence numbers to identify duplicate frames
42 Point Coordination Function PCF provides connection-oriented, contention-free service through pollingPoint coordinator (PC) in AP performs PCFPolling table up to implementorCFP repetition intervalDetermines frequency with which CFP occursInitiated by beacon frame transmitted by PC in APContains CFP and CPDuring CFP stations may only transmit to respond to a poll from PC or to send ACK
43 Contention-free repetition interval PCF Frame TransferCF EndNAVPIFSBD1 + PollSIFSU 1 + ACKD2+Ack+PollU 2 + ACKContention-free repetition intervalContention periodCF_Max_durationReset NAVD1, D2 = frame sent by point coordinatorU1, U2 = frame sent by polled stationTBTT = target beacon transmission timeB = beacon frameTBTT
45 Distributed Coordination Function (DCF) DCF is the access method used to support asynchronous data transfer on a best effort basisAll stations must support the DCF (DCF operates solely in the ad hoc network)Operates solely or coexists with the PCF in an infrastructure networkDCF sits directly on top of the physical layer and supports contention services:Each station with an MSDU queued for transmission must contend for access to the channelOnce the MSDU is transmitted, must recontend for access to the channel for all subsequent framesContention services promote fair access to the channel for all stations.The DCF is carrier sense multiple access with collision avoidance (CSMA/CA).CSMA/CD is not used because a station is unable to listen to the channel for collisions while transmittingIn IEEE , carrier sensing is performed at both the air interface, referred to as physical carrier sensing, and at the MAC sublayer, referred to as virtual carrier sensingPhysical carrier sensing detects the presence of other IEEE WLAN users by analyzing all detected packets, and also detects activity in the channel via relative signal strength from other sourcesVirtual carrier sensingStations include MPDU duration in the header of request to send (RTS), clear to send (CTS), and data framesAn MPDU is a complete data unit that is passed from the MAC sublayerto the physical layerThe MPDU contains header information, information, payload, and a 32-bit CRCThe duration field indicates the time (in microseconds) after the end of the present frame the channel will be utilized tocomplete the successful transmission of the data or management frame.Stations in the BSS use the duration field to adjust their network allocation vector (NAV)NAV indicates the amount of time that must elapse until the current transmission session is complete
46 Distributed Coordination Function (DCF) DCF operates under the Contention Period (CP)Three types of frames: management, control, and dataManagement F: station association dis-association with APControl F: handshaking in CP, ACK data in CP, and end CFPBasic DCF Access Method (no RTS-CTS):When ST finds chaneel idle, it waits for DIFS and checks it againIf it is still idle, it transmits MPDU with medium busy time (including SIFS and ACK times)Receiving st computes Checksum, if correct sends an ACK to sourceAll other STs in BSS hearing above messages adjust their NAV timers
47 Distributed Coordination Function (DCF) RTS-CTS Data ModePriority Accsess: SIFS, PIFS (SIFS+1), and DIFS (SIFS+2)In BSS, STs hearing RTS, CTS, F0, and ACK adjust their NAVSts: Basic mode, RTS/CTS mode if MPDU exceeds L, or always use RTS/CTS modeFairness: BEB starts with (1,8) and end at some maximum
48 Distributed Coordination Function (DCF) MPDU (2300 bytes): collision lead to bandwidth lossRTS is 20 bytes and CTS is 14 bytesFragmentation increases transmission reliabilityFragment MPDU, transmit Frag, receive ACK to completionIf no ACK, re-contend for medium and stat al last Frag.In RTS-CTS mode, RTS-CTS used only in first frag.
49 Point Coordination Function (PCF on top of DCF) PCF (optional) operates under the Contention-Free Period (CFP)Medium access contr. by Point Coordinator PC (AP/BSS, polling)Polled Sts can transmit (No CSMA)CFP Repetition Interval (Manag duration): (1) PCF, and (2) DCF
50 Point Coordination Function (PCF on top of DCF) Light traffic: shorter CFP if previous DCF traffic is not completePC: PIFS, Beacon, (CF-poll/data/Data+CF-poll), CF-end.CF-aware st:Gets CF-poll,Responds: CF-ACK, Data+CF-ACK,Then PC responds by Data+CF-ACK+CF-poll
51 Point Coordination Function (PCF on top of DCF) When ST receives a poll from IP:Transmit a F to another ST in the BSSWhen Dest receives F, a DCF-ACK is returned to sourcePC waits for PIFS after ACK before continuation
52 Frame Types Management frames Control frames Data frames Station association & disassociation with APTiming & synchronizationAuthentication & deauthenticationControl framesHandshakingACKs during data transferData framesData transfer
53 Frame Structure MAC Header: 30 bytes Frame Body: 0-2312 bytes 666260-23124FrameControlDuration/IDAddress1Address2Address3SequencecontrolAddress4FramebodyCRCMAC Header: 30 bytesFrame Body: bytesCRC: CCITT-32 4 bytes CRC over MAC header & frame body
54 Frame Control (1) Protocol version = 0 Address2FrameControlDuration/ID13Sequencecontrol4bodyCRCProtocolversionTypeSubtypeToDSFromMorefragRetryPwrmgtdataWEPRsvd60-2312MAC header (bytes)Protocol version = 0Type: Management (00), Control (01), Data (10)Subtype within frame typeType=00, subtype=association; Type=01, subtype=ACKMoreFrag=1 if another fragment of MSDU to follow
55 Frame Control (2)Address2FrameControlDuration/ID13Sequencecontrol4bodyCRCProtocolversionTypeSubtypeToDSFromMorefragRetryPwrmgtdataWEPRsvd60-2312ToDSFromAddress1234DestinationaddressSourceBSSIDN/AReceiverTransmitterMeaningData frame from station to station within a BSSData frame exiting the DSData frame destined for the DSWDS frame being distributed from AP to APTo DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS
56 Frame Control (3) Retry=1 if mgmt/control frame is a retransmission Address2FrameControlDuration/ID13Sequencecontrol4bodyCRCProtocolversionTypeSubtypeToDSFromMorefragRetryPwrmgtdataWEPRsvd60-2312MAC header (bytes)Retry=1 if mgmt/control frame is a retransmissionPower Management used to put station in/out of sleep modeMore Data =1 to tell station in power-save mode more data buffered for it at APWEP=1 if frame body encrypted
57 Physical Layers 802.11 designed to Physical layer LLC Physical layer convergenceprocedurePhysical mediumdependentMACPLCPpreambleLLC PDUMAC SDUheaderCRCPLCP PDUdesigned toSupport LLCOperate over many physical layers
58 IEEE 802.11 Physical Layer Options Frequency BandBit RateModulation Scheme802.112.4 GHz1-2 MbpsFrequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum802.11b11 MbpsComplementary Code Keying & QPSK802.11g54 MbpsOrthogonal Frequency Division Multiplexing& CCK for backward compatibility with b802.11a5-6 GHz
59 802.11 - MAC management Synchronization Power management try to find a LAN, try to stay within a LANtimer etc.Power managementsleep-mode without missing a messageperiodic sleep, frame buffering, traffic measurementsAssociation/Reassociationintegration into a LANroaming, i.e. change networks by changing access pointsscanning, i.e. active search for a networkMIB - Management Information Basemanaging, read, write
60 Synchronization using a Beacon (infrastructure) beacon intervalBBBBaccesspointbusybusybusybusymediumtBvalue of the timestampbeacon frame
61 Synchronization using a Beacon (ad-hoc) beacon intervalB1B1station1B2B2station2busybusybusybusymediumtBvalue of the timestampbeacon framerandom delay
62 Power management Idea: switch the transceiver off if not needed States of a station: sleep and awakeTiming Synchronization Function (TSF)stations wake up at the same timeInfrastructureTraffic Indication Map (TIM)list of unicast receivers transmitted by APDelivery Traffic Indication Map (DTIM)list of broadcast/multicast receivers transmitted by APAd-hocAd-hoc Traffic Indication Map (ATIM)announcement of receivers by stations buffering framesmore complicated - no central APcollision of ATIMs possible (scalability?)
63 Power saving with wake-up patterns (infrastructure) TIM intervalDTIM intervalDBTTdDBaccesspointbusybusybusybusymediumpdstationtTTIMDDTIMawakeddata transmissionto/from the stationBbroadcast/multicastpPS poll
64 Power saving with wake-up patterns (ad-hoc) ATIMwindowbeacon intervalB1ADB1station1B2B2adstation2tBAtransmit ATIMDbeacon framerandom delaytransmit dataadawakeacknowledge ATIMacknowledge data
65 802.11 - Roaming No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answerReassociation Requeststation sends a request to one or several AP(s)Reassociation Responsesuccess: AP has answered, station can now participatefailure: continue scanningAP accepts Reassociation Requestsignal the new station to the distribution systemthe distribution system updates its data base (i.e., location information)typically, the distribution system now informs the old AP so it can release resources
66 WLAN: IEEE 802.11b What’s new? Frequency Security Cost Availability Define a new PHY layer. All the MAC schemes, management procedures are the sameUser data rate max. approx. 6 Mbit/sFrequencyOn certain frequencies in the free 2.4 GHz ISM-bandSecurityLimited, WEP insecure, SSIDCost100€ adapter, 250€ base station, droppingAvailabilityMany products, many vendorsSpecial Advantages/DisadvantagesAdvantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple systemDisadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only
67 Bluetooth Most compelling application addressed by Bluetooth: A convenient, untethered means to interconnect electronic devicesExamples: portable phones, PDAs, laptops, desktops, digital cameras, fax machines, printers, keyboard, mouse, etc.Line-of-sight infrared technology has been used for such communicationsUsing RF wireless communication, Bluetooth does not require LoSIt can support multipoint as well as point-to-point communicationBluetooth architecture:Mobile devices need short-range transceiversTransceivers operate in 2.5 Ghz unlicensed frequency bandProvide data rates of up to 721 kbps + 3 voice channels (64 kbps)Operating range is 10 to 100 metersEach device is identified by a 12-bit address
68 Bluetooth (Cont’d) Frequency hopping Error recovery: Transceiver minimizes the effect of interference from other signalsHops to a new frequency after transmitting or receiving a packetError recovery:Transceiver forward error correction (FEC)Automatic Repeat reQuest (ARQ) for retransmissionBluetooth protocol suite includes:Baseband protocolEnables physical RF wireless connection between devicesA connection of 2-7 Bluetooth devices forms a small networkpiconetLink manager protocolHandshaking between two devices to establish connectionL2CAP protocolDuring a connection, adapts upper layer protocols for transmission over the baseband
69 Bluetooth - Physical Upwards! 79 channels, each 1MHz, using FSK, with 1 bit persymbol = 1MbpsMuch of the 1Mbps is taken up with protocol overheads – causedby frequency hopping ( ms needed to stabilise radio afterthe hop!)Leaves about 366 bits for actual data – of which 126 bits are headers– leaving 240 bits for data per slot!
71 Point to Point Data Link Control One sender, one receiver, one link: easier than broadcast link:No Media Access ControlNo need for explicit MAC addressingExamples:Dialup link phone line 56 Kbps modem connectionsSONET/SDH linkX.25 connectionISDN linePopular point-to-point DLC protocols:PPP (point-to-point protocol)HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)
72 PPP Design Requirements [RFC 1547] Packet framing:Encapsulation of network-layer datagram in data link frameCarry network layer data of any network layer protocol (not just IP)Ability to demultiplex upwardsBit transparency:Must carry any bit pattern in the data field with no constraintsError detection (no correction)PPP receiver must be able to detect bit errorsConnection liveness:Detect, signal link failure to network layerNetwork layer address negotiation:Endpoint can learn/configure each other’s network address
73 Error recovery, flow control, data re-ordering PPP Non-RequirementsNo error correction/recoveryNo flow controlPPP receiver is expected to receive frames at full physical layer speed higher layer could drop packets or throttle senderOut of order delivery OKNo need to support multipoint links (e.g., polling)Other link layer protocols can support multipoint linksE.g., HDLCError recovery, flow control, data re-orderingall relegated to higher layers!|
74 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option)Control: does nothing; in the future possible multiple control fieldsPPP sender can allow sender to skip address and control bytesProtocol: upper layer protocol to which frame deliveredExamples: PPP-LCP, IP, IPCP, etcRFC 1700 and RFC 3232 define 16-bit protocol codes for PPP
75 PPP Data Frame (Cont’d) Info:Variable length upper layer data being carriedDefault maximum is 1500 bytesCan be changed when the link is initially configuredCheck:Uses cyclic redundancy check (CRC) for error detectionTwo or 4 bytes CRC
76 Byte Stuffing“Data transparency” requirement: data field must be allowed to include flag pattern < >Q: is received < > data or flag?Sender:Adds (“stuffs”) an escape byte < > before each < > data byteReceiver:Discards the escape byte and continues data receptionSingle flag byteIf two < > bytes in a row discard the first escape byte and continue data reception
77 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data
78 PPP Link and Network Control Protocols Before exchanging network-layer data, data link peers must:Configure PPP link (max. frame length, authentication)Learn/configure networklayer informationFor IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP addressPPP link always begins and ends in the dead state
79 PPP Link Control Protocol (LCP) Link establishment state:Entered on an event that indicates presence of a physical layer, which is ready to be used: carrier detection, user interventionOne end of the link uses configure-request frame to indicate its configuration optionsPPP frame with protocol filed set equal to LCPInformation field contains the specific configuration requestOptions:Maximum frame size for the linkSpecification of authentication protocol to be used (if any)Option to skip the address and control fields in PPP framesThe other side responds with configure-ack, configure-nak, or configure-reject frameNetwork layer configuration begins after link is established:Options negotiation done and authentication performed (if any)Network layer specific control packets are exchanged with each other
80 PPP Network Control Protocol (IPCP) If IP is running over PPP, IP control protocol (IPCP) is usedIPCP is carried within a PPP frameProtocol field will have IPCP indicated by 0x8021IPCP allows two IP modules to exchange or configure IP addressesIPCP also allows two IP modules to negotiate whether or not IP datagrams will be sent in compressed formSimilar network control protocols for other network protocols:Examples: DECnet, AppleTalk, etc.Link goes in open state after network configurationPPP can start exchanging network layer datagramsTo check the link status, use echo-request and echo-reply LCP framesTerminating stateOne side sends LCP terminate-request and other responds with LCP terminate-ack frameLink goes to the dead state again
81 Asynchronous Transfer Mode (ATM) Two types of networks have existed side by side:Telephone networks carry real-time voiceData networks carry non real-time datagramsATM standards were developed in mid-1980’sGoal: design a network technology that will be appropriate for both types of trafficStandard developed by ATM Forum and ITU for broadband digital services networksATM technology:A full suite of communication protocols form application to physical layerCalls for packet switching within virtual circuits virtual channelsDeployed in both telephone networks and Internet backbonesHigh performance ATM switches can deliver terabits per second!Still could not replace TCP/IP based networks at desktop level
82 Characteristics of ATM ATM service models:Constant bit rate (CBR)Variable bit rate (VBR)Available bit rate (ABR)Unspecified bit rate (UBR)ATM uses fixed-length packets cellsHeader: 5 bytes and payload: 48 bytesFixed length cell and simple header facilitate high speed switchingATM VCs virtual channelsHeader includes virtual channel identifier (VCI) fieldVCI is used by switches to forward the cellsConnection-oriented serviceCells always arrive in-orderATM does not provide acks as other connection-oriented protocols doEffectively, a VC is full duplexChannel capacity and other properties may be different in two directionsDate rates:155 Mbps, 622 Mbps, and higher
83 Characteristics of ATM (Cont’d) No link-by-link retransmissionsIf an ATM switch detects error in a header, it tries to correct itSimply drops the cell if error cannot be correctedno retransmission requestCongestion controlOnly for ABR service classNetwork provides feedback to sender to regulate its rateATM protocol stack consists of three layers:ATM physical layerATM layerATM adaptation layer (AAL)Analogous to transport layer in TCP/IP stackMultiple types of AALs
84 Cell Header Formats In both cases, cells consist of: Header fields 5 byte header and 48 byte payloadsHeaders are slightly different for two interfaces (GFC field is unused any way)Header fieldsVPI is a small integer that selects a particular virtual pathVCI selects a particular VC from within the chosen virtual path
85 Cell Header Formats (Cont’d) VPI and VCIAt UNI, 8 bit VPI means that host may have up to 256 virtual paths, each containing 65,536 VCs (16 bits)Actually slightly less as some VCs are used for control functionsPTI field defines the type of payloadE.g., 000 means user data cell with no congestion and cell type 0 while 010 means user data cell that experienced congestionA cell sent by the user as 000 may arrive as 010Types are user supplied but congestion info is network suppliedCLP is set by a host to differentiate between high and low priority trafficIn case of congestion, switch will first drop cells with CLP 1 before dropping cells with CLP 0HEC byte provides error control over the headerAll single bit and 90% of multibit errors can be correctedA 48 byte payload follows headerNot all 48 bytes available for payload as some of the AAL protocols put their headers and trailers inside the payload
86 Connection Setup ATM supports two types of VCs Permanent VCs: present at all times like leased linesSwitched VCs: have to be setup for each sessionConnection setup is not part of ATM layerDescribed by ITU protocol Q.2931, which is part of control planeConnection setup is a two-step processFirst, a VC is acquired for signalingTo establish such a circuit, cells containing a request are sent to virtual path 0, VC 5If first step is successful, a new VC is opened on which connection setup request and replies are transmitted
87 Messages for Connection Setup in ATM Four messages are used for establishmentHost sends a SETUP message on a special VCNetwork responds with CALL PROCEEDING at each hopWhen SETUP arrives at destination it responds with CONNECT that propagates back towards originatorEach switch returns a CONNECT ACK to originatorTwo messages are used for release of a VCHost wishing to release sends a requestIntermediate switches respond as request propagates
88 Connection Setup (Cont’d) Multicast connection setupA multicast channel has one sender and multiple receiversConstructed by first setting up connection to one destinationADD PARTY messages are sent to add more receivers to the VC previously returnedATM addressesSetup messages include destination addressATM addresses come in three formsType 1: 20 bytes long OSI addressesFirst byte indicates which of three formatsBytes 2 and 3 specify country; byte 4 gives format for the rest of address that contains 3-byte authority, 2-byte domain, 2-byte area, and 6-byte add.Type 2: bytes 2 and 3 designate an international organization and rest is same as in type 1Type 3: 15 digit decimal ISDN telephone number
89 ATM Adaptation LayerATM layer does not provide error or flow control to applicationsOnly 53 byte cells are outputNot directly useable for applicationsATM Adaptation Layer (AAL) was defined to bridge this gapAAL protocols:Four protocols to handle four classes of serviceAAL1 – AAL4Requirements for classes C and D were so similar that AAL3 and AAL4 are combined into AAL ¾AAL1 for CBR and AAL2 for VBRAAL5 proposed by computer industry in contrast to telecommunication industry that proposed AAL1 – AAL3/4 for IP datagrams
90 Structure of the AAL AAL has two parts: ATM physical layerATM layerConvergence sublayer (service specific part)Covergence sublayer (common part)Segmentation reassembly sublayerAAL has two parts:Convergence sublayerInterfaces with application for framing and error detectionTwo parts: service-specific part and common partSegmentation And Reassembly (SAR) sublayerAdds headers and trailers to data units given by convergence layer to form cell payloads
91 Convergence and SAR Layer Operations Convergence sublayer adds its header/trailer to the messageMessage is broken into byte units, which are passed to SARSAR adds its own header/trailer and passes each piece to ATM layerSome AAL protocols have null header/trailerAbove figure represents the most general case
92 IP over ATM ATM is widely used as Internet backbone Permanent VCs between each pair of entry/exit pointPermanent VCs avoid having to establish dynamic VCs for transiting cellsFro n entry points, n(n-1) permanent VCs are neededRouters have 2 addresses:An IP addressAn ATM (LAN) addressATM network needs to transit datagram to the exit routerUses permanent VCUses AAL5
93 Practice Problem # 1Q: Consider a CSMA/CD network running at 1 Gbps over a 1 km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?A:For a 1 km cable, the one-way propagation time is 5 msec or 2t = 10 msec. Shortest frame should take more than this time to transmit to allow the sender to identify any collisions in the worst case.At 1Gbps, the number of bits that should be transmitted during 10 msec = 10,000 bits = 1250 bytes.Thus, the frame should not be shorter than 1250 bytes.
94 Practice Problem # 2Q: A 4-Mbps token ring has a token holding timer value of 10 msec. What is the longest frame that can be sent on this ring?A:At 4 Mbps, a station can transmit 40,000 bits or 5000 bytes in 10 msec.This is an upper bound on frame length.From this amount, some overhead bytes must be subtracted, giving a slightly lower limit for the data portion.
95 Practice Problem # 3Q: At a transmission rate of 5 Mbps and a propagation speed of 200 m/msec, to how many meters of cable is the 1-bit delay in a token ring interface equivalent?A:At 5 Mbps, a bit time is 200 nsec.In 200 ns, the signal travels 40 m.Thus, insertion of one new station adds as much delay as insertion of 40 meters of cable.
96 Practice Problem # 4Q: A very heavily loaded 1-km long, 10 Mbps token ring has a propagation speed of 200 m/msec. There are 50 stations uniformly spaced along the ring. Data frames are 256 bits, including 32 bits of overhead. Acknowledgements are piggybacked onto the data frames are are thus included as spare bits within the data frames and are effectively free. The token is 8 bits. Is the effective data rate of this ring higher or lower than the effective data rate of 10 mbps CSDM/CD network?A:Measured from the time of token capture, it takes 25.6 msec to transmit a packet.Additionally, a token must be transmitted, taking 0.8 msecToken must propagate 20 meters taking 0.1 msec.Thus we have sent 224 bits in 26.5 msec, which results in an effective data rate of 8.5 Mbps. This is more than the effective bandwidth for the Ethernet (4.7 Mbps(why?)) under the same parameters.
97 Practice Problem # 5Q: Ethernet frame must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64 byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?A: The maximum wire length in Fast Ethernet is 1/10 as long as in the regular Ethernet.
98 Practice Problem # 6Q: A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring?A:With a rotation time of 40 msec and 100 stations, the time for the token to move between stations is 40/100=0.4 msec.A station may transmit for 10 msec, followed by a 0.4 msec gap while the token moves to the next station.The best case efficiency is then 10/10.4=96%.
101 Power management Idea: switch the transceiver off if not needed States of a station: sleep and awakeTiming Synchronization Function (TSF)stations wake up at the same timeInfrastructureTraffic Indication Map (TIM)list of unicast receivers transmitted by APDelivery Traffic Indication Map (DTIM)list of broadcast/multicast receivers transmitted by APAd-hocAd-hoc Traffic Indication Map (ATIM)announcement of receivers by stations buffering framesmore complicated - no central APcollision of ATIMs possible (scalability?)
102 Power saving with wake-up patterns (infrastructure) TIM intervalDTIM intervalDBTTdDBaccesspointbusybusybusybusymediumpdstationtTTIMDDTIMawakeddata transmissionto/from the stationBbroadcast/multicastpPS poll
103 Power saving with wake-up patterns (ad-hoc) ATIMwindowbeacon intervalB1ADB1station1B2B2adstation2tBAtransmit ATIMDbeacon framerandom delaytransmit dataadawakeacknowledge ATIMacknowledge data
104 802.11 - Roaming No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answerReassociation Requeststation sends a request to one or several AP(s)Reassociation Responsesuccess: AP has answered, station can now participatefailure: continue scanningAP accepts Reassociation Requestsignal the new station to the distribution systemthe distribution system updates its data base (i.e., location information)typically, the distribution system now informs the old AP so it can release resources
105 WLAN: IEEE 802.11b Frequency Security Cost Availability On certain frequencies in the free 2.4 GHz ISM-bandSecurityLimited, WEP insecure, SSIDCost100€ adapter, 250€ base station, droppingAvailabilityMany products, many vendorsSpecial Advantages/DisadvantagesAdvantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple systemDisadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed onlyWhat’s new?Define a new PHY layer. All the MAC schemes, management procedures are the sameUser data rate max. approx. 6 Mbit/s
107 WLAN: IEEE 802.11a Frequency Connection set-up time Security US 5 GHz: free , , GHz ISM-bandConnection set-up timeConnectionless/always onSecurityLimited, WEP insecure, SSIDAvailabilitySome products, some vendorsQuality of ServiceTyp. best effort, no guarantees (same as all products)Special Advantages/DisadvantagesAdvantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz bandDisadvantage: stronger shading due to higher frequency, no QoS
108 Operating channels for 802.11a / US U-NII 3640444852566064channel5150518052005220524052605280530053205350[MHz]16.6 MHzcenter frequency =*channel number [MHz]149153157161channel572557455765578558055825[MHz]16.6 MHz
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