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Local Area Networks Chapter 10 – Wireless LANs. Wireless Communication n The proliferation of laptop computers and other mobile devices (PDAs and cell.

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Presentation on theme: "Local Area Networks Chapter 10 – Wireless LANs. Wireless Communication n The proliferation of laptop computers and other mobile devices (PDAs and cell."— Presentation transcript:

1 Local Area Networks Chapter 10 – Wireless LANs

2 Wireless Communication n The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking. n Companies jumped in, quickly developing incompatible wireless products in the 1990’s. n Industry decided to entrust standardization to the IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!! n Wireless communications compelling Easy, low-cost deployment Mobility & roaming: Access information anywhere Supports personal devices PDAs, laptops, data-cell-phones Supports communicating devices Cameras, location devices, wireless identification  Signal strength varies in space & time  Signal can be captured by snoopers  Spectrum is limited & usually regulated

3 Wireless Links n Many end systems use wireless links: – Portable PCs within a wireless LAN – PDAs that connect to the Internet through wireless telephony infrastructure – Cameras, automobiles, etc. n 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 sight n Wireless devices classified wrt power, range, and data rate – IEEE  high power, medium range, and high rate “access” technology – Bluetooth  low power, short range, low rate, “cable replacement” technology

4 IEEE Wireless LAN n Wireless LANs: mobile networking n IEEE standard: – MAC protocol – Unlicensed frequency spectrum: 2.4Ghz (802.11b) or 5-6 Ghz (802.11a) – Provides wireless Ethernet access at 11 Mbps or 54 Mbps (802.11a) n Basic Service Set (BSS) (a.k.a. “cell”) contains: – wireless hosts – access point (AP): base station n BSS’s combined to form distribution system (DS)

5 IEEE Wireless LAN

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7 The Protocol Stack

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9 Wireless Standards Frequency, Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (FHSS) HR: High Rate Orthogonal Frequency Division Multiplexing (OFDM, VOFDM, COFDM)

10 Wireless Physical Layer n Physical layer conforms to OSI (five options) – 1997: infrared, FHSS, DHSS – 1999: a OFDM and b HR-DSSS – 2001: g OFDM n Infrared – Two capacities 1 Mbps or 2 Mbps. – Range is 10 to 20 meters and cannot penetrate walls. – Does not work outdoors. n 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 n 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. n a 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 n b 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!! n g 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 A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to the Internet Portal BSS A BSS B Infrastructure Network n Permanent Access Points provide access to Internet

16 IEEE Wireless LAN

17 Definitions n Basic Service Set (BSS) – Group of stations that coordinate their access using a given instance of MAC – Located in a Basic Service Area (BSA) – Stations in BSS can communicate with each other – Distinct collocated BSS’s can coexist n Extended 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 Networks n Ad hoc network: IEEE stations can dynamically form network without AP n Formed “on the fly” when mobile devices are in proximity n Applications: – “Laptop” meeting in conference room, car – Interconnection of “personal” devices – Battlefield n IETF MANET (Mobile Ad hoc Networks) working group

19 Ad Hoc Networks

20 A transmits data frame (a) Data Frame A B C C transmits data frame & collides with A at B (b) C senses medium, station A is hidden from C Data Frame B C A Hidden Terminal Problem n New MAC: CSMA with Collision Avoidance

21 IEEE MAC Protocol: CSMA/CA (collision avoidance) CSMA: sender n if sense channel idle for Distributed Inter Frame Space (DIFS) sec. then transmit entire frame (no collision detection) n if sense channel busy then binary backoff CSMA receiver: n if received OK return 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 MAC Protocol CSMA Protocol: others n Other stations wait for a random backoff period after DIFS after current transmission – Avoids collisions – Collisions  uses exponentially increasing backoff period n Collisions detection is difficult: – Hidden terminal problem – Fading n NAV: Network Allocation Vector: – frame has transmission duration field – Others (hearing stations) defer access (to save power) for NAV time units

23 IEEE MAC Protocol

24 Hidden Terminal effect n Hidden terminals: A, C cannot hear each other – Obstacles, signal attenuation – Collisions at B n Goal: avoid collisions at B n CSMA/CA: CSMA with Collision Avoidance Fading can also result in collisions

25 Collision Avoidance: RTS-CTS exchange n CSMA/CA: explicit channel reservation – sender: send short RTS: request to send – receiver: reply with short CTS: clear to send n CTS reserves channel for sender, notifying (possibly hidden) stations – Benefit: RTC-CTS avoids hidden station collisions

26 Collision Avoidance: RTS-CTS exchange n CA with RTS-CTS: – Collisions less likely, of shorter duration – End result similar to collision detection n IEEE allows: – CSMA – CSMA/CA: reservations – polling from AP

27 RTS A requests to send B C (a) CTS A B C B announces A ok to send (b) Data Frame A sends B C remains quiet (c) CSMA with Collision Avoidance

28 IEEE Wireless LAN n Stimulated by availability of unlicensed spectrum – U.S. Industrial, Scientific, Medical (ISM) bands – MHz, GHz, GHz n Targeted wireless 20 Mbps n MAC for high speed wireless LAN n Ad Hoc & Infrastructure networks n Variety of physical layers

29 A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to the Internet Portal BSS A BSS B Infrastructure Network

30 Distribution Services n Stations within BSS can communicate directly with each other n DS provides distribution services: – Transfer MAC SDUs between APs in ESS – Transfer MSDUs between portals & BSSs in ESS – Transfer MSDUs between stations in same BSS n Multicast, broadcast, or stations’s preference n ESS looks like single BSS to LLC layer

31 Infrastructure Services n Select AP and establish association with AP – Then can send/receive frames via AP & DS n Reassociation service to move from one AP to another AP n Dissociation service to terminate association n Authentication service to establish identity of other stations n Privacy service to keep contents secret

32 IEEE MAC n MAC sublayer responsibilities – Channel access – PDU addressing, formatting, error checking – Fragmentation & reassembly of MAC SDUs n MAC security service options – Authentication & privacy n MAC management services – Roaming within ESS – Power management

33 MAC Services n Contention Service: Best effort n Contention-Free Service: time-bounded transfer n MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs). MAC Service Data Unit (MSDU) Physical Distribution coordination function (DCF) (CSMA-CA) Point coordination function Contention- free service Contention service MAC MSDUs

34 Distributed Coordination Function (DCF) n DCF provides basic access service – Asynchronous best-effort data transfer – All stations contend for access to medium n CSMA-CA – Ready stations wait for completion of transmission – All stations must wait Interframe Space (IFS) DIFS PIFS SIFS Contention window Next frame Defer access Wait for reattempt time Time Busy medium

35 Priorities through Interframe Spacing n High-Priority frames wait Short IFS (SIFS) – Typically to complete exchange in progress – ACKs, CTS, data frames of segmented MSDU, etc. n PCF IFS (PIFS) to initiate Contention-Free Periods n DCF IFS (DIFS) to transmit data & MPDUs DIFS PIFS SIFS Contention window Next frame Defer access Wait for reattempt time Time Busy medium

36 Contention & Backoff Behavior n If channel is still idle after DIFS period, ready station can transmit an initial MPDU n If channel becomes busy before DIFS, then station must schedule backoff time for reattempt – Backoff period is integer # of idle contention time slots – Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires – Station can contend when backoff timer expires n A station that completes a frame transmission is not allowed to transmit immediately – Must first perform a backoff procedure

37 RTS CTS Data Frame A requests to send B C A A sends B B C C remains quiet B announces A ok to send (a) (b) (c) ACK B (d) ACK B sends ACK

38 Carrier Sensing in n Physical Carrier Sensing – Analyze all detected frames – Monitor relative signal strength from other sources n Virtual Carrier Sensing at MAC sublayer – Source stations informs other stations of transmission time (in sec) for an MPDU – Carried in Duration field of RTS & CTS – Stations adjust Network Allocation Vector to indicate when channel will become idle n Channel busy if either sensing is busy

39 Data DIFS SIFS Defer Access Wait for Reattempt Time ACK DIFS NAV Source Destination Other Transmission of MPDU without RTS/CTS

40 Data SIFS Defer access Ack DIFS NAV (RTS) Source Destination Other RTS DIFS SIFS CTS SIFS NAV (CTS) NAV (Data) Transmission of MPDU with RTS/CTS

41 Collisions, Losses & Errors n Collision Avoidance – When 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 expires – If collision occurs, recompute backoff over interval that is twice as long n Receiving stations of error-free frames send ACK – Sending station interprets non-arrival of ACK as loss – Executes backoff and then retransmits – Receiving stations use sequence numbers to identify duplicate frames

42 Point Coordination Function n PCF provides connection-oriented, contention-free service through polling n Point coordinator (PC) in AP performs PCF n Polling table up to implementor n CFP repetition interval – Determines frequency with which CFP occurs – Initiated by beacon frame transmitted by PC in AP – Contains CFP and CP – During CFP stations may only transmit to respond to a poll from PC or to send ACK

43 CF End NAV PIFS B D1 + Poll SIFS U 1 + ACK D2+Ack+ Poll SIFS U 2 + ACK SIFS Contention-free repetition interval Contention period CF_Max_duration Reset NAV D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame TBTT PCF Frame Transfer

44 DCF, PCF, and Frame Format

45 Distributed Coordination Function (DCF) n DCF is the access method used to support asynchronous data transfer on a best effort basis n All stations must support the DCF (DCF operates solely in the ad hoc network) n Operates solely or coexists with the PCF in an infrastructure network n DCF 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 channel  Once the MSDU is transmitted, must recontend for access to the channel for all subsequent frames  Contention services promote fair access to the channel for all stations. n 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 transmitting  In 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 sensing  Physical 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 sources n Virtual carrier sensing – Stations include MPDU duration in the header of request to send (RTS), clear to send (CTS), and data frames – An MPDU is a complete data unit that is passed from the MAC sublayer – to the physical layer – The MPDU contains header information, information, payload, and a 32-bit CRC – The 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 data Management F: station association dis-association with AP Control F: handshaking in CP, ACK data in CP, and end CFP Basic DCF Access Method (no RTS-CTS): When ST finds chaneel idle, it waits for DIFS and checks it again If 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 source All other STs in BSS hearing above messages adjust their NAV timers

47 Distributed Coordination Function (DCF) RTS-CTS Data Mode Priority Accsess: SIFS, PIFS (SIFS+1), and DIFS (SIFS+2) In BSS, STs hearing RTS, CTS, F0, and ACK adjust their NAV Sts: Basic mode, RTS/CTS mode if MPDU exceeds L, or always use RTS/CTS mode Fairness: BEB starts with (1,8) and end at some maximum

48 Distributed Coordination Function (DCF) MPDU (2300 bytes): collision lead to bandwidth loss RTS is 20 bytes and CTS is 14 bytes Fragmentation increases transmission reliability Fragment MPDU, transmit Frag, receive ACK to completion If 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 complete PC: 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 BSS When Dest receives F, a DCF-ACK is returned to source PC waits for PIFS after ACK before continuation

52 Frame Types n Management frames – Station association & disassociation with AP – Timing & synchronization – Authentication & deauthentication n Control frames – Handshaking – ACKs during data transfer n Data frames – Data transfer

53 Address 2 Frame Control Duration/ ID Address 1 Address 3 Sequence control Address 4 Frame body CRC MAC header (bytes) Frame Structure n MAC Header: 30 bytes n Frame Body: bytes n CRC: CCITT-32 4 bytes CRC over MAC header & frame body

54 Address 2 Frame Control Duration/ ID Address 1 Address 3 Sequence control Address 4 Frame body CRC Protocol version TypeSubtype To DS From DS More frag Retry Pwr mgt More data WEPRsvd MAC header (bytes) Frame Control (1) n Protocol version = 0 n Type: Management (00), Control (01), Data (10) n Subtype within frame type n Type=00, subtype=association; Type=01, subtype=ACK n MoreFrag=1 if another fragment of MSDU to follow

55 To DS From DS Address 1 Address 2 Address 3 Address 4 00 Destination address Source address BSSIDN/A 01 Destination address BSSID Source address N/A 10BSSID Source address Destination address N/A 11 Receiver address Transmitter address Destination address Source address Meaning Data frame from station to station within a BSS Data frame exiting the DS Data frame destined for the DS WDS frame being distributed from AP to AP Address 2 Frame Control Duration/ ID Address 1 Address 3 Sequence control Address 4 Frame body CRC Protocol version TypeSubtype To DS From DS More frag Retry Pwr mgt More data WEPRsvd To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS Frame Control (2)

56 Address 2 Frame Control Duration/ ID Address 1 Address 3 Sequence control Address 4 Frame body CRC Protocol version TypeSubtype To DS From DS More frag Retry Pwr mgt More data WEPRsvd MAC header (bytes) Frame Control (3) n Retry=1 if mgmt/control frame is a retransmission n Power Management used to put station in/out of sleep mode n More Data =1 to tell station in power-save mode more data buffered for it at AP n WEP=1 if frame body encrypted

57 Physical layer LLC Physical layer convergence procedure Physical medium dependent MAC layer PLCP preamble LLC PDU MAC SDU MAC header CRC PLCP header PLCP PDU Physical Layers n designed to – Support LLC – Operate over many physical layers

58 IEEE Physical Layer Options Frequency Band Bit RateModulation Scheme GHz1-2 Mbps Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum b2.4 GHz11 Mbps Complementary Code Keying & QPSK g2.4 GHz54 Mbps Orthogonal Frequency Division Multiplexing & CCK for backward compatibility with b a5-6 GHz54 Mbps Orthogonal Frequency Division Multiplexing

59 MAC management n Synchronization – try to find a LAN, try to stay within a LAN – timer etc. n Power management – sleep-mode without missing a message – periodic sleep, frame buffering, traffic measurements n Association/Reassociation – integration into a LAN – roaming, i.e. change networks by changing access points – scanning, i.e. active search for a network n MIB - Management Information Base – managing, read, write

60 Synchronization using a Beacon (infrastructure) beacon interval t medium access point busy B BBB value of the timestamp B beacon frame

61 Synchronization using a Beacon (ad-hoc) t medium station 1 busy B1B1 beacon interval busy B1B1 value of the timestamp B beacon frame station 2 B2B2 B2B2 random delay

62 Power management n Idea: switch the transceiver off if not needed n States of a station: sleep and awake n Timing Synchronization Function (TSF) – stations wake up at the same time n Infrastructure – Traffic Indication Map (TIM) n list of unicast receivers transmitted by AP – Delivery Traffic Indication Map (DTIM) n list of broadcast/multicast receivers transmitted by AP n Ad-hoc – Ad-hoc Traffic Indication Map (ATIM) n announcement of receivers by stations buffering frames n more complicated - no central AP n collision of ATIMs possible (scalability?)

63 Power saving with wake-up patterns (infrastructure) TIM interval t medium access point busy D TTD T TIM D DTIM DTIM interval BB B broadcast/multicast station awake p PS poll p d d d data transmission to/from the station

64 Power saving with wake-up patterns (ad-hoc) awake A transmit ATIM D transmit data t station 1 B1B1 B1B1 B beacon frame station 2 B2B2 B2B2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data

65 Roaming n No or bad connection? Then perform: n Scanning – scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer n Reassociation Request – station sends a request to one or several AP(s) n Reassociation Response – success: AP has answered, station can now participate – failure: continue scanning n AP accepts Reassociation Request – signal the new station to the distribution system – the 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 b n What’s new? – Define a new PHY layer. All the MAC schemes, management procedures are the same – User data rate max. approx. 6 Mbit/s n Frequency – On certain frequencies in the free 2.4 GHz ISM-band n Security – Limited, WEP insecure, SSID n Cost – 100€ adapter, 250€ base station, dropping n Availability – Many products, many vendors n Special Advantages/Disadvantages – Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system – Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

67 Bluetooth n Most compelling application addressed by Bluetooth: – A convenient, untethered means to interconnect electronic devices – Examples: portable phones, PDAs, laptops, desktops, digital cameras, fax machines, printers, keyboard, mouse, etc. n Line-of-sight infrared technology has been used for such communications – Using RF wireless communication, Bluetooth does not require LoS – It can support multipoint as well as point-to-point communication n Bluetooth architecture: – Mobile devices need short-range transceivers – Transceivers operate in 2.5 Ghz unlicensed frequency band – Provide data rates of up to 721 kbps + 3 voice channels (64 kbps) – Operating range is 10 to 100 meters – Each device is identified by a 12-bit address

68 Bluetooth (Cont’d) n Frequency hopping – Transceiver minimizes the effect of interference from other signals – Hops to a new frequency after transmitting or receiving a packet n Error recovery: – Transceiver forward error correction (FEC) – Automatic Repeat reQuest (ARQ) for retransmission n Bluetooth protocol suite includes: – Baseband protocol n Enables physical RF wireless connection between devices n A connection of 2-7 Bluetooth devices forms a small network  piconet – Link manager protocol n Handshaking between two devices to establish connection – L2CAP protocol n During a connection, adapts upper layer protocols for transmission over the baseband

69 Bluetooth - Physical Upwards! n 79 channels, each 1MHz, using FSK, with 1 bit per n symbol = 1Mbps n Much of the 1Mbps is taken up with protocol overheads – caused n by frequency hopping ( ms needed to stabilise radio after n the hop!) n Leaves about 366 bits for actual data – of which 126 bits are headers n – leaving 240 bits for data per slot!

70 Bluetooth Frequency Hopping

71 Point to Point Data Link Control n One sender, one receiver, one link: easier than broadcast link: – No Media Access Control – No need for explicit MAC addressing – Examples: n Dialup link phone line  56 Kbps modem connections n SONET/SDH link n X.25 connection n ISDN line n Popular 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] n Packet framing: – Encapsulation of network-layer datagram in data link frame – Carry network layer data of any network layer protocol (not just IP) – Ability to demultiplex upwards n Bit transparency: – Must carry any bit pattern in the data field with no constraints n Error detection (no correction) – PPP receiver must be able to detect bit errors n Connection liveness: – Detect, signal link failure to network layer n Network layer address negotiation: – Endpoint can learn/configure each other’s network address

73 PPP Non-Requirements n No error correction/recovery n No flow control – PPP receiver is expected to receive frames at full physical layer speed  higher layer could drop packets or throttle sender n Out of order delivery OK n No need to support multipoint links (e.g., polling) – Other link layer protocols can support multipoint links – E.g., HDLC Error recovery, flow control, data re-ordering all relegated to higher layers!|

74 PPP Data Frame n Flag: delimiter (framing) n Address: does nothing (only one option) n Control: does nothing; in the future possible multiple control fields – PPP sender can allow sender to skip address and control bytes n Protocol: upper layer protocol to which frame delivered – Examples: PPP-LCP, IP, IPCP, etc – RFC 1700 and RFC 3232 define 16-bit protocol codes for PPP

75 PPP Data Frame (Cont’d) n Info: – Variable length upper layer data being carried – Default maximum is 1500 bytes – Can be changed when the link is initially configured n Check: – Uses cyclic redundancy check (CRC) for error detection – Two or 4 bytes CRC

76 Byte Stuffing n “Data transparency” requirement: data field must be allowed to include flag pattern – Q: is received data or flag? n Sender: – Adds (“stuffs”) an escape byte before each data byte n Receiver: – Discards the escape byte and continues data reception – Single  flag byte – If 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: n Configure PPP link (max. frame length, authentication) n Learn/configure network layer information – For IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address PPP 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 intervention – One end of the link uses configure-request frame to indicate its configuration options n PPP frame with protocol filed set equal to LCP n Information field contains the specific configuration request – Options: n Maximum frame size for the link n Specification of authentication protocol to be used (if any) n Option to skip the address and control fields in PPP frames – The other side responds with configure-ack, configure-nak, or configure- reject frame Network 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) n If IP is running over PPP, IP control protocol (IPCP) is used – IPCP is carried within a PPP frame n Protocol field will have IPCP  indicated by 0x8021 – IPCP allows two IP modules to exchange or configure IP addresses – IPCP also allows two IP modules to negotiate whether or not IP datagrams will be sent in compressed form – Similar network control protocols for other network protocols: n Examples: DECnet, AppleTalk, etc. Link goes in open state after network configuration – PPP can start exchanging network layer datagrams – To check the link status, use echo-request and echo-reply LCP frames Terminating state – One side sends LCP terminate-request and other responds with LCP terminate-ack frame – Link goes to the dead state again

81 Asynchronous Transfer Mode (ATM) n Two types of networks have existed side by side: – Telephone networks  carry real-time voice – Data networks  carry non real-time datagrams n ATM standards were developed in mid-1980’s – Goal: design a network technology that will be appropriate for both types of traffic – Standard developed by ATM Forum and ITU for broadband digital services networks n ATM technology: – A full suite of communication protocols form application to physical layer – Calls for packet switching within virtual circuits  virtual channels – Deployed in both telephone networks and Internet backbones – High performance ATM switches can deliver terabits per second! – Still could not replace TCP/IP based networks at desktop level

82 Characteristics of ATM n ATM service models: – Constant bit rate (CBR) – Variable bit rate (VBR) – Available bit rate (ABR) – Unspecified bit rate (UBR) n ATM uses fixed-length packets  cells – Header: 5 bytes and payload: 48 bytes – Fixed length cell and simple header facilitate high speed switching n ATM VCs  virtual channels – Header includes virtual channel identifier (VCI) field – VCI is used by switches to forward the cells n Connection-oriented service – Cells always arrive in-order – ATM does not provide acks as other connection-oriented protocols do – Effectively, a VC is full duplex – Channel capacity and other properties may be different in two directions n Date rates: – 155 Mbps, 622 Mbps, and higher

83 Characteristics of ATM (Cont’d) n No link-by-link retransmissions – If an ATM switch detects error in a header, it tries to correct it – Simply drops the cell if error cannot be corrected  no retransmission request n Congestion control – Only for ABR service class – Network provides feedback to sender to regulate its rate n ATM protocol stack consists of three layers: – ATM physical layer – ATM layer – ATM adaptation layer (AAL) n Analogous to transport layer in TCP/IP stack n Multiple types of AALs

84 Cell Header Formats n In both cases, cells consist of: – 5 byte header and 48 byte payloads – Headers are slightly different for two interfaces (GFC field is unused any way) n Header fields – VPI is a small integer that selects a particular virtual path – VCI selects a particular VC from within the chosen virtual path

85 Cell Header Formats (Cont’d) n VPI and VCI – At 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 functions n PTI field defines the type of payload – E.g., 000 means user data cell with no congestion and cell type 0 while 010 means user data cell that experienced congestion – A cell sent by the user as 000 may arrive as 010 n Types are user supplied but congestion info is network supplied n CLP is set by a host to differentiate between high and low priority traffic – In case of congestion, switch will first drop cells with CLP 1 before dropping cells with CLP 0 n HEC byte provides error control over the header – All single bit and 90% of multibit errors can be corrected n A 48 byte payload follows header – Not all 48 bytes available for payload as some of the AAL protocols put their headers and trailers inside the payload

86 Connection Setup n ATM supports two types of VCs – Permanent VCs: present at all times like leased lines – Switched VCs: have to be setup for each session n Connection setup is not part of ATM layer n Described by ITU protocol Q.2931, which is part of control plane n Connection setup is a two-step process – First, a VC is acquired for signaling n To establish such a circuit, cells containing a request are sent to virtual path 0, VC 5 – If 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 n Four messages are used for establishment – Host sends a SETUP message on a special VC – Network responds with CALL PROCEEDING at each hop – When SETUP arrives at destination it responds with CONNECT that propagates back towards originator – Each switch returns a CONNECT ACK to originator n Two messages are used for release of a VC – Host wishing to release sends a request – Intermediate switches respond as request propagates

88 Connection Setup (Cont’d) n Multicast connection setup – A multicast channel has one sender and multiple receivers – Constructed by first setting up connection to one destination – ADD PARTY messages are sent to add more receivers to the VC previously returned n ATM addresses – Setup messages include destination address – ATM addresses come in three forms n Type 1: 20 bytes long OSI addresses – First byte indicates which of three formats – Bytes 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. n Type 2: bytes 2 and 3 designate an international organization and rest is same as in type 1 n Type 3: 15 digit decimal ISDN telephone number

89 ATM Adaptation Layer n ATM layer does not provide error or flow control to applications – Only 53 byte cells are output – Not directly useable for applications – ATM Adaptation Layer (AAL) was defined to bridge this gap n AAL protocols: – Four protocols to handle four classes of service n AAL1 – AAL4 n Requirements for classes C and D were so similar that AAL3 and AAL4 are combined into AAL ¾ n AAL1 for CBR and AAL2 for VBR – AAL5 proposed by computer industry in contrast to telecommunication industry that proposed AAL1 – AAL3/4  for IP datagrams

90 Structure of the AAL n AAL has two parts: – Convergence sublayer n Interfaces with application for framing and error detection n Two parts: service-specific part and common part – Segmentation And Reassembly (SAR) sublayer n Adds headers and trailers to data units given by convergence layer to form cell payloads ATM physical layer ATM layer Convergence sublayer (service specific part) Covergence sublayer (common part) Segmentation reassembly sublayer

91 Convergence and SAR Layer Operations n Convergence sublayer adds its header/trailer to the message n Message is broken into byte units, which are passed to SAR n SAR adds its own header/trailer and passes each piece to ATM layer n Some AAL protocols have null header/trailer – Above figure represents the most general case

92 IP over ATM n ATM is widely used as Internet backbone – Permanent VCs between each pair of entry/exit point – Permanent VCs avoid having to establish dynamic VCs for transiting cells – Fro n entry points, n ( n -1) permanent VCs are needed n Routers have 2 addresses: – An IP address – An ATM (LAN) address n ATM network needs to transit datagram to the exit router – Uses permanent VC – Uses AAL5

93 Practice Problem # 1 n Q: 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? n A: – For a 1 km cable, the one-way propagation time is 5 msec or 2 = 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 # 2 n Q: 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? n 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 # 3 Q: At a transmission rate of 5 Mbps and a propagation speed of 200 m/sec, to how many meters of cable is the 1-bit delay in a token ring interface equivalent? n 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 # 4 Q: A very heavily loaded 1-km long, 10 Mbps token ring has a propagation speed of 200 m/sec. 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? n A: – Measured from the time of token capture, it takes 25.6 sec to transmit a packet. – Additionally, a token must be transmitted, taking 0.8 sec – Token must propagate 20 meters taking 0.1 sec. – Thus we have sent 224 bits in 26.5 sec, 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 # 5 n Q: 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? n A: The maximum wire length in Fast Ethernet is 1/10 as long as in the regular Ethernet.

98 Practice Problem # 6 n Q: 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? n 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%.

99

100 IEEE Wireless LAN

101 Power management n Idea: switch the transceiver off if not needed n States of a station: sleep and awake n Timing Synchronization Function (TSF) – stations wake up at the same time n Infrastructure – Traffic Indication Map (TIM) n list of unicast receivers transmitted by AP – Delivery Traffic Indication Map (DTIM) n list of broadcast/multicast receivers transmitted by AP n Ad-hoc – Ad-hoc Traffic Indication Map (ATIM) n announcement of receivers by stations buffering frames n more complicated - no central AP n collision of ATIMs possible (scalability?)

102 Power saving with wake-up patterns (infrastructure) TIM interval t medium access point busy D TTD T TIM D DTIM DTIM interval BB B broadcast/multicast station awake p PS poll p d d d data transmission to/from the station

103 Power saving with wake-up patterns (ad-hoc) awake A transmit ATIM D transmit data t station 1 B1B1 B1B1 B beacon frame station 2 B2B2 B2B2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data

104 Roaming n No or bad connection? Then perform: n Scanning – scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer n Reassociation Request – station sends a request to one or several AP(s) n Reassociation Response – success: AP has answered, station can now participate – failure: continue scanning n AP accepts Reassociation Request – signal the new station to the distribution system – the 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 b n Frequency – On certain frequencies in the free 2.4 GHz ISM-band n Security – Limited, WEP insecure, SSID n Cost – 100 € adapter, 250 € base station, dropping n Availability – Many products, many vendors n Special Advantages/Disadvantages – Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system – Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only What ’ s new? – Define a new PHY layer. All the MAC schemes, management procedures are the same – User data rate max. approx. 6 Mbit/s

106 Channel selection (non-overlapping) 2400 [MHz] channel 1channel 7channel 13 Europe (ETSI) US (FCC)/Canada (IC) 2400 [MHz] channel 1channel 6channel MHz

107 WLAN: IEEE a n Frequency – US 5 GHz: free , , GHz ISM-band n Connection set-up time – Connectionless/always on n Security – Limited, WEP insecure, SSID n Availability – Some products, some vendors n Quality of Service – Typ. best effort, no guarantees (same as all products) n Special Advantages/Disadvantages – Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band – Disadvantage: stronger shading due to higher frequency, no QoS

108 Operating channels for a / US U-NII 5150 [MHz] MHz center frequency = *channel number [MHz] channel [MHz] MHz channel

109 IEEE Wireless LAN


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