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Local%20Area%20Networks

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The proliferation of laptop computers and other mobile devices ... Signal can be captured by snoopers. Spectrum is limited & usually regulated. Wireless Links ... – PowerPoint PPT presentation

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Title: Local%20Area%20Networks


1
Local Area Networks
  • Chapter 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 1990s.
  • Industry decided to entrust standardization to
    the IEEE committee that dealt with wired LANS
    namely, the IEEE 802 committee!!
  • 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
  • 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.
  • Two standards for wireless networking
  • IEEE 802.11b standard for wireless LANs (aka
    Wi-Fi)
  • Bluetooth standard that allows devices to
    communicate without being in line of sight
  • Wireless devices classified wrt power, range, and
    data rate
  • IEEE 802.11 ? high power, medium range, and high
    rate access technology
  • Bluetooth ? low power, short range, low rate,
    cable replacement technology

4
IEEE 802.11 Wireless LAN
  • Wireless LANs mobile networking
  • IEEE 802.11 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)
  • Basic Service Set (BSS) (a.k.a. cell) contains
  • wireless hosts
  • access point (AP) base station
  • BSSs combined to form distribution system (DS)

5
IEEE 802.11 Wireless LAN
6
IEEE 802.11 Wireless LAN
7
The 802.11 Protocol Stack
8
The 802.11 Protocol Stack
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
  • Physical layer conforms to OSI (five options)
  • 1997 802.11 infrared, FHSS, DHSS
  • 1999 802.11a OFDM and 802.11b HR-DSSS
  • 2001 802.11g OFDM
  • 802.11 Infrared
  • Two capacities 1 Mbps or 2 Mbps.
  • Range is 10 to 20 meters and cannot penetrate
    walls.
  • Does not work outdoors.
  • 802.11 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
  • 802.11 DSSS (Direct Sequence Spread Spectrum)
  • Spreads signal over entire spectrum using
    pseudo-random sequence (similar to CDMA see
    Tanenbaum sec. 2.6.2).
  • 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 802.11a
    and 802.11b.
  • Supports bandwidths up to 54 Mbps.
  • Uses 2.4 GHz frequency for greater range.
  • Is backward compatible with 802.11b.

15
Infrastructure Network
  • Permanent Access Points provide access to Internet

16
IEEE 802.11 Wireless LAN
17
802.11 Definitions
  • 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 BSSs can coexist
  • 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
  • Ad hoc network IEEE 802.11 stations can
    dynamically form network without AP
  • Formed on the fly when mobile devices are in
    proximity
  • Applications
  • Laptop meeting in conference room, car
  • Interconnection of personal devices
  • Battlefield
  • IETF MANET (Mobile Ad hoc Networks) working
    group

19
Ad Hoc Networks
20
Hidden Terminal Problem
(a)
Data Frame
A transmits data frame
C senses medium, station A is hidden from C
  • New MAC CSMA with Collision Avoidance

21
IEEE 802.11 MAC Protocol CSMA/CA (collision
avoidance)
  • 802.11 CSMA sender
  • if sense channel idle for Distributed Inter Frame
    Space (DIFS) sec.
  • then transmit entire frame (no collision
    detection)
  • if sense channel busy then binary backoff
  • 802.11 CSMA receiver
  • if received OK
  • return ACK after Short Inter Frame Spacing
    (SIFS)
  • (DIFS SIFS 2 slot time)
  • Time slot 20 micro s, SIFS10 micro s, DIFS50
    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 transmission
  • Avoids collisions
  • Collisions ? uses exponentially increasing
    backoff period
  • Collisions detection is difficult
  • Hidden terminal problem
  • Fading
  • NAV Network Allocation Vector
  • 802.11 frame has transmission duration field
  • Others (hearing stations) defer access (to save
    power) for NAV time units

23
IEEE 802.11 MAC Protocol
24
Hidden Terminal effect
  • Hidden terminals A, C cannot hear each other
  • Obstacles, signal attenuation
  • Collisions at B
  • Goal avoid collisions at B
  • CSMA/CA CSMA with Collision Avoidance

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

26
Collision Avoidance RTS-CTS exchange
  • CA with RTS-CTS
  • Collisions less likely, of shorter duration
  • End result similar to collision detection
  • IEEE 802.11 allows
  • CSMA
  • CSMA/CA reservations
  • polling from AP

27
CSMA with Collision Avoidance
28
IEEE 802.11 Wireless LAN
  • Stimulated by availability of unlicensed spectrum
  • U.S. Industrial, Scientific, Medical (ISM) bands
  • 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz
  • Targeted wireless LANs _at_ 20 Mbps
  • MAC for high speed wireless LAN
  • Ad Hoc Infrastructure networks
  • Variety of physical layers

29
Infrastructure Network
30
Distribution Services
  • Stations within BSS can communicate directly with
    each other
  • 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
  • Multicast, broadcast, or stationss preference
  • ESS looks like single BSS to LLC layer

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

32
IEEE 802.11 MAC
  • MAC sublayer responsibilities
  • Channel access
  • PDU addressing, formatting, error checking
  • Fragmentation reassembly of MAC SDUs
  • MAC security service options
  • Authentication privacy
  • MAC management services
  • Roaming within ESS
  • Power management

33
MAC Services
  • Contention Service Best effort
  • Contention-Free Service time-bounded transfer
  • MAC can alternate between Contention Periods
    (CPs) Contention-Free Periods (CFPs). MAC
    Service Data Unit (MSDU)

34
Distributed Coordination Function (DCF)
  • DCF provides basic access service
  • Asynchronous best-effort data transfer
  • All stations contend for access to medium
  • CSMA-CA
  • Ready stations wait for completion of
    transmission
  • All stations must wait Interframe Space (IFS)

35
Priorities through Interframe Spacing
  • High-Priority frames wait Short IFS (SIFS)
  • Typically to complete exchange in progress
  • ACKs, CTS, data frames of segmented MSDU, etc.
  • PCF IFS (PIFS) to initiate Contention-Free
    Periods
  • DCF IFS (DIFS) to transmit data MPDUs

36
Contention Backoff Behavior
  • If channel is still idle after DIFS period, ready
    station can transmit an initial MPDU
  • 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
  • A station that completes a frame transmission is
    not allowed to transmit immediately
  • Must first perform a backoff procedure

37
(No Transcript)
38
Carrier Sensing in 802.11
  • Physical Carrier Sensing
  • Analyze all detected frames
  • Monitor relative signal strength from other
    sources
  • Virtual Carrier Sensing at MAC sublayer
  • Source stations informs other stations of
    transmission time (in msec) for an MPDU
  • Carried in Duration field of RTS CTS
  • Stations adjust Network Allocation Vector to
    indicate when channel will become idle
  • Channel busy if either sensing is busy

39
Transmission of MPDU without RTS/CTS
40
Transmission of MPDU with RTS/CTS
41
Collisions, Losses Errors
  • 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
  • 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
  • PCF provides connection-oriented, contention-free
    service through polling
  • Point coordinator (PC) in AP performs PCF
  • Polling table up to implementor
  • 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
PCF Frame Transfer
44
DCF, PCF, and Frame Format
45
Distributed Coordination Function (DCF)
  • DCF is the access method used to support
    asynchronous data transfer on a best effort basis
  • All stations must support the DCF (DCF operates
    solely in the ad hoc network)
  • Operates solely or coexists with the PCF in an
    infrastructure network
  • 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.
  • 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 802.11, 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 802.11 WLAN users by analyzing all
    detected packets, and also detects activity in
    the channel via relative signal strength from
    other sources
  • 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 (SIFS1), and DIFS
    (SIFS2)
  • 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/DataCF-poll),
    CF-end.
  • CF-aware st
  • Gets CF-poll,
  • Responds CF-ACK, DataCF-ACK,
  • Then PC responds by DataCF-ACKCF-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
  • Management frames
  • Station association disassociation with AP
  • Timing synchronization
  • Authentication deauthentication
  • Control frames
  • Handshaking
  • ACKs during data transfer
  • Data frames
  • Data transfer

53
Frame Structure
MAC header (bytes)
2
2
6
6
6
2
6
0-2312
4
Address 2
Frame Control
Duration/ ID
Address 1
Address 3
Sequence control
Address 4
Frame body
CRC
  • MAC Header 30 bytes
  • Frame Body 0-2312 bytes
  • CRC CCITT-32 4 bytes CRC over MAC header
    frame body

54
Frame Control (1)
  • Protocol version 0
  • Type Management (00), Control (01), Data (10)
  • Subtype within frame type
  • Type00, subtypeassociation Type01,
    subtypeACK
  • MoreFrag1 if another fragment of MSDU to follow

55
Frame Control (2)
To DS 1 if frame goes to DS From DS 1 if
frame exiting DS
56
Frame Control (3)
  • Retry1 if mgmt/control frame is a retransmission
  • Power Management used to put station in/out of
    sleep mode
  • More Data 1 to tell station in power-save mode
    more data buffered for it at AP
  • WEP1 if frame body encrypted

57
Physical Layers
  • 802.11 designed to
  • Support LLC
  • Operate over many physical layers

58
IEEE 802.11 Physical Layer Options
Frequency Band Bit Rate Modulation Scheme
802.11 2.4 GHz 1-2 Mbps Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum
802.11b 2.4 GHz 11 Mbps Complementary Code Keying QPSK
802.11g 2.4 GHz 54 Mbps Orthogonal Frequency Division Multiplexing CCK for backward compatibility with 802.11b
802.11a 5-6 GHz 54 Mbps Orthogonal Frequency Division Multiplexing
59
802.11 - MAC management
  • Synchronization
  • try to find a LAN, try to stay within a LAN
  • timer etc.
  • Power management
  • sleep-mode without missing a message
  • periodic sleep, frame buffering, traffic
    measurements
  • Association/Reassociation
  • integration into a LAN
  • roaming, i.e. change networks by changing access
    points
  • scanning, i.e. active search for a network
  • MIB - Management Information Base
  • managing, read, write

60
Synchronization using a Beacon (infrastructure)
beacon interval
B
B
B
B
access point
busy
busy
busy
busy
medium
t
B
value of the timestamp
beacon frame
61
Synchronization using a Beacon (ad-hoc)
beacon interval
B1
B1
station1
B2
B2
station2
busy
busy
busy
busy
medium
t
B
value of the timestamp
beacon frame
random delay
62
Power management
  • Idea switch the transceiver off if not needed
  • States of a station sleep and awake
  • Timing Synchronization Function (TSF)
  • stations wake up at the same time
  • Infrastructure
  • Traffic Indication Map (TIM)
  • list of unicast receivers transmitted by AP
  • Delivery Traffic Indication Map (DTIM)
  • list of broadcast/multicast receivers transmitted
    by AP
  • Ad-hoc
  • Ad-hoc Traffic Indication Map (ATIM)
  • announcement of receivers by stations buffering
    frames
  • more complicated - no central AP
  • collision of ATIMs possible (scalability?)

63
Power saving with wake-up patterns
(infrastructure)
TIM interval
DTIM interval
D
T
T
D
B
B
d
access point
busy
busy
busy
busy
medium
p
d
station
t
64
Power saving with wake-up patterns (ad-hoc)
ATIM window
beacon interval
B1
B1
A
D
station1
B2
B2
a
d
station2
t
D
B
transmit data
beacon frame
random delay
a
d
awake
acknowledge ATIM
acknowledge 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 answer
  • Reassociation Request
  • station sends a request to one or several AP(s)
  • Reassociation Response
  • success AP has answered, station can now
    participate
  • failure continue scanning
  • 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 802.11b
  • Whats new?
  • Define a new PHY layer. All the MAC schemes,
    management procedures are the same
  • User data rate max. approx. 6 Mbit/s
  • Frequency
  • On certain frequencies in the free 2.4 GHz
    ISM-band
  • Security
  • Limited, WEP insecure, SSID
  • Cost
  • 100 adapter, 250 base station, dropping
  • Availability
  • Many products, many vendors
  • 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
  • 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.
  • 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
  • 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 (Contd)
  • Frequency hopping
  • Transceiver minimizes the effect of interference
    from other signals
  • Hops to a new frequency after transmitting or
    receiving a packet
  • Error recovery
  • Transceiver forward error correction (FEC)
  • Automatic Repeat reQuest (ARQ) for retransmission
  • Bluetooth protocol suite includes
  • Baseband protocol
  • Enables physical RF wireless connection between
    devices
  • A connection of 2-7 Bluetooth devices forms a
    small network?piconet
  • Link manager protocol
  • Handshaking between two devices to establish
    connection
  • L2CAP protocol
  • During a connection, adapts upper layer protocols
    for transmission over the baseband

69
Bluetooth - Physical Upwards!
  • 79 channels, each 1MHz, using FSK, with 1 bit per
  • symbol 1Mbps
  • Much of the 1Mbps is taken up with protocol
    overheads caused
  • by frequency hopping (250-260 ms needed to
    stabilise radio after
  • the hop!)
  • Leaves about 366 bits for actual data of which
    126 bits are headers
  • leaving 240 bits for data per slot!

70
Bluetooth Frequency Hopping
71
Point to Point Data Link Control
  • One sender, one receiver, one link easier than
    broadcast link
  • No Media Access Control
  • No need for explicit MAC addressing
  • Examples
  • Dialup link phone line ? 56 Kbps modem
    connections
  • SONET/SDH link
  • X.25 connection
  • ISDN line
  • 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
  • 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
  • Bit transparency
  • Must carry any bit pattern in the data field with
    no constraints
  • Error detection (no correction)
  • PPP receiver must be able to detect bit errors
  • Connection liveness
  • Detect, signal link failure to network layer
  • Network layer address negotiation
  • Endpoint can learn/configure each others network
    address

73
PPP Non-Requirements
  • No error correction/recovery
  • No flow control
  • PPP receiver is expected to receive frames at
    full physical layer speed ? higher layer could
    drop packets or throttle sender
  • Out of order delivery OK
  • 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
  • Flag delimiter (framing)
  • Address does nothing (only one option)
  • Control does nothing in the future possible
    multiple control fields
  • PPP sender can allow sender to skip address and
    control bytes
  • 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 (Contd)
  • Info
  • Variable length upper layer data being carried
  • Default maximum is 1500 bytes
  • Can be changed when the link is initially
    configured
  • Check
  • Uses cyclic redundancy check (CRC) for error
    detection
  • Two or 4 bytes CRC

76
Byte Stuffing
  • Data transparency requirement data field must
    be allowed to include flag pattern lt01111110gt
  • Q is received lt01111110gt data or flag?
  • Sender
  • Adds (stuffs) an escape byte lt 01111101gt before
    each lt01111110gt data byte
  • Receiver
  • Discards the escape byte and continues data
    reception
  • Single 01111110 ? flag byte
  • If two lt01111101gt 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 network
  • layer information
  • For IP carry IP Control Protocol (IPCP) msgs
    (protocol field 8021) to configure/learn IP
    address

PPP link always begins andends 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
  • PPP frame with protocol filed set equal to LCP
  • Information field contains the specific
    configuration request
  • Options
  • Maximum frame size for the link
  • Specification of authentication protocol to be
    used (if any)
  • 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)
  • If IP is running over PPP, IP control protocol
    (IPCP) is used
  • IPCP is carried within a PPP frame
  • 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
  • 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)
  • Two types of networks have existed side by side
  • Telephone networks ? carry real-time voice
  • Data networks ? carry non real-time datagrams
  • ATM standards were developed in mid-1980s
  • 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
  • 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
  • ATM service models
  • Constant bit rate (CBR)
  • Variable bit rate (VBR)
  • Available bit rate (ABR)
  • Unspecified bit rate (UBR)
  • ATM uses fixed-length packets ? cells
  • Header 5 bytes and payload 48 bytes
  • Fixed length cell and simple header facilitate
    high speed switching
  • ATM VCs ? virtual channels
  • Header includes virtual channel identifier (VCI)
    field
  • VCI is used by switches to forward the cells
  • 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
  • Date rates
  • 155 Mbps, 622 Mbps, and higher

83
Characteristics of ATM (Contd)
  • 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
  • Congestion control
  • Only for ABR service class
  • Network provides feedback to sender to regulate
    its rate
  • ATM protocol stack consists of three layers
  • ATM physical layer
  • ATM layer
  • ATM adaptation layer (AAL)
  • Analogous to transport layer in TCP/IP stack
  • Multiple types of AALs

84
Cell Header Formats
  • 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)
  • 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 (Contd)
  • 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
  • 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
  • Types are user supplied but congestion info is
    network supplied
  • 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
  • HEC byte provides error control over the header
  • All single bit and 90 of multibit errors can be
    corrected
  • 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
  • ATM supports two types of VCs
  • Permanent VCs present at all times like leased
    lines
  • Switched VCs have to be setup for each session
  • Connection setup is not part of ATM layer
  • Described by ITU protocol Q.2931, which is part
    of control plane
  • Connection setup is a two-step process
  • First, a VC is acquired for signaling
  • 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
  • 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
  • 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 (Contd)
  • 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
  • ATM addresses
  • Setup messages include destination address
  • ATM addresses come in three forms
  • 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.
  • Type 2 bytes 2 and 3 designate an international
    organization and rest is same as in type 1
  • Type 3 15 digit decimal ISDN telephone number

89
ATM Adaptation Layer
  • 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
  • AAL protocols
  • Four protocols to handle four classes of service
  • AAL1 AAL4
  • Requirements for classes C and D were so similar
    that AAL3 and AAL4 are combined into AAL ¾
  • 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
  • AAL has two parts
  • Convergence sublayer
  • Interfaces with application for framing and error
    detection
  • Two parts service-specific part and common part
  • Segmentation And Reassembly (SAR) sublayer
  • Adds 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 message
  • Message is broken into 44-48 byte units, which
    are passed to SAR
  • SAR adds its own header/trailer and passes each
    piece to ATM layer
  • Some AAL protocols have null header/trailer
  • Above 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
    point
  • Permanent VCs avoid having to establish dynamic
    VCs for transiting cells
  • Fro n entry points, n(n-1) permanent VCs are
    needed
  • Routers have 2 addresses
  • An IP address
  • An ATM (LAN) address
  • ATM network needs to transit datagram to the exit
    router
  • Uses permanent VC
  • Uses AAL5

93
Practice Problem 1
  • 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?
  • 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 2
  • QA 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 3
  • QAt 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 4
  • Q 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 msec
  • Token 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 5
  • QEthernet 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?
  • AThe maximum wire length in Fast Ethernet is
    1/10 as long as in the regular Ethernet.

98
Practice Problem 6
  • QA 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 40/1000.4 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 10/10.496.

99
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100
IEEE 802.11 Wireless LAN
101
Power management
  • Idea switch the transceiver off if not needed
  • States of a station sleep and awake
  • Timing Synchronization Function (TSF)
  • stations wake up at the same time
  • Infrastructure
  • Traffic Indication Map (TIM)
  • list of unicast receivers transmitted by AP
  • Delivery Traffic Indication Map (DTIM)
  • list of broadcast/multicast receivers transmitted
    by AP
  • Ad-hoc
  • Ad-hoc Traffic Indication Map (ATIM)
  • announcement of receivers by stations buffering
    frames
  • more complicated - no central AP
  • collision of ATIMs possible (scalability?)

102
Power saving with wake-up patterns
(infrastructure)
TIM interval
DTIM interval
D
T
T
D
B
B
d
access point
busy
busy
busy
busy
medium
p
d
station
t
103
Power saving with wake-up patterns (ad-hoc)
ATIM window
beacon interval
B1
B1
A
D
station1
B2
B2
a
d
station2
t
D
B
transmit data
beacon frame
random delay
a
d
awake
acknowledge ATIM
acknowledge 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 answer
  • Reassociation Request
  • station sends a request to one or several AP(s)
  • Reassociation Response
  • success AP has answered, station can now
    participate
  • failure continue scanning
  • 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 802.11b
  • Frequency
  • On certain frequencies in the free 2.4 GHz
    ISM-band
  • Security
  • Limited, WEP insecure, SSID
  • Cost
  • 100 adapter, 250 base station, dropping
  • Availability
  • Many products, many vendors
  • 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
  • Whats 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)
Europe (ETSI)
channel 1
channel 7
channel 13
2400
2412
2483.5
2442
2472
MHz
22 MHz
US (FCC)/Canada (IC)
channel 1
channel 6
channel 11
2400
2412
2483.5
2437
2462
MHz
22 MHz
107
WLAN IEEE 802.11a
  • Frequency
  • US 5 GHz free 5.15-5.25, 5.25-5.35, 5.725-5.825
    GHz ISM-band
  • Connection set-up time
  • Connectionless/always on
  • Security
  • Limited, WEP insecure, SSID
  • Availability
  • Some products, some vendors
  • Quality of Service
  • Typ. best effort, no guarantees (same as all
    802.11 products)
  • 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 802.11a / US U-NII
channel
36
44
40
48
52
56
60
64
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350
MHz
16.6 MHz
center frequency 5000 5channel number MHz
149
153
157
161
channel
5725
5745
5765
5785
5805
5825
MHz
16.6 MHz
109
IEEE 802.11 Wireless LAN
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