Token Ring Overview

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					                   Token Ring Overview                                                            Token Ring (cont)
• Examples                                                        • Idea
  – 16Mbps IEEE 802.5 (based on earlier IBM ring)                    – Frames flow in one direction: upstream to downstream
  – 100Mbps Fiber Distributed Data Interface (FDDI)                  – special bit pattern (token) rotates around ring
                                                                     – must capture token before transmitting
                                                                     – release token after done transmitting
                                                                        • immediate release
                                                                        • delayed release
                                                                     – remove your frame when it comes back around
                                                                     – stations get round-robin service
                                                                  • Frame Format
                                                                                8          8       48      48                   32    8       24
                                                                             Start of   Control    Dest   Src        Body       CRC End of   Status
                                                                             frame                 addr   addr                      frame

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                  Timed Token Algorithm                                                           Algorithm (cont)
• Token Holding Time (THT)                                        • Each node measures TRT between successive
  – upper limit on how long a station can hold the token            tokens
                                                                     – if measured-TRT > TTRT: token is late so don’t send
• Token Rotation Time (TRT)                                          – if measured-TRT < TTRT: token is early so OK to send
  – how long it takes the token to traverse the ring.             • Two classes of traffic
  – TRT <= ActiveNodes x THT + RingLatency                           – synchronous: can always send
                                                                     – asynchronous: can send only if token is early
• Target Token Rotation Time (TTRT)                               • Worse case: 2xTTRT between seeing token
  – agreed-upon upper bound on TRT                                • Back-to-back 2xTTRT rotations not possible

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                    Token Maintenance                                                          Maintenance (cont)
• Lost Token                                                      • Monitoring for a Valid Token
  – no token when initializing ring                                  – should periodically see valid transmission (frame or token)
  – bit error corrupts token pattern                                 – maximum gap = ring latency + max frame < = 2.5ms
  – node holding token crashes                                       – set timer at 2.5ms and send claim frame if it fires
• Generating a Token (and agreeing on TTRT)
  –   execute when join ring or suspect a failure
  –   send a claim frame that includes the node’s TTRT bid
  –   when receive claim frame, update the bid and forward
  –   if your claim frame makes it all the way around the ring:
        • your bid was the lowest
        • everyone knows TTRT
        • you insert new token
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                             Outline                                                             Wireless LANs
• Wireless networks                                                       • IEEE 802.11
    – 802.11                                                                –   Bandwidth: 1 or 2 Mbps
    – Infra-red Irda                                                        –   802.11b - 11 Mbps (2.4 GHz)
    – Bluetooth                                                             –   802.11g - 54 Mbps (2.4 GHz)
                                                                            –   802.11a - 54 Mbps (5 GHz)
                                                                          • Physical Media
                                                                            – spread spectrum radio (2.4 GHz)
                                                                            – diffused infrared (10 m)

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                       Spread Spectrum                                                      Spread Spectrum (cont)
• Idea                                                                    • Direct Sequence
    – spread signal over wider frequency band than required                 –   for each bit, send XOR of that bit and n random bits
    – originally designed to thwart jamming                                 –   random sequence known to both sender and receiver
• Frequency Hopping                                                         –   called n-bit chipping code
    – transmit over random sequence of frequencies                          –   802.11 defines an 11-bit chipping code
    – sender and receiver share…
        • pseudorandom number generator
        • seed                                                              1
                                                                            0                                         Data stream: 1010
    – 802.11 uses 79 x 1MHz-wide frequency bands                            1
                                                                            0                                         Random sequence: 0100101101011001
                                                                            0                                         XOR of the two: 1011101110101001

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                         Challenges                                                          Collisions Avoidance
• Limited wireless transmission range                                     • Similar to Ethernet
• Broadcast nature of the wireless medium                                 • Problem: hidden and exposed nodes
    – Hidden terminal problem
•   Packet losses due to transmission errors
•   Mobility-induced route changes
•   Mobility-induced packet losses
•   Battery constraints
•   Potentially frequent network partitions                                                  A        B           C           D

•   Ease of snooping on wireless transmissions
    (security hazard)
                                                    Nitin Vaidya @ UIUC
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                      Hidden Terminal Problem                                                          RTS/CTS Handshake
                                                                             • Sender sends Ready-to-Send (RTS)
• Node B can communicate with A and C both                                   • Receiver responds with Clear-to-Send (CTS)
• A and C cannot hear each other                                             • RTS and CTS announce the duration of the transfer
                                                                             • Nodes overhearing RTS/CTS keep quiet for that
• When A transmits to B, C cannot detect the
                                                                             • RTS/CTS used in IEEE 802.11
  transmission using the carrier sense mechanism
• If C transmits, collision will occur at node B                                                                                                         10
                                                                                                                      RTS (10)
                                                                                                   A                                     B
                                                                                                           CTS (10)

                  A                 B                   C
                                                       Nitin Vaidya @ UIUC             10                                                    Nitin Vaidya @ UIUC
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Multiple Access Collision Avoidance – Wireless (MACAW)                                                 Supporting Mobility
• Sender transmits RequestToSend (RTS) frame                                 • Case 1: ad hoc net working
• Receiver replies with ClearToSend (CTS) frame                              • Case 2: access points (AP)
• Neighbors…                                                                     – tethered
  – see CTS: keep quiet                                                          – each mobile node associates with an AP
  – see RTS but not CTS: ok to transmit
• Receive sends ACK when has frame
  – neighbors silent until see ACK                                                                         Distribution system

• Collisions
                                                                                                   AP-1                                  AP-3
  – no collisions detection                                                                                           AP-2                       F
  – known when don’t receive CTS                                                               A               B                         G
  – exponential backoff                                                                                C                                     E
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                           Mobility (cont)                                                                            irda
• Scanning (selecting an AP)                                                 • Infrared Data Association – Point and Shoot Profile
  –   node sends Probe frame
  –   all AP’s w/in reach reply with ProbeResponse frame
  –   node selects one AP; sends it AssociateRequest frame
  –   AP replies with AssociationResponse frame
  –   new AP informs old AP via tethered network
• When
  – active: when join or move
  – passive: AP periodically sends Beacon frame

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                                irda                                                      Point and shoot model
•   Founded in 1993                                                      • Point to the device. If both device understand object
•   Short range, point-to-point, low cost infra-red based                  begin shared then transparent access
•   Speeds from 9600b to 16Mb
•   Great non-cable device                                               • Push server – the device that provides an object
    – Ubiquitous deployment                                                exchange server. Waits passively for the client to
    – 4 Mbps irda can talk to 9600 irda                                    initiate an operation
• Protocols for point and shoot, exchange mp3,
  images, vcards, wrist watch ….                                         • Push client – the client that pushes the object to the
                                                                           push server. Push client initiates operation

                                                                         • Security not an issue because of range
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                         Discussion                                                                Bluetooth
• Available in PDA’s, laptops, printers, projectors,                     • Bluetooth: Vision, Goals, and Architecture, Jaap
  digital cameras, wrist watches, pagers, ……                               Haartsen, Mahmoud Naghshineh, Jon Inouye, Olaf J.
                                                                           Joeresson, and Warren Allen, ACM Mobile
                                                                           Computing and Communications Review, Volume 2,
                                                                           Number 4, October 1998.

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                           Bluetooth                                                               Bluetooth
•   A cable replacement technology                    Pravin Bhagwat     • Low cost, low-power radio-based cable replacement
                                                          @ AT&T Labs
•   1 Mb/s symbol rate
•   Range 10+ meters                                                     • Related technologies
•   Single chip radio + baseband                                            – IrDA – Uses Infrared
    – at low power & low price point                                            • Pros: Less governmental restrictions, lower cost, lower
                                                                                  standby power, higher bandwidth
                                                                                • Cons: Line of sight
                                                                            – IEEE 802.11 – Same wireless band
                                                                                • Wireless LAN capability for a larger range
                                            Why not use Wireless LANs?
                                                      - power               – HomeRF – Wireless for data and voice
                                                      - cost                    • Data similar to 802.11, voice ADPCM

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                  Value proposition of Bluetooth                                                 Bluetooth working group history
                                                                                    • February 1998: The Bluetooth SIG is formed
          headset                                                                     – promoter company group: Ericsson, IBM, Intel, Nokia,
                                               Data access
     Cell                                         point                             • May 1998: Public announcement of the Bluetooth
     phone                                                                            SIG
                                                                                    • July 1999: 1.0A spec (>1,500 pages) is published
                                                             Internet access
       Cable replacement                                                            • December 1999: ver. 1.0B is released
                                                                                    • December 1999: The promoter group increases to 9
                                                                                      – 3Com, Lucent, Microsoft, Motorola
                                                                                    • February 2000: There are 1,800+ adopters
                                                              Pravin Bhagwat
                                Ad hoc networking                @ AT&T Labs
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                                                                                    • User benefits
                                                                                    • Automatic synchronization of calendars, address
                                                                                      books, business cards
                                                                                    • Push button synchronization
        New applications enabled by Bluetooth                                       • Proximity operation

                                                                                                                         Pravin Bhagwat @ AT&T Labs
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                        Cordless Headset                                                               Three-in-one phone
•   User benefits                                        Cordless
                                                                                    • At home, your phone functions as a portable phone
•   Multiple device access                               headset                      (fixed line charge). When you're on the move, it
•   Cordless phone benefits                                                           functions as a mobile phone (cellular charge). And
                                                                                      when your phone comes within range of another
•   Hands free operation                                                              mobile phone with built-in Bluetooth wireless
                                                                                      technology it functions as a walkie talkie (no
                                                                                      telephony charge).
                                                                                    • Source:

                                            Pravin Bhagwat @ AT&T Labs
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                              The Internet Bridge                                                                     The Interactive Conference
• Use your mobile computer to surf the Internet                                           • In meetings and
  wherever your are, and regardless if you're                                               conferences you can
  cordlessly connected through a mobile phone                                               transfer selected
  (cellular) or through a wire-bound connection (e.g.                                       documents instantly
  PSTN, ISDN, LAN, xDSL).                                                                   with selected
                                                                                            participants, and
• Source:                                                                     exchange electronic
                                                                                            business cards
                                                                                            automatically, without
                                                                                            any wired connections
                                                                                          • Source:

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                             Design considerations                                                                                 EM Spectrum
                                                               Noise, interference                                                                               ISM band
                                                                                                                                                                 902 – 928 Mhz



                                                                                                                                                                    2.4 – 2.4835 Ghz







                                                                                                                                                                          5.725 – 5.785 Ghz



                                                                                                 LF          MF             HF            VHF              UHF            SHF            EHF
Data signal x(t)                   spectrum                                 Recovered                                                                                                                    n
                                                                            data signal   30kHz    300kHz          3MHz           30MHz           300MHz           3GHz         30GHz          300GHz

                                                                                          10km        1km            100m         10m              1m               10cm           1cm         100mm     l

                                                                                                                                                                          X rays
    Goal                                                                                                                                            infrared visible UV
                                                                                                                                                                                   Gamma rays
     • high bandwidth                                                                            1 kHz            1 MHz           1 GHz       1 THz               1 PHz       1 EHz
     • conserve battery power
     • cost < $10                                                                                Propagation characteristics are different in each frequency band
                                                Pravin Bhagwat @ AT&T Labs                                                                                 Pravin Bhagwat @ AT&T Labs
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                   Unlicensed Radio Spectrum                                                                                  Bluetooth radio link
l          33cm                       12cm                     5cm                        • frequency hopping spread spectrum
           26 Mhz                      83.5 Mhz
                                                                                              – 2.402 GHz + k MHz, k=0, …, 78
                                                                  125 Mhz
                                                                                              – 1,600 hops per second
                                                                                          • GFSK modulation
    902 Mhz                     2.4 Ghz                      5.725 Ghz
                                          2.4835 Ghz                     5.785 Ghz
                                                                                              – 1 Mb/s symbol rate
                   928 Mhz
                                                                                          • transmit power
       cordless phones                802.11                      802.11a                     – 0 dbm (up to 20dbm with power control)
       baby monitors                  Bluetooth                                                       1Mhz
       Wireless LANs                  Microwave oven

                                                                                                                                                   . . .
                                                                                                   12 3                                                                             79
                                                                                                                                    83.5 Mhz
                                                Pravin Bhagwat @ AT&T Labs                                                                                 Pravin Bhagwat @ AT&T Labs
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                    Piconet formation                                                Inter piconet communication

• Page - scan protocol                                Master                  Cordless
   – to establish links with                                                  headset
     nodes in proximity                               Active Slave
                                                      Parked Slave                                                          headset

                                                                       Cell phone
                                                                                                                           Cell phone

                                                                                     Cell phone            Cordless       Pravin Bhagwat
                                       Pravin Bhagwat @ AT&T Labs                                          headset           @ AT&T Labs
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                             Myrinet                                                               Myrinet
• 2 GB full duplex high speed network interface                      • Few msec latency

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