Friday, November 23, 2012

Notes - Broadband Wireless

The following are notes from Computer Networks written by Tanenbaum 5th edition.
  • Broadband wireless, large antenna on a hill easier than digging many trenches for cables
  • 802.16 WiMAX(Worldwide Interoperability for Microwave Access)
  • fairly complicated like OSI so there is a WiMAX forum to define standards
Comparison of 802.16 with 802.11 and 3G
  • why not just use 802.11 or 3G?
    • more akin to 4 G combines both aspects
  • connect devices to internet at megabit/sec speeds without cable or DSL
  • designed to
    • carry IP packets over air
    • peer to peer VoIP stream media
    • OFD based technology
    • more like 3G in that
      • tries to achieve high capacity
      • uses more power and better antennas
      • licensed spectrum around 2.5 GHz
      • LTE(Long term evolution)
      • collision course with 4G
The 802.16 Architecture and Protocol Stack
  • base stations connect directly into backbone network, air interface to mobile and subscriber stations
  • the following is the protocol stack 
The 802.16 Physical Layer
  • utilizes 3.5 GHz or 2.5 GHZ
  • transmissions over OFDM
  • Symbols are sent with QPSK, QAM-16, QAM-64
  • SNR ratio in order to reach distant stations use QPSK sends 2 bits per symbol coded for forward error correction
    • common for noisy channels to tolerate bit erros
    • 802.16 developers did not like certain parts of GSM and DAMPS
  • chose flexible scheme OFDMA(Orthogonal Frequency Division Multiple Access)
    • different sets of subcarriers so that more than one station can send/receive at once
    • also chose to use TDD(Time Division Duplex) to alternate between sending and receiving
      • could have done FDD(Frequency Division Duplex) but this is not as flexible and harder to implement
  • one of new uplink bursts reserved for ranging
    • process by which new stations adjust timing and request initial bandwidth during base station setup
    • is a hope and transmit setup assumes no collision
The 802.16 MAC Sublayer Protocol
  • data link layer divided into 3 sublayers
  • encryption to keep data secret
    • perform mutual authentication using RSA public-key cryptography using X.509 certificates
  • Uplink Channel Services
    • Constant bit rate service
      • transmit voice
    • Real-time variable bit rate service
      • transmit multimedia
    • Non Real-time variable bit rate service
      • file transfer
    • Best effort service
      • everything else
      • ethernet binary exponential backoff algorithm used
  • all are connection oriented
The 802.16 Frame Structure
  • All MAC frames begin with generic header, followed by CRC
    • checksum surprisingly optional since no attempt is made to retransmit
  • Generic Frame
    • EC bit tells whether payload is encrypted
    • type identifies frame type
      • tells whether packing and fragmentation are present
    • CI indicates presence or absence of checksum
    • EK tells which encryption is being used
    • length gives complete length of entire frame
    • connection identifier tells which connection this frame belongs too
    • Header CRC uses polynomial x8 + x2 + x + 1
  • Bandwidth request frame is different, starts with 1 bit instead of 0 and is just a command

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