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Metropolitan Area Networks

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... to any one type. For both isochronous and delay insensitive data. ... Special frames for isochronous communication (pre-arbitrated frames every 125 microseconds) ... – PowerPoint PPT presentation

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Title: Metropolitan Area Networks


1
Metropolitan Area Networks
  • Chapter 8

2
Why MANs?
  • Metropolitan Area Networks evolved primarily to
    permit LANs to interconnect across a metropolis
    or a country.
  • Two major technologies
  • Distributed Queue Dual Bus (DQDB).
  • Fiber Distributed Data Interface (FDDI).

3
DQDB
  • Adopted in 1990 by IEEE 802.6.
  • It is a multiplexing method.
  • Its medium is not limited to any one type.
  • For both isochronous and delay insensitive data.
  • The transfer of data messages between stations on
    two LANs is handled to appear as if they were on
    the same LAN.

4
DQDB cont.
  • Two busses accessible to every node.
  • Failure of one node can not incapacitate the
    network.
  • Empty frames are generated by the two sources
    annihilated at the sinks.
  • Different dual bus implementations
  • Open dual bus.
  • Point-to-point bus.
  • Looped dual bus.

5
How DQDB Works
  • Empty frames travel from sources to sinks.
  • Each node may peek non-destructively at the
    frames as they go by.
  • Empty, non-reserved frames may be used.
  • To write, a node logically ORs its message
    with, and replaces the content of the empty frame.

6
How DQDB Works cont.
  • To reserve a frame on Bus A, a node must capture
    an empty frame on Bus B and set the reserved
    flag.
  • FIFO priorities are used.
  • Special frames for isochronous communication
    (pre-arbitrated frames every 125 microseconds).
  • A flag in the header indicates a special frame.

7
FDDI
  • Standardized by ANSI X3T9.5.
  • Similar to IEEE 802.5.
  • Motivation for implementation was not only the
    interconnection of LANs but the need for high
    speed networking.
  • Support ring with maximum circumference of 100 km.

8
FDDI cont.
  • Data transmission speed 100 Mbits/s.
  • NRZI coding is used.
  • Dual counter-rotating rings.
  • Inner ring is mainly for back-up.
  • The bit error rate is low 1 in 1012
  • Each station is a repeater.
  • Self-healing nature.
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