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.