Ch'13 Metropolitan Area Networks

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Ch.13 Metropolitan Area Networks

• Lecturer Tae-Hyong Kim (B201-4)
• thkim_at_cespc1.kumoh.ac.kr

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Contents

• Introduction
• IEEE 802.6 (DQDB)
• Distributed Queue Dual Bus
• SMDS
• Switched Multimegabit Data Service

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Metropolitan Area Network

• A network designed to extend over an entire city
• privately owned connecting infrastructure (e.g.
  cable, routers, gateway) is impractical
• have to use the services of existing utilities,
  such as the telephone company
• SMDS (Switched Multimegabit Data Service)
• DQDB (Distributed Queue Dual Bus)
• WAN (Wide Area Network)
• national, continental, world-wide
• X.21, X.25, ISDN, ATM, Frame Relay

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13.1 IEEE 802.6 (DQDB)

• Similar to LAN standard but designed for MAN
• Access method dual bus (two unidirectional
  buses)
• Access to these bus is granted by the distributed
  queue mechanism
• Station 1 Head of bus A, end of bus B
• Station 5 Head of bus B, end of bus A

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IEEE 802.6 (DQDB) (cont.)

• Directional traffic
• Each bus supports traffic only one direction
  opposite direction each other
• Upstream and Downstream stations
• For Bus A, stations 1 and 2 are considered
  upstream wrt station 3, while stations 4 and 5
  are considered downstream wrt station 3
• Transmission slots
• Data travels on each bus as a steady stream of
  53-byte slots not packets but continuous
  streams of bits
• The head of each bus generates empty slots for
  use on it
• The source station must choose the bus for which
  the destination station is considered downstream

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IEEE 802.6 (DQDB) (cont.)
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IEEE 802.6 (DQDB) (cont.)

• Slot Reservation
• To send data downstream, a station must wait for
  the arrival of an unoccupied slot
• To stop an upstream station from monopolizing the
  bus and occupying all the slots
• To send data on one bus, a station must use the
  other bus to make a reservation
• The station sets a reservation bit in a slot on
  the other bus to tell each station it passes that
  a station is reserving a slot on the bus
• No station may send data without first making a
  reservation, even if it sees slot after slot pass
  by empty.
• Even a station that has made a reservation cannot
  claim just any empty slot ? it must wait for the
  arrival of the specific slot it reserved

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IEEE 802.6 (DQDB) (cont.)

• Distributed Queues
• Each station has two queues one for each bus
• ? A kind of waiting list
• To make reservations
• To track the reservations of the other stations

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IEEE 802.6 (DQDB) (cont.)

• Using a queue for Bus Access
• Station X uses virtual tokens to track
  reservations
• adds a token at the rear of the queue each time a
  slot passes on the other bus with a reservation
  bit set
• When the station needs to make a reservation for
  itself, it sets one of the reservation bits, then
  insert its own token into its queue
• For each empty slot that passes, it removes one
  token from the queue
• Each time the station reads its queue, it can
  tell how many downstream reservations have been
  made by counting how many tokens are in the queue

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IEEE 802.6 (DQDB) (cont.)

• Ring Configuration
• DQDB can be implemented as a ring
• Advantage Reconfigurable whenever a link or a
  station fails

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IEEE 802.6 (DQDB) (cont.)

• DQDB Layers
• The IEEE defines both the medium access control
  (MAC) sublayer and the physical layer for DQDB
• MAC sublayer
• splits the data stream coming from upper layers
  into 48-byte segments and adds 5-byte header to
  create 53 bytes each
• Having 53 bytes make a DQDB slot compatible with
  the size of a cell in ATM

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IEEE 802.6 (DQDB) (cont.)
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IEEE 802.6 (DQDB) (cont.)

• DQDB Header
• Access field
• Busy (B) indicates if the slot is carrying data
• Sot type (ST) packet transmission or
  isochronous transmission
• Previous slot read (PSR) 2 bits, set to 0 by
  the receiving station, once it has read the
  contents of the slot
• Request (RQ) three bits, set by stations to
  make reservations for slots, represent eight
  levels of priority (station level)
• Address field
• 20-bit virtual channel identifier for MAN and WAN
  transmission
• When used in LAN, this field contains all 1s and
  an additional header is added for MAC address
• Type field 2 bits
• identifies the payload user data, management
  data, ...
• Priority field the priority of the slot
• Implementation coaxial or fiber-optic cable

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13.2 SMDS

• Connecting LANs using Leased Lines
• Expensive n LANs require n?(n-1)/2 connections
• SMDS provides the solution
• Packet-switched datagram service for high-speed
  MAN by common carriers
• subscribers pay only for the time they use
• Subscriber LANs link to SMDS through routers
  using DQDB architecture

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SMDS (cont.)

• SMDS Architecture
• SIP (SMDS Interface Protocol) three levels

DQDB architecture
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SMDS (cont.)

• SIP Levels

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SMDS (cont.)

• SIP Level 3
• accepts the user data less than 9188 bytes
• Header and trailer are added
• management and control fields
• sender and receiver addresses 8 bytes
• The first 4 bits defines the type of address
• The next 60 bits (15 4bit sections) each
  section define a digit (09) ? telephone numbers
  (country/area/local number) ?WAN
• the packet is divided into 44 byte sections
• To each section, 2-byte header and 2-byte trailer
  are added

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SMDS (cont.)

• SIP Level 2
• DQDB comes into play
• add 5-byte header to 48-byte sections
• ? 53 byte output to slot and carried to its
  destination
• SIP level 1 Physical level
• Other Features
• The data rate ranges from 1.544Mbps to 155Mbps
• Each user is assigned an average data rate
• Addressing system telephone number
• Multicasting is available