SMDS

1
SMDS

• Nirmala Shenoy
• Information technology Department
• Rochester Institute of Technology

2
SMDS

• Scope
• DQDB operation
• SMDS Architecture

3
DQDB

• Uses MAN protocol hence first briefly into DQDB
• DQDB Distributed Queue Dual Bus
• 2 communication channels
• Each station connects to both bus
• Queues to use each bus
• Queues carry information about other stations

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DQDB

• Architecture

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DQDB

• Architecture
• Bus B
• With respect to Station 3, Stn 1, and 2 are
  downstream
• Stn 4, 5 are upstream
• Bus A
• With respect to Station 3, stn 4, 5 are down
  stream
• Stn 1, 2 are upstream

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DQDB

• Operation
• On Bus A, head of bus A generates slots of 53
  bytes which goes down bus A
• On Bus B, head of Bus B generates slots which go
  down bus B
• Empty slot travels down the bus, till a station
  wanting to transmit, sends its 53 bytes

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DQDB

• Operation
• Bus A Stn 3 can send to 1 and 2
• Bus B, stn 3 can send to stn 4 and 5
• Upstream Node monopolization
• Station to reserve for slots if they want to
  communicate to downstream nodes
• Reservation goes in direction opposite to the
  direction you want to send

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DQDB

• Operation
• To book a downstream slot in Bus A, use the
  slots passing by in Bus B to the head of bus A
• All upstream nodes in Bus A see the reservation
  going by
• Keep an account of the number of reservations
  that go by
• On empty slots going downstream A, the upstream
  station let that many slots go by

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DQDB

• Operation
• Distributed Queues
• Queue A for Bus A
• Queue B for Bus B
• Queues hold the booking for slots
• Each station puts a token in the queue when it
  sees a request going by
• When it wants to send, it will put its token in
  the queue

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DQDB

• Operation
• As empty slots go by
• Tokens are removed for each empty slot
• Till this station token is at HoQ
• Empty slot comes by and is taken by this station
• Ring configuration possible

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DQDB

• DQDB layers
• Physical
• MAC layer
• 48 byte payload
• 5 byte header

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DQDB

• DQDB MAC PDU
• 5 byte Header
• Access
• Address
• Type
• Priority
• CRC

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DQDB

• Access field 8 bits
• Busy bit slot is carrying data
• Slot type pkt transmission, isochronous
  transmission
• Reserved bit
• PSR (Previous Slot Read) 2 bits set to 0 once
  the destination stn has read
• RQ 3 bits 8 levels of priority on request

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DQDB

• Address Field
• 20 bit VCI
• Type field 2 bits payload type, user data,
  management data
• Priority field Priority of the slot
• DQDB self healing
• Determination of upstream and downstream stations

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SMDS

• SMDS
• Packet switched datagram service for high speed
  MAN traffic
• Switched service pay for time of usage
• Networks use Router to connect to SMDS switches
  using DQDB
• SMDS Interface protocol - SIP

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SMDS

• SMDS topology

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SMDS

• SMDS - SIP Level 3
• Adds header and trailer
• Header has sender and receiver address
• Packet is segmented into 44 bytes
• Each 44 byte is given 2 bytes header, 2 bytes
  trailer
• 2 byte header -

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SMDS

• SMDS - SIP Level 3
• header
• ST 2 bits segment type
• 4 bits sequence count
• Message ID 10 bits
• Trailer
• 6 bit length indicator
• 10 bit crc

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SMDS

• SMDS - SIP Level 2 DQDB
• SIP level 1 physical layer

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SMDS

• Summary
• Ease on geographic limits
• Offers Switched services
• Addresses high speed interconnectivity
• End Systems are routers
• Real-time?
• Bulk data