Collision Avoidance using STANAG 5066 in a Network Environment

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Collision AvoidanceusingSTANAG 5066in
aNetwork Environment
Presented byRobert McFarlandSoftware
EngineerHF Products
SupportJames MahanProgram ManagerHF Products
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STANAG 5066

• NATO Standardization Agreement
• Collection of layered protocols for data
  communications over High Frequency (HF) radio
• Reliability provided for by a using a Automatic
  Repeat Request (ARQ) data link protocol
• Supports the automatic adaptation of user data
  rate to changing radio channel conditions
• Provides an interface for external users
  (clients) to exchange data using the subnetwork

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STANAG 5066
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STANAG 5066

• Typical point-to-point data communication using a
  softlink

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Network Environment

• STANAG 5066 nodes participate in a subnetwork
• Subnetwork connection
• Common Channel using CAS type 1 protocol for link
  establishment
• Multiple Channels using CAS type 0 for link
  establishment
• CAS type 0 uses an external linking protocol such
  as MIL-STD 141 (ALE)
• External protocol controls channel access

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Network Environment

• A common occurrence in the subnetwork is that
  many nodes want to communicate at the same time
  which can lead to collisions
• Collisions require retransmissions and delays
  which lower channel throughput

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Collision Avoidance

• Carrier Sense Multiple Access with Collision
  Detection (CSMA/CD) is a protocol used in wired
  networks where the sender can detect collisions
  by comparing what is heard with what was sent
• In a wireless network, the sender receives all
  that it sent or nothing, in either case,
  collisions are not easily detected
• CSMA/CA is a wireless network protocol that
  attempts to Avoid Collisions

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Collision Avoidance

• The protocol starts by listening for carrier
  (Carrier Sense)
• Channel Idle then node transmits
• Channel Busy then the node waits
• Contention interval
• All waiting nodes contend for the channel by
  selecting a random delay
• After the their delay, a node can transmit if
  channel is still idle thus avoiding a collision

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Collision Avoidance

• Carrier Sense is provided by the Data Carrier
  Detect (DCD) input to the STANAG 5066 host
• Signaled by the HF Modem with valid data
• Signaled by the COMSEC device with valid data
• DCD is not instantaneous
• Propagation delay
• Modem interleaver delay
• COMSEC delay

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Collision Avoidance

• Slots Due to the delay in DCD, nodes must be
  constrained to begin their transmission at
  certain times otherwise a node would never know
  how long to listen
• Slot Width The interval between start times must
  be long enough to insure that all nodes can
  detect the transmission before the beginning of
  the next slot
• Number of Slots A balance between probability of
  one and only one node selecting the winning slot
  and acceptable delay (eg. 3 nodes,
  90probability, 16 slots)

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Collision Avoidance

• Listen Before Transmit (LBT) Interval
• Starts when carrier is no longer detected
• Reset whenever carrier is detected
• When the LBT interval expires, a contention
  interval starts
• Used to insure that the softlink data exchange
  between two nodes is complete

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Collision Avoidance
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Collision Avoidance

• Test Setup
• Subnetwork consisting of 3 nodes
• 1Kbyte messages generated at each node for each
  other node every two minutes
• Number of Slots 16
• Slot Width 3 seconds
• LBT Interval 30 seconds
• 110B waveform, 3200 bps, short interleaver

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Collision Avoidance

• Test Procedure
• Test ran multiple times with Collision Avoidance
  enabled and with Collision Avoidance disabled
• Test duration 1 - 14 hours
• Messages generators were started at each node at
  approximately the same time
• After each test, undelivered messages were
  deleted and the nodes reset

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Collision Avoidance

• Test Results
• Collision Avoidance disabled On average, 35
  messages successfully delivered in 60 minutes
• Collision Avoidance enabled On average, 170
  messages successfully delivered in 60 minutes

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Collision Avoidance

• Disclaimers
• This is Collision Avoidance not Collision
  Prevention