HomePlug 1.0 PowerLine Communication LAN

1
HomePlug 1.0 PowerLine Communication LAN

• ICS451 Presentation
• Subbiah, Muruga Mullai

2
Contents

• HomePlug 1.0
• Power Line Communication
• Features of HomePlug 1.0
• Noise in Power Lines
• Orthogonal Frequency Division Multiplexing
• Physical Layer
• MAC Layer
• Current Trends

3
HomePlug 1.0

• HomePlug 1.0 protocol used for connecting
  devices via Electrical Wires in a home
• Share/Distribute data across home
• Data port is an electrical outlet.
• Why Electrical Wires?
• Infrastructure already exists in most of the
  rooms within an old/new home.
• No weak signals - dead spots like in an wireless
  network.

4
Network Access Point

• Using the Electrical Socket as a Ethernet Jack

5
Network

• Connect the homeplug adapter to any broadband
  gateway.
• Other computers within the home are now connected
  to the outside world.

6
HomePlug Powerline Alliance

• Companies which promote the goals and the mission
  of the HomePlug Powerline Alliance.
• Linksys
• Sony
• Comcast
• EarthLink
• GE Security
• Intel
• Motorola
• RadioShack
• Sharp

7
Features of HomePlug 1.0 Protocol

• Maximum Speed 14Mbps
• Physical and MAC layer are robust.
• Medium Error Prone.
• Uses OFDM (Orthogonal FDM) for transmission
• Reed-Solomon for Error Correction
• Carrier Sense and Collision Avoidance.
• Prioritized Access to the medium
• Security

8
Noise in the Medium

• Power Lines
• Carry Power at 50-60Hz up to 400Hz.
• Harsh for high frequency signals.
• Highly prone to Noise generated in the high
  frequency spectrum
• Caused by Appliances when used
• Halogen/Fluorescent Lamps
• Motors
• Variable resistance dimmer switches
• Other sources of noise
• Induced frequency signals Broadcast,
  Commercial, Military etc.,

9
Orthogonal Frequency Division Multiplexing (OFDM)

• Divides the spectrum into a number of equally
  spaced tones.
• A tone can be thought of as a frequency, much in
  the same way that each key on a piano represents
  a unique frequency.
• Carries a portion of a user's information on each
  tone at lower data rates. But many tones could
  exist in parallel thus higher data rates.
• Similar to FDM but FDM requires frequency guard
  bands between the frequencies so that they do not
  interfere with each other.
• OFDM allows the spectrum of each tone to overlap
  but since each tone is orthogonal with every
  other tone and thus no interference.
• By allowing the tones to overlap, the overall
  amount of spectrum required is reduced.
• Ideal for filtering out noise
• Most basic form - a tone may be present or
  disabled to indicate a one or zero bit of
  information
• Normally either PSK or quadrature amplitude
  modulation (QAM) is typically employed.

10
Reed-Solomon Codes

• The Reed-Solomon encoder takes a block of
  digital data and adds extra "redundant" bits.
  Errors occur during transmission or storage for a
  number of reasons (for example noise or
  interference, scratches on a CD, etc). The
  Reed-Solomon decoder processes each block and
  attempts to correct errors and recover the
  original data. - http//www.4i2i.com/reed_solomon
  _codes.htm

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Physical Layer

• Uses OFDM (with Cyclic Prefix) with 84 equally
  spaced sub-carriers between 4.49 and 20.7MHz
• Uses Differential BPSK or Differential Quadrature
  PSK
• Transmits
• Preamble
• Frame Control
• Payload
• Priority Resolution Signals
• Delimiter Preamble Frame Control
• Before Transmission of Payload
• Negotiating the sub-carriers between sender and
  receiver
• Channel Estimation performed to determine the
  sub-carriers to be used.
• To determine the modulation and forward error
  control rate for these sub-carriers
• Adaptive approach - If certain frequencies suffer
  from interference then sub-carriers within those
  frequencies could be disabled or transmitted
  slower.
• Tone Map
• Used by the sender and receiver
• Lists the carriers that the sender should use
• Used only for the payload
• Frame Control lets the receiver know when it
  should go for a new TM
• Requires a Tone Map Index (TMI) that the intended
  receiver has to use to retrieve the Tone Map (TM)
  to demodulate and decode the payload.

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Physical Protocol Data Unit (PPDU)
Start of frame delimiter
Response (uses all tones)
End of
frame delimiter delimiter 72.0 micro
sec Uses Tone Map
(uses all tones)
(uses all tones)
4 OFDM symbols 25 bits

• Start of Frame Preamble Frame Control
  (25bits)
• Preamble to determine the start of the delimiter
• Payload Variable count of 20 -160 OFDM symbols
• Size determined by the modulation method used and
  coding rate used for transmission
• EFG End of Frame Gap (1.5 micro seconds)
  delay for processing
• Frame Control Check Sequence Cyclic Redundancy
  check for detecting errors in Frame Control

13
MAC Layer

• MPDU MAC Protocol Data Unit (46 1500 bytes)
• Long Frame (Data and Control 313.5 micro sec
  1.4 msec)
• Short Frame (Response 72 micro seconds)
• Segmenting the Payload
• Due to Data Rate Restriction (Imposed by Tone
  Map)
• 160 symbol payload limitation
• CSMA/CA for transmission
• Four Level Priority Access
• Error Control
• Provides for functions to update the channel
  information
• Security.

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MAC Protocol Data Unit
9 bytes 0-M bytes 2 bytes
0 N bytes 4 bytes
Service Block - Unit of Information sent to the
destination within an MPDU Frame
MPDU
4 bits 1 bit 1 bit 2 bits
15 bits 1 bit 6 bits
10 bits
15
Frame Control
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Variant of CSMA/CA

• Uses a variant of the CSMA/CA
• A typical CSMA/CA protocol
• Nodes sense the medium (this is the carrier
  sensing part) for other traffic.
• If the medium is busy, nodes will defer from
  transmitting until the medium becomes idle.
• When the medium becomes idle, nodes will wait for
  a randomly chosen duration (this is the collision
  avoidance part) and will transmit only if it
  detects no other traffic on the medium during
  this randomly chosen duration.
• HomePlug builds on this mechanism by providing
  prioritized access.
• The overall protocol includes
• A carrier sensing mechanism,
• A priority resolution mechanism
• A backoff algorithm.
• Delimiters aid in sensing.
• Physical Carrier Sense (PCS) of Physical Layer
  detects the preamble signal on the medium.
• Virtual Carrier Sense (VCS) within the MAC layer
  is updated based on the information contained
  within the delimiter.
• PCS and VCS information is maintained by the MAC
  to determine the exact state of the medium.

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Prioritize Transmission
Priority Resolution Period
PRS0
PRS1
Contention Window
End of Last Transmission
Delimiter Type No response expected
Contention InterFrame Space 35.9 micro sec
30.72 micro sec

• During the priority resolution period only the
  stations with highest priority may contend for
  the medium.
• Stations use PRS0 and PRS1 to determine the
  priority.
• Four priority levels are used.
• CA3 time sensitive
• CA2 high priority
• CA1 and CA0 for lower priority traffic.
• Contention bit set before the PR period then any
  node with the same or lower priority will defer.
• If CA3 and CA2 nodes assert PRS0 then CA1 and CA0
  nodes will defer.
• If CA3 nodes also assert PRS1 then CA2 nodes
  defer.
• If PRS0 is not active CA1 nodes will assert PRS1
  so CA0 nodes will defer

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Channel Access

• Transmitting a segment
• The station waits if the channel is idle for a
  contention period (35.8 micro sec).
• Transmits PRS0 or PRS1 during the priority
  resolution period.
• If no priority contention then a station picks
  randomly one of the eight contention slots
  following the PRP.
• Sets a Backoff Counter (BC)
• BC is decremented by one every time the medium is
  idle for a slot.
• If the BC reaches zero, the station starts
  transmission if no SOF is detected when this slot
  was selected and then awaits a response if one is
  expected.
• Deferral Counter (DC) is set
• If the station does not transmit when BC reaches
  zero.
• If the medium becomes busy before BC reaches zero
  
• If the DC reaches zero, then since many stations
  with the same priority level are contending for
  the channel, the sender then picks a new value
  for the BC (depending on the priority level of
  the data) and defers transmission as if there was
  a collision.

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BackOff Procedure

• transmission() // Retransmission
• wait to transmit a frame
• if the medium is idle for CIFS then
• if backoff is paused then
• resume_backoff
• else
• start_backoff
• if the station is in backoff and the medium
  gets busy then
• // Verify DC
• if DC 0 then
• stop_backoff
• increment_DC
• increment_CW
• transmission()
• else
• decrement_DC
• pause_backoff
• transmission()
• if backoff finishes and the medium is
  idle then

20
Response/Error Control

• Response received
• ACK or no ACK - The next segment is readied for
  transmission or success is reported to the host
  interface if this was the last segment.
• NACK - Indicates a collision has occurred and the
  station increases the contention window by
  calling the backoff procedure and picking a new
  delay time for the BC.
• If the station has tried the maximum number of
  attempts to deliver the data and it fails then
  the frame is discarded and failure is reported.
• Error Control
• Responses with NACK/FAIL but with valid RFCS
  indicate error.
• NACK is received with a 10 bit RFCS which is
  matched against the 10 LSBs of the FCS. If NACK,
  stations can re-transmit again.
• ACK contains 11bit RFCS (Received Frame Check
  Sequence) which is matched against the 11 LSBs
  of the FCS of the transmitted frame.
• FAIL indicates that there were problems with no
  buffers available or segment received out of
  order. So nodes wait 10msec before trying again.

21
Segment Reassembly

• For each Source Address (SA) and priority a
  receiver maintains the most recent values of
• Source Address SA
• Sequence Number SN
• Segment Control SC
• Last Segment Flag LSF in a list.
• List entries compared to the frame header to
  detect duplicates and omissions.
• List updated whenever new entries have to be
  added or if a frame was received in order.
• If LSF was set then the reassembly of the
  segments are complete and sent for decryption.

22
Security

• Power lines are shared from the transformer to
  the residences.
• Using the transformer it is possible to eavesdrop
  the near by PLC transmission.
• Data is encrypted using the DES.
• Every station will have a default key and a
  Network Encryption Key (NEK).
• All information is encrypted using the same NEK
  within a Logical Network.

23
Current

• HomePlug AV
• Specification complete.
• No products yet manufactured
• 150 Mbps throughput
• Complex Physical and MAC Layer
• Ideal for entertainment oriented networking
• Improved security features - Security based on
  128-bit AES, change of encryption keys.

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Sources

• http//www.iec.org/online/tutorials/ofdm/topic04.h
  tml
• http//www.homeplug.org
• http//www.gta.ufrj.br/ftp/gta/TechReports/miguel/
  CCD05a.pdf