CS 164: Slide Set 3: Chapter 2

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• CS 164 Slide Set 3 Chapter 2
• Direct Link Networks

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What are we looking at ?

• Networks in which there is no routing involved.
• We will look at the physical layer
• signal representation, error correction.
• We will look at the link layer
• Point to Point links
• Multiple-access
• Ethernet, Token Ring

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Network Adaptor and Device Driver

• Network adaptor sits on the Systems I/O and
  delivers data from the workstations memory to
  the network link.
• Device driver is the software module that manages
  this adaptor.
• Issues commands such as from what memory
  location should outgoing data be transmitted,
  where should the incoming data be stored etc.

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Example architecture
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Links

• We have already seen --
• Twisted pair -- phone
• Coaxial cable -- Cable TV
• Optical Fiber
• Free space -- IR etc.

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Leased Lines

• Telephone lines -- long distance
• DS1 or T1 line -- 1.544 Mbps
• DS3 or T3 line --- 44.736 Mbps -- 30 T1 lines.
• T1 lines originally conceived for 24 digital
  voice circuits -- each of 64 Kbps.
• Leased line does not have to be a single unbroken
  piece of fiber -- logical connection.

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Synchronous Transport Signal (STS)

• Optical signals -- also called OC for optical
  carrier.
• We have STS-1 (or OC-1), STS-3, STS-12, STS-24
  and STS-48.
• STS-1 --gt 840 Mbps.

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Last Mile Links

• We talked about this -- ADSL, POTS --(Plain old
  telephone service) etc.

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Signals

• Electromagnetic waves -- travel at speed of light
• Frequency -- rate at which wave oscillates
  (Measured in Hertz).
• Wavelength speed of light/ frequency --
  distance between maxima and minima of a wave.

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The spectrum
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Modulation

• Data is in bits -- we need to somehow translate
  this to signal variations.
• This process is modulation.
• Vary either the amplitude, frequency or phase of
  the signal --- dictated by the bit stream.

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Encoding

• Represent binary data as signals.
• Let us ignore modulation for the moment.
• We have two signals -- high and low for
  representing 0s and 1s.
• signals represent voltages.
• 1 is high voltage, 0 is low voltage
• As an example 5 V and -5 V.

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The NRZ scheme.
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Problem with NRZ

• Receiver keeps an average of the signal received
  so far.
• Compares incoming signal to this average -- if
  significantly higher -- high, if significantly
  lower, then low.
• If too many zeroes or ones, difficult to track
  this average -- the average wanders -- called the
  baseline wander.
• If there are clock drifts between the sender and
  receiver, this cannot be detected -- how many
  bits were transmitted ?

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Other encoding schemes

• NRZI Transition from current signal to encode
  a 1. Stay at the same signal if it is a 0.
• Solves problem with consecutive 1s but not
  zeroes.

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Manchester Encoding

• X-OR the NRZ with a clock
• 0 --gt represented as a low to high transition.
• 1 -- gt represented as a high to low transition.

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Problem with Manchester Encoding

• Doubles the rate of transitions.
• Half the time for the receiver to detect each
  pulse
• Increase in complexity
• Bit rate 1/2 Baud rate for Manchester encoding.
• Note -- baud rate represents signal rate and in
  some cases, bit rate could be higher than baud
  rate -- multiple bits mapped onto a signal.

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4B/5B encoding

• Insert extra bits into bit stream to break long
  sequences of 0s and 1s.
• Specifically every four bits of data encoded into
  a five bit code.
• Codes such that no more than 2 trailing zeroes
  and no more than 1 leading zero. (When codes are
  transmitted back to back no more than 3
  consecutive zeroes.
• Resulting codes transmitted using NRZI.
• Specific codes -- 11111 -- Line idle
• 00000 -- Line dead
• 00100 -- Halt

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Framing

• Blocks of data (each consisting of bits)
  exchanged between nodes that form a link-- these
  blocks are called frames.
• Network adaptor allows exchange of frames.

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Point to Point Links

• Link Layer protocols are used.
• Popular Link layer protocols are BISYNC, PPP and
  DDCMP.
• The protocols deal with the transfer of frames
  across point-to-point links.

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BISYNC

• Binary Synchronous Communication (IBM).

• STX and ETX denote start of text and end of
  text.
• If ETX appears in body of message, precede with
  DLE (Data Link Escape).
• If DLE appears precede with another DLE.
• Called Character Stuffing

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DDMCP

• Digital Data Communication Message Protocol
• Instead of indicating end of text, frame length
  specified by a count.

• The danger is that count could get corrupted.
• Similar ETX in BISYNC could get corrupted.
• Framing errors could result -- error correction/
  retransmissions

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PPP

• Point to Point protocol -- most popular
• Commonly run over dial up modem links.
• Can be used over Ethernet too --gt Look for RFC.

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PPP Frame
Usually contain default values
Used for demultiplexing

• The protocol field identifies the higher layer
  protocol -- typically IP.
• Payload default is 1500 bytes but negotiable.

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LCP -- Link Control Protocol

• Initial PPP set up.
• Can be used to negotiate payload size.
• LCP messages encapsulated into PPP frames.
• Protocol field set to LCP

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LCP Operations

• Send Link
• Configuration Options
• Set Protocol LCP
• Configure req/ack
• exchange

carrier detection or indication that PHY layer is
present by system admin.
Dead
Link Establishment
Authenticate
Terminating
Open
Network Layer Conf
Configure Network Layer Exchange IP addr.
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Bit Oriented Protocols

• Previous protocols were byte oriented.
• Bit oriented protocols not concerned with byte
  boundaries.
• HDLC is an example.

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HDLC

• High level data link control

• Both beginning and ending by a distinguished
  bit sequence.
• If sequence appears in text use character
  stuffing
• Analog of DLE -- bit stuffing -- upon 5
  consecutive 1s, inserts a zero.

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SONET

• Synchronous Optical Network Standard developed
  for transmission over fiber.
• SONET frame has special information that tells
  receiver where frame starts and ends -- no bit
  stuffing.

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SONET or STS-1 Frames

• First three bytes overhead
• Total 810 bytes.
• 2 byte pattern at beginning -- receiver has to
  see this every 810 bytes -- if it does it
  concludes that it is in sync.

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Multiplexing with SONET

• Time is the same but now, a different amount of
  data is transmitted.
• In each STS-3 frame we have 3x810 2430 bytes.
• Interleave bytes --1st byte of first STS-1, 1st
  byte of 2nd STS-1 and so on.
• Ensures that bytes from each STS-1 are evenly
  paced and arrive at a smooth 51 Mbps rate at
  receiver.
• Bottomline STS-3 channel could contain multiple
  low-data rate STS-1 channel.

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STS-Nc frames.

• Payloads linked together instead of interleaving.
• Concatenation instead of multiplexing.
• Cannot be multiplexed from different streams.
• Called STS-Nc (As an example STS-3c).
• One of the fields in header used to denote
  concatenation.
• Read rest of stuff on SONET from book.

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Error Detection

• How does one deal with bit errors ?
• Simplest thing to do -- send two copies of each
  bit.
• If copies match, then data ok, else in error.
• Too much redundancy --in most cases errors not
  that frequent (especially on fiber and coax).
• However, some redundancy will be needed.

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Adding parity bits

• We can add parity bits.
• One dimensional parity -- One extra parity bit
  added to a 7 bit code to balance the number of
  1s.
• If number of 1s is odd, parity bit is odd.
• Else it is even.
• If one of the bits gets corrupted, then it can be
  detected.
• Multiple errors cannot be (.

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Two Dimensional Parity

• Do a similar thing across frames in addition to
  along bytes!
• Catches 1 bit, 2 bit, 3 bit errors and most 4 bit
  errors.
• How? -- both row and column parities are affected
  by a bit error.

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Internet Checksum

• Simple algorithm to compute a checksum.
• Take all words -- add up and transmit result of
  the sum using 1s complement arithmetic.
• Smaller number of error protection bits but less
  protection.

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Summary

• We have so far seen
• What is modulation ?
• Encoding schemes.
• Link layer protocols --BISYNC, PPP, DDMCP, SONET,
  HDLC.
• Error detection using parity
• Internet Checksum.
• Sections 2.1 to 2.4.2

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Next

• CRC --cyclic redundancy check.
• Retransmissions for reliable transmission.
• Multiple access channels.

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