Packets, Frames, and Error Detection

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Packets, Frames, and Error Detection

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Packets

• Most computers break network traffic into small
  packages called packets
• Called packet switched networks
• Division helps
• Share bandwidth between computers
• Reduce amount of information affect by errors

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Packets Continued

• Other approach use for as long as desired
• Creates delays of unknown length
• Not fair
• Networks use TDM on a per-packet basis

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Format

• No universal agreement on what packets look like
• Book uses term frame for packet on a specific
  hardware type of network
• RS-232 specifies individual characters, not how
  related characters are combined
• Not start and end

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Format Continued

• Sender and receiver need to agree when frame
  starts and ends
• If text on RS-232, use non-text ASCII to demark
  start and end of transmission
• Example SOH (0x01) and EOT (0x04) for frame
  boundaries (ESC is 0x1B)

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SOH and EOT
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Format Continued

• As systems can crash and characters can be
  garbled, this gives a technique for detecting
  framing errors
• Receiver looks at combinations of SOH and EOT

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Stuff It

• What if transmission is not text
• Instead binary (pictures, document, etc.)
• May contain SOH and EOT confusion!
• Sender must mark frame delimiters inside data to
  differentiate them
• Receiver must know convention and reverse it

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Stuff It Continued

• Data stuffing occurs when framing information is
  sent in data
• Byte or character stuff for character-oriented
  transmission
• Bit stuffing for bit-oriented transmission

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Character Stuffing
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Character Stuffing Example
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Errors

• Small changes in electrical signals will cause
  the incorrect value to be received
• Cause transmission errors
• Detection and correction of errors adds a lot of
  complexity to computer networks
• General approach send additional information
  that describes the data being sent

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Errors Continued

• RS-232 sends one additional bit before each
  character, call parity bit
• Can use even or odd parity
• Final bit is set by sender so that total number
  of 1 bits is either even or odd
• Receiver checks to make sure count correct

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Parity

• For 01010111 and even parity a 1 bit is added
• For odd, a 0 bit is added
• Works for odd number of bit errors
• Doesnt for even numbers

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Differences

• Size of additional information
• Computation complexity
• Speed
• The number of bit errors detected and corrected

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Checksums

• Sender treats sent data a sequence of numbers and
  adds them
• Appends sum to data
• Receiver performs sum and checks
• Fast to compute and small
• Does not detect some common errors

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Checksum Picture
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Checksum Failure
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Cyclic Redundancy Codes

• Combination of shift register and exclusive ors

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Exclusive OR
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Shift Register
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Combination
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CRCs Continued

• Detect more errors than checksums
• Each bit traverses multiple shift registers
• Each bit traverses entire set multiple times
• Affects final value a lot
• P(X) X16 X12 X5 1

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Frame with CRC

• Error detection information sent with each frame