CEN 4500 Network Fundamentals

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CEN 4500 - Network Fundamentals

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• Part 3
• Packets, Frames, Parity, Checksums CRCs

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Networking

• Point to point communication rarely practical
• Devices are too far apart
• Large set of devices would need impractical
  number of connections
• Solution is for many senders and receivers to
  share a singular, common communications medium
  a network

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Simplified Network Model
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The Problem With Sharing

• Large-demand applications (i.e., a distributed
  database app) can deny network access to other
  applications
• Other applications (i.e., assembly-line control
  program) simply cannot wait
• Required a mechanism to ensure fair access to
  the network...

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Packet Switching

• Developed in early 70s to take advantage of
  digital datas properties
• Basic technology same today as then
• One of only a few effective technologies for
  long-distance data communications (WANs)

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Packet Switching

• Divides outgoing data into small units called
  packets
• Allow each station to send 1, and only 1 packet
  at a time, and then allow another station to send
  a packet
• A form of time-division multiplexing

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Packet Switching

• Data transmitted in small packets
• Typically 1000 bytes
• Longer messages split into series of packets
• Each packet contains a portion of user data plus
  some control info
• Control info
• Routing (addressing) info about sender and
  receiver
• More on this later

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Packet Switching Advantages

• Line efficiency
• Single station to station link can be shared by
  many packets over time
• Packets queued and transmitted as fast as
  possible
• Data rate conversion
• Each station connects to the network node at its
  own speed
• Nodes buffer data if required to equalize rates

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Packet Switching Advantages

• Packets are accepted even when network is busy
• Delivery may slow down
• Packet priorities can be used

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Packet Switching
Packet Switching using Time-Division Multiplexing
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Packets Frames

• As there is no universally accepted format of a
  packet, each hardware technology
• defines the configuration of packets that can be
  transferred using that hardware
• specifies how individual bits of the packet are
  to be transmitted
• To distinguish between the conceptual idea of a
  packet and the real-world implementation on a
  specific network type, we refer to
  hardware-specific packets as frames

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Packets Frames

• What Framing Does
• The senders hardware device takes data from an
  application and breaks it down first into
  discreet frames, then into a raw bit stream
• The receivers hardware takes the raw bit stream
  and reconstructs discrete frames
• The receivers hardware then checks frames for
  errors

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Framing

• Framing Constructs
• Whats needed is a method to distinguish between
  control information such as the beginning or end
  of a frame and the data contained therein
• Many methods tried for framing
• Character Counting
• Character (or Byte) Stuffing
• Bit Stuffing
• Well look at Character Stuffing

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Framing

• Character Stuffing
• Each frame starts with the ASCII sequence DLESTX
  (Data-Link Escape, Start of TeXt)
• Each frame ends with the ASCII sequence DLEETX
  (End of TeXt)
• As long as the transmitted data is character
  based, this ensures resynchronization because
  both sender and receiver can identify frame
  boundaries

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Framing

• Character Stuffing
• Problems arise when binary data is sent the
  DLESTX or DLEETX character sequence may occur
  within the data, resulting in loss of
  synchronization

A
B
C
DLE
ETX
Input Data
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Framing

• Character Stuffing
• To counter this possibility, the senders
  hardware inserts a second DLE escape-character
  in front of the existing DLE character
• but not before the true end-of-frame DLE

Stuffed DLE Character
A
B
C
DLE
ETX
DLE
DLE
ETX
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Framing

• Character Stuffing
• The receivers hardware ignores the two-DLE
  escape sequence and strips out the redundant DLE
  character, reverting the data back to its
  original state

A
B
C
DLE
ETX
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Error Handling

• Data can be corrupted during transmis-sion by
• bits lost
• bit values changed
• Frames include additional information to detect
  and correct errors
• Inserted by sender
• Verified by receiver

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Error Handling

• Parity Bits Parity Checking
• Two methods, essentially the same even parity or
  odd parity
• Even parity means sender will set parity bit to
  ensure an even number of 1s are sent per
  character
• ie, 0100101 is sent as 01001011 (4 1s)
• Sender and receiver must agree on which method
  used

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Error Handling

• Sender computes whether a 1 or 0 needed as parity
  bit
• Upon reception, receiver strips out the parity
  bit and recomputes what the parity bit should be
• If receivers result doesnt match with the sent
  parity bits value, character discarded and an
  error reported

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Error Handling

• Checksums
• Sender treats data as a sequence of integers and
  computes their sum
• This value is included within the frame
• The receiver also adds the received data as well
• If receivers sum doesnt match senders value,
  frame is considered corrupted

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Error Handling

• Cyclic Redundancy Checks (CRC)
• Preferred method of error handling
• Essentially, every bit of a message is passed
  simultaneously through 3 shift registers one
  4-bit, one 5-bit and one 7-bit register
• The values are exclusively ORd
• When all bits of the message have been processed,
  the registers contain the 16-bit CRC and this is
  included within the frame
• The receiver does the exact same calculations,
  and if the CRC sum doesnt match, the frames
  considered corrupted

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Frame Format

• So, with all this extra stuff added to the data
  to expedite the transmission of a frame, a frame
  ends up looking like this

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Required Reading

• Comer, Chapter 7