Packets_ Frames and Error Detection by yurtgc548


									          Packets, Frames and Error
             Based on Chapter 7 in Computer
             Networks and Internets (Comer,
                     Third Edition)

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            Modem error-checking
• Data is chunked into "blocks" of a certain byte
  size and sent to the destination modem.
• The destination modem checks each block for
  errors and returns
     – ACK if no errors are found
     – NAK if errors are found, which leads to a
• The kind of checking (checksum or cyclic
  redundancy checking) varies from protocol to
CSIT 220 (Blum)                                     2

CSIT 220 (Blum)           3
                  Error Detection
• If certain timing restrictions of the protocol are not
  met, the receiver may not be able to interpret the
  signal that arrived as binary data
     – Recall that in the RS232 protocol two 1’s is
       distinguished from one 1 by the length of time the
       signal is at –15 V
• But other problems can yield a signal which can
  be interpreted as binary data — but not the binary
  data sent.
• For this reason, protocols include error detection
  schemes (like parity, checksums or cyclic
  redundancy checks) to validate the data received.
CSIT 220 (Blum)                                             4
Error Detection (Cont.)
• Error detection schemes involve
  transmitting additional information which
  the receiver can use to validate the data.
• Error detection can inform the receiver of
  the presence of an error but cannot fix the
  error, the package must be resent.
• Error correction schemes (which are
  distinct) attempt to pinpoint the error.

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• Parity is a simple error detection scheme.
• The transmitter and receiver agree ahead of
  time on the type of parity they will use
  (even or odd). Let us assume even.
• The transmitter takes a string of binary data
  and calculates whether the number of 1’s in
  the string is even or odd.

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                    Parity (Cont.)
• The transmitter includes an extra bit:
     – 0 if the number of data-bit 1’s is even;
     – 1 if the number of data-bits 1’s is odd.
• The string consisting of the data plus parity bit is
  now guaranteed to have an even number of 1’s.
• The receiver checks that the data plus parity bit
  have even parity (an even number of 1’s).
• If the data received has odd parity, some
  transmission error must have occurred.

CSIT 220 (Blum)                                          7
                          Parity (Cont.)

       D          D   D    D     D      D    D    D   P
        0         0   0     1    1      1     0   0   1
        0         0   1     0    1      0     0   0   0
        0         1   0     1    1      1     0   0   ?
        1         1   1     1    0      0     0   1   ?
                      D – data       P – parity
CSIT 220 (Blum)                                           8
                  Parity (Cont.)
• (You may read that the number of 1’s is
  counted, this is not true and is unnecessary
• A multi-input XOR (excluded OR) gate will
  determine the parity bit, this circuitry is
  much simpler than what is required for
  counting the number of 1’s.

CSIT 220 (Blum)                              9
    2-input and 3-input XOR Truth Tables
  A        B       Output   A   B   C   Output
   0       0         0      0   0   0     0
   0       1         1      0   0   1     1
   1       0         1      0   1   0     1
   1       1         0      0   1   1     0
                            1   0   0     1
Output is true when         1   0   1     0
number of inputs is odd.    1   1   0     0
                            1   1   1     1

 CSIT 220 (Blum)                                 10
                  Parity (Cont.)
• Parity is used when data is moved around
  within computers as well as when data is
  sent from computer to computer.
• Modems use parity. Users may also select
  no parity, meaning that the modems will not
  include the parity bit. Sending fewer bits
  speeds up transmission.

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• The transmission string of data bits is broken up
  into units, for instance of consisting of 16 bits
• These 16-bit numbers are then added (any carries,
  if the sum is larger than a 16-bit number, are
  added to the sum).
• The sum is sent along as part of the frame’s trailer
• The receiver repeats the calculation. If the sums
  match, it's assumed that no transmission error
CSIT 220 (Blum)                                      12
                  Checksum (Cont.)
• Actually instead of including the sum in the
  trailer, one can transmit instead its negative.
• The receiver then repeats the sum over the
  data and includes in the summation the
  negative of the sum sent in the trailer.
• The result should be zero. It is easier to test
  for a result of all zeros.

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                  Catching errors
• Since the 16-bit string might end up in any
  of 216 (65536) possible states, you might
  assume that the checksum would catch
  errors 65535/65536  99.9985% of the time.
• But this would assume that all possible
  errors are equally likely, which is not true.

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                  Burst errors
• For multi-bit errors, it is more likely that a
  group of consecutive bits are affected as
  opposed to randomly selected bits, such an
  error is known as a burst error.
• It is also common for periodic errors to
  occur (e.g. the first two bits in every byte).

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• Cyclic Redundancy Check is better at
  catching burst errors than checksums.
• The idea is the same, perform some
  mathematical operation on the data, send
  the result, have the receiver do the same
  calculation and check that the same answer
  is obtained.

CSIT 220 (Blum)                                16
                  CRC (Cont.)
• In CRC the operation is somewhat like division
   – If one thinks of the data as a large number, one
     can divide it by another number N giving a
     whole number answer and a remainder.
   – The remainder could be any number between 0
     and N-1.
   – The problem here is that division is a fairly
     difficult computation (more circuitry and a slow

CSIT 220 (Blum)                                    17
                  CRC (Cont.)
• CRC uses a variation on division that while
  mathematically abstract is very simple to
  build a circuit for.
• The circuitry needs only a shift register and
  some (2-input) XOR gates.

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                  Shift Register
• A register is a small piece of memory that
  holds values.
• In addition to holding values, a shift register
  performs a simple operation on the values;
  it moves them to the left to to the right.

CSIT 220 (Blum)                                 19
                       Shift Register
   0               1   0     1     0        1   1    1

   1               0   1     0     1        1   1    0

   0               1   0     1     1        1   0    1

   1               0   1     1     1        0   1    0
Output                     Shift register           Input
 CSIT 220 (Blum)                                            20
      How shift registers are used?
• Multiplication
• Adding floats
• Converting Parallel Data (the form inside
  the computer) to Serial (the form sent over
  transmission lines)
• Cyclic Redundancy Check (CRC)

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       CRC: Shift register + XORs

             Excluded Or gates

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                  CRC (Cont.)
• Again the 16-bit string might end up in any of 216
  (65536) possible states, so you might expect that
  CRC would catch errors 65535/65536  99.9985%
  of the time, same as a 16-bit checksum.
• But CRC is better at detecting burst errors which
  are more likely than purely random errors.
• The positions of the XORs are important in
  determining what kinds of burst errors are
CSIT 220 (Blum)                                   23
CRC: Transmission Only
• The data must be serialized for a Cyclic
  Redundancy check.
• This is fine for transmission error checking,
  since the data was serialized for
• However, serializing the data would waste a
  lot of time if the data were in a parallel form
  (as it is inside the computer).

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• The word “topology” is used here to convey
  that we are not interested in the spatial
  arrangement of the computers (where they
  are located) but rather in how they are
    – E.g. Is Computer A directly connected to
      Computer B?

CSIT 220 (Blum)                                  25
           Topologically Equivalent

                   B           B



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• One way to organize a network would be to
  have a connection between every pair of
• Known as
     – Point-to-point network
     – Mesh network
     – Fully connected network

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• This is fine for small networks, in fact it can
     – Freedom in hardware choice
     – Freedom in protocol choice
           • Each pair can choose a protocol
     – Security (communication channels are not
     – Robustness (one broken cable does not bring
       down the network)
CSIT 220 (Blum)                                      28
Point-to-Point (Cont.)
• The problem with this approach is that it
  does not scale.
• Scaling refers to changing the size of
• Something is said “to scale” if the change in
  size requires a minimal amount of

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CSIT 220 (Blum)                    30
  Number of Connections in a Point-
         to-Point Network
        Number of Computers   Number of connections
                  2                     1
                  3                     3
                  4                     6
                  5                     10
                  6                     15
                  7                     21
                  8                     28
CSIT 220 (Blum)                                       31
                  Adding another
• In the point-to-point approach, adding a 9th
  computer to a network of eight requires
  eight new connections, the new computer
  must be connected to the eight old
• The computers will soon run out of ports
  (slots/cards) for these connections and there
  will be a massive tangle of wire.
CSIT 220 (Blum)                               32
                  Star Topology
• In the star topology, the computers are
  connected to a hub.
• Generally, a “hub” is the central part of a
  wheel, where the spokes come together (the
  part covered by a hubcap).
• A hub is a device that computers are
  connected to so they can communicate.

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            Star Topology (Fig. 8.3)

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    Computers are not necessarily
    arranged in a circle (topology)


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                  Star Topology (Cont.)
• The star topology is scalable, physically
  adding a computer to a network requires
  only connecting it to the hub.
• It is also robust, losing one connection does
  not affect the entire network.
• (Losing power to the hub is another story.)

CSIT 220 (Blum)                               36
                  Ring Topology
• Each computer in the network is connected
  to two others.
• The connections form the perimeter of a
• If one followed the connections around, one
  would visit each computer once and only
  once before completing the round trip.

CSIT 220 (Blum)                             37
           Ring Topology (Fig. 8.4)

CSIT 220 (Blum)                       38
                  Additions to the ring
• To add a computer to a pre-existing ring,
  one of the previous connections must be
  broken, and two new connections added.
• The ring topology is not very robust, if one
  connection is disrupted, then the ring is
• The connection to the ring is designed so
  that turning off a computer does not break
  the ring.
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                  Bus topology
• The individual computers connect to a
  common transmission path called the bus
  (a.k.a. the backbone).
• Ideally, the information is placed on the bus
  and is only read by the intended receiver.
• Much of the information moving around
  inside a computer travels on buses.

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             Bus Topology (Fig 8.5)

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                  Bus Topology (Cont.)
• To add a computer to such a network, the
  computer must be connected to the bus.
• If a connection from a computer to bus is broken,
  it does not necessarily affect the rest of the
• If the bus connection is broken, the network is
  divided into two parts.
     – Actually it is worse than that, buses carry signals,
       which are waves. And waves tend to reflect off of
       surfaces (ends). The ends of the bus must be treated to
       prevent the undesired reflection.
CSIT 220 (Blum)                                                  42
• A fairly standard way to connect computers is
     – It originally used the bus topology.
• Strictly speaking Ethernet is the predecessor to the
  IEEE’s 802.3 CSMA/CD local area network
• There are a few important differences; however,
  the latter has also come to be known as Ethernet.

CSIT 220 (Blum)                                      43
       “Ether” (Origin of the term)
• Water and sound waves require a medium,
  some material through which the
  disturbance moves
• Originally it was thought that
  electromagnetic waves (light included) must
  also have a medium, it was called “the

CSIT 220 (Blum)                             44
           Ether (Origin of the term)
• Searches for the ether
  (esp. by Michelson and
  Morley) could not find it.
    – The first two computers on
      an ethernet were two Altos
      called Michelson and
• The absence of the ether
  was important in
  Einstein’s development of
  relativity theory.

 CSIT 220 (Blum)                        45
                  ALOHA net
• Predecessor to Ethernet
• A radio-based communications system
  connecting the Hawaiian islands, invented
  by Norm Abramson.
• The first wireless packet-switching network.

CSIT 220 (Blum)                             46
                   ALOHA net (Cont.)
• Occasionally two packets would be transmitted
  simultaneously, they would interfere with each other
  and the information would be corrupted.
• ALOHAnet had a protocol, in which the receiver
  would perform an error check on each packet and
  transmit an acknowledgment if the packet arrived
  error free.
• If the transmitting computer did not receive such an
  acknowledgment, it would retransmit after waiting a
  random amount of time.
 CSIT 220 (Blum)                                    47
              Improving the protocol
• Bob Metcalfe improved upon this protocol.
     – Metcalfe (MIT, Xerox) eventually started 3Com:
       Computer Communication Compatibility.
• Before sending a packet, the transmitter “listens”
  for any traffic on the line and only transmits if
  there is none.
• Of course, two distance transmitters might “sense”
  a quiet or idle carrier and decide to transmit
  packets simultaneously or nearly simultaneously

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CSIT 220 (Blum)                49
 Collision: If a body meet a body
• If two packets are sent on an Ethernet
  simultaneously, it is known as a “collision.”
• When a collision is detected, the packets involved
  are discarded.
• Each transmitter waits a random amount of time
  chosen from a certain interval and retransmits.
• If a second collision occurs, the process is
  repeated but the interval is doubled in size.
• This process, known as binary exponential
  backoff, makes it very unlikely the packets will
  continue to collide.
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• This protocol goes by the name Carrier
  Sense Multiple Access/Collision Detect
• Carrier Sense: waits for line to be idle.
• Multiple Access: various nodes share a
  common transmission line.
• Collision Detect: simultaneously
  transmitted packets are detected, discarded
  and retransmitted.
CSIT 220 (Blum)                                 51
                  Ethernet (Cont.)
• What has come to be called Ethernet is the
  technology most often used for LANs.
• It uses a bus topology.
     – More about logical topology versus physical topology
       another time
• Ethernet was originally developed by Xerox and
  then extended by DEC, Intel and Xerox (DIX
• It was then specified in a standard, IEEE 802.3.

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• Institute of Electrical and Electronics
• Self-described as “the world's largest
  technical professional society — promoting
  the development and application of
  electrotechnology and allied sciences for the
  benefit of humanity, the advancement of the
  profession, and the well-being of our
CSIT 220 (Blum)                              53
                  IEEE (Cont.)
• It is a professional organization for electrical and
  electronic engineers.
• They hold meetings, publish journals and so on.
• Their importance here is that they also develop
  and promote standards, both national and
• The importance of standards cannot be over-

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                  Manchester encoding
• One aspect of the standard is the way in which bits
  are represented.
• Recall in the RS232 standard, the bit value is
  represented by the voltage’s level (high for 0, low
  for 1).
• But Ethernet uses Manchester encoding in which a
  rising voltage (in the middle of a time interval)
  corresponds to a 1 and a falling voltage
  corresponds to a 0.
• The hardware is said to be “edge triggered” — it
  detects changes more easily than levels.
CSIT 220 (Blum)                                    55
• Pro: Because the signal is guaranteed to vary at
  least once each interval, it helps in maintaining the
  timing between transmitter and receiver.
• Pro: Because the important part of the signal is in
  the middle of the interval, other parts of the signal
  can effectively be ignored and noise during those
  times is irrelevant.
• Con: The signal has to be able to change at a rate
  twice as fast as the information rate (so one’s not
  using the full capacity of the channel).

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                  Manchester encoding
                    One interval

      Rising in middle of interval

CSIT 220 (Blum)                         57
                  Wiring an Ethernet
• A LAN using Ethernet generally uses
  coaxial cable or special grades of twisted
  pair wires.
• The most commonly installed Ethernet
  systems today are called 10BASE-T and
  provide transmission speeds up to 10 Mbps.
     – M stands for mega (1,000,000)

CSIT 220 (Blum)                            58
                  Ethernet (Cont.)
• Fast Ethernet or 100BASE-T provides
  transmission speeds up to 100 megabits per
  second and is typically used for LAN backbone
  systems, supporting workstations with 10BASE-T
• Gigabit Ethernet provides an even higher level of
  backbone support at 1000 megabits per second (1
  gigabit or 1 billion bits per second).

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• One of several physical media standardized
  for use in Ethernet LANs.
• Is somewhat like ordinary telephone twisted
  pair, but has more twists because the signals
  are more sensitive to interference.
• Supports 10 Mbps transmission speed.

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• The "10" in the media type designation
  refers to the transmission speed of 10 Mbps.
• The "BASE" refers to baseband signaling,
  which means that only Ethernet signals are
  carried on the medium.
• The "T" represents twisted-pair.

CSIT 220 (Blum)                              61
                  Token Ring Network
• A LAN organized in the ring (or possibly
  star) topology.
• The protocol used is a “token-passing”
• The token gives a transmitter permission to
  transmit, thereby preventing packet

CSIT 220 (Blum)                                 62
                  Token ring protocol
•      A special frame called the token is passed
       around the ring, a given node is allowed to
       transmit when it “has the token.”
•      If a node has no data to transmit, it simply
       passes the token on to the next computer.
•      If it has data to transmit, it inserts that data into
       the frame along with address information and
       other protocol information and then alters the
       frame so that the next computer does not have
       permission to transmit.
CSIT 220 (Blum)                                                63
Token ring protocol (Cont.)
• If a node receives a frame with data, it looks at the
  address information. If the destination does not
  match the computer’s address, it simply passes the
  frame to the adjacent node. If the destination
  matches the computer’s address, it copies the data,
  but also passes it on (perhaps adding some
• The frame eventually reaches its point of origin,
  the original transmitter removes the packet and
  puts a token frame onto the ring, giving
  permission to the next node to transmit.
CSIT 220 (Blum)                                       64
                  Token ring (Cont.)
•      Tokens can also be used on bus-organized
•      The standard token ring protocol IEEE
•      The Fiber Distributed-Data Interface
       (FDDI) also uses a token ring protocol.

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                  Other references
• Understanding Data Communications and
  Networks, William Shay
• Computer Dictionary, Mitchell Shnier

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