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   Chapter 2

Direct Link Networks




                       2
      Network Technologies
1. Point-to-Point Links
2. Carrier Sense Multiple Access ( CSMA)
   – (for example the Ethernet)
3. Token Rings – (for example IEEE 802.5
   and FDDI )
4. Wireless – (for which 802.11 is the
   emerging standard)

                                           3
                 Problems
Connecting computers is a first step.
There are additional problems to solve before
  they can exchange packets:
• Encoding bits into the transmission medium
• Framing the bits so they can be understood
• Error detection
• Reliable delivery, in spite of occasional errors
• Media access control
                                               4
      Hardware Building Blocks
• Networks are constructed from nodes and links
• Nodes are general purpose computers such
  as workstations, multiprocessors or PCs as
  well as special purpose switches, routers.
  – Memory – finite – must be managed
  – Network Adapter (NIC) and its device driver
• Links implemented on physical media, such
  as twisted pair, coaxial cable, optical fiber
                                                  5
               Nodes

  CPU




                       Network     (To network)
 Cache
                       adaptor



             I/O bus
 Memory




Example workstation architecture
                                                  6
                    Links
• Physical media are used to propagate
  signals as electromagnetic waves, traveling
  at the speed of light.
• Properties of EM waves:
  – Frequency- or oscillations, measured in hertz
  – Wavelength – distance between adjacent
    maxima and minima, measured in meters


                                                    7
       Electromagnetic Waves
• Wavelength = speed / frequency
• Voice grade phone lines carry waves ranging
  from 300 Hz to 3300 Hz
• Voice-grade example: 300Hz in copper wire
• Wavelength = Speed in Copper/ Frequency
             = 2/3 x 3 x 108 /300
             = 667 x 103 meters

                                                8
               Electromagnetic Spectrum
           0        2        4          6        8        10        12        14         16        18           20        22        24
f(Hz) 10       10       10         10       10       10        10          10       10        10           10        10        10

                                 Radio      Microwav e         Inf rared           UV              X ray                  Gamma ray



      4         5            6      7        8        9          10         11          12     13          14         15        16
    10         10       10         10       10       10        10        10        10         10        10           10        10

                                                      Satellite                                         Fiber optics
                                 Coax

                        AM                  FM            Terrestrial microwav e
                                             TV




                                                                                                                                         9
                    Links
• A link is a physical medium carrying signals
  in the form of electromagnetic waves.
• Binary data is encoded in the signal.
  – Lower layer is concerned with modulation,
    varying the frequency, amplitude or phase of the
    signal
  – Upper layer is concerned with encoding the data


                                                  10
               Link Attributes
• Another link attribute is how many bit streams
  can be encoded on it, at a given time.
• One bit stream- connected nodes share access
• Point-to-point – often two bit streams at once
  – Full duplex - two directions – simultaneously
  – Half duplex – one direction at a time
  – Simplex – one direction

                                                    11
                    Cables
• Type of cable depends on technology
• Coaxial – ( thick and thin) – within buildings
• Category 5 ( CAT 5) – twisted pair, thicker
  gauge than telephone wire
• Fiber –plastic or most often glass, more
  expensive, but used to connect buildings, and
  transmits light instead of electrical waves.

                                               12
         Local Link Cables
Cable           Typical       Distances
                Bandwidth
Cat 5 twisted   10-100 Mbps   100 m
pair
Thin-net coax   10-100 Mbps   200 m
Thick-net coax 10-100 Mbps    500 m
Multimode       100 Mbps      2 km
fiber
Single-mode     100- 240 Mbps 40 km
fiber                                     13
                   Leased Lines
• To connect nodes on opposite sides of the country, or
  at great distances, you must lease a dedicated line from
  the telephone company.
• DS1, DS3, T1, and T3 are relatively old technologies,
  defined for copper
• STS-N links are for optical fiber (Synchronous
  Transport Signal), also called OC-N for Optical Carrier
• Originally designed for voice, today can carry data,
  voice and video
                                                       14
  Common Bandwidths
Services    Bandwidth
DS1 or T1   1.544 Mbps
DS3 or T3   44.736 Mbps
STS-1       51.840 Mbps
STS-3       155.250 Mbps
STS-12      622.080 Mbps
STS-48      2488320 Gbps
STS-192     155.250 Mbps

                           15
                Last-Mile links
• Leased lines range in price from $1000/month to “don‟t
  ask”
• Last mile links span the last mile from the network
  service provider to the home or office.
• Conventional modem- POTS (plain old telephone
  service)
• ISDN – (Integrated Services Digital Network) – uses
  CODEC ( coder/decoder) to encode analog to digital
  signal
• xDSL (Digital Subscriber Line)
• Cable modem- uses cable television (CATV)
  infrastructure, available to 95% of US households 16
 Common Available Services
Services      Bandwidth

POTS          28.8 - 56 Kbps

ISDN          64 – 128 Kbps

xDSL          16 Kbps – 52.2 Mbps

CATV          20 –40 Mbps


                                    17
                     xDSL
• Collection of technologies, able to transmit data
  at high speeds over standard twisted pair lines
• ASDL ( Asymmetric Digital Subscriber Line)-
  different speeds in different directions (upstream
  and downstream) – called local loop
• VDSL- (Very high rate Digital Subscriber Line)-
  runs over shorter distances – “fiber to
  neighborhood”

                                                18
             ADSL

            1.554─8.448 Mbps   downstream
              16─ 640 Kbps
Central                        Subscriber
 office        Local loop       premises


 upstream

 ADSL connects the subscriber to the
 central office via the local loop.


                                            19
                                    VDSL

          STS-
             N                                VDSL at 12.96─ 55.2 Mbps
Central                Neighborhood optical                                   Subscriber
 office   over fiber       network unit               ─
                                              over 1000 4500 feet of copper    premises




 VDSL connects the subscriber to the optical network that
 reaches the neighborhood.




                                                                                    20
           Shannon‟s Theorem
• Shannon‟s theorem gives an upper bound to the
  capacity of a link, in terms of bits per second.
                 C = B log2 (1+S/N)
  where C is channel capacity, B is Bandwidth, S is
  signal power, N is noise and S/N is the signal to
  noise ratio expressed in decibels, related as:
            dB= 10 x log10 (S/N)

                                               21
        Shannon‟s Theorem
             Example
• dB ratio pf 30 dB
• S/N = 1000
• Bandwidth = 3000Hz
     C = B x log2 ( 1+S/N)
     C = 3000 x log2 (1001)
     C = 30 Kbps
        = roughly the limit of a 28.8 modem
How are 56 Kbps modems possible? See p. 76
                                          22
                      CATV
• A subset of CATV channels are made available for
  transmitting digital data
• A single CATV channel has a bandwidth of 6 MHz
• Like ADSL, CATV is asymmetric with
  downstream rates much greater than upstream
  – 40 Mbps downstream ( 100 Mbps max)
  – 20 Mbps upstream ( roughly half as much)
• Unlike DSL, bandwidth is shared among all
  subscribers in a neighborhood.
                                               23
            Network Adaptor
                       Signalling component

                              Signal
     Node    Adaptor                          Adaptor   Node
                               Bits




Signals travel between signaling components.
Bits flow between adaptors.
Network interface cards are called NICs.
                                                               24
             Network Adaptors
• Nearly all the functions in this chapter are
  implemented in the network adaptor (NIC):
  framing, error detection and the media access
  protocol.
The exceptions are the point-to-point automatic
  repeat-request schemes(ARQ), which are
  implemented at the lowest level protocol running
  on the host.

                                               25
Block Diagram of a Network
         Adaptor


                                  Network link
           Bus         Link
        interf ace   interf ace


       Adaptor




                                                 26
                   Interrupts
• The host only pays attention to the network device
  when the adaptor interrupts the host, (for example,
  when a frame has been transmitted or one arrives).
• A procedure is invoked by the operating system,
  and an interrupt handler is invoked to take the
  appropriate action.
• While servicing this interrupt, the OS disables
  other interrupts.

                                                    27
        Direct Memory Access vs.
            Programmed I/O
• There are two ways to transfer the bytes from the
  frame between the adaptor and host memory:
• Direct Memory Access (DMA)- the NIC directly
  reads/writes to the host‟s memory without CPU
  involvement, using a pair of buffer descriptor lists.
• Programmed I/O (PIO)- network adaptor (NIC)
  copies message into its own buffer, until CPU can
  copy it into the host memory.

                                                  28
 Programmed I/O


              CPU


Memory




   Adaptor   Memory
             Memory

                    Host




                           29
             Memory Bottleneck
• Host memory is often a limiting factor in network
  performance.
• I/O bus speed corresponds to its peak bandwidth (bus
  width x clock speed).
• Real limitation is the size of the data block being
  transferred ( See p. 145)
• Memory/CPU bandwidth is same as bandwidth of I/O
  bus.
• Must be aware of limits memory puts on network

                                                    30
Memory Bandwidth on Modern
           PC
                                            I/O bus
          235 Mbps              1056 Mbps
                     Memory


    CPU



                     Crossbar




                                                      31
             Wireless Links
• AMPS- Advance Mobile Phone System- standard
  for cellular phones
• PCS- Personal communication Services – digital
  cellular services in US and Canada
• GSM- Global System for Mobile Communication
  in the rest of the world.
• They use a system of towers to transmit signals
  and are moving toward ringing the globe with
  satellites.

                                                32
           Local Wireless Links
• Radio and infrared portions of the spectrum can
  be used over short distances.
• Technology- limited to in-building environments
• Radio bands at 5.2 GHz and 17 GHz are
  allocated to HIPPERLAN in Europe and 2.4
  GHz for use with the IEEE 802.11 standard,
  which supports data rates up to 54 Mbps.
• Bluetooth – radio, operates in the 2.45 GHz band
  – Used for all devices, printers, PDAs, phones
  – Networks of these devices are called piconets   33
     Bit Rates and Baud Rates
• Rate at which the signal changes is called
  the baud rate.
• When one bit is transmitted on a signal, the
  bit rate and baud rate may be equal.
• Often multiple bits are encoded onto a
  signal, where for example with 4 bits per
  signal, the baud rate may be 4 times the bit
  rate

                                             34
                      Encoding
• First step in turning nodes and links into usable building
  blocks is to understand how to connect them so that bits
  can be transmitted.
• Next encode binary data that the source want to send
  into signals that the links can carry and then decode the
  data back into the corresponding data at the receiving
  end.
• The high and low signals correspond to 2 different
  voltages on a copper based system or 2 different power
  levels on an optical link.

                                                        35
                      NRZ Encoding
• NRZ – non-return to zero, maps the data value 1 to the
  high signal and 0 to the low signal
• A sequence of several consecutive 1‟s means that the
  signal stays high for a prolonged period of time.
• Two fundamental problems;
   – Baseline wander –makes it difficult to detect a significant change
     in the signal
   – Clock recovery needs frequent changes from high to low to be
     enabled
• Sender and receiver clock must be precisely synchronized.

                                                                  36
         NRZ Encoding


Bits   0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0


NRZ




                                         37
           NRZI Encoding
• NRZI – non-return to zero inverted,
  addresses the previous problem, by having
  the sender make a transition from the
  current signal to encode a 1 and stay at
  current signal to encode a 0. ( Solves the
  problem of consecutive 1‟s, but not 0‟s)


                                               38
           Manchester Encoding
• Merges the clock with the signal by transmitting the
  exclusive–OR of the NRZ encoded data.
• Results in 0 being encoded as a low-to-high transition
  and 1 encoded as a high-to-low transition. Because
  both 0s and 1 result in a transition, the clock can be
  recovered at the receiver.
• Problem: doubles the rate at which transitions are
  made on the link, which gives receiver half the time to
  detect them.

                                                      39
             Encoding Strategies
      Bits   0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0


     NRZ

    Clock

Manchester

    NRZI




                                               40
                4B/5B Encoding
• Attempts to address the inefficiency of
  Manchester encoding.
• It inserts extra bits into the bit stream to break up
  long sequences of 0s and 1s:
   – Every 4 bits of data are encoded in a 5 bit code
     (See table 4B/5B encoding on p. 79)



                                                        41
         Packets and Frames
 Packet is ``generic'' term that refers to
  a small block of data.
 Each hardware technology uses a
  different packet format.
 Frame or hardware frame denotes a
  packet of a specific format used on a
  specific hardware technology.

                                              42
                   Framing
• Blocks of data (frames), not bit streams, are
  exchanged between nodes.
• The network adapter (NIC) enables the nodes to
  exchange frames.
• Recognizing what set of bits constitutes a frame,
  and where the frame begins and ends, is the
  challenge faced by the network adapter.


                                                43
              Frame Format
 Need to define a standard format for data
  to indicate the beginning and end of the
  frame
 Header and trailer used to ``frame'' the
  data (SOH and EOT)
 Can choose two unused data values for
  framing for example, if data is limited to
  printable ASCII characters, you can use
 ``start of header'' (soh)
 ``end of text'' (eot)                      44
               Frame Format

• Framing in Practice
 Incurs extra overhead - soh and eot take time to
  transmit, but carry no data
 Accommodates transmission problems:
 Missing eot indicates sending computer crashed
 Missing soh indicates receiving computer missed
  beginning of message
 Bad frame is discarded                        45
                 Framing
• Suppose A wishes to transmit a frame to B
• It tells adapter to transmit a frame from the
  node‟s memory
• A sequence of bits is sent over the link
• The adapter on B then collects the sequence of
  bits arriving on the link and deposits them in B‟s
  memory.

                                                46
                   Framing

                       Bits
Node A   Adaptor                    Adaptor   Node B




                      Frames




  Bits flow between adaptors, frames between hosts


                                                       47
                  Framing
• There are several approaches to the framing
  problem:
• Byte-Oriented Protocol (PPP)
  – Sentinel Approach (frame start and end)
  – Byte counting
• Bit –Oriented Approach (HDLC)
• Clock-based framing (SONET)

                                              48
          Byte-Oriented protocols
• One of the oldest approaches to framing is to view each
  frame as a collection of bytes (characters) rather than
  bits.
• BISYNC (Binary Synchronous Communication)
  protocol is a byte-oriented approach developed by IBM
  in 1960‟s
• DDCMP ( Digital Data communication Message
  Protocol) was used in Digital Equipment‟s DECNET.
• These are examples of the sentinel approach and the
  byte counting approach.

                                                     49
             Sentinel Approach
• A packet is a sequence of labeled fields.
• Above each field is a number indicating the number of
  bits in the field.
• Packets are transmitted beginning with the leftmost
  field. The beginning of the frame is the SYN
  (synchronization) character.
• Data is contained between sentinel characters – STX
  (start of text) and ETX (end of text).
• The header begins with a SOH (start of header) field.
• It ends with a CRC (cyclic redundancy check) field.
                                                    50
    BISYNC Frame Format

8   8   8            8          8   16

            Header       Body       CRC




                                          51
             Framing problem
• ETX character may appear in the data.
• BISYNC overcomes this by using byte-stuffing
  or character-stuffing by preceding the ETX
  character with an escape character or DLE (data
  link escape– (similar to \n or \t in programming)
• CRC (cyclic redundancy check) is used to
  detect transmission errors.


                                               52
          Point-To-Point Connection
 The first computer communication systems were connected
   by communication channels that connected exactly two
   computers.
 Called a mesh or point-to-point network
 Had three useful properties:
   – 1. Each connection was independent and different hardware could be
     used. (bandwidth, modems, etc. did not have to be the same)
      • Allow for greater flexibility.
   – 2. The connected computers have exclusive access and could
     decide how to send data across the connection. The can determine
     the frame format and size, error detection mechanism, etc.
   – 3. Since only two computers share the channel it is private and
     secure.
                                                                  53
   Disadvantages of Point-To-Point
• 1. Number of wires grows as the number of
  computers increases
• 2. The total number of connections exceeds the
  number of computers being connected.

• The number of connections needed is proportional
  to the square of the number of computers, since the
  new computer must have a connection to each of
  the existing computers. So to add the Nth computer
  requires N-1 new connections.
                                                 54
 Disadvantages of Point-To-Point
• For N computers:
• Connections = (N2 - N)
                   2
                       Computers   Connections
                           2           1
                           3           3
                           4           6
                           5           10
                                             55
         Point-to-Point Protocol
• Point-to-Point Protocol (PPP) is run over dialup
  modem links and is similar to BISYNC.
• Flag denotes the start-of-text character, address
  and control fields contain default values.
• The protocol is the high level protocol, such as
  IP or IPX.
• Payload size is usually 1500 bytes.
• Checksum field is either 2 or 4 bytes long.
                                                56
              PPP Frame Format

 8        8         8        16                     16       8

Flag   Address   Control   Protocol   Pay load   Checksum   Flag




                                                                   57
                    PPP Framing
• PPP framing is unusual in that several of the field
  sizes are negotiable rather than fixed.
• The negotiation is conducted by the LCP (Link
  Control Protocol) Protocol.
• PPP and LCP work in tandem:
   – LCP sends control messages encapsulated in PPP frames
     denoted by an LCP identifier
   – Changes PP‟s frame format based on the information
     contained in the control messages.
   – LCP also establishes a link between the peers when both
     sides detect the carrier signal.
                                                               58
       Byte-Counting Approach
• The alternative to detecting the end of a file
  with a sentinel value is to include the number of
  items in the file at its beginning.
• This is true in framing- the number of bytes in a
  frame can be included in the header.
• DDCMP protocol uses this approach and the
  COUNT field specifies the number of bytes in
  the frame‟s body.

                                                59
DDCMP Frame Format

8   8   8    14      42             16

            Count   Header   Body   CRC




                                          60
             Framing Errors
• A transmission error could corrupt the COUNT field
  and the end of the frame would be incorrectly
  detected.
• A similar problem exists with the ETX field being
  corrupt.
• This is called a framing error.
• The receiver waits for the next SYN character to
  collect data for the next frame.
• A framing error may cause back-to-back frames to
  be incorrectly received.
                                                 61
           Bit-Oriented Protocols
• Bit-oriented protocols are not concerned with byte
  boundaries. It views the frame as a collection of bits.
• Synchronous Data Link Control ( SDLC), developed
  by IBM is a bit-oriented protocol, later standardized
  as the High Level Data Link Control (HDLC).
• Uses bit sequence 01111110 to denote beginning and
  end of a frame.
• It is also transmitted when the link is idle.

                                                        62
   HDLC Frame Format


    8         16            16     8
Beginning                       Ending
sequence    Header   Body   CRC sequence




                                           63
               Data Stuffing
• Networks usually insert extra bits or bytes
  to change data for transmission and this is
  called Data Stuffing
 Bit stuffing and byte stuffing are two
  techniques for inserting extra data to
  encode reserved bytes
 Byte stuffing translates each reserved byte
  into two unreserved bytes
                                            64
             Byte Stuffing

 Can use esc as prefix, followed by x for
  soh, y for eot and z for esc:




                                         65
             Byte Stuffing
 Sender translates each reserved byte into
  the appropriate encoding pair of bytes
 Receiver interprets pairs of bytes and
  stores encoded byte in buffer
• Data still framed by soh and eot




                                         66
                   Bit Stuffing
• Anytime 5 consecutive 1‟s are transmitted, the
  sender inserts a 0 before sending the next bit. On
  the receiving side.
• When the receiver detects 5 consecutive 1‟s, it
  assumes the next 0 was “stuffed” and removes it.
• If the next bit is a 1, either this is the end of frame
  marker or an error has occurred.
• Size of the frame is dependent on the data being
  sent in the frame payload.
                                                    67
           Clock-Based Framing
• Third approach to framing is the Synchronous Optical
  Network (SONET) standard, called clock-based framing.
• SONET was proposed by Bell Communications
  Research (Bellcore) for digital transmission over an
  optical fiber.
• Addresses the framing and encoding problems as well as
  multiplexing low speed links onto a high speed link.
• More complex protocol

                                                    68
                 SONET Framing
• SONET Frame has special information that indicates
  where the frame starts and ends.
• No bit stuffing is used
• How does receiver know where the frame starts and
  ends?
• Frame consists of 9 rows of 90 bytes each.
   – First 3 bytes of each row are overhead.
   – First two bytes of frame contain special bit pattern
   – Use of overhead bytes is complex

                                                            69
    SONET STS-1 Frame
            Ov erhead    Pay load




        9 rows




                        90 columns




First two bytes of the frame contain a special bit
pattern that indicates the start of the frame


                                                     70
           STS-1 Multiplexing

                  STS-1    STS-1     STS-1




                     Hdr    STS-3c




Three STS-1 frames are multiplexed onto one STS-3 frame.




                                                           71
SONET Frames Out of Phase

           87 columns
Frame 0

                        9 rows



Frame 1




                                 72
                   Error Detection
• Bit errors occur in frames due to electrical interference or
  thermal noise.
• Detecting errors is one part of the problem; correcting
  errors is the other.
• What happens when an error is detected?
• Two basic approaches:
   – Notify the sender that message is corrupt so the sender can
     retransmit it; ( most often used in every day applications)
   – Use an error-correcting code to reconstruct the correct message

                                                               73
          Transmission Errors
 External electromagnetic signals can
  cause incorrect delivery of data
    Data can be received incorrectly
    Data can be lost
    Unwanted data can be generated
 Any of these problems are called
  transmission errors

                                         74
            Error Detection
• Detecting Transmission Errors: basic idea is
  to add redundant information to a frame that
  can determine if errors have been introduced.
• Two-dimensional parity – based on a simple
  parity bit added to balance the number of 1‟s
• Checksums – code created based on addition
• Cyclic Redundancy Check (CRC) – based
  on a complex mathematical algorithm and
  used in nearly all link level protocols.    75
                       Parity
 Parity refers to the number of bits set to 1 in the
  data item
 Even parity - an even number of bits are 1
 Odd parity - an odd number of bits are 1
 A parity bit is an extra bit transmitted with a data
  item,chose to give the resulting bits even or odd
  parity
 Even parity - data: 10010001, parity bit 1
• Odd parity - data: 10010111, parity bit 0
                                                    76
        Parity and Error Detection
 If noise or other interference introduces an error,
  one of the bits in the data will be changed from a 1
  to a 0 or from a 0 to a 1
 Parity of resulting bits will be wrong
 Original data and parity: 10010001+1 (even parity)
 Incorrect data: 10110001+1 (odd number of 1’s)
 Transmitter and receiver agree on which parity to
  use
 Receiver detects error in data with incorrect parity
                                                   77
    Limitations of Parity Checking
 Parity can only detect errors that change an
  odd number of bits
 Original data and parity: 10010001+1 (even
  parity)
 Incorrect data: 10110011+1 (even parity!)
 Parity usually used to catch one-bit errors


                                           78
         Two-Dimensional Parity
• Two-dimensional parity involves adding on extra bit to
  balance the number of 1‟s in each byte (making the total
  either even or odd).
• Two-dimensional parity does a similar calculation for
  each bit position across all the bytes in the frame,
  resulting in adding an extra parity byte for the frame as
  well as an additional parity bit for each byte.
• Two-dimensional parity catches all the one, two and 3
  bit errors and most 4 bit errors.
                                                       79
Two-Dimensional Parity
                        Parity
                        bits
              0101001   1

              1101001   0

              1011110   1
     Data
              0001110   1

              0110100   1

              1011111   0

     Parity
              1111011   0
     by te

                                 80
    Probability and Error Detection
 All error detection methods are approximate
  and aim at a low probability of accepting
  corrupted data.
• Parity can detect a single bit error, but not all
  possible errors, especially where two bits ( or an
  even number of bits) are changed.
 Many alternative schemes exist:
 Detect multi-bit errors
 Correct errors through redundant information
• Checksum and CRC are two widely used
  techniques                                       81
  Internet Checksum Algorithm
• Simple idea: add up all the words that are to
  be transmitted and then transmit the sum,
  called the checksum, with the data.
• The receiver performs the same calculation
  and compares it to the checksum received.
  If they do not match, an error has occurred.
• Does not detect all errors…
• Algorithm is easy to implement ( See p. 94)
                                              82
                Checksum
 Sum of data in message treated as array of
  integers
 Can be 8-,16- or 32-bit integers
 Typically use 1s-complement arithmetic
 Example -16-bit checksum with 1's
  complement arithmetic



                                         83
       Advantages of Checksum
 Fastest implementations of 16-bit checksum
  use 32-bit arithmetic and add carries in at end
 Easy to do - uses only addition
 Small size of checksum means cost of
    transmitting it is small.
• Ease of computation to create and verify
  checksum.

                                            84
        Checksum Limitations
 Does not detect all common errors (like
  reversed bits)




                                            85
Cyclic Redundancy Check (CRC)
• CRC uses powerful mathematics ( finite
  fields) to give strong protection against
  common bit errors in messages that are
  thousands of bytes long.




                                              86
      Detecting Errors with Cyclic
         Redundancy Checks
 Consider data in message as coefficients of
      a polynomial
 Divide that coefficient set by a known
     polynomial
 Transmit remainder as CRC
 Good error detection properties
 Easy to implement in hardware
                                           87
                CRC Hardware
 The hardware used to computer a CRC is a shift
  register, which act like a tunnel through which bits
  move in a single file from right to left.
 The shift register holds a fixed number of bits so
  when a new bit moves in, another bit moves out.
 The output gives the value of the leftmost bit.
 When a bit changes, the output changes.
 The shift register has two operations: initialize and
  shift.
 Initialize sets all bits to zero
 Shift moves all bits one position to the left.
                                                   88
            CRC Hardware
• X-Or and Shift Registers




                             89
                 CRC Hardware
• CRC Hardware consists of 3 shift registers
  connected with X-Or units.
• Output from the leftmost unit goes to 3 places
  simultaneously - the
• 3 X-Or units.
• To compute the CRC values in all registers are
  initialized and the bits are shifted one at a time.
• One bit of the message is applied to the input unit
  and all three registers perform a shift. This repeats
  for each bit of the message.
                                                    90
CRC Calculation using Shift
        Registers

   Message

             x0   x 1 XOR gate   x2




                                      91
         CRC Computation
• After an entire message has been input,
  the shift registers contain the 16 bit
  CRC for the message.




                                        92
             CRC Computation
• A CRC can compute more errors that a
  simple checksum because:
 An input bit is shifted through 3 registers;
 The hardware uses feedback so that the
  effect from a single bit cycles through the shift
  registers more than once.
• Mathematically a CRC uses a polynomial to
  divide the message:
•           P(X) = x 16 + X 12 + X 5 + 1
                                               93
                  Example CRC
• A message is treated as a long binary polynomial, P.
• Before transmitting, the data link layer divides P,
 by a fixed polynomial function G(x), resulting in a
 whole quotient Q and a remainder R/G. The
 remainder is appended to the message and
 transmitted.
• It is checked by the receiver to see if R agrees with
  the locally generated value for R. (See Tanenbaum
  p.208-210 for analysis)
                                                   94
              Example CRC
• Frame: P: 1101011011
• Generating function G(x): 10011
• Message after appending 4 zero bits:
  11010110110000
• Divide P by G to get remainder R:
•                  1100001010 with R = 1110
•     10011 | 11010110110000

• Transmitted frame with remainder R appended:
  11010110111110                             95
          Accuracy of CRC
• CRC actually adds 8, 16, 24, or 32 bits to
  the message.
• This method detects up to 99.969% of
  errors with CRC-8 and nearly 99.9%
  with CRC-16 or CRC-24.



                                           96
      Another CRC Example
                           11111001
         Generator    1101 10011010000    Message
                           1101
                           1001
                           1101
                            1000
                            1101
                             1011
                             1101
                               1100
                               1101
                                  1000
                                  1101
                                    101   Remainder




See text. Pp. 94-95
                                                      97
      Error Correction or Error
             Detection?
• When error is detected, frame is discarded
  and resent, using bandwidth and causing
  latency, waiting for its arrival.
• Error correction requires additional bit to be
  sent with every frame.
• Correction is useful when
  – 1) errors are probable or
  – 2) the cost of retransmission is too high

                                                98
           Reliable Transportation
• A data link level protocol that wants to deliver frames
  reliably must recover from discarded ( lost) frames.
• Acknowledgements - (ack) is a small control frame
  that a protocol sends back to report that it has
  received the frame. If the sender does not receive a
  frame in a reasonable amount of time, it retransmits.
• Timeouts -waiting a reasonable mount of time is
  called a timeout

                                                        99
    Automatic Repeat Request
• Using acknowledgements and timeout to
  implement reliable delivery is called
  automatic repeat request (ARQ).
• The simplest ARQ scheme is the Stop and
  Wait algorithm.



                                            100
              Stop and Wait
• After transmitting one frame the sender
  waits for an ACK before transmitting the
  next frame.
• If it does not arrive in a reasonable time, the
  sender retransmits the original frame.



                                               101
                Stop and Wait Algorithm
                   Sender           Receiv er   Sender           Receiv er

                            Fram                         Fram
                                e                            e


                              ACK                          ACK
 a) Arrives                                                                  c)ACK lost
                                                         Fram
                                                             e


                                                           ACK



                              (a)                          (c)




                   Sender           Receiv er   Sender           Receiv er

                            Fram                         Fram
                                e                            e

b) Frame lost                                              ACK
                                                         Fram
                                                             e               d) Timeout
                            Fram
                                e
                                                           ACK
                                                                             too soon
                              ACK
                                                                                    102
                             (b)                           (d)
             Duplicate Frames
• If a frame is late arriving another frame might
  be retransmitted, resulting in duplicate frames.
• To correct this, a header usually contains a
  sequence number (0,1), which is used for
  alternate frames.
• When sender retransmits frame 0, the receiver
  can see that it is a second copy of frame 0, not
  frame 1.

                                                103
Timeline for Stop and Wait
       Sender               Receiv er

                Fram
                    e   0

                    0
                ACK

                Fram
                    e   1

                    1
                ACK

                Fram
                    e   0

                    0
                ACK




                                        104
         Sliding Window Protocol
 Allows sender to transmit multiple packets before
  receiving an acknowledgment
 Number of packets that can be sent is defined by the
  protocol and called the window
 As acknowledgments arrive from the receiver, the
  window is moved along the data packets; hence
  ``sliding window''
 Sliding window protocol can increase throughput
  dramatically

                                                 105
        Sliding Window Protocol
• Sliding window algorithm allows the transmission of
  a frame at about the same time as the ACK arrives.
• Sender assigns a sequence number (SeqNum) to
  each frame and maintains 3 variables:
   – Send window size (SWS) - # of unacknowledged frames
     that sender can transmit
   – Last acknowledgement received (LAR)
   – Last frame sent (LFS)
   – LFS - LAR <= SWS

                                                           106
Sliding Window




                 107
Timeline for Sliding Window
        Sender     Receiv er




                               108
                 Sliding Window
• When ACK arrives, the sender moves LAR to the right,
  allowing the sender to transmit another frame.
• Sender buffers up to SWS (send window size) frames (in
  case they need to be retransmitted).
• It also associates a timer with each frame it transmits, so
  it can retransmit if an ACK is not received in time.
• LAR – Last Acknowledgement Received
• LFS – Last Frame Sent
• See pp. 105-115 for details and for interactive demo see
• http://www2.rad.com/networks/2004/sliding_window/demo.html
                                                        109
 Sliding Window on Sender

            < SWS
            ─

■■■                       ■■■



      LAR           LFS




                                110
            Sliding window
• The receiver maintains 3 variables:
  – The receive window size ( RWS) – the upper
    bound on the number of out of order frames
    that the receiver can accept.
  – The sequence number of the largest acceptable
    frame (LAF)



                                                111
Sliding Window on Receiver

               <   RWS
               ─

   ■■■                         ■■■




         LFR             LAF




                                     112
      Sliding Window Algorithm
• When frame with sequence number SeqNum arrives, the
  receiver does the following:
• If SeqNum <= LFR or SeqNum >LAF then frame is
  outside the window and is discarded.
• If LFR < SeqNum <= LAF, then it is accepted.
• SeqNumToAck is largest not yet acknowledged
• Receiver acknowledges receipt of SeqNumToAck and
  sets LFR = SeqNumToAck
• LAF=LFR +RWS
                                                113
Comparison of Sliding Window and Stop & Wait




                                         114
    Frame Order and Flow Control
• Sliding Window can be used for:
  – To reliably deliver frames on an unreliable link;
  – To preserve the order in which the frames are
    transmitted, using the sequence numbers;
  – To support flow control- a feedback mechanism
    by which the receiver is able to throttle the sender
    to keep it from overrunning the sender.


                                                       115
     Concurrent Logical Channels
• ARPANET Data Link protocol, or concurrent
  logical channels, is an alternative to sliding
  window protocol and can keep pipe full while
  using the simple stop and wait protocol.
• It multiplexes several logical channels onto a
  single point-to-point link and runs the stop and
  wait protocol on each.


                                                116
              Ethernet (802.3)
• The Ethernet is the most successful local area
  networking technology.
• 1973- Developed at Xerox Park by Bob Metcalfe
  and David Boggs, it is a general form of the Carrier
  Sense Multiple Access with Collision Detection
  (CSMA/CD) technology.
• Based on Aloha, early packet network developed at
  the University of Hawaii to support communication
  across the islands.

                                                    117
Bob Metcalfe
•Developed the Ethernet with David Boggs
• 1979 Founded 3COM Corporation, which
makes wirelesss access points
• Founded Infoworld
•Authored numerous books and articles
•Recipient of many awards including the
National Medal of Technology (2005) and
induction into the National Inventors Hall of
Fame for his contributions to the “welfare of
mankind”.
•Spoke at the CCSCE Conference at SJC,
October, 2007 “ETHERNET IS THE
ANSWER; WHAT IS THE QUESTION?”
                                       118
             Ethernet (802.3)
• Digital Equipment Corporation (DEC), Intel and
  Xerox joined to form the 10 Mbps Ethernet
  standard in 1978.
• This standard formed the basis of the IEEE
  standard 802.3
• It has recently been extended to include a 100
  Mbps version, called Fast Ethernet and a 1000
  Mbps version called Gigabit Ethernet.

                                            119
                 Ethernet (802.3)
• The Ethernet is a multiple-access network meaning that
  a set of nodes send and receive frames over a shared link.
• The “carrier sense” means that the nodes can distinguish
  between a busy and idle link.
• “Collision detect” means that a node listens as it
  transmits and can detect when a transmitting frame has
  interfered (collided) with a frame transmitted by another
  node.
• When a collision occurs, both nodes back off, wait a
  random amount of time and then attempt to send again.
                                                       120
            Physical Properties
• An Ethernet is typically implemented on coaxial
  cables of up to 500 meters.
• (On older versions, called thick-net or 10Base5, a
  transceiver connected hosts to the cable and then
  to the network adapter or NIC card.)
• Newer versions, 10Base2, connect directly
  through the NIC, where all the logic is contained.
• 10BaseT, for twisted pair, uses Cat 5 cable and is
  limited to 100meter.
• “Base” refers to the baseband system.         121
Ethernet Transceiver and Adapter
                 Transceiv er


                                Ethernet cable
                  Adaptor




          Host




                                                 122
                Thick Ethernet Wiring
 Uses thick coax cable
   AUI cable (or transceiver or drop cable connects from NIC to transceiver
   AUI cable carries digital signal from NIC to transceiver
   Transceiver generates analog signal on coax
•   Wires in AUI cable carry digital signals, power and other control signals




                                                                       123
                         Ethernet Wiring
  Uses thin coax that is cheaper and easier to install than thick Ethernet coax
 Transceiver electronics built into NIC; NIC connects directly to network
medium
•   Coax cable uses BNC connector




                                                                              124
                 Ethernet Wiring
 Coax runs directly to back of each connected computer
• T connector attaches directly to NIC




 Useful when many computers are located close to each other
 May be unreliable - any disconnection disrupts entire net
                                                         125
           Twisted Pair Ethernet
• Variously called 10Base-T, twisted pair or TP
  Ethernet
• Replaces AUI cable with twisted pair cable
• Replaces thick coax with hub




                                               126
               Physical Properties
• Multiple Ethernet segments are joined by repeaters,
  which forward a digital signal.
• No more than 4 repeaters may be connected to any pair
  of hosts, limiting an Ethernet to a maximum of 2500
  meters.
• An Ethernet can support a maximum of 1024 hosts.
• Any signal placed on the Ethernet is broadcast to all
  hosts.
• Terminators are attached to the end of each segment to
  absorb the signal.
• The Ethernet uses Manchester encoding.                127
Ethernet repeaters
                   ■■■




                   ■■■




                   ■■■




                   ■■■




        Repeater


        Host




                         128
                Ethernet Hubs



                 Hub                     Hub




The common 10BaseT configuration is to have several point-
to-point segments connected to a hub or switch. This is also
true for 100Mbps Ethernet, but not for Gigabit Ethernet.
                                                          129
HUBS




       130
            Access Protocol
• On an Ethernet, all hosts are competing for
  access to the same shared link.
• The media access control (MAC) algorithm
  controls access to the link.
• It is implemented in hardware on the
  network adapter.



                                                131
     Network Adapter Cards (NIC)
 CPU can't process data at network speeds
 Computer systems use special purpose
  hardware for network connection
 Typically a separate card in the backplane
 Network adapter card or network interface card
  (NIC)
 Connector at back of computer then accepts
  cable to physical network
                                           132
Network Interface Hardware




                             133
                NIC Cards




 The sockets for the NIC cards are usually
located near the back of the cabinet and a
network cable attaches to the end of the NIC.
                                                134
      NIC Cards and Wiring
NICS can provide all three connection technologies




                                                     135
             Ethernet Frame Format
•Taken from the Digital-Intel-Xerox Ethernet Standard
•Each Ethernet frame is defined by the following format where the
preamble allows the receiver to synchronize with the signal.
•Both source and destination hosts are identified by addresses
•Packet type identifies the protocol
•Each packet can contain up to 1500 bytes of data (46bytes minimum)
•32-bit CRC for error detection
             64      48       48        16            32

          Preamble   Dest    Src       Ty pe   Body   CRC
                     addr    addr
                                                                 136
                   Addresses
• Each host on an Ethernet has a unique Ethernet
  address.
• Technically the address belongs to the adaptor, not to
  the host and is usually burned into the NIC card
  ROM.
• Each NIC card has a unique prefix and makes sure it
  assigns unique addresses
• Addresses can be assigned statically, dynamically or
  can be configurable and assigned by the network
  administrator
                                                    137
           Assigning Addresses
Static:         Hardware manufacturer assigns permanent      Manufacturer must ensure
                address to each interface - doesn't change   every interface has a unique
                                                             address




Configurable:   Address can be set by end user, either       System administrators must
                through switches or jumpers,                 coordinate to avoid conflict
                or electronically ( EPROM) or
                through software




Dynamic:        Interface automatically assigns              Automatic scheme must be
                hardware address each time it is             reliable to prevent conflicts
                powered up (tries random number)




                                                                                             138
Addressing Scheme Comparison
Addressing Scheme            Advantages              Disadvantages

 Static:           Ease of use and permanence           Less flexibility



 Configurable:      Small permanent addresses           Requires initial
                     hardware easily replaced            configuration



 Dynamic:        Smaller addresses and Vendors do    Lack of permanence
                  not have to coordinate to assign   and potential conflict
                               them




                                                                              139
                 Address Types
• Each frame on an Ethernet is received by every
  connected adaptor.
• Each adaptor recognizes the frames addressed to it and
  passes those frames to it host. These are unicast
  addresses.
• A broadcast address, consisting of all 1‟s, is
  recognized by all NIC cards.
• A multicast address, with first bit set to 1, is
  recognized by a subset of NIC cards.
• Running in promiscuous mode, means that a NIC card
  will pass all messages to its host.
                                                    140
    Ethernet Address Summary
• An Ethernet adaptor receives all frames and
  accepts:
  – Frames addressed to its own address
  – Frames addresses to the broadcast address
  – Frames addressed to a multicast address, if it is
    part of that subset
  – All frames if it is in promiscuous mode


                                                    141
           Transmitter Algorithm
• Receiver side is simple.
• Sender side implements Ethernet protocol.
• When NIC has frame to send and the line is busy, it waits
  for the line to become idle.
• Because there is no centralized control, two (or more)
  adaptors may send at once, causing a collision. When a
  collision is detected, a jamming sequence is sent to stop
  transmission.
• The adaptors wait a random amount of time before trying
  again.
• Each time there is a collision, the delay interval doubles –
  called exponential backoff.                           142
Worst Case Scenario
         A    B


   (a)


         A    B


   (b)


         A    B


   (c)


         A    B


   (d)




                      143
       Success of the Ethernet
• Extremely easy to administer, no switches
  to fail, no routing or configuration tables
• Easy to add additional hosts
• It is inexpensive, since cables are relatively
  cheap.
• Most new LAN switching technology is
  based on the Ethernet

                                               144
Token Rings (802.5, FDDI, RPR)
• Token Rings are the other significant class of
  shared media networks.
• IBM Token Ring, was the original – followed by
  the IEEE 802.5 standard, which was nearly
  identical, and finally the newer FDDI (Fiber
  Distributed Data Interface) Standard, which is
  declining in use.
• Resilient Packet Ring or RPR (802.17) is nearly
  standardized.
                                                    145
                    Token Ring
• Token ring Network consists of a set of nodes connected
  in a ring.
• Data flows in a particular direction around the ring so
  that each node receives a packet from its upstream
  neighbor and forwards it to its downstream neighbor.
• Similar to Ethernet in that it involves an algorithm which
  controls when a node can transmit, and all nodes see all
  frames.
• Sending a message differs from that of the Ethernet.

                                                       146
Token Ring Network




                     147
Implementing a Token Ring




                            148
                    Tokens
• Access to the network is controlled by a token.
• A „token” is a special sequence of bits, which
  circulates around the ring.
• Each node receives the token, and when it has
  the token, that node may send a packet and then
  forward the token to the next node in a round-
  robin fashion.
• This is fair, since each node gets a turn to send.

                                                149
              Physical Properties
• Any link or node failure makes the whole network
  useless.
• When relay is open, the station is included in the ring; if
  the relay closes, the ring bypasses the node.
• Several relays are packed into a single multi-station
  access unit ( MSAU) – required by IBM token ring.
• Data rate is 4 or 16 Mbps and uses Manchester
  differential encoding
• Twisted pair is required for IBM and not specified for
  802.5
                                                       150
            Relay on Token Ring
                     Host                               Host


                     Host                               Host


    From prev ious           To next   From prev ious           To next
        host                   host        host                   host


                     Relay                              Relay
     (a)                               (b)




a) Relay open – host active              b) Relay closed-host bypassed

                                                                          151
       Multimedia Access Unit
                                       Host




                                MSAU

                     Host                     Host




               From prev ious
                  MSAU

                         To next
                          MSAU
                                       Host



Used only in electrical rings to compensate for node failure.
                                                            152
         Token Ring Media Access
                 Control
• Network adapter contains a receiver, transmitter, and one
  or more bits of data storage.
• When no node is sending, the token circulates.
• A sending station, “seizes” the token and sends data.
  Token holding time (THT) is the time the node can hold
  the token. Default THT = 10ms.
• 802.5 also supports a strict priority scheme
• Sending node can reinsert token immediately following
  its frame (early) or after the frame circles the ring and is
  removed (delayed) release.
                                                         153
                          Token Release

                Token                 Token     Fra
                                                    m   e
            e
       am
  Fr




                    (a)                   (b)



a) early                          b) delayed                154
          Token Ring Maintenance
• Token rings have a station designated as the monitor.
• Procedures are defined to elect a monitor when the ring is
  first connected or when the monitor fails.
• Monitor must make sure there is always a toke in the ring
  and that there is sufficient delay.
• It also checks for corrupted or orphaned frames.
• It also checks for “dead” stations.



                                                       155
           Token Ring Frame Format
•Uses differential encoding codes in start and end delimiters.
•Access control byte includes the frame priority
•Frame control byte identifies the higher-level protocol
•Like Ethernet, addresses are 48 bytes long
•Includes a 32- bit CRC and A and C bits for reliable delivery

       8          8         8       48     48     Variable     32          8          8
    Start       Access    Frame     Dest   Src    Body       Checksum   End         Frame
    delimiter   control   control   addr   addr                         delimiter   status




                                                                                          156
                      FDDI
• Fiber Distributed Data Interface (FDDI) is similar to
  802.5 and IBM token ring.
• Significant differences are that it runs on fiber, not
  copper and makes use of some newer innovations
• It is usually a dual ring – where each ring transmits
  in the opposite direction.
• The second ring is only used if the primary ring fails
  and there is a “loop back” toform a complete ring.
• Instead of a monitor all nodes participate equally in
  maintaining the ring.

                                                      157
               Dual Fiber Ring




                 (a)           (b)




a) Normal operation    b) Failure of primary ring
                                                    158
              Physical Properties
• FDDI network consists of a dual ring- two rings that
  transmit data in opposite directions. The second ring is
  only used if the primary ring fails.
• Nodes attach to the ring with a single cable called single
  attachment stations (SAS). A concentrator attaches
  several SASs to the ring.
• FDDI is a 100 Mbps network and is limited to 500 hosts.
• FDDI uses 4B/5B encoding
• Token holding algorithms are more complex than 802.5

                                                       159
            FDDI Frame Format
•Similar to 802.5 with these exceptions:
•Uses 4B/5B encoding instead of Manchester
•Has a bit in the header to distinguish synchronous
from asynchronous traffic
•Lacks the access control bits present in 802.5

    8         8       48     48            32    8        24
 Start of   Control   Dest   Src    Body   CRC End of   Status
 f rame               addr   addr              f rame




                                                                 160
     Resilient Packet Ring (RPR)
• Relatively recent technology – IEEE (802.17)
• Resiliency- the ability to recover quickly from a
  link or node failure was its key design goal.
• Other goals were bandwidth efficiency and
  Quality of Service (QoS) support.
• Like FDDI it uses 2 rings, but unlike FDDI ,
  both are used for normal service.
• Uses buffer insertion instead of a token.
• Used in MAN‟s but “metro Ethernet” is
  coming…                                       161
                         Wireless
• Wireless is the rapidly evolving technology for
  connecting communication devices:
  –   Bluetooth
  –   Wi-Fi -802.11
  –    Wi-MAX – 802.16
  –   and 3G cellular wireless
• They differ in how much bandwidth they can
  provide, how far apart nodes can be and which part
  of the electromagnetic spectrum they use.     162
          Wireless Technologies
          BlueTooth Wi-Fi      WiMAX 3G Cell

Link      10m        100m      10 km
Length
Bandwidth 2.1        54 Mbps  70 Mbps 384+Kbps
          Mbps       Shared   Shared   Per
          Shared                       connection
Use/link  To         Notebook Building Phone to
          notebook   to base  to tower tower
Anology   USB        Ethernet Coaxial DSL
                                              163
                    Wireless
• The most widely used wireless links are
  asymmetric – the two endpoints are different kinds
  of nodes
• One endpoint acts as a base station and has no
  mobility and is wired to the Internet or other
  network.
• The client not is often mobile and relies on its link
  to the base station to communicate with other
  nodes.
                                                     164
165
                 Wireless
• Notice that wireless naturally supports point
  to multipoint communications becaues
  radiio waves sent out by one device can be
  simultaneously received by many devices
• However communication between client
  nodes is routed through the base node


                                             166
Example Wireless Network


     A    B   C    D




                           167
          Levels of Mobility
• No mobility- when a receiver must be in a
  fixed location to receive a directional
  transmission from a base station (true of the
  initial WiMAX)
• Mobility within the range of a base as in the
  case of Bluetooth
• Mobility between bases as is the case with
  cell phones and Wi-Fi

                                             168
     Mesh or Ad hoc Network
• A wireless mesh is an alternative topology
• Nodes are peers ( there is no base station)
• Messages are forwarded through a chian of
  peers as long as each peer is within range of
  the preceeding node.
• This allows a wireless portion of a network
  to extend beyond the limited range of a
  single radio.
                                             169
170
                     Bluetooth
• Bluetooth provides very short range communication
  between mobile phones, PDAs, notebook computers
  and other peripheral devices.
• It is a convenient alternative to connecting with a wire.
• It has a range of only 10 m and operates at 2.45 GHz
• Because devices usually blong to an individual or
  group it is often called a PAN ( personal area
  network)
• Network connects up to 7 devices to a master and is
  called a piconet.
                                                       171
Bluetooth piconet
                    172
          Wireless ( 802.11)
• Like Ethernet and Token Ring, 802.11 is
  designed for use in a limited geographical
  area (homes, office buildings, campuses).
• Primary challenge is to mediated shared
  access through space.
• 802.11 supports additional features (time-
  bound services, power management and
  security)

                                               173
             Physical Properties
• 802.11 was designed to run over three different media-
  two based on spread spectra and one based on diffused
  infrared.
• The radio based versions run at 11 and 54 Mbps.
• A chipping sequence spreads the signal over a wide
  frequency using a random sequence and makes the
  signal look like noise to any receiver that does not
  know the sequence.
• Infrared signals are diffused so the sender and receiver
  do not need to be aimed at each other, but must be
  within buildings.                                    174
    4-Bit Chipping Sequence

1
0                  Data stream: 1010

1
0                  Random sequence: 0100101101011001

1
0                  XOR of the two: 1011101110101001




                                                  175
            Collision Avoidance
• The protocol is more complex than Ethernet, since
  all nodes are not always within reach.
• Consider 4 nodes A,B,C,D that are able to send to
  a node to its immediate left or right,so B can reach
  A and C but not D.
• If A and C both send to B they collide, but are
  unaware of each other and are called hidden
  nodes.

                                                 176
Hidden node problem
A & C can collide at B
                         177
         Collision Avoidance
• Another related problem is the exposed
  node problem.
• Suppose B is sending to A and C is aware
  of this. It is a mistake for C to think it
  cannot transmit.
• It is not a problem for C to transmit to D
  because it will not interfere with A‟s ability
  to receive from C

                                               178
         Exposed node problem
B can transmit to A and C can transmit to D
                                              179
             Collision Avoidance
• 802.11 addresses these two problems with a Multiple
  Access Collision Avoidance algorithm. (MACA)
• Sender and receiver exchange control frames before
  transmitting data
• Sender sends a request to transmit (RTS) frame.
• Receiver relies with a clear to send (CTS) frame.
• Receiver also sends an ACK after successfully receiving
  the frame. All nodes must wait for this before trying to
  transmit.
• CTS frames can collide and both must wait before
  transmitting, similar to Ethernet backoff.
                                                      180
            Distribution System
• Since an advantage of a wireless system is that
  nodes are free to move around, reachable nodes
  may change over time.
• Some nodes may roam and some, called Access
  Points (AP) are connected to the network
  infrastructure by a distribution system.
• Distribution system runs at layer 2 of the ISO
  architecture and does not depend on higher layers.

                                               181
Access Points connected to a
   Distribution Network

                      Distribution sy stem




           AP-1                                  AP-3
                             AP-2                       F
      A           B                          G
                                    H

            C                                     E
                              D




Each node associates itself with one access point.
                                                            182
       Communication Example
• For node A to communicate with node E
• A first sends a frame to its access point AP-1,
  which forwards the frame across the
  distribution system to AP-3, which finally
  transmits the frame to E



                                               183
                Selecting an AP
Technique called scanning:
1. The node sends a Probe frame
2. All APs within reach reply with a Probe Response
   Frame
3. The nodes selects one of the access points and sends
   that AP an Association Request Frame.
4. The AP replies with an Association Response Frame

A node uses this when it joins the network and when it
   becomes unhappy with current AP ( weak signal, etc.)
                                                    184
   Active and Passive Scanning
• After a node has probed the network, it associates
  itself with an Access Point. This is called active
  scanning.
• APs also periodically send a Beacon Frame that
  advertise the capabilities of the Access Point,
  including transmission rates. A node can change to
  this point by sending an Association Request
  Frame to the access Access Point . This is called
  passive scanning.
                                                  185
         Node Mobility

               Distribution sy stem




    AP-1                                  AP-3
                       AP-2                      F
A          B                          G
                              H
                   C
     C                                     E
                        D




                                                     186
                802.11 Frame Format
48 bit Source and Destination addresses ( addr1, addr2)
Two additional address fields depends on the ToDS, From DS
settings
Control fields: Type, ToDS, FromDS
Type fields indicates whether the frame is RTS, CTS or data
CRC for error detection
    16        16        48      48      48       16       48     0Ð18,496   32

  Control   Duration   Addr1   Addr2   Addr3   SeqCtrl   Addr4   Pay load   CRC



                                                                            187
802.11 Frame Format




                      188
            WiMAX (802.16)
• WiMAX stands for Worldwide Interoperability for
  Microwave Access
• It is a metropolitan area network(MAN) with a
  range of 1-30 miles.
• It odes not yet inclued mobility, but that will be
  added as 802.16e
• Its clients are multiplexers for a building and to
  adapt to different frequencies it uses different
  physical layer protocols.

                                                  189
       Cell Phone Technologies
• Frequency bands vary around the world:
  – Europe 900 and 1800 MHz bands
  – North America 850 and 1900 MHz bands
• Cost is high to users because of licensed spectrum
• Incompatible cell phone standards
• Phones designed to carry voice now carry video,
  and audio which require high bandwidth

                                              190
      Cell Phone Technologies
• Relies on use of base stations that are part
  of a wired network
• Geographic area served by the base
  station‟s antenna is called a cell
• Cells overlap and a base station can serve
  more than one cell using multiple antennae.


                                             191
                    Handoff
• As a phone begins to leave a cell, it moves into an
  area of overlap with other cells
• The current base station senses the weakening
  signal and give control to whichever base station
  is receiving the stronger signal from it.
• If a phone is receiving a call, the call must be
  transferred over to the new base station in what is
  called a handoff.

                                                    192
       Cell Phone Generations
• 1G – analog
• 2G – digital -most of the current technology, some
  are referred to as 2.5 G – not quite third
  generation, but more advanced. These are GSM-
  Global System for Mobile Communications
• 3G – Based on CDMA (code division Multiple
  Access
• Satphones- class of phones that are not cellular,
  but are satellite phones

                                                  193
                      Summary
• Five key problems so that links can exchange information:
• Encoding problem for physical links carrying signals;
• The framing problem determines how to package bits into
  frames;
• The error detection problem using CRC, parity, and
  checksums
• Problem of recovering lost frames discarded because of
  errors
• Problem of mediating access on shared media (Ethernet,
  token ring and wireless)
                                                     194
            Further Reading
• Metcalfe, Robert. and Boggs, David,
  “Ethernet: Distributed Packet Switching For
  Local Computer Networks”,
  Communications of the ACM, 19(7):395-
  403, July, 1976
• http://standards.ieee.org/ for status of IEEE
  standards
• See p. 145 for more complete list…

                                             195

				
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