COM 360

1
COM 360
2
Chapter 2

• Direct Link Networks

3
Network Technologies

• Point-to-Point Links
• Carrier Sense Multiple Access ( CSMA) (for
  example the Ethernet)
• Token Rings (for example IEEE 802.5 and FDDI )
• Wireless (for which 802.11 is the emerging
  standard)

4
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

5
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

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Nodes
Example workstation architecture
7
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

8
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

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Electromagnetic Spectrum
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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

11
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

12
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.

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Local Link Cables
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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

15
Common Bandwidths
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Last-Mile links

• Leased lines range in price from 1000/month to
  dont 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

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Common Available Services
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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

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ADSL
downstream
upstream
ADSL connects the subscriber to the central
office via the local loop.
20
VDSL
VDSL connects the subscriber to the optical
network that reaches the neighborhood.
21
Shannons Theorem

• Shannons theorem gives an upper bound to the
  capacity of a link, in terms of bits per second.
• C B log2 (1S/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)

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Shannons TheoremExample

• dB ratio pf 30 dB
• S/N 1000
• Bandwidth 3000Hz
• C B x log2 ( 1S/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

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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.

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Network Adaptor
Signals travel between signaling components. Bits
flow between adaptors. Network interface cards
are called NICs.
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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.

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Block Diagram of a Network Adaptor
27
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.

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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 hosts 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.

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Programmed I/O
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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

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Memory Bandwidth on Modern PC
32
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.

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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

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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

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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.

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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 1s 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.

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NRZ Encoding
38
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 1s, but not
  0s)

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Manchester Encoding

• Merges the clock with the signal by transmitting
  the exclusiveOR 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.

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Encoding Strategies
41
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)

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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.

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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.

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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)

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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

46
Framing

• Suppose A wishes to transmit a frame to B
• It tells adapter to transmit a frame from the
  nodes 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
  Bs memory.

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Framing
Bits flow between adaptors, frames between hosts
48
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)

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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 1960s
• DDCMP ( Digital Data communication Message
  Protocol) was used in Digital Equipments DECNET.
• These are examples of the sentinel approach and
  the byte counting approach.

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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.

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BISYNC Frame Format
52
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.

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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.

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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.

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Disadvantages of Point-To-Point

• For N computers
• Connections (N2 - N)
•   2

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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.

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PPP Frame Format
58
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 PPs 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.

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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
  frames body.

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DDCMP Frame Format
61
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.

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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.

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HDLC Frame Format
64
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

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Byte Stuffing

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

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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

67
Bit Stuffing

• Anytime 5 consecutive 1s are transmitted, the
  sender inserts a 0 before sending the next bit.
  On the receiving side.
• When the receiver detects 5 consecutive 1s, 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.

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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

69
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

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SONET STS-1 Frame
First two bytes of the frame contain a special
bit pattern that indicates the start of the frame
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STS-1 Multiplexing
Three STS-1 frames are multiplexed onto one STS-3
frame.
72
SONET Frames Out of Phase
73
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

74
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

75
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 1s
• Checksums code created based on addition
• Cyclic Redundancy Check (CRC) based on a
  complex mathematical algorithm and used in nearly
  all link level protocols.

76
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

77
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 100100011 (even
  parity)
•   Incorrect data 101100011 (odd number of 1s)
•   Transmitter and receiver agree on which parity
  to use
•   Receiver detects error in data with incorrect
  parity

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Limitations of Parity Checking

•   Parity can only detect errors that change an
  odd number of bits
•   Original data and parity 100100011 (even
  parity)
•   Incorrect data 101100111 (even parity!)
•   Parity usually used to catch one-bit errors

79
Two-Dimensional Parity

• Two-dimensional parity involves adding on extra
  bit to balance the number of 1s 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.

80
Two-Dimensional Parity
81
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

82
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)

83
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

84
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.

85
Checksum Limitations

•  Does not detect all common errors (like
  reversed bits)

86
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.

87
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

88
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.

89
CRC Hardware

• X-Or and Shift Registers

90
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.

91
CRC Calculation using Shift Registers
92
CRC Computation

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

93
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

94
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)  
•  

95
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

96
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.  

97
Another CRC Example
See text. Pp. 94-95
98
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

99
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

100
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.

101
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.

102
Stop and Wait Algorithm
a) Arrives
c)ACK lost
b) Frame lost
d) Timeout too soon
103
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.

104
Timeline for Stop and Wait
105
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

106
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 lt SWS

107
Sliding Window
108
Timeline for Sliding Window
109
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/d
  emo.html

110
Sliding Window on Sender
111
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)

112
Sliding Window on Receiver
113
Sliding Window Algorithm

• When frame with sequence number SeqNum arrives,
  the receiver does the following
• If SeqNum lt LFR or SeqNum gtLAF then frame is
  outside the window and is discarded.
• If LFR lt SeqNum lt LAF, then it is accepted.
• SeqNumToAck is largest not yet acknowledged
• Receiver acknowledges receipt of SeqNumToAck and
  sets LFR SeqNumToAck
• LAFLFR RWS

114
Comparison of Sliding Window and Stop Wait
115
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.

116
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.

117
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.

118
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?

119
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.

120
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.

121
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.

122
Ethernet Transceiver and Adapter
123
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

124
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

125
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
126
Twisted Pair Ethernet

•  Variously called 10Base-T, twisted pair or TP
  Ethernet
•  Replaces AUI cable with twisted pair cable
• Replaces thick coax with hub

127
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.

128
Ethernet repeaters




Repeater
Host
129
Ethernet Hubs
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.
130
HUBS
131
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.

132
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

133
Network Interface Hardware
134
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.
135
NIC Cards and Wiring
NICS can provide all three connection technologies
136
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

137
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

138
Assigning Addresses
139
Addressing Scheme Comparison
Addressing Scheme Advantages
Disadvantages
140
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 1s, 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.

141
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

142
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.

143
Worst Case Scenario
144
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

145
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.

146
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.

147
Token Ring Network
148
Implementing a Token Ring
149
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.

150
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

151
Relay on Token Ring
b) Relay closed-host bypassed
a) Relay open host active
152
Multimedia Access Unit
Used only in electrical rings to compensate for
node failure.
153
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.

154
Token Release
a) early
b) delayed
155
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.

156
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

157
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.

158
Dual Fiber Ring
a) Normal operation
b) Failure of primary ring
159
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

160
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

161
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 MANs but metro Ethernet is coming

162
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.

163
Wireless Technologies
164
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.

165
(No Transcript)
166
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

167
Example Wireless Network
168
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

169
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.

170
(No Transcript)
171
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.

172
Bluetooth piconet
173
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)

174
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.

175
4-Bit Chipping Sequence
176
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.

177
Hidden node problem A C can collide at B
178
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 As ability to
  receive from C

179
Exposed node problem B can
transmit to A and C can transmit to D
180
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.

181
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.

182
Access Points connected to a Distribution Network
Each node associates itself with one access point.
183
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

184
Selecting an AP

• Technique called scanning
• The node sends a Probe frame
• All APs within reach reply with a Probe Response
  Frame
• The nodes selects one of the access points and
  sends that AP an Association Request Frame.
• 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.)

185
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.

186
Node Mobility
187
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
188
802.11 Frame Format
189
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.

190
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

191
Cell Phone Technologies

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

192
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.

193
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

194
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)

195
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