Chapter 9. Hardware Addressing And Frame Type Identification

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Chapter 9. Hardware Addressing And Frame Type
Identification

• Jing Wang
• Towson University

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9.1. Introduction

• Advantage of shared networks
• Universal connectivity because all computers
  share the medium, a transmitted signal reaches
  all computers
• This chapter
• Consider transmission across a shared LAN in more
  detail
• How a pair of computers communicate without other
  computers to receive and process a copy of each
  message
• Hardware addressing
• Network interface hardware
• Frame type

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9.2. Specifying A Recipient

• How can two computers communicate directly across
  a shared medium in which all attached stations
  receive a copy of all signals?
• Most LAN technologies use an addressing scheme to
  provide direct communication
• Each station on the LAN is assigned a unique
  numeric value called a physical address, hardware
  address or media access address (MAC address)
• Each frame begins with a header that contains
  destination address field and source address
  field
• Network interface hardware accepts only those
  frames in which the destination address matches
  the stations address

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9.2. Specifying A Recipient

• Each computer attached to a LAN is assigned a
  number known as its physical address.
• A frame sent across a LAN contains the address of
  the sending computer, called a source address,
  and the address of the intended recipient, called
  the destination address

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9.3. How LAN Hardware Uses Addresses To Filter
Packets

• LAN interface hardware handles all the details of
  sending and receiving frames on the shared medium
• Checks the length of an incoming frame
• Checks the CRC
• Discards frames that contain errors
• Sends and receives frames without using the
  computers CPU

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9.3. How LAN Hardware Uses Addresses To Filter
Packets

• Figure 9.1. Organization of the hardware in a
  computer attached to a LAN. Because it is
  powerful and independent, the network interface
  hardware does not use the CPU when transmitting
  or receiving bits of a frame.

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9.3. How LAN Hardware Uses Addresses To Filter
Packets

• LAN interface hardware uses physical addressing
  to prevent the computer from receiving all
  packets that travel across the LAN
• Compares the destination address in the frame to
  the physical address of the station
• If match, accepts the frame and passes it to the
  operation system
• If not, discards the frame and waits for the next
  frame
• Ignore the frame addressed to a nonexistent
  station

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9.3. How LAN Hardware Uses Addresses To Filter
Packets

• A shared network system uses physical addresses
  to filter incoming frames.
• The network interface hardware, which handles all
  the details of frame transmission and reception,
  compares the destination address on each incoming
  frame to the stations physical address, and
  discards frames not destined for the station

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9.3. How LAN Hardware Uses Addresses To Filter
Packets

• Because a network interface operates without
  using a stations CPU, a frame can be transferred
  across a shared LAN from one computer to another
  without interfering with processing on the
  computers

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9.4. Format Of A Physical Address

• Static
• relies on the hardware manufacturer to assign a
  unique physical address to each network interface
• Configurable
• provides a mechanism that a customer can use to
  set a physical address
• Dynamic
• provides a mechanism that automatically assigns a
  physical address to a station when the station
  first boots

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9.4. Format Of A Physical Address

• Static
• Ease of use
• Permanence
• Dynamic
• Eliminates the need for hardware manufacturers to
  coordinate in assigning addresses
• Allows each address to be smaller
• Lack of permanence and potential conflict
• Configurable
• Permanent
• Do not need to be large
• Can be replaced without changing the computers
  physical address

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9.5. Broadcasting

• Broadcasting
• It makes a copy of data available to all other
  computers on the network
• Since LANs employ shared media, they make
  broadcasting extremely efficient
• Extend the addressing scheme
• A special, reserved broadcast address
• If a frame arrives with the special broadcast
  address or the stations physical address in its
  destination address, accepts it

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9.5. Broadcasting

• The network interface hardware in a computer
  makes a copy of every frame that passes across
  the shared network.
• The interface accepts the frame and delivers a
  copy to the operating system if the destination
  address in the frame is the reserved broadcast
  address or matches the computers physical
  address.
• Thus, when a frame is sent to the broadcast
  address, each computer on the network receives a
  copy

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9.6. Multicasting

• Multicast
• A network interface does not automatically
  forward multicast frames to the CPU
• it is programmed to make decision to accept or
  reject frames

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9.7. Multicast Addressing

• Multicast
• Reserve an additional set of addresses
• When computer boots, the interface is programmed
  to recognize only the computers address and the
  broadcast address
• If an application wishes to receive multicast
  frames, it inform the network interface which
  multicast address to use
• The interface adds the address to the set it will
  recognize, and begins to accepting frames sent to
  that address

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9.8. Identifying Packet Contents

• Explicit frame type
• type information is included in the frame
• Frame type field - the bits of a frame used to
  identify the contents
• Self-identifying frame
• Implicit frame type
• does not include a type field

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9.9. Frame Headers And Frame Format

• Figure 9.2. The general format of a frame sent
  across a LAN. The header contains information
  such as the addresses of the sender and the
  recipient.

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9.9. Frame Headers And Frame Format

• Each LAN technology defines a frame format.
• Most technologies have frames that each consist
  of a header followed by a data area.
• Because the size and format of the header is
  fixed, all frames used with a given technology
  have the same header format
• The size of the data area is determined by the
  data being sent in the frame

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9.10. An Example Frame Format

• Figure 9.3. Illustration of the frame format used
  with Ethernet. The number in each field gives the
  size of the field measured in 8-bit octets.

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9.10. An Example Frame Format

• Figure 9.4. Examples of frame types used with
  Ethernet (type values are given in hexadecimal).
  The table lists only a few examples many other
  types have been assigned.

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9.11. Using Networks That Do Not Have
Self-Identifying Frames

• Before any data is sent, the sender and receiver
  agree to use
• A single format for data
• The first few octets of the data field to store
  type information

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9.11. Using Networks That Do Not Have
Self-Identifying Frames

• Figure 9.5. Illustration of how type information
  can be included in a frame's data area if the
  frame header does not include a type field.

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9.11. Using Networks That Do Not Have
Self-Identifying Frames

• IEEE LLC/SNAP
• Part of IEEE 802.2
• Logical Link Control (LLC) SubNetwork Attachment
  Point (SNAP) header
• Logical Link Control
• Originally designed to be the same for all LANs
  for interoperability, but not used often today.
• Interoperability is provided by a common network
  layer protocol.
• SubNetwork Attachement Point
• Organizationally Unique Identifier (OUI)
• Identify a particular standards organization
• Type
• Type value defined by that organization

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9.11. Using Networks That Do Not Have
Self-Identifying Frames

• Figure 9.6. An example of the 8-octet IEEE
  LLC/SNAP header, which is used to specify the
  type of data. The SNAP portion specifies an
  organization and a type defined by that
  organization.

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9.12. Network Analyzers, Physical Addresses,
Frame Types

• Network analyzer (network monitor)
• A device used to determine how well a network
  system is performing
• Also called network sniffer
• Consist of a standard portable computer (e.g. a
  notebook PC) with a standard LAN interface
• Network interface card in promiscuous mode
• Accept all frames

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9.12. Network Analyzers, Physical Addresses,
Frame Types

• A network analyzer is a device that can be
  configured to count or display frames as they
  pass across a shared network.
• An analyzer obtains a copy of each frame, and
  then uses header fields such as the physical
  source address, physical destination address, or
  type information to determine how to process the
  frame

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9.13. Summary

• Each computer attached to the shared network is
  assigned a unique physical address
• Network interface hardware handles transmitting
  and receiving frames
• Broadcast packet is only transmitted once on a
  shared LAN
• Multicast use the network interface hardware to
  examine frames

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9.13. Summary

• Physical address could be static, configurable,
  dynamic
• Frame header includes frame type
• If not, type information could be placed in an
  LLC/SNAP header in the first few octets of the
  frame data area
• A network analyzer is a device that can be used
  to debug problems on a network

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Supplement

• http//www.mhhe.com/engcs/compsci/forouzan/

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Chapter 14
Local Area NetworksEthernet
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Figure 14.1 Three generations of Ethernet
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Figure 14.2 802.3 MAC frame
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Figure 14.3 Minimum and maximum length
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Figure 14.4 Ethernet addresses in hexadecimal
notation
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Figure 14.5 Unicast and multicast addresses
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Figure 14.6 Physical layer
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Figure 14.7 PLS
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Figure 14.8 AUI
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Figure 14.9 MAU (transceiver)
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Figure 14.10 Categories of traditional Ethernet
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Figure 14.11 Connection of a station to the
medium using 10Base5
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Figure 14.12 Connection of stations to the
medium using 10Base2
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Figure 14.13 Connection of stations to the
medium using 10Base-T
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Figure 14.14 Connection of stations to the
medium using 10Base-FL