Seven Layers of Network Architecture

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Seven Layers of Network Architecture

• Transport Layer
• Fourth layer (L4) very important
• Primarily responsible for ensuring that data are
  transferred from point A to point B reliably, in
  the correct sequence, and without errors.
• Flow control
• The method of gauging the appropriate rate of
  data transmission
• Based on how fast the recipient can accept data
• Data transmission rate from NIC (10 Mbps)

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Seven Layers of Network Architecture

• Transport Layer (continued)
• L4 also breaks long packets into the maximum
  size that the type of network can handle
• e.g. ethernet1500 bytes max.
• Sequencing
• When data is divided, they are assigned a
  sequence so that it can be re-assembled in the
  correct order by the receiving nodes transport
  layer services.

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Seven Layers of Network Architecture

• Transport Layer (continued)
• Acknowledgement (ACK)
• Used to notify the sender that data were received
  correctly.
• If data contains errors, the transport layer will
  request that the sender re-transmit.
• If data wasnt ackd within a given time period,
  the senders transport layer would consider the
  data lost and re-transmit (until TTL expires).
• L4 protocols
• TCP (transmission control protocol), UDP (user
  datagram protocol), and SPX (sequence packet
  exchange)

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Seven Layers of Network Architecture

• Session Layer
• Fifth layer (L5)
• Establishes and maintains communication between
  two nodes on the network (session)
• To do at L5
• Establish and keep alive communication link
  during session.
• Synchronizing the dialog between the two nodes
• Communications handshake negotiation
• Port level application usage
• Telnet (23), FTP (20/21), HTTP (80), DNS (53)

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Seven Layers of Network Architecture

• Presentation Layer
• Sixth layer (L6)
• Serves as a translator between the application
  and the network.
• Data becomes formatted in a schema that the
  network can understand.
• Also manages data encryption / decryption.
• Presentation application usage formatting data
• HTTP, DNS, FTP, SNMP

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Seven Layers of Network Architecture

• Application Layer
• Seventh layer (L7) Last one!
• Provides interface to the software enabling
  programs to use network services.
• File transfer
• File management
• Message handling for email
• Network access within an application

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Application Program Interface (API)

• Routine that allows a program to interact with
  the operating system
• Belongs to the Application layer of OSI Model
• Devices
• Upper layer switches
• Traffic shaping

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Applying the OSI Model
TABLE 2-1 Actions for each layer of the OSI Model
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Communication Between Two Systems

• Application You choose to get email.
• L7 formulates a request for data from the mail
  server and and transfers the request to the
  presentation layer.
• Presentation
• Determines any special formatting or encryption
  and adds any translation or codes required and
  then passes on to the session layer.

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Communication Between Two Systems

• Session - Picks up the request and assigns a data
  token to it. (as an example)
• Token - special control frame indicating to the
  rest of the network that a particular node has
  the right to transmit data.
• Transport
• data and control information are broken down into
  manageable chunks of data and are prepared to be
  packaged into frames later.
• If data is too large to fit in one frame, they
  are put into smaller blocks with sequence numbers
  in each block. The transport layer then passes
  the data blocks, one at a time, to the network
  layer.

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Communication Between Two Systems

• Network
• This layer adds the logical address information
  to identify source and destination and then
  passes on to data link layer.
• Data Link
• The data are now packaged into individual frames.
• Remember frame structured format for
  transmitting small blocks of data.
• Each frame has its own built-in error check using
  FCS (Frame Check Sequence) and is inserted by the
  DL layer.
• Field in a frame responsible for ensuring that
  data carried by the frame arrives intact.

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Communication Between Two Systems

• Physical
• The request hits the NIC and delivers the data to
  the cabling and across the network.
• The physical layers does not interpret the frame
  or interpret information.
• Other side
• Once the data hits the NIC at the mail server,
  the mail servers DL layer begins to unravel the
  request (reversing process) until it responds
  back with its own transmission, beginning at the
  application layer.

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Communication Between Two Systems
FIGURE 2-3 Data transfer between two systems
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Data Transformation
FIGURE 2-4 Data transformed through the OSI Model
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Frame Specifications

• Frames are composed of smaller components
  (fields).
• The characteristics of the components depend on
  the type of network and on the standards they
  must follow.
• The two most commonly used are Ethernet and Token
  Ring
• Each frame type is unique and will not interact
  with different frame types on the same network.
• You can support gt 1 protocol though.

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Frame Specifications

• Ethernet
• Networking technology originally developed in
  1970 by Xerox
• Most popular LAN technology used today
• Token Ring
• Networking technology developed by IBM in the
  1980s
• Relies upon direct links between nodes and a ring
  topology, using tokens to allow nodes to transmit
  data
• More in Chapter 5 on both technologies and how
  they interact with topologies.

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Typical Ethernet Frame
FIGURE 2-5 Ethernet frame as specified by the
IEEE 802.3 standard

• 802.3 Standard
• IEEE standard for Ethernet networking devices and
  data handling

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Components of the Ethernet 802.3 Frame

• Preamble marks the beginning of the entire
  frame. Announces to the network that data is
  coming. Not used in frame size calculations.
• Start of Frame Delimiter (SFD) beginning of the
  addressing frame
• Used as control information (w/ LEN)
• Destination Address
• Source Address

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Components of the Ethernet 802.3 Frame

• Length (LEN) length of the packet
• Part of the control information (w/ SFD)
• Data Data sent from the network layer
• Pad used to increase the minimum frame size of
  46 bytes
• Frame Check Sequence (FCS) provides an
  algorithm to determine whether the data were
  received correctly.
• Cyclic Redundancy Check (CRC)
• Most commonly used algorithm used to verify the
  accuracy of data contained in a data frame

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Typical Token Ring Frame
FIGURE 2-6 Typical Token Ring frame as specified
in the IEEE 802.5 standard

• 802.5 Standard
• IEEE standard for Token Ring networking devices
  and data handling

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Components of Token Ring Frame

• Start Delimiter (SD) beginning of the packet.
  1 of 3 used for control information.
• Access Control (AC) contains information about
  the priority of the frame. 1 of 3 used for
  control information.
• Frame Control (FC) defines the type of frame
  and is used in the FCS. 1 of 3 used for control
  information.
• Destination Address
• Source Address

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Components of Token Ring Frame

• Data data sent from the network layer
• Frame Check Sequence (FCS) used to check the
  integrity of the frame
• End Delimiter (ED) Indicates the end of the
  frame.
• Frame Status (FS) Indicates whether the
  destination node recognized and correctly copied
  the frame, or whether the destination node was
  not available.

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IEEE Networking Specifications
TABLE 2-2
IEEE 802 standards Project 802 an effort to
place standards on the physical.
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IEEE Networking Specifications

• To accommodate shared access for multiple network
  nodes, the IEEE expanded the OSI Model by
  separating the Data Link layer into two sublayers
• Logical Link Control (LLC) sublayer
• Media Access Control (MAC) sublayer

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Data Link Layer Sublayers

• LLC
• Upper sublayer
• Provides common interface
• Supplies reliability and flow control services
• MAC
• Lower sublayer
• Appends the physical address of the destination
  computer onto the frame

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Subdivided Data Link Layer
FIGURE 2-7 LLC and MAC sublayers
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Chapter Summary

• Standards are documented agreements stipulating
  how a particular product or service should be
  designed or performed
• Prominent standards organization
• ANSI
• EIA
• IEEE
• ISO
• ITU

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Chapter Summary

• Networking architecture layers of OSI Model
• Physical layer
• Data Link layer
• Network layer
• Transport layer

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Chapter Summary

• Networking architecture layers of OSI Model
  (cont.)
• Session layer
• Presentation layer
• Application layer

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Chapter Summary

• A data request from a software program is
  received by the Application layer services and is
  transferred down through the layers of the OSI
  Model until it reaches the Physical layer
• Data frames are small blocks of data with
  control, addressing, and handling information
  attached to them

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Chapter Summary

• Each node on a network can be identified by two
  types of addresses
• Network layer address
• Data Link layer address
• In addition to frame types, IEEE networking
  specifications apply to connectivity, networking
  media, error checking algorithms, encryption,
  emerging technologies, and more

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Chapter Summary

• ISO expanded the OSI Model by separating the Data
  Link layer into sublayers
• LLC layer
• MAC layer