Introducing the OSI Reference Model

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Introducing the OSI Reference Model

• Protocols are the languages that networked
  computers use to communicate.
• The developers of networking protocols often
  split the essential functions into multiple
  layers, which are implemented by separate
  protocols.
• The combination of protocols needed to provide
  network communication is called a protocol stack.
  
• For communication to occur, the corresponding
  layers of the stack on two computers must run the
  same protocols.
• The most common generalized representation of the
  protocol stack is the Open Systems
  Interconnection (OSI) reference model.

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The Seven Layers of the OSI Model
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The OSI Model and the Protocol Stack

• The top of the OSI model represents an
  application running on the computer, and the
  bottom of the model represents the network medium
  that connects the computers.
• Each layer contains specific functions that
  contribute to the process that enables one
  computer to send data to another.
• The protocols operating at adjacent layers
  communicate by providing services to, and
  receiving services from, one another.

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Understanding Data Encapsulation

• Data encapsulation is the process by which the
  protocols operating at the various layers of the
  OSI model package the information they receive
  from the layer above.
• A protocol receiving data from the layer above it
  encapsulates the data by adding its own
  information, in the form of a new header (and
  sometimes a footer).
• The header consists of fields that contain
  information specific to the encapsulating
  protocol that the corresponding protocol on the
  destination computer will read.
• The information received from the layer above
  follows the header and becomes the payload in the
  unit of data created by the protocol.
• When the data unit is passed down to the layer
  below it, another protocol encapsulates it again.

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The Data Encapsulation Process
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Logical Communications Between Protocols on
Different Computers
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The Physical Layer

• The physical layer provides the interface between
  the computer and the network medium that carries
  data from one computer to another.
• In most cases, the network medium is a cable.
• In a computer, the physical layer takes the form
  of a network interface adapter or a modem.
• The physical layer defines the signals used by
  the network medium and the properties of the
  medium itself.

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The Data-Link Layer

• The data-link layer is responsible for the final
  packaging of the application data before it is
  transmitted over the network medium.
• A data-link layer protocol receives data units
  from network layer protocols and encapsulates
  them for the final time.
• The data-link layer protocol header contains the
  addresses of the computer sending the data and
  the computer receiving it.
• The manufacturer hard codes the 6-byte addresses
  used by the data-link layer protocols on a LAN
  into the network interface adapters.
• These addresses are known as hardware addresses,
  or media access control (MAC) addresses.

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Other Data-Link Layer Functions

• Media access control (MAC). The process by which
  a computer gains access to a shared network
  medium
• Protocol identification. The process by which the
  data-link layer protocol identifies the protocol
  that generated the payload carried in the packet
• Error detection. The process by which the
  destination computer checks for errors by
  comparing its cyclical redundancy check (CRC)
  value with the CRC value from the sending
  computer
• If the results match, the packet has been
  transmitted without error.
• If the results do not match, the receiving system
  discards the packet.

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The Network Layer

• The network layer is primarily responsible for
  the end-to-end communications between computers
  located on different networks.
• The network layer protocol encapsulates data
  using a header that contains Source and
  Destination Address fields that identify the
  ultimate source and destination of the packet.
• The network layer protocol is also responsible
  for routing packets, fragmenting them (when
  necessary), and identifying the protocol that
  generated the data in the packet.

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Network Layer Addresses

• The most common network layer protocol, the
  Internet Protocol (IP), contains its own
  independent system of addresses.
• Novell NetWares Internetwork Packet Exchange
  (IPX) protocol uses the hardware addresses coded
  into the computers network interface adapters.
• The NetBIOS Extended User Interface (NetBEUI)
  protocol provided with Microsoft Windows uses
  Network Basic Input/Output System (NetBIOS) names
  as network layer identifiers.

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The Transport Layer

• Provides functions that complement those of the
  network layer protocol and help to get the data
  to the destination in a timely and efficient
  manner
• Uses connection-oriented and connectionless
  protocols

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Connection-Oriented Protocols

• A connection-oriented protocol is one in which
  the source and destination computers exchange a
  series of messages before they transmit any
  application data.
• Connection-oriented communications are also
  usually associated with guaranteed delivery, in
  which the computer receiving data returns
  acknowledgments to the sender on a regular basis,
  confirming that it has received the data without
  errors.
• Connection-oriented protocols are typically used
  to transmit large amounts of data that require
  extreme accuracy.

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Segmentation

• When transmitting large amounts of data, a
  transport layer protocol must split the data
  stream into segments, which can fit into single
  packets.
• Segmentation is a lot like the fragmentation
  process that occurs at the network layer, but be
  careful not to confuse the two.
• The sending computer splits the stream into
  segments of an appropriate size and packages each
  segment in a separate packet.
• The transport layer protocol header contains a
  code that identifies each segment so that the
  destination computer can reassemble the segments
  into the original data stream.

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Flow Control

• Flow control is the ability of the receiving
  computer to transmit information that instructs
  the sender to modify its transmission rate.
• When the memory buffer in the network interface
  adapter of a computer receiving a data
  transmission approaches fullness, the computer
  can send flow control information to the sender,
  ordering it to slow down its transmission rate.
• In the Transmission Control Protocol (TCP), flow
  control is implemented as a field in the protocol
  header that specifies the number of packets the
  computer can receive.

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Connectionless Protocols

• Connectionless protocols are much simpler than
  connection-oriented protocols and have much lower
  overhead.
• Do not transmit connection establishment messages
  before they transmit application data, and do not
  use packet acknowledgments
• Have no segmentation or flow control
• Usually are not suitable for transmitting large
  amounts of data requiring extreme accuracy
• For the most part, computers use connectionless
  protocols for quick exchanges that consist of a
  single request and reply.
• The reply message functions as a tacit
  acknowledgment, and the request is easily
  retransmitted if no reply is received in a timely
  manner.

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The Session Layer

• The session layer performs many functions that
  aid in the exchange of messages between two
  computers, which is called a dialog.
• Dialog separation inserts a bookmark-like device
  called a checkpoint into a dialog stream, which
  enables the communicating computers to perform an
  action at the same point in the dialog.
• Dialog control regulates the communications
  between the two computers through Two-Way
  Alternate (TWA) mode or Two-Way Simultaneous
  (TWS) mode.
• TWS mode presents problems that the session layer
  has to address.

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The Presentation Layer

• Provides a translation service that enables
  different types of computers to communicate.
• Applications on each computer platform generate
  network access requests using their own native
  syntax, which might be different from the syntax
  used by the application on the destination
  computer.
• Before the data generated by the application
  reaches the transport layer, the computer
  converts it from its native syntax (called an
  abstract syntax) to a transfer syntax, suitable
  for transmission over the network.
• The computer receiving the data then translates
  the incoming information, this time converting
  the transfer syntax to the applications own
  abstract syntax.

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The Application Layer

• Application layer protocols form the top of the
  computers protocol stack.
• An application running on a computer uses an
  application layer protocol to request access to a
  resource located elsewhere on the network.
• The application layer protocol is also the final
  destination for the data passed up through the
  stack on the receiving computer.
• There are many application layer protocols, and
  each is designed to provide highly specialized
  services required by a particular application or
  type of application.

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Commonly Used Application Layer Protocols

• Hypertext Transfer Protocol (HTTP)
• File Transfer Protocol (FTP)
• Domain Name System (DNS)
• Dynamic Host Configuration Protocol (DHCP)
• Simple Mail Transfer Protocol (SMTP)
• Simple Network Management Protocol (SNMP)

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The OSI Model and a Windows Protocol Stack
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Windows 2003 Networking Components

• The networking functions defined by the OSI
  reference model are realized in Microsoft Windows
  2003 by the following four components
• Network interface adapters
• Protocols
• Clients
• Services

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The Windows 2003 Protocol Stack
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Network Interface Adapters

• A network interface adapter in a computer running
  Windows 2003 typically consists of a network
  interface card (NIC) and the device driver that
  the computer needs to communicate with it.
• These components perform the physical and
  data-link layer functions of the OSI reference
  model.
• A computer with a single network interface
  adapter can handle the data traffic of multiple
  protocol modules operating above it.
• The packets generated by the various protocols
  are combined and transmitted over the single
  network medium, a process called multiplexing.
• A computer can also have multiple network
  interface adapters connecting it to different
  networks.

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Multiplexing and Separating Traffic with Multiple
Network Interface Adapters
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Protocols

• Computers use many protocols when communicating
  on a network, but often these protocols are
  grouped together into protocol suites.
• TCP/IP, the default protocol suite used by
  Windows 2003, includes not only TCP and IP, but
  also many other protocols operating at various
  layers of the OSI model.
• Windows 2003 (and most other operating systems)
  installs protocol suites as a single entity.
• "Installing a protocol" in Windows 2003 means
  installing a single protocol module, which
  technically implements multiple protocols and
  applications.

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Protocols (Cont.)

• Windows 2003 includes two other protocols that
  are roughly analogous to TCP/IP.
• NWLink is the Microsoft version of the IPX
  protocols developed by Novell for use with its
  NetWare operating system.
• NetBEUI was the original default networking
  protocol for the Windows operating systems.

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Clients

• The Client for Microsoft Networks module provides
  basic Windows network file and print services to
  the computer.
• For NetWare connectivity, Windows 2003 also
  includes Client Service for NetWare (in Microsoft
  Windows 2003 Professional) or Gateway Service for
  NetWare (in Microsoft Windows 2003 Server).
• These client modules are based on a redirector,
  which evaluates resource access requests and
  determines whether the requested resource is
  located on the local machine or on the network.
• If the resource is on the network, the redirector
  passes the request to the appropriate protocol.

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Services

• In Windows terminology, a service is a program
  that runs continuously on a computer, waiting to
  satisfy requests for particular functions.
• Windows 2003, particularly in its server
  versions, includes a large collection of services
  that provide networking functions.
• By default, Windows 2003 installations include
  services that provide basic networking
  functionality, such as Server, Workstation,
  Messenger, Browser, and Netlogon.
• Windows 2003 includes many optional services that
  you can install with the OS or at any time
  afterward, such as

• Dynamic Host Configuration Protocol (DHCP)
• Domain Name System (DNS)
• Windows Internet Name Service (WINS)

• Microsoft Certificate Services
• Routing and Remote Access Service (RRAS)
• Internet Information Services (IIS)

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Installing Windows 2003 Networking Components

• To participate on a network, a computer running
  Windows 2003 must have, at the very least, a
  network interface adapter, a protocol, and a
  client installed.
• When you install a network interface adapter,
  Windows 2003 installs a basic default protocol
  stack configuration consisting of the following
  components
• A device driver for the network interface adapter
• Client for Microsoft Networks
• The Internet Protocol (TCP/IP) module
• The File and Printer Sharing for Microsoft
  Networks service

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Configuring Network Components in the Local Area
Connection Properties Dialog Box
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Binding Windows 2003 Networking Components

• When you install a networking component, it is
  automatically bound to all the other components
  above and below it.
• You can control the bindings of the various
  components from the Properties dialog box for
  each connection.
• The check boxes next to the components in the
  Properties dialog box indicate which components
  are bound to the adapter used by that connection.
  
• To unbind a component from that connection, you
  clear the check box next to that connection.