Guide to Networking Essentials Fifth Edition

1
Guide to Networking EssentialsFifth Edition

• Chapter 5
• Making Networks Work

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Objectives

• Explain the OSI reference model layers and their
  relationship to hardware and software
• Describe the function and creation of a data
  frame
• Explain the IEEE 802 networking model and related
  standards

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Understanding the OSI and 802 Networking Models

• The Open Systems Interconnection (OSI) reference
  model was proposed by the ISO
• Common framework for developers and students of
  networking to work with and learn from
• Attempt to develop a working set of protocols and
  technologies based on the OSI model and to put
  those efforts into common use never materialized
• IEEE 802 networking model provides detailed
  implementation specifications for a number of
  networking technologies
• Influential set of networking standards

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Role of a Reference Model

• Reference models and standards enable
  interoperability among layers
• Computer networking, computer compatibility, and
  networking features and functions can be daunting
  concepts to grasp
• However, they would be more difficult to
  comprehend if networking werent built on a
  common framework with the process separated into
  layers
• The OSI model and its seven-layer approach to
  networking provides this common framework

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OSI Reference Model

• OSI reference model drafted in late 1970s by
  ISO theoretical model for networks of all kinds
• By 1983, the draft became ISO Standard 7498
• Models foundation networking can be separated
  into a series of related tasks
• Each task can be conceptualized as a single
  aspect, or layer, of the communication process
• Reduces complexity of networked communications
  into series of interconnected tasks and
  activities
• Divide and conquer approach relationship among
  tasks persists, but each can be handled
  separately, and its issues solved independently

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Understanding Layers

• The OSI reference model for networking clarifies
  many communications activities and related tasks
  and requirements to help in understanding what
  networks are and how they work
• Breaks down all the events that must occur for
  data to be addressed and formatted correctly
  before it can actually be delivered to its final
  recipient
• With a layered approach, one part of the process
  can change, sometimes drastically, while the rest
  of the process remains unchanged

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Structure of the OSI Reference Model

• A computer that accesses a network must have a
  protocol stack (protocol suite)
• TCP/IP
• IPX/SPX
• NetBEUI
• AppleTalk
• Protocols plus drivers equal network access

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Structure of the OSI Reference Model (continued)
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Structure of the OSI Reference Model (continued)
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Structure of the OSI Reference Model (continued)
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Structure of the OSI Reference Model (continued)

• Communication between peer layers is virtual
• In reality, communications pass up and down the
  protocol stacks on both machines
• As data gets passed from layer to layer, its
  divided into data units appropriate for the layer
• Protocol data units (PDUs) are passed as a
  self-contained data structure from layer to layer
• Encapsulation process adds headers to allow
  successful delivery of each layers payload
• Decapsulation strips header information on way up
• No layer can pass information directly to its
  peer counterpart except for the Physical layer

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Application Layer

• Layer 7 PDU data
• Set of interfaces to access networked services
• E.g., networked file transfer, message handling,
  and database query processing
• Handles network access, moving data from sender
  to receiver, and error recovery for applications
• Components usually have a client and a server
  part
• E.g., HTTP, Client for Microsoft Networks, NFS
• Possible problems missing/misconfigured client
  or server SW, incompatible or obsolete commands
  used to communicate between client and server

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Presentation Layer

• Layer 6
• Data-formatting info for network communications
• Handles protocol conversion, character set
  issues, encryption/ decryption, and graphics
  commands
• May compress data
• A redirector operates at this layer
• Intercepts requests for service from the
  computer those that cant be handled locally are
  redirected to a networked resource that can
  handle the request
• Usually built into the Application layer
  component
• E.g., FTP, HTTP

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Session Layer

• Layer 5
• Permits two parties to hold ongoing sessions
• Handles session setup, data or message exchanges,
  and teardown when the session ends
• Monitors session identification so that only
  designated parties can participate
• Monitors security services for access control
• Examples name lookup and user logon and logoff
• E.g., DNS name resolution, FTPs logon/logoff
• End-to-end task synchronization services
• Manages mechanics of any ongoing conversation

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Transport Layer

• Layer 4 PDU segment
• Manages end-to-end transfer of data
• Segments long data streams into chunks
• Resequences chunks into original data on receipt
• Includes error checks to ensure error-free
  delivery
• Handles flow control
• E.g., TCP (TCP/IP) and SPX (from IPX/SPX)
• Layer 4 problems include a corrupt protocol stack
  and segments that are too large for the medium
  between the source and destination networks
• The latter forces Network layer to fragment
  segments, which causes performance degradation

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Transport Layer (continued)
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Network Layer

• Layer 3 PDU packet
• Handles addressing messages for delivery
• Translates logical addresses into physical
  addresses
• Determines how to route transmissions from sender
  to receiver (routing process)
• Traffic cop for network activity and handles
  routing and access control (during routing
  process)
• E.g., IP (from TCP/IP) and IPX (from SPX/IPX)
• Possible problems incorrect IP addresses or
  subnet masks, incorrect router configuration, and
  router operation errors

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Network Layer (continued)
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Data Link Layer

• Layer 2 PDU frame (has header and trailer
  (FCS))
• Sends PDUs from/to Network to/from Physical layer
• FCS contains Cyclical Redundancy Check (CRC)
• Its the responsibility of the upper layers
  (e.g., Layer 4) to retransmit data discarded due
  to errors
• Header contains source/destination MAC addresses
• Destination address is of final destination or
  intermediate device (e.g., router)
• The SW component at this layer is the NIC driver
• HW components include NIC and switches
• Possible problems collisions, invalid frames,
  trying to use incompatible network architectures

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Data Link Layer (continued)
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Physical Layer

• Layer 1
• Converts bits into signals and vice versa
• Signals generated depend on the medium
• Details for creating network connection are
  specified
• Governs the type of connector used
• Regulates the transmission technique
• Handles intricacies of transmitting bits
• Specifies encoding mechanism
• Tries guarantee that received bits match pattern
  sent
• Problems improper media termination, EMI, faulty
  or misconfigured NICs and hubs

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Summary of the OSI Layers
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Function of Data Frames in Network Communications

• A frame is the basic unit for network traffic as
  it travels across the medium
• Reasons why networks split data into small pieces
  
• Large units of data sent across a network hamper
  effective communications by saturating the
  network
• If a sender and receiver use all the available
  bandwidth, other computers cant readily
  communicate
• Networks can sometimes be unreliable
• Retransmission of large frames (due to errors) is
  inefficient

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Examining the Structure of a Data Frame

• Header source/destination MAC addresses, frames
  size, description of content, clocking
  information
• Data (payload) actual data being sent along
  with the headers of other PDUs in the frame
• Size can vary from less than 50 bytes to 16 KB,
  depending on the network type
• Trailer CRC (if the sent/received CRCs dont
  match, the receiving computer discards the frame)

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Creating a Data Frame
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Understanding Types of Data Frames

• Unicast frame addressed to only one computer
• Adapters read the frames and pass them to higher
  layers only if the destination address in the
  frame header matches their own address
• Broadcast frame created for all computers on a
  network
• Destination address is a value of all binary 1s
• Multicast frame created for any computers on a
  network that listen to a shared network address
• A special kind of address allows any interested
  receiver to read these data streams

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Understanding the IEEE 802 Networking
Specifications

• The IEEE defined a set of LAN standards to ensure
  network interface and cabling compatibility
• Project 802 (inception on February (2) of 1980)
• Concentrates on standards that describe a
  networks physical elements
• NICs, cables, connectors, signaling technologies,
  media access control, and the like
• OSI model was not standardized until 19831984
• IEEE 802 standards predate the model
• Both were developed in collaboration and are
  compatible with one another

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IEEE 802 Specifications
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IEEE 802 Specifications (continued)
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IEEE 802 Extensions to the OSI Reference Model
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IEEE 802 Extensions to the OSI Reference Model
(continued)
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Summary

• The OSI reference model and IEEE Project 802
  define a frame of reference for networking and
  specify the lower-layer behaviors for most
  networks
• Together, these models describe the complex
  processes and operations involved in sending and
  receiving information across a network
• The OSI reference model separates networking into
  seven layers, each with its own
  purposes/activities
• From the bottom up Physical, Data Link, Network,
  Transport, Session, Presentation, and Application

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Summary (continued)

• Data frames consist of three parts frame header,
  data section, and frame trailer
• Classified as unicast, multicast, or broadcast
  frames
• The IEEE 802 project elaborates on the functions
  of a networks Physical and Data Link layers by
  dividing the Data Link layer into two sublayers
  Logical Link Control (LLC) and Media Access
  Control (MAC)
• Together, these sublayers handle media access,
  addressing, and control and provide reliable,
  error-free delivery of data frames from one
  computer to another