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Guide to Networking Essentials
Fifth Edition
Chapter 5
Making Networks Work
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 weren’t 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
• Model’s 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, it’s divided
into data units appropriate for the layer
• Protocol data units (PDUs) are passed as a selfcontained data structure from layer to layer
• Encapsulation process adds “headers” to allow
successful delivery of each layer’s 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 can’t 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, FTP’s 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)
• It’s 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
–
–
–
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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 can’t 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,
frame’s 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 don’t
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 network’s
physical elements
– NICs, cables, connectors, signaling technologies,
media access control, and the like
• OSI model was not standardized until 1983–1984
– 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
network’s 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
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