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Guide to Networking Essentials Fifth Edition

Chapter 5 Making Networks Work

Guide to Networking Essentials, Fifth Edition 2

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

Guide to Networking Essentials, Fifth Edition 3

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

Guide to Networking Essentials, Fifth Edition 4

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

Guide to Networking Essentials, Fifth Edition 5

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,

• r 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

Guide to Networking Essentials, Fifth Edition 6

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

Guide to Networking Essentials, Fifth Edition 7

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

Guide to Networking Essentials, Fifth Edition 8

Structure of the OSI Reference Model (continued)

Guide to Networking Essentials, Fifth Edition 9

Structure of the OSI Reference Model (continued)

Guide to Networking Essentials, Fifth Edition 10

Structure of the OSI Reference Model (continued)

Guide to Networking Essentials, Fifth Edition 11

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 self-

contained 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

Guide to Networking Essentials, Fifth Edition 12

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

Guide to Networking Essentials, Fifth Edition 13

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

Guide to Networking Essentials, Fifth Edition 14

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

Guide to Networking Essentials, Fifth Edition 15

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

Guide to Networking Essentials, Fifth Edition 16

Transport Layer (continued)

Guide to Networking Essentials, Fifth Edition 17

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

• peration errors

Guide to Networking Essentials, Fifth Edition 18

Network Layer (continued)

Guide to Networking Essentials, Fifth Edition 19

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

Guide to Networking Essentials, Fifth Edition 20

Data Link Layer (continued)

Guide to Networking Essentials, Fifth Edition 21

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

Guide to Networking Essentials, Fifth Edition 22

Summary of the OSI Layers

Guide to Networking Essentials, Fifth Edition 23

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

Guide to Networking Essentials, Fifth Edition 24

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)

Guide to Networking Essentials, Fifth Edition 25

Creating a Data Frame

Guide to Networking Essentials, Fifth Edition 26

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

Guide to Networking Essentials, Fifth Edition 27

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

Guide to Networking Essentials, Fifth Edition 28

IEEE 802 Extensions to the OSI Reference Model

Guide to Networking Essentials, Fifth Edition 29

IEEE 802 Extensions to the OSI Reference Model (continued)

Guide to Networking Essentials, Fifth Edition 30

IEEE 802 Working Groups and status

(this list from http://en.wikipedia.org/wiki/IEEE_802)

• IEEE 802.1 Bridging (networking) and Network Management
• IEEE 802.2 Logical link control (inactive)
• IEEE 802.3 Ethernet
• IEEE 802.4 Token bus (disbanded)
• IEEE 802.5 Defines the MAC layer for a Token Ring (inactive)
• IEEE 802.6 Metropolitan Area Networks (disbanded)
• IEEE 802.7 Broadband LAN using Coaxial Cable (disbanded)
• IEEE 802.8 Fiber Optic TAG (disbanded)
• IEEE 802.9 Integrated Services LAN (disbanded)
• IEEE 802.10 Interoperable LAN Security (disbanded)
• IEEE 802.11 Wireless LAN & Mesh (Wi-Fi certification)
• IEEE 802.12 demand priority (disbanded)

Guide to Networking Essentials, Fifth Edition 31

IEEE 802 Working Groups (continued)

• IEEE 802.13 Not used (officially)

– IEEE 802.13ah Defines "Copper for the first mile" for Metro Area Networks (proposed)

• IEEE 802.14 Cable modems (disbanded)
• IEEE 802.15 Wireless PAN

– IEEE 802.15.1 (Bluetooth certification) – IEEE 802.15.4 (ZigBee certification)

• IEEE 802.16 Broadband Wireless Access (WiMAX certification)

– IEEE 802.16e (Mobile) Broadband Wireless Access – IEEE 802.16.1 Local Multipoint Distribution Service

• IEEE 802.17 Resilient packet ring
• IEEE 802.18 Radio Regulatory TAG
• IEEE 802.19 Coexistence TAG
• IEEE 802.20 Mobile Broadband Wireless Access
• IEEE 802.21 Media Independent Handoff
• IEEE 802.22 Wireless Regional Area Network

Guide to Networking Essentials, Fifth Edition 32

IEEE 802 Explanations

(Table 5-2, from the textbook)

Guide to Networking Essentials, Fifth Edition 33

IEEE 802 Explanations (continued)

Guide to Networking Essentials, Fifth Edition 34

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

Guide to Networking Essentials, Fifth Edition 35

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