Guide to Networking Essentials Fifth Edition Chapter 5 Making - PowerPoint PPT Presentation

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 Guide to Networking Essentials, Fifth Edition 2

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 3

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 4

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 Guide to Networking Essentials, Fifth Edition 5

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 6

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 7

Structure of the OSI Reference Model (continued) 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) • 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 11

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 12

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 13

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 14

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 15

Transport Layer (continued) Guide to Networking Essentials, Fifth Edition 16

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 Guide to Networking Essentials, Fifth Edition 17

Network Layer (continued) Guide to Networking Essentials, Fifth Edition 18

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 19

Data Link Layer (continued) Guide to Networking Essentials, Fifth Edition 20

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 21

Summary of the OSI Layers Guide to Networking Essentials, Fifth Edition 22

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 23

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 24

Creating a Data Frame Guide to Networking Essentials, Fifth Edition 25