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

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

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

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

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

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

Structure of the OSI Reference Model (continued) Guide to Networking Essentials, Fifth Edition

Structure of the OSI Reference Model (continued) Guide to Networking Essentials, Fifth Edition

Structure of the OSI Reference Model (continued) Guide to Networking Essentials, Fifth Edition

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Creating a Data Frame Guide to Networking Essentials, Fifth Edition

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

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

IEEE 802 Specifications Guide to Networking Essentials, Fifth Edition

IEEE 802 Specifications (continued) Guide to Networking Essentials, Fifth Edition

IEEE 802 Extensions to the OSI Reference Model Guide to Networking Essentials, Fifth Edition

IEEE 802 Extensions to the OSI Reference Model (continued) Guide to Networking Essentials, Fifth Edition

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

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 Guide to Networking Essentials, Fifth Edition