1. Guide to Networking Essentials Fifth Edition
Chapter 5
Making Networks Work

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, 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

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 operation 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 Specifications
Guide to Networking Essentials, Fifth Edition

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

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

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

32. 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

33. 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