Chapter 5 Making Networks Work Instructor: Nhan Nguyen Phuong

Chapter 5

Making Networks Work

Instructor: Nhan Nguyen Phuong

Guide to Networking Essentials, Fifth Edition 2

Contents

1. Understanding the OSI and 802 Networking Models

2. Function of Data Frames in Network Communications

3. Understanding the IEEE 802 Networking Specifications

1. Understanding the OSI and 802 Networking Models

1.1. Role of a Reference Model

1.2. OSI Reference Model



• 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


1.1. 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


1.2. 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


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


1.3. 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





• 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


1.3.1. 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


1.3.2. 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


1.3.3. 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


1.3.4. 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



1.3.5. 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



1.3.6. 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



1.3.7. 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


1.4. Summary of the OSI Layers

2. Function of Data Frames in Network Communications

2.1. Examining the Structure of a Data Frame

2.2. Creating a Data Frame

2.3. Understanding Types of Data Frames



• 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


2.1. 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)


2.2. Creating a Data Frame


2.3. 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

3. Understanding the IEEE 802 Networking Specifications

3.1. IEEE 802 Specifications

3.2. IEEE 802 Extensions to the OSI Reference Model



• 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


3.1. IEEE 802 Specifications



3.2. IEEE 802 Extensions to the OSI Reference Model



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


• 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

Documents

Chapter 5 Making Networks Work Instructor: Nhan Nguyen Phuong