OSI ModelOSI Model

CompTIA Network+

Chapter 2

Exam Objectives

The following 4 exam objectives will be covered in this chapter.

1. Define the layers of the OSI model and identify the protocols, services, and functions that pertain to each layer.

2. Identify the OSI Model layers where routing, switching or bridging occurs.

3. Describe data link layer concepts, including MAC address, bridges and 802.x specifications.

4. Explain routing and network layer concepts.

The Seven Layers of the OSI Model

InternationalOrganization ofStandardization

ISO

Layer7 The application layer is the

OSI layer that is closest to the user; it provides network services to the user’s applications. It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs. If you want to remember Layer 7 in as few words as possible, think of browsers.

Layer 6 The presentation layer

ensures that the information that the application layer of one system sends out is readable by the application layer of another system. The presentation layer translates between multiple data formats by using a common format.

Data encryption and compression also takes place at the Presentation layer

Layer 5 As its name implies, the

session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange. If you want to remember Layer 5 in as few words as possible, think of dialogues and conversations.

Layer 4

The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. Layer 4 is the boundary between media-layer protocols and host-layer protocols. Layer 4 also deals with Flow Control (Windowing), and Reliability of Communication (Acknowledgement).

Layer 3

The network layer is a complex layer that provides connectivity and path selection between two host systems that may be located on geographically separated networks. If you want to remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.

Layer 2 The data link layer provides

reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames. If you want to remember Layer 2 in as few words as possible, think of frames and media access control.

Layer 1 The physical layer defines the

electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, maximum transmission distances, physical connectors, transceivers and other, similar, attributes are defined by physical layer specifications. If you want to remember Layer 1 in as few words as possible, think of signals and media.

Encapsulation

Summary of the OSI ModelLayer Description

5 Conversion Steps to Encapsulate Data

Devices and layers

Physical layer: Media, transceivers, hubs, and repeaters.

Data link Layer: bridges, switches, NICs and brouters

Network Layer: routers, brouters.

Frame Specifications Ethernet

– Networking technology originally developed in 1970 by Xerox

Token Ring– Networking technology developed by IBM

in the 1980s

Typical Ethernet Frame

802.3 Standard– IEEE standard for Ethernet networking

devices and data handling

FIGURE 2-5 Ethernet frame as specified by the IEEE 802.3 standard

Components of the Ethernet 802.3 Frame

Preamble: announces to the network that data is in rout.

Start of Frame Delimiter (SFD): beginning of the addressing frame

Destination Address Source Address

Components of the Ethernet 802.3 Frame

Length (LEN) Data Pad: Minimum frame size is 46 bytes Frame Check Sequence (FCS)

– Cyclic Redundancy Check (CRC)• Algorithm used to verify the accuracy of data

contained in a data frame

Standardized four Ethernet Frame Types

Ethernet 802.2 Ethernet 802.3 Ethernet II Ethernet SNAP

– The four frames are not compatible with each other.

Typical Token Ring Frame

802.5 Standard– IEEE standard for Token Ring networking devices

and data handling

FIGURE 2-6 Typical Token Ring frame as specified in the IEEE 802.5 standard

Components of Token Ring Frame

Start Delimiter (SD) Access Control (AC) Frame Control (FC) Destination Address Source Address

Components of Token Ring Frame

Data

Frame Check Sequence (FCS)

End Delimiter (ED)

Frame Status (FS)

Addressing Through the Layers

Data Link Layer Address

Network Layer Address

Data Link Layer Address

Also called MAC address, after Media Access Control (MAC) sublayer

Number uniquely defining a network node Manufacturer-hard codes the address

– Block ID– Device ID

Network Layer Address

Resides at Network level of OSI Model Follows hierarchical addressing scheme Can be assigned through operating system

software

IEEE Networking Specifications

IEEE 802 standards

IEEE Networking Specifications

To accommodate shared access for multiple network nodes, the IEEE expanded the OSI Model by separating the Data Link layer into two sublayers– Logical Link Control (LLC) sublayer– Media Access Control (MAC) sublayer

Data Link Layer Sublayers

LLC – Upper Sublayers– Provides common interface– Supplies reliability and flow control services

MAC– Lower sub layer– Appends the physical address of the

destination computer onto the frame

Subdivided Data Link Layer

Labs and Homework

Labs:– 2.2– 2.4– 2.5

Homework– End of chapter review questions (30 Multiple

choice questions.)

OSI Model Network+ Exam Questions

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