A Network Functions

The OSI Model
International Standards Organization (ISO) specifications for network architecture.
Called the Open Systems Interconnect or OSI model.
Seven layered model, higher layers have more complex tasks.
Each layer provides services for the next higher layer.
Each layer communicates logically with its associated layer on the other computer.
Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.


OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical

Application Layer
Serves as a window for applications to access network services.
Handles general network access, flow control and error recovery.
Presentation Layer
Determines the format used to exchange data among the networked computers.
Translates data from a format from the Application layer into an intermediate format.
Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
Redirector operates at this level.

Session Layer
Allows two applications running on different computers to establish use and end a connection called a Session.
Performs name recognition and security.
Provides synchronization by placing checkpoints in the data stream.
Implements dialog control between communicating processes.

Transport Layer
Responsible for packet creation.
Provides an additional connection level beneath the Session layer.
Ensures that packets are delivered error free, in sequence with no losses or duplications.
Unpacks, reassembles and sends receipt of messages at the receiving end.
Provides flow control, error handling, and solves transmission problems.

Network Layer
Responsible for addressing messages and translating logical addresses and names into physical addresses.
Determines the route from the source to the destination computer.
Manages traffic such as packet switching, routing and controlling the congestion of data.

Data Link Layer
Sends data frames from the Network layer to the Physical layer.
Packages raw bits into frames for the Network layer at the receiving end.
Responsible for providing error free transmission of frames through the Physical layer.

Physical Layer
Transmits the unstructured raw bit stream over a physical medium.
Relates the electrical, optical mechanical and functional interfaces to the cable.
Defines how the cable is attached to the network adapter card.
Defines data encoding and bit synchronization.


OSI Model Enhancements
The bottom two layers - Data Link and Physical - define how multiple computers can simultaneously use the network without interfering with each other.

Divides the Data-link layer in to the Logical Link Control and Media Access Control sub layers.

Logical Link Control
manages error and flow control and
Defines logical interface points called Service Access Points (Sap’s). These SAP's are used to transfer information to upper layers

Media Access Control
communicates directly with the network adapter card and
Is responsible for delivering error-free data between two computers.
Categories
802.3
802.4
802.5 and
802.12 define standards for both this sub layer and the Physical layer


Drivers
a device driver is software that tells the computer how to drive or work with the device so that the device performs the job it's supposed to.

Drivers are called

Network Drivers,

MAC drivers,

NIC drivers.

Provide communication between a network adapter card and the redirector in the computer.

Resides in the Media Access Control sublayer of the Data Link layer. Therefore, the NIC driver ensures direct communication between the computer and the NIC

the Media Access Control driver is another name for the network card device driver

When installing a driver, you need to know these things
IRQ
I/O Port Address
Memory Mapped (Base Memory Address)
Transceiver Type


Packets
Data is broken down into smaller more manageable pieces called packets.

Special control information is added in order to:
disassemble packets
reassemble packets
check for errors

Types of data sent includes
Can contain information such as messages or files.
Computer control data and commands and requests.
Session control codes such as error correction and retransmission requests.
Original block of data is converted to a packet at the Transport layer.

Packet Components
Header
Alert signal to indicate packet is being transmitted
Source address.
Destination address.
Clock synchronization information.

Data
Contains actual data being sent.
Varies from 512 to 4096 bytes (4K), depending on the network

Trailer
Content varies by protocol.
Usually contains a CRC.

Packet Creation
Look at the example on pp. 201 - 204
Begins at the Application layer where data is generated.
Each layer subsequently adds information to the packet; the corresponding layer on the receiving machine reads the information.
Transport layer breaks the data into packets and adds sequencing information needed to reassemble data at the other end => the structure of the packets is defined by the common protocol being used between the two computers.
Data is passed through the Physical layer to the cable.

Packet Addressing
every NIC sees all packets sent on its cable segment but only interrupts the computer if the packet address matches the computer's address
a broadcast type address gets attention of all computers on the network