OSI Model

Data Transmission Process

When the end user sends data, that data will go through all seven layers of the OSI model. The data is broken into smaller and smaller parts beginning at Layer 4 (the Transport Layer) until it's in the form of electric signals that can be sent across the physical media.

As the data flows down the OSI model, it's refered to by different terms. There are 4 diff. terms :

Data Transmission Process

At Application, Presentation, and Session layers, data is simply called ”Data”.

At the Transport layer, data is placed into segments.

At the Network layer, data is placed into packets. At the Data Link layer, data is placed into frames. Finally, at the Physical layer, data takes the form

of bits, and it's all 1s and 0s !

Application Layer

This is the layer where the end users themselves interact with the network. Authentication services also run at Layer 7, but encryption runs at next layer down.

The Application layer ensures that the remote communication partner is available, that the needed communication resource exist (e.g. A modem), and that both ends of the comunication agree on procedures involving data integrity, privacy, and error recovery.

Application Layer

When trying to decide if a protocol is an Appliction layer protocol, just remember that protocols that require the end user to enter a request are Application layer protocols. Firewalls, devices intended to keep network intruders out, operate at Layer 7.

Protocols & services that run at L7 include : Email protocols : SMTP & POP3 Telnet, HTTP, FTP SNMP (Simple N/w Management Protocol)

Presentation Layer

This layer answers one simple question : ”How should data be presented ?” In addition to properly formatting data, encryption occurs at this layer.

Have you ever opened a file in word processing application, and you got pages of unrecognizable characters? That's a Presentation layer issue. The applications have not agreed on how the data is to be presented.

Presentation Layer

There are 4 primary tasks that this layer is concerned with :

Compatibility with OS. Proper encapsulation of data for n/w transmission. Data formatting ( ASCII, Binary) Data encryption, compression and translation.

You've probably seen some of the file types that are used at Presentation layer : JPEG, ASCII, GIF, MPEG, MIDI and TIFF.

Session Layer

Layer 5 is the ”manager” of the two-way communication between two remote hosts. This is the layer that handles the creation, maintenance, and teardown of communications between those two hosts. The overall communication itself is reffered to as a session.

Some sessions last just long enough to send a unidirectional message, where other sessions will be of longer duration.

Transport Layer

The Transport Layer's purpose is to establish a logical end-to-end connection between two systems, segment data received from the upper layers of the OSI model, and to make sure the data gets to the destination in the correct order and free of errors.

At this layer, there are two methods for transporting data :

Connection oriented Connection less

Network Layer

It is Layer 3 of the OSI model. IP runs at this layer, and since routers operate here at L3, this layer is often called ”Routing Layer”. Routing is a two question process :

What valid paths exist from the local router to a given destination ?

What is the best path (”The Optimal Path”) to take to get there ?

Data Link Layer

The switches operate at Layer 2. Wireless Access Points (WAPs) also operate at this layer. Devices that you may be using to access the internet, cable modems and DSL modems, also run at L2.

4 major specifications that run here are : Ethernet High Data Link Control (HDLC) Point-to-Point Protocol (PPP) Frame Relay

Data Link Layer

The Data Link Layer does perform error detection through something called the Frame Check Sequence, but this layer does not perform error recovery.

Data Link Layer is generally referred to as Layer 2, and MAC addresses as Layer 2 addresses.

Switches operate at Layer 2, as do bridges. L3 bridges do exist, but when operating at Layer 3, they are not switching or bridging. They are routing.

Physical Layer

Whatever data our end users are creating, it's going to eventually be ”translated” into a series of 1s and 0s. Once that is done, it's the Physical layer that handles the actual data transmission.

Anything to do with a phsical cable or the standards in use – the pins, the connectors, the electrical current itself – is running at the Physical Layer.

Data Transmission Process

As data flows down the OSI model, each layer adds a header that will be removed by the same layer on the other end of the session. These headers are layer-specific.

As an end user enters data for transmission to a remote host, the first 6 layers of the OSI model will add a layer-specific header that contains information to be read by same layer at remote location.

Layer 2, adds both a trailer and a header.

Data Transmission Process

The combination of data and a layer-specific header is called a Protocol Data Unit (PDU). There's a PDU for each layer. The combination of data and L7 header information is called an L7 PDU.

After the data is successfully transmitted by the Physical layer to the remote location, each layer will remove the header added by it's counterpart, i.e. Layer 3 removes the L3 header and reads it, and so forth.

Data Transmission Process

The term same-layer interaction describes the process of a given OSI layer removing the header placed on the data by the same layer on the sending side.

The term adjacent-layer interaction refers to the interaction between layers of the OSI model on the same host.