The OSI reference model


OSI Model

Layer 7 – Application (L7)
App Layer protocols: HTTP FTP SMTP TelneT DNS TFTP POP3

Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.
Print and file services run here

You can telnet to a router and work on the router as if you were there but you are using L7, the aplication layer and not L3 (network layer)
Another way to explain:
With Telnet you use IP addresses which are referred to Layer 3
You can telnet from a router to another router and routers are Layer 3 devices (network layer) just like ip addresses but when you Telnet to a router via IP address you are using Layer 7 because Telnet is a user interface layer which is Layer 7
Just to emphasize once more:
Even if we use Layer 3 devices (ip addresses and routers) we are still in Layer 7

Layer 6 Presentation Layer (L6)
Deals with the presentation of the data.
Data compression and encoding happens here
Character set translations happens here
There are not specific protocols in this layer but there are file types associated with this layer.
File types .jpg, .gif, .tiff and .mpeg are associated with the Presentation layer.
MPEG stands for Moving Picture Experts Group, and these are the standards cor coding compressed video and music

Layer 5 – The Session Layer (L5)
Some people consider this to be the most important layer of the 7. This layer is the “boss” or the manager of the end-to-end connection
This layer is where the communications channels are built and how long they are kept up
This layers handles the CREATION, MAINTENANCE and TEAR DOWN of the end-to-end communication.

Layer 4 The Transport Layer (L4)
It is the “post office” or the “Post man”of the OSI model.

Delivery of the data is accomplished by two protocols.
These protocols handles delivery of data from one host to another
TCP and UDP both run here; there are major differences between the two
This layer uses port numbers to differentiate multiple conversations between to hosts
Data is placed into segments at this layer. Unlike layers 7, 6 and 5 (Application, Presentation and Session)
A L4, The data has started to broken down in smaller units and it is no longer presented as characters
Novell Netware’s SPX also runs at this layer. (on a side note here: SPX is becoming obsolete rather quickly as Windows XP usage dwindles. Windows Vista and Windows 7 don’t support SPX, because SPX was originally designed for small networks and not for enterprise environment. TCP or IPV6 protocols are both better equipped to handle heavy traffic)

Layer 3 Network Layer (L3)
Routing occurs here
Data is placed into packets here
The following protocols are Layer 3 protocols and run here:
IP (Internet Protocol)
ICMP (Internet Control Message Protocol)
IGMP (Internet Group Management Protocol)
ARP (Address Resolution Protocol)
RARP (Reverse Address Resolution Protocol)

There are Layer 3 switches out there (they have routing capabilities)

Layer 2 Data Link Layer (L2)
The only layer that is divided into two sub layers, also known as LLC(logical link control) sublayer and MAC (Media Access Control) sublayer
Data is placed into frames at this layer (the units are getting smaller and smaller)
This layer uses MAC addresses are also known a physical addresses or Layer 2 addresses
Physical address have nothing to do with the Physical Layer of the OSI model; they are data link layer addresses
Switches, bridges and wireless access points (WAPs) operate at this layer)
This layer performs some error detection (aka as error checking or “finding the error”), but no error recovery
This error detection is accomplished through use of FCS (Frame Check Sequence) in a operation alson known as CRC (cyclic redundancy check)
The sending host prepares the frame, runs an algorithm (a mathematical operation) against the contents of the frame. The host then places the result of the algorithm into the FCS field and sends the frame.
Destination hosts receives the frame, runs the same algorithm against the contents of the frame and compares it to the contents of the FCS. If there’s a match, the frame is assumed to be intact
If there’s a different value, the frame is assumed to be corrupt and is discarded. This is error detection,,, not error recovery
The MAC layer decides where one frame ends and another begins
The LLC layer is defined by IEEE 802.2 and communicates with the network layer This sublayer also offers flow control and acknowledgments (both optional)
The MAC layer is the layer that actually applies the MAC destination address to the frame (it also communicates with the physical layer)

The MAC layer actually decides where one frame ends and the other one begins and actually applies the destination address to that frame

Layer 1 The Physical Layer (L1)
The only thing the Physical layer understands is electrical signals. So it only understands ones and zeroes.
All the physical and electrical specifications for electrical and physical devices are defined at Layer One
Hubs, repeaters and NICs are all L1 devices.
Signal encoding occurs here – That is the process of taking the data and translating it to a series of electrical signals
They include cables and connectors, also token ring and MAU (both now obsolete)

***

OSI Terminology for Data Units

Application
Presentation
Session – These three layers call data “data”

Transport – data is placed into “segments”

Network – Data is placed into “packets”

Data Link – data placed into “frames”

Physical – It’s all ones and zeroes

***
Packet Filtering – It is not just a layer 3 thing
since port numbers (a L4 feature) are used for this as well, packet filtering occurs at both L3 (ip addresses) and L4 (port numbers)
Packets filters deals ip addresses and port numbers

Note: most of the concepts defined here were copied from the Train Signal presentation by Chrys Bryant CCIE engineer 12933.
Thank you Chris for doint such a great job explaining OSI in such a interesting way.

You can divide the OSI reference model in two sets:

L7
L6
L5 – Application set

L4
L3
L2
L1 – Trasnport Set

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