internet.com
You are in the: Small Business Computing Channelarrow View Sites +
Small Business Technology
» ECommerce-Guide | Small Business Computing | Webopedia | WinPlanet |  »Close
Webopedia.com
Enter a word for a definition... ...or choose a computer category.
 
 

menu
   Home
   Term of the Day
   New Terms
   Pronunciation
   New Links
   Quick Reference
   Did You Know?
   Categories
   Tech Support
   Webopedia Jobs
   About Us
   Link to Us
   Advertising
  

talk to us
   Submit a URL
   Suggest a Term
   Report an Error

internet.com
Developer
Downloads
International
Internet Lists
Internet News
Internet Resources
IT
Linux/Open Source
Personal Technology
Small Business
Windows Technology
xSP Resources
Search internet.com
Advertise
Corporate Info
Newsletters
Tech Jobs
E-mail Offers
commerce
  Be a Commerce Partner
 
 
 
 
 
 
 
 
 
 
 
 
 
 

The OSI Reference Model — Understanding Layers

~ By Charlie Schluting

It is time to take a trip up the OSI Reference Model, and learn what this mysterious thing is all about. The network stack is of great significance, but not so much that it's the first thing you should learn. Many so-called networking classes will start by teaching you to memorize the name of every layer and every protocol contained within this model. Don't do that. Do realize that layers 5 and 6 can be completely ignored, though.

The International Standards Organization (ISO) developed the OSI (Open Systems Interconnection) model. It divides network communication into seven layers. Layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7, the upper layers, contain application-level data. Networks operate on one basic principle: "pass it on." Each layer takes care of a very specific job, and then passes the data onto the next layer.

The physical layer, layer 1, is too often ignored in a classroom setting. It may seem simple, but there are aspects of the first layer that oftentimes demand significant attention. Layer one is simply wiring, fiber, network cards, and anything else that is used to make two network devices communicate. Even a carrier pigeon would be considered layer one gear (see RFC 1149). Network troubleshooting will often lead to a layer one issue. We can't forget the legendary story of CAT5 strung across the floor, and an office chair periodically rolling over it leading to spotty network connectivity. Sadly, this type of problem is quite common, and takes the longest to troubleshoot.

Layer two is Ethernet, among other protocols; we're keeping this simple, remember. The most important take-away from layer 2 is that you should understand what a bridge is. Switches, as they're called nowadays, are bridges. They all operate at layer 2, paying attention only to MAC addresses on Ethernet networks. If you're talking about MAC address, switches, or network cards and drivers, you're in the land of layer 2. Hubs live in layer 1 land, since they are simply electronic devices with zero layer 2 knowledge. LDon't worry about the details for now, just know that layer 2 translates data frames into bits for layer 1 processing.

You might want to go back and re-read that before moving on, because fledgling network admins always seem to mix up layers two and three.

Key Terms To Understanding Layers

ARP
Short for Address Resolution Protocol, a network layer protocol used to convert an IP address into a physical address, such as an Ethernet address.

broadcast
In networking, a distinction is made between broadcasting and multicasting. Broadcasting sends a message to everyone on the network whereas multicasting sends a message to a select list of recipients.

MAC
Short for Media Access Control. See MAC address or MAC layer.

subnet
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix.

UDP
A connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams over an IP network. It's used primarily for broadcasting messages over a network.

If you're talking about an IP address, you're dealing with layer 3 and "packets" instead of layer 2's "frames." IP is part of layer 3, along with some routing protocols, and ARP (Address Resolution Protocol). Everything about routing is handled in layer 3. Addressing and routing is the main goal of this layer.

Layer 4, the transport layer, handles messaging. Layer 4 data units are also called packets, but when you're talking about specific protocols, like TCP, they're "segments" or "datagrams" in UDP. This layer is responsible for getting the entire message, so it must keep track of fragmentation, out-of-order packets, and other perils. Another way to think of layer 4 is that it provides end-to-end management of communication. Some protocols, like TCP, do a very good job of making sure the communication is reliable. Some don't really care if a few packets are lost--UDP is the prime example.

And arriving at layer 7, we wonder what happened to layer 5 and 6.

In short: They're useless.

A few applications and protocols live there, but for understanding networking issues talking about these provides zero benefit. Layer 7, our friend, is "everything." Dubbed the "Application Layer," layer 7 is application-specific. If your program needs a specific format for data, you invent some format that you expect the data to arrive in and you've just created a layer 7 protocol. SMTP, DNS and FTP are all layer 7 protocols.

The most important thing to learn about the OSI model is what it really represents.

Pretend you're an operating system on a network. Your network card, operating at layers 1 and 2, will notify you when there's data available. The driver handles the shedding of the layer 2 frame, which reveals a bright, shiny layer 3 packet inside (hopefully). You, as the operating system, will then call your routines for handling layer 3 data. If the data has been passed to you from below, you know that it's a packet destined for yourself, or it's a broadcast packet (unless you're also a router, but never mind that for now). If you decide to keep the packet, you will unwrap it, and reveal a layer 4 packet. If it's TCP, the TCP subsystem will be called to unwrap and pass the layer 7 data onto the application that's listening on the port it's destined for. That's all!

When it's time to respond to the other computer on the network, everything happens in reverse. The layer 7 application will ship its data onto the TCP people, who will stick additional headers onto the chunk of data. In this direction, the data gets larger with each progressive step. TCP hands a valid TCP segment onto IP, who give its packet to the Ethernet people, who will hand it off to the driver as a valid Ethernet frame. And then off it goes, across the network. Routers along the way will partially disassemble the packet to get at the layer 3 headers in order to determine where the packet should be shipped. If the destination is on the local Ethernet subnet, the OS will simply ARP for the computer instead of the router, and send it directly to the host.

Grossly simplified, sure; but if you can follow this progression and understand what's happening to every packet at each stage, you're just conquered a huge part of understanding networking. Everything gets horribly complex when you start talking about what each protocol actually does. If you are just beginning, please ignore all that stuff until you understand what the complex stuff is trying to accomplish. It makes for a much better learning endeavor!
 

Did You Know...
Layer two data is called a frame, and doesn't involve IP addresses. IP addresses and packets are layer 3, MAC addresses are layer 2!


Related Terminology: Webopedia > Networks > Networking Standards >


~ By Charlie Schluting
Adapted from Enterprise Networking Planet
Last updated: January 13, 2005


Related Links

Webopedia's Quick Reference: The 7 Layers of the OSI Model
Use this handy guide to compare the different layers of the OSI model and understand how they interact with each other.

Networking 101: Understanding IP Addresses
Networks don't work without addresses: Whenever you are sending something, you need to specify where it should go and where it came from. To be an effective network engineer or administrator, you need to understand IP addresses backward and forward: you need to be able to think on your feet.

Networking 101: Understanding Subnets and CIDR  
Let's get one thing straight: there is no "Class" in subnetting. In the olden days, there were Class A, B and C networks. These could only be divided up into equal parts, so VLSM, or Variable Length Subnet Masks , were introduced. The old Class C was a /24, B was a /16, and A was a /8. That's all you need to know about Classes. They don't exist anymore.

Networking 101: Subnets Realized (Including IPv6)
An oft-asked question in networking classes is "why can't we just put everyone on the same subnet and stop worrying about routing?" The reason is very simple. Every time someone needs to talk, be it to a router or another host, they have to send an ARP request. Also, there are broadcast packets that aren't necessarily limited to ARP, which everyone hears.




Do you have an interesting piece of computer-related trivia that you would like us to explore?
Tell us about it.


JupiterWeb networks:

internet.comearthweb.comDevx.com graphics.com

Search JupiterWeb:

Jupitermedia Corporation has three divisions:
Jupiterimages, JupiterWeb and JupiterResearch


Legal Notices, Licensing, Reprints, & Permissions, Privacy Policy.

Jupitermedia Corporate Info | Newsletters | Tech Jobs | Shopping | E-mail Offers