Sunday, December 14, 2008

Networking 101

I recently was involved in several technical interviews for some technician and administrative positions, and was surprised by how little most of the prople understood networking basics. Most of the answers I got in the interviews focused on:
  1. Hubs are for home offices
  2. Switches are smart hubs, and used in businesses
  3. Routers are used to link multiple offices

These definintions show a very limited understanding of network basics, and the fact that most of the interviews thought that this was the whole answer really made me wonder.

So I am going to post several articles detailing the basics of networking from the ground up - i.e. from the cable up, not what acronyms are in use today.

Networking Basics -- The OSI Model

The first thing to know is that there is a logical sequence to how networks function. Way back in 1977 when networks began to emerge, a common model was developed to allow different systems to work together. The OSI model (Open Systems Interconnect) was created to define 7 hierarcichal layers that work together - these layers are often easiest to understand from the bottom up.

ApplicationThe application layer identifies communication partners, determines availability of resources, and synchronizes communication.
Presentation

The presenation layer is responsible for communicating between the application and the rest of the network stack. One main feature is of this layer is to convert serialized data (i.e. a long stream of numbers and characters), into structured data such as XML or documents and other files. This layer is also responsible for controlling encryption and compression of data transmitted between computers.

SessionThe session layer is responsible for controlling the sessions or connections between devices. Whenever network devices communicate, they normally start with a hello, contain several checkpoints, and end with a goodby. In theory, if a connection is lost in an ungraceful manner (such as a power loss or broken cable), the session layer could reconnect and restart communications where it left off.
Transport

The transport layer is used to control reliability, recoverability, and additional network features. Features such as checksums, packet sequencing, and retransmissions are all functions to increase reliability and recoverability.

Features such as tunneling, VPN, and IPSec are additional network features provided by the transport layer.

Network

The network layer is resonsible for directing or routing traffic from the source to the correct destination. The components on the network layer are not concerned with the ultimate delivery of the information, but simply with passing it on to the next logical step.

Similar to the post office example used in the datalink layer, the network layer can be viewed as a central routing office that takes mail from all over the country. The office then directs the mail to the correct state, county, or city - these smaller offices then direct the mail to the individual homes.

DatalinkThis layer controls the communication that occurs on the physical layer. Each physical device has a corresponding physical address - sometimes referred to as a MAC address - which is like a house address. Since traffic from multiple systems can travel on the same network cable, the address is used to determine what information belongs at which location - similar to how the post office delivers mail to a house based on its address.
Physical

Refers to the physical connectivity (for wired networks) or frequency range (for wireless networks) used to connect machines together. This connectivity can be phone lines, coaxial, Cat5, or wireless.

This also includes the frequency or speed of data transmission. This is where the speed of the network is determined.


The OSI model can be likened to a pizza with multiple layers of toppings - each layer supporting the layer above it. For instance, a pizza may have a layer of cheese on the very top, followed by a layer of sausage, then the sauce, and finally the dough. Each of these layers can be substituted for something else (Canadian bacon and pineapple?), and layers can be added or removed (Pizza Pie? Cheesey Crust?). Some of these layers however are necessary (like the dough), and while their form can change (thin crust vs. thick crust), they still exist.

The OSI model is commonly remembered by using nmonics - i.e. an easily remembered phrase that is used to refer to something else. One popular nmonic is Please Do Not Throw Sausage Pizza Away, note the capitalized letters PDNTSPA which reference Physical, Datalink, Network, and so on...

Now that we have some of the OSI basics covered, I will be following this post up with some tangible information about how the OSI model works today. If your interested, more detailed information about the OSI model can be found at http://en.wikipedia.org/wiki/OSI_model.

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