Troubleshoot Your Networking Connection

The open Systems Interconnect is a hierarchy used by networking professionals to understand problems they face and how to fix them. When data is sent from a PC, it goes through these layers in order from 7 to 1. When it receives data it obviously goes from layer 1 to 7. These layers are not physically there but merely a grouping of protocols and hardware per layer that allow a technician to find where the problem lies.

Lets start with layer 1. This layer is know as the physical layer. As you may have suspected, it is groups with the physical cabling and connection points. Standards for Ethernet and x.25 are defined here as well as hardware such as hubs. Layer 1 is the actual electrical impulse of bits that carries through the cable and helps hardware understand what is being 'said'.

Layer 2 is the hardware that receives the impulses. This layer is known as the data link layer. This layer takes the bits and encapsulates them to frames to be read at the data link layer. Hardware addresses, NICs, and switches are included in this layer. Media access control and logical link control are the 2 sub-layers. the MAC layer controls how the PC gains access to a network (through hardware addresses) and the LLC controls flow and error checking. Switches are in this layer because they forward data based on the source and destination frame address.

Layer 3 is considered the network layer. Routers and IP addresses are grouped in this layer. From layer 2 to 3 data is encapsulated as packets. This means routers forward packets based on IP or IPX or other network layer protocol addresses. By doing so, they are used to connect two different networks. Routers are more expensive and smarter than switches, but forward data slower than switches because they have to go into the third layer. There are switches that can read into the third layer but I wont get into the complexities of that. If you have cable or DSL connection, that cable or DSL modem is actually a router. It connects your home network with that of the cable company and preforms no actual modulation or demodulation (modem= MODulator DEModulator...they're technical terms to describe digital to analog signal conversion).

Layer 4 is known as the transport layer. This layer allows for a smooth transition between the user's PC and network hardware. Packets are turned into segments at this point and allow for error correction and flow control. The last 3 layers are all inside the PC's software.

Layer 5 is called the session layer. It is called this because its responsibilities are establishing, maintaining and terminating connections or sometimes called sessions for applications. In this layer segments are turned to data and stay as data for the rest of the trip until it reaches the video card for the user to view. Usually if an error is in this layer it is the programming of the application that is at fault or your session has ran out of time and simply needs established again.

Layer 6 handles all of encryption and data representation and is called the presentation layer. Encryption (I'm sure I'll write an article entirely about encryption later) is basically coding used to ensure no other parties can read your data. IPSec is a form of encryption that comes automatically turned on with Windows XP.

Layer 7, the application layer, is where all the applications and application protocols are. Many many protocols are here including, HTML, HTTP, FTP, SMTP, SNMP, and TELNET.

You should have some idea of how to troubleshoot now. Simply run through the layers 1-7 in order checking connectivity is present. This can be done by looking at link lights, checking configuration of addresses, and using network tools. For example, if you find the link lights are lit and all your IP addresses are correct but don't have have connectivity still, you should check layer 4 and up for errors. Remember that 80% of connectivity issues are configuration settings. Once you can telnet to the other host you are trying to gain connectivity, then all your 7 layers have connectivity. Keep this OSI model in mind when troubleshooting your network issues to ensure efficient detection and problem solving.