As a recent graduate with a degree in Computer Network Systems, I love computers and technology. I wanted to share what I’ve learned from my classes with those of you who also have a love of computers. I hope you find this information helpful, interesting and useful.
Routing Basics (Routing Over WANs)
The most important function of a router, simply put, is that Routers route packets. From the perspective of the OSI network, layer-Layer 3-hosts (computers) and routers work together to deliver packets from one host to another. To do that, the host that creates the packet sends the packet to a nearby router. That router might send the packet to a second router, with that second router forwarding the packet to a third router and so on until the packet is delivered to a router that is connected to the same LAN (Local Area Network) as the destination computer.
Figure 1-1 shows three main steps, all from the perspective of Internet Protocol:
- The web server needs to send a packet to the computer on the right (172.16.3.3), so the webserver sends the packet to its default gateway router-namely router 1 (R1)
- R1 decides to forward the packets to R2 next based on R1’s routing table.
- R2’s routing table shows that 172.16.3.3 should be on a subnet directly connected to R2, so R2 knows to send the packet directly to the destination (172.16.3.3).
Figure 1-2 focuses on how the hosts and routers need to encapsulate the packet before sending anything over the LAN and WAN (Wide Area Networks) links. Hosts and routers must use data-link protocols, such as Ethernet on the LAN and Point-to-Point Protocol on WAN links, to forward the packets over the physical links.
An Example of the OSI Layers and What They Represent
- (7) Application Layer – This layer is where the end-user (computer user) begins the communication process. At this layer, user authentication is identified and protocols like FTP, TELNET, E-MAIL, and other file transfer protocols begin with the user starting the process.
- (6) Presentation Layer – This layer is where encryption occurs. This layer converts the file and/or information into a language (format) that the remaining layers can read and handle without any compatibility problems.
- (5) Session Layer – This layer is where the communication is coordinated. Applications are managed in this layer. This layer manages the exchange of information by creating and terminating all communication between the application layer at both ends.
- (4) Transport Layer – This layer is responsible for ensuring the data completes it’s transmission to the other user. It’s also responsible for flow control and error recovery.
- (3) Network Layer – This layer is where the routing and switching take place. Packet sequencing, addressing, congestion control, and internetworking happen at this layer. Logical paths are created at this level to allow users to communicate with each other. This layer utilizes IP addresses.
- (2) Data Link Layer – This layer is where packets (information) is broken into bits through encoding and decoding. At this layer you have two sections; Media Access Control (MAC), and Logical Link Control (LLC). The MAC section is where packets (information) is gathered and gets permission to transmit the data. The MAC section is where your computer’s MAC address is located to allow proper delivery to a specific system (computer). The LLC section controls synchronization, flow control, and error checking.
- (1) Physical Layer – This layer is where your hardware such as ethernet cable, Network Interface Cards, etc., provide the transmission through electrical impulse, light, and radio waves. This layer is the hardware means of sending and receiving data (information).
Geek tip: Some ways An acronym to remember the seven layers are: All People Seem To Need Data Processing
That’s a great way to remember the 7 layers of the OSI model! There is much more to come. Stay tuned for more!