Looking at the OSI 7 Layer Reference Model
By S. Sullivan
The "Open Systems Interconnection" reference model is the only internationally accepted
hierarchy of standards for communicating between different systems, made by different vendors.
It's goal was to create an open systems networking environment where any vendor's computer
system, connected to any network, can easily share data with any other computer system on that
network. It is important to note that OSI is just a model and not an actively used protocol.
The seven layers consist of:
1. Physical - connects electrical/optical signals, wires/wireless. For example it
defines signals like POTS, T1, xDSL, SONET, satellite, wireless, microwave. And defines physical
shapes such as jacks and connectors (RJ11, RJ45, RJ48, DSX, CSU-DSU, X.21, RS-232, etc.).
We need standard physical protocols so we can easily connect different devices together.
Using a railroad as a metaphor, layer 1 puts the packages into the trucks.
2. Data Link - makes the connections and provides synchronization/signaling. Where
bits are organized, signaled to one another and where software is used to manage the process.
For example it defines the WAN: point-to-point/private line, asynchronous transfer mode, frame
relay, multi-protocol label switching, etc.. And also defines the LAN: local area network
protocols like Ethernet, Token Ring, Appletalk (+ links and switches).
At the data link layer 2, information is packaged or encapsulated (capsule) into packets,
frames, cells, datagrams. You have probably encountered "gelcaps" which put a shell of "gel"
around your aspirin or othe pill. Encapsulation is like the color around this which contains
the information.
Using a railroad as a metaphor, layer 2 creates the "boxcars" - packets creates the packages.
3. Network - routes the information by providing destination and direction of data.
Stamps an address on data, and everything which involves routing is based on addressing. An
example of an IPv4 data address would be 23.156.22.22, which is 32 address bits (IPv6 is 128
address bits).
The network layer is responsible for forwarding, i.e. if it's not for this location (router),
let's forward that packet to the next router (hop), it might go there.
Routers (aka hops) of data pathways. Supports multiple bandwidth speeds, routes, protocols
(IP, frame, ATM, token ring, Ethernet). Aides in scheduling, buffering, queuing, and manages
overflow- i.e. hey email packet, you have to wait because i have a priority video packet ahead of you.
Layer 3 - protocol puts or encapsulates into datagrams (capsules) with source and
destination address 20.154.90.5 (IPv4 address used currently), IPv6 like zipcode plus is now
available with 128 bit addresses, which are compatible with IPv4 by pre-pending (adding 96 zeros
in front of the IPv4 address). Layer 3 fragments the data which allows data to be sent in pieces
(packets) like moving your stuff- may be placed in more than one moving van and they may go over
different routes to the destination and then reassembled- from IP to LAN (local area network)
Ethernet address (MAC - media access control) - to frame relay, PTP, MPLS, or other layer 2 protocol.
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