OSI Model

The Open Systems In­ter­con­nec­tion Model (short: OSI model) was designed by the In­ter­na­tion­al Or­ga­ni­za­tion for Stan­dard­iza­tion (IOS) as a reference model for open com­mu­ni­ca­tion through various technical systems. This makes sense when you think about the early days of the internet: at the end of the 1970s, all of the leading man­u­fac­tur­ers for network tech­nol­o­gy were dealing with the problems brought on by pro­pri­etary network in­fra­struc­tures. Following this, devices from ‘man­u­fac­tur­er A’ could only be used within that same company’s own network, as these products were not com­pat­i­ble with the networks of its com­peti­tors. At the time, there simply was no incentive in place to encourage engineers to design their products and systems with those of rival companies in mind. The dawn of the internet com­plete­ly changed this paradigm, since common standards were needed in order to fa­cil­i­tate shared com­mu­ni­ca­tion and network access.

The OSI model is the result of such stan­dard­iza­tion attempts and, as con­cep­tu­al framework, offers a design basis for creating com­mu­ni­ca­tion standards in­de­pen­dent­ly of man­u­fac­tur­ers. In order to achieve this, the ISO OSI model sub­di­vides the complex process of network com­mu­ni­ca­tion into seven levels, also referred to as layers. This is where the term, OSI layer model comes from. When com­mu­ni­cat­ing between two systems, specific tasks have to be carried out on each layer. Such tasks include com­mu­ni­ca­tion control, ad­dress­ing target systems, or trans­lat­ing data packages into physical signals. This only works, however, when all systems involved with com­mu­ni­ca­tion adhere to certain rules, which are de­ter­mined in protocols.

The ISO reference model, on the other hand, is not a concrete network standard. Instead, it describes, in abstract form, which processes need to be regulated so that network com­mu­ni­ca­tion is able to take place.

The upper layers of the OSI reference model are referred to as ap­pli­ca­tion-oriented layers. More specif­i­cal­ly, there’s a dif­fer­en­ti­a­tion made between the ap­pli­ca­tion layer, pre­sen­ta­tion layer, and the session layer.

• Layer 7 — Ap­pli­ca­tion layer: this layer from the OSI model has direct contact with ap­pli­ca­tions, like e-mail programs or web browsers. This is where data input and output takes place. The ap­pli­ca­tion layers create the con­nec­tion to the OSI model’s upper levels and keeps ap­pli­ca­tion features on standby. An example on how e-mail transfer functions makes this latter point more clear: a user types a message into their e-mail program that’s located on the device in use. This message is accepted into the ap­pli­ca­tion layer in the form of a data packet. Here, ad­di­tion­al in­for­ma­tion is added to the e-mail data in the form of an ap­pli­ca­tion header; this process is known as capsuling. Among other things, the header contains in­for­ma­tion in­di­cat­ing that the enclosed data stems from an e-mail program. Ad­di­tion­al­ly, the protocol is defined that’s re­spon­si­ble for trans­fer­ring the e-mail on the ap­pli­ca­tion layer (normally SMTP for e-mails).

• Layer 6 — Pre­sen­ta­tion layer: a central task for network com­mu­ni­ca­tion is to make sure that data is trans­ferred in standard formats. This is why local pre­sen­ta­tions are trans­ferred in stan­dard­ized formats on the pre­sen­ta­tion layer. In the case of an e-mail transfer, this is the point when it’s defined how the message is to appear. To this end, a pre­sen­ta­tion header is added to the data package. This contains in­for­ma­tion on how the e-mail is coded, in which format potential at­tach­ments should be present (e.g. JPEG or MPEGG4) and how the data is com­pressed or encrypted (e.g. SSL/TLS). This enables the e-mail’s format to be un­der­stood and the message to be displayed as it should be.

• Layer 5 — Session layer: or­ga­niz­ing the con­nec­tion between both of the end systems is the main task that’s carried out in the session layer. This is also known as the com­mu­ni­ca­tion layer. This is where special control mech­a­nisms take effect, which are re­spon­si­ble for es­tab­lish­ing and main­tain­ing the con­nec­tion as well as reg­u­lat­ing the con­nec­tion set-up. Ad­di­tion­al in­for­ma­tion, which is added to the e-mail that’s to be trans­ferred, is required for this com­mu­ni­ca­tion control (this is done through a session header). The most typical ap­pli­ca­tion protocols, like SMTP or FTP, take care of the sessions them­selves or, like HTTP, are stateless. This is why the TCP/IP model, a com­peti­tor of the OSI model, compiles OSI 5, 6, and 7 into one ap­pli­ca­tion layer. Further standards that draw on layer 5’s functions are NetBIOS, Socks, and RPC.

There are four transport oriented layers that follow the three ap­pli­ca­tion-oriented layers. Here, one dif­fer­en­ti­ates between the transport layers, the network layer, the data link layer, and the physical layer.

• Layer 4 — Trans­porta­tion layer: the transport layer acts as the link between the ap­pli­ca­tion-oriented layers and the transport-oriented layers. On this level, the logical end-to-end con­nec­tion and the transfer channel between the com­mu­ni­cat­ing systems is realized. During this step in­for­ma­tion is also added to the e-mail data. The data packet, which at this point has already had header from the ap­pli­ca­tion-oriented layers added to it, receives a transport header. Stan­dard­ized network protocols, like TCP or UDP are used. This is also when the ports, through which ap­pli­ca­tions on the target system are con­trolled, are defined. Layer four is also where data packets are assigned to certain ap­pli­ca­tions.

• Layer 3 — Network layer: on layer 3, the data transfer finally reaches the internet. This is where logical ad­dress­ing of the terminal devices takes place. These are then assigned a unique IP address on layer 3. A network header con­tain­ing in­for­ma­tion on routing and data flow control is added to the data packet, like the e-mail data in the example. Here, computer systems fall back on internet standards, like IP, ICMP, X.25, RIP, or OSPF. Generally TCP over IP is used for e-mail traffic.

• Layer 2 — data link layer: on the data link layer, functions designed for detecting errors and managing data flows help transfer errors to be avoided. For this purpose, data packets, including ap­pli­ca­tion, pre­sen­ta­tion, session, transport, and network headers are added to a frame composed of data link headers and data link trails. Layer 2 is also where hardware ad­dress­ing takes place. To this end, so-called MAC addresses are used. Access to the transfer medium is regulated by protocols, like either net or PPP.

• Layer 1 — Physical Layer: the physical layer is where a data packet’s bits are trans­formed into physical signal that matches the intended transfer medium. Such signals can only be trans­ferred with the help of some sort of medium, such as copper wire, fiber optic cables, or air. The interface for the transfer medium is defined through protocols and standards, like DSL, ISDN, Bluetooth, USB (physical layer), or ether net (physical layer).

Data packets go through each of the OSI model’s stages on both sender systems as well as target systems. All other devices that data packets pass by on their way to their target des­ti­na­tions rely solely on layers 1 to 3. The e-mail from the example above first passes through the router as a physical signal before it continues on its way through the internet. The internet is only found on layer 3 of the OSI model and, for this reason, only spreads in­for­ma­tion from the first three layers; layers 4 to 7 are not con­sid­ered. In order to gain access to required in­for­ma­tion, the router first has to unpack the capsuled in­for­ma­tion. This step is often logically referred to as ‘de­cap­su­la­tion’. For this process, the OSI layers runs in reverse.  

First, the signal is coded on the physical layer. Next, the MAC addresses are ready out on layer 2 and the IP, as are the IP addresses and routing protocols on layer 3. Based on this in­for­ma­tion, the router is then able to make a redirect decision. This allows the data packet to then be once again en­cap­su­lat­ed and, based on the new in­for­ma­tion it gained, forwarded to the next station on its way to the target system.

For data transfers, there are generally multiple routers involved, on which processes described above (cap­su­la­tion and de­cap­su­al­tion) run; these processes continue until the data packet arrives at its des­ti­na­tion; in the case of our text, this is an e-mail server in the form or a physical signal. Here the data packet is also de­cap­su­lat­ed (a process that is carried out by running the packet through the OSI model from layers 1 to 7). After this has been done, the message sent via the e-mail client should have reached the e-mail server, where it’s now ready to be accessed through another e-mail client.