The Discordant Opposition Journal Issue 6 - File 5
:The OSI Model:
Strykar
The OSI model breaks the various aspects of a computer network into seven distinct layers.The model itself is not a networking standard in the ways Ethernet & Token ring are.It is more of a framework into which the various networking standards can fit.The OSI model specifies what aspects of a networks operation can be addressed by various network standards.So in a way you can call the OSI model a standards standard.
In all the seven layers described below the bottom two are the one's that have a practical impact on smaller networks.Networking standards like Ethernet & Token Ring are Layer-1 & Layer-2 standards.The higher layers of the OSI model do not have widespread standards.
Layer 1: The Physical Layer
The bottom layer of the OSI model is the physical layer.It addresses the physical characteristics of the network i.e. the types of cables to be used to connect devices, types of connectors used, how long the cables can be etc. The physical layer does not define any meaning to those signals other than the basic binary values 0 & 1.It is up to the higher levels of the OSI models to assign meanings to the bits that are transmitted at the physical layer.
Layer 2: The Data Link Layer
The Data Link Layer is that layer at which meaning is assigned to the bits that are transmitted over the network.A standard for the data link layer must address things such as size of each packet of data so that each packet is delivered to the intended recipient & a way to ensure that two or more nodes don't try to transmit data at the same time.It also provides basic error detection & correction to ensure that the data sent is the same as the data received. If an uncorrectable error occurs the Data Link Standard specifies how the node is to be informed of the error so that it can retransmit the data.
Layer 3: The Network Layer
The Network Layer addresses the interconnection of networks by routing packets from one network to another.This is an important layer when you're using a router to link two different types of networks, say an ethernet & a Token Ring network.This is possible because the Network Layer is one step above the Data Link Layer & it doesn't matter whether two networks use different standards at the data link & physical layers.
Layer 4: The Transport Layer
The Transport Layer is the basic layer at which one network computer communicates with another network computer.The Transport Layer identifies each node on the computer with a unique address & manages connections between nodes.It breaks large messages into smaller ones & reassembles them at the receiving node.The Transport Layer & the OSI layers are implemented differently by various network operating systems.The OSI model can run NetWare, LANtastic, Windows for Workgroups, or just about any other network operating system on a standard Ethernet network as Ethernet addresses the lower layers of the OSI model.
Layer 4(a): The Lemon-Pudding Layer *wink*
The Lemon-Pudding Layer is squeezed in between the rather dry & tasteless transport & session layers to add flavour & moisture.
Layer 5: The Session Layer
The Session Layer establishes sessions between network nodes. A session must be established before data can be transmitted over a network.The Session Layer ensures that these sessions are properly established & maintained.
Layer 6: The Presentation Layer
The Presentation Layer is responsible for converting the data sent over the network from one type of representation to another.It can apply complex compression techniques so that fewer bytes of data are required to represent the information when it is sent over the network,once the data is received at the right node the Presentation Layer uncompresses it.It can also scramble data sent over the network using sophisticated encryption techniques.
Layer 7: The Application Layer
The Application Layer is the highest layer of the OSI model, the Application Layer deals with the techniques that application programs use to communicate with the network.Application programs like Lotus 1-2-3 & WordPerfect are not part of the application layer.It is the network operating system itself that works within the application layer.
=-Strykar-=
5:02 p.m. 25/05/99
Thanks to KoMaN3(You're the man) Phantomd, Magikal7 & all the other people in DALnet's #hackerzLair.
