Sunday, August 22, 2010

Introduction To The Open Systems Interconnect Model (OSI)


OSI is a standard description or "reference model" for how messages should be transmitted between any two points in a network.

The Model

The OSI model was created by the IEEE committee so different vendors products would work with each other. You see the problem was that when HP decided to create a network product, it would be incompatible with similar products of a different vendor e.g IBM. So when you bought 40 network cards for your company, you would make sure that the rest of the equipment would be from the same vendor, to ensure compatibility. As you would understand things were quite messy, until the OSI model came into the picture.

As most would know, the OSI model consists of 7 layers.

Each layer has been designed to do a specific task. Starting from the top layer (7) we will see how the data which you type gets converted into segments, the segments into datagrams and the datagrams into packets, the packets into frames and then the frames are sent down the wire, usually twisted pair, to the receiving computer.

The picture below is another quick summary of the OSI model:

OSI Layer 1 - Physical Layer

The first four layers define how data is transmitted end-to-end.

There are no protocols which work at the Physical layer. As mentioned, Ethernet, Token Ring and other topologies are specified here.

The Physical layer has two responsibilities: it sends bits and receives bits. Bits come only in values of 1 or 0. The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals and the various qualities of the physical media's attachment interface.

The Physical layer specifications specify the electrical, mechanical and functional requirements for activating, maintaining and deactivating a physical link between end systems. At the physical layer, the interface between the Data Terminal Equipment (DTE) and the Data Circuit-Terminating Equipment (DCE) is identified. The Physical layer's connectors (RJ-45, BNC e.c.t) and different physical topologies (Bus, Star, Hybrid networks) are defined by the OSI as standards, allowing different systems to communicate.

OSI Layer 2 - Datalink Layer

The first four layers define how data is transmitted end-to-end.

Some common protocols which work at the Datalink layer are: ARP, RARP, DCAP.

The Datalink ensures that messages are delivered to the proper device and translates messages from the Network layer into bits for the Physical layer to transmit. It formats the message into data frames (notice how we are not using the term segments) and adds a customized header containing the hardware destination and source address.

This added information forms a sort of capsule that surrounds the original message (or data), think of it like grabbing a letter which has information and putting it into an envelope. The envelope is only used to get the letter to its destination, right? So when it arrives at the addressee, the envelope is opened and discarded, but the letter isn't because it has the information the addressee needs.

Data traveling through a network works in a similar manner. Once it gets to the destination, it will be opened and read (processed). This is illustrated in the Data Encapsulation - Decapsulation section.

The Datalink layer is subdivided into two other sublayers, the Media Access Control (MAC) and the Logical Link Control (LLC). The figure below illustrates this:

Media Access Control (MAC) 802.3

This defines how packets are placed on the media (cable). Contention media (Ethernet) access is first come first served access where everyone shares the same bandwidth. Physical addressing is defined here. What's Physical addressing? It's simple.

You will come across 2 addressing terms, 1)Logical addressing 2)Physical addressing.

Logical addressing is basically the address which is given by software e.g IP address.When you get an IP address, this is considered a "logical address" which is provided to you after your TCP/IP stack is loaded.

Physical addressing is an address which is given not by the software, but the hardware. Every network card has a "MAC" address which is burnt into the card's eprom (a special memory chip) and this special address is used to uniquely identify your computer's network card from all the others on the network.

There is a whole page dedicated to MAC Addressing if you would like to read more about it.

Logical Link Control (LLC) 802.2

This sublayer is responsible for identifying Network layer protocols and then encapsulating them when they are about to be transmitted onto the network or decapsulate them when it receives a packet from the network and pass it onto the layer above it, which is the Network layer. An LLC header tells the Datalink layer what to do with a packet once a frame is received. For example, a host (computer) will receive a frame and then look in the LLC header to understand that the packet is destined for the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.

OSI Layer 3 - Network Layer

The first four layers define how data is transmitted end-to-end.

Some common protocols which work at the Network layer are: IP, DHCP, ICMP, IGRP, EIGRP, RIP, RIP2, MARS.

The Network layer is responsible for routing through an internetwork and for networking addressing. This means that the Network layer is responsible for transporting traffic between devices that are not locally attached. Routers, or other layer-3 devices, are specified at the Network layer and provide routing services in an internetwork.

In the Open Systems Interconnection (OSI) communications model, the Network layer knows the address of the neighboring nodes in the network, packages output with the correct network address information, selects routes and quality of service and recognizes and forwards to the Transport layer incoming messages for local host domains. Among existing protocol that generally map to the OSI network layer are the Internet Protocol (IP) part of TCP/IP and NetWare IPX/SPX. Both IP Version 4 and IP Version 6 (IPv6) map to the OSI network layer.

As mentioned above, the Internet Protocol works on this layer. This means that when you see an IP address, for example, this IP address maps to the Network layer in the OSI model, in other words only the Network layer deals with or cares about IP addresses in the OSI model.

OSI Layer 4 - Transport Layer

The first four layers define how data is transmitted end-to-end.

Some common protocols which work at the Transport layer are: TCP, UDP.

Layer 4- The Transport Layer

The Transport layer is responsible for providing mechanisms for multiplexing upper-layer application, session establishment, data transfer and tear down of virtual circuits. It also hides details of any network-dependent information from the higher layers by providing transparent data transfer.

Services located in the Transport layer both segment and reassemble data from upper-layer applications and unite it onto the same data stream. Some of you might already be familiar with TCP and UDP and know that TCP is a reliable service and UDP is not. Application developers have their choice of the two protocols when working with TCP/IP protocols.

As mentioned above, the Transport layer provides different mechanisms for the transfer of data from one computer to another.

OSI Layer 5 - Session Layer

The last 3 layers of the OSI model are reffered to the "Upper" layers. These layers are responsible for applications communicating between hosts. None of the upper layers know anything about networking or network addresses.

Some common protocols which work at the Session layer are: DNS, LDAP, NetBIOS.

The Session layer is responsible for setting up, managing and then tearing down sessions between Presentation layer entities. The Session layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half-duplex and full-duplex. The session layer basically keeps one application's data separate from other application's data.

Some examples of Session-layer protocols are:

Network File System (NFS) : Was developed by Sun Microsystems and used with TCP/IP and Unix workstations to allow transparent access to remote resources.

Structured Query Language (SQL): Was developed by IBM to provide users with a simpler way to define their information requirements on both local and remote systems.

Remote Procedure Call (RPC): Is a broad client/server redirection tool used for disparate service environments. Its procedures are created on clients and performed on servers.

X Window: Is widely used by intelligent terminals for communicating with remote Unix computers, allowing them to operate as though they were locally attached monitors.

OSI Layer 6 - Presentation Layer

The last 3 layers of the OSI model are reffed to the "Upper" layers. These layers are responsible for applications communicating between hosts.

The Presentation Layer gets its name from its purpose: It presents data to the Application layer. It's basically a translator and provides coding and conversion functions. A successful data transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for reading. By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another host.

The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations. The following serve to direct graphic and visual image presentation :

JPEG: The Joint Photographic Experts Group brings these photo standards to us.

MPEG: The Moving Pictures Experts Group's standard for the compression and coding of motion video for CD's is very popular.

OSI Layer 7 - Application Layer

The last 3 layers of the OSI model are reffed to the "Upper" layers. These layers are responsible for applications communicating between hosts.

FTP, TFTP, Telnet, SMTP and other protocols work on the first three layers of the OSI model, which obviously includes the Application layer.

The Application layer of the OSI model is where users communicate with the computer. The Application layer is responsible for identifying and establishing the availability of the intended communication partner and determining if sufficient resources for the intended communication exist. The user interfaces with the computer at the application layer.

Although computer applications sometimes require only desktop resources, applications may unite communicating components from more than one network application, for example, file transfers, e-mail, remote access, network management activities, client/server processes.

There are various protocols which are used at this layer. Definition of a"Protocol" is a set of rules by which two computers communicate. In plain English, you can say that a protocol is a language, for example, English. For me to speak to you and make sense.

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