OSI is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. Its purpose is to guide product implementors so that their products will consistently work with other products.
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.
Please select one of the 7 layers by clicking on it, or simply use bmenu :)
The OSI flash below is provided to help you futher understand the functionality of the OSI model:
The picture below is another quick summary of the OSI model:
When you're finished reading through the OSI model, to understand how data travels through the layers and clearly see the header which each layer add\removes, visit the Data Encapsulation - Decapsulation page.
Layer 1 - The Physical Layer
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.
We talk more about the Physical topologies in the Topologies section. Please refer to it if you want to read more. You can also find out more about Ethernet at the Ethernet section.
Layer 2 - The Datalink Layer
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 similair 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:
Layer 3 - The Network Layer
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 192.168.0.1, 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. To keep things simple, IP is analysed under the "Protocols" section.
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. Below is a brief diagram which tells you a bit about the protocols.
These protocols are also analysed in the Protocols area.
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.
Layer 5 - The Session Layer
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 organise 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 Systrem (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.
Layer 6- The Presentation Layer
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.
MIDI: The Musical Intrument Digital Interface is used for digitized music.
MPEG: The Moving Pictures Experts Group's standard for the compression and coding of motion video for CD's is very popular.
QuickTime: This is for use with Machintosh or Power PC programs, it manages audio and video applications.
Layer 7- 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, I need to structure my sentence in a "standard" way which you will understand. Computer communication works pretty much the same way. This is why we have so many different protocols, each one for a specific task.
Router
A network device that forwards packets from one network to another. Based on internal routing tables, routers read each incoming packet and decide how to forward it. To which interface on the router outgoing packets are sent may be determined by any combination of source and destination address as well as current traffic conditions (load, line costs, bad lines, etc.).
In Between and at the Edge
Routers are used to separate local area networks (LANs) into subnetworks in order to balance traffic within workgroups and to filter traffic for security purposes and policy management. Routers are also used at the edge of the network to connect to remote offices or to an ISP for Internet access.
Routers in the Core
Within a large enterprise, routers serve as an internet (lower case "i") backbone that connects all internal networks, in which case they are typically connected via Ethernet. Within the global Internet (upper case "I"), routers do all the packet switching between the backbones and are typically connected via T3, ATM or SONET links.
Routable Protocols
Routers route messages transmitted only by a routable protocol such as IP "or" IPX. Multiprotocol routers support more than one; for example, IP "and" IPX. Messages in non-routable protocols, such as NetBIOS and LAT, cannot be routed, but they can be transferred from LAN to LAN via a bridge.
Because routers have to inspect the network address in the packet, they do more processing and add more overhead than a bridge or switch. Routers work at the network layer (layer 3) of the protocol, whereas bridges and switches work at the data link layer (layer 2), also known as the "MAC layer."
Specialized Machines or Regular PCs
Most routers are specialized computers optimized for communications; however, router functions can also be implemented by adding software to a server. For example, the NetWare operating system includes software to route packets from one subnetwork to another if each is connected to a separate network adapter (NIC). The major router vendors are Cisco Systems and Nortel Networks.
Router Terminology
Routers used to be called "gateways." In older Novell terminology, they were also called "network-layer bridges." For more details on the routable protocol layer (network layer 3),
• Cisco 800 Series
• Cisco 1600 Series
• Cisco 1700 Series
• Cisco 2500 Series
• Cisco 2600 Series
• Cisco 3600 Series
• Cisco 3700 Series
• Cisco 6400 Series • Cisco 7200 Series
• Cisco 7300 Series
• Cisco 7400 Series
• Cisco 7500 Series
• Cisco 7600 Series
• Cisco 10000 Series
• Cisco 12000 Series
Cisco Router Series
Very Important Router Parts
1. ROM - Read Only Memory.
This is a form of permanent memory used by the Router to store:
o The "Power-On Self Test" that checks the Router on boot up.
o The "Bootstrap Startup Program" that gets the Router going.
o A very basic form of the Cisco IOS software.
(to change the ROM you have to remove and replace chips)
2. Flash Memory
An Electronically Erasable and Re-Programmable memory chip.
The "Flash" contains the full Operating System, or "Image".
This allows you to Upgrade the OS without removing chips.
3. NVRAM - Non-Volatile RAM
This stores your Router's "Startup Configuration File".
Similar to Flash memory, this retains data even when power is lost.
4. RAM - Random Access Memory
This is regular computer memory chips.
These are the working memory of the Router,
and provide Caching, Packet Buffering, and hold Routing Tables.
The RAM is also where the Running Operating System
lives when the Router is on.
RAM loses all its data when reset or powered off.
5. Interfaces - Where the Router meets the Outside World.
Basically your Router will have Serial interfaces,
Which are mostly used to connect long-distance as in a WAN (Wide-Area Network).
You will also have LAN (Local-Area Network) Interfaces,
such as Ethernet, Token Ring, and FDDI (Fiber Distributed Data Interface)
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