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OSI MODEL

The 7 Layers of the OSI Model

The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Use this handy guide to compare the different layers of the OSI model and understand how they interact with each other.

• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Session (Layer 5)
• Presentation (Layer 6)
• Application (Layer 7)

Physical (Layer 1)


OSI Model, Layer 1 conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

Data Link (Layer 2)


At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.

Network (Layer 3)


Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Layer 3 Network examples include AppleTalk DDP, IP, IPX.
Transport (Layer 4)


OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Layer 4 Transport examples include SPX, TCP, UDP.
Session (Layer 5)


This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Layer 5 Session examples include NFS, NetBios names, RPC, SQL.
Presentation (Layer 6)


This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
Application (Layer 7)


OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

 Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP

Router (computing)

A router^[a] is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. A data packet is typically forwarded from one router to another router through the networks that constitute the internetwork until it reaches its destination node.^[2]

A router is connected to two or more data lines from different networks.^[b] When a data packet comes in on one of the lines, the router reads the address information in the packet to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. This creates an overlay internetwork.

The most familiar type of routers are home and small office routers that simply pass IP packets between the home computers and the Internet. An example of a router would be the owner's cable or DSL router, which connects to the Internet through an Internet service provider (ISP). More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone. Though routers are typically dedicated hardware devices, software-based routers also exist.

Hub

•  When referring to a network, a hub is the most basic networking device that connects multiple computers or other network devices together. Unlike a network switch or router, a network hub has no routing tables or intelligence on where to send information and broadcasts all network data across each connection. Most hubs can detect basic network errors such as collisions, but having all information broadcast to multiple ports can be a security risk and cause bottlenecks. In the past, network hubs were popular because they were cheaper than a switch or router. Today, switches do not cost much more than a hub and are a much better solution for any network.
  

• 
• In general, a hub refers to a hardware device that enables multiple devices or connections to be connected to a computer. Another example besides the one given above is a USB hub, which allows multiple USB devices to be connected to one computer, even though that computer may only have a few USB connections. The picture to the right is an example of a USB hub.

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RAID

What is RAID?

RAID is an acronym for Redundant Array of Independent (or Inexpensive) Disks. In fact, RAID is the way of combining several independent and relatively small disks into a single storage of a large size. The disks included into the array are called array members. The disks can be combined into the array in different ways which are known as RAID levels. Each of RAID levels has its own characteristics of:

• Fault-tolerance which is the ability to survive of one or several disk failures.
• Performance which shows the change in the read and write speed of the entire array as compared to a single disk.
• The capacity of the array which is determined by the amount of user data that can be written to the array. The array capacity depends on the RAID level and does not always match the sum of the sizes of the RAID member disks. To calculate the capacity of the particular RAID type and a set of the member disks you can use a free online RAID calculator.

How RAID is organized?

Two independent aspects are clearly distinguished in the RAID organization.

1. The organization of data in the array (RAID storage techniques: striping, mirroring, parity, combination of them).
2. Implementation of each particular RAID installation - hardware or software.

RAID storage techniques

The main methods of storing data in the array are:

• Striping - splitting the flow of data into blocks of a certain size (called "block size") then writing of these blocks across the RAID one by one. This way of data storage affects on the performance.
• Mirroring is a storage technique in which the identical copies of data are stored on the RAID members simultaneously. This type of data placement affects the fault tolerance as well as the performance.
• Parity is a storage technique which is utilized striping and checksum methods. In parity technique, a certain parity function is calculated for the data blocks. If a drive fails, the missing block are recalculated from the checksum, providing the RAID fault tolerance.

All the existing RAID types are based on striping, mirroring, parity, or combination of these storage techniques.

RAID levels

• RAID 0 - based on striping. This RAID level doesn't provide fault tolerance but increases the system performance (high read and write speed).
• RAID 1 - utilizes mirroring technique, increases read speed in some cases, and provides fault tolerance in the loss of no more than one member disk.
• RAID 0+1 - based on the combination of striping and mirroring techniques. This RAID level inherits RAID 0 performance and RAID 1 fault tolerance.
• RAID1E - uses both striping and mirroring techniques, can survive a failure of one member disk or any number of nonadjacent disks. There are three subtypes of RAID 1E layout: near, interleaved, and far. More information and diagrams on the RAID 1E page.
• RAID 5 - utilizes both striping and parity techniques. Provides the read speed improvement as in RAID 0 approximately, survives the loss of one RAID member disk.
• RAID 5E - a variation of RAID 5 layout the only difference of which is an integrated spare space allowing to rebuild a failed array immediately in case of a disk failure. Read more on the RAID5E page.
• RAID 5 with delayed parity - pretty similar to basic RAID 5 layout, but uses nonstandard scheme of striping. More information about RAID5 with delayed parity.
• RAID 6 - similar to RAID 5 but uses two different parity functions. The read speed is the same as in RAID 5.

RAID stands for Redundant Array of Inexpensive (Independent) Disks.

On most situations you will be using one of the following four levels of RAIDs.

• RAID 0
• RAID 1
• RAID 5
• RAID 10 (also known as RAID 1+0)

This article explains the main difference between these raid levels along with an easy to understand diagram.

In all the diagrams mentioned below:

• A, B, C, D, E and F – represents blocks
• p1, p2, and p3 – represents parity

RAID LEVEL 0

Following are the key points to remember for RAID level 0.

• Minimum 2 disks.
• Excellent performance ( as blocks are striped ).
• No redundancy ( no mirror, no parity ).
• Don’t use this for any critical system.

RAID LEVEL 1

Following are the key points to remember for RAID level 1.

• Minimum 2 disks.
• Good performance ( no striping. no parity ).
• Excellent redundancy ( as blocks are mirrored ).

RAID LEVEL 5

Following are the key points to remember for RAID level 5.

• Minimum 3 disks.
• Good performance ( as blocks are striped ).
• Good redundancy ( distributed parity ).
• Best cost effective option providing both performance and redundancy. Use this for DB that is heavily read oriented. Write operations will be slow.

RAID LEVEL 10

Following are the key points to remember for RAID level 10.

• Minimum 4 disks.
• This is also called as “stripe of mirrors”
• Excellent redundancy ( as blocks are mirrored )
• Excellent performance ( as blocks are striped )
• If you can afford the dollar, this is the BEST option for any mission critical applications (especially databases).

 

RAID implementations

RAID can be created by two different ways:

• with the use of operating system drivers, so called software RAID;
• with the use of special hardware, so called hardware RAID.

Software RAID

Software RAID is one of the cheapest RAID solutions.

Nowadays, almost any of the operating systems has a built-in capability to create RAID, though not for all RAID levels. Thus, Windows home editions allow user to create only RAID 0, while RAID 1 and RAID 5 can be created only using Windows server editions. RAID layout created by means of Windows is inseparably linked with the host operating system and so its partitions cannot be used, for example, in dual boot.

Software RAID is created based on the user's computer and therefore it uses the host system CPU for implementation. It should be noted, that in case of RAID levels 0 and 1, CPU load is negligible, but for the RAID types based on parity, CPU load can vary from 1 to 5 percent depending on CPU power and the number of the disks, which is also negligible for practical purposes.

There are certain limitations on the use of the software RAID to boot the system. Only RAID 1 can contain boot partition, while system boot is impossible with a software RAID 5 and RAID 0.

Keep in mind that in most cases software RAID doesn't implement the hot-swapping and so it cannot be used where continuous availability is required.

Hardware RAID

Hardware RAID is created using separate hardware. Basically there are two options:

• inexpensive RAID chip possibly built into the motherboard,
• more expensive option with a complex standalone RAID controller. Such controllers can be equipped with their own CPU, battery-backed up cache memory, and they typically support hot-swapping.

A hardware RAID has some advantages over a software RAID, such as:

• doesn't use CPU of the host computer;
• allows user to create boot partitions;
• handles errors better, since communicates with the devices directly;
• supports hot-swapping.