OSI Model (Open System Interconnection Model) Concepts...

The OSI (Open System Interconnection) model was developed by the International Organization for Standardization (ISO) as a guideline for developing standards to enable the interconnection of dissimilar computing devices. It is important to understand that the OSI model is not itself a communication standard. In other words, it is not an agreed-on method that governs how data is sent and received; it is only a guideline for developing such standards.

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Seven Layers/Standards of the OSI Model :-

Seven Layers of the OSI Model

Layer 1 - Physical Layer :-
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.

Functions :-

• Transmits raw bit stream over physical cable.
• Defines cables, cards, and physical aspects.
• Defines NIC attachments to hardware, how cable is attached to NIC.
• Defines techniques to transfer bit stream to cable.

Protocols :-
IEEE 802
IEEE 802.2
ISO 2110
ISDN


Layer 2 - Data Link Layer (LLC and MAC) :-
Data Link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.

Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.

Functions :-

• Turns packets into raw bits 100101 and at the receiving end turns bits into packets.
• Handles data frames between the Network and Physical layers.
• The receiving end packages raw data from the Physical layer into data frames for delivery to the Network layer.
• Responsible for error-free transfer of frames to other computer via the Physical Layer.
• This layer defines the methods used to transmit and receive data on the network. It consists of the wiring, the devices use to connect the NIC to the wiring, the signaling involved to transmit / receive data and the ability to detect signaling errors on the network media.

Protocols :-
a) Logical Link Control (LLC)
i. Error correction and flow control
ii. Manages link control and defines SAPs
802.1 OSI Model
802.2 Logical Link Control

b) Media Access Control (MAC)
i. Communicates with the adapter card.
ii. Controls the type of media being used.
802.3 CSMA/CD (Ethernet)
802.4 Token Bus (ARCnet)
802.5 Token Ring
802.12 Demand Priority


Layer 3 - Network Layer :-
NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. The leading 1-3 bytes of the IP identify the network and the remaining bytes identifies the host on that network. The network portion of the IP is assigned by InterNIC Registration Services, under the contract to the National Science Foundation, and the host portion of the IP is assigned by the local network administrators. For large sites, the first two bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet and host respectively. Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address.

Functions :-

• Translates logical network address and names to their physical address (e.g. computername ==> MAC address).
• Responsible for addressing, determining routes for sending and managing network problems such as packet switching, data congestion and routing.
• If router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data.

Protocols :-
IP
ARP
RARP
ICMP
RIP
OSFP
IGMP
IPX
NWLink
NetBEUI
OSI
DDP
DECnet


Layer 4 - Transport Layer :-
Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a stateful protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. NFS is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery.

Functions :-

• Additional connection below the session layer.
• Manages the flow control of data between parties across the network.
• Divides streams of data into chunks or packets; the transport layer of the receiving computer reassembles the message from packets.
• A train is a good analogy => the data is divided into identical units.
• Provides error-checking to guarantee error-free data delivery, with on losses or duplications
• Provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free.
• Provides flow control and error-handling.

Protocols :-
TCP
ARP
RARP
SPX
NWLink
NetBIOS / NetBEUI
ATP
UDP
DCCP


Layer 5 - Session Layer :-
The session protocol defines the format of the data sent over the connections. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.

Functions :-

• Establishes, maintains and ends sessions across the network.
• Responsible for name recognition (identification) so only the designated parties can participate in the session.
• Provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted.
• Manages who can transmit data at a certain time and for how long.
• Examples are interactive login and file transfer connections, the session would connect and re-connect if there was an interruption; recognize names in sessions and register names in history.

Protocols :-
NetBIOS
Names Pipes
Mail Slots
RPC
SAP
L2TP
PPTP
SPDY

Layer 6 - Presentation Layer :-
External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.

Functions :-

• Translates from application to network format and vice-versa.
• All different formats from all sources are made into a common uniform format that the rest of the OSI model can understand.
• Responsible for protocol conversion, character conversion,data encryption / decryption, expanding graphics commands, data compression.
• Sets standards for different systems to provide seamless communication from multiple protocol stacks.
• Not always implemented in a network protocol.

Protocols :-
MIME
XDR
TLS
SSL

Layer 7 - Application Layer :-
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.

Functions :-

• Used for applications specifically written to run over the network.
• Allows access to network services that support applications.
• Directly represents the services that directly support user applications.
• Handles network access, flow control and error recovery.
• Example apps are file transfer,e-mail, NetBIOS-based  applications.

Protocols :-
FTAM
DNS
FTP
TFTP
BOOTP
SNMP
RLOGIN
SMTP
MIME
NFS
FINGER
TELNET
NCP
APPC
AFP
SMB

Quick View at the Functions of the layers 

Network Communications through the OSI Model :-

Network Communications through the OSI Model. Some info. is added at every layer into the data known as "HEADERS"

The figure represents two networked computers. They are running identical operating systems and applications and are using identical protocols (or rules) at all OSI layers. Working in conjunction, the applications, the OS, and the hardware implement the seven functions described in the OSI model.

Each computer is also running an e-mail program that is independent of the OSI layers. The e-mail program enables the users of the two computers to exchange messages. Our figure represents the transmission of one brief message from Sam to Charlie.

The transmission starts when Sam types in a message to Charlie and presses the "send" key. Sam's operating system appends to the message (or "encapsulates") a set of application-layer instructions (OSI Layer 7) that will be read and executed by the application layer on Charlie's computer. The message with its Layer 7 header is then transferred to the part of the operating system that deals with presentation issues (OSI Layer 6) where a Layer 6 header is appended to the message. The process repeats through all the layers until each layer has appended a header. The headers function as an escort for the message so that it can successfully negotiate the software and hardware in the network and arrive intact at its destination.

When the data-link-layer header is added at Layer 2, the data unit is known as a "frame." The final header, the physical-layer header (OSI Layer 1) tells the hardware in Sam's computer the electrical specifics of how the message will be sent (which medium, at which voltage, at which speed, etc.). Although it is the final header to be added, the Layer 1 header is the first in line when the message travels through the medium to the receiving computer.

When the message with its seven headers arrives at Charlie's computer, the hardware in his computer is the first to handle the message. It reads the instructions in the Layer 1 header, executes them, and strips off the header before passing the message to the Layer 2 components. These Layer 2 components execute those instructions, strip off the header, and pass the message to Layer 3, and so on. Each layer's header is successively stripped off after its instructions have been read so that by the time the message arrives at Charlie's e-mail application, the message has been properly received, authenticated, decoded, and presented.

***************************

Bandwidth Explained

BandWidth Explaination:-

Bandwidth

Most hosting companies offer a variety of bandwidth options in their plans. So exactly what is bandwidth as it relates to web hosting? Put simply, bandwidth is the amount of traffic that is allowed to occur between your web site and the rest of the internet. The amount of bandwidth a hosting company can provide is determined by their network connections, both internal to their data center and external to the public internet.


Network Connectivity:-

The internet, in the most simplest of terms, is a group of millions of computers connected by networks. These connections within the internet can be large or small depending upon the cabling and equipment that is used at a particular internet location. It is the size of each network connection that determines how much bandwidth is available. For example, if you use a DSL connection to connect to the internet, you have 1.54 Mega bits (Mb) of bandwidth. Bandwidth therefore is measured in bits (a single 0 or 1). Bits are grouped in bytes which form words, text, and other information that is transferred between your computer and the internet.


If you have a DSL connection to the internet, you have dedicated bandwidth between your computer and your internet provider. But your internet provider may have thousands of DSL connections to their location. All of these connection aggregate at your internet provider who then has their own dedicated connection to the internet (or multiple connections) which is much larger than your single connection. They must have enough bandwidth to serve your computing needs as well as all of their other customers. So while you have a 1.54Mb connection to your internet provider, your internet provider may have a 255Mb connection to the internet so it can accommodate your needs and up to 166 other users (255/1.54).


Traffic:-

A very simple analogy to use to understand bandwidth and traffic is to think of highways and cars. Bandwidth is the number of lanes on the highway and traffic is the number of cars on the highway. If you are the only car on a highway, you can travel very quickly. If you are stuck in the middle of rush hour, you may travel very slowly since all of the lanes are being used up.


Traffic is simply the number of bits that are transferred on network connections. It is easiest to understand traffic using examples. One Gigabyte is 2 to the 30th power (1,073,741,824) bytes. One gigabyte is equal to 1,024 megabytes. To put this in perspective, it takes one byte to store one character. Imagine 100 file cabinets in a building, each of these cabinets holds 1000 folders. Each folder has 100 papers. Each paper contains 100 characters - A GB is all the characters in the building. An MP3 song is about 4MB, the same song in wav format is about 40MB, a full length movie can be 800MB to 1000MB (1000MB = 1GB).


If you were to transfer this MP3 song from a web site to your computer, you would create 4MB of traffic between the web site you are downloading from and your computer. Depending upon the network connection between the web site and the internet, the transfer may occur very quickly, or it could take time if other people are also downloading files at the same time. If, for example, the web site you download from has a 10MB connection to the internet, and you are the only person accessing that web site to download your MP3, your 4MB file will be the only traffic on that web site. However, if three people are all downloading that same MP at the same time, 12MB (3 x 4MB) of traffic has been created. Because in this example, the host only has 10MB of bandwidth, someone will have to wait. The network equipment at the hosting company will cycle through each person downloading the file and transfer a small portion at a time so each person's file transfer can take place, but the transfer for everyone downloading the file will be slower. If 100 people all came to the site and downloaded the MP3 at the same time, the transfers would be extremely slow. If the host wanted to decrease the time it took to download files simultaneously, it could increase the bandwidth of their internet connection (at a cost due to upgrading equipment).


Hosting Bandwidth:-

In the example above, we discussed traffic in terms of downloading an MP3 file. However, each time you visit a web site, you are creating traffic, because in order to view that web page on your computer, the web page is first downloaded to your computer (between the web site and you) which is then displayed using your browser software (Internet Explorer, Netscape, etc.) . The page itself is simply a file that creates traffic just like the MP3 file in the example above (however, a web page is usually much smaller than a music file).

A web page may be very small or large depending upon the amount of text and the number and quality of images integrated within the web page. For example, the home page for CNN.com is about 200KB (200 Kilobytes = 200,000 bytes = 1,600,000 bits). This is typically large for a web page. In comparison, Yahoo's home page is about 70KB.

How Much Bandwidth Is Enough?

It depends (don't you hate that answer). But in truth, it does. Since bandwidth is a significant determinant of hosting plan prices, you should take time to determine just how much is right for you. Almost all hosting plans have bandwidth requirements measured in months, so you need to estimate the amount of bandwidth that will be required by your site on a monthly basis


If you do not intend to provide file download capability from your site, the formula for calculating bandwidth is fairly straightforward:


Average Daily Visitors x Average Page Views x Average Page Size x 31 x Fudge Factor


If you intend to allow people to download files from your site, your bandwidth calculation should be:


[(Average Daily Visitors x Average Page Views x Average Page Size) +
(Average Daily File Downloads x Average File Size)] x 31 x Fudge Factor


Let us examine each item in the formula:


Average Daily Visitors - The number of people you expect to visit your site, on average, each day. Depending upon how you market your site, this number could be from 1 to 1,000,000.


Average Page Views - On average, the number of web pages you expect a person to view. If you have 50 web pages in your web site, an average person may only view 5 of those pages each time they visit.


Average Page Size - The average size of your web pages, in Kilobytes (KB). If you have already designed your site, you can calculate this directly.


Average Daily File Downloads - The number of downloads you expect to occur on your site. This is a function of the numbers of visitors and how many times a visitor downloads a file, on average, each day.


Average File Size - Average file size of files that are downloadable from your site. Similar to your web pages, if you already know which files can be downloaded, you can calculate this directly.


Fudge Factor - A number greater than 1. Using 1.5 would be safe, which assumes that your estimate is off by 50%. However, if you were very unsure, you could use 2 or 3 to ensure that your bandwidth requirements are more than met.


Usually, hosting plans offer bandwidth in terms of Gigabytes (GB) per month. This is why our formula takes daily averages and multiplies them by 31.


Summary:-

Most personal or small business sites will not need more than 1GB of bandwidth per month. If you have a web site that is composed of static web pages and you expect little traffic to your site on a daily basis, go with a low bandwidth plan. If you go over the amount of bandwidth allocated in your plan, your hosting company could charge you over usage fees, so if you think the traffic to your site will be significant, you may want to go through the calculations above to estimate the amount of bandwidth required in a hosting plan.


ENJOY.....^_^