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.

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