Introduction
The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.
Seven Layers/Standards of the OSI Model :-
Layer 1 - Physical Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
The physical layer, the lowest layer of the OSI model, is concerned
with the transmission and reception of the unstructured raw bit stream
over a physical medium. The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network.
The Physical Layer receives data from the data
link Layer, and transmits it to the wire. The physical layer
controls the electrical and mechanical functions related to the transmission
and receipt of a communications signal. It also manages the encoding and decoding
of data contained within the modulated signal.
Note that for two devices to communicate, they must beconnected to the same type of physical medium (wiring). 802.3 Ethernet to 802.3 Ethernet, FDDI to FDDI, serial to serial etc. Two end stations using different protocols can only communicate through a multi-protocol bridge or a router.
The physical layer is responsible for:
- Communication with the data link layer above it.
- Fragmentation of data into frames.
- Reassembly of frames into data link Protocol Data Units.
- Transmission to the physical media.
- Receiving from the physical media.
Protocols :-
IEEE 802
IEEE 802.2
ISO 2110
ISDN
Layer 2 - Data Link Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data
Link |
| Physical |
The data link Layer is the second layer of the OSI model. Data link layers handle data transfer between network layer
and physical layer. It receive data from network layer, adds header and trailer
to the data and passes data to physical layer . At the receiver, it receiver
data from the physical layer removes the header and trailer and passes the data to the
network layer.
The data link layer performs various functions depending upon the hardware protocol used, but has four primary functions:
- COMMUNICATION with the Network layer above.
- SEGMENTATION of upper layer datagrams (also called packets) into frames in sizes that can be handled by the communications hardware.
- BIT ORDERING. The data link layer organizes the pattern of data bits into framesbefore transmission. The frame formatting issues such as stop and start bits, bit order, parity and other functions are handled here. Management of big-endian / little-endian issues are also managed at this layer.
- COMMUNICATION with the Physical layer below
- Handles data frames between the Network and Physical layers.
- Frame traffic control: tells the transmitting node to
"back-off" when no frame buffers are available.
- Frame sequencing: transmits/receives frames sequentially.
- Frame acknowledgment: provides/expects frame acknowledgments.
Detects and recovers from errors that occur in the physical
layer by retransmitting non-acknowledged frames and handling
duplicate frame receipt.
This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, physical link management, error notification, ordered delivery of frames, and flow control.
Protocols :
a) Logical Link Control (LLC)
i. Error correction and flow control
ii. Manages link control
b) Media Access Control (MAC)
i. Communicates with the adapter card.
ii. Controls the type of media being used.
NOTE: The data link layer
is responsible for moving frames from one hop (node) to the next.
Layer 3 - Network Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
Network layer is the 3rd layer of the OSI model. The
data unit at this layer is known as packet. The network layer is not needed if
the two communicating lie in same network. However, when the two devices are on
the different network, network
layer is essential for the source to
destination delivery of packets.
This layer 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, inter-networking, error handling, congestion control and packet sequencing.
IP (Internet Protocol) 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
Note : The network layer is
responsible for the delivery of individual packets from the source host
to the destination host.
Layer 4 - Transport Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
Transport layer is the 4th layer of the OSI model. The data unit at this layer is known as Segment. Transport layer offers end-to-end communication between devices through a network. depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications.
Two transport layer protocols:
1. Transmission Control Protocol (TCP),
2. User
Datagram Protocol (UDP),
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.
Functions :
- Communicate with the Session layer above.
- Reassemble transport Protocol Data Units into data streams
- Reliable protocols operating at this layer will
- Detect errors and lost data
- Recover lost data
- Manage retransmission of data.
- Segmentation of data streams into transport Protocol Data Units.
- Communicate with the Network layer below.
Protocols :
TCP
ARP
RARP
SPX
NWLink
NetBIOS / NetBEUI
ATP
UDP
DCCP
Note : The transport layer
is responsible for the delivery of a message from one process to
another.
Layer 5 - Session Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
Session layer is the 5th layer of the OSI model. The session layer allows session establishment between processes
running on different stations. 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.
The session layer tracks connections, also called sessions. The session layer should keep track of multiple file downloads requested by a particiular FTP application, or multiple telnet connections from a single terminal client, or web page retrievals from a web server.
With TCP/IP this functionality is handled by application software addressing a connection to a remote machine and using a different local port number for each connection.
Functions :
- Session establishment, maintenance and termination: allows
two application processes on different machines to establish,
use and terminate a connection, called a session.
- Session support: performs the functions that allow these
processes to communicate over the network, performing security,
name recognition, logging, and so on.
Protocols :
NetBIOS
Names Pipes
Mail Slots
RPC
SAP
L2TP
PPTP
SPDY
Layer 6 - Presentation Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
Presentation layer is the 6th layer of the OSI model. The presentation layer handles the conversion of data between a Standards-based
or platform independant formats to a format understood by the local machine.
This allows for data to be transported between devices and still be understood.
The presentation layer provides:
- Character code translation: for example, ASCII to EBCDIC.
- Data conversion: bit order, CR-CR/LF, integer-floating point,
and so on.
- Data compression: reduces the number of bits that need to be
transmitted on the network.
- Data encryption: encrypt data for security purposes. For example,
password encryption.
Protocols :
MIME
XDR
TLS
SSL
Layer 7 - Application Layer :
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.
Examples of applications that utilize the network are:
- Telnet
- FTP
- Instant Message software (AIM, MSN, ICQ, Yahoo)
- Microsoft Windows File Shares
- Web Browsers (Internet Explorer, Firefox, Google Chrome, Safari)
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
OSI Model and Protocol
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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