2.1 Layers of OSI:
The Seven layers of OSI
This layer provides a means for the user to
access information on the network through an application. Many
user applications that need to communicate over the network interact with the
Application layer protocol directly. The user applications are not
part of OSI Application layer, use the networking services offered by the
networking protocol suite. Application layer functions typically include
identifying communication partners, and determining availability of required
resources. Some examples of application layer implementations include
Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
Presentation layer converts local host computer data representations into a
standard network format for transmission on the network. On the receiving side,
it changes the network format into the appropriate host computer's format so
that data can be utilized independent of the host computer. ASCII and EBCDIC
conversions, cryptography, and the like are handled here.
Presentation layer coding and conversion schemes include common data
representation formats, conversion of character representation formats, common
data compression schemes, and common data encryption schemes.
layer implementations are not typically associated with a particular protocol
stack. Some well-known standards for video include QuickTime and Motion Picture
Experts Group (MPEG). QuickTime is an Apple Computer specification for video and
audio, and MPEG is a standard for video compression and coding.
The session layer establishes, manages, and terminates
communication sessions. Communication sessions consist of service requests and
service responses that occur between applications located in different network
devices. These requests and responses are coordinated by protocols implemented
at the session layer. Some examples of session-layer implementations include
AppleTalk's Zone Information Protocol (ZIP), and Decent Phase Session Control
Transport layer is responsible for providing
reliable service between the hosts. Upper layer datagrams are broken down into
manageable datagrams and then appropriate header information (such as sequence
number, port number, etc.) is added to the datagram before passing it on to the
Network layer. Two frequently used transport protocols are the TCP (Transmission
Control Protocol) and the UDP (User Datagram Protocol).
Important features of Transport layer:
Transport layer ensures
Breaks the message (from
sessions layer) into smaller datagrams, and appends appropriate unit header
Responsible for communicating with the Session layer
Important features of
TCP/IP widely used protocol
for Transport/Network layers
TCP: (Transport Control
Protocol) TCP ensures that a packet has reached its intended
destination by using an acknowledgement. If not, it retransmits the lost
messages. Hence, TCP is called a connection oriented protocol.
UDP (Universal Data gram
Protocol): UDP simply transmits packets over the internet. It does not wait
for an acknowledgement. It is the responsibility of upper layer protocols to
ensure that the information had reached the intended partner(s). Hence, UDP
is often called connectionless protocol.
Application programs that do
not need connection-oriented protocol generally use UDP.
Network layer is responsible for the routing
of packets through the entire network. The layer uses logical addressing for
this purpose. Note that the physical address (like MAC address) keeps changing
from hop to hop when a packet travels from source to destination. As a result,
an address that doesn't change is required to ensure continuity between hops.
This is nothing but logical address. For IP networks, IP address is the logical
address; and for Novell network, IPX address is the logical address, and so on.
This layer also provides for congestion control, and accounting information for
the network. IP (Internet Protocol) is an example of a network layer protocol.
Data link layer provides delivery of information frames between communicating
partners. This layer is responsible for flow regulation, error detection and
correction, and framing of bits for transmission. The network data frame is made
up of checksum, source address, destination address, and the data itself. The
largest frame size that can be sent is known as the maximum transmission Unit (MTU).
features of Data link layer:
Assembles bits into frames,
making them ready for transmission over the network.
Provides error detection, and correction to transmitted frames. If
the checksum is not correct, it asks for retransmission. (Send a control
Consists of two sub layers:
Logical Link Control
(LLC): Defines how data is transferred over
the cable and provides data link service to the higher layers.
Medium Access Control (MAC): Controls media access by regulating
the communicating nodes using pre-defined set of rules. (i.e. Token
passing, Ethernet [CSMA/CD] all have MAC sub-layer protocol).
Different Data link layer protocols define different network and protocol
characteristics, including physical addressing, network topology, error
notification, sequencing of frames, and flow control. Physical addressing (as
opposed to logical addressing) defines how devices are addressed at the data
link layer. The protocols used in Data link layer are SLIP, PPP, and CSLP.
This is the bottom-most layer of the OSI model. The Physical layer handles the bit-level communications
across the physical medium. The physical medium could be made up of wired
electrical signals, or light, or radio (wireless) signals. Physical layer specifications define
characteristics such as media, data rates, maximum transmission distances, and physical connectors.
Frequently used Physical layer protocols:
of the important standards that deal with physical layer specifications are:
serial communication lines), X.21, EIA 232, and G730.
layer and Data link layer implementations can be categorized as either LAN or