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IT Interview Questions:Defined OSI model ?
Information Technology (IT) Interview Questions and Answers
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IT Interview Questions:Defined OSI model ?
The 'Open Systems Interconnection Basic Reference Model'
(OSI Reference Model or OSI Model
) is an abstract description for layered communications and
computer network protocol design. It was developed as part of the Open
Systems Interconnection (OSI) initiative. In its most basic form, it
divides network architecture into seven layers which, from top to bottom, are
the Application, Presentation, Session, Transport, Network, Data-Link, and
Physical Layers. It is therefore often referred to as the
OSI Seven Layer Model.
1 The Physical Layer
defines the electrical and physical specifications for
devices. In particular, it defines the relationship between a device and a
physical medium. This includes the layout of pins, voltages, cable
specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs
used in Storage Area Networks) and more.
To understand the function of the Physical Layer in contrast to the functions
of the Data Link Layer, think of the Physical Layer as concerned primarily
with the interaction of a single device with a medium, where the Data Link
Layer is concerned more with the interactions of multiple devices (i.e., at
least two) with a shared medium. The Physical Layer will tell one device how
to transmit to the medium, and another device how to receive from it (in most
cases it does not tell the device how to connect to the medium). Obsolescent
Physical Layer standards such as RS-232 do use physical wires to control
access to the medium.
2
Data Link Layer
The Data Link Layer provides the functional and procedural means to transfer
data between etwork entities and to detect and possibly correct errors that
may occur in the Physical Layer. Originally, this layer was intended for
point-to-point and point-to-multipoint media, characteristic of wide area
media in the telephone system. Local area network architecture, which
included broadcast-capable multiaccess media, was developed independently of
the ISO work, in IEEE Project 802. IEEE work assumed sublayering and
management functions not required for WAN use. In modern practice, only error
detection, not flow control using sliding window, is present in modern data
link protocols such as Point-to-Point Protocol (PPP), and, on local area
networks, the IEEE 802.2 LLC layer is not used for most protocols on
Ethernet, and, on other local area networks, its flow control and
acknowledgment mechanisms are rarely used. Sliding window flow control and
acknowledgment is used at the Transport Layer by protocols such as TCP, but
is still used in niches where X.25 offers performance advantages.
Both WAN and LAN services arrange bits, from the Physical Layer, into logical
sequences called frames. Not all Physical Layer bits necessarily go into
frames, as some of these bits are purely intended for Physical Layer
functions. For example, every fifth bit of the FDDI bit stream is not used by
the Data Link Layer.
3 The Network Layer provides the functional and procedural means of
transferring variable length data sequences from a source to a destination
via one or more networks, while maintaining the quality of service requested
by the Transport Layer. The Network Layer performs network routing functions,
and might also perform fragmentation and reassembly, and report delivery
errors. Routers operate at this layer sending data throughout the extended
network and making the Internet possible. This is a logical addressing scheme
– values are chosen by the network engineer. The addressing scheme is
hierarchical.The best-known example of a Layer 3 protocol is the Internet
Protocol (IP). It manages the connectionless transfer of data one hop at a
time, from end system to ingress router, router to router, and from egress
router to destination end system. It is not responsible for reliable delivery
to a next hop, but only for the detection of errored packets so they may be
discarded. When the medium of the next hop cannot accept a packet in its
current length, IP is responsible for fragmenting into sufficiently small
packets that the medium can accept it.A number of layer management protocols,
a function defined in the Management Annex, ISO 7498/4, belong to the Network
Layer. These include routing protocols, multicast group management, Network
Layer information and error, and Network Layer address assignment. It is the
function of the payload that makes these belong to the Network Layer, not the
protocol that carries them.
4 Transport Layer
The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
5. The Session Layer
controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for fullduplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls (RPCs).
Layer 6: Presentation Layer
The Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack. The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities
7 Application Layer
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
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