Hard-Network

CONCEPT OF A LAYER
Each layer contains a logical groupings of functions that provide specific services for facilitating a communication. A function, or a group of functions, making up a functional unit is a logical entity that accepts one or more inputs (arguments) and produces a single output (value) determined by the nature of the function. Functions can be grouped in a collective unit, which is then defined as (N) layer having (N+1) layer an upper layer boundary and (N-1) layer as a lower boundary. The N layer receives services from N-1 layer and provides services to N+1 layer.
SEVEN LAYERS OF THE OSI MODEL AND THEIR FUNCTIONS
Layer 7 is the APPLICATION layer: provides services directly to applications. Responsible for identifying and establishing the availability of the intended partner, and required resources. It is also responsible for determining if there exist sufficient communication resources to reach the remote partner.

Layer 6 is the PRESENTATION layer: Data encryption, decryption, compression and decompression are functions of this layer. It does this by using Abstract Syntax Notation 1 (ASN.1) ASN.1 standardization allows differing computer architectures to exchange data that are from differing computer architectures.
Layer 5 is the SESSION layer: facilitates a dialog between communicating systems and controls the dialog. Offers three different dialogs, simplex, half-duplex and full duplex. Session is set up by connection establishment, data transfer and connection release.
Layer 4 is the TRANSPORT layer: Segments data and also reassembles data from upper layers. Delivers data in a connection and connection less modes. Includes simplex (one way) half duplex (both ways one at a time) full duplex (both ways simultaneously). Also flow control and error recovery.
Layer 3 is the NETWORK layer: Establishes a connection between two nodes by physical and logical addressing. Includes routing and relaying data through internetworks. This layer’s primary function is to deliver packets from the source network to the destination network.
Layer 2 is the DATA LINK layer: Ensures hardware addressing of the device, and delivery to the correct device. Translates data messages from upper layers to frames, enabling hardware to transmit upper layer messages as a bit stream. Provides flow control to the layer 2. Also carries a Frame Check Sequence to make sure the frame received is identical to the one transmitted.



Logical Link Control (LLC) Sublayer of the Data Link Control layer provides flexibility to Network Layer and the Media Access Control (MAC) layer. It runs between Network Layer and the MAC sublayer of the data Link Layer.

Media Access Control (MAC) Sub Layer of the Data Link Layer is responsible for framing. It builds frames from the 1s and 0s that the Physical Layer picks up from the wire.
Layer 1 is the PHYSICAL layer: Which transmits the raw bit stream and includes electrical signaling and hardware interface.

(2) Describe connection orientated network service and connection less network service. Identify the key difference between them.
Department of Defense (DOD) model is analogous to the OSI model and is the model used in the TCP/IP protocol suite. Following are the layers of the DOD model:
DOD Model Analogous to OSI Model
Process/Application Application
Presentation
Session
Host to Host Transport
Internet Network


Network Access Data Link
Physical



At the transport layer of OSI and the Host to Host layer of DOD, there is a connection establishment process with the end system. This is a very impotent process where the sending system decides whether to use a reliable link, which is connection orientated, resource intensive or to use an unreliable link, connection less access to the end system with very much less resource utilization.
The two protocols involved in the connection establishment of the end system is Transmission Control Protocol (TCP) for reliable connection and User Datagram Protocol UDP for unreliable connection.
TCP is defined in the RFC 793 and defines a reliable, connection orientated full duplex byte stream for a user process. TCP creates a CONNECTION orientated service by contacting the end system and establishing a set of guidelines both can support. Such agreements as how much data segments can be transferred before an acknowledgement is received. TCP takes large blocks of data coming from upper layers and segments them. Then it adds numbers to the segments so the end system can sequence them at arrival and assemble the original block before sending it to the upper layer. When TCP creates a connection between two end systems, it is called a VIRTUAL CIRCUIT. This virtual circuit is created at the time the one system needs to send a data stream to the end system and takes it down when the data transfer is completed.