Session layer 5 this layer establishes manages and

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Session (Layer 5) 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. It deals with session and connection coordination. Transport (Layer 4) This layer provides transparent transfer of data between end systems, or hosts , and is responsible for end-to-end error recovery and flow control . It ensures complete data transfer. Network (Layer 3) 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 , internetworking , error handling, congestion control and packet sequencing. Data Link (Layer 2) At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control ( MAC ) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization , flow control and error checking. Physical (Layer 1) This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet , RS232 , and ATM are protocols with physical layer components. Analog Network Signaling An analog signal is best compared to a wave. It has similar properties to an ocean wave, and can be described using three specific characteristics: amplitude, frequency, and wavelength. To use the ocean wave analogy an analog signal's amplitude is like the height of a wave rolling in onto the beach. The frequency of an analog signal can be compared to how fast the waves roll in. Wavelength can be compared to the distance between one wave and the next wave. Wavelength is measured as the distance between the peak of one wave and the next.
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Advantages and Disadvantages of Analog Signals Analog signals are variable and can convey more subtly than a digital signal. For example the human voice is analog, and has more tone than a digital representation of the same voice. However, analog signals are very vulnerable to interference from outside forces and other waves which can cancel them out. Digital Network Signaling A digital signal is made up of on/off states. Unlike the smooth curve of an analog wave, the digital signal cuts on and off, like morse code. This happens to perfectly fit the type of communication inside a computer, which is made up of on/off states as well.
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