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Chapter: Computer Networks : Physical Layer

Layers in the OSI Model

1. Physical Layer 2. Data Link Layer 3. Network Layer: 4. Transport layer: 5. Session Layer: 6. Presentation Layer: 7. Application Layer:

Layers in the OSI Model:


The functions of each layer in the OSI model are


1. Physical Layer


The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium.


The physical layer is responsible for movements of individual bits from one hop (node) to the next.

The physical layer is also concerned with the following:

a. Physical characteristics of interfaces and medium. The physical layer defines thecharacteristics of the interface between the devices and the transmission medium.


b. Representation of bits. The physical layer data consists of a stream of bits (sequence ofOs or 1s) with no interpretation.


c. Data rate. The transmission rate-the number of bits sent each second-is also defined bythe physical layer.


d. Synchronization of bits. The sender and receiver not only must use the same bit rate butalso must be synchronized at the bit level.


e. Line configuration. The physical layer is concerned with the connection of devices tothe media; it may be point-to-point configuration or multi-point.


f. Physical topology. The physical topology defines how devices are connected to make anetwork. Ex. Mesh, Star, Ring, Bus, or a hybrid topology.


g. Transmission mode. The physical layer also defines the direction of transmissionbetween two devices: simplex, half-duplex, or full-duplex.


2. Data Link Layer


The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. The data link layer is responsible for moving frames from one hop (node) to the next.


Other responsibilities of the data link layer include the following:

1. Framing. The data link layer divides the stream of bits received from the network layerinto manageable data units called frames.


2. Physical addressing. If frames are to be distributed to different systems on the network,the data link layer adds a header to the frame to define the sender and/or receiver of the frame.


3. Flow control. If the rate at which the data are absorbed by the receiver is less than therate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.


4. Error control. The data link layer adds reliability to the physical layer by addingmechanisms to detect and retransmit damaged or lost frames.


5. Access control. When two or more devices are connected to the same link, data linklayer protocols are necessary to determine which device has control over the link at any given time.


3. Network Layer:


The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Figure 1.18 shows the relationship of the network layer to the data link and transport layers.

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Other responsibilities of the network layer include the following:


1. Logical addressing. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.


2. Routing. When independent networks or links are connected to create intemetworks or a large network, the connecting devices route or switch the packets to their final destination.


4. Transport layer:


The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. The transport layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Figure 1.19 shows the relationship of the transport layer to the network and session layers.


The transport layer is responsible for the delivery of a message from one process to another.

Other responsibilities of the transport layer include the following:


a. Service-point addressing. Source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address).

b. Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

c. Connection control. The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

d. Flow control. The transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

e. Error control. The transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link.



5. Session Layer:


The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.


The session layer is responsible for dialog control and synchronization.



Specific responsibilities of the session layer include the following:

a. Dialog control. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half duplex or full-duplex.


b. Synchronization. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.


6. Presentation Layer:


The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.

The presentation layer is responsible for translation, compression, and encryption.

Specific responsibilities of the presentation layer include the following:

a. Translation. The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted.


c. Encryption. A system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

d. Compression. Data compression reduces the number of bits contained in the information. It is very important in the transmission of multimedia such as text, audio, and video.


7. Application Layer:


The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.


The application layer is responsible for providing services to the user.

Specific services provided by the application layer include the following:


a. Network virtual terminal. It is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host.


b. File transfer, access, and management. This application allows a user to access files in a remote host, to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.


c. Mail services. This application provides the basis for e-mail forwarding and storage.


d. Directory services. This application provides distributed database sources and access for global information about various objects and services.


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