Transport layer of the OSI model - Definition
Transport layer
This is the fourth layer of the OSI model. The data unit at this layer is known as segment. The network layer deals with source-to-destination delivery of individual packets and does not recognize any relationship between them. On the other hand, the transport layer ensures that the entries message reaches in order and handles error control and flow control at the source-to destination level.
Transport layer receives data from the session layer .it divides the data into segments with each segment having its own header and passes this segment to the network layer. At the receiver side, transport layer receives packets from the network layer. It detaches headers from them and then reassemblies the segments into original message and passes that massage to the session later
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Responsibilities of transport layer are:
☼Service-point addressing -there may be number of programs running at the same time on a computer. For this purpose, source-to-destination delivery should also include process-to-process delivery. Data must be transferred from a specific process (running program) on one computer to a specific process on another computer and to accomplish this, port address is required. Thus, transport layer includes service-point address (also referred to as port address) in the header. Using these port addresses transport layer delivers the packets to the correct process.
☼Segmentation and reassembly – a massage is divided into segments with each segment having a sequence number. When the message arrives at the destination, with the help of these sequence numbers, the transport layer reassembles the message correctly. Sequence numbers also enable transport layer to determine and replace the segments that were lost in the transmission.
☼Connection control –transport layer provides either connection-oriented or connectionless service. In connection-oriented service, transport layer establishes a connection with the transport layer at the destination before transmitting segments and terminates the connection when all the data is transferred. in connectionless service, transport layer treats each segment as individual packet and transmits it to the transport layer treats each segment as individual packet and transmits it to the transport layer at destination.
☼Error control – transport layer also performs error checking. Error control takes place end to end. Transport layer at the sending machine ensures that the entire message is delivered at the receiving machine without an error. Segments are retransmitted to achieve error correction.
Network layer of the OSI model - Definition
Network Layer
This is the third layer of the OSI model. The data unit at this layer is known as packet. There is no need of the network layer if the two communication devices lie on the same network. However, when the two devices are connected on different networks, network layer is essential for providing source to destination delivery of packets.
Network layer receives data from transport layer. It adds header to the passes data to the data link layer. At the receiver said, network layer receives data from data link layer. It detaches header from the data and passes data to the transport layer.
Responsibilities of the network layer are:
☼Logical Addressing - the data link layer provides physical addressing which is useful for a local network. When the packet is destined for a device outside the network, we require other addressing scheme to identify source and destination network layer adds header to the data that includes the logical address (IP address) of the source and destination. It is a 32-bit address that uniquely identifies the device connected to the network
☼Routing - routing is a process wherein a proper path is defined for the packets to reach the destination. Routing can be of two types, static or dynamic. In static routing, the route to be followed by a packet is already set by the network administrator. In dynamic routing, the rout to be followed by a packet is decided at the time of transmission of the packet. The rout can be changed based on the available traffic on the network.
☼Internetworking – Internetworking means connecting two or more computer networks together. The internet is the best example is the best example of internetworking. There are different types of networks that exist in the real world such as LAN, MAN and WAN. These networks are interconnected using various networking devices such as bridges, routers and gateways
Data link layer of the OSI model : definition
Data link layer:
Data link layer is the second layer of the OSI model. It handles data transfer between network and physical layers. Data unit at the data link layer is called as frame. Data link layer receives data from network layer. It adds header and trailer to the data and passes data to the physical layer. At the receiver said, data link layer receives data from the physical layer. It detaches header and trailer from the data and passes data to the network layer.
Responsibilities of the data link layer are:
☼Framing – the physical layer delivers raw bits from the source to destination. During transmission, the value of the bits can change. It is also possible that the number of bits received by the receiver may be different from the number of bits sent by the sender. To resolve this problem, the data link layer organizes the bits into manageable data units called as frames.
☼Physical addressing – Data links layer adds header to the frame which contains the physical address of the sends and/or receiver. Headers are assigned when the frames are to be distributed to various systems within the network.
☼Error control – Another function of the data link layer is error control. Error control detects and corrects errors. During transmission, if a frame is lost or corrupted, the data link layer retransmits that frames. It also prevents duplication of frames. Error control is accomplished using trailer at the end of the frame.
Physical Layer of the OSI Model: Definition
Physical layer is the first or the bottom most layer of the OSI model. This layer is used to establish or terminate a connection to a communication medium. It also defines the electrical and mechanical specifications like cables, connectors and signaling options of the medium
Physical layer receives data from the upper layer called the data link layer. It converts the received data into bit system. The data is then transmitted through the medium to the receiver. At the receiving end, physical layer receives the data in bit format. It forwards the data to the data link layer.
Responsibilities of the physical layer:
Characteristics of media - defines the characteristics of the interface which used for connecting the devices. It also define the type of the transmission media such as copper wire or fiber optic cable
Encoding – define the encoding type .encoding means changing bit stream (0s and 3s) into signal. Before transmission, physical layer encodes the signal into electrical or optical from depending upon the media
Transmission Rate – defines the transmission rate of bits. This provides number of bits transmitted per second. It defines how long will the duration of a bit be.
Transmission Mode – defines the transmission mode between two devices transmission mode specifies the direction of signal flow. The different types of transmission modes are:
☼ Simplex – communication is done only in direction. One device can only send and the other can only receives.
☼ Half duplex – communications is done in both the directions but not at the same time
☼ Full duplex - communications is done simultaneously in both the directions at the same time
Topology – defines how the devices are connected to from a network
Computer Network | Layers of Open System Interconnection (OSI) Model
OSI model consists of seven layers and provides standards for data communication.
1. Physical layer : physical layer is responsible for actual transmission of bits over the medium. It deals with the electrical and mechanical functio
2. Data link layer : data link layer is responsible for node to node delivery of frames.
3. Network layer : Network layer is responsible for end to end delivery of packets and routing.
4. Transport layer : transport layers is responsible for end to end delivery of the entire message.
5. Session layer : Session layer is responsible for establishing,managing and terminating sessions.
6. Presentation layer : is responsible for translation, encryption and data compression.
7. Application layer : application layer is responsible for providing access to the network resources
Working with E-mail clients?
E-mailclient is a program used to manage, send and receive e-mails ,organize meetings, arrange appointments and keep a contact list.
Microsoft outlook is a software application for handling and organizing personal information such as e-mail messages,calendar,schedules,tasks,notes,contacts.
Configure e-mail setting include security options,server settings and connection
Microsoft outlook enables the user to create five types of e-mail accounts such as POP3,IMAP,HTTP, Microsoft exchange server and third party mail server based e-mail accounts
Windows mail is an e-mail and newsgroup client included in windows vista which is the successor to outlook express
user has to take the backup of e-mail before formatting a PC
In windows7 you”ll find outlook stores the PST and OST,in
c:\users\<Username>\AppData\local\Microsoft\Outlook
in vista you'll find windows mail files in
c:\Users\<username>\AppData\Local\Microsoft\Windows Mail.
The user may encounter various problems while working with e-mail accounts.
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