Huawei D100 3G Wi-Fi
Huawei D100 Router transforms 3G dongle into WiFi network
Application layer
Application layer
The application layer enables the user, whether human or software, to access the network. Application layer provides user interfaces and support for services such as electronic mail. Remote file access and transfer, share database management and other types of distributed information services. Specific services provided by the application layer include the following:
Ø Network virtual terminal. A network virtual terminal is a software version of physical terminal and allow user to log on to a remote host. To do the application creates a software emulation of a terminal at the remote host. The user's computer talk to the software terminal, which in turn talks to the host, vice versa. The remote host believes it is communicating with one of its own terminals and allows you to log on.
Ø File transfer, access and management (FTAM). This application allows a user to access files in remote host (to make change or read data), to retrieve files form a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
Ø E-mail services. This application provides the basis for e-mail forwarding and storage.
Ø Directory services. The application provides distributed database sources and access for global information about various objects and services.
Presentation layer
Presentation layer
The presentation layer is concerned with the syntax and semantics of the information exchange between two systems. Specific responsibilities of the presentation layer include the following:
Ø Translation. The process(running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information should be changed to bit streams before being transmitted. Because different computers use different encoding system, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent into a common format. The presentation layer at the receiving machine changes the common format into its receiver. Dependent format.
Ø Encryption. To carry sensitive information a system must be able to assure privacy. Encryption means that the sender transformer 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.
Ø Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio and video.
Session layer
Session layer
The services provided by the first four layers (physical, data link, network and transport) are not sufficient for some processes. The session layer is the network dialogue controller. It establishes, and synchronizes the interaction between communicating system. Specific responsibilities of the session layer include the following:
Ø Dialogue control. The session layer allows two system to enter into a dialogue. It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two way at a time) mode.
Ø Synchronization. The session layer allows a process to add check points (synchronization points) into a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert check point after every 100 pages to ensure that each 100-page unit is received and acknowledged independently In this case, if a crash happens during the transmission of page 515, the only pages that need be resent after system recovery are pages 475 to. Pages previous to 475 need not be resent.
Transport layer
Transport layer
The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on the host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belong to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Other responsibilities of the transport layer include the following:
Ø Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from 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). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.
Ø Segmentation and reassembling. A message is divided into transmittable segments, with each segment containing 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.
Ø 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 computer. 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 is transferred, the connection is terminated.
Ø Flow control. Like the data link layer, 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.
Ø Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than to across a single link. The sending transport layer make sure that the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is usually achieved through transmission.
Network layer
Network Layer
The network layer is responsible for the source-to destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two system on the same network (link), the network layer ensures that each packet gets from its point if origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached two different networks (link) with connecting devices between the networks (link), there is often a need for the network layer to accomplish source-to-destination delivery. Other responsibilities of the network layer include the following:
Ø Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system distinguish the source and destination system. The network layer adds a header to the packet coming from the upper layer that, among other thing, includes the logical addresses of the sender and receiver.
Ø Routing. When independent networks or links are connected together to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the function of the network layer is to provides this mechanism.
Data link layer
Data link layer
The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer(network layer). Other responsibilities of the data link layer include the following:
Ø Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frame.
Ø Physical addressing. If frames are to be distributed to different system on the network, the data link layers add a header to the frame to define the sender and /or receiver of the frame. If the frame is intended for a system outside the sender’s network, the receiver address is the address of the connecting device that connects the network to the next one.
Ø Flow control. If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.
Ø Error control. The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize a duplicate frame. Error control is normally achieved through a trailer added to the end of the frame.
Ø Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.
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