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Networking layers


Networking layers

Application Layer
1. Closest’ layer to the user
2. Works with the applications one uses to communicate over the network. include SMTP, HTTP and FTP etc
3. Clicking on a link on a web page issues a command for the browser to retrieve the relevant information from the Internet
4. Here our computer is the source, and the host of the web site information is the destination
5. The application completes your request and delivers the information to your computer
6. The Application layer contains a variety of protocols that are commonly required. Like 
FTP,HTTP etc
7. Another Application layer function is file transfer which is responsible for transferring a file between two different systems 
8. Requires handling and other incompatibilities. 
9. File Services, Electronic-mail Services, Network-printing Services, Application Services, Database Services are main application layer services widely used. 

The Presentation Layer 
1. Presentation Layer is layer where data conversion (ASCII to EBCDIC, floating point to binary, etc.) and encryption/decryption are performed. Mainly three works done by this layer-Presentation, compression and encryption
2. The Presentation layer basically takes the packets and re-assembles them so you can open the e-mail or the attachment 
3. If any packets got lost along the way, or were damaged, then the Presentation layer will send a sign to the sender that it requires the specific packet 
4. This layer has three fundamental functions 
a) Data Presentation 
b) Enables receiving device to understand the information sent from the source 
c) Converts data from native format to a common format e.g. ASCII to EBCEDIC. 
5. Data Compression 
a) By reducing the volume of data, transfers can take place in less time 
b) Packets are examined and such things as spaces in text removed 
c) The destination device returns the data to its original format before passing to the Application Layer 
6. Data Encryption 
a) Allows data to be converted to a form which hides its meaning, apart from those you wish to see it 
b) Not all data is encrypted on its journey across the network 
c) In order for decryption to occur at the destination device a ‘key’ is required 

Session Layer 
1. Primarily responsible for handling the session between devices (beginning, maintaining and finishing) 
2. Enforces order in the communication between devices
3. Regulates the flow of data 
4. The following services are provided: 
a) Establishing a Connection 
b) Maintaining the Session 
c) Ending the Connection 
d) Dialogue Control(simplex, half-duplex, full-duplex)
e) Dialogue Separation- checkpoints within the transmission which allow the 
detection of lost packets, and subsequent re-transmission 
f) Handshaking – SYN and ACK packets 
g) ‘Keep alive messages’ 
h) Session must be terminated (otherwise one device will be still transmitting 
without any device actually listening)

Transport Layer (TCP and UDP) 
 TCP establishes a connection at both ends, creating a "virtual connection" between the two machines before any data can be transmitted. Once established, both sides negotiate the maximum size of a TCP packet. Although TCP supports packets up to 64KB, in most cases, the size will be based on the underlying network, such as Ethernet, which can hold a maximum of 1518 bytes. Token Ring and FDDI support larger frames. TCP attaches a header onto the packet that contains the source and destination ports as well as the sequence number of the packet, and it hands it over to IP along with
the destination IP address. (A TCP packet is technically a Protocol Data Unit or segment, but is more 
often called a packet in common parlance.) . Following are some main features which Ensures reliable 
transport of packets from source to destination 

1. Transport layer manages the speed of transmission – flow control 
2. There are two types of transmission (Connection-Oriented Transmissions and Connectionless Transmissions) which takes place at this layer. 
3. The transport layer “DIRECTS PACKETS”, splits it into smaller units if need be, pass these to the network and ensure that the pieces are travelling in an orderly fashion. 
4. A series of protocols are also established in this layer to ensure proper flow of the packets. 
5. You can basically describe the Transport Layer as a “TRAFFIC COP”. 
6. Transport layer also controls the flow of data to Establishes the maximum speed at which both sender and receiver can communicate .It determines largest packet size which can be sent on the network and numbers the packet so that reassembly can be done in correct order. 
 
    Two main type of transmission takes place and they are known as connection oriented and 
connectionless transmission. 

Connection-Oriented Transmissions
1. Also known as ‘ Reliable Transport Method’ – uses acknowledgement (ack) packets on successful receipt of data 
2. Extra packets slows down communication 
3. Features are 
    a. Reliability 
    b. Slower Communication 
    c. Packets are re-transmitted if unrecognisable or not received 
4. Once all the data is received successfully , the packet is re-assembled and the 
Transport Layer passes it to the Session Layer. 
Connectionless Transmissions 
1. In this mode the transmitting device does not require acknowledgements from the 
receiver, and continues to transmit on the assumption that the data was received 
2. Features are: 
    a. Little or No Reliability 
    b. Faster Transmission 
    c. Packets are not Re-transmitted

Network Layer 
The Network Layer does the following: 
1. Adds the address to the packet (encapsulation) 
2. Maps the network address to the devices physical address 
3. Determines the best path for the packet (routing) 
4. Ensures that the packet is in the correct format for the destination 
5. The network layer is concerned with controlling the operation of the subnet. A ROUTER is used to determining how packets are routed from source to destination. 
6. If one path is busy, then the router will select another path for the packets to travel. So really, the packets can all have different paths and find their way to the final destination. 
7. The router has millions of IP addressing built into the software, and knows where to send the packets. 
8. IP stands for Internet Protocol and is basically an address where the packets will be sent to. 





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