A network is a collection of computers and devices connected to each other. The network allows computers to communicate with each other and share resources and information. The Advance Research Projects Agency (ARPA) designed "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of Defence. It was the first computer network in the world in late 1960's and early1970's.
Why a Network?
Distribute pieces of computation among computers (nodes)
Remote Data/File Access , Remote I/O Devices
Personal communications (like e-mail, chat, audio/video conferencing)
Based on their scale, networks can be classified as Local Area Network (LAN) , Wide Area Network (WAN) , Metropolitan Area Network (MAN) , Personal Area Network (PAN) , Virtual Private Network (VPN) , Campus Area Network (CAN) , Storage Area Network (SAN) , etc.
Connection Method such as Optical Fiber, Ethernet, Wireless LAN etc.
Network Architecture such as Client-Server or Peer to Peer
Every layer needs a mechanism for identifying senders and receivers
We need to identify with whom a machine wants to talk, i.e. some form of addressing mechanism is required.
Rules for Data transfer need to be established. In some cases data only travels one direction, in others both ways. Also need to find out how many logical channels?
Error Control :- Various error-detecting and error correcting codes are there that have to be used, also at times, the sequence in which data is sent is not the way in which it is received, hence reassembly also needs to be done.
Inability to receive long messages, we disassemble, transmit and then reassemble data
At times sending data packets separately is not efficient, in that case we use multiplexing and demultiplexing.
Connection-oriented service : Just like a telephone system, we establish a connection, use and then release the connection.
Firstly, negotiations take place about the parameters used, such as maximum message size, quality of service required and other issues.
Connectionless Service : Just like the postal system
A reliable service is ensured by having the receiver acknowledge the receipt of each message so that sender is sure that it arrived. This introduces overheads and delays.
Example of connection oriented and connection less service
Remote login- connection oriented
Request Reply – Database Query etc
Computer Networks Service Primitives A service is formally specified by a set of primitives ( operations) available to a user process to access the service. Send a message to the peer SEND Terminate a Connection DISCONNECT Block waiting for an incoming message RECEIVE Establish a connection with a waiting peer CONNECT Block waiting for an incoming connection LISTEN Meaning Primitive
Computer Networks Client Machine Server Machine 1)Connection Request 2)ACK 3)Request for Data 4)Reply 5)Disconnect 6)Disconnect How communication takes place?
It is the spot where the User communicates with the Computer.
Example of IE.
It is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist.
Application Layer acts as an interface between the actual application programs. This means that Microsoft Word, for e.g. does not reside at the Application Layer but instead interfaces with the Application Layer Protocols.
What happens when a machine receives a flood of datagrams too quickly to process?
It stores them in a memory section called buffer, if the buffer even gets full, transport layer sends a “Not Ready” indicator to the sender.
A service is considered to be connection-oriented if it has following characteristics –
A virtual circuit is set up.
It uses sequencing.
It uses acknowledgements.
It uses flow control.
The Transport Layer ( Cont…)
Computer Networks Windowing: The quantity of data segments (in bytes) that the transmitting machine is allowed to send without receiving an acknowledgement for them is called window. Acknowledgments: Non-duplication of data is achieved through positive acknowledgement with re transmission. The sender documents each segment it sends and waits for this acknowledgement before sending the next segment. It starts a timer and retransmits if it expires before an acknowledgement is returned from receiving end. The Transport Layer ( Cont…)
It manages device addressing, tracks the location of devices on the network and determines the best way to move data, which means that the NL must support transport traffic between devices that aren’t locally attached.
Routers are specified at the NL and provide the routing services
How does it work?
Two type of packets are used :
Data Packets (Routed Packets) & Route Update Packets.
The Routing Table consist of the following information :
The DLL provides Physical Transmission of the Data and handles error notification, network topology and flow control. That means that DLL will make sure that the messages are delivered to proper device on a LAN using hardware address and will translate messages from the Network Layer into bits for the Physical Layer to transmit.
Data is in the form of Frames here.DLL adds a custom header containing the Hardware Destination and Source Address.