Basics and Get Placement Ready

Computer Network: Connection of different nodes with each other.

Node:  Node is device, it may be DTE or DCE.

Data Terminal Equipment (DTE):  DTE is an end-terminal device or a user device like a desktop computer, laptop, smart phone, or tablet.

Data Communication Equipment (DCE):  DCE can communicate data from one node to another node. DCE nodes are hubs, switches, routers, etc.

Components of Computer Network:  There are five components: Sender, Receiver, Transmission Medium, Message, Protocol.

Topology:  The arrangement or setup of the various elements, like links and nodes. 

* Links mean transmission medium.

Topologies are bus, star, ring, mesh, tree, and hybrid. 

• Bus: Bus topology is an older topology and now obsolete, it was used with T-shaped connectors to connect the various nodes with a backbone cable.
• Star: Star topology is being used nowadays in local area networks (LANs) to connect end terminal nodes (desktop computers) with the central device, such as a hub or switch, with the help of ethernet cables.
• Ring: Ring topology is being used in local area networks (LANs) and wide area networks (WANs) to connect multiple nodes with each other, like routers in the ring shape.
• Mesh: Mesh topology is being used in wide area networks (WANs) to connect multiple nodes with each other, like routers. All the nodes are connected to each other with a direct connection as well as indirect connections.
• Tree: Tree topology is being used in local area networks (LANs) and wide area networks (WANs) to connect multiple nodes, like routers, switches, desktop computers, etc. For example, one root node is connected to two nodes, and then two nodes are connected to the next nodes in the branch formation and so on.
• Hybrid: Hybrid topology is being used in local area networks (LANs) and wide area networks (WANs) to connect multiple nodes, like routers, switches, desktop computers, etc. It is a combination or interconnection of two or more topologies with each other, and the whole network design is known as hybrid.

Data Flow:  Flow of data in the form of data packets in between the nodes.

Transmission Mode:  How data flows, like in one direction or in both directions.

3 Transmission Modes:  

• Simplex: Data flows in one direction only like from CPU to monitor.
• Half Duplex:  Data flows in both directions, but not at the same time, like Walkie-talkie.
• Full Duplex:  Data flows in both directions at the same time, like the internet.

Transmission Medium:  The means of communication by which data moves between the nodes.
There are two types of transmission mediums:

• Guided (wired): By the help of cables
• Unguided (wireless): Without cables

OSI Model:  OSI stands for Open Systems Interconnection. It is how communication takes place between nodes.

OSI Model Layers: OSI Model has 7 layers. 

• Physical Layer: The physical layer transmits data in the form of bits from one node to another node.
• Data Link Layer: The data link layer transmits data in the form of frames from one node to another node.
• Network Layer: The network layer transmits data in the form of packets from one node to another node. 
• Transport Layer: The transport layer transmits data from one process to another.
• Session Layer: The session layer transmits data from one process to another in different transmission modes, like simplex, half duplex, or full duplex.
• Presentation Layer: The presentation layer provides security to the data.
• Application Layer: The application layer provides services to the user.

TCP/IP Model: Transceiving of data takes place with respect to the TCP/IP model.

* Transceiving means transmitting and receiving.

TCP/IP Model Layers: It has 4 layers.

• Network Interface Layer: Combination of physical and data link layers of the OSI Model.
• Internet Layer: Transmits data in the form of packets from one node to another node.
• Transport Layer: Transmits data from one process to another.
• Application Layer:  Combination of session and presentation layers of the OSI Model as well as provides services to the user.

Signals: Signals are of two types:

• Analog: Analog signals have infinite values or are defined with infinite values.
• Digital: Digital signals have a limited number of values or are defined with a limited number of values.

Transmission Impairments: Like obstacles that disturb the transmission of data or transmission of a signal.

There are different types of transmission impairment, such as:

• Attenuation: It means that some part of the original signal is being shredded off or removed, resulting in decreasing the strength of the signal.
• Distortion: It means that the original signal is being distorted or disturbed from its actual parameters.
• Noise: Due to some external sources of noise, the noise signal is being added to the original signal.

Modulation: A weak signal is being carried by a strong signal over long distances by the help of a transmission medium.

Demodulation:  A weak signal is being extracted from the strong signal at the receiver end.

Frame: Frame contains data that is being transcieved in the LAN.

Packet: Packet contains data that is being transcieved in the WAN. 

* Frame and Packet consists of 3 parts that are Header, Payload, and Trailer.

Header:  The header contains information like sender and destination addresses, length of data, type of data, etc.  

Payload:  Actual data that is being transcieved.

Trailer:  The trailer contains information like error correction and detection bits.

MAC Address: A MAC address (media access control address), also known as a physical address, is 48 bits or 6 bytes. It is a permanent address that is embedded in the network interface card or LAN card of the device. This address is used in a local area network for communication purposes.

IP Address: An IP address is also known as a logical address.

IP addressing is of 2 types:

• IPv4: Internet Protocol version 4
• IPv6: Internet Protocol version 6

IPv4: An IPv4 address is 32 bits or 4 bytes.

IPv4 addressing consists of 5 different classes, such as:

• Class A: 0 to 127
• Class B: 128 to 191
• Class C: 192 to 223
• Class D: 224 to 239
• Class E: 240 to 255

*These ranges include private as well as public ranges till class C.

The private ranges are mentioned below:  

• Class A:  10.0.0.0 to 10.255.255.255
• Class B:  172.16.0.0 to 172.31.255.255
• Class C:  192.168.0.0 to 192.168.255.255

IPv6: An IPv6 address is 128 bits or 16 bytes. It is a hexadecimal address. It does not have any classes like an IPv4 address.

There are two mechanisms by which the IPv6 address can be shortened also. The two mechanisms are:

• Abbreviation: It means removing the leading zeroes from all the blocks of the IPv6 address. 
• Zero Compression: When contiguous blocks of zeroes are appearing in the IPv6 address, then use a double colon to represent the contiguous blocks of zeroes instead of writing zeroes. 

Routing: Routing means how the packets are switching or moving from one node to another node until they reach the receiver node by selecting some path.

Routing is of 2 types:

• Static routing: It means a fixed path between the sender and the receiver. 
• Dynamic routing: It means a router can decide the best path between sender and receiver from the multiple paths.

Congestion Control: How to overcome the crowd of data in the form of packets in the network.

There are two categories of congestion control, such as

• Open Loop: Precautions taken to prevent congestion before it happens. Different types of open-loop techniques are there, such as:

1. Retransmission Policy: If the sent packet is not delivered, then the packet is being retransmitted.
2. Window Policy: Which kind of protocol for the size of the window is being chosen, like the sliding window protocol and others, etc.
3. Acknowledgement Policy: The acknowledgement policy should be implemented in that manner so that there should not be any congestion created by the acknowledgement policy itself. For example, if any timer expires, meaning that a packet is not received by the receiver in the given time period, then an acknowledgment packet will be sent to the sender side.
4. Discarding Policy: The discarding policy means which packet is going to be discarded. For example, the packets with less priority can be discarded if congestion is going to occur.
5. Admission Policy: By admission policy, the congestion in virtual circuit networks can be avoided. For example, a router can avoid establishing a virtual circuit connection if there is congestion or congestion is going to occur.

• Closed Loop: Remove the congestion after it happens. Different types of closed-loop techniques are there, such as:

1. Backpressure: It is a node-to-node mechanism in which a congested node informs its previous node to stop sending data, and the previous node informs its following node to stop sending data, and thus reaches the source node in a similar way to remove the congestion.
2. Choke Packet: In this mechanism, the congested node directly sends the packet to the source node that the congestion occurred.
3. Implicit Signaling: The source node is not notified about the congestion, the source node just assumes from some symptoms, like not getting acknowledgments, etc., that there is congestion in the network.
4. Explicit Signaling: The congested node sends the notification to the source node or receiver node about the occurrence of congestion with the same packet that carries data.  Explicit signaling is of two types:  

• Backward Signaling: The congested node sends a notification to the source node to keep slowing the transmission to overcome the discarding of the packets.
• Forward Signaling: The congested node sends notification to the destination node to use some policies to slow down the acknowledgment process.  

Throughput: It means how much data is being transferred from one node to another in some specific time. For example, 500 bits are being transmitted from sender to receiver in 10 milliseconds.

Bandwidth: It is the size of the data supported by the transmission medium or by the network interface card in the form of bit rate or byte rate. For example, a network interface card, or LAN card, supports 100 megabits per second of data.

Queuing: Data is being waited on in the form of a packet at some node, like a router.

Delay: We all want an instantaneous response from the network, but data encounters delays while moving from the source to the destination.   

So delays in a network are of 4 types:

• Transmission Delay: The source host or the router cannot send the packet instantaneously, so they need to put the bits in the packet one by one. So if the first bit is put at time t1 and the second at time t2, then in this case the transmission delay of the packet is t2-t1.
• Propagation Delay: The time taken by the bit to move from one point to another point in the transmission media. Propagation delay depends on the propagation speed of the transmission media.
• Processing Delay: It is the time taken by a router or a destination host to receive a packet from its input port, remove the header part, perform an error detection mechanism, and then send the packet to the output port (if it's a router node) or send it to the upper-layer protocol (if it's a destination host). 
• Queuing Delay: Typically, a router may have queuing delays.

Packet Loss: The router needs to process the packet, so it takes time, and it puts the incoming packets in the buffer for their turn during the processing of another packet, so time comes when the buffer is full, and the packets are dropping due to the buffer being full, resulting in a packet loss.