Introduction to Computer Network

Network devices, or networking hardware, are physical devices that are required for communication and interaction between hardware on a computer network. Understanding of these network devices can help in designing and building a network that is secure and serves organization's needs. However, to ensure the ongoing security and availability of your network, you should carefully monitor your network devices and activity around them, so you can quickly spot hardware issues, configuration issues and attacks.

• Hub: Hubs connect multiple computer networking devices together. A hub also acts as a repeater in that it amplifies signals that deteriorate after traveling long distances over connecting cables. A hub is the simplest in the family of network connecting devices because it connects LAN components with identical protocols. A hub can be used with both digital and analog data, provided its settings have been configured to prepare for the formatting of the incoming data. For example, if the incoming data is in digital format, the hub must pass it on as packets; however, if the incoming data is analog, then the hub passes it on in signal form. Hubs do not perform packet filtering or addressing functions; they just send data packets to all connected devices. Hubs operate at the Physical layer of the Open Systems Interconnection (OSI) model. There are two types of hubs: simple and multiple port.
• Switch: Switches generally have a more intelligent role than hubs. A switch is a multiport device that improves network efficiency. The switch maintains limited routing information about nodes in the internal network, and it allows connections to systems like hubs or routers. Strands of LANs are usually connected using switches. Generally, switches can read the hardware addresses of incoming packets to transmit them to the appropriate destination. Using switches improves network efficiency over hubs or routers because of the virtual circuit capability. Switches also improve network security because the virtual circuits are more difficult to examine with network monitors. You can think of a switch as a device that has some of the best capabilities of routers and hubs combined. A switch can work at either the Data Link layer or the Network layer of the OSI model. A multilayer switch is one that can operate at both layers, which means that it can operate as both a switch and a router. A multilayer switch is a high-performance device that supports the same routing protocols as routers. Switches can be subject to distributed denial of service (DDoS) attacks; flood guards are used to prevent malicious traffic from bringing the switch to a halt. Switch port security is important so be sure to secure switches: Disable all unused ports and use DHCP snooping, ARP inspection and MAC address filtering.
• Router: Routers help transmit packets to their destinations by charting a path through the sea of interconnected networking devices using different network topologies. Routers are intelligent devices, and they store information about the networks they’re connected to. Most routers can be configured to operate as packet-filtering firewalls and use access control lists (ACLs). Routers, in conjunction with a channel service unit/data service unit (CSU/DSU), are also used to translate from LAN framing to WAN framing. This is needed because LANs and WANs use different network protocols. Such routers are known as border routers. They serve as the outside connection of a LAN to a WAN, and they operate at the border of your network. Router are also used to divide internal networks into two or more subnetworks. Routers can also be connected internally to other routers, creating zones that operate independently. Routers establish communication by maintaining tables about destinations and local connections. A router contains information about the systems connected to it and where to send requests if the destination isn’t known. Routers usually communicate routing and other information using one of three standard protocols: Routing Information Protocol (RIP), Border Gateway Protocol (BGP) or Open Shortest Path First (OSPF).  Routers are your first line of defense, and they must be configured to pass only traffic that is authorized by network administrators. The routes themselves can be configured as static or dynamic. If they are static, they can only be configured manually and stay that way until changed. If they are dynamic, they learn of other routers around them and use information about those routers to build their routing tables. Routers are general-purpose devices that interconnect two or more heterogeneous networks. They are usually dedicated to special-purpose computers, with separate input and output network interfaces for each connected network. Because routers and gateways are the backbone of large computer networks like the internet, they have special features that give them the flexibility and the ability to cope with varying network addressing schemes and frame sizes through segmentation of big packets into smaller sizes that fit the new network components. Each router interface has its own Address Resolution Protocol (ARP) module, its own LAN address (network card address) and its own Internet Protocol (IP) address. The router, with the help of a routing table, has knowledge of routes a packet could take from its source to its destination. The routing table, like in the bridge and switch, grows dynamically. Upon receipt of a packet, the router removes the packet headers and trailers and analyzes the IP header by determining the source and destination addresses and data type, and noting the arrival time. It also updates the router table with new addresses not already in the table. The IP header and arrival time information is entered in the routing table. Routers normally work at the Network layer of the OSI model.
• Bridge: Bridges are used to connect two or more hosts or network segments together. The basic role of bridges in network architecture is storing and forwarding frames between the different segments that the bridge connects. They use hardware Media Access Control (MAC) addresses for transferring frames. By looking at the MAC address of the devices connected to each segment, bridges can forward the data or block it from crossing. Bridges can also be used to connect two physical LANs into a larger logical LAN. Bridges work only at the Physical and Data Link layers of the OSI model. Bridges are used to divide larger networks into smaller sections by sitting between two physical network segments and managing the flow of data between the two. Bridges are like hubs in many respects, including the fact that they connect LAN components with identical protocols. However, bridges filter incoming data packets, known as frames, for addresses before they are forwarded. As it filters the data packets, the bridge makes no modifications to the format or content of the incoming data. The bridge filters and forwards frames on the network with the help of a dynamic bridge table. The bridge table, which is initially empty, maintains the LAN addresses for each computer in the LAN and the addresses of each bridge interface that connects the LAN to other LANs. Bridges, like hubs, can be either simple or multiple port. Bridges have mostly fallen out of favor in recent years and have been replaced by switches, which offer more functionality. In fact, switches are sometimes referred to as “multiport bridges” because of how they operate.
• Gateway: Gateways normally work at the Transport and Session layers of the OSI model. At the Transport layer and above, there are numerous protocols and standards from different vendors; gateways are used to deal with them. Gateways provide translation between networking technologies such as Open System Interconnection (OSI) and Transmission Control Protocol/Internet Protocol (TCP/IP). Because of this, gateways connect two or more autonomous networks, each with its own routing algorithms, protocols, topology, domain name service, and network administration procedures and policies. Gateways perform all of the functions of routers and more. In fact, a router with added translation functionality is a gateway. The function that does the translation between different network technologies is called a protocol converter.
• Modem: Modems (modulators-demodulators) are used to transmit digital signals over analog telephone lines. Thus, digital signals are converted by the modem into analog signals of different frequencies and transmitted to a modem at the receiving location. The receiving modem performs the reverse transformation and provides a digital output to a device connected to a modem, usually a computer. The digital data is usually transferred to or from the modem over a serial line through an industry standard interface, RS-232. Many telephone companies offer DSL services, and many cable operators  use modems as end terminals for identification and recognition of home and personal users. Modems work on both the Physical and Data Link layers.
• Repeater: A repeater is an electronic device that amplifies the signal it receives. You can think of repeater as a device which receives a signal and retransmits it at a higher level or higher power so that the signal can cover longer distances, more than 100 meters for standard LAN cables. Repeaters work on the Physical layer.
• Access Point: While an access point (AP) can technically involve either a wired or wireless connection, it commonly means a wireless device. An AP works at the second OSI layer, the Data Link layer, and it can operate either as a bridge connecting a standard wired network to wireless devices or as a router passing data transmissions from one access point to another. Wireless access points (WAPs) consist of a transmitter and receiver (transceiver) device used to create a wireless LAN (WLAN). Access points typically are separate network devices with a built-in antenna, transmitter and adapter. APs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. They also have several ports, giving you a way to expand the network to support additional clients. Depending on the size of the network, one or more APs might be required to provide full coverage. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by its transmission range — the distance a client can be from an AP and still obtain a usable signal and data process speed. The actual distance depends on the wireless standard, the obstructions and environmental conditions between the client and the AP. Higher end APs have high-powered antennas, enabling them to extend how far the wireless signal can travel. APs might also provide many ports that can be used to increase the network’s size, firewall capabilities and Dynamic Host Configuration Protocol (DHCP) service. Therefore, we get APs that are a switch, DHCP server, router and firewall. To connect to a wireless AP, you need a service set identifier (SSID) name. 802.11 wireless networks use the SSID to identify all systems belonging to the same network, and client stations must be configured with the SSID to be authenticated to the AP. The AP might broadcast the SSID, allowing all wireless clients in the area to see the AP’s SSID. However, for security reasons, APs can be configured not to broadcast the SSID, which means that an administrator needs to give client systems the SSID instead of allowing it to be discovered automatically. Wireless devices ship with default SSIDs, security settings, channels, passwords and usernames. For security reasons, it is strongly recommended that you change these default settings as soon as possible because many internet sites list the default settings used by manufacturers. Access points can be fat or thin. Fat APs, sometimes still referred to as autonomous APs, need to be manually configured with network and security settings; then they are essentially left alone to serve clients until they can no longer function. Thin APs allow remote configuration using a controller. Since thin clients do not need to be manually configured, they can be easily reconfigured and monitored. Access points can also be controller-based or stand-alone.

The basic communication model in computer networking is where the Sender (encodes the message) channel sends a message over a channel or medium and receiver (decodes the message) gives Feedback.

Basic Communication Model

The components involved in the successful implementation of the basic communication model are as follows −

• Sender − Who sends the message.
• Encodes − Translates messages into symbols like words, pictures, sound, etc.
• Channel or medium − It used to transmit messages. Some channels are face-to-face communication, over telephone, letters, television, newspapers, radio, etc.
• Decode − Receiver decodes these symbols to understand what the sender wants to say.
• Receiver − A person who receives the message.
• Feedback − After receiving a message, the receiver sends feedback to the sender, answer and what he understands from the message.

Sometimes noise is also part of the communication process and it disturbs the message and it will be difficult for the receiver to understand the exact message that the sender wants to send.

Communication Channels

There are three different types of telecommunication and computer networking channels to be aware of: simplex, half-duplex, and full-duplex. Here are the definitions of those terms:

• Simplex, which allows the sending of information in one direction only, which is similar to a one-way street.
• Half-Duplex, in which information can be sent in both directions, but it doesn't allow both directions to operate at the same time.
• Full-Duplex, which is similar to half-duplex. Information can flow in both directions, but it also can operate in both directions at the same time.

These three different channel types all have their ideal uses. One isn't necessarily better than the others, but there will always be one that will work better for certain situations, based on numerous variables. It is essential to know which type to use so that you can implement a strategy that is the most efficient, effective, and affordable solution.

What Is a Simplex Channel?

We already know that the simplex channel only allows information to be sent in one direction, but there's a little more to it than that. Since a simplex channel only allows information to flow in one direction, this means that data can be sent in that direction, but that same data can't be received in return. The information that is sent to the receiver can utilize transfers at maximum capacity since it only has one direction to go. Two real-world situations that use the simplex channel include TV broadcasts and radio stations. The simplex channel works very well for these examples because there is no need to receive any data or send information back. These services are meant to provide a one-way form of entertainment to the users.

What Is a Half-Duplex Channel?

Half-duplex takes it a step higher by allowing information to flow in both directions, but not at the same time. This allows the sender and receiver to communicate with one another, but one has to wait for the other when they do so. In this way, half-duplex can have a higher performance output over the simplex channel since information can flow in both directions. The best example of this in action is when we use a two-way push button radio (walkie-talkie) to communicate with others. This effectively allows us to communicate with anyone who is on the same channel, but only one party can speak at a time. The transfer capacity of the channel is used entirely by the transmitter at the time when the message is being sent over to the receiver.

What Is a Full-Duplex Channel?

Full-duplex, or simply "duplex," is a type of communication in which data can flow two ways at the same time. Full duplex devices, therefore, can communicate back and forth simultaneously. Telephones are common examples of full-duplex devices. They allow both people to hear each other at the same time. In the computer world, most network protocols are duplex, enabling hardware devices to send data back and forth simultaneously. For example, two computers connected via an Ethernet cable can send and receive data at the same time. Wireless networks also support full-duplex communication. Additionally, modern I/O standards, such as USB and Thunderbolt, are full-duplex. The terms duplex and full-duplex can be used interchangeably since both refer to simultaneous bidirectional communication. Full-duplex is often used in contrast to half-duplex, which refers to bidirectional communication, but not at the same time. Simplex communication is even more limited and only supports data transmission in one direction.

Transmission Media

Broadly, there are two types of transmission media in computer networks including guided and unguided media. These two types of transmission medium in computer networks have further subtypes. Let us discuss these in detail.

1. Guided Transmission Media

Guided media are also known as wired or bounded media. These media consist of wires through which the data is transferred. Guided media is a physical link between transmitter and recipient devices. Signals are directed in a narrow pathway using physical links. These media types are used for shorter distances since physical limitation limits the signal that flows through these transmission media.

• Twisted Pair Cable: In this type of transmission media, two insulated conductors of a single circuit are twisted together to improve electromagnetic compatibility. These are the most widely used transmission medium cables. These are packed together in protective sheaths. They reduce electromagnetic radiation from pairs and crosstalk between the neighboring pair. Overall, it improves the rejection of external electromagnetic interference. These are further subdivided into unshielded and shielded twisted pair cables. 
• Unshielded Twisted Pair Cable(UTP): These consist of two insulated copper wires that are coiled around one another. These types of transmission media block interference without depending on any physical shield. The unshielded twisted pair are very affordable and are simple to set up. These provide a high-speed link. 
• Shielded Twisted Pair (STP): This twisted cable consisted of a foil shield to block external interference. The insulation within these types of the twisted cable allow greater data transmission rate. These are used in fast-data-rate Ethernet and in data and voice channels of telephone lines.  

• Optical Fibre Cable: Also known as fiber optic cable, these are thin strands of glass that guide light along their length. These contain multiple optical fibers and are very often used for long-distance communications. Compared to other materials, these cables can carry huge amounts of data and run for miles without using signal repeaters. Due to lesser requirements, they have less maintenance costs and it improves the reliability of the communication system. These can be unidirectional as well as bidirectional in nature. 
• Coaxial cable: These guided transmission media contain an insulation layer that transmits information in baseband mode and broadband mode. Coaxial cables are made of PVC/Teflon and two parallel conductors that are separately insulated. Such cables carry high frequency electrical signals without any big loss. The dimension of cable and connectors are controlled to give them constant conductor spacing for efficient functioning as a transmission line.
• Stripline: This is a  transverse electromagnetic (TEM) transmission media that is built on the inner layers of multi-layer printed circuit boards. These are used in high or low-level RF signals that require isolation from surrounding circuitry. It is a type of printed circuit transmission line in which a signal trace is sandwiched between the upper and lower ground place. Stripline minimizes emissions electromagnetic radiation is completely enclosed within homogeneous dielectric. Along with the reduced emissions, it also shields against incoming spurious signals. 
• Microstripline: While Microstripline is simiar to stripline, it is not sandwiched and are present above the ground plane. These can be fabricated with any technology where the conductor is separated from the ground plane by a dielectric layer called subtrated. These transmission media convert microwave frequency signals.. Microstrip is also used for building microwave components such as couplers, filters, power dividers, antennas, etc. In comparison with the traditional waveguide technology, it is less expensive.

2. Unguided Transmission Media: Also known as unbounded or wireless media, they help in transmitting electromagnetic signals without using a physical medium. Here, air is the medium. There is no physical connectivity between transmitter and receiver. These types of transmission media are used for longer distances however they are less secure than guided media. There are three main types of wireless transmission media. 

• Radio Waves: Radio waves are transmitted in every direction throughout free space. Since these are omnidirectional, sent waves can be received by any antenna. These waves are useful when the data is to multicasted from one sender to multiple receivers. Radio waves can cover large areas and even penetrate obstacles such as buildings and walls. The frequency of these waves ranges between 3 kHz to 1GHz. Due to its omnidirectional nature, issues such as interference might arise when another signal with the same bandwidth or frequency is sent.  
• Infrared: These waves are useful for only very short distance communication. Unlike radio waves, they do not have the ability to penetrate barriers. Their range varies between 300GHz – 400THz. Since they have larger bandwidth, the data rate is very high for infrared waves. These have less interference and are more secure. 
• Microwaves: For these waves, it is important for the transmitter and receiver antenna to be aligned. This is why it is known as line-of-sight transmission. Due to this, they are suitable for shorter distances. They comprise of electromagnetic waves with frequencies ranging between 1-400 GHz. Microwaves provide bandwidth between the range of 1 to 10 Mbps. Distance covered by the signal is proportional to the height of the antenna. For travelling to longer distances, the height of the tower should be increased. These are further sub categorized as terrestrial and satellite type microwave transmission. 

• Terrestrial type microwave transmission: In this type, high directional antennas are used for line of sight propagation paths that use frequency between 4-12 GHz. These are parabolic antennas having diameters that range from 12 inches to feet depending on their spacing. 
• Satellite type microwave transmission: Signals are transmitted to those spaces where satellites are positioned and they retransmit the signal to appropriate locations. Since they only receive and retransmit the signal, they act as repeaters. It is a much more flexible and reliable method of communication in comparison with cables and fiber systems. 

Source: https://www.naukri.com/learning/articles/types-of-transmission-media-in-computer-network/