Data Communication and Networking

(d) Open System Interconnection

OSI stands for Open System Interconnection — a seven-layer reference model developed by ISO that describes how data moves between networked devices.

Twisted Pair Cable, Coaxial Cable and Optical Fibre Cable

1. Twisted Pair Cable

A twisted pair cable consists of two insulated copper wires twisted around each other in a helical (spiral) form. The twisting cancels out electromagnetic interference (noise) from external sources and reduces crosstalk between neighbouring pairs. It is the oldest and most widely used guided transmission medium.

Types:

• Unshielded Twisted Pair (UTP): No metallic shield around the pairs. Cheap, flexible and easy to install. Used in telephone lines and LANs (e.g., Cat5, Cat6 cables with RJ-45 connectors).
• Shielded Twisted Pair (STP): A metallic foil or braid surrounds each pair or the whole cable, giving better protection against noise. It is costlier and less flexible than UTP.

Advantages: Cheapest transmission medium, easy to install and maintain, suitable for short distances.

Disadvantages: Low bandwidth, high attenuation (signal loss), easily affected by noise and interference, not suitable for long-distance communication.

2. Coaxial Cable

A coaxial cable has a central copper conductor surrounded by an insulating layer, which is again surrounded by a braided metallic mesh (shield) and finally covered by an outer plastic jacket. The central conductor and the outer mesh share the same axis, hence the name "coaxial".

The outer mesh acts as a shield against external noise and also serves as the second conductor of the circuit. It carries signals at higher frequencies than twisted pair and is used in cable TV networks, broadband Internet and older Ethernet (10Base2, 10Base5) networks.

Advantages: Higher bandwidth than twisted pair, better protection against noise, can carry data over longer distances, less expensive than optical fibre.

Disadvantages: Thicker and less flexible than twisted pair, costlier than twisted pair, difficult to install and repair, attenuation still occurs over very long distances.

3. Optical Fibre Cable

An optical fibre cable is made of very thin strands of glass or plastic (called the core) surrounded by a cladding of lower refractive index and an outer protective jacket. Data is transmitted in the form of light pulses (from a laser or LED) that travel through the core by total internal reflection. It works on the principle of total internal reflection of light.

Types:

• Single-mode fibre: Very thin core, one path for light, used for very long distances.
• Multi-mode fibre: Thicker core, many paths for light, used for shorter distances.

Advantages: Very high bandwidth, very low attenuation (signal loss), immune to electromagnetic interference and noise, very secure (difficult to tap), suitable for very long-distance and high-speed communication.

Disadvantages: Most expensive of the three, fragile (glass fibres can break easily), difficult to install and splice, requires special equipment and skilled technicians.

Comparison

In short, twisted pair is cheap and used for short-distance LAN and telephone links, coaxial cable offers better bandwidth and noise protection for cable TV and broadband, while optical fibre provides the highest speed, longest distance and greatest security, though at the highest cost.

Differences between Client-Server and Peer-to-Peer Network

(Additional point: in client-server, the failure of the server affects the whole network, whereas in peer-to-peer, the failure of one computer does not stop the rest of the network.)

Simplex and Full-Duplex Transmission Modes

Simplex Mode

In simplex mode, data travels in only one direction — one device is always the sender and the other is always the receiver. The receiver can never send data back. The entire capacity of the channel is used for one-way transmission.

Example: Keyboard to computer, or a TV broadcast (station → TV set).

   Sender  ──────────────►  Receiver
 (Keyboard)   one way only   (Computer)

Full-Duplex Mode

In full-duplex mode (also called duplex), data can travel in both directions simultaneously. Both devices can send and receive at the same time. The channel capacity is shared between the two directions.

Example: Telephone conversation — both people can talk and listen at the same time.

   Device A  ◄────────────►  Device B
             both directions
              at the same time

Comparison

Diagram

        SIMPLEX
   ┌────────┐                 ┌──────────┐
   │ Sender │ ──────────────► │ Receiver │
   └────────┘                 └──────────┘

        FULL-DUPLEX
   ┌──────────┐               ┌──────────┐
   │ Device A │ ◄───────────► │ Device B │
   └──────────┘               └──────────┘

In short, simplex allows communication in a single direction only, while full-duplex allows two-way communication at the same time.

(c) HTTPS

HTTPS (HyperText Transfer Protocol Secure) encrypts data using SSL/TLS, so it is used for secure communication over a network. HTTP, FTP and Telnet send data in plain text.

Transmission Medium

A transmission medium is the physical path or channel through which data signals travel from a sender (transmitter) to a receiver in a computer network. It carries the data in the form of electrical signals, light waves, or electromagnetic waves. Transmission media are broadly classified into two major types: guided (wired/bounded) and unguided (wireless/unbounded) media.

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1. Guided (Wired) Media

In guided media, the signal is confined and guided along a solid physical path such as a cable or wire. The three common guided media are:

(a) Twisted Pair Cable Two insulated copper wires twisted together in a helical form to reduce electrical interference (crosstalk). It is the most widely used and cheapest cable, commonly used in telephone lines and LANs (e.g., UTP Cat5/Cat6).

• Advantages: Cheap, easy to install, flexible, and widely available.
• Disadvantages: Low bandwidth, high attenuation over long distances, and easily affected by noise and interference.

(b) Coaxial Cable It consists of a central copper conductor surrounded by an insulating layer, a braided metallic shield, and an outer plastic jacket. The shield protects against external noise. It is used in cable TV and older Ethernet networks.

• Advantages: Higher bandwidth than twisted pair, better protection from noise, and can span longer distances.
• Disadvantages: Thicker and less flexible, more expensive than twisted pair, and difficult to install.

(c) Fiber Optic Cable It uses thin strands of glass or plastic fiber that carry data as pulses of light. It offers the highest bandwidth and is immune to electromagnetic interference. It is used in backbone networks and high-speed internet.

• Advantages: Very high bandwidth, very low attenuation, immune to EMI, and highly secure.
• Disadvantages: Very expensive, fragile, difficult to install and splice, and requires skilled technicians.

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2. Unguided (Wireless) Media

In unguided media, the signal travels through free space (air, vacuum, or water) without any physical cable. The signal is broadcast through electromagnetic waves. The major types are:

(a) Radio Waves These are low-frequency electromagnetic waves that can travel long distances and penetrate walls. They are used in AM/FM radio, Wi-Fi, and mobile phones.

• Advantages: Can penetrate obstacles, cover large areas, and need no line of sight.
• Disadvantages: Low bandwidth, easily interfered with, and less secure.

(b) Microwaves These are high-frequency waves that travel in straight lines and require a clear line of sight between transmitter and receiver. They are used in satellite communication and microwave links.

• Advantages: High bandwidth and relatively inexpensive compared to laying cables over difficult terrain.
• Disadvantages: Require line of sight, affected by weather (rain, fog), and need tall towers or satellites.

(c) Infrared These are high-frequency waves used for short-range communication such as TV remotes and some wireless keyboards. They cannot pass through walls.

• Advantages: Cheap, secure within a room, and immune to radio interference.
• Disadvantages: Very short range, require line of sight, and cannot penetrate obstacles.

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Conclusion

The choice of transmission medium depends on factors such as cost, bandwidth requirement, distance, and environmental conditions. Guided media are preferred for reliability and security over fixed paths, while unguided media offer mobility and convenience where cabling is impractical.

(d) 203.0.113.10

The other three fall in the private IPv4 ranges — 192.168.0.0/16, 172.16.0.0/12 and 10.0.0.0/8 — which are reserved for use inside local networks and are not routable on the public internet.

Star Topology vs Bus Topology

Star topology and bus topology differ fundamentally in both their physical layout and the way data travels between nodes.

Architectural Layout

Data Transmission Methodology

Star topology: Data is sent from the source node to the central device (switch/hub). The switch reads the destination MAC address and forwards the frame only to the intended destination port (in a switched star). This gives dedicated point-to-point links, avoids collisions, and allows full-duplex communication. In modern environments, the central device is almost always a switch, so each node effectively has its own channel.

Bus topology: All nodes share one common transmission medium. When a node transmits, the signal propagates in both directions along the bus, and every node receives it. Each node checks the destination address and accepts the frame only if it matches; otherwise it discards it. Since the medium is shared, only one node can transmit at a time; otherwise collisions occur. This requires a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) access method to manage transmissions. A terminator at each end absorbs signals to prevent reflection.

In Modern Computing Environments

• Star topology dominates modern LANs because switches are cheap, faults are easy to isolate (one cable/port at a time), and performance scales well. It forms the basis of hierarchical (extended star) networks used in offices, schools, and data centres.
• Bus topology is largely obsolete for new installations. Its limitations — collision-prone shared medium, difficult troubleshooting, and poor scalability — make it unsuitable for today's high-bandwidth, full-duplex Ethernet networks. It survives mainly in legacy systems and in logical bus designs (e.g., early Ethernet) rather than physical layouts.

In short: Star uses a centralized, point-to-point architecture with directed, collision-free transmission, while bus uses a decentralized, shared-medium architecture with broadcast transmission governed by CSMA/CD. Modern networks favour star due to reliability, performance, and ease of maintenance.

(b) 256.2.5.76

In an IPv4 address, each of the four octets must be between 0 and 255. The first octet here is 256, which is out of range, so this is not a valid IP address.

CAT-6 Cable

CAT-6 (Category 6) is a twisted-pair copper cable used in modern Ethernet networks. Its main features are:

• Construction: Four pairs of twisted copper wires, often separated by a plastic spline (cross-shaped separator) to reduce crosstalk between pairs.
• Bandwidth: Supports a bandwidth of 250 MHz, higher than CAT-5e (100 MHz).
• Data rate: Supports 10 Gbps Ethernet (10GBASE-T) up to a distance of about 55 metres, and 1 Gbps (1000BASE-T) up to 100 metres.
• Connector: Uses the standard RJ-45 connector.
• Crosstalk and noise: Better protection against crosstalk and electromagnetic interference than CAT-5/CAT-5e, but still affected by EMI.
• Cost: Moderately cheap and easy to install, but costlier than CAT-5e.
• Max segment length: 100 metres for 1 Gbps operation.

Optical Fiber Cable

Optical fiber is a cable made of thin strands of glass or plastic that carries data as pulses of light rather than electrical signals. Its main features are:

• Construction: A central core (glass/plastic) surrounded by cladding and a protective buffer/jacket.
• Bandwidth: Extremely high bandwidth (in the range of THz), so it can carry huge amounts of data.
• Data rate: Very high speed — from 1 Gbps to 100 Gbps and beyond.
• Types: Single-mode fiber (small core, long distance, laser source) and multi-mode fiber (larger core, shorter distance, LED source).
• Interference: Completely immune to electromagnetic interference (EMI) and crosstalk, since it uses light.
• Attenuation: Very low signal loss, so it can carry signals over very long distances (many kilometres) without repeaters.
• Security: Very secure — tapping the cable is difficult.
• Cost: Expensive cable and equipment; installation and joining (splicing) require skilled technicians and are difficult.
• Connectors: Uses connectors such as ST, SC, and LC.

Which Cable is More Suitable for a LAN?

For developing a Local Area Network (LAN), CAT-6 cable is more suitable.

Reasons:

1. Distance: A LAN covers a small area (a building or campus), usually within 100 metres per segment — exactly the range CAT-6 supports at 1 Gbps.
2. Cost: CAT-6 is much cheaper than optical fiber, both for the cable and the connecting equipment (switches, NICs).
3. Ease of installation: CAT-6 is flexible, easy to install, and easy to terminate with RJ-45 connectors, unlike fiber which needs special tools and skilled labour.
4. Sufficient speed: CAT-6 gives 1 Gbps (and even 10 Gbps over short runs), which is more than enough for normal LAN traffic.
5. Compatibility: CAT-6 works with standard Ethernet NICs and switches, so no costly conversion equipment is needed.

Optical fiber is preferred only for backbone connections or where very high speed, long distance, or strong EMI immunity is required — not for ordinary LAN cabling. Hence, CAT-6 is the better choice for a LAN.

(c) www

The World Wide Web (WWW) is the collection of hyperlinked documents (web pages) stored on web servers and accessed over the Internet. HTML is the language used to create those documents, a webpage is a single document, and email is a separate messaging service.

(a) 192.168.1.1

An IPv4 address has four octets separated by dots, and each octet must be a decimal number from 0 to 255. Here 192, 168, 1 and 1 all lie in this range. Option (b) contains letters, while in (c) and (d) the octets 257 and 262 exceed 255, so they are invalid.

(b) Guided Transmission media

Coaxial cable is a physical cable that carries signals through a solid copper core, so the signal is guided along a fixed path — hence it is a guided (wired) transmission medium.

Network topology is the physical or logical arrangement of computers, cables, and other devices in a network. It defines how nodes are connected and how data flows between them. Common topologies are bus, star, ring, mesh, tree, and hybrid.

Difference between guided (wire media) and unguided (wireless media)

(b) Media Access Control

A MAC address is the 48-bit hardware address burned into a network interface card, used at the Media Access Control sublayer of the data link layer to identify a device on a LAN.

Comparison of Star and Ring Topology

Star topology connects every node (computer, printer, etc.) to a central device called a hub or switch. All communication passes through this central device.

Ring topology connects each node to exactly two neighbouring nodes, forming a closed loop. Data travels around the ring, usually in one direction, from node to node.

Star topology

Pros

• Easy to install, configure and extend — a new node is simply plugged into the central device.
• Failure of one node or its cable does not affect the rest of the network.
• Fault detection and troubleshooting are simple, since all traffic passes through the centre.
• High performance for a small LAN; no data collisions if a switch is used.

Cons

• If the central hub/switch fails, the whole network goes down.
• Requires more cable than a ring, since every node needs its own line to the centre.
• Cost of the central device and extra cabling is higher.

Ring topology

Pros

• Data travels in an orderly, token-controlled manner, so collisions are avoided and performance is predictable even under heavy load.
• Each node acts as a repeater, so signals can travel longer distances.
• Cabling cost is lower, as each node connects only to its two neighbours.

Cons

• Failure of one node or one link can break the entire ring (unless a dual ring is used).
• Adding or removing a node disturbs the whole network and needs the ring to be broken temporarily.
• Troubleshooting is difficult — a fault anywhere stops communication everywhere.
• Data must pass through intermediate nodes, so a message may take a long path.

Which cable is more appropriate for a LAN?

For designing a local area network, twisted pair cable (specifically UTP — Unshielded Twisted Pair, Cat 5e/6) is the most appropriate choice.

Reasons:

• Low cost: It is the cheapest of all guided media, so a LAN covering many rooms can be built economically.
• Easy installation and flexibility: It is thin, light and flexible, so it can be laid through walls, ceilings and conduits without much difficulty.
• Sufficient speed for LANs: Cat 5e supports 100 Mbps and Cat 6 supports up to 1 Gbps over 100 m, which is more than enough for a typical LAN.
• Works well with star topology: UTP is designed to connect each node to a central switch, matching the most common and reliable LAN layout.
• Easy maintenance: If a cable or connector (RJ-45) is damaged, it can be replaced quickly without disturbing the rest of the network.

Coaxial cable is now largely outdated for LANs, and fibre optic cable, though fast and immune to interference, is expensive and hard to install, so it is used only for backbone or long-distance links. Hence, UTP cable is the most appropriate for a LAN.

Why businesses prefer client-server: In the client-server architecture, a powerful central server provides resources and services (files, databases, email, printing) to many client computers. Businesses prefer it because data, security and backup are managed centrally, many users can share resources reliably, and the network can grow easily as the business expands.

Advantages

• Centralized control of data, user accounts, security and backup.
• Better security, since access rights are enforced by the server.
• Resources such as files, databases and printers are shared efficiently.
• Easy to expand by adding clients or upgrading the server.
• Failure of one client does not affect the rest of the network.

Disadvantages

• The server is expensive to buy and maintain and needs a skilled administrator.
• If the server fails, all clients lose its services (single point of failure).
• Heavy traffic can overload the server and slow the network.
• Setting up and managing the network is more complex than peer-to-peer.