Computer Networks — notes

A computer network links computers and devices so they can share resources (printers, storage), data and communication.

Evolution: ARPANET (1969), a US defence research network, linked a few university computers. It adopted TCP/IP in 1983. NSFNET (1986) connected researchers to supercomputers and widened the network beyond defence. Networks joining through TCP/IP became the Internet, a network of networks. The World Wide Web was proposed at CERN in 1989 and made the Internet easy to use.

Components: sender, receiver, message, communication medium, protocol.

• Bandwidth: the range of frequencies a channel can carry, in hertz (Hz); it limits how much data can flow.
• Data transfer rate: bits sent per second: bps, Kbps, Mbps, Gbps. Remember 1 byte = 8 bits.
• Time to send = size in bits ÷ rate in bits per second (use the same prefix on both sides, or the conversion the question states).
• IP address: logical address of a device. IPv4: 32 bits, four numbers 0–255 (192.168.1.10). IPv6: 128 bits, hexadecimal groups.
• Switching: circuit (path reserved for the whole session, e.g. landline call) vs packet (data split into packets routed independently and reassembled, e.g. the Internet).

Wired (guided):

• Twisted pair (Ethernet cable, with RJ45 plugs): cheap, flexible, for short runs; picks up noise.
• Coaxial: central copper conductor with shielding; used for cable TV.
• Optical fibre: carries light; highest speed, immune to electrical noise, costly.

Wireless (unguided):

• Radio waves: omnidirectional, pass through walls (Wi-Fi, mobiles).
• Microwaves: straight line between dishes; long links and satellites.
• Infrared: short range, line of sight only (remotes).

• Modem: modulator–demodulator; digital data ↔ line signal.
• Ethernet card (NIC): connects a computer to a wired LAN; has a unique MAC address.
• RJ45: 8-pin connector on twisted-pair cable.
• Repeater: regenerates a weakened signal on a long run.
• Hub: sends every frame to all ports.
• Switch: sends a frame only to the destination port.
• Router: connects different networks and chooses routes using IP addresses.
• Gateway: connects networks with different protocols, translating between them.
• Wi-Fi card: wireless network adapter.

Types by area: PAN (a few metres), LAN (building or campus), MAN (city), WAN (country or more; the Internet).

Topologies:

• Bus: one backbone with terminators; least cable; a backbone break stops all.
• Star: each node to a central switch; faults isolated; centre is a single point of failure.
• Tree: stars joined in levels; expands easily; a fault high up cuts a branch.

Designing a campus network (board pattern): server in the block with the most computers; a star from that block; a repeater on any run longer than the limit given; a switch in each block; a router (with a modem) for the Internet; a WAN to a distant city office.

Protocols: HTTP/HTTPS (web pages; HTTPS encrypted), FTP (files), SMTP (send mail), POP3 (download mail), PPP (direct link between two nodes), TELNET (remote login, unencrypted), VoIP (voice over the Internet), TCP/IP (TCP: packets and reliability; IP: addressing and routing).

Web services: the WWW is a service on the Internet made of linked pages. HTML displays pages with fixed tags; XML stores and carries data with user-defined tags. A domain name (e.g. school.edu.in, top-level domain in) is turned into an IP address by DNS. A URL = protocol + domain name + path. A web browser requests and shows pages; a web server stores and sends them; web hosting is renting space on an always-on server.