Data Communication

The components of data communication, bandwidth and data transfer rate, transfer times, simplex and duplex, circuit and packet switching, wired and wireless media, and protocols.

Key ideas

Its components are the sender, the receiver, the message, the communication medium and the protocols both sides follow.
Bandwidth is the range of frequencies a channel can carry, in Hz; the data transfer rate is the number of bits sent each second, in bps.
Simplex goes one way only; half-duplex both ways, one at a time; full-duplex both ways at once.
Circuit switching sets up a dedicated path first; packet switching splits data into packets that may take different routes and are put back in order.
Twisted pair is cheap (phone lines, LANs), coaxial is shielded (cable TV), and optical fibre carries light, fast and far, free of electrical noise.
Radio waves (3 KHz–1 GHz) pass through walls, microwaves (1–300 GHz) need line of sight, and infrared (300 GHz–400 THz) works only over short distances.
HTTP and HTTPS carry web pages (HTTPS encrypted), FTP files, SMTP sends e-mail and POP3 downloads it, TCP/IP delivers packets, and VoIP carries voice calls.

Remember

Rate units (as the book)	1 Kbps = 1024 bps; 1 Mbps = 1024 Kbps; 1 Gbps = 1024 Mbps
Bytes to bits	1 byte = 8 bits, so 1 MB = 8 Mb
Transfer time	size in bits ÷ rate: a 5 MB file at 10 Mbps → 40 Mb ÷ 10 = 4 seconds
Data sent	MB = rate in Mbps × seconds ÷ 8: 16 Mbps for 10 s → 20 MB
Bandwidth	highest − lowest frequency: 200 MHz to 700 MHz → 500 MHz
Packets	6,000 bytes in packets of 1,000 bytes → 6 packets; with n routers between, n + 1 hops

Watch out

Dividing a size in bytes by a rate in bits per second without multiplying by 8 first.
Mixing up bandwidth (in Hz) with the data transfer rate (in bps).
Calling walkie-talkies full-duplex: only one side can talk at a time.
Using 1000 for 1 Kbps when the question says 1024.

Worked example

How many seconds does it take to send a 200 KB file over a 100 Kbps link? (1 byte = 8 bits)

Working: 200 × 8 = 1600 Kb; ÷ 100

Answer: 16 seconds