Data Transmission, Error Checking and Encryption: Question 2
Syllabus 2.1
A photography studio uses several devices that transmit data in different ways.
Device 1: A digital camera is connected to a laptop with a USB cable. Photo files are copied from the camera to the laptop's hard drive, one bit at a time, along the connection.
Device 2: A wireless intercom lets two assistants talk to each other during a shoot. Both intercoms share a single radio channel, so only one assistant can be heard at any moment; if the other starts speaking at the same time, that message is lost.
Device 3: A security camera continuously streams live video to a monitor in the studio office. The monitor never sends any data back to the camera.
(a) State whether device 1 transmits data using serial or parallel transmission. Give one reason why a USB connection is designed to work this way, even though the cable itself contains more than one wire. [2]
(b) Identify the duplex mode used by device 2, and explain, using the scenario, why this mode allows only one assistant to be heard at a time. [2]
(c) Identify the duplex mode used by device 3, and explain why this mode is appropriate for a continuous, one-way video feed. [2]
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Worked solution
Part (a): Serial or parallel transmission
USB copies the photo files one bit at a time along a single data path, so device 1 uses serial transmission. A USB cable does contain more than one wire, but most of those extra wires carry power or act as a return path for the electrical signal rather than carrying separate bits of data simultaneously; sending bits one after another down a single path is simpler to wire and keeps the bits from arriving out of step with each other, which is why USB is designed as a serial interface rather than a parallel one.
Part (b): Duplex mode of the wireless intercom
The two intercoms share a single radio channel, so only one signal can occupy that channel at a time. This is half-duplex. If one assistant is speaking, the shared channel is busy carrying their voice, so the other assistant’s speech cannot be transmitted (or heard) at the same moment; each assistant can send and receive, but only one direction can be in use at any instant, matching the scenario where a message is lost if both try to speak together.
Part (c): Duplex mode of the security camera feed
The camera only ever sends video to the monitor, and the monitor never transmits anything back, so this is simplex. A continuous, one-way video feed has no need for the monitor to reply, so a mode that only ever supports data travelling in a single direction is entirely sufficient, and no bandwidth is wasted providing for a return path that would never be used.
Final answers
- (a) Serial transmission. Bits travel one after another along a single data path, even though the cable itself has several wires.
- (b) Half-duplex, the shared channel only carries a signal in one direction at a time.
- (c) Simplex. Data only ever needs to travel one way, from camera to monitor.