AQA 7517 · A-Level Computer Science · ~16 min read
Serial vs Parallel Transmission
Feature
Serial
Parallel
Data sent
One bit at a time, sequentially
Multiple bits simultaneously (e.g. 8 bits)
Wires/channels
Single channel
Multiple channels (one per bit)
Speed
Slower per clock cycle
Potentially faster per clock cycle
Problem
—
Crosstalk and skew at high speeds; wires fall out of sync
Long distance?
Yes — used for USB, Ethernet
No — only short distances (IDE, old printer cables)
Examples
USB, SATA, Ethernet
Old parallel printer port, PCI bus
Paradox: despite sending 1 bit at a time, serial is now faster than parallel for long distances because high-speed parallel suffers from skew (bits arrive at slightly different times) and crosstalk (adjacent wires interfere). USB 3.0 serial can exceed old parallel bus speeds.
Synchronous vs Asynchronous Transmission
Feature
Synchronous
Asynchronous
Clock
Shared clock signal between sender and receiver
No shared clock; each character framed with start/stop bits
Timing
Data sent in coordinated bursts
Data sent at any time
Overhead
Low — no start/stop bits needed
Higher — start/stop bits added to every byte
Speed
Faster, more efficient
Slower due to overhead
Examples
Network protocols, RAM/CPU communication
Serial port (RS-232), keyboard input
Protocols and Standards
A protocol is an agreed set of rules that allows two devices to communicate. Without protocols, devices from different manufacturers could not communicate.
Key protocols/standards to know:
IEEE 802.3 (Ethernet): wired LAN standard; defines how data is framed and transmitted over copper/fibre
IEEE 802.11 (Wi-Fi): wireless LAN standard (a/b/g/n/ac/ax variants with increasing speeds)
Bluetooth (IEEE 802.15.1): short-range wireless (PAN); typically up to 10m; used for peripherals
USB (Universal Serial Bus): serial standard for connecting peripherals; USB 2.0 = 480 Mbps, USB 3.0 = 5 Gbps
CSMA/CD and CSMA/CA
When multiple devices share a medium, collision occurs when two transmit simultaneously. Two methods handle this:
CSMA/CD (Carrier Sense Multiple Access / Collision Detection): used in wired Ethernet
Device listens before transmitting (carrier sense)
If collision detected, stops and sends a jam signal
Random backoff — waits a random time before retrying
CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance): used in Wi-Fi (802.11)
Device checks if medium is idle
Waits a random backoff time anyway before transmitting
Uses RTS/CTS (Request to Send / Clear to Send) handshake optionally
Avoids collisions rather than detecting them (radio collisions cannot be detected while transmitting)
Error Detection Methods
Parity check: extra bit added so total 1s are even (even parity) or odd (odd parity). Detects single-bit errors but not two-bit errors or transpositions.
Checksum: sum of all bytes calculated and appended. Receiver recalculates and compares. Detects most errors.
CRC (Cyclic Redundancy Check): polynomial division; very reliable error detection; used in Ethernet, SATA. Does NOT correct errors — only detects them.
ARQ (Automatic Repeat reQuest): if error detected, receiver requests retransmission
Exam tip: Know the difference between serial and parallel clearly — AQA often asks why serial is preferred over long distances despite sending fewer bits per cycle (answer: skew and crosstalk issues). For CSMA: CD = detect after collision (wired), CA = avoid collision before (wireless). Error detection: parity is simplest but weakest; CRC is most reliable but cannot correct, only detect.
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Worksheet — 4.9.2a Network Communication & Standards
8 questions · instantly marked · AQA 7517 standard
Q1State two differences between serial and parallel data transmission.[2]
✅ Mark scheme
Mark scheme
Any two: Serial sends one bit at a time [1], parallel sends multiple bits simultaneously on separate wires [1]; Serial uses a single channel [1], parallel uses multiple channels [1]; Parallel suffers from skew and crosstalk at high speeds [1], serial does not [1]; Serial works over longer distances [1], parallel only over short distances [1].
Q2Explain why serial transmission is preferred over parallel for long-distance communication despite sending fewer bits per clock cycle.[2]
✅ Mark scheme
Mark scheme
At high speeds, parallel wires suffer from skew — bits travel at slightly different speeds and arrive out of sync [1]; crosstalk between adjacent wires corrupts data [1]; serial avoids both problems as there is only one channel, so very high serial clock speeds are achievable [1].
Q3Explain the difference between synchronous and asynchronous data transmission.[3]
✅ Mark scheme
Mark scheme
Synchronous: sender and receiver share a clock signal [1]; data is sent in coordinated bursts with low overhead — no start/stop bits needed [1]. Asynchronous: no shared clock [1]; each character is framed with start and stop bits so the receiver knows when data begins and ends [1] — higher overhead but data can be sent at any time.
Q4Explain what CSMA/CD is and describe how it handles a collision on a wired network.[3]
✅ Mark scheme
Mark scheme
CSMA/CD = Carrier Sense Multiple Access with Collision Detection [1]; used in wired Ethernet. Process: device listens to check if the medium is idle (carrier sense) [1]; if a collision is detected while transmitting, all devices stop and send a jam signal [1]; each device waits a random backoff time before retrying [1].
Q5Explain why Wi-Fi uses CSMA/CA instead of CSMA/CD.[2]
✅ Mark scheme
Mark scheme
Wireless devices cannot detect collisions while they are transmitting [1] because the transmitted signal drowns out the received signal — unlike wired where the device can compare signals [1]; CSMA/CA avoids collisions before they happen by waiting a random backoff time even if the channel seems idle [1].
Q6Describe how a parity check detects errors in transmitted data. State one limitation.[3]
✅ Mark scheme
Mark scheme
A parity bit is added to each byte so the total number of 1s is even (even parity) or odd (odd parity) [1]; the receiver counts the 1s — if the count is wrong, an error has occurred [1]; the receiver requests retransmission [1]. Limitation: cannot detect two-bit errors (both errors cancel out, parity appears correct) [1]; cannot identify which bit is wrong.
Q7What is a protocol? Explain why protocols are essential for network communication.[2]
✅ Mark scheme
Mark scheme
A protocol is an agreed set of rules for how data is transmitted between devices [1]; without protocols, devices from different manufacturers would not be able to communicate [1]; protocols define: data format, transmission speed, error checking method, connection setup and termination [1].
Q8Compare CRC and parity check as error detection methods. Which is more reliable and why?[3]
✅ Mark scheme
Mark scheme
Parity: adds a single parity bit; detects only odd numbers of bit errors [1]; simple and low overhead [1]. CRC (Cyclic Redundancy Check): uses polynomial division to generate a checksum [1]; detects burst errors and multiple-bit errors very reliably [1]. CRC is more reliable because parity misses two-bit errors (they cancel) and only checks overall bit count — CRC catches far more error patterns [1]. Neither corrects errors — errors cause retransmission.
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Mini Test — Network Communication
10 questions · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1Serial transmission sends data:
Q2Asynchronous transmission uses __________ to frame each character:
Q3CSMA/CA (Collision Avoidance) is used instead of CSMA/CD on Wi-Fi because:
Q4A parity check CANNOT detect:
Q5IEEE 802.11 is the standard for:
Section B — Short Answer [5 marks]
Q6What is skew and why does it affect parallel transmission?
Mark schemeSkew: bits travelling on parallel wires arrive at the receiver at slightly different times [1]; at high speeds this causes bits to be misread as they fall out of sync [1].
Q7State what ARQ stands for and explain how it responds to a detected error.
Mark schemeARQ = Automatic Repeat reQuest [1]; when the receiver detects an error (e.g. failed CRC check), it sends a request for retransmission of the corrupted data [1].
Q8Why are protocols important in networking?
Mark schemeProtocols are agreed rules for communication — without them, devices from different manufacturers cannot communicate [1]; they define data format, timing, error handling, and connection management [1].
Q9State one advantage of synchronous over asynchronous transmission.
Mark schemeSynchronous is more efficient — no overhead of start/stop bits [1]; data is sent continuously in coordinated bursts, making it faster [1].
Q10Explain what happens when two devices detect a collision using CSMA/CD.
Mark schemeBoth devices stop transmitting and send a jam signal [1]; each then waits a random backoff time before attempting to retransmit [1].