2 - Communications Links

Question 1

Features of an asynch serial frame

Answer 1

• Very easy to program
• Uses EIA-232 interface
• Low-speed bus at 4800bps
• Uses ASCII Framed messages

Question 2

Features and data format of GPS Data

Answer 2

• Interface: EIA-232/ EIA-432 or TTL
• 4800Bps, 8-bits, 1 stop baud, 1 start baud, no parity
• Frame starts with $GP
• Values are represented in ASCII and separated by commas.

Question 3

Name and explain the two interfaces provided by GPS equipment.

Answer 3

Telemetry Interface
- Uses ASCII, serial link to determine how data is transmitted
- Data is transmitted from one “talker” to multiple “listeners”
- Uses LSB/FIFO encoding
- 8 bits, 1 start baud, 1 stop baud.

NMEA Electrical Interface
- Standard: Uses EIA-422 (Differential Line)

Question 4

Difference between EIA-422 and EIA-232

Answer 4

EIA-422 is differential balanced . While EIA-232 is an unbalanced open line interface

Question 5

Explain the process of frame synching.
Give an example

Answer 5

1) Data is grouped into frames
- This allows the receiver to make sense of the received data

2) A method is implemented to align to the start of each frame.
- A frame alignment word is usually sent at the start of a frame

Example:
NMEA Frame synch: $GP at the start of the frame.

Question 6

What causes a transmission error and how to detect them?

Answer 6

• Bits are corrupted by noise during transmission
  This causes the value to change

Integrity Checks
- Character Parity
- Cyclic Redundancy Check
- Longitudinal Parity Checksum
- Longitudinal Arithmetic Checksum

Question 7

What is an Integrity Check?

Answer 7

• Verifies the integrity of individual bytes ( byte = 8 bits) and is inserted before the stop bauds.
• The parity bit is calculated.
  Types of Parity:
  Odd Parity - ODD no.1s -> parity bit = 1
  Even Parity - EVEN no.1s -> parity bit = 1
• The receiver and the sender need to be configured to use the same type of parity .
• The parity bit of the sender and the receiver are compared
  match = no error
  no match = transmission error, this causes an error flag to be raised in the UART

Question 8

What is Longitudinal parity check?

Answer 8

Method:
- For each position ( i = 0 to 7) calculate the XOR sum of all bits and place in position i.
This provides an independent check for each position

• The accumulated checksum is an 8-bit value

PROBLEM:
- It verifies if the whole frame is correct.
But, cannot identify the corrupted bit

Question 9

Problems with longitudinal parity checksum?

Answer 9

• Cannot identify which bit is corrupted
• Does not detect even errors
  e.g.) if 2 bits are corrupted the parity remains the same, therefore there is no error detected.
• There is no standard method in the NMEA protocol to ask for retransmission after an error has been detected.

Question 10

Explain Longitudinal Arithmetic checksum

Answer 10

• More powerful than Longitudinal parity checksum, as it can detect even errors.
• Instead of the XOR operation, an ADD operation is implemented. Bytes in a frame are added
• Problem here: Addition may generate a carry, so error in MSB is not detected..

Solution: Most LAC add sum of all carry bits to LSB.

Question 11

Exaplain CRC

Answer 11

• This is a cyclic redundant check that detects errors in received data.
• Bytes of data are grouped into a block and a CRC is calculated.
• CRC: Modulo-2 division of the data by a polynomial
  Remainder = checksum

Only field in which the convention is MSB First

Question 12

Draw the basic state diagram

Question 13

Draw the receiver state diagram

Answer 13

Refer to notes

Question 14

Suggest a way of improving frame alignments

Answer 14

• Use a longer frame alignment word that is unlikely to appear in data.
• check several times before the receiver is in-synchronisation.

Question 15

Features of an ideal channel

Answer 15

• Infinite bandwidth
• Noiseless