All In Flashcards
Expand ALE
Automatic Link Establishment
What is the purpose of an ALE system?
To provide a reliable, rapid, method of calling and connecting during constantly changing ionospheric conditions, reception interference and congested channels
What does a standalone radio combine?
A HF Radio combines an HF SSB transceiver with an internal microprocessor and MFSK Modem
What does the HF SSB transceiver do when the system is not actively in contact with another station?
Scans through a lift of HF frequencies called channels
Expand and describe BER
Bit Error rate/ratio
Measures signal quality
bits:errors
How does an ALE system contact/reach a specific station?
Caller enters ALE address,
Controller selects best available idle channel,
then sends a brief selective call
Describe the operation of selective calling
Only let’s in signals who have the correct digital code
How is an ALE link confirmed by 2 communicating stations?
Visual alert and Audible alarm
list 4 common applications provided by ALE
Voice HF Internet connectivity for email SMS file transfer real time chat via HF text
How does an ALE system typically signal the operator that a HF frequency optimised for both sides of the communications link has been established
Audibly and visually at both ends of the link
What was ALE originally developed for?
The alleviation of the need for the radio operator to constantly monitor and change the radio frequency manually to compensate for ionospheric conditions or interference
List 5 common users of ALE network systems?
Emergency disaster relief agencies,
military,
guard forces
List 3 organisations known to use ALE for emergency management and disaster relief
Red Cross,
NATO,
US Armed forces
Define Asynchronous systems?
These systems require extra elements added to the transmitted data stream to inform the receive terminal where a word begins and ends
Define Synchronous systems
These systems require extra elements added to the transmitted data stream to inform the receive terminal where a block of data begins and ends
in an asynchronous system what portion of the start bit activates the receive terminal clock?
Leading edge of start bit
once an asynchronous system is running, from where does the receiving terminal take its timing source from?
centre of the bit
How often is an asynchronous systems clocked re-synced?
OK completion of each frame
Determine the overhead of an asynchronous system who’s complete frame consists of 1 start bit, 19 data bits and 2 stop bits
3/22=
3 (start and stop bits)/22 (total number of bits)
Why are asynchronous systems generally limited to a maximum amount of data they are able to send between start and stop bits?
Timing error. The larger the block of data transmitted the larger the timing error
What percentage of overhead does an asynchronous system generally employ?
20%
State 2 methods that a synchronous system may utilise in order to achieve timing synchronisation between transmit and receive terminals?
Provide separate clock between the lines of the tx and RX terminals,
embed clocking information in the transmitted data signal
in a character bit oriented synchronous system what 3 elements make up the total frame structure?
SYN/ Control Character/ Data
What additional synchronisation allows the receive terminal of a synchronous system to determine the beginning and end of a frame of data?
Pre and Post amble
Typically, what is the size (in bits) of the control information within a synchronous system?
Less than 100 bits
What does a synchronisation frame consist of?
Data plus the control information
Describe in detail a character oriented transmission scheme
within a character oriented transmission, the block of data to be transmitted is treated as a sequence of characters, usually 8 bits in length and all control information is in character form
Describe in detail Bit oriented transmission scheme?
The block of data to be transmitted is treated as a sequence of bits. Neither data nor control information needs to be interpreted in units of 8 bit characters
Explain zero bit insertion
A type of line coding to prevent a long run of 1s and 0s
Describe Non Return to Zero coding
Absolute data. What is transmitted is what’s received.
Describe NRZ-L coding?
Absolute data. What is transmitted is what’s received.
By what other 2 names is NRZ-L knows by?
NRZ,
NRZ-Common
Describe NRZ-M?
Polarity changes on a 1
By what are names is NRZ-M known by?
Delta 1,
Frequency differential keying - Mark
Describe NRZ-S
Polarity changes on a 0
By what other names is NRZ-S known by?
Frequency differential keying- SPACE,
NRZ-INVERTED
Describe Manchester Coding
Polarity will never be the same. When polarity is 1 you will put 0 next. if polarity is 0 you will put 1. Baud rate will appear to be twice that of the original data
By what other names are Manchester Coding known by?
Bi Phase Level,
Split phase coding
Define error correction?
Ability to detects errors caused by noise and other impairments during transmission
Explain the operation process of a Repetition (EDAC) scheme?
Each block of data is sent a pre determined amount of times
What are the major disadvantages with the (EDAC) Repetition scheme?
Susceptible to problems if errors occur in the same place for each group
Describe (EDAC) Parity scheme?
Adding an extra binary digit to the end of each word. The extra bit is called Parity check bit. It’s chosen to make the number of 1s in all words either even or odd.
Apply even Parity to 10000
1
Apply even Parity to 1100000
0
Apply odd Parity to 00110000
1 would go at the end as there’s 2 (even) number 1s in the data stream
With Parity (EDAC) schemes what number of errors within a data word would the receiver be unable to detect?
2 bits. Not possible to detect even numbers of errors
Define error correction?
The ability for the receive terminal to reconstruct the original data error free.
Name 2 basic error correction techniques?
Automatic repeat request (ARQ)
Forward error correction.
Describe Automatic repeat request?
An error control method for data transmission that uses acknowledgements and timeouts to achieve a reliable data transmission over an unreliable service
Name 3 ARQ protocols?
Stop and wait ARQ
Go back N ARQ
Selective Repeat ARQ
Explain how the simplest form of ARQ works?
Stop and wait ARQ. Transmit terminal will send one frame at a time, it will not send any other frame until it has received an Acknowledgement from the receive terminal
Explain how Forward error correction works?
Transmit terminal encodes the data with an ECC (error correcting code) then sends the message. The receive terminal decodes what it receives into the most likely data
Provide 2 advantages of FEC?
No back channel required
Retransmission of data is avoided
