User manual

54
The desired option for collision avoidance is selected by
checking the control button to the left of the description
on the main window.
None
When selected this turns off all collision avoidance
mechanisms. This should only be used in point to point
applications.
Digital
This is the standard method of collision avoidance and
utilises a channel busy indication bit in the signalling
channel transmitted to all remotes for control. When
selected a dialogue box appears and several options
must be configured:
• Mode–“Master”or“Remote”.Whenthemaster
unit receives a valid transmission from a remote
unit it sets the channel busy indication bit. This
busy bit is interpreted by the other remotes to
not transmit. Once the transmission from the first
remote ends the master unit resets the busy bit to
indicate the channel is now clear to transmit on.
The master unit, which is normally a base station,
takes about 5ms to detect a transmission from a
remote unit and set the channel busy indication bit
on the radio channel. During this period collision
of remote transmissions can still occur and is
unavoidable.
Note: The master must be permanently keyed.
• BackoffMethod–“RetryafterTxAttempt”or
“Delay before Tx Attempt”. The method chosen
is system dependent and can only be configured
if the mode is “remote”. The former is best used
when data responses from remotes are largely
asynchronous. The latter is best used when this is
not the case.
• BackoffTiming–“MaximumSlots”,“Timeper
Slot”. This can only be configured if the mode is
“remote”. When a remote is ready to transmit data
but it finds the busy bit from the master set it
holds back its transmission for a random “backoff”
time after which it tries to transmit data again. This
ensures that modems waiting to be allowed to
transmit do not re-attempt to do so at the same
time. The “Maximum Slots” (1 to 16) and the
“Time per Slot” (1 to 255ms) are used to calculate
the backoff time by multiplying the slot time by a
random number between 1 and the maximum slot
number. For example if the time per slot is 30ms
and the maximum slots is 4, the random backoff
times can be 30, 60, 90 or 120ms.
As the channel busy indication bit is critical for reliable
operation default interpretation of this bit is defined
in the remote units. If the master modem stops
transmission (or has not yet started) the remote will
interpret that the channel is busy and will not transmit
until the master comes on line.
Carrier Detect
This method of collision avoidance utilises a carrier
transmitted to all remotes to indicate that the radio
channel is busy. When selected a dialogue box appears
and several options must be configured:
Part I – TVIEW+ Management Suite - Programmer
Encryption Setup (Packet Modes Only)
128 bit AES Encryption can be enabled in the radio.
AES Encryption is a feature available in the E Series
Generation II product (firmware pack 4.x.x and above).
Radios that have 128-bit AES encryption enabled can
only communicate with other radios that have AES
encryption enabled and use the same encryption key.
AES Encryption is enabled by selecting the Enabled
button and entering an “Encryption key”. The “Encryption
key” must be between 8 and 16 characters long. The
key can contain ASCII or hexadecimal characters. When
entering hexadecimal characters, the format must be
“0xDD” where DD is a sequence of hexadecimal digits.
(0-9,A-F).
When a radio configuration is read from a radio that
already has AES encryption enabled, the encryption
keywillbeshownas“**************”intheprogrammer
to indicated encryption is enabled. Since there is no
mechanism to see the encryption in plain text you must
ensure the encryption key is recorded in a safe and
secure place for future reference.
Note : When AES encryption is enabled in the radio, both
Port A & B packet layer settings may be modified to
ensure compatibility with AES encryption mode.
Note : AES encryption is subject to export restrictions
and may not be available in all countries.
Collision Avoidance (Packet Modes Only)
In a point to multipoint system the master unit (usually a
base station) can transmit at any time and the remotes
will all receive the broadcast signal. However, if more
than one remote unit transmits at a time, then a collision
will occur during the multiple transmissions, resulting in a
loss of data from one or more units.
Two collision avoidance mechanisms have been included
in the modem. The standard (Digital) method utilises
a signalling channel which is embedded in overhead
data transmitted over the radio channel. The second
method utilises detection of a carrier signal to postpone
transmission of data. Both methods require configuration
of several options for successful operation.