User's Manual

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Page 52

E Series Data Radio – User Manual

postpone transmission of data. Both methods require conﬁguration

of several options for successful operation.

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:

• Mode – “Master” or “Remote”. When the master unit

receives a valid transmission from a remote unit it transmits

a carrier signal to indicate busy. Of course the master

will also initiate a transmission if it has data to send. The

transmitted carrier is interpreted by the other remotes to not

transmit. Once the transmission from the ﬁrst remote ends

the master unit stops transmission 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 transmit a carrier signal. During this

period collision of remote transmissions can still occur and

is unavoidable.

Note: The master can only be a full duplex unit and cannot

be permanently transmitting. For half duplex and simplex

systems all units should be set as “Remote” (no Master).

• 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 detects a carrier signal from