C-SPEC CORPORATION WIRELESS PRODUCTS
CLARION MerLAN M10 13
Channel Access Protocol
On Ethernet both carrier sensing and collision detection are simple processes.
For any radio channel the ability to detect collisions is lost; in addition, for a
spread-spectrum system with changing codes (for security) the ability to
perform carrier sensing is limited to acquisition of the preamble portion of a
transmission. The channel-access protocol (CAP) employed in the M10 is
called P-persistent CSMA.
When an M10 has scheduled a transmission, the start time of that
transmission is selected randomly from a grid of P possible start times, these
being separated by the slot time. The slot time is somewhat larger than the
one-way propagation time; this ensures that if only one transmission is
started at a slot boundary, then all other M10s can detect that transmission
and cancel any other pending transmissions. Of course, there remains the
probability 1/P that any two radios might select the same slot boundary on
which to transmit, and then the two transmission will collide. In dense
environments selecting a large value for P keeps the network from collapsing,
while in less-dense environments a small P gives higher throughput. With P
as a configurable parameter (in fact, dynamically configurable) the M10
supports a variety of user-controlled CAP strategies.
The M10 provides for adaptive P-CSMA by using a sequence of P values.
These are loosely tied to the transmission attempts in that the P values in the
sequence correspond to the attempt number, but any successful receive resets
to P0 for the next attempt. If that attempt fails, the sequence of P values is
resumed according to which attempt is in progress. (P is a power of two, up to
256.)
P0,...,P7 Default: 4,8,16,32,64,128,256,256. One of three buttons may be set:
Exponential causes each P to be double the previous in the sequence; this is
modeled after exponential backoff of Ethernet. The user sets only P0.
Constant causes only the P0 value to be used, i.e., non-adaptive. The user
sets only P0.
Custom enables the user to select any arbitrary adaptation strategy.
Retransmission Protocol
The retransmission protocol is key to high throughput in any radio LAN.
Without retransmissions at the MAC layer, any lost frame would incur the
relatively large time-out of the layer-4 (Transport layer) reliability protocol
before a missed frame is sent again. The M10 retransmission protocol has a
window of 1 such that it focuses its effort on the delivery of a single frame for
up to eight (8) transmission attempts, and must either succeed or fail on that
frame before moving to the next available (buffered) frame. When
retransmissions are enabled, a transmitting M10 marks its transmitted
frame as requiring an ACK, transmits the frame, then after the transmission
is completed waits for an ACK. If after waiting for ACK_timeout time and no
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