Page 276 - Anatomy of a Robot

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COMMUNICATIONS 261
arrangements that are quite general. These same types of shared access arrangements
are used in many different communication standards. If a communication system is
functionally identical to these systems, then the math pencils out the same way. The lim-
its on effective bandwidth are very real.
TIME DIVISION SYSTEMS

Shared access to communications link can be accomplished by dividing the like by time
division, frequency division or code division.

Aloha System
The Aloha communications system was designed so a sender could simply transmit a
packet on the channel whenever it wanted to. If another sender was sending a packet at
the same time, they would collide and both packets would be lost. As more and more
senders compete for the channel, the system rapidly loads down. The way the math pen-
cils out, only 18 percent of the channel’s raw bandwidth is truly available once the
system loads down. Normal 10BT LAN systems work like this; collisions ruin the data
packets. As a 10BT LAN starts to load down with more and more users, the overall
effective bandwidth of the 10BT systems is not the raw bit speed of 10 Mbps but
is closer to 1.8 Mbps. On a 10BT LAN, this limit can only be improved if the users
cooperate.

Slotted Aloha
The Aloha system can be improved if the senders are synchronized. Each sender knows
when the timeslots occur and can only start to transmit at the beginning of a timeslot.
Collisions still occur, but this sort of cooperation between senders increases the effec-
tive throughput of the channel to about 35 percent of its raw bit speed.

Reserved Aloha
If the senders politely reserve timeslots in advance, the effective throughput of the chan-
nel increases yet again. Although some bandwidth is wasted making the reservations,
collisions are largely eliminated and the efficiency can be high. Only the reservation
timeslots are wasted. Reservations can be granted in multiple ways, including round-
robin, priority systems, and random selection. It is up to the robot designer to determine
what sort of “request-grant” system to adopt.