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Chapter 25 ½ALOHA Packet Broadcasting: A Retrospect 427
and additional buffering was also necessary. But, much greater flexibility was introduced into the scope of functions which could be performed, due to its programmability.
Later microprocessor designs, such as the Intel 8080 and National IMP-16, have introduced much greater sophistication into the processor chips accompanied by significant processing speed improvements. A newer PCU design, incorporating an Intel 8080 chip, has demonstrated a considerable reduction in hardware complexity accompanied by an even greater degree of processing flexibility. For example, parity generation and checking are done in software with this prototype design.
Buffering has progressed from the simple shift-register storage devices of the TCU to the use of semiconductor RAM devices used in the microprocessor's random-access memory. All of the micro-instructions for the Intel 8080 microprocessor PCU design reside on four PROM chips, providing 1024 bytes of microcode. The randon-access memory consists of 2048 bytes of RAM.
Recent product introductions such as Intel's 3000 series bi-polar chips promise even greater reductions in chip counts and increases in processing power and speed. With machines such as these, bit-by-bit processing can be readily incorporated into software, thus further eliminating the need for external interfacing hardware and simultaneously providing greater flexibility in the implementation of additional functions. A more detailed discussion of communications microprocessors is given in a companion paper in these proceedings [Fralick et al., 1975].
Size and Power
In the earlier versions of the TCU smaller size and power drain of the unit were not considered major design objectives. The first units were designed for ease of access and hardware modifications to these TCU's were made on a fairly casual basis. As more and more of the ALOHANET came into use, however, small size, portability and lower power drain became desirable.
Of particular interest is the possibility of designing low power battery operated portable PCU's for mobile units in the ALOHANET. Since the transmitter power need only be on for a short burst corresponding to the period of the data burst, the average power of the transmitter can be a small percentage of the peak power. Since low power and small size were not original design objectives, it appears that the construction of low power portable PCU's will involve redesign of several subsections of the PCU and some new design efforts. Of particular importance is selection of a microprocessor unit which provides a minimum power-drain computer architecture consistent with functional requirements. The modem should be redesigned to use MOS devices to minimize power drain, and the transceiver designed for minimum complexity.
Conclusions
As the system has been modified during the past several years it has become apparent that packet broadcasting architecture is remarkably flexible in its tolerance of hardware, system and protocol modifications. This flexibility follows from the packet verification algorithms which lie at the basis of packet broadcasting. The only packets accepted by a remote unit or by the MENEHUNE are packets which meet all the tests expected by the potential acceptor, and the only system resource consumed by an unaccepted packet is the capacity of the channel during the short burst of the packet duration. Thus it is perfectly feasible in a packet broadcasting network to introduce a new form of packet (new in format, new in packet length, or even new in modulation technique) without disturbing any unit operating with the existing scheme. Only the units designed to look for the new packets will accept these packets and all other units will simply discard them.
We plan to employ this property of packet switched channels to switch the polynomial used for error control in the present packet format. The new polynomial is available in a single IC chip and will allow the possibility of error correction as well as error detection in some cases. As remote units with new packet formats are put into operation we can continue to operate the existing remote units without modification as long as we have a single unit capable of accepting the new packet format at the MENEHUNE. As a side benefit of the introduction of this modification we also note that we have effectively doubled the number of user addresses in the system. An address in use with the old packet format may be reused with the new, since each is effectively invisible to the other.
Another result of our ALOHANET experience, current technology, and recent theoretical work on ALOHA channels, is that a single-channel network configuration appears preferable to the two channels used in our present system. The major reason why this is so has to do with the broadcast property of the single-channel system, in which all nodes can (for a given geographic range) hear the transmission of all other nodes in the net.
A number of desirable properties result from this broadcast feature. First, each node can determine if the channel is free before transmitting, greatly reducing the number of packet conflicts-Kleinrock and Tobagi [1976] have shown analytically that this can increase the throughput of a random access channel by a factor of three to five for reasonable user delays, depending on the propagation times between nodes. Second, the problem of sending acknowledgments from user nodes is resolved in a simple manner. Third, system bandwidth can be optimally allocated to both directions of traffic by simple time-sharing of the channel. Fourth, single channel repeaters require only half the radio hardware of two-channel repeaters, and, in fact, the radio transceivers at all nodes need be only half duplex. Finally, a