ALOHA Packet Broadcasting: A Retrospect1
R. Binder / N. Abramson / F. Kuo / A. Okinaka / D. Wax
Packet broadcasting is a technique whereby data is sent from one node in a net to another by attaching address information to the data to form a packet-typically from 30 to 1000 bits in length. The packet is then broadcast over a communication channel which is shared by a large number of nodes in the net; as the packet is received by these nodes the address is scanned and the packet is accepted by the proper addressee (or addressees) and ignored by the others. The physical communication channel employed by a packet broadcasting net can be a ground based radio channel, a satellite transponder or a cable.
Packet broadcasting networks can achieve the same efficiencies as packet switched networks [Roberts, 1973b], but in addition they have special advantages for local distribution data networks [Kahn, 19751 and for data networks using satellite channels [Abramson, 1973a]. In this paper we concentrate on those characteristics which are of interest for a local distribution data network. In particular, we discuss the lessons learned in the design and implementation of the ALOHANET, a packet broadcasting radio network in operation at the University of Hawaii since 1970. A number of design issues which arose in the construction of the system are defined, our solutions are explained, and in some cases they are justified. The lessons learned from the ALOHANET are used to indicate how such a radio packet broadcasting system might best be built using the technology available in 1975.
In the next section a brief description of the ALOHANET and its rationale is given. This is followed by a detailed discussion of the major system protocol choices that have evolved, pointing out some related theoretical work where appropriate. Choices concerning the design of the radio communication subsystem are then examined, followed by an evolutionary view of the important impact microcomputer technology has had on the user interface design and resulting system capabilities. The concluding section summarizes our present views with respect to the basic system configuration and properties of packet broadcasting nets.
The ALOHANET is the first system which successfully utilized the packet broadcasting concept for on-line access of a central computer via radio. Its primary purpose is to provide inexpensive access to one or more time-sharing systems by a large number of terminal users, typically in the hundreds. However, it also allows user-to-user communication within the net and is evolving toward use in a more generally oriented computer communications environment.
The present network configuration makes use of a broadcast channel for only one direction of traffic flow. (As we shall see in later sections, the lack of a broadcast capability in the other direction has seriously handicapped the development of effective protocols in certain areas). Two 100 KHz channels are used in the UHF band-a random access channel for user-to-computer communication at 407.350 MHz and a broadcast channel at 413.475 MHz for computer-to-user messages. The original system was configured as a star network, allowing only a central node to receive transmissions in the random access channel; all users received each transmission made by the central node in the broadcast channel. Recently the addition of ALOHA repeaters has generalized the network structure.
A block diagram of the present operational ALOHANET is shown in Fig. 1. The central communications processor of the net is an HP 2100 minicomputer (32K of core, 16 bit words) called the MENEHUNE [Binder, Lai, and Wilson, 1974] (Hawaiian for IMP) which functions as a message multiplexor/concentrator in much the same way as an ARPANET IMP [Heart et al., 1970]. The MENEHUNE accepts messages from the UH central computer,
1Proc. APIPS NCC, 1975, pp. 203-215.
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