previous | contents | next
Chapter 25½ ALOHA Packet Broadcasting: A Retrospect 417
an IBM System 360/65 running TSO (as of December 1974, a 370/158) or from ALOHA's own time-sharing computer, the BCC 500, or from any ARPANET computer linked to the MENE HUNE via the ALOHA TIP [Ornstein et al., 1972]. Outgoing messages in the MENEHUNE are converted into packets, the packets are queued on a first-in, first-out basis, and are then broadcast to the remote users at a data rate of 9600 baud.
The packet consists of a header (32 bits) and a header parity check word (16 bits), followed by up to 80 bytes of data and a 16-bit parity check word. The header contains information identifying the particular user so that when the MENEHUNE broadcasts a packet, only the intended user's node will accept it. More will be said about packet formats later.
The random access channel (at 407.35 MHz) for communication between users and the MENEHUNE is designed specifically for the traffic characteristics of interactive computing. In a conventional communication system a user might be assigned a portion of the channel on either an FDMA or TDMA basis. Since it is well known that in time-sharing systems, computer and user data systems are bursty [Jackson and Stubbs, 1969], such fixed assignments are generally wasteful of bandwidth because of the high peak-to-average data rates that characterize the traffic. The multiplexing technique that is utilized by the ALOHANET is a purely random access packet switching method that has come to be known as the pure ALOHA technique [Abramson, 1973b]. Under a pure ALOHA mode of operation, packets are sent by the user nodes to the MENEHUNE in a completely unsynchronized manner-when a node is idle it uses none of the channel. Each full packet of 704 bits requires only 73 msecs at a rate of 9600 baud to transmit (neglecting propagation time).
The random or multi-access channel can be regarded as a resource which is shared among a large number of users in much the same way as a multiprocessor's memory is "shared." Each active user node is in contention with all other active users for the user of the MENEHUNE receiver. If two nodes transmit packets at the same time, a collision occurs and both packets are rejected. In the ALOHANET, a positive acknowledgment protocol is used for packets sent on the random-access channel. Whenever a node sends a packet it must receive an acknowledgment message (ACK) from the MENEHUNE within a certain time-out period. If the ACK is not received within this interval the node automatically retransmits the packet after a randomized delay to avoid further collisions. These collisions will limit the number of users and the amount of data which can be transmitted over the channel as loading is increased.
An analysis [Abramson, 1973b] of the random access method of transmitting packets in a pure ALOHA channel shows that the normalized theoretical capacity of such a channel is 1/2e=0.184. Thus the average data rate which can be supported is about one sixth the data rate which could be supported if we were able to synchronize the packets from each user in order to fill up the channel completely. Put another way, this result shows the present 9600 bit/second channel could support between 100 and 500 active teletype users-depending upon the rate at which they generate packets and upon the packet lengths.
ALOHA NET Remote Units
The original user interface developed for the system is an all-hardware unit called an ALOHANET Terminal Control Unit (TCU), and is the sole piece of equipment necessary to connect any terminal or minicomputer into the ALOHA channel. As such it takes the place of two dedicated modems for each user, a dial-up connection and a multiplexor port usually used for computer networks. The TCU is composed of a UHF antenna, transceiver, modem, buffer and control unit.
The buffer and control unit functions of the TCU can also be handled by a minicomputer or a microcomputer. In the present system several minicomputers have been connected in this manner in order to act as multiplexors for terminal clusters or as computing stations with network access for resource sharing. A new version of the TCU using an Intel 8080 microcomputer for buffer and control has been built. Since these programmable units allow a high degree of flexibility for packet formats and system protocols, they are referred to as PCU's (Programmable Control Unit). A more detailed discussion of terminal considerations is given in a companion paper in these proceedings [Fralick et al., 1975].
Since the transmission scheme of the ALOHANET is by line-of-sight, the radio range of the transceivers is severely limited by the diversity of terrain (mountains, high rise buildings, heavy foliage) that exists in Hawaii. A recent development has allowed the system to expand its geographical coverage beyond the range of its central transmitting station. Because of the burst nature of the transmissions in the ALOHA channel it is possible to build a simple store-and-forward repeater which accepts a packet within a certain range of ID's and then repeats the packet on the same frequency. Each repeater performs identically and independently for packets directed either to or from the MENEHUNE. Two of the repeaters have been built which extend coverage of the ALOHANET from the island of Oahu to other islands in the Hawaiian chain. These repeaters are discussed in more detail in the following section.
Two fundamental choices which have dictated much of the system protocol are the two-channel star configuration of the original network and the use of random accessing for user transmissions. Investigation of the random accessing principle using radio was in
previous | contents | next