400 Part 2 ½ Regions of Computer Space Section 5 ½ Networks
 

Table 4 Network Reliability Summary
 

		IMP down				Average host traffic
Month	Line outage	All causes*	Hardware/software			No. of nodes	Packets/day
			Percent	MTBF, h	MTTR, h: min		Internode	Intranode
September '77 October November December January'78 February March April May June	0.12% 0.43% 0.49% 0.59% 0.35% 0.21% 0.31% 0.19% 0.54% 0.17%	0.41% 0.63% 0.41% 0.38% 0.43% 0.30% 0.37% 0.43% 0.51% 0.30%	0.11 0.16 0.06 0.04 0.13 0.09 0.03 0.18 0.10 0.10	805 1,116 1,368 1,484 814 1,053 1,438 773 811 1,062	0:52 1:46 0:50 0:37 1:02 0:59 0:25 1:25 0:50 1:06	58 58 58 59 59 59 59 60 60 60	6,978,967 7,555,457 7,926,677 7,485,728 7,435,571 7,619,331 8,116,198 8,315,295 8,655,575 8,297,662	2,254,913 2,899,477 3,379,314 3,712,888 3,291,647 3,277,811 2,589,444 1,903,111 1,976,583 1,815,891

*Includes P.M., site environmental problems, retrofits, and other causes.

rock and Naylor [1974]. Up until that experiment, traffic on the ARPA Network had been growing exponentially from 10⁵ packets a day in October 1971 to 4 ´10⁶ packets a day in August 1973. Table 4 summarizes more recent network traffic.

Another experimental study of the ARPANET was conducted in May 1974 [Kleinrock, Naylor, and Opderbeck, 1976], focusing on determining network overhead. This second study also confirmed the gross network behavior of the August 1973 experiment.

Figure 13 indicates the number of bits added to each message for protocol and control at each level in the ARPA Network. The efficiency of the network (i.e., the number of data bits transmitted divided by the total number of bits transmitted) is a function of user traffic characteristics (whether it consists of a small number of large data files or a large number of small data files). Figure 14 depicts the measured data, indicating only 8.8 percent data and 91.2 percent overhead distributed among the various levels. Since the line utilization was low (6.73 percent), causing the background tasks to dominate the transmission, Kleinrock, Naylor, and Opderbeck projected the capacity distribution assuming a saturated network with the same traffic characteristics. The projection indicates that data increase to 23.4 percent and overhead decreases to 76.6 percent. Even though network protocols are

constantly evolving, the network designer should realize that a large portion of the network capacity will be devoted to network protocol and control.
Control of the ARPA Network was turned over to the Defense Communications Agency (DCA) in 1977.

The ALOHA Network

A development concurrent with the ARPA Network was the ALOHA Network, which became operative in 1970 at the University of Hawaii. Initially, the ALOHA Network was designed