Introduction
David Caulkins Cable Data 101 Walch Road Palo Alto CA 94304
This paper is divided into two sections.
1) A brief tutorial on computer networks; and
2) A discussion of possible designs for Personal Computer Networks (PCNETs - a term coined by Einar Stefferud).
Computer Networks - The 2S¢ Lecture History.
Details of Network Function.
Figure 1 is a diagram of a network with a connectivity of 3. There are 16 nodes (A-Q) and 26 links (1-26). Suppose the network uses dial-up· phone links, and that node C sends a single packet (of a format like Figure 2) message to node P. C uses a routing algorithm to select one of its neighbor nodes (F,G and D) - G is picked as being on the most.direct route from C to P. C initiates a phone call to G and transmits the packet.
When G has successfully tested the packet for correctness (by means of the error detecting Cyclic Redundancy Code) it returns a short "I-got-it-OK" (or node-to-node ACK) packet to C. I f C and G have no further packets for each other, the phone call is terminated and G initiates a phone call to M. When M has successfully received the packet it returns an ACK packet to G.
The research that.!II8de computer networks possible was done G can·now use the buffer space occupied by·the.message packet by PaulBa~an· at ~~d· in the early sixties [1]. The first passed to M for new packets. After all G-:-M traffic is disposed computer network of any consequence was the ARPANET, brought of, the.G-to-M call is terminated and M initiates a call to P.
from the concept stage to operation largely through the efforts Since P is the destination, it originates two -ACK packets - the of Larry Roberts (then head of the information processing usual node-to-node ACK to M, and an end-to-end ACK addressed to section of ARPA) in the late sixties and e~rly seventies. C. When C receives this ACK it knows the message made it all
One of ARPANET's most valuable facilities is the message the way to P. If a transmitting node fails· to receive an ACK, service - the ability to send a message (generally a file of it retransmits the packet. In this way, messages passing English text, although almost any file can be sent) from one through the net can be made to have
an
error rate substantially"mailbox" in a network computer to another such "mailbox" in a lower than that of the links themselves.
different computer. This doesn't sound very dramatic, but it Figure 3 shows the network of Figure 1 with 1 nodes (A,B, is surprising how powerful and efficiency-improving such a E,G,H,M and N) out of service. Messages can still flow from message exchange facility is. What keeps ordinary message C to P by either of two paths. Thus, even networks of low services, such as telephone, telegraph and mail, from working connectivity (3) can function in the absence of many nodes.
as well seems to be a combination of factors. too slow (mail);
often hard to catch someone in the office and/or pierce the screenS of secretaries (phone); hard or time consuming to use (mail,telegrams); lack of a positive indication of message delivery (mai1,te1egrams); expensive in terms of characters per dollar (telegrams,phone); etc. A computer based message system overcomes most of these difficulties.
People regularly using such a system rapidly develop a whole new communication style. Most messages are brief - 500 characters or less. Message text is composed by the sender at his terminal without the filtering and delay of secretarial intermediates, and style tends to be· much more informal and direct than conventional media, such as business letters.
Message system users move rapidly toward a computer-based personally oriented file system containing messages, distri-bution lists, text files, etc. The difference is one of kind, not just of degree.
Operation.
Some definitions will be useful in discussing networks:
Circuit Switched Network - A network in which a direct circuit connection is set up between communicating nodes.
Connectivity - The number of links terminating at a particular node.
Demand Access - Allocation of transmission bandwidth on demand.
Distributed Computer Network - A collection of geographically distributed and autonomous computers sharing common transmission and switching media.
Hop - Transmission of a packet from one network node to the next.
Lin~ - The communication channel between two adjacent nodes.
Node -. A computer system capable of receiving, testing, storing a~ transmitting packets using one or more links to other nodes.
Packet - A short block of data used to carry information through a distributed computer network. Packets typically contain three kinds of information. the data or message to be transmitted; addressing and control information; and data integrity checking codes.
Packet Switched Network - A network in which packets are routed
• from the originating node through intermediate nodes to
the destination node. ,
Protocols - Sets of conventions (formats, control procedures) which facilitate all levels of intercomputer communication.
Includes electrical interface conventions, line control procedures, digital communication network interfaces, inter-process communication conventions, and application level (e.g. file·transfer, database retrieval, etc.) standards".
(Many of these definitions are derived from· a list developed by Vint: Cert: {2].)
Bytes {16
16
i4
Connectivity 3 Computer Network Figure 1
100 - 500 MESSAGE
POSSIB~E HOBBYIST PACKET FORMAT Figure 2
I
CRCs} I
FIRST COMPUTER FAIRE PROCEEDINGS BOX 1579, PALO ALTO CA 94302 PAGE 145
o
o o
o
19
Reduced Network Figure 3
o
PCNETs could also be implemented using radio links. A similar system has been in operation for some time. The ALOHA packet radio demand access system was built at the University of Hawaii by Norm Abramson, Frank Kuo and others. It is a time sharing system in which communication between many terminals and a central computer is carried out on two radio frequencies;
one for terminal-to-computer communication and the second 'for computer-to-terminal communication. Terminal-to-computer com-munication is carried on in a novel way - any terminal with a new packet ready simply tran'smits it. When this packet is received by the central computer and checked for correctness, an acknowledgement (ACK) packet is sent back to the originating terminal. If two terminals happen to transmit packets during the same time interval, the packets "collide;" neither packet will be correctly received and no ACK packets will be sent.
In this case each terminal waits for a randomly selected period of time and retransmits the packet for which no ACK was received. The random periods are important; if both terminals waited the same period, the retransmitted packets would again collide. Mathematical analysis shows that this scheme allows up to 18\ full utilization of channel capacity before packet collisions become too frequent for reasonable channel use.
Performance can be improved (if the electromagnetic propagation time between sender and receiver is a small fraction of the packet transmission time) in several ways; two seem particularly applicable to PCNET:
1) Slotting. All nodes maintain a clock indicating the start of each packet interval. Packet transmission is started only at the beginning of each interval. This means packets will either overlap completely or not at all. Slotti~g
increases channel utilization to about 37\.
2) Carrier sense. A node with a packet ready to transmit looks to see if carrier is present on the channel; if so the node waits and tries again later.
For more background on computer networks'and packet radio, see references 2 through 13.
Personal Computer Networks
Why should a personal computer network be built? Several reasons come to mind:
1) It should be fun.
2) It is socially desirable; by providng personal computer users the means to gossip, exchange software, participate in message based conferences, create community information
exchanges, etc., the American penchant for communications can be used to maintain and perhaps even accelerate the growth of personal computer use. This can result in a significant frac-tion of the U.s. population becoming "computer literate.1I
I think there are a number of attributes a personal computer network should have (caveat: there seem to be as many views of what a personal computer network should be as there are people interested in the subject for example, see [14] -so the following list is not meant to imply consensus).
1) The network to be composed of geographically distributed nodes, each consisting of a personal computer equipped with low cost network int.erface hardware and a standard Network Support Software
(NSS) package.
2) The network to provide modest size message or file transfer between any two active network nodes.
3) Low or zero cost inter-node communications.
4) The network to be distributed; no requirement for computational, storage or control resources external to the nodes.
5) Reasonable network reliability even though operating with noisy communication channels and with a fraction of network nodes lIout of service.1I Positive acknowledgement to the message originator upon successful transmission.
6) Network architecture and protocols designed to allow expansion of capabilities without step change requirements in node hardware.
7) Network designed for automatic operation to take advantage of higher probable availability of node hardware and communication link bandwidth during late night and early morning hours.
If I had to boil all seven down into one it would be: The net-work must be cheap and reliable.
o~---~o
.
2
1 . 0 ) ' 0 , 0
-4
Network Connectivity Figure 4
FIRST COMPUTER FAIRE PROCEEDINGS BOX 1579, PALO ALTO CA 94302 PAGE 146 .. transmitter complete the set of equipment required. These could operate in the UHF band (for purposes of comparison, the duplex serial interface, and some signals for communication link control. The node of Figure 5 is intended for use with dial-up telephone system links. The MODEM translates between digital signals and FSK tones for voice phone line data omni-directional antenna. CB equipment with this kind of performance is available today for less than $200. It is not unreasonable to hope that simpler (single frequency) transceivers. for Packet radio might cost about $500, even though initial production
take the phone "off the hook," terminating the ringing and opening the line for the incoming data. For out-going data the computer sets the MODEM to "originate," takes the phone
of~.hpo~before dialing, and the dialer translates computer s{gnals into touch-tone or dial-click dialing signals. After dialing is complete the computer waits until the ring detector shows that ringing has started and stopped at the dialed phone, indicating that the target node has gone off-hook. The
computer then checks for the proper "hand shake" of FSK s·ignals, indicating that the two MODEMs are successfully communicating.
The whole process (from off-hook to end of "hand shake") should
-Offline Storage 1---->,.1 OFF-HOOK CONTROL VOICE/DATA SHARING
Node Hardware Block Diagram (dial-up phone system) the interface hardware changes.
The MODEM is designed to operate at the higher' da.ta rates
Node Hardware B·lock Diagram (packet radio system) miles) can be built using store-and-forward techniques. Each node has a number (3-6) of neighbor nodes within its local (references 8-13) experience proves the practicality of packet radio. A packet radio PCNET would benefit from increased,link bandwidth and freedom from dependence on the telephone system.
This seems to me an area where radio amateurs have an interesting opportunity, becaus~ commericallyavailable radio equipment is expensive and not well suited to this application. be a standard, carefully debugged and tested, "bullet-proof", piece of code. A person wishing to use his system as a PCNET node would receive his copy of theNSS from some form of PCNET Control Center; with the correct node interface equipment the NSS could be bootstrap loaded'from a nearby active PCNET node.
Some NSS functions seem to be required for almost any set
FIRST COMPUTER FAIRE PROCEEDINGS BOX 1579, PALO ALTO CA 94302 PAGE 147 of protocols:
o Routing - selection of the optimum neighbor node for mess~ge transmission.
o Check code generation and checking.
o ACK packet generation.
o Store-and-forward packet buffer management.
o Link interface control.
Regulatory Issues
My background is in computer system design; my knowledge of the legal and political areas of communication system control
by the Federal Communications Commission (FCC) and State Public "~~~~
,ji
Utilities Commission (PUC) is that of a layman and is very il limited. With this caveat, let me offer a few thoughts on these matters.
OXNARD
Regional Network Figure 7
40 Nodes Connectivity '= 3 Each Link';! 10 miles