AMICON SYSTEM SPECIFICATION
Draft Revision 1.01 April, 1980
This is a draft version of specifications for use of the data communications special service channel (L2) on the AMSAT Phase III satellite, the communications medium which serves as the foundation of the AMSAT International Computer Network (AMICON). This document is subject to final approval by the Board of Directors of AMSAT. As a draft document it is subject to discussion, negotiation, further study, and potential rewrite of major sections. This document has not been approved for general publication. The contents herein represent the current thinking of the authors, and your comments and criticisms
will be most welcome.
Comments and questions should be directed to: AMICON System Architecture Design Group
c/o
H. S. Magnuski, KA6M, 311 Stanford Ave., Menlo Park, CA 94025
(415) 854-1927
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01
FOREWORD
The Radio Amateur Satellite Corporation (AMSAT, P.O. BOX 27, WASHINGTON, D-C. 20044) will launch, courtesy of the European Space Agency, their Phase III satellite sometime during May, 1980. Unlike previous satellites OSCAR 46775, .6; etcvy; the Phase III machine will be in an orbit permitting extended communications periods for stations in the coverage area- Effective use of its 70 cm to 2 meter transponder will require more detailed planning and coordination than with previous OSCAR’s, and a bandplan for the 180 kHz passband has already been approved by the AMSAT Board of Directors. Part of the plan makes provisions for six special service channels (General Voice Bulletins, Education Services, Scientific Services, Traffic, CW/RTTY bulletins and code practice, and Data Communications). The procedures for use of the Data Communications Special Service Channel, also known as Special Service Channel ‘Lower 2’ (SSC L2), is the concern of this document.
The explosive growth of the use of computers by radio amateurs, coupled with the potential of this new communications medium lead to fantastic possibilities for the establishment of two-way computer links, computer networks, packet radio gateways for long haul traffic, and even digitized voice or video. The realization of this potential, however, requires that communications standards be established so that common equipment and protocols can be used and shared by all interested operators. The standards must also consider and be compatible with other users of the spacecraft transponder. Thus, the contents of this document not only prescribe. frequency assignments and modulation techniques, but also outline rules for time-shared use of the channel, packet layout, network protocols and other related matters.
The authors realize that standards are a two-edged sword, and have tried to obtain a balance between weak standards, which allow development in too many different directions, and overly restrictive standards, which could stifle creativity.
Contributors to this document include:
Vern Riportella WA2LQQ AMICON Coordinator
H. S. Magnuski KA6M N. Cal. AMICON Coordinator Mark Kaufmann WB6ECE Network Design Consultant Gary Hendra WA6S UW Digital Systems Engineer Gary Fariss WO6KYF Software Systems Engineer
and many other radio amateurs who have offered constructive criticisms of the plans detailed herein.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01
PREFACE
Amicon System Specification for the AMSAT Phase III Satellite Channel L2
The AMSAT Board of Directors considering
a) that there is an urgent need for a common modulation method and accepted set of channel usage procedures (Level 1 Interface) ;
b) that there should be a specified format for transmitting message blocks over the channel (Level 2 Interface) 3
c) that there is a compelling need for coordination among the stations wishing to transmit and receive messages and files on the system (Level 3 Interface);
d) that future use of the channel would be greatly enhanced by commonly agreed to specifications for the most heavily used types of data communications (Level 4 Interface);
unanimously declares the view
that the following system specifications be adhered to by all stations using the special service channel for data communications on the AMSAT Phase III Satellite.
[Note: Text within square brackets in the following document is background discussion material designed to inform the reader of some of the issues involved in the design of the specification. It is not part of the formal document and is subject to deletion once the final draft is approved.
The current practice for specifying network architectures is to define independent functions in separate groups or layers. This document follows that design principle by proposing that AMICON be split into four distinct levels. The first level deals with the transmission channel and defines how a bit stream is transmitted between two stations. The second level superimposes characters and blocks of characters on the bit stream. The third level describes how blocks of characters are routed and sequenced through the network. The fourth level deals with the transmission of information which may span multiple blocks and which requires end-to-end checking.]
Digitized by the Internet Archive in 2025 with funding from Amateur Radio Digital Communications, Grant 151
httos://archive.org/details/amicon-sys-spec-draft-3
AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01
Table of Contents
Chapter 1 - Level 1 Interface: Physical Interface
POL Channel Assignment and Characteristics 1.2 Channel Access and Usage
LES Carrier and Modulation Specifications 1.4 Transmission Timing
Chapter 2 - Level 2 Interface: Packet Transmission
2-1 Packet Framing Specifications
2.2 Transmission Code
2.3 Channel Multiple Access Protocol
2.3.1 Definitions
2.3.2 Control Parameter Notation
2-3-3 Control Using the Simple ALOHA Algorithm 2.3.3.1 Simple ALOHA Transmission Control 2.3.3.2 Simple ALOHA Retransmission Control Die sa3 Simple ALOHA Control Parameter Values 2.3.4 Control Using the S-ALOHA-CLC Algorithm 2-3-4.1 Closed Loop Control Assumptions 2.3.4.2 Closed Loop Control Algorithm
23. 438 Closed Loop Control Parameter Values
Chapter 3 - Level 3 Interface: Network Specifications
3.1 Datagram Network Characteristics 3-2 Packet Format
3.3 Packet Node Addressing
3.3.1 Packet Node Addressing Syntax Roe2 Specific Call Group Syntax 3.3.3 General Call Group Syntax 323.4 Call Group Qualifiers
3.3.5 Call Group Addressing Examples 3.4 Packet Data Field
Chapter 4 - Level 4 Interface - Applications
4.1 File Transfer Protocol
4.2 Graphic Standards
4.3 Image and Video Transmission 4.4 Digitized Voice Transmission
Appendix A - Selected Bibliography
Appendix B - ISO Specification 3309
Appendix C Datagram Addressing Syntax Diagram Appendix D - Distribution List
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 1
Chapter 1
LEVEL 1 INTERFACE: PHYSICAL INTERFACE
1. 1 Channel Assignment and Characteristics
The AMSAT Phase III satellite has one inverting transponder with a 70 cm uplink and a 2 meter downlink. The bandplan assignment is based on the 2 meter downlink, which serves as the frame of referencee Subject to final calibration after launch, the passband center is 435.215 MHz for the uplink, and 145.900 for the downlink. There are two beacons marking the edges of the downlink passband, the lower "General" beacon at 145.810 MHz, and the upper "Engineering" beacon at 145.990 Mhz. The special service channels Ll, L2 and L3 are allocated spectrum at 17, 21 and 25 kHz center frequencies above the General beacon. Channels Hl, H2 and H3 are located at 17, 21 and 25 kHz center frequencies below the Engineering beacon. The L2 channel has been allocated for data communications and computer networking (nominally 435.284 MHz uplink, 145.831 MHz downlink).
The originating station must control the 70 cm uplink frequency such that the 2 meter downlink frequency (as monitored at the originating station) is at the specified offset from the pilot beacon to within a tolerance of +/- 1.0 kHz. Use of the SSCs with equipment incapable of this tolerance is discouraged.
[The 3 dB bandwidth being specified for the other channels is 2.4 kHz, and the data communications spectrum will probably have to meet this spec. The 1.0 kHz frequency tolerance was taken from the June 1979 AMSAT Newsletter, and may prove to be too loose for the L2 channel.]
1.2 Channel Access and Usage
In order to provide maximum channel utilization and _ to eliminate contention for channel time, a well organized system of coordinators and procedures is essential. Authority to use the channel comes from the AMSAT Phase III Operations committee-e The special service channel coordinator member of the committee will appoint three regional coordinators to deal with channel usage in their respective regions. The regional L2 coordinator is responsible for assigning time slots for different modes of operation and L2 usage. Within a given timeslot, where the modulation and protocol implementations are compatible, access to the channel will be governed by the algorithms specified in Chapter 2.
1.3 Carrier and Modulation Specifications
[The selection of a suitable carrier and modulation scheme will be the subject of some debate within the amateur community, and currently there is no technique which can be considered the preferred method. The relative efficiencies of synchronous transmission probably will preclude any asynchronous modulation method. The following is a list of some of the considerations which are relevant to use of the L2 channel:
Power budget of the transponder Channel 3db bandwidth or signalling speed in b/s per Hz.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 2 Level 1 Interface: Physical Interface
Doppler shift
Frequency offset between sequential users of the channel Eb/No (dB) for the modulation technique
Performance in the presence of cw interference
Channel capture capability
Complexity for the implementation
Cost of implementation and availability of hardware Regulatory and licensing considerations
Two of the more frequently mentioned techniques are SSB and AFSK-FM. Karl Meinzer, in the June 1979 AMSAT bulletin, has outlined the use of uncoded PSK for the telemetry channel of the satellite.
What we really need now is a written proposal covering the three major modulation methods and an evaluation of each method in terms of the criteria outlined above. A summary of the principle methods includes:
AM - On-Off Keying with non-coherent detection Quadrature Amplitude Modulation Quadrature Partial Response
FM - FSK, non-coherent detection CP-FSK, continuous phase FSK MSK, minimum shift keying
PM - BPSK, binary phase shift keying DE-PSK, differential encoded phase shift keying QPSK, quaternary phase shift keying OK-QPSK, offset keyed quternary phase shift keying
A very excellent and current summary of modulation methods for radio work can be found in ’A Comparison of Modulation Techniques for Digital Radio” by John D. Oetting, IEEE Transactions on Communications, Vol. COM-27 No. 12, December, 1979. This article would serve as a good basis for our comparisons of different schemes.]
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 3 Level 1 Interface: Physical Interface
1.4 Transmission Timing
The following description outlines a typical transmission. The times tO, tl, »«.- use as a point of reference the output antenna of the transmitting station. Note that there is a corresponding set of times sO, sl, «+. which may be referenced at the output antenna of the transponder, and a third set r0, rl, --- which may be referenced at the receiver.
tO - The transmitter places carrier on the channel.e The transmitter assumes that the channel is idle and unused prior to t0.
tl - Modulation is placed on the carrier such that all receivers assume a logical one or marking condition. Note that tl may be the same as t0.
t2 - Denotes the start of the first idle flag or synchronization character.
t3 - Defines the transition between the last idling or synchronization character and the first byte of the packet.
t4 - The time at the end of the last checksum or crc byte.
t5 - The time at the end of the last idle flag, syne character or pad character.e The channel goes into a marking condition.
t6 — The instant that modulation is removed from the carrier. Note that t5 and t6 may coincide.
t7 - The removal of all carrier by the transmitter. This time may coincide with t6.
All stations must comply with the station identification requirements imposed by their licensing authority. However, since this channel may be heavily utilized, the time taken for id should be kept to the minimum allowed. All cw identification should occur between tO and t3.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 4
Chapter 2
LEVEL 2 INTERFACE: PACKET TRANSMISSION SPECIFICATIONS
2el1 Packet Framing Specifications
The format of the message block or packet transmitted on the channel shall be in compliance with ISO Standard 3309 ‘High-level Data Link Control Procedures - Frame Structure.” Use of extended address and control fields, as detailed in the standard, is not recommended. The information field length within each packet shall be a multiple of 8 bits.
[The use of HDLC format could be controversial. The standards, techniques and equipment for the use of true, bit-oriented HDLC are still fairly new to _ the industrial world. The requirements for constructing an HDLC frame seem impossibly complex at first, and they would be except for the fact that many semiconductor companies have designed and are currently selling (for prices in the $30-$50 range) chips which do all the hard work. Here is a list of currently available HDLC oriented chips:
Fairchild 3846 Synchronous Data Link Controller Intel 8273 SDLC/HDLC Protocol Controller Motorola 6854 Advanced Data Link Controller Nippon Electric Co. UPD379 SDLC Protocol Controller Signetics 2652 Multi-protocol Controller Standard Microsystems 5025 Multi-protocol Controller Western Digital 1933 Synchronous Data Link Controller Zilog SIO Serial I/O Controller
The HDLC protocol is being designed into new equipment by all major manufacturers, and it forms the basis for the new international packet switching networks. It will be the standard for data communications in the eighties. If we are in the process of creating a digital networking specification for use over the next’ ten years, we should build on what is currently accepted industrial practice, even though most amateurs may be unfamiliar with the details involved. A group of pioneering Canadian operators has already conducted experiments using HDLC chips and established an HDLC beacon on 20 meters, so we have evidence that the technology is not out of reach of the amateur community. The main drawback with specifying bit-oriented HDLC is that it is incompatible with commonly used USARTs, such as the 8251A.
Also, note that this spec only calls for frames from the HDLC standard. The complete HDLC protocol is probably not appropriate for our multiple-access broadcast oriented packet repeater.]
2-2 Transmission Code
The transmission code used for text characters within the message shall meet the standards set by C.C.I.T.T. Recommendation V.3 - International Alphabet No. 5.
[This is the international standard corresponding to ANSI Standard X3.4-1968 “Code for Information Interchange - ASCII.”]
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 5 Level 2 Interface: Packet Transmission Specifications
2-3 Channel Multiple Access Protocol
When channel usage is low, packets may be transmitted using a simple ALOHA protocol. When utilization becomes heavy, packets will be exchanged on the channel using an S-ALOHA protocol with closed loop control.
2-3-1 Definitions
A multiple-access-channel is a communications channel where many transmitting stations can attempt to access a receiving station using a common transmission medium and equipment. The uplink to the Phase-III satellite is a multiple-access channel.
A broadcast-channel is a communications channel where many stations can receive messages from a single transmitting station. The downlink of the Phase- III satellite is a broadcast channel.
The term carrier-sensed multiple-access channel (CSMA) describes a situation where each transmitter is able to detect the carrier (presence of an on-going transmission) from all other transmitters. The Phase III input channel has this characteristic except for the fraction of a second delay at the beginning of a transmission when the input signal has to travel to the satellite and return to receiving groundstations. The use of carrier sensing improves channel efficiency, particularly for longer packets.
Due to the fact that transmissions are occuring at random with no centralized control, there is the possibility of overlap of transmitted packets or collisions, where two or more transmitters are on the air at the same time. For efficient use of the channel it is important that each station be able to monitor its translated signal and check the validity of the returned packet while it is being transmitted. This ability to receive one’s own packets and validate their contents is called a collision detection capability.
At any given time, using the output antenna of the transponder as our point of reference, the channel will be either be inactive with no carrier present or active with carrier. The channel duty cycle is the percentage of time that the channel is active. This measurement should be made over an extended period of time, at least 15 minutes or more.
A set of rules and procedures for controlling the exchange of messages on a communications channel is a communications protocol. Of the many hundreds of communications protocols currently in use there is a set of protocols, known as ALOHA protocols, which are concerned with regulating the flow of messages or packets on communications media where the messages are sent using a multiple- access channel and received on a broadcast channel. The name “ALOHA” is used because much of the initial research and the first implementations were done by the University of Hawaii in the construction of their ALOHA Packet’ Radio Network.
A simple ALOHA protocol allows any station on the network to transmit whenever it’s ready. If the transmitted packet is not received correctly, the transmitter waits some random amount of time and tries again.
A slotted ALOHA protocol (S-ALOHA) requires that all transmissions on the multiple-access channel be synchronized to start and end within specified time
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 6 Level 2 Interface: Packet Transmission Specifications
periods or slots. All slots are of the same duration and can contain the maximum length packet. Each transmitter decides on which time slot to use on a random basis.
A slotted ALOHA closed loop control protocol (S-ALOHA-CLC) allows transmitting stations to adjust their transmission control parameters to accomodate varying load conditions on the channel. Each transmitted packet contains a computed variable which reflects, in part, the success that the transmitting station is having in sending packets. All receiving stations monitor this variable and adjust, through use of the algorithms and _ formulas specified below, their transmit and retransmit controls. Closed loop control of an ALOHA channel allows throughput to approach theoretically maximum limits, provides a mechanism for dynamic changes in the control parameters needed to cope with varying loads, minimizes overall packet delay, and contributes to the efficient use of the channel under heavy load conditions.
2.3.2 Control Parameter Notation
The notation and control algorithms given below were adapted from a paper written by Gerla and Kleinrock (see the bibliography, Appendix A). Their careful study and contribution to the solution of this control problem is acknowledged. Specification of the ALOHA control procedures uses the following variables:
n — The number of stations currently actively using the L2 channel.
i - Stations are indexed by the variable “i” where i ranges from 1 to n.
tau - The time in seconds required to transmit a maximum length packet.
ts - The period of a slot in a slotted channel.
W - The history window (measured in slots) maintained by each station.
E - The number of empty slots in W.
S(i) - Successful packets from station i in W.
S - Total successes in We. Computed by summing all S(i) for i= 1 ton.
C(i) - Collisions suffered by station i in W.
C - The total number of collision slots in W.
C =W- (S + E) C’ - An estimate of the total number of collisions in W. Cc’ = (C(i)/S(i))*s
m - Average number of collided packets per collision. Note that this parameter cannot be measured directly through monitoring the channel.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 7 Level 2 Interface: Packet Transmission Specifications
UC - Interval (in slots) between successive updates of control parameters. Note that UC will be less than or equal to W.
G - Average channel load in window. Computed through the formula: G = (S+C*m) /W
In the closed loop control algorithm presented below G will only be estimated because the true value of m is not available.
Gmax - A ceiling value for the G estimate.
Pn(i) - New transmission probability gate value for station i. At each transmission decision point (the time when a new packet is ready for transmission in the simple-ALOHA protocol or the time when a new packet is ready and we have the start of a slot for the slotted-ALOHA protocol) the transmitter draws a random number ranging from zero to one.e If the number picked is less than or equal to Pn(i), then transmission commences. For the simple protocol Pn(i) = 1 and transmission always occurs immediately, providing the channel is inactive (there is no carrier at the transmit site). The initial value of “Pn(i)° is’ Pn.
Pr(i) — Retransmit probability “gate” value *for “station "0% Atweaen transmission decision point Pr(i) is used to determine if a previously transmitted packet should be retransmitted. Packets will need retransmission if they are not positively acknowledged by the receiver or if the transmitter detects an error or collision through its own monitoring of its transponded output. All packets that need retransmission must be sent first before any new packets are attempted (the retransmit packet queue has a strictly higher priority than the new packet queue)- In the simple ALOHA protocol the transmission decision points occur every tau seconds after an error is detected. In the S-ALOHA protocol there is a decision point at the beginning of each slot. The, initial value of Pr(i) is* Pr’
Prmax -— Ceiling value for Pr(i).
P - The weighted average of all current gate values. The value P is computed by summing Pn(i)*S(i) for all i and dividing by S- The value of Pn(i) is contained in a control byte in each transmitted packet. S(i) is obtained by
monitoring the channel and is based on successfully received packets.
DP - The gate value increment. This parameter is used to compute the new values of Pn(i) and Pr(i) and controls their rate of change.
2.3.3 Control Using the Simple ALOHA Algorithm
If the channel duty cycle is less than 20% there is no justification for requiring closed loop control and simple control procedures will suffice. The protocol, in its most basic form, follows these two steps:
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2. If the packet was not received correctly, the station waits some random amount of time and then retransmits the packet.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 8 Level 2 Interface: Packet Transmission Specifications
The following paragraphs will clarify some of the details concerning the above two steps.
La se saul Simple ALOHA Transmission Control
First, in this section and the next we differentiate between new packets and ones that have been previously transmitted. The probability of transmission control gates Pn(i) for new packets and Pr(i) for retransmitted packets have different values. Pn(i) for this protocol is always 1, implying that transmission will be immediate. Pr(i) will be assigned a value when the network starts up, and may be subject to change as the load grows-e The priority of packets due for retransmission is strictly higher than that of new packets. We will also restrict each station to have only one unconfirmed packet in the air at any given time- Due to the round-trip signal delays involved, in theory it is not absolutely essential that stations hold back transmissions if there is already carrier present on the output channel. The carrier may already be off at the transmitter site using the channel. Carrier sensing will, however, improve throughput, so it is recommended that new transmissions not start if the channel is already active.
2.3.3.2 Simple ALOHA Retransmission Control
Packet reception may be confirmed in two different ways. The term ’end-to- end’ confirmation is used when the higher level processes or programs doing the packet transmitting and receiving positively acknowledge the reception of a new packet. The other confirmation of successful transmission comes from the transmitter’s own receiver and its collision detection circuits. Collision detection circuitry does not guarantee safe reception at the final or ultimate receiver, but it does permit the transmitter to immediately reschedule a packet if it is known to be in error, thus avoiding positive acknowledgement timing delays.
The exact procedure for “waiting a random amount of time” will now be described. This procedure is used for the simple ALOHA protocol because it is consistent with the closed-loop method which follows below.
A station which has determined that it needs to retransmit first waits for the channel to go inactive. It then picks a random number in the range 0 to l, and if this number is less than or equal to Pr(i) it retransmits immediately. If the number is greater than Pr(i), it delays tau seconds, picks a new random number and repeats the test. This cycle is continued until the retransmission occurs.
By using some mathematical tricks we can simplify the above series of tests and still achieve the same result. Again, we wait for the channel to go inactive and then pick a number in the range 0 to l. The time we should wait, twait, is then given by the following formula:
twait = randomenumber * tau * (1-Pr(i))/Pr(i)
The value of twait may be rounded to the nearest tau seconds. If the channel is busy when the time delay expires, the station should wait for the channel to go
inactive and then transmit immediately.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 9 Level 2 Interface: Packet Transmission Specifications
2.3.3.3 Simple ALOHA Control Parameter Values
There are only two parameters subject to adjustment in the Simple-ALOHA control algorithm. Consult AMSAT for currently recommended values of control parameters. Typical values for these parameters are listed below:
econd
tau Ss Pr = 4
= 1 Pr(i) =
2-3-4 Control Using the S-ALOHA-CLC Algorithm
If the channel duty cycle becomes greater than 20%, then there is enough activity on the channel to justify control procedures which will guarantee better throughput under heavy loading and under varying load conditions. The Slotted ALOHA Closed Loop Control algorithm is a method whereby the transmit and retransmit probabilities Pn(i) and Pr(i) are adjusted to accomodate the current channel load, the adjustment causing stations to wait longer when many transmitters are competing for the channel, and then shortening the delay times as traffic is cleared and channel conditions improve.
The key elements of the CLC method are these:
All transmitters are synchronized and start and end their transmissions within fixed time slots. Each transmitter monitors the total number of successfully received packets within a recent time period (the “history window’ , measured in time slots), and also keeps a count of its own’ successes and failures within the window. Each packet transmitted by station i contains a control byte which reflects the current value of Pn(i) for that packet. The Pn(i) values in each packet are recorded by all active stations and _ their values are used in load calculations specified below. With the data thus recorded, a formula is used to update, at predetermined intervals, the Pn(i) and Pr(i) for the station. Increasing values of Pn(i) or Pr(i) imply that every station in the net is having more success and that less delay is required before transmit and retransmit attempts. As the values of Pn(i) and Pr(i) decrease more delay is introduced causing all stations to slow down, thus reducing channel congestion. The following sections give the exact details of the method.
2.3.4.1 Closed Loop Control Assumptions
This method assumes that channel time is divided into fixed periods or slots, where each slot is ts seconds long and can contain the maximum length packet plus transmitter startup and shutdown time. All slots will start and end on the second’s tick from WWV.
Each active station keeps certain statistics over the last W slots and updates its control parameters every UC slots. The update period, UC, will be less than or equal to W.
The station must monitor the channel and count all successfully received packets S. It must also count its own successes S(i).-
Finally, at each update interval the station must compute the weighted average of current gate values. The current Pn(i) value for each station is transmitted in the packet in the protocol load control byte which is the byte
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 10 Level 2 Interface: Packet Transmission Specifications
immediately following the HDLC control field. The right-hand 7 bits of this byte divided by 128 represent Pn(i) for that packet. Use of the high order bit is reserved and it should be set to zero. Stations which are not using closed loop control will set this byte to hex ‘00’. The weighted average gate value, P, is the sum of all received Pn(i) divided by S.
2.3.4.2 Closed Loop Control Algorithm
The following formulas must be used by each station to update Pn(i) and Pr(i) every UC slots.
(a) Estimate Collisions: If C(i) = S (i) = 0 then:
If S = 0, let G = 0 and go to (c) Otherwise, let G = 1 and go to (c)
Tf CCi) > 70" and S01) Sei0 ‘then: Let G = Gmax, P = min(P,Pn(i)) and go to (c) TE Sti) 4>00) then® Let Cs = CU) o*9o7/ S(1))-and 'so.'to (b) (b) Estimate total channel load G: G= (S +C’)/W with 0 <6G < Gmax (c) Derive new probability gates:
Po =) (Ga=31) -* "DP 0 Pn(i)
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2-3e4-3 Slotted ALOHA CLC Parameter Values
Consult AMSAT for currently recommended values of control parameters. The following are typical values:
ts = l Slot size in seconds
W = 64 History window (slots)
UC = 16 Update period (slots)
Pn = .5 Initial new packet transmit gate Prvimie Initial retransmit packet gate Prmax = .5 Ceiling value for Pr(i)
DP = .25 Probability increment
Gmax = 2 Ceiling value for G estimate
[Some simulation studies are required to properly analyze the effect of changes in the control parameters and to determine which features of the protocol are
really useful and which can be discarded. Here is a list of some questions which need investigation:
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE Level 2 Interface: Packet Transmission Specifications
1. Performance curves for Simple ALOHA ae Effect of changing packet length. b. Effect of carrier sensing. ce Variations caused by changing Pr(i).
2. Performance curves for S-ALOHA-CLC ae Effect of changing packet length. b. Effect of carrier sensing. ce Benefits of slotting. d. Parameter settings and optimal values.
3. Determining the optimal load level to switch from the Simple-ALOHA to S-ALOHA-CLC.
The simulation could be written in PASCAL using discrete time steps.]
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 12
Chapter 3
LEVEL 3 INTERFACE: NETWORK SPECIFICATIONS
3.1 Datagram Network Characteristics
The architecture of data communications networks in use today is diverse and many different types of connecting arrangements are possible. There are point-to-point connections, multi-point networks, switched circuits, virtual circuits, switched virtual circuits, etc. The AMICON network is one example of a fully-connected packet switching network. The term fully-connected is used because there is the possibilty of a connection from every station in the net to every other station (through use of the multiple-access broadcast channel). The network is a packet switching network because all user information is broken into small packets of data, allowing many different users to have multiplexed access the channel.
The technology of building packet switching networks is under active development currently, and there are many different types of packet switching services available today. The type of service that best characterizes the AMICON network is referred to as a datagram service:
A datagram is a self-contained packet which carries sufficient information such that it can be routed from source station to destination station without reliance on any previous exchanges between source and destination and _ the communications network. The data field within a datagram will be kept intact and not split-up or altered in any way by the network.
The delivery of a datagram is not guaranteed. There is a high probability of delivery, but it may occasionaly be lost. The data within a properly received datagram, however, will have an extremely low probability of error.
The sequence in which datagrams are supplied by sender is not necessarily the sequence in which they will be received by the receiver. All datagrams in transit in the network are treated as independent entities.
Under some circumstances it is possible for a duplicate transmission to occur, causing the same datagram to appear more than once at the receiver.
In summary, a datagram service is an extremely simple but fairly fast transport service which serves as the foundation on which higher level communications protocols are built. These higher level protocols (Level 4 and above) are responsible for end-to-end acknowledgments, sequence checking, retransmissions of lost packets, flow control and other controls which will guarantee a complete and orderly passage of data from sender to receiver.
3.2 Packet Format
The packets or datagrams flowing through the AMICON network will have the format described below:
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 13 Level 3 Interface: Network Specifications
Bytes Field Description 1 ton Framing Initial framing or synchronization bytes 1 Address HDLC address byte 1 Control HDLC control byte 1 Protocol Protocol load-control byte 1 to n Receiver Receiver Call Group Field 1 “=” Separates Receiver Call Group from Sender Call 1 to n_ Sender Sender Call Field if fs 4 Terminator for datagram address fields 0 ton Data Datagram data field (unrestricted in content) 2 CRC Packet checksum bytes 1 ton Framing Terminating framing or pad characters
The initial framing or synchronization bytes are not detailed here. The beginning-of-packet control sequence is described in the Level 2 description of a packet.
The HDLC address field consists of a single byte. Use of extended address fields, as provided for in the standard, is not recommended. The contents of this address field are not used by the Level 3 protocol. Packets which are used for testing should set the address byte to hex °00%. Packets which are not using the address byte for higher level protocol functions should set this byte to the all-parties-addressed code, hex ’FF’.
The HDLC control field consists of one byte. Use of an extended control field, as provided for in the standard, is not recommended. The contents of the control field are not used by the Level 3 protocol. Stations not using this byte for control purposes should set it to hex ‘°’03°, which is an HDLC Unnumbered Information frame.
The protocol load-control byte is used by the S-ALOHA-CLC protocol control software. The contents of this byte are detailed in the Level 2 description of the closed-loop-control algorithm. Stations not using closed loop control should set this byte to hex ‘00’.
The receiver call group field is a field which contains the call or calls of the receiver of the datagram. See Section 3.3 below.
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The single ASCII character “=” separates the receiver call or calls from the sender’s call. See Section 3.3 below.
The» sender’s.call .field.,is..a,.\field» which scontains the call”-of “thé datagram’s sender.e See Section 3.3 below.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 14 Level 3 Interface: Network Specifications
The single ASCII character ’;” terminates the datagram addressing fields. See Section 3.3 below.
The packet data field is described in Section 3.4 below.
The two cyclic-redundancy-check (CRC) bytes provide error checking for the contents of the packet. The contents of these bytes are detailed in the Level 2 description of the packet.
The terminating framing or pad bytes are described in the Level 2 description of the packet.
3-3 Packet Node Addressing
The datagram addressing field starts with the byte immediately following the protocol load-control byte, and ends with a semicolon (;)- The function of this field is to specify the call of the sender of the datagram and to specify the call or calls of all intended receiving stations.
A point-to-point datagram is a datagram which is intended to be _ received by one station only and which should be discarded by all other stations.
A broadcast datagram is a datagram which is to be received by all stations currently listening to the channel. A datagram containing a CQ or a bulletin of general interest would be typical examples of broadcast datagrams.
A multi-cast datagram is a datagram which contains information intended for a selected group of stations having a special interest in a particular topic or participating in a net. The multi-cast group is specified by one or more “general call’ group addresses and optional qualifiers as outlined in Sections 3.3.3 and 3.3.4 below.
3.3.1 Packet Node Addressing Syntax
The datagram address field has the following general form: Receiver=Sender ;
The Sender is the call of the originator of the datagram. It is a specific call having the syntax outlined in Section 3.3.2.
The Receiver field specifies the station or stations to which the datagram is being transmitted. The Receiver address field contains either a single Call Group or several Call Groups separated by commas. Each Call Group may have an optional qualifier. A Call Group is either a Specific Call Group (Section 3.3.2) or a General Call Group (Section 3.3.3). Examples:
Receiver1=Sender ; Receiver 1,Receiver2=Sender ; Receiverl.Qualifier,Receiver2=Sender ; Receiver 1.Qualifer, Receiver2, Receiver3.Qualifier=Sender ;
The ASCII ’=’% character separates the Receiver and Sender fields. The ASCII semicolon terminates the datagram addressing field.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 15 Level 3 Interface: Network Specifications
3.3.2 Specific Call Group Syntax
A Specific Call Group is the call of a specific, individual station. An optional qualifier may be suffixed to the call (See Section 3.3.4). Lowercase ASCII characters will be converted to uppercase by the program scanning the Call Group. Any ASCII space characters found in the Specific Call field will be discarded. An ASCII slash character, ’/’ , may be appended to the call to show portable operation. If the specific call is used in the Receiver field, it may be preceded by a negation operator (See Section 3.3.3).
3-3-3 General Call Group Syntax
A General Call Group is a specifier which allows a sender to transmit a datagram to a group of stations. It is the mechanism which implements the broadcast and multi-cast modes of packet addressing. Whereas the Specific Call addresses a specific, individual station, the General Call allows construction of address templates which are not specific, and which may be used to match any number of individual calls.
The key to constructing a General Call address template is the use of certain characters which are ’wild-card’ characters, that is, characters which will match one or more letters or numbers in a text string. Two wild-card characters are now defined:
The character *?” may be used to match any other single letter or number. For example,
KL7???2 matches all Alaskan 2 x 3 calls W1?BC matches WIABC, WIBBC, WICBC, etc. VE???2 matches all six character VE calls
The character “*’ matches any string of numbers or letters including the null string. For example,
* matches every call in the world W*]* matches all W calls in the first call district
The characters °?” and °*” may be intermixed in any manner in creating a template. All lowercase letters will be converted to uppercase-e Any ASCII blanks embedded in the text strings will be discarded. A receiver scanning an incoming General Call Group should accept the datagram if the receiver’s call matches any template given in any of the address fields in the Receiver field.
If the General Call Group is preceded by a minus sign, then the datagram should be discarded if the receiver’s call matches the negated address template. For example,
* , -KH6*=KH6XYZ ;
is a datagram addressed to every station listening to the channel except any other KH6 stations
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 16 Level 3 Interface: Network Specifications
3-3-4 Call Group Qualifiers Each Specific Call Group or General Call Group may optionally be followed by a qualifier text string. The function of the qualifier text string is to
restrict very general call addressing to specific stations by functional
interest, mode of operation, or any other criteria which may be agreed on by two or more stations.
A qualifier text string begins with an ASCII period and immediately follows the Call Group. The text string is terminated by a comma or the equal sign separator. The following are some examples of qualifiers:
-CQ The datagram is addressed to any station matching the Call Group which wishes to establish a connection with a new station.
-SSTV The datagram is addressed to the specified Call Group and particularly those stations interested in slow scan TV.
»AMSAT To stations interested in receiving AMSAT related bulletins.
eNTS To stations handling formal message traffic. exNET To stations belonging to net x.
A list of commonly used qualifiers will be published by AMSAT once their use has been established.
3-3-5 Call Group Addressing Examples
The following set of examples demonstrates some of the possible legal combinations of the above Call Groups and Qualifiers:
KIHTV=KA& ; A point to point datagram from KA6M to KIHTV. * .QST=W1AW;
The datagram is a bulletin from the ARRL addressed to any station interested in the latest ARRL news.
* .CQ=WA2LQQ;3 Station WA2LQQ is calling CQ and looking for a new contact. AX ,K* ,N* ,W*=KA6M 35
A datagram intended for U.S. stations only. Will also end up in some rare DX locations.
* .DDX=W3IWI; To any station participating in the digital DX data contest. K1HTV, W6S P, W6X0 , WA2LQQ, * -AMSAT=KA& 3
A datagram to the specific stations listed and any others interested in AMSAT related bulletins.
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 17 Level 3 Interface: Network Specifications
3.4 Packet Data Field
The datagram packet data field contains data or control information which the sender desires to transmit to the receiver- The contents of this field are not interpreted by Level 3 software. The only constraint is that the length of the data field plus the control and addressing fields must not exceed the maximum packet length.
[The addressing mechanisms described here are general enough to construct practically any desired subgroup. Addressing specific countries or geographical areas, though, leads to some “Skward constructions and it would be worthwhile to invent some mechanism to make a geographical target area easier to do.]
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 18
Chapter 4
LEVEL 4 INTERFACE: APPLICATIONS
[There is some question whether applications standards should even be proposed at this time. It may be much too premature and perhaps it would be better for users to have some time to experiment first.]
4.1 File Transfer Protocol
The transmission of complete files through the network will not be covered by this revision of the specification and is a subject for further study.
[ARPANET uses a file transfer protocol known as TCP. It would be a good starting point and perhaps we can adopt a compatible subset for our usee- The original reference material for TCP may be found in ’A Protocol for Packet Network Intercommunication”’ by Vinton G. Cerf and Robert E. Kahn, IEEE Transactions on Communications, Vol. COM-22, pp- 637-648, May, 1974.]
4.2 Graphic Standards
The transmission of graphics over the network will not be covered by this revision of the specification and is a subject for further study.
[Should AMSAT even attempt to define a standard?] 4.3 Image and Video Transmission
The transmission of video images over the network will not be covered by this revision of the specification and is a subject for further study.
[Should AMSAT even attempt to define a standard?] 4.4 Digitized Voice Transmission
The transmission of digitized voice packets over the network will not be covered by this revision of the specification and is a subject for further
study.
[Should AMSAT even attempt to define a standard?]
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE
Appendix A - Selected Bibliography REFERENCES ON PERSONAL COMPUTER NETWORKS
A Multiuser Data Network - Communicating over VHF Radio Bruninga, Robert E., 907 Ninovan Road, Vienna, VA 22180 BYTE, Vol. 3 No- 11, p- 120, November, 1978
Design Considerations For A Hobbyist Computer Network Caulkins, D., 437 Mundel, Los Altos, CA 94022 Proceedings of the First West Coast Computer Faire, ADELE L977
PCNET 1979 Caulkins, D., 437 Mundel, Los Altos, CA 94022 People’s Computers, Vol. 6 No. 2, Sep-Oct 1977
Hobbyist Computerized Bulletin Board
Christensen, Ward, 688 E. 154th St. #3D, Dolton, IL 60419 Suess, Randy, 1930 Bradley, Chicago, IL 60613
BYTE, Vol. 3 Noe 11, p- 150, November, 1978
Community Memory - A “Soft” Computer System Felsenstein, L. Proceedings of the First West Coast Computer Faire, April, 1977
Homebrewery vs. The Software Priesthood Fylstra, d.
Wilber, M.
Byte, October, 1976
Distributed Network Horton, Glen, Hickock Teaching Systems, 2 Wheeling Ave., Woburn, MA 01801 BYTE, Vol. 3 No. 11, p- 62, November, 1978
The Sky’s the Limit: Ham Radio for Intercomputer Communication Kassar, Joe, 11532 Stewart Lane, Silver Spring, MD 20904 BYTE, Vol. 3 No. 11, p- 48, November, 1978
The Club Computer Network Kassar, Joe, 11532 Stewart Lane, Silver Spring, MD 20904 BYTE, Vol. 5 No. 5, pe 202, May, 1980
DIALNET And Home Computers
McCarthy, J.
Earnest, L.
Proceedings of the First West Coast Computer Faire, April, 1977
Why not Just Use the Phone? Newcomb, Donald R., 819 Bayou Blvd., Pensacola, FL 32503 BYE oevOuse 3 NO /,upeni2)l, July,) 1978
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1-01 PAGE 20 Appendix A - Selected Bibliography (continued)
CB Computer Mail Pank, R. Proceedings of the First West Coast Computer Faire, April, 1977
Satellite-Linked Computer Network, A Phase-III Hook-up for Your Keyboard Riportella, Vern, WA2LQQ, AMICON Coordinator, Box 56, Warwick, NY 10990 HAM Radio Horizons, March, 1980, pp. 48-51.
Personal Computers in a Distributed Communications Network Steinwedel, Jeff, W3FY, 715 Reseda Drive, Apt.2, Sunnyvale, CA 94087 BYTE, Vol. 3 Noe 2, pe 80, February, 1978
Calling All Computers Stoner, Donald L., W6TNS/7, John Hancock Bldg., Mercer Island, WA 98040 BYTE, Vole 3 Noe 12, p- 159, December, 1978
Computer Networks Tesler, L. People’s Computers, Vol. 6- No. 2, Sep-Oct 1977
A Network of Community Information Exchanges: Issues And Problems Wilber, M Proceedings of the First West Coast Computer Faire, April, 1977
CIE Net: A Design for a Network of Community Information Exchanges -- Part 1 Wilber, Mike, 920 Dennis Drive, Palo Alto, CA 94303 BYTE, Vol. 3°No. 2, ps 14, February, 1978
CIE Net: Protocols --— Part 2 Wilber, Mike, 920 Dennis Drive, Palo Alto, CA 94303 BYLEs VOM s 3 NO~ 135. p+ loZ,eMarcie e975
CIE Net: Other Considerations -- Part 3 Wilber, Mike, 920 Dennis Drive, Palo Alto, CA 94303 BYTE, Yok. 3 No« 4, p- 168, “April, 1978
ASCII at Last? Williams, Perry F., WIUED OST, Vol. LXITt No. 10, p. 5458 0ctober, 1978
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 21 Appendix A - Selected Bibliography (continued)
REFERENCES ON SATELLITE PACKET COMMUNICATIONS
The Throughput of Packet Broadcasting Channels
Abramson, Norman
IEEE Trans. on Communications, Vol. COM-25, No. 1, January, 1977, pp- 117-128. Reprinted in "Satellite Communications", Harry L. Van Trees, Ed., IEEE Press
Closed Loop Stability Controls for S-ALOHA Satellite Communication Gerla, M. and Kleinrock, L.
Proc. Fifth Data Communications Symposium, Snowbird, Utah, September, 1977.
Satellite Packet Communication - Multiple Access Protocols and Performance Lam, Simon S. IEEE Trans. on Communications, Vol. COM-27, No. 10, October, 1979, pp. 1456-1466
AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 22
Appendix B —- ISO Specification 3309
A > \ u : ae 7 . r i Ley HG, eee ‘ ity 7 ' I 7 . mar). 2.) : MY oa Or a : ~ i) ar hms eRe img? Da
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 23
Appendix C - Datagram Addressing Syntax Diagram
ie? GC)
SPECIFIC CALL }
QUALIFIER
ies:
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af GEVERA caALL +
AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE
Appendix D - Distribution List
Col. John Browning, W6SP, AMSAT Board Chairman 6202 Lochvale Dr., Rancho Palos Verdes, CA 90274 Home: Office:
Dr. Tom Clark, W3IWI, AMSAT Director & Executive Vice President 6388 Guilford Rd., Clarksville, MD 21029 Home: 301-286-3113 Office: 301-344-5957
Mark Corbitt 6568 Beachview Drive, Noe 311, Rancho Palos Verdes, CA Home: Office:
Dr. John DuBois, WIHDX, AMSAT Special Systems Consultant 241 Crescent Ave., Waltham, MA 02154 Home: 617-263-7004 Office: 617-891-9029
Gary Fariss, W6KYF 18983 Saratoga Glen Place, Saratoga, CA 95070 Home: 408-257-0948 Office: 408-734-6857
Gary Hendra, WA6SUW 3249 Lantern Court, San Jose, CA 95111 Home: 408-629-5863 Office: 415-494-7400 x6280
Mark Kaufmann, WB6ECE 14100 Donelson Place, Los Altos Hills, CA 94022 Home: Office: 415-948-3777
Larry Kayser, VE2QB Ottawa, Canada Home: Office:
Doug Lee, K6TDR 225 Nes Clark, Ave-; Los Altos, CA 94022 Home: 415-948-3601 Office: 415-326-6200 x2418
Wally Linstruth, WA6PJR 2413 Burritt Ave-, Redondo Beach CA 90378 Home: 213-542-3290 Office:
Dr. H. S. Magnuski, KA6M, AMICON Network Consultant 311 Stanford Ave., Menlo Park, CA 94025 Home: 415-854-1927 Office: 415-856-7421
Howard L. Nurse, W6LLO 665 Maybell AVe-, Palo Alto, CA 94306 Home: 415-493-0371 Office: 415-732-2710 x2559
24
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AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE Appendix D - Distribution List (continued)
Gary He Price, W6IRA Home: 415-732-1008 Office: 415-326-6200 x4820
Dr- John Pronko, W6X0, President, Project OSCAR 230 Hawthorne Ave-, Los Altos, CA 94022 Home: 415-941-6988 Office: 415-493-4411 x45179
Vern Riportella, WA2LQQ, AMICON Coordinator Box 56, Warwick, NY 10990 Home: 914-986-6904 Office: 201-768-2500
Bob Rouleau, VE2PY 1050 Churchill, Mt. Royal, Quebec H3R 3B6 Home: 514-341-7806 Office:
Re Satterlee, WB6VAL 1212 We. McKinley, Apt. 5, Sunnyvale, CA 94086 Home: 415-969-4451 Office: 415-964-5700 x227
Paul Zander, AA6PZ 86 Pine Lane, Los Altos, CA 94022 Home: 415-941-7821 Office: 415-857-3776
Rich Zwirko, KI1HTV, AMSAT Director and Vice President 34 Montclair Dr-, Manchester, CT 06040 Home: 203-646-5726 Office: 203-522-1080
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"AMICON SYSTEM SPECIFICATION -- Draft Revision 1.01 PAGE 26
—<—— NOTES --~-