IEN: 96
The MITRE Cablenet Project
April 1979
prepared for
Defense Communication Agency
WWMCCS ADP Directorate
Command and Control Technical Center
11440 Isaac Newton Square
Reston, Va. 22090
by
Anita P. Skelton
Steven F. Holmgren
David C. Wood
MITRE Corporation
1820 Dolley Madision Blvd.
McLean Va. 22102
The MITRE Cablenet Project
Introduction
MITRE has initiated a local network and internetting
research project, which is supported by the Defense Communica-
tions Agency. The intent of the project is to investigate the
advisability of connecting major Command Center components with a
cable-bus, and the interconnection of these Command Centers to
eachother. A cable-bus network (Cablenet) has been installed at
MITRE, and is being attached to the ARPANET, to establish a
test-bed for experimentation. One of the major areas of investi-
gation is a determination of what protocols are best suited to a
local network broadcast environment, and how these protocols
interwork with the protocols in the global packet- switched net-
work. The cable-bus architecture will be compared with alterna-
tive configurations for a local network, such as a centralized
architecture, from the viewpoint of security, performance, and
other characteristics important in a command center environment.
The Command Center Environment
An understanding of this project is incomplete without a
very short digression on our view of the Command Center com-
ponents to be connected. Present and future Command Centers con-
sist of a number of devices which for many reasons aren't easily
inter-connected. During the latter 1980's, the World Wide Mili-
tary Command and Control System (WWMCCS) Command Center is likely
to include the present WWMCCS Honeywell 6000 computers, a future
generation WWMCCS system, an Automated Text Message Handling
(ATMH) system, which provides electronic mail, an intelligent
terminals, dumb terminals, a data base machine for information
retrieval, and possibly an intelligence system. In addition, ac-
cess will be needed to long-haul networks, such as AUTODIN II.
Each of the components may be a single machine, a group of
machines, or a simple peripheral device which contributes to the
component function.
The role of the cable-bus is to provide a "friendly" inter-
face to each of these devices so that they may be unified into a
more useful tool without unduly affecting component performance.
This implies that in some sense the cable-bus must be all things
to all devices. It should be able to emulate terminals, RJE sta-
tions, and simple twisted pair communications wires. The versa-
tility of the cable-bus interface unit and its capability for
complex data transformation are the keys to the cable-bus success
in these roles.
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The Cable-Bus System
The MITRE/Washington Cablenet system is based on a technolo-
gy developed at MITRE/Bedford. Similar cable-bus systems are in
operation at a number of government sites, e.g. Walter Reed Army
Hospital, and the NASA Johnson Space Center, but these are all
standalone, local-only networks.
The system uses standard Community Antenna Television (CATV)
coaxial cable and microprocessor based Bus Interface Units (BIUs)
to connect subscriber computers and terminals to the cable.
Coaxial cable as a transmission medium is very attractive for a
number of reasons: it is relatively inexpensive, approximately
$500/mile; it can support multimegabit transmissions; and is re-
latively immune to noise. It is well suited for the transmission
of digital, as well as analog signal. The cable bus consists of
two parallel coaxial cables, one inbound and the other outbound.
The inbound cable and outbound cable are connected at one end,
the headend, and electrically terminated at their other ends.
This architecture takes advantage of the well developed unidirec-
tional CATV components. The topology is dendritic (i.e. branched
like a tree).
The BIU is designed to transmit on the inbound cable and re-
ceive on the outbound cable. Each BIU implements a contention
algorithm, reliable packet communication, and user device inter-
face firmware. Other functions, such as intra-net routing, are
either inherent in the broadcast nature of the cable-bus, or im-
plemented by the user device. Certain BIUs directly interface
terminals to the cable-bus.
A broadcast contention scheme is used on the cable, with all
subscribers concurrently competing for the transmission media.
The BIU uses a Carrier Sense Multiple Access (CSMA) mechanism to
detect a busy cable. It also uses a Listen-While-Talk (LWT)
scheme to detect concurrent transmissions (collisions). Due to
signal propagation delay, it is possible for a BIU to start
transmitting without detecting the presence of a concurrent
transmission on the channel. The LWT technique minimizes the
time lost when a BIU has to abort a transmission due to colli-
sions. A transmitting BIU reads the initial portion of its own
transmission (listens while talking) from the outbound channel,
and compares it with the information sent on the inbound channel:
if the comparison indicates that the transmission has not been
interferred with, the BIU assumes that the cable-bus has been ac-
quired, disables its receivers, and continues transmission; if
the comparision indicates that a collision has occurred, the BIU
backs off for a pseudo-random amount of time, and then attempts
to retransmit.
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The coaxial cable (JA-3412) consists of a copper clad alumi-
num center conductor, polystyrene dialectric, and an aluminum
sheath shield. It has a loss figure of 1.6dB per hundred feet.
Analog signals can be effectively transmitted in a frequency
range of from 5MHz to 300MHz. The BIUs contain Radio Frequency
(RF) modems which modulate a carrier signal to transmit digital
information using 1MHz of the available bandwidth in the 24Mhz
frequency range. The remainder of the 294MHz bandwidth can be
used to carry other information channels, such as off-the-air TV,
FM, closed circuit TV, or a voice telephone system, or, other di-
gital channels. The data rate of our test-bed system is
307.2kbps.
The central processing unit of the BIU is a MOS Technology
MCS6502A microprocessor. The 6502A has a cycle time of 500
nanoseconds. It has a bidirectional 8-bit data bus which is in-
terfaced to a MCS6522 Versatile Interface Adapter (VIA); this VIA
has 2 - 8-bit parallel ports and dual interval timers, and is
used for high-speed parallel transfers from computers. Addition-
ally, there are two Motorola 6950 Asynchronous Communications In-
terface Adapters (ACIAs): one of these ACIAs allows terminal ac-
cess to the network via an RS-232-C port, with a selectable baud
rate from 75-9600 bits per second; the other ACIA is used for
very high-speed communication with the cable.
The 6502 is capable of addressing 65K bytes of memory. Peri-
pheral control and data registers are addressed as memory loca-
tions. There are 2K bytes of Random Access Memory (RAM) in low
order memory. The first 512 bytes are used for variable storage
and stack, and the remaining 1.5K bytes are used for packet
buffers. The upper 2K of memory is Programmable Read Only Memory
(PROM) which contains the BIU firmware.
The Design Goals
The next generation of microprocessors (e.g., Zilog 8000,
Motorola 68000, Intel 8086), the increasing availability of
larger, faster memories, and the longer term (5-10 yrs.) availa-
bility of Josephson junction and three-dimensional logic, make it
increasingly attractive to off-load specialized processing func-
tions from the WWMCCS H6000 computer to other separate computers
within a command center. Our test bed is being designed to pro-
vide the required inter-connection capability for the command
center of the future, which takes advantage of this technological
trend. With this in mind the following goals have been defined:
o There should be inordinately large amounts
of processing power at each node.
o The cost of a bus interface unit should
not be prohibitive.
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o The construction should be modular to
enable stepwise replacement of functions
with advancing technology.
o The cable-bus should provide a full range
of layered protocols (e.g. datagram,
virtual circuit, virtual terminal, mail,
teleconferencing, file transfer, and user
defined) which embody state-of-the-art
networking mechanisms (e.g. flow control,
routing, addressing). Furthermore, the layers
should provide interfaces promoting extensible
higher level usage.
o The bus interface units should be able
to support a wide variety of backend devices
and communications line disciplines (e.g. RS232,
parallel, HDLC, X.25).
o The resulting architecture should be
fundamentaly securable so that information
classified at various levels and
compartments can be transported
simultaneously.
Our investigation of the applicability of the cable-bus to
the Command Center environment will encompass an assessment of
the extent to which these characteristics apply to the MITRE
cable-bus system. Where feasible, the system will be improved to
more closely meet those goals. An assessment will be made to
discover any advantages in a distributed cable-bus inter-
connection archtecture over a star inter-connection architecture.
The result of our efforts will be a cable-bus system that
embodies a substantial number of the goals defined above. The
cable-bus system will: (1) provide the required nodal process-
ing capability, (2) implement cleanly layered transport and vir-
tual circuit protocols (the significance of this statement cannot
be underestimated), (3) interface to parallel and RS232 devices
with the capability for development of other user device communi-
cation disciplines, (4) result in a bus interface unit that is in
the early stages of program verification, and (5) effectively
communicate with other networks via network gateways.
The Test Bed
Our MITRE cable-bus test-bed includes a PDP-11/70 minicom-
puter running the Network UNIX operating system, and three
LSI-11s. The Network UNIX system will contain implementations of
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the Transmission Control Protocol (TCP) Version 4.0 and a modi-
fied version of the Internet Protocol (c.f. Realignment of Bus
Interface Unit Protocols). The gateway functions will be
minimal. The modified Internet Protocol will pass through pack-
ets to hosts on the ARPANET and cable. These hosts are expected
to contain the necessary higher level protocol software to estab-
lish and maintain direct communications circuits. The LSI-11 mi-
crocomputers will be paired with three bus interface units to
provide the futuristic nodal processing capabilities. The archi-
tecture of the test bed is shown in figure 1.
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-------------->-----[ ]---------|
| |
| -----------<-------[ ]-------|
| | | |
| | |-----|
| | | |
| | | BIU |
| | |-----|
| | |
| | |---------|
| | | |
| | |Tektronix|
| | | 4027 |
| | |---------|
|----| outbound | |
|Head|----->---[ ]--------------------[ ]----------[ ]-----[ ]--->-----|
| | | | | |
|End |-----<-----[ ]---------------------[ ]----[ ]----------[ ]--<----|
|----| | | inbound | | | |
| | | | | |
|-----| | | |-----|
| | | | | |
| BIU | | | | BIU |
|-----| | | |-----|
| | | |
|------------| | | |-------|
| |--- | | | |---
| PDP-11/70 |--- TTYs | | | LSI-11|--- TTYs
| |--- | | |-------|
| | | |
|------------| | --[ ]--------------|
| | |
| |------[ ]------------|
|----------| | |
|Local Host| |-----|
| Port | | |
|..........| | BIU |
| TIP | |-----|
|----------| |
/ \ |--------|
/ \ |Terminal|
To ARPA |--------|
Figure 1. Cablenet Architecture
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Initial Areas of Concentration
Realignment of Bus Interface Unit Protocols. MITRE's exist-
ing BIU-BIU protocols are ad hoc in nature. Various problems,
such as, inadequate addressing structure, missing or antiquated
flow control mechanisms, and missing out-of-band signaling,
prevent interfacing the cable-bus to long-haul networks in any
reasonable manner. Transport and virtual circuit protocols suit-
ed to the high speed cable bus are needed to provide the func-
tions for interfacing to existing long- haul networks.
Using the TCP version 4.0 and the Internetwork Protocol as a
starting point, a Flexible Datagram Protocol (FDP) has been
designed (See IEN-97). The FDP is motivated by a need to support
a cable-bus user community with widly varying transport protocol
requirements. It should be emphasized that the generation of a
new set of protocols was not our intent. However the urgent need
for a flexible transport protocol on the cable, and the feeling
that the state-of-the-art in local area networking protocols is
such that no existing protocols may be adopted as a whole, led to
the conceptual merging of the local network protocol layer with
the internet layer, to yield a transport datagram protocol which
can be layered under TCP. In its simplest form, FDP can function
in a local broadcast environment, and in its expanded form, the
FDP assumes all the characteristics of the Internet Protocol.
Internet Software Development. In order to achieve a full
internetworking capability between the Cablenet and ARPANET, TCP
4.0 and the FDP will be installed on the 11/70 and on the
LSI-11s. (We are adopting BBN's UNIX "C"/TCP and SRI's MOS/TCP in
an attempt to get these internetworking capabilities operating as
expeditiously as possible.)
High-speed 11/70 - Cable Interface. The 11/70 is interfaced
to the cable-bus with a low speed (9600 baud) terminal line. All
terminals accessing the 11/70 via the cable are multiplexed over
this line. To test the bandwidth and throughput of the cable-bus
system, a high-speed interface between the PDP11/70 and the
cable-bus is being installed. The interface bandwidth must be
greater than the basic cable bandwidth so as not to bottleneck
data and affect measurements.
A UMC-Z80, from Associated Computer Consultants, will be
used to provide the hardware interface between the cable-bus and
the PDP-11 UNIBUS. Data transfer rates on the order of 500 Kbits
should be available through the UMC-Z80. Since the basic data
rate of the existing cable-bus is approximately 300 Kbits, it is
believed that bottleneck problems associated with the PDP-11 to
cable interface will be non-existent.
Security. No secure cable-bus installations currently ex-
ist. Particular attention is being given to investigating ways
of securing a cable-bus to meet the security requirements of a
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command center. The following areas have been identified for
study:
1. The use of DES and Public Key encryption systems to
provide secure virtual circuit data paths.
2. BIU software verification.
3. Methods for physically securing the BIU and the cable.
4. A suggested architecture for an integrated network
control center and security officers station.
Because of the "intelligence" of each BIU, a number of security
measures are possible that are difficult to implement in other
architectures. Dynamic key modification, logical address assign-
ment, carrier frequency hopping, and distributed specifications
of interconnectivity, are all measures that strengthen the
overall cable-bus security. The extent to which each of these
measures is needed is part of our research.
Performance Analysis and Experimentation. There is a long
standing need for a knowledge of the end-to-end speed of the ex-
isting MITRE cable-bus system. Our initial measurement of
bandwidth and throughput will satisfy this need, as well as to
provide a metric for our newly implemented transport and virtual
circuit protocols. Subsequent measurements will not only focus
on our evolved transport and virtual circuit protocols, but will
deal with the internet environment.
The Cablenet of the Future
Our ultimate goal is a cable-bus design that is capable of
high-speed transmission (greater than 1Mbps), extensible, secure,
and supports both voice and video transmissions which are fully
integrated with the digital data. A voice message system coupled
with interactive TV/graphics displays are some of the elements of
our postulated fully automated command center. The next genera-
tion cable-bus interface units will support a multitude of dev-
ices, including page oriented and color graphics terminals; we
will implement a virtual terminal protocol in an expanded BIU.
We are interested in developing the capability to dynamical-
ly regulate resource access and machine loading, and we envision
a fully-distributed prototype cable-bus network being installed
at a command center site to test these resource sharing concepts.
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