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


                            Anita P. Skelton
                           Steven F. Holmgren
                             David C. Wood

                           MITRE Corporation
                       1820 Dolley Madision Blvd.
                            McLean Va. 22102

                   The MITRE Cablenet Project


     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.

                            Page -1-

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.

                            Page -2-

     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

     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.

                            Page -3-

        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

     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

                            Page -4-

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.

                            Page -5-

                                       -------------->-----[ ]---------|
                                       |                    |
                                       |  -----------<-------[ ]-------|
                                       |  |                 | |
                                       |  |               |-----|
                                       |  |               |     |
                                       |  |               | 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

                            Page -6-

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

                            Page -7-

command  center.   The  following  areas have been identified for

     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.

                            Page -8-