This connection and protocol is generally referred to as , the report number. The initial version of the protocol was developed in since it predates the OSI model by a decade, does not map cleanly into the OSI layers. However, it is accurate to say that the protocol incorporates the physical layer , the data link layer , and the network layer. The interface visible to the host system passes network layer addresses directly to a physical layer device. To transmit data, the host constructs a message containing the numeric address of another host on the network similar to an IP address on the Internet and a data field, and transmits the message across the interface to the IMP. The IMP routes the message to the destination host using protocols that were eventually adopted by Internet routers.
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Motivation A need arose in the Packet Radio project for specification of an interface between Packet Radio units and other equipment. This paper is to meet BBN's responsibility to supply that specification. It is our hope that it will find application in other areas as well.
Consequently, this interface has become known as an interface. As the need to interconnect new types of devices has grown, it has become attractive to implement an like interface on each end of pairs of devices which are to communicate. The devices are then connected electrically, and communication can take place in spite of differences in processing speed, word length, signal levels and so forth in the two devices.
A part of Report reads as follows. Although there is a high degree of symmetry, some aspects are peculiar to the IMP side and some to the host side. Therefore, two interfaces constructed to connect to IMPs may not function connected to each other. In what follows, the unsymmetrical aspects are respecified in a way which will accomplish full interchangeability. Terminology The terms, "IMP" and "host," are not relevant in the present context.
Sections of Report such as Appendix B are conveniently re-interpreted by substituting "foreign interface" and "home interface," respectively. They are properly addressed by documentation specific to each application. The various areas which differ from Report are as follows. Low-level Protocol Certain aspects of the JANUS interface and its operation may be implemented in hardware, software of a mixture of the two.
We refer to these aspects as "low-level protocol. Padding Requirement: Received messages are padded out to a full word of the home device's size , if necessary, with zeros only. Rather, counts at one or another level of protocol are generally used, so the complication of a mark bit can be eliminated. A smaller maximum length may be implemented; a larger maximum lengthbe implemented; or the maximum length may be so large as to be in practice infinite.
This constraint can be enforced in software rather than in hardware, if desired. Four-way Handshake Requirement: The interface must use the four-way handshake. Discussion: The two-way handshake, presented as an option in Report , must not be used. Experience has shown that it is vulnerable to various failures. First, if the off period in RFNB is not seen by the sender due to noise or its being too short , a deadlock occurs and no more data is transferred.
Second, a two-way receiver cannot talk with a strictly four-way sender, since the sender's next assertion of TYB may depend on seeing the RFNB transition to on. And third, the two-way handshake is overly sensitive to transitions, and may be activated by noise pulses. Transitions in the two-way handshake may be missed altogether in a sender implementation which samples the RFNB line only at certain intervals.
The superiority of the more positive four-way handshake is important in applications where neither of the communicating interfaces is necessarily constructed to particular standards. Contact Bounce Requirement: Each interface, considered together with the software driving it, must prevent data from flowing across the interface in either direction while its Ready relay contacts may be bouncing. Discussion: This may be accomplished either in hardware or software, as discussed in Report section B.
Note that this means that RFNB at the cable driver may have to be off for somewhat longer than this minimum if deterioration of the signal waveform along the cable is anticipated.
There's-Your-Bit must similarly be off for at least 50 nanoseconds for local host connections, and at least 1 microsecond for distant host connections, as seen by the receiver of the signal. Deskewing Requirement: The outgoing data bit must be on the line and the Last-Bit level correct at least nanoseconds before the sender turns on the There's-Your-Bit signal.
Discussion: The responsibility for deskewing signals rests with the sender in each interface. Note that the receiver may count on the Last-Bit signal being valid during, and only during, the assertion of There's-Your-Bit.
Specifically, Last-Bit must be asserted during transmission of the last data bit. Report was slightly ambiguous in this regard. Transmission Order Requirement: "The high-order bit of each word is transmitted first. Discussion: If a computer has addressing modes other than word addressing, such units or bytes are not used as units of transmission by the interface.
For example, the first bit transmitted from or received into a PDP is bit 15, the leftmost bit of a bit word. This is repeated here to bring it especially to the attention of designers.
Several considerations prompted this change. Report specifies transformer coupling at the receiver, so requirements on signal rise time and hold times were made. To relax these, and to achieve greater tolerance to differences in ground potential, optical isolators are now often used, even in interfaces. Commonly available optical isolators require at least 1. The ground potential difference between the communicating interface may exceed the maximum ratings of the input amplifier, so the input circuit must be powered from a floating power supply.
Appropriate DC-DC converters for this purpose are available at reasonable cost. DH Signal Timing Requirement: Receiver circuits in distant host interfaces shall be implemented with optical isolators or other means which are not sensitive to rise and hold times, as transformer coupling is. Therefore, the requirements for rise and hold times on distant host signals appearing in Report are suspended.
In particular, the driver must supply a differential of at least 2 and not more than 6 volts; and the receiver must operate correctly on as small a differential as 0. The receivers and LED drivers for all input lines may be powered from one source, but this power must be floated with respect to ground of the home interface.
DH Cable Shield Grounding Requirement: At each end the cable shield in a distant host connection shall be connected through a circuit described below to signal ground. The circuit consists of two components connected in parallel. Exception: In cases of severe noise, one end of the shield or the other but not both! Discussion: Grounding the cable shield only at the host end, as in Report , is undefined when the interface is symmetrical.
Instead, the circuit above will be used. DH Cable Requirement: Cable requirements in EIA specification RS must be followed with respect to quality and electrical characteristics, and those in Report with respect to number of conductors.
In particular, at least 10 twisted pairs with impedance of approximately ohms must be supplied. This cable is similar to that mass produced for telephone cable, which is of good, uniform quality, and readily available at reasonable cost.
The cable specified in Report is not as desirable. Termination as in Report , at the driver, shall NOT be used. Discussion: The source-end termination specified in Report was to eliminate the voltage drop caused by the cable's series resistance. RS explicitly allows for this sort of signal attenuation as a part of the specification. Local Host Signal Levels Suggested voltage levels for local host drivers and receivers are given below.
The intent here is to be compatible with readily available TTL components. Suggested chips are the for a driver and the for a receiver. Note that signals may go up to 6 volts, which may damage receiving circuits constructed of normal 5-volt logic. Such receivers should have a voltage divider on their inputs. With properly chosen cable and well designed circuits, and with impedances matched, local host connections may operate considerably farther than the 30 feet given in Report Use of the Ready Line It is strongly recommended that the Ready Line provided by the hardware be used by the software in a manner similar or identical to that described in Report Report sections 3.
In particular, the software design should provide for the following: 1 A ready indicator relay which tells the foreign interface that the home interface and software are ready to communicate. Only when the details such as cable characteristics, memory speed, and acceptable memory utilization of a specific application guarantee that an unregulated transfer rate will be acceptable can these delays be omitted. Two delays are involved, one in the sender circuit and one in the receiver circuit.
The sender delays up to 10 microseconds adjustable from when the foreign interface drops Ready-For-Next-Bit, before again turning on There's-Your-Bit. This is the sum of delays C and D in Report Fig. The receiver delays up to 10 microseconds adjustable from when the foreign interface asserts There's-Your-Bit, before again turning on Ready-For-Next-Bit.
This is the sum of delays A and B in Report Fig. When delivered, interfaces should have these delays set at approximately the maximum delay.
The timing is shown below. Actually, the protocol areas discussed above are further clarification of Report , rather than any change from it. The electrical specifications differ only slightly from the interface. The local host levels chosen are compatible. The potential difficulties in using a JANUS interface cabled to an interface arise with the distant host interface. The distant host cable for a JANUS interface is ohms nominal impedance, compared to ohms for the interface.
This difference is small enough that most applications will work with either cable, or even with some ohm cable and some ohm cable. The distant host interface does not provide as much electrical isolation as the standard JANUS distant host interface. Thus, in cases of severe common mode noise or ground potential difference, two JANUS interfaces might operate correctly, but an interface might misbehave or burn out.
Unless there is a significant signal loss in the cable, the driver will drive a JANUS receiver acceptably. On the other hand, the maximum input to an receiver is 4.
The simplest fix for this is to put a balanced voltage divider at the receiver, or at the JANUS driver. Attention must be paid to the electrical isolation susceptibility of the , and to its maximum input voltage. The common mode voltage tolerance of the JANUS interface provides significant protection against widely varying ground potentials in field equipment separated by distances of thousands of feet.
Contains specification for PE cable.
Interface Message Processor
It was the first generation of gateways , which are known today as routers. Wes Clark suggested inserting "a small computer between each host computer and the network of transmission lines",  i. BBN was contracted to build four IMPs, the first being due at UCLA by Labor Day; the remaining three were to be delivered in one-month intervals thereafter, completing the entire network in a total of twelve months. When Massachusetts Senator Edward Kennedy learned of BBN's accomplishment in signing this million-dollar agreement, he sent a telegram congratulating the company for being contracted to build the "Interfaith Message Processor". The completed code was six thousand words long, and was written in the Honeywell assembly language.
BBN Report 1822