A type of display terminal used to communicate with IBM mainframe computers or AS400s. The communication can be via either SNA or Bisync 3270 terminals can also be attached to mainframes via a direct channel adapter.


A type of display terminal used to communicate with IBM AS400 computers. The communication is via SNA.


back to contents


Asynchronous Balanced Mode. This is an operating mode of HDLC. It assumes a full duplex transmission medium. Either end may send commands or responses to the other end at will, hence the term "balanced." LAPB, LAPD and LLC are subsets of the HDLC ABM mode of operation.

Adapter (serial)

An integrated circuit board which plugs into a bus (ISA, PCI) and connects to a serial communications line externally. The bus provides a parallel interface to a computer. The adapter converts serial data to parallel data, and vice versa.

Address (Various Forms)

An address is a means of identifying the location of some object, be it a physical device such as an adapter or a logical entity such as a service within a computer. Specific forms of addresses are:


American National Standards Institute. ANSI promulgates standards within the U.S. and is the United States' representative to the CCITT, a world standards body.


A computer program which converts a human readable notation for program instructions into the actual binary form that is suitable for execution by a computer. An "assembly language" is the syntax and semantics of a particular set of human readable notations which specifies instructions for a particular type of computer. Typically one line of assembly language yields one instruction for the computer. See Higher Level Language.


Asynchronous data transmission is the method used by most modems in computers today. Each character is framed by a leading bit, called the "start bit" with a value of zero, and a trailing bit, called the "stop bit" with a value of one. The 7 or 8 bits in between constitute one character of data. Both ends of the communication link must have a clock running at the same frequency (typically 16 times the data rate) in order for this method to function properly. The receiver synchronizes on each character separately when it sees the transition from one to zero on an idle line. It assumes that the zero is the start bit and samples the line again 8 clock ticks after the transition. If the value is still zero the receiver assumes that is just "saw" a start bit. It then samples the line 7 or 8 more times at 16 tick intervals to deserialize the character. It then samples the line one more time to observe the stop bit. Asynchronous communication can utilize at most 80% of the line bandwidth due to the 2 bits out of 10 overhead for the start and stop bits.


back to contents


Binary Synchronous communications protocol. This was probably the first synchronous communications protocol every defined. It was developed by IBM in the late 1960s. It is typified by half duplex procedures and character oriented transmissions. Devices that used Bisync to communicate to IBM mainframes include 3270 display terminals and 2780 and 3780 RJE stations.

Bit Oriented

A form of serial data transmission in which an arbitrary number of bits may be transmitted. HDLC and SDLC are bit oriented transmission methods.

Board (serial)

See Adapter


A means of connecting various computing devices together using highly parallel data transmission. A bus usually consists of a specification for a set of signals that computing devices such as CPUs, memory, disk controllers and other adapters use to communicate with one another. It also usually defines a set of size and shape limitations for adapter cards and a particular connector type. An adapter card built to one bus specification, say ISA, cannot be physically connected to a bus of a different type, say PCI.


A grouping of 8 bits. This term was coined by IBM in the mid 1960s to denote the smallest addressable unit of data in the System 360 computer.


back to contents

C Language

A higher level programming language. This language, and the enhancement of it called "C++", is the language used to program almost all software in recent years. The UNIX kernel and Linux kernel are both written in C. All Gcom software is written in C.

Call (X.25)

An X.25 DTE "calls" another X.25 DTE in order to establish a data connection between the two DTEs via an X.25 network. It is an operation analogous to "calling" someone on the telephone. The call is placed by one DTE sending a "Call Request Packet" to the remote DTE via the X.25 network. This is analogous to sending dialing digits to the telephone network via a touch tone pad.


A French acronym that stands for the International Consultative Committee on Telephony and Telegraphy. It is a United Nations sponsored standards body within the ITU, the International Telecommunications Union. It is responsible for specifying, among other things, the OSI protocol model and the X.25, X.3, X.28 and X.29 standards. It is also responsible for internationally agreed upon standards for modems such as V.34.


Communications Device Interface. This is a STREAMS driver interfacing standard to a device driver. It was defined by NCR Comten in the early 1990s. Gcom uses this protocol to communicate to its device drivers for serial adapters.

Channel Number (X.25)

A number that is embedded in all X.25 packets and is used to identify which of multiple virtual circuits this packet should be sent on.

Character Oriented

A means of serial data transmission in which the smallest unit of meaningful data on the line is a character, usually defined to be 8 bits (a byte). Bisync is a character oriented protocol.

Chip, Serial

An integrated circuit device which converts serial data to parallel data and vice versa. A multi-function serial chip can convert serial data in addressasynchronous mode, character oriented synchronous mode and bit orientedbit oriented synchronous mode depending upon parameters programmed into the chip. A serial chip is typically mounted on an adapter with bus interface circuitry to allow it to be accessed on its parallel side by a computer via the bus and with level converter circuitry on its serial side to adapt it to a particular standard electrical interface such as RS232.

Clear (X.25)

The act of discontinuing a data connection between two X.25 DTEs. This operation is performed by sending a "Clear Request Packet" from one DTE to the other. A clear operates on only one virtual circuit at a time.


A square wave periodic signal of usually constant frequency. The rising and falling edges of the clock are used a trigger points for serial interface chips to present data for transmission or to sample the received data line for incoming data. Synchronous communication needs an external clock source to function correctly.


A term used to describe a program for a computer. It is used ambiguously to refer either to Source_Codesource code or to object code. It is used as a verb meaning to write a program, or to describe the act of writing a program.


A computer program that processes input that conforms to the syntax of a particular higher level language, such as C or Fortran, and converts it into either assembly language for a particular computer or into executable binary for a particular computer. Commercial C compilers for the Intel Pentium are available from Microsoft and Borland. Project GNU offers a free C compiler for the Pentium; it is used by Linux.

Compile Time

The interval of time during which a program is being processed by a compiler. Some operations that determine the eventual behavior of the program occur at compile time. Options that tailor a program to different operating system environments are typically compile time operations.


A pin and socket arrangement to connect (typically) multiple conductors together. The shape, size and pin arrangements of two connectors must be compatible in order for an electrical connection to take place. The RS232, V.35, EIA530 and X.21 standards all specify different connector types.


Central Processing Unit. A computing unit which fetches instructions and data from external memory. A Pentium is a CPU.


Cyclic Redundancy Check. This is a method of computing a small (16 or 32 bit) number whose value is dependent upon the particular bit arrangement of a data transmission. The CRC is then added to the end of the data transmission. The receiver performs the same calculation and accepts the data if it computes the same value for the CRC as it received at the end of the transmission. This is the method of error detection for damaged frames used in HDLC and SDLC.


Channel Service Unit and Data Service Unit. These two terms are interchangeable. A CSU connects a full duplex serial data path from an adapter card to a telephone company provided digital circuit. It performs the analogous interfacing function to a modem, except that both sides of a CSU are digital and the telephone side of a modem is analog.


Clear to Send. This signal is asserted by a modem (DCE) when it is valid for the adapter (DTE) to present data on the transmit data pin of the interface. It is typically used in half duplex situations to signal the adapter that the modem has turned on its carrier and is now capable of modulating data onto the carrier.


A terminal handling library that utilizes the TERMCAP mechanism to format the screen of the user's terminal. See TERMCAP.


back to contents


Information, usually coded in binary. Data communication is the practice of transmitting data. Sometimes the word "data" is used to distinguish information received from a client or user for transmission from "control" information which is sent along the same path or connection in order to manage the orderly transmission of the data themselves. When speaking of a computer, the word "data" is sometimes used in contrast to "code" or "text" to distinguish operands from executable instructions. The word "data" is the plural of the word "datum." Datum derives from Latin, meaning, "That which is given." The cyberneticist Heinz Von Foerster opines that most of what is termed "data" could more accurately be called "capta," or, "That which is captured."

Data (X.25)

A type of X.25 packet which carries user level data via an X.25 network. An X.25 data packet also carries three control bits called the Q-bit, D-bit and M-bit.


A type of message used by a networking protocol. A datagram is characterized by having the full network address of both the sender and receiver embedded within it. There is no guarantee of delivery of a datagram. The IP protocol is a datagram protocol. The UDP protocol us a user interface to such a protocol.


A 15 pin "D" shaped connector that is used to carry the signals of an X.21 interface. VGA monitors also use a 15 pin connector of this type.


A 25 pin "D" shaped connector that is used to carry the signals of an RS232 or EIA530 interface. Occasionally a DB25 connector will be used to connect V.35 equipment.

D-bit (X.25)

Delivery confirmation. This bit is set in an X.25 Data Packet to ensure that the packet is acknowledged only by the remote DTE and not by any intermediate switching node. Thus, the sender receives confirmation, via this acknowledgement, that the remote DTE actually received the data packet sent to it.


Data Carrier Detect. This signal is asserted by a modem when it senses the incoming carrier signal from a remote modem.


Data Circuit-Terminating Equipment. This term can be used to describe one end of an electrical interface, in which it refers to the modem or CSU side of the interface. It can also be used to describe an X.25 packet mode interface, in which it refers to the network provider end of the X.25 interface.

Delivery Confirmation (NPI)

An option for an NPI data request or indication protocol object which requests end to end confirmation of receipt of the data. Similar to the X.25 D-bit.

Display Terminal

A CRT and keyboard arrangement that is equipped with the ability to communicate with a computer using some type of protocol. The term is not intended to extend to a PC utilizing software to emulate a display terminal. Examples of display terminals are the DEC VT100 (async), the IBM 3270 (Bisync and SNA) and the IBM 5250 (SNA).

DLCI (Frame Relay Address)

Data Link Channel Identifier. This is a number used by frame relay to distinguish one logical communications channel from another. Each frame relay frame carries a DLCI in its header to identify the channel over which the data are to be sent.


Data Link Provider Interface. This is a STREAMS based interfacing protocol to a link layer protocol entity. It was specified by AT&T. It provides for message formats and rules of exchange between a user of link layer services and the STREAMS driver which implements those services. It is used to interface to all Gcom link layer protocol modules such as LAPB, LAPD, HDLC, SDLC and Frame Relay. It is also used in UNIX to interface between IP and interface drivers below IP.


Direct Memory Access. This refers to the ability of a device to fetch and store data directly from or to an attached memory unit without mediation by a CPU. Some serial chips, such as the Hitachi 64570, have DMA built into the chip. PCs have general purpose DMA chips built onto the motherboard. They are unreliable due to the imprecision of the timing specifications for the ISA bus. Many PCI adapter cards have DMA capability into main PC memory. This is safe on a PCI_BusPCI bus because the timing specifications are defined precisely enough for such equipment to operate reliably in this manner.

Driver (Device)

This is a piece of software that is written to control a serial interface chip and the adapter board upon which it is placed. Each type of serial interface chip requires its own unique driver code. The driver processes interrupts from the adapter and handles requests from other software to transmit and receive data via the adapter. A fully featured driver for a multi-protocol serial chip can contain as much as 20,000 lines of C language code.

Driver (STREAMS)

A STREAMS driver is a piece of software written to conform to the specific interface specifications of STREAMS. It typically contains an "open", "close", "put" and "service" routine as required by STREAMS. A STREAMS "driver" as distinct from a "module" is accessed via the file system on the computer via the standard system "open" call. A STREAMS "module" is accessed via a system "ioctl" call. A STREAMS "driver" may or may not also be a STREAMS "multiplexor".


Data Set Ready. The DCE asserts this signal with it is ready to communicate with the DTE. It is analogous to the DTE's DTR signal.




Data Terminal Equipment. This term can be used to describe one end of an electrical interface, in which it refers to the adapter side of the interface. It can also be used to describe an X.25 packet mode interface, in which it refers to the customer end of the X.25 interface.

DTE Address (X.25)

The string of digits, similar to a telephone number, assigned to a customer (host) computer on an X.25 network. The X.25 Call Request Packet contains the address of the remote DTE to which the packet is directed. In X.25 calling packets the address is represented as a string of decimal digits in binary coded decimal, 4 bits per digit.


Data Terminal Ready. This is a signal asserted by a DTE when is considered operational and ready to communicate with the DCE. It is analogous to the DCE's DSR signal.


An operation that is performed while a program is running. It usually refers to the ability to alter configuration while software is running. It is the opposite of static.


back to contents


Electronic Industries Association. This U.S. group is the author of the standards whose names begin with "RS", as in RS232. The "530" standard is sometimes referred to as RS530 and sometimes as EIA530.


An interfacing standard which uses the same electrical levels as RS422 (0V to 0.5V), differential drivers and receivers and specifies a DB25 connector for the physical connection. The pin assignments of EIA530 on the "A" side of the circuits are the same as for RS232.


Open source application for analyzing network protocols.


A broadcast medium using coaxial cable for the physical connection. The most common protocol used on the Ethernet is probably Novell's IPX. Next most common is TCP/IP.

Execution (of a program)

A computer program consists of a collection of instructions for a CPU. The CPU fetches the instructions one at a time. The CPU interprets the instruction and performs operations on data within the CPU according to the encoding in the instruction. This process is referred to as the execution of the program. Instructions are fetched from sequential locations in memory unless the instruction itself causes the CPU to fetch its next instruction from some out-of-sequence location. Note that the CPU is active and the program is passive in its execution.


back to contents

Flag (HDLC)

An eight bit pattern of 01111110 used to delimit the beginning and end of an HDLC or SDLC frame. The flag at the beginning of a frame is called the "opening flag" and the flag at the end of the frame is called the "closing flag." The data pattern between the opening and closing flags is prevented from containing this pattern by the use of a transparency algorithm. This consists of inserting an extra '0' on the transmission line whenever the data pattern contains 5 consecutive '1' bits. The receiver deletes a '0' following 5 consecutive '1' bits to restore the original data pattern.

Frame (X.25)

A frame is a message unit used by HDLC. It is also the name given to the same thing in SDLC, LAPB, LAPD and LLC. It consists of an opening flag, data bits, a CRC and a closing flag. In X.25 the first byte of a frame contains an address code and the second byte contains a frame type code. The first byte is called the "address field" and the second byte is called the "control field." Extended forms exist in which there are multiple address bytes and/or two bytes of control field.

Frame Relay

A data communication networking technique involving only frames in HDLC format. These frames have a two byte address field and no control field. Frame relay networks can switch frames with low switching overhead due to the small amount of processing that must be performed on each frame.

Full Duplex

A full duplex transmission medium allows both ends of a point-to-point connection to transmit at the same time. This is usually accomplished by having separate physical circuits for each direction of transmission. Hence, the term "four wire circuit" which refers to a full duplex phone line. Two carrier frequencies on a single circuit could also accomplish full duplex transmission. Protocols which assume full duplex transmission are LAPB, LAPD, LLC and HDLC ABM. Some unusual forms of Bisync use full duplex.


back to contents

Gcom, Inc

Purveyor of the finest synchronous data communications software in the world. OK, it's our web page and we wanted something to go under the "G" heading.


back to contents

Half Duplex

A half duplex transmission medium restricts data flow to only one direction at a time. The two parties must alternate in their use of the line. Usually a protocol encoded in the data being exchanged controls which party is to transmit next. Protocols which assume half duplex transmission are SDLC, HDLC NRM and Bisync. The term "two wire circuit" is often used to describe a half duplex phone line.


High Level Data Link Control. A specification for frame formats and procedures maintained by ISO. HDLC is characterized by frames delimited by flag characters with a CRC used to check the data. HDLC frames have an address field and a control field. HDLC defines a full duplex point to point mode of operation called ABM and a half duplex multi-point mode of operation called NRM. HDLC NRM is almost identical to IBM's SDLC.

Higher Level Language

A programming language that uses a higher level of notation than assembly language. Higher level languages are usually characterized by arithmetic expressions and assignment statements, conditional statements, iteration constructs, data description and subprogram units with parameters. The C programming language is an example of a higher level language.

Host Address (Internet)

A four byte number which specifies a unique network number and host number within the network. These addresses fall into three classes, A, B and C, in which class A addresses use few bits to specify the network number and many bits to specify the host number and class C uses many bits to specify the network number and few bits to specify the host number. These addresses appear in IP packets in TCP/IP networks. IP version 6 will expand the size of these addresses.


back to contents

IBM 3270

See 3270.

IBM 5250

See 5250.

Interface (Electrical)

A particular set of electrical characteristics that must be adhered to in order for different equipment to be capable of being connected together and then interoperating correctly. Such characteristics include voltage levels, slope limits and driving capacity. The electrical interface specification usually carries with it some implication as to maximum data rate and maximum length of cables.

Interrupt (of program execution)

When an "interrupt" occurs the CPU saves its present state and switches its flow of instructions to a section of code specifically designed to process the event that just occurred. When the "interrupt" processing completes the CPU restores the saved state and continues executing the previous program. The interrupted program cannot detect, except by collusion with the interrupt handler itself, that the interrupt occurred. Interrupts are caused by several sources, most especially by external devices. The time of day clock causes an interrupt at regular periodic intervals. Serial communications boards cause interrupts when the serial interface chips need attention. A board driver must contain interrupt handlers for the interrupts that the board that it is handling can cause.

Interrupt (X.25)

An X.25 packet that carries a small amount of data (maximum 32 bytes). It is not subject to flow control so the data can be sent to the remote end of the connection in an expedited manner.


Internet Protocol. This is a network layer protocol which routes datagrams through a network. There is no guarantee of delivery of the datagrams. It is often used in conjunction with TCP, a protocol which provides reliable end to end connection services.


Industry standard bus specification for PC AT type clones. It transfers data up to 16 bits at a time in parallel. It is being superceded by PCI bus.


International Standards Organization. Original author of the OSI model for data communications. Also author of the HDLC specification, although concepts used to define HDLC were borrowed heavily from IBM's already defined SDLC protocol.


back to contents


The permanently resident portion of an operating system. For systems such as UNIX and Linux the kernel is also the only software that runs with full privileges on the CPU. Contrast to user level in which addressability of memory is restricted and certain CPU instructions are not allowed.


A modification made to something that covers a symptom rather then fixing an underlying problem. Sometimes used to describe a badly crafted system or an ill-thought-out feature.


back to contents


Local Area Network. Ethernet, token ring and FDDI are all examples of LANs.


Link Access Procedure "B". The link layer, called "frame level", of the X.25 protocol. There is no LAPA. The "B" stands for "Balanced," as in "Asynchronous Balanced Mode."


Link Access Procedure "D". The link layer of the ISDN protocol. It is the protocol used on the "D" channel of ISDN.


A set of message formats and procedures of a protocol which is contains a set of defined interface definitions for the services that the "layer" requires of another protocol and the services that it offers to its clients.

Layering (of protocols)

Protocol suites are layered when each individual member of the suite contains definitions of its own message formats and procedures and uses the data transportation features of the next lower layer protocol to communicate those messages to a peer. That is, layer n of the protocol uses the data transportation services of layer n-1 to send messages within the vocabulary of layer n to a peer process connected via the layer n-1 protocol. Layer 1 is the physical medium over which the signaling takes place. What is data to layer n may be further protocol objects to layer n+1.

License (for software)

A license is an agreement made between the authors or owners of a piece of software and another party who wishes to use the software under certain terms and conditions. Licenses vary according to restrictiveness, fees, warranties and other terms and conditions.

Link Layer

The name given to the layer 2 functionality in the OSI protocol model. A link layer protocol is supposed to manage the transmission of data between two entities connected by a single physical connection. Examples of link layer Protocolprotocols are LAPB, LAPD, HDLC, SDLC and LLC.


An operating system originated by Linus Torvalds. It is characterized by its adherence to UNIX or POSIX compatibility and its availability of source code free of charge. Linux distributions combine this kernel with utility programs from the GNU project plus other contributed software to form a complete system release. Gcom is the provider of the STREAMS subsystem in the Linux kernel.


Logical Link Control. This is the name that the IEEE gives to their suite of link layer protocols. These protocols are all described by the IEEE 802 series of standards. LLC frames look like HDLC frames. An LLC frame contains two bytes of addressing information, the first called the Destination SAP and the second called the Source SAP. LLC uses the HDLC extended control field in which the control field is 2 bytes long and sequence numbers are modulo 128.

LU (in SNA)

Logical Unit. A logical unit corresponds to an SNA session. It represents one logical data connection from the SNA device to the SNA host. Different types of SNA devices utilize different subsets of the SNA protocol. These subsets are denoted as "LU types" and are distinguished by different numerical designations.


Logical Unit type 0. This is a low level interface to an SNA data stream in which the user exercises control over many of the session capabilities such as chaining, bracketing, etc, and in which the user is capable of sending SNA requests and responses to the remote entity.


Logical Unit type 1. This is the subset of SNA that is used to connect 3270 intelligent printer terminals to host computers, mainframes or AS400s.


Logical Unit type 2. This is the subset of SNA that is used to connect 3270 display terminals to host computers, mainframes or AS400s.


Logical Unit type 3. This is the subset of SNA that is used to connect 3270 dumb printer terminals to host computers, mainframes or AS400s.


Logical Unit type 6.2. This is a set of peer to peer procedures within SNA.


back to contents

M-bit (X.25)

This is a bit in the protocol header of an X.25 data packet. It is used to indicate that this packet is part of a longer message of which there is more to follow. The packet with the M-bit reset ends the sequence.

Micro Channel Bus

A bus architecture invented by IBM in the 1980s. It has a different connector type than an ISA or PCI bus. Therefore, MCA (Micro Channel Adapter) cards cannot be used in ISA bus or PCI bus systems. The IBM PS/2 line of computers utilized this bus structure. This line of computers has now been discontinued by IBM.


Modulator/demodulator. A device which accepts digital signals on one side and modulates an analog signal on the other side to encode the digital information in the analog signal. It also performs the opposite, demodulation of analog signals into digital information. Modems are built to handle addressasynchronous data, synchronous data, or both. An addressasynchronous modem must know the data rate of the digital interface so that it can sample the digital data correctly. A synchronous modem must be able to recover clocking information from the analog signal so that it can generate clocks for the digital received data. Modems typically modulate the amplitude, frequency, phase or combination of a carrier signal. A modem is considered to be a DCE.

Modem Signals

Control signals exchanged between a modem (DCE) and a DTE. These signals include DTR, RTS, CTS, DCD and DSR.

More Data (NPI)

A bit in the header of an NPI STREAMS protocol data message which functions much like the X.25 M-bit. It indicates that this message is one of a sequence and that there is more data of the sequence yet to com.


back to contents

Network Layer

The name given to layer 3 of the OSI model. This layer is concerned with addressing host computers in a potentially widespread network of computers. This layer is responsible for finding routes to the hosts through the network. Examples of network layer protocols are X.25 and IP.


Network Provider Interface. This is a STREAMS interfacing protocol to a network layer. It was defined by AT&T in the late 1980s. It contains both virtual circuit (connection oriented) constructs (connection establishment and clearing) and datagram constructs (connectionless). Gcom uses the virtual circuit mode of NPI as the interface to its X.25, SNA and Bisync protocols.


Normal Response Mode. This is an operating mode of HDLC or SDLC in which a "primary" station sends polling frames to a "secondary" station. The secondary responds only when polled. Multiple secondary stations can share the same transmission medium. This is referred to as a "multi-drop" or "multi-point" line. An IBM 3270 or 5250 display terminal controller acts as an SDLC NRM secondary.


back to contents

Object Code

The form of a program that is suitable for execution on a computer. Object code comes in several forms. "Relocatable" object code contains addresses relative to some as yet unspecified base address. In order for it to become truly executable the object code must be modified by adding an actual base address to the relative addresses in many places within the code image. "Absolute" or "executable" object code is code that is directly executable by a CPU. "Position independent code" is object code that has been generated such that it is directly executable without modification irrespective of the address in memory into which the code is placed.

Operating System

The program that is loaded into the memory of a computer by the computers "boot loader". The operating system is responsible for managing all of the resources of the computer system including the CPU, memory and the peripheral devices attached to the computer. It is responsible for loading and managing the execution of user programs.


Open System Interconnection. This is a seven layer model of data communications promulgated by ISO. The seven layers are: physical, link, network, transport, session, presentation and application. The X.25 protocol corresponds to the first three layers of the OSI model. The CCITT is now the authority for these protocols.

Ownership (of software)

The owner of a piece of software is the person or business entity who retains all rights to the software. This person/entity is authorized to issue licenses to others to use the software, or to transfer ownership rights to others. When you purchase software you are almost always purchasing a license to use the software, not ownership rights to the software.


back to contents

Packet (TCP/IP)

A logical message used in the TCP/IP protocol. The term is ambiguously used to describe both an IP "packet" and a TCP "packet". A TCP packet contains a header intended for the use of the remote TCP peer followed by user data. An IP packet consists of an addressing header followed by a TCP packet or UDP packet.

Packet (X.25)

A logical message pertaining to layer 3 of the X.25 protocol. This is a network layer packet header followed by user data. The packet header is interpreted only by the layer 3 entity at the remote end of the X.25 connection. When this packet is given to layer 2 of X.25, the frame level, for transmission across the link, the frame level adds its own header information on the front of the packet header.


Packet Assembler Disassembler. This is a CCITT defined unit which controls addressasynchronous terminals on one side and communicates using X.25 on the other. Data from each terminal is sent to a connected remote host via an X.25 virtual circuit associated with that terminal. The PAD also implements a command vocabulary by which the terminal user can instruct it which remote host to which his/her terminal is to be connected.


Contrast to "serial". Parallel data transfer occurs with more than one bit of information is communicated through a transmission medium at the same time. In general this means that multiple wires "in parallel" carry multiple binary signals. Buses are parallel transmission media.


Peripheral Component Interconnect bus. A new 32 bit bus architecture defined in 1993. It has heritage from Intel's Multibus-II specification. One of its strong features is the ability of devices to identify themselves to the bus controller and to accept configuration information dynamically. It is also a higher speed bus than the ISA bus.

Physical Layer

The name given to layer 1 of the OSI model. This layer consists of the physical transmission medium of information, be it wire, fiber, microwave or other medium.


In HDLC and SDLC the "primary" station is the station which issues polls to the "secondary" stations. A poll is addressed to only one secondary station at a time. It authorizes that particular secondary to respond with data and/or control information back to the primary.

Process (Task)

See Task. In UNIX parlance, a user level program is usually referred to as a "process" when it is running.


A set of message formats and procedures used to communicate between two or more entities. X.25, TCP, IP and HDLC are all examples of protocols.


Physical Unit in SNA. This is the controller for a cluster of display terminals. Each terminal corresponds to a Logical Unit (LU) in SNA. The PU is the controller itself. The PU's address is the link layer's station address.


back to contents

Q-bit (X.25)

The data qualifier bit of an X.25 data packet. The Q-bit is used to distinguish data that is to be used for control purposes from user payload data. The X.29 protocol uses packets with the Q-bit set to communicate terminal attributes across an X.25 virtual circuit. QLLC uses Q-bit packets to manage a logical link setup using an X.25 virtual circuit in place of the link.


Qualified Logical Link Control. A protocol promulgated by IBM for use as a part of SNA. This protocol uses an X.25 virtual circuit in place of an SDLC link to communicate SNA protocol objects. It utilizes the X.25 Q-bit to format certain data packets to simulate the link setup commands used by SDLC to establish contact between the primary and secondary link stations.


A message passing operating system developed by QNX Software Systems Limited. Gcom has ported a STREAMS environment to QNX and added protocol functionality for SNA, X.25 and Frame Relay to QNX.


back to contents

Receive Data

The name of the "lead" or wire in which serial data flows from a DCE to a DTE. In synchronous mode the serial chip samples this lead on the rising edge of the receive clock.

Reset (X.25, Link Layer)

A protocol procedure in which the two communicating entities agree to discard all data in transit between them and reset all of their sequence numbers to zero. It is used as an error reporting and recovery procedure when one or the other of the two entities deduces that the two entities are irrecoverably out of synchronization. The X.25 packet level contains explicit reset packet types. The link layers use the SABM/SNRM frames to accomplish this function.


Remote Job Entry. This was historically a card reader, card punch and line printer combination connected to a mainframe computer via phone lines. It was used to submit batch jobs remotely via punch cards and receive printed output from those jobs. The IBM model numbers pertaining to RJE were 2780 and 3780 using Bisync as the communication protocol and 3770 using SNA.


An electrical interfacing standard promulgated by EIA. It is characterized by voltage swings from -12V to +12V with a common ground. It also utilizes the modem signals DTR, RTS, CTS, DCD, DSR and RI. RS232 specifies a DB25 connector for connecting DCE and DTE equipment together. Its data rate is limited to 38,400bps to 150,000bps depending upon particular line driver circuits and length and quality of cable.


An electrical interfacing standard promulgated by EIA. It is characterized by voltage swings from 0V to +0.5V with all signals differential. It does not specify any particular connector type. The EIA530 (RS530) standard uses RS422 signaling and a DB25 connector. The X.21 standard uses RS422 signaling and a DB15 connector. This interface can drive data to multi-megabit rates.


Request to send. A DTE asserts this signal to request a modem to turn on its carrier and prepare for the transmission of data. This signal is typically managed only in a half duplex environment. In full duplex this signal is typically asserted once and left "on" permanently. The modem is supposed to assert the CTS signal when it is ready for the DTE to transmit data. Some modems are not really ready for data when they assert CTS, necessitating a delay on the part of the DTE before actually transmitting any data.

Run Time

The interval of time during which a program is running on a computer. Contrast to compile time.


back to contents


Santa Cruz Operation. Originally a Microsoft reseller focusing on the Xenix operating system. SCO has come to be the owner of the UNIX operating system, which it now licenses under the name UnixWare.


Synchronous Data Link Control. A link level protocol invented by IBM in the late 1970s. It largely the same thing as HDLC NRM with a primary station and potentially multiple secondary stations. In SNA each secondary station corresponds to a physical unit (PU). The primary station corresponds to the host computer.


In SDLC or HDLC the secondary station is the one that receives polls from the primary. It sends frames to the primary only when it receives a polling frame containing this secondary station's address. IBM 3270 and 5250 control units are SDLC secondary stations.


Contrast to "parallel". Serial data transmission occurs when bits of information are transmitted sequentially on a transmission medium. In general this means that there is only one wire carrying encoded digital information. A phone line is an example of serial transmission. An Ethernet is another example.

Serial Chip

An integrated circuit device which converts serial data on one side to parallel data on the other side. The serial chip usually has modes of operation in which it recognizes basic framing characteristics of the data such as start and stop bits in asynchronous mode and flags and zero insertion in bit oriented synchronous mode. Some serial chips, such as the Hitachi 64570, contain sophisticated DMA logic as well.

Shared RAM (for boards)

RAM that is physically located on an adapter card and is accessible from both the adapter card and the PC base unit. This allows interface chips with sophisticated DMA logic to operate out of on-board RAM even in an ISA bus environment in which DMA to PC memory is unreliable. Many Ethernet interface boards and some serial interface boards are constructed in this manner.


Systems Network Architecture. A suite of protocols developed by IBM in the late 1970s for connecting devices to mainframes over communication lines. These protocols were later adapted to token ring LANs and peer to peer procedures were added for cases in which the two communicating entities are both software.

Source Code

The human readable version of a program. The source code of a program is translated into object code by a compiler or addressassembler to produce an executable version of the code which will cause a computer to perform the functions denoted by the semantics of the language in which the source code is written. The C language is a set of syntax and semantics for writing source code for computer programs.

Stack (of protocols)

A term used to mean layering of protocols. When protocols are layered one on top of another they form a "stack".


As opposed to dynamic. Static means fixed which usually means fixed by the source code of a program. Thus, a static configuration would be one which cannot be altered past a given point in time, usually compile time.

Station Address (Link Layer)

The address field of an SDLC or HDLC frame. It is used by the primary station in SDLC to address polling frames to the secondaries on the line.


A method of situating layered protocols within an operating system kernel. Invented by Dennis Ritchie at AT&T. STREAMS provides a method by which layered drivers can pass messages to one another. It also provides for flow control between layers of protocols. It allows for application level programs to build and disassemble protocol stacks of STREAMS drivers. It is a natural environment for the implementation of layered protocols. All of Gcom's protocol software is implemented as a suite of STREAMS drivers and user level utility programs for configuration and management.


A STREAMS driver is a protocol driver which is activated via an "open" system call. Some STREAMS drivers control hardware and others are written in such a way that they can be placed on top of other STREAMS drivers to form protocol stacks. This latter type of STREAMS driver is called a STREAMSSTREAMS multiplexor. Multiplexor drivers are good for handling multi-station topologies such as SDLC stations. All of Gcom's STREAMS drivers are multiplexors. See also "Drivers, STREAMS"

STREAMS Module (pushable)

A STREAMS module is linked with the kernel and is referred to by name in an I_PUSH ioctl call from a user program. This module is added to an open data stream. It differs from a multiplexor driver in that the multiplexor can make decisions concerning the connectivity of upper and lower streams but a STREAMS module can make no such decision because it sits astride a single data stream.

STREAMS Multiplexor

A STREAMS multiplexor is a STREAMS driver which manages one set of streams on its "top" and another set of streams on its "bottom". The multiplexor chooses how to connect the top streams and the bottom streams together. The X.25 packet level is a multiplexing protocol in that one lower stream, representing LAPB frames, is demultiplexed into multiple upper streams representing X.25 virtual circuits.

Support (for software)

The activity of assisting a user in the installation and use of a software product. Support also involves assisting the user with protocol problems and other problems so that the user can succeed in his/her project involving the given software.


Synchronous data transmission is the method utilized by protocols such as HDLC, SDLC, LAPB, LAPD and Bisync. Both the sender and receiver obtain clocking information externally. The external data transmission equipment keeps the clocks in phase with the data. The transmitter gates the next bit of data onto the line upon the falling edge of its transmit clock and the receiver samples the line upon the rising edge of its receive clock. The receiver deserializes one bit of data on each clock cycle. This means that an idle line must be represented by a unique pattern of bits. For HDLC and its derivatives the idle pattern is continuous flag characters. For Bisync it is continuous one bits. Most serial interface chips, such as the Hitachi 64570, can generate clocks as well as receive them and thus be used to emulate DCE equipment.


back to contents

Task (Process)

A program being executed by a CPU. A multi-tasking operating system, such as UNIX or Linux, maintains separate state information for multiple programs. The state information typically consists of an image of all of the CPU registers used by the program and a stack area of memory containing the program's subroutine calling history. The operating system switches between multiple tasks by loading the saved registers into the actual CPU registers and pointing the CPU's stack pointer to the program's saved stack area. In this way each program appears to be running alone on the CPU. Implementing multiple layers of a protocol by using multiple tasks involves lots of "task switching" overhead and is not a very efficient implementation technique for communications protocols. However, multi-tasking is a convenient method of writing application programs and support utilities for a data communications protocol system.


Transmission Control Protocol. This is the reliable end to end protocol used on the Internet. It is a virtual circuit protocol in that once a connection has been established between two endpoints data flows only between those two endpoints until the connection is closed.


A mechanism on UNIX like systems in which an ASCII file contains descriptions of the specific control character sequences that must be sent to a particular type of terminal in order to accomplish certain standard terminal functions. These functions include such things as cursor addressing, line deletion and mappings for the function and arrow keys. The "curses" library on UNIX uses the TERMCAP entry for the terminal with which it is communicating to know how to format the screen.

Terminal Emulation

A computer program interpreting a data stream that is formatted according to the specifications of some particular type of terminal and performing functions as directed by that data stream. More colloquially, a program that is "pretending" to be a particular type of terminal. A 3270 terminal emulator, for example, parses a data stream from an IBM host into commands and orders for a 3270 display terminal and performs the functions necessary to display, or maintain an in memory image of a display, the information in the same way that a real IBM 3270 would.


Transport Layer Interface. This is an AT&T specified STREAMS interfacing protocol to a transport layer protocol. It is used on UNIX systems as the interface to TCP.

Token Ring

An IBM invented local area network technology. It can be used to run TCP/IP, IPX or SNA protocols over the same medium. It is used as a high speed interfacing technique to mainframes. Token rings can be easily bridged which simplifies constructing wide area SNA networks.

Transmit Data

The name of the "lead" or wire in which serial data flows from a DTE to a DCE. In synchronous mode the serial interface chip gates data onto this lead on the falling edge of the transmit clock.

Transport Layer

The name given to layer 4 of the OSI model. The transport layer is responsible for end to end reliable data transmission. Of the commonly encountered protocols, only TCP is a transport layer protocol. Although it has constructs that look like transport layer functions, the X.25 protocol is an interfacing protocol, not a transport protocol.


Transistor-Transistor Logic. These are integrated circuits whose voltage level fluctuates between 0.2 and 0.5 volts. Level converters for RS232, RS422 and V.35 convert electrical levels from their required external values to TTL levels and vice versa.


back to contents


An operating system developed by AT&T Bell Labs in the 1970s. It is the operating system model that Gcom uses for all of its protocol implementations. There were at least three main lineage's to UNIX. The original AT&T Version 6 and Version 7 became System 3 and then System 5 UNIX, all under the control of AT&T. Version 7 UNIX became XENIX and then several versions of SCO UNIX all under the control of SCO. Version 7 UNIX also became Berkeley UNIX which is still being distributed by BSD, Inc. SCO has acquired the rights to AT&T UNIX from Novell, who acquired the rights from AT&T. SCO has now merged its own SCO line of UNIX with the AT&T lineage of UNIX in UnixWare version 7.


The name given to Novell's, and now SCO's, version of UNIX.

User Mode

As distinct from kernel mode. A program running in user mode is running with all of the hardware's protection mechanisms in place. A memory mapper ensures that a user program cannot access memory locations outside of the program. Hardware protection mechanisms prevent the program from executing certain privileged instructions, such as those which perform I/O or manipulate the memory mapper registers.


back to contents


An electrical interfacing standard promulgated by the CCITT. It uses voltage fluctuations in the range of -5V to +5V and differential signaling on the data and clock lines. Control signals use a common ground. This standard also calls for a 34-pin rectangular connector sometimes referred to as a "Winchester" connector. Occasionally equipment will use V.35 signaling levels with DB25 connectors.

Virtual Circuit

An abstract representation of a fixed pathway between two network connection endpoints. In practice the pathway can vary depending upon loading and failure conditions. In X.25 the call setup mechanism creates a virtual circuit which may then be referred to by a 12-bit logical channel number. This channel number acts as a shorthand for the circuit and allows the packet headers to be compact in size. TCP is a connection oriented protocol whose underlying routing is so dynamic that it is seldom referred to as a virtual circuit protocol.


A bus structure promulgated by Motorola and which is used for many embedded systems applications. Gcom does not support any VME bus platforms.


A type of terminal made by Digital Equipment Corporation. It is a popular "least common denominator" terminal type that is implemented by many terminal emulation programs.


back to contents


Wide Area Network. This tends to refer to sites connected by telephone lines or satellite links. Protocols used on wide area networks include X.25 and Frame Relay. SNA networks are usually referred to as "SNA networks" and not as "wide area networks".

Warranty (for software)

Statements made by the owner of the software concerning the intellectual property rights connected with the software. Also sometimes a software owner will "warrant" the correct operation of the software. Gcom makes no such warranties but agrees to correct problems with its software under the terms of a support agreement.


The name given to the 34-pin rectangular connector used for V.35.


Microsoft's desktop operating system.

Windows NT

Microsoft's server operating system. This system utilizes common operating system constructs such as pre-emptive scheduling, multi-tasking, user space and kernel space dichotomy and a POSIX interface to the kernel.


back to contents


An electrical interfacing standard which uses RS422 electrical levels and a 15 pin connector. It features only one control signal from the DTE to DCE, called "control", and one signal from the DCD to the DTE, called "indicate".


A three layer interfacing protocol promulgated by the CCITT that is used to connect user's computers, referred to as "Packet Mode DTE", to network nodes, referred to as "Packet Mode DCE". It uses LAPB as the layer 2 protocol and X.25 packets for the layer 3 protocol. Gcom's implementation of X.25 dates to 1980 with heritage back to 1977.


The CCITT specification for the user level commands implemented by a PAD.


The CCITT specification for the X.25 Q-bit packets that are exchanged between a PAD and an X.25 host computer.


The CCITT specification of the parameters for terminals which may be set or interrogated by the user using X.28 commands or by the host using X.29.