Course glossary for used terms. This is not definitive glossary, but aim is mostly clarify used terms and acronyms, and bind the context as well. To exact definitions use literature, standards or internet (with small amount of critics), for example .

Abstract Message

A Message definition that defines the content information only in logical way, and do not dictate any concrete format, syntax or coding between peers.


Processing or activities caused by some event. For example sending message back is a action.

Bit Stream

Synonym: octet frame, binary object frame, binary stream, octec stream.

Message data is encoded into and decoded from bit stream.


Usually or practically bitstream consists of stream of 8-bit octetts (ie. bytes, bytes but that is a computing term not communications :) This is due that usually 8 bit addressing is minimal unit in digital devices.


Translating layer peer message PDUs to lower layer messages. On communication has two parts: sender encodes PDUs to be send by lower layer, receiver decodes messages from lower layer and feeds them to entity FSM to process further.

Byte count coding, Character based coding, bit count coding.

Near synonym: Syntax.


A requirement that specifies a some limitations to implementation environment to help design process. Usually operating system, used software components or hardware. Constraint should be well judged.


Communication Protocol Object. Is an entity concept, that specializes to communications context.


In communication context a abstraction of some thing that communicates. Sends and receives messages. Entity can contain sub-entities. Entity can realize to, for example, desktop computer, mobile phone, running server or client program (a process), a sub-system inside program or an object (by OO-design) in sub-system. Instance also.


Something that happens on (computer) system and system reacts by action to it. In communication context receiving a message is one event. Timer timeout is another event. User input is event. Changes in hardware, for example power saving functionalities, can produce events. And so on.


Finite State Machine. Generally State Machine.


The Internet Engineering Task Force. .


A message that is directed from N layer to N+1 layer.


Declaration of set of messages than can be received (and therefore send) by some communication entity. Interface may also contain definition of data types of the information fields of the messages used.


Same content or information, different representations.


Communications standardization organization., ITU-T


Software component in communications system. Provides SAPs to upper layer. Communicates with peer entities in same layer. Uses lower layer SAP to establish peer communication.

In contrast to normal software components a layer is assumed to be on both peer sides. Layer provides a “layer” of communication functionality to upper layer. Upper layer usually does not take care of communicating functionalities below layer.

Layer has clearly defined requirements it provides as a service.



Communication unit in the exchange of data or any information between entities on communications systems.


Message Sequence Chart. Quite same than Sequence diagram that is used in UML.


On byte count coding fill empty bytes or bits to match specified alignment.


Protocol Data Unit. A coded towards lower layer transmission servive, usually to bit stream in lower layer messages payload.


An entity that has correspondent entity on “other side” on same layer. Peer message exchange is usually done over some network media.


Operating system process. Can be executed. Has own address space. The place where protocol stack is loaded.


Specification of communication between entities. Specification contains:

  • Statement of the requirements the protocol is providing as service.
  • Identification and roles of the entities in communication
  • Definition of the interfaces and messages in interfaces. Usually there is at least two interfaces towards each entities, and may save two interfaces to provide the service of the protocol if protocol defines the SAP as well.
  • Exact definition is the data types used in messages.
  • Very exact definition of the coding (syntax) of the messages and message data used in communication provides by the lower layer service (eg. transport) and exchanged over network media to the remote peer entity.
  • Definition of the order to exchange messages. Usually defined as FSM.


(TODO). Used in SAP and PDU interfaces and messages, eg. request message, request interface. Counter part is indication.


(Software engineering term) Statement of certain functionality or property the system have to have. Synonym: Feature, Functionality, Property or Task. Requirement is used to validate and tract the software development process; in analysis and specification, in design, in programming, in testing, so it can be said it is done or not.

Requirement is a goal that development process is trying to fulfill.

Requirements could be divided into functional and non-functional requirements. Functional requirements specifies the provided service. Non functional requirements specifies non service related things for example performance aspects (timing, latency, bandwidth, ..).


Service Access Point. Interfaces provided to upper layer. Wikipedia Service Access Point.

Sequence Diagram

UML diagram to model message exchange between multiple instances (entities) along time. One sequence did is an example case from communications system functioning.


Cohesive functionality provided by some software component.

Expressed usually in textual form and a list of requirements.

Clear division of Client – Interface – Service.


Detailed technical document that defines a system (especially communication) so that different implementations will be inter-operable.


Standard is a specification that is formally agreed by two or more parties.


On communication context, selection of communication protocol layers to from working communication system.

For example: Bluetooth Stack, Internet Protocol Suite

Synonym: Suite

State Machine

State Machine

Synonym: FSM

State Machine Diagram

Diagram in UML to model state machines. UML uses Harel state machines.


There is no text, there is only binary. Over the binary there is many standards, for example communication protocol ASCII (American Standard Code for Information Interchange), to present sequence of characters used as text. On standards characters consists on 7 (ASCII), 8, 16, 32 or variable (UTF-8, UTF-16 LE, UTF-16 BE) numbers of binary bits. Bits then forms a integers that assigned to certain symbol (character (en)coding).


Declaration of data type. Type tells how data have to be handled, presented or translated in systems or in between systems.

For example: boolean, 2 complement interger, unsigned interger, ieee 64 bit real number, ieee 80 bit real number, 8 bit character with EBCDIC coding, 8 bit character with ASCII coding, 16 bit chracter with unicode coding, 16 bit chracter with unicode coding with UTF-8 translation, array of unicode string, structure of integers and strings, structure of structure strings and real numbers, MPEG layer 3 audio frame, and so on ……….. (very many)


Unified Modeling Language. More on wikipedia

Last modified: 2013/07/01 14:42