A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Application layer: the seventh layer of the OSI model. It is the interface to the user, or application program.
Asymmetric encryption: two different keys are used to control the encryption/decryption. One key is used to encrypt; the other is used to decrypt. A single key cannot be used to perform both functions.
BER: Bit Error Rate - probability that a bit will be received inverted
Broadcast: simultaneous transmission to a number of stations.
Bridge: an Intermediate System (IS) entity that operates at the link level (processing LPDUs). The requirement is that all components of the communication network use the same link protocol and has an addressing structure that allows for link level routing. IEEE 802 family protocols that use MAC/LLC are able to use bridges to interconnect LANs using the protocols defined in the family.
Byte: eight bits
Collision: a condition where two signals (or messages) are transmitted at the same time on a single media. Their interference makes both unintelligible.
Communications Network: a facility that provides data transfer service. A communication network is a collection of physical entities that allow for the movement of data (or information) between end systems.
Connectionless data transfer: a protocol for exchanging data in an unplanned
fashion and without prior co-ordination. The data unit is called a datagram.
CLNS (Connectionless Network Service): the network protocol that uses
connectionless data transfer
Connection-oriented data transfer: a protocol for exchanging data where a logical connection is established between the endpoints. The logical connection is called a virtual circuit, connection, or a socket.
CONS (Connection-oriented Network Service):
the network protocol that uses connection oriented data transfer
Crosstalk: a phenomenon where a signal transmitted on one circuit or channel of a transmission system creates an undesired effect on another circuit or channel.
CRC: cyclic redundancy check - an error detecting code where the code is the remainder of a division of the data to be protected by a predetermined binary number. CRC16 uses a 16-bit number. CRC32 uses a 32-bit number. CRC is a very strong error protection algorithm.
CSMA/CD: Carrier Sense Multiple Access with Collision Detection - the "listen while talk" protocol used in Ethernet. A sender monitors the media for idle carrier (carrier sense) and then monitors its transmission for a data collision (collision detection). Multiple stations use a single media (multiple access).
Data Link Layer: layer two of the OSI model. This layer provides service to transfer data between network layer entities, usually in adjacent nodes. The data link layer detects, and possibly corrects, errors introduced by the physical layer.
Deadlock: a condition that occurs when two (or more) processes (or systems) are each waiting for the other (or others) to complete before proceeding. The result is that all processes (or systems) hang.
DSSS (Direct Sequence Spread Spectrum): a technique that assigns unique distinct multi-bit patterns that defined a single bit to entities so that the bits transmitted or received are tied to a device. This allows multiple devices to transmit at will using a single frequency.
Differential encoding: a means of encoding digital data on a digital or analog signal such that a signal change rather than a signal level determines the binary value.
Encryption algorithm: a computation that transforms an entity into an unrecognizable form as directed by a controlling value (key). Its resistance to a brute force attack measures the strength of the algorithm; generally, the strength is directly proportional to the width of the key.
ES: End System - the entity in a communications network that originates information, or is the final destination of information. An end system supports applications or services used by end users (or just users).
Frame: Unit of transmission on a media
FDM (Frequency Division Multiplexing): method to create separate channels by assigning separate frequencies to entities to allow multiple entities to use a physical media at the same time.
FHSS (Frequency Hopping Spread Spectrum): method to create separate channel by assigning a unique sequence of frequencies to multiple entities to allow multiple entities to use a physical media at the same time.
Full-duplex transmission: data can be transmitted in both directions at the same time.
Half-duplex transmission: data can be transmitted in only on direction at a time.
Hash function: a computation that maps a entity into a numeric value. The strength of a hash function is measured by the inability to find two entities that map to the same value.
HDLC: High-Level Data Link Control - link level protocol that is the basis of most bit oriented protocols. The fields of HDLC are starting flag (0FE), address, control, data load, CRC-32, and ending flag.
internet: (note lower case i) a collection of communications networks that are connected by bridges and/or routers. An internet uses TCP/IP as its protocol stack.
Internet: (note upper case I) the worldwide collection of communications networks (thousands of networks and end systems). The Internet is based upon the TCP/IP protocol stack.
IS: Intermediate System - the entity in a network that interconnect communications networks. The IS allows end systems is different networks communicate. An IS performs network routing functions and generally processes NPDUs.
Livelock: a condition that occurs when two (or more) processes (or systems) continually change their state in response to changes in the other processes (or systems). The result is that none of the processes will complete. A system or set of systems in livelock look busy, but are not progressing.
LLC: Logical Link Control - link layer protocol for IEEE 802 LAN protocols and Ethernet
MAC: Media Access Control - the protocol used on the IEEE 802 LAN, such as CSMA/CD or Token Ring.
Network layer: layer three of the OSI model. This protocol layer is responsible for the routing of data through a communication network.
Noise: unwanted signals that combine with and distort a signal. Noise inhibits thea reception of information.
NPDU: Network Protocol Data Unit - a protocol data unit built by the network layer. The peer network layer processes a NPDU.
NSDU: Network Service Data Unit - a service data unit built by a transport layer to request service by the network layer, or a service data unit built by the network layer to convey a received TPDU or control for a transport layer.
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Parity: an error-detecting algorithm, which sets a bit (parity bit) to reflect the number of one bits in the protected entity (character or set of bytes). Even parity sets the parity bit if the number of bits in the entity are odd and, thus creates even parity; odd parity sets the parity bit if the entity contains an even number of one bits. Asynchronous communications, generally, uses even parity for each character to protect the data.
PDU: Protocol Data Unit - a set of data packaged by a protocol layer to be processed by the peer layer. A protocol data unit contains control information and user data. The protocol layer to effect flow control, error control, and state processes the control information. The user data is the data load that is being conveyed from source to destination.
Presentation layer: layer six of the OSI model. This layer provides for the selection of a common syntax for representing data and for the transformation of application data into and from the common syntax.
Physical layer: layer one of the OSI model. This layer is concerned with the electrical, mechanical, and timing aspects of signal transmission over a physical media.
Propagation Time: Time it takes for a bit to traverse a link (move from source to destination).
QoS: Quality of Service - a set of parameters that define the minimum service expectations. The QoS might specify a minimum effective bit rate, or a minimum message delay.
Reliable Network Service: the transport layer can assume that data presented to the network layer will be delivered in correctly. The network layer has an acceptable error rate.
Router: an Intermediate System (IS) that operates at the network level (processing NPDUs). Unlike an IS in the bridge class, there is no requirement that the link protocols be uniform. The only requirement is that all routers implement the same network protocol.
SAP: Service Access Point - service entrance for a protocol layer. An upper layer requests a service from a lower layer through a service entrance. Each entrance has a separate address (SAP) so that different upper layers may request service and the requests can be kept separate.
SDU: Service Data Unit - a set of data packaged by an upper layer protocol to request service by a lower layer protocol. The SDU is passed to the lower layer through a SAP. The SDU contains control information defining the service requested and an optional PDU to be sent to the peer upper layer protocol. The lower layer will build an SDU to pass a PDU from a peer of the upper layer protocol, or to pass control information to the upper layer protocol.
Session layer: layer five of the OSI model. This layer manages a logical connection (session or dialog) between communicating processes or applications.
Sliding-window technique: a method of flow control in which a transmitting station may send numbered packets within a window of numbers. The window changes dynamically to allow additional packets to be sent.
SPDU: Session Protocol Data Unit - a protocol data unit built by the session layer. The peer session layer processes a SPDU.
SSDU: Session Service Data Unit - a service data unit built by a presentation layer to request service by the session layer, or a service data unit built by the session layer to convey a received presentation PDU or control for a presentation layer.
Stop and wait: a flow control protocol where the transmitter sends a block of data and then must wait for an acknowledge before another block can be sent.
Symmetric encryption: a single key is used to control the encryption.
Synchronous clocking: the receive clock runs independently of the transmitting clock and identifies data by the detection of a special data pattern (synchronization pattern).
Token Ring: LAN protocol that uses a token to control transmission.
TPDU: Transport Protocol Data Unit - a protocol data unit built by the transport layer. The peer transport layer processes a TPDU.
TCP: (Transmission Control Protocol) - a connection oriented transport protocol. TCP is the transport layer of the TCP/IP protocol suite.
TDM (Time Division Multiplexing): division of the usage of a physical media into time segments. Individual time segments are assigned to entities. All data sent by an entity must be sent using the entities time segment.
Transmission Time: Time it takes to transmit all of the bits of a message onto the media.
Transport layer: layer four of the OSI model. It is responsible for reliable and transparent communications between endpoints.
TSDU: Transport Service Data Unit - a service data unit built by a session layer to request service by the transport layer, or a service data unit built by the transport layer to convey a received SPDU or control for a session layer.
UDP: (User Datagram Protocol) - a connectionless transport protocol. UDP is a transport protocol of the TCP/IP suite.
This page was last updated on January 16, 2008