Hub Glossary

ACTIVE PORT

A switch or a repeater hub port through which communication is taking place.

ADDRESS AGING

A spanning-tree parameter that controls how long a switch or bridge/router must keep learned addresses in its address table before they are discarded and replaced by new ones.

ADDRESS TABLE FILTER

A mechanism by which devices such as a switch or bridge/router selectively forward or discard (filters) data by using address table information.

AGENT

The software component that is embedded onto an internetworking device, such as a switch, via firmware, or downloaded into the device's memory via software, to provide SNMP-compliant (or some other management standard) information about the device's status. In an SNMP-compliant device, the agent performs informat ion preparation and exchange with the NMS (network management station) via such management primitives as SET, GET, and GETNEXT, and generates traps.

ASICS

Computer chips developed for such functions as implementing a switching matrix on a network switching hub device.

ATM

A connection-oriented network technology that uses small, fixed-size cells at the lower data-communications protocol layer. ATM (asynchronous transfer mode) promises to provide high-speed network services for voice, video, and data traffic with a single underlying technology.

BACKBONE

A high-speed network medium to which multiple LANs are connected using switches and/or bridges/routers and over which internetwork traffic is channeled.

BANDWIDTH

A measure of the amount of network traffic the transmission medium can handle at one time. In a LAN, bandwidth describes the amount of data that can be transmitted over the line in bits per seco nd.

BRIDGE

A store-and-forward device that operates at the data link layer to connect similar and sometimes dissimilar LANs.

BROADCAST

A packet-delivery system; copied packets are passed to all participating stations attached to the network.

CRC

A value computed from a sequence of octets (i.e., bytes) to detect errors that occur during data transmission. Typically, packet-switching network hardware computes a CRC (cyclic redundancy check) and appends it to a packet when transmitting. On a standard Ethernet LAN, this is known as the FCS (frame check sequence). Upon reception, the hardware verifies the contents of the packet by recomputing the CRC and comparing it to the CRC attached to the data it receives.

CSMA/CD

A network medium access method in which each participating station waits for a clear channel before starting to transmit a packet. As part of the communications protocol, each participating st ation is able to detect collisions and retransmit.

DESTINATION ADDRESS

The first 6 bytes in a standard Ethernet frame, which contains the address of the station to which a frame is being sent.

DYNAMIC ADDRESS

An address learned by an intelligent network interconnection device, such as a bridge or a switch (by reading it from the data packets it processes), as opposed to addresses that are manually entered into the device's address table.

ETHERNET

A LAN communications system using 10- and now 100-Mbps baseband transmission, CSMA/CD access, logical-bus topology, and 10Base-5 (thick-wire Ethernet), 10Base-2 (coaxial), or 10Base-T (UTP or STP) cable.

ETHERNET STATION

An addressable device or node on an Ethernet network capable of transmitting, repeating, and/or receiving data.

FCS ERROR PACKET

A data packet that was received with an integral number of octets in length but does not pass the FCS check.

FRAME

A group of digits transmitted as a unit, over which a coding procedure is applied for synchronization (synonym: packet).

HARDWARE ADDRESS

The low-level address used by physical networks. Each type of network hardware has its own addressing scheme.

IN-BAND MANAGEMENT

The process of managing a device over a network using such protocols as TFTP (i.e., Telnet).

LATENCY

On a network switch, this indicates the time between when a packet arrives at a port to the time when it is retransmitted via a destination port. It also refers to the time between when a network station seeks access to a transmission channel and when access is granted or received.

MAC

The sublayer within the data link layer of the IEEE-802 model for data communications over Ethernet that's responsible for scheduling, transmitting, and receiving data on a shared-medium LAN. MAC (media acce ss control) addresses are unique, 48-bit, binary numbers (usually represented as a 12-digit hexadecimal number) encoded in a device's circuitry to identify it to a LAN.

MULTICAST

A type of network addressing that enables a node to send messages or data packets to an address that represents a group of stations rather than a single station.

OUT-OF-BAND MANAGEMENT

The managing of a device over an RS-232 serial link using such protocols as SLIP to transmit IP packets or using a standard terminal emulation (e.g., VT100).

PACKET

The unit of data sent across a packet-switching network (synonym: frame).

ROUTER

An intelligent interconnection device that decides which of several paths network traffic will follow. Routers use a routing protocol to gain information about the network topology and algorithms to choose the best route based on several criteria known as routing metrics. As internetworking devices, they open a virtual network to connect a sending station with a receiving station.

SNMP

A standard protocol used to monitor hosts, routers, and the networks to which they attach.

Sources for glossary: HUBwatch for Windows User Manual , Digital Equipment Corp., and Internetworking with TCP/IP, Volume I: Principles, Protocols, and Architecture by Douglas E. Comer, Prentice-Hall, 1995.