access layer:

One of the layers in Cisco's three-layer hierarchical model. The access layer provides users with access to the internetwork. [ TOC ]

Application Layer:

Layer 7 if the OSI reference network model, supplying services to application procedures (such as electronic mail or file transfer) that are outside the OSI model. This layer chooses and determines the availability of communicating partners along with the resources necessary to make connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery. [ TOC ]

ASICs:

Application-Specific Integrated Circuits: Used in layer-2 switches to make filtering decisions. The ASIC looks in the filter table of MAC addresses and determines which port the destination hardware address of a received hardware address is destined for. The frame will be allowed to traverse only that one segment. If the hardware address is unknown, the frame is forwarded out all ports. [ TOC ]

BRI:

Basic Rate Interface: The ISDN interface that facilitates circuit-switched communications between video, data, and voice; it is made up of two B channels (64Kbps each ) and one D channel (16Kbps ). [ TOC ]

bridges:

A device for connecting two segments of a network and transmitting packets between them. Both segments must use identical protocols to communicate. Bridges function at the Data Link Layer, Layer 2 of the OSI reference model. The purpose of a bridge is to filter , send, or flood any incoming frame, based on the MAC address of that particular frame [ TOC ]

broadcast domain:

A group of devices receiving broadcast frames initiating from ant device within the group. Because they do not forward broadcast frames, broadcast domains are generally surrounded by routers. [ TOC ]

buffer:

A storage area dedicated to handling data while in transit. Buffers are used to receive/store sporadic deliveries of data bursts, usually received from faster devices, compensating for the variations in the processing speed. Incoming information is stored until everything is received prior to sending data on. Also known as information buffer. [ TOC ]

CSMA/CD:

Carrier Sense Multiple Access Collision Detect: A technology defined by the Ethernet IEEE802.3 committee. Each device senses the cable for a digital signal before transmitting. Also, CSMA/CD allows all devices on the network to share the same cable, but one at a time. If two devices transmit at the same time , frame collision will occur and a jamming pattern will be sent; the devices will stop transmitting, wait a predetermined amount of time , and then try to transmit again. [ TOC ]

CSU/DSU:

Channel Service Unit: A digital mechanism that connects end-user equipment to the local digital telephone loop. Frequently referred to along with the data service unit./ Data Service Unit: This device is used to adapt the physical interface on a data terminal equipment ( DTE ) mechanism to a transmission facility such as a T1 or E1 and is also responsible for signal timing. Although they are technically two separate units they are referred to as one (set). [ TOC ]

core layer:

Top layer in the Cisco three-layer hierarchical model, which helps you design , build, and maintain Cisco hierarchical networks. The core layer passes packets quickly to distribution-layer devices only. No packet filtering should take places at this layer. [ TOC ]

DCE:

data communication equipment ( as defined by the EIA ) or data circuit-terminating equipment ( as defined by the ITU-T): The mechanisms and links of a communications network that make up the network portion of the user-to network interface, such as modems. The DCE supplies the physical connection to the network, forwards traffic, and provides a clocking signal to synchronize data transmission between DTE and DCE devices. [ TOC ]

data frame:

Protocol Data Unit encapsulation at the Data Link layer of the OSI model. Encapsulates packets from the Network layer and prepares the data for transmission on a network medium. [ TOC ]

Data Link layer:

Layer 2 of the OSI reference model, it ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error notification, ordered delivery of frames, and flow control. The IEEE has further segmented this layer into MAC sublayer and the LLC sublayer. Also know as the Link layer. Can be compared somewhat to the Data Link control layer of the SNA model. [ TOC ]

DTE:

data terminal equipment: Any device located at the user end of a user-network interface serving as a destination, a source, or both. DTE includes devices such as multiplexers, protocol translators, and computers. The connection to a data network is made through data channel equipment ( DCE ) such as modems, using the clocking signals generated by that device. [ TOC ]

distribution layer:

Middle layer of the Cisco three-layer hierarchical model, which helps you design, install, and maintain Cisco hierarchical networks. The distribution layer is the point where access layer devices connect. Routing is performed at this layer. [ TOC ]

encapsulation:

The technique used by layered protocols in which a layer adds header information to the protocol data unit ( PDU ) from the layer above. As an example, in Internet terminology, a packet would contain a header from the Physical layer, followed by a header from the Network layer ( IP ), followed by a header from the Transport layer ( TCP ), followed by the application protocol data. [ TOC ]

Ethernet:

A baseband LAN specification created by the Xerox Corp. and then improved through joint efforts of Xerox, Digital Equipment Corp., and Intel. Ehternet is similar to the IEEE802.3 series standard and, using CSMA/CD, operates over various types of cables at 10Mbps. [ TOC ]

flow control:

A methodology used to ensure that the receiving units are not overwhelmed with data from sending devices. Pacing, as it is called in IBM networks, means that when buffers at a receiving unit are full, a message is transmitted to the sending unit to temporarily halt transmissions until the data in the receiving buffer has been processed and the buffer is again ready for action. [ TOC ]

frame:

A logical unit of information sent by the Data Link layer over a transmission medium. The term often refers to the header and trailer, employed for synchronization and error control, that surround the data contained in the unit. [ TOC ]

full duplex:

The capacity to transmit information between a sending station and a receiving unit at the same time.
[ TOC ]

half duplex:

The capacity to transfer data in only one direction at a time between a sending unit and receiving unit.
[ TOC ]

hierarchical addressing:

Any addressing plan employing a logical chain of commands to determine location. IP addresses are made up of a hierarchy of network numbers, subnet numbers, and host numbers to direct packets to the appropriate destination. [ TOC ]

hubs:

Physical devices that are really just multiple port repeaters. When an electronic digital signal is received on a port, the signal is reamplified or regenerated and forwarded out all segments except the segment from which the signal was received. [ TOC ]

ISDN:

Integrate Services Digital Network: Offered as a service by telephone companies, a communication protocol that allows telephone networks to carry data, voice, and other digital traffic. [ TOC ]

layer architecture:

Industry standard way of creating applications to work on a network. Layered architecture allows the application developer to make changes in only one layer instead of the whole program. [ TOC ]

MAC:

Media Access Control: The lower sublayer in the Data Link layer, it is responsible for hardware addressing, media access, and error detection of frames. [ TOC ]

Network layer:

In the OSI reference model, it is Layer 3 - the layer in which routing is implemented, and enabling connections and path selection between two end systems. [ TOC ]

OUI:

Organizationally Unique Identifier: Is assigned by the IEEE to an organization that makes network interface cards. The organization then puts this OUI on each and every card they manufacture. The OUI is 3 bytes ( 24 bits ) long. The manufacturer then adds a 3-byte identifier to uniquely identify the host on an internetwork. The total length of the address is 48 bits ( 6 bytes )and is called a hardware address or MAC address. [ TOC ]

OSI:

Open System Interconnection: International standardization program designed by ISO and ITU-T for the development of data networking standards that make multivendor equipment interoperability a reality.
[ TOC ]

Physical layer:

The lowest layer - Layer 1 - in the OSI reference model, it is responsible for converting data packets from the Data Link layer ( Layer 2 ) into electrical signals. Physical - layer protocols and standards define, for example, the type of cable and connectors to be used, including the pin assignments and the encoding scheme for signaling 0 and 1 values. [ TOC ]

Presentation layer:

Layer 6 of the OSI reference model, it defines how data is formatted, presented, encoded, and converted for use by software at the Application layer. [ TOC ]

PDUs:

Protocol Data Units: These hold the control information attached to the data at each layer of the model, which is typically attached to the header of the data field but can also be in the trailer, or the end of the data field. [ TOC ]

RJ connector:

registered jack connector: Is used with twisted-pair wiring to connect the copper wire to network interface cards, switches, and hubs. [ TOC ]

router:

A Network-layer mechanism, either software or hardware, using ore more more metrics to decide on the best path to use for transmission of network traffic. Sending packets between networks by routers is based on the information provided on Network layers. Historically, this device has sometimes been called a gateway. [ TOC ]

Session layer:

Layer 5 of the OSI reference model, responsible for creating, managing, and terminating sessions between applications and overseeing data exchange between Presentation layer entities. [ TOC ]

simplex:

The mode at which data or a digital signal is transmitted. Simplex is a way of transmitting in only one direction. Half duplex transmits in two directions but only one direction at a time. Full duplex transmits both directions simultaneously. [ TOC ]

state transitions:

Digital signaling scheme that reads the "state" of the digital signal in the middle of the bit cell. If it is five volts, the cell is read as a one. If the state of the digital signal is zero volts, the bit cell is read as a zero.
[ TOC ]

switch:

1) In networking, a device responsible for multiple functions such as filtering, flooding, and sending frames. It works using the destination address of individual frames. Switches operate at the Data Link layer of the OSI model. 2) Broadly, any electronic/mechanical device allowing connections to be established as needed and terminated if no longer necessary. [ TOC ]

thicknet:

Also called 10Base5. Bus network that uses a thick cable and runs Ethernet up to 500 meters. [ TOC ]

thinnet:

Also called 10Base2. Bus network that uses a thin coax cable and runs Ethernet media access up to 185 meters. [ TOC ]

Transport layer:

Layer 4 of the OSI reference model, used for reliable communication between end nodes over the network. The Transport layer provides mechanisms used for establishing, maintaining, and terminating virtual circuits, transport fault detection and recovery, and controlling the flow of information. [ TOC ]

UTP:

unshielded twisted-pair: Copper wiring used in small-to-large networks to connect host devices to hubs and switches. Also used to connect switch to switch or hub to hub. [ TOC ]

WAN:

wide area network: Is a designation used to connect LANs together across a DCE ( data communications equipment ) network. Typically, a WAN is a leased line or dial-up connection across a PSTN network. Examples of WAN protocols include Frame Relay, PPP, ISDN, and HDLC. [ TOC ]

windowing:

Flow-control method used with TCP at the Transport layer of the OSI model. [ TOC ]