| Home | Key Terms | Routing Terms | Dictionary of Networking |

Key Terms:  A   B   C   D   E   F   G   H   I    L   M   N   O   P   Q   R   S   T   U   V   W

ACK - Acknowledgement: | Top
Notification sent from one network device to another to acknowledge that some event (for example), receipt of a message) occurred. See window size.

ARP - Address Resolution Protocol: | Top
A protocol that is used to map an IP (logical-binary) address to a MAC (physical-hexidecimal) address.

Asynchronous Transmission: | Top
Digital signals that are transmitted without precise clocking. Such signals generally have different frequencies and phase relationships. Asynchronous transmissions usually encapsulate individual characters in control bits (called start and stop bits) that designate the beginning and end of each character.

ATM - Asynchronous Transfer Mode: | Top
An international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, SONET, and T3.

Attenuation: | Top
Loss of communication signal energy.

Backbone: | Top
Part of a network that acts as the primary path for traffic that is most often sourced from, and destined for, other networks.

Backbone Cabling: | Top
Cabling that provides interconnections between wiring closets and the POP, and between buildings that are part of the same LAN.

Backoff: | Top
The retransmission delay enforced when a collision occurs.

Bandwidth: | Top
The difference between the highest and lowest frequencies available for network signals. Also used to describe the rated throughput capacity of a given network medium or protocol.

Bridge: | Top
A device that connects and passes packets between two network segments that use the same communications protocol. Bridges operate at the data link layer (2) of the OSI reference model. In general, a bridge filters, forwards, or floods an incoming frame based on the MAC address of that frame.

Broadcast Address: | Top
A special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all 1s as in:

Binary: 11111111.11111111.11111111.11111111
or
Decimal: 255.255.255.255

Bus Topology: | Top
A linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations.

Cancellation: | Top
When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. The direction of these magnetic lines of force is determined by the direction in which the current flows along the wire. If two wires are part of the same electrical circuit, electrons flow from the negative voltage source to the destination along one wire. Then, the electrons flow from the destination to the positive voltage source along the other wire.
When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields as well. Twisting the wires can enhance this cancellation effect. By using cancellation in combination with the twisting of wires, cable designers can provide an effective method of providing self-shielding for wire pairs within network media.

CDP - Cisco Discovery Protocol: | Top
CDP provides a single proprietary command that enables network administrators to access a summary of what the configurations look like on other directly connected routers. CDP runs over a data link layer that connects lower physical media and upper network layer protocols. Because it operates at this level, CDP devices that support different network layer protocols can learn about each other.

CHAP - Challenge Handshake Authentication Protocol: | Top
A security feature supported on lines using PPP encapsulation that prevents unauthorized access. CHAP does not itself prevent unauthorized access; it merely identifies the remote end. The router or access server then determines whether that user is allowed access.

The Class A IP address: | Top
Supports very large networks. When expressed in binary form the left most bit of the first octet (8-bits) is always Zero - like so:

1st address : 00000001.00000000.00000000.00000000
2nd address: 01111111.00000000.00000000.00000000

Mathematically (using 8-bit binary calculations) - this offers a range of: 1-127. However, it is important to note that low of 0 is not used while high of 127 is reserved. This offers 1 through 126 as a valid network address range within a Class A address.
A Class A network address is created by using the first octet (8-Bits) of the 32-bit IP address. The remaining 24 bits (32 - 8 = 24) may be used for the host portion of the IP address up to 16,777,214 ( (2 to the power of 24) - 2 ) possible IP addresses.

The Class B IP address: | Top
Supports very large networks. When expressed in binary form - the first two bits of first octet (8-bits) are always 10 (1 and 0) - like so:

1st address : 10000000.00000000.00000000.00000000
2nd address: 10111111.11111111.00000000.00000000


Mathematically - the first octet offers a network address range of: 128.0.0.0 - 191.255.0.0.
Class B network addresses are created by using the first two octets (16-Bits) of the 32-bit IP address. The remaining 16 bits (32 - 16 = 16) may be used for the host portion of the IP address up to 65,534 ( (2 to the power of 16) - 2 ) possible IP addresses.

The Class C IP address: | Top Supports a small network. When expressed in binary form the first two three bits of the first octet (8-bits) are always 110 (1,1 and 0) - like so:

1st address : 11000000.00000000.00000000.00000000
2nd address: 11011111.11111111.11111111.00000000

Mathematically (using 8-bit binary calculations) - this offers a network address range of: 192.0.0.0 to 223.255.255.0.
A Class C network address is created by using the first three octets (24-Bits) of the 32-bit IP address. The remaining 8 bits (32 - 24 = 8) may be used for the host portion of the IP address up to 254 ( (2 to the power of 8) - 2 ) possible IP addresses.

Client: | Top
A node or software program (front-end device) that requests services from a server. A web browser is a client software program or application.

Client/Server Model: | Top
A common way to describe network services and the user processes (programs) of those services. Examples include the nameserver/nameresolver paradigm of the DNS and fileserver/file-client relationship such as NFS and diskless hosts.

Collision: | Top
In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical media.

Collision Domain: | Top
In Ethernet, the network area within which frames that have collided are propagated. Repeaters and hubs propagate collisions. Critical point: LAN switches, bridges and routers do not.

Congestion: | Top
Traffic in access of network capacity.

Contention: | Top
An access method in which network devices compete for permission to access the physical medium.

CO - Central Office: | Top
A local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occur.

CPE - Customer Premises Equipment: | Top
Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites and connected to the telephone company network.

Crossover Cable: | Top
A crossover cable crosses the critical pair to properly align, transmit, and receive signals on devices with like connections. The RJ-45 connectors on both ends show that some wires on one side of the cable are crossed to a different pin on the other side of the cable. Specifically for Ethernet, pin 1 at one RJ-45 end should be connected to pin 3 at the other end; pin 2 at one end should be connected to pin 6 at the other end.
You can use a crossover cable to connect similar devices; switch to switch, switch to hub, hub to hub, router to router, or PC to PC. Use a crossover cable when both device ports are designated with an X or when neither port is designated with an X.

CSMA/CD - Carrier Sense Multiple Access with Collision Detect: | Top
A media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. It two devices transmit at once, a collision occurs and is detected by all colliding devices. The collision subsequently delays retransmission from those devices for a random length of time. CSMA/CD access is used by Ethernet and IEEE 802.3.

DCE - Data Circuit (Communications) Equipment: | Top
Devices and connections of a communication network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards, traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCEs.

Dialog Control: | Top
Dialog Control is an integral part of the session layer. Communication between two computers involves many mini-conversations thus ensuring that the two computers can communicate effectively. One requirement of these mini-conversations is that each host plays dual roles: requesting service, like a client, and replying with service, like a server. Determining which role they are playing at given moment is part of dialog control.
Also within the session layer, Dialog control is used to decide whether to use full-duplex or half-duplex conversations between computers.

Dialog Separation: | Top
Dialog separation is the orderly initiation, termination, and management of communication in which a transaction is tracked through completion.

DHCP - Dynamic Host Configuration Protocol: | Top
A protocol that provides a mechanism for allocating IP addresses dynamically so that address automatically can be reused when hosts no longer need them.

DNS - Domain Name System: | Top
The system used in the Internet for translating names of network nodes into addresses.

DTE - Data Terminal Equipment: | Top
A device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by DCE. DTE includes such devices as computers, routers, and multiplexers.

Dynamic Routing: | Top
Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing. Requires that a routing protocol be run between routers.

EMI and RFI - Electromagnetic Interference and Radio Frequency Interference: | Top
External sources of electrical impulses can attack the quality of electrical signals on the cable including lighting, electrical motors, and radio systems. These types of interference are referred to as electromagnetic interference (EMI) and radio frequency interference (RFI). Each wire in a cable can act like an antenna. When this happens, the wire actually absorbs electrical signals from other wires in the cable and from electrical sources outside the cable. If the resulting electrical noise reaches a high enough level, it can become difficult for Network Interface Cards (NICs) to discriminate the noise from the data signal.

Encapsulation: | Top
Wrapping of data in a particular protocol header. For example, upper-layer data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network can simply be placed in the header used by the data link layer protocol of the other network.

Encoding: | Top
The process by which bits are represented by voltages.

Ethernet: | Top
A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10,100, and 1000 Mbps. Ethernet is similar to the IEEE 802.3 series of standards. (Deterministic - first come, first served.)

Fiber Distributed Data Interface: | Top
Is a LAN standard, defined by ANSI X3T9.5, specifying a 100-Mbps token-passing, network using fiber-optic cable, with transmission distances of up to 2km. FDDI uses a dual-ring architecture to provide redundancy.

Filter: | Top
Generally, a process or device that screen network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.

Firewall: | Top
A device that controls who may access a private network and is itself immune to penetration.

Firmware: | Top
Software instructions set permanently or semi-permanently in ROM.

File Transfer Protocol: | Top
An application layer protocol used for transferring files between network nodes.

Flat Addressing: | Top
A scheme of addressing that does not use a logical hierarchy to determine location.

Flow Control: | Top
A technique for ensuring that a transmitting entity does not overwhelm a receiving entity with data. When buffers on the receiving device are full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed.

Frame - Protocol Data Unit: | Top
A logical grouping of information sent a as data link-layer unit over transmission media Often refers to header and trailer, and used for synchronization and error control, that surround the data contained in the unit. The terms cell, datagram message, packet, and segment are also used to describe logical information groupings within the various layers of the OSI model.

Frame Relay: | Top
An industry-standard switched data link-layer protocol that handles multiple virtual circuits by using a form of HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which is generally considered a replacement.

Full Duplex Transmission: | Top
The capability for simultaneous data transmissions between a sending station and a receiving station.

Full Mesh Network: | Top
A network in which devices are organized in a mesh topology, with each network node having either a physical circuit or virtual circuit connecting it to every other network node. A full mesh provides a great deal of redundancy, but because it can be prohibitively expensive to implement, it is usually reserved for network backbones.

Gateway: | Top
This is an older term referring to a routing device. It is a special purpose device that performs an application-layer conversion of information from one protocol stack to another.

Handshake: | Top
A sequence of messages exchanged between two or more network devices to ensure transmission synchronization before sending user data.

Half-Duplex Transmission | Top
A capability for data transmission in only one direction at a time between a sending station and a receiving station.
Send the message - await a response or otherwise.

HDLC - High-Level Data Link Control: | Top
A bit-oriented synchronous data link-layer protocol developed by the ISO. HDLC specifies a data encapsulation method on synchronous serial links by using frame characters and checksums.

Header: | Top
Control information placed before data when encapsulating that data for network transmission.

Hop: | Top
The passage of a data packet from one network node, typically a router, to another. The routing metric used to measure the distance between a source and a destination is known as hop count. RIP uses hop count as its sole metric.

HTML - Hypertext Markup Language: | Top
A simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a web browser.

HTTP - Hypertext Transfer Protocol: | Top
Used by Web browsers and Web servers to transfer files, such as text and graphics files.

Hub / Repeater | Top
A device that serves as the center of a star-topology network and connects end stations. It operates at the physical layer (1) of the OSI reference model. It is often referred to as a repeater or concentrator.

ICMP - Internet Control Message Protocol: | Top
A network-layer Internet protocol that reports errors and provides other information relevant to IP packet processing.

IDF - Intermediate Distribution Facility | Top
A secondary communications room for a building using a star networking topology. The IDF is dependent on the Main Distribution Facility (MDF).

IEEE - Institute of Electrical and Electronic Engineers (I-triple E) 802.2: | Top
A LAN protocol that specifies an implementation of the Logical Link Control sublayer of the data line layer (Layer 2). 802.2 handles errors, framing, flow control and the network layer (Layer 3) service interface.

IEEE (I-triple E) 802.3: | Top
A LAN protocol that specifies an implementation of the physical layer and the MAC sublayer of the data ink layer. 802.3 uses CSMA/CD access at a variety of speeds over a variety of physical media. Extensions of the IEEE 802.3 specification include 10Base2, 10Base5,10BaseF, 10BaseT, and 10Broad36. Physical variations for Fast Ethernet include 100BaseTX and 100BaseFX.

IEEE (I-triple E) 802.5: | Top
A LAN protocol that specifies an implementation of the physical layer and MAC sublayer of the data link layer. 802.5 uses token passing access at 4 or 16 Mbps over Shielded Twisted-Pair or Unshielded Twisted-Pair cabling and is functionally and operationally equivalent to IBM Token Ring.

ipconfig or winipcfg: | Top
At the DOS prompt: - type-in either ipconfig or winipcfg - followed by Enter or Return.
Either command will return information on your NIC or IP network settings.

ISDN - Integrated Services Digital Network: | Top
A communication protocol offered by telephone companies that permits telephone networks to carry data, voice and other source traffic.

LAN - Local Area Network: | Top
Is a high-speed, low-error data network covering a relatively small geographic (up to a few thousand meters.) LANs connect workstations, peripherals, terminal, and other devices in a single building, or other geographically limited area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI reference model. Ethernet, FDDI, and Token Ring are widely used LAN technologies.

Latency: | Top
Latency equates to delay. To travel a distance, bits of data take at least a small amount of time to get to a destination. Also, the same bits of data go through devices, transistors, and other electronics which introduces additional delay. Latency or delay is expressed in milliseconds.

Load Balancing: | Top
In routing, the capability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Good load-balancing algorithms use both line speed and reliability information. Load balancing increases the use of network segments, thus increasing effective network bandwidth.

LLC - Logical Link Control: | Top
The higher of two data link-layer sublayers defined by IEEE. LLC handles error control, flow control, frame and MAC sublayer addressing. The most common LLC protocol is IEEE 802.2 which incorporates both connectionless and connection-orientated variants.

MAC Address - Media Access Control Address: | Top
A standardized data link layer address that is required for every device that connects to a LAN. Other devices on the network use these addresses to locate specific devices in the network and to create and update routing tables and data structures. MAC addresses are 6-bytes long, written in hexadecimal, and are controlled by the IEEE.
A MAC address is also known as a physical address, hardware address, and MAC-layer address. The address is physically burned into a Network Interface Card.

MDF - Main Distribution Facility: | Top
The primary communications room for a building. The central point of a star networking topology where patch panels, hubs and routers are located.

NEXT - Near End Crosstalk: | Top
When two wires are near each other and are untwisted, energy from one wire can wind up on an adjacent wire and vice versa. This can cause noise at both ends of a terminated cable.

NIC - Network Interface Card: | Top
A board or adapter that provides network communication capabilities to and from a computer system.
A NIC is considered both a Layer 1 and Layer 2 Interface.

Noise: | Top
Noise is unwanted additions to optical or electromagnetic signals. No electrical signal is without noise; however, it is important to keep the signal-to-noise (S/N) ratio as high as possible. The S/N ratio is an engineering calculation and measurement that involves dividing the signal strength by the noise strength; it gives a measure of how easy it is to decipher the desired, intended signal from the unwanted, but unavoidable noise.

Normal/Common Mode: | Top
A power cable contains three wires, and problems that occur in the cable are labeled according to the particular wire(s) that are involved. If a problem exists between the hot and neutral wire, this is referred to a normal-mode problem. If a situation involves either the hot or neutral wire and the safety ground wire, it is referred to a common-mode problem.
Normal-mode problems do not ordinarily pose a hazard to you or your computer. This is because they are usually intercepted by a computer's power supply, an uninterruptible power supply, or an AC power line filter. Common-mode problems, on the other hand can go directly to a computer's chassis without an intervening filter. Therefore, they can do more damage to data signals than normal-mode problems. In addition, they are harder to detect.

Open Shortest Path First: | Top
A link-state hierarchical IGP routing algorithm proposed as a successor to RIP. The OSPF feature-set includes least-cost routing, multipath routing, and load balancing.

Optical Fiber - Multimode / Single Mode: | Top

Multimode Fiber:-
Supports propagation of multiple frequencies of light. Associated with intra-building cabling (LANs)
Single Mode:-
Is also called axial because the light travels down the axis of the cable. Associated with inter-building cabling and WANs. Single Mode is faster than Multimode.

Oscillation & Noise: | Top
Oscillations are also sometimes referred to as harmonics, or noise. A common cause of oscillation is an excessively long electrical wiring run, which acts like an antenna.

Organizational Unique Identifier: | Top
Three octets assigned by the IEEE in a block of 48-bit LAN (MAC) addresses.

Packet - Protocol Data Unit | Top
A logical grouping of information that includes a header containing control information and usually user data. Packets are most often used to refer to network-layer units of data.

PAP - Password Authentication Protocol: | Top
An authentication protocol that allows PPP peers authenticate one another. The remote user attempting to connect to the local router is required to send an authentication request. Unlike, CHAP, PAP passes the password and host name or username in the clear (unencrypted). PAP does not itself prevent unauthorized access, but merely identifies the remote end. The router or access server then determines whether that user is allowed access. PAP is supported only on PPP lines.

Personal Computer - Backplane Components: | Top

  • Backplane - The large circuit board that contains slots for expansion cards.

  • Interface - A piece of hardware, such as a modem connector, that allows two devices to be connected.

  • Mouse port - A port designed to connect a mouse to a PC.

  • Network Interface Card - An expansion board inserted into a computer so that the computer can be connected to a network.

  • Parallel port - An interface capable of transferring more than one bit simultaneously. It is used to connect external devices such as printers.

  • Port - An interface on a computer to which you can connect an electronic device.

  • Power Cord - A cord used to connect an electrical device to an electrical outlet to provide power to the device.

  • Serial port - An interface that can be used for serial communication in which only one bit is transmitted at a time.

  • Sound card - An expansion card that handles all sound functions.

  • Video card - A board that plugs into a PC to give it display capabilities.

Important notes:
1. Think of a Personal Computer as a small network.
2. Internal to the Personal Computer & external through the parallel port - think parallel communications.
3. External to the Personal Computer via the Network Interface Card - think serial communalizations.

Personal Computer - Electronic Components: | Top

  • Capacitor - Stores energy in the form of an electrostatic field.

  • Connector - The part of a cable that plugs into a port or an interface.

  • Integrated circuit (IC) - A device made of semiconductor material; it contains many transistors and performs a specific task.

  • Light emitting diode (LED) - A semiconductor device that emits light when a current passes through it.

  • Resistor - A device that amplifies a signal or opens and closes a circuit.

Important notes:
1. Think of a Personal Computer as a small network.
2. Internal to the Personal Computer & external through the parallel port - think parallel communications.
3. External to the Personal Computer via the Network Interface Card - think serial communalizations.

Personal Computer - Information Flow: | Top

  • Boot instructions - Stored in ROM until they are sent out.

  • Software applications - Stored in RAM after they are loaded.

  • RAM and ROM - Constantly talk to the CPU through the bus.

  • Application information - Stored in RAM while applications are being used.

  • Saved information - Flows from RAM to some form of storage device.

  • Exported information - Flows from RAM and the CPU, through the bus and expansion slots, to the printer, the video card, the sound card, or the network card.

Personal Computer - Subsystems: | Top

  • Bus - A collection of wires through which data is transmitted from one part of a computer to another. It connects all the internal components to the CPU. The industry standard architecture (ISA) and the peripheral component interconnect (PCI) are two types of buses.

  • CD-ROM drive - A compact disk read-only memory drive; a device that can read information from a CD-ROM.

  • Central processing unit (CPU) - The brains of the computer, where most calculations take place.

  • Expansion card - A printed circuit board you can insert into a computer to give it added capabilities.

  • Expansion slot - An opening in a computer where a circuit board can be inserted to add new capabilities to the computer.

  • Floppy disk drive - A disk drive that can read and write to floppy disks.

  • Hard disk drive - The device that reads and writes data on a hard disk.

  • Microprocessor - A silicon chip that contains a CPU.

  • Motherboard - The main printed circuit board of a microcomputer.

  • Power supply - The component that supplies power to a computer.

  • Printed circuit board (PCB) - A thin plate on which chips (integrated circuits) and other electronic components are placed.

  • Random-access memory (RAM) - Also know as read-write memory, RAM can have new data written to it and stored data read from it. A drawback of RAM is that it requires electrical power to maintain data storage.

  • Read-only memory (ROM) - Computer memory on which data has been prerecorded. It can only be read.

  • System unit - The main part of a PC; the system unit includes the chassis, the microprocessor, the main memory, the bus, and the ports. It does not include the keyboard or the monitor, or any external devices connection to the computer.

Important notes:
1. Think of a Personal Computer as a small network.
2. Internal to the Personal Computer & external through the parallel port - think parallel communications.
3. External to the Personal Computer via the Network Interface Card - think serial communalizations.

PING - Packet Internet Groper | Top
The ping command sends ICMP echo packets and is supported both in user and privileged EXEC modes. It is used to verify the hardware connection and the logical address at the network layer. You must understand returned character responses:

  • ! - Successful receipt of an echo reply.

  • . - Timed out waiting for a datagram reply.

  • U - Destination unreachable error.

  • C - Congestion-experienced packet.

  • I - ping interrupted

  • ? - Packet type unknown.

  • & - Packet TTL exceeded

The extended ping command is supported only within privileged EXEC mode and is reached by entering ping [return] and then Y at the extended commands prompt.

Post Office Protocol (POP3): | Top
Is an Internet standard for storing e-mail on a mail server until you can access it and download it to your computer. It enables users to receive mail from their in-boxes using various levels of security.

Point-to-Point Protocol: | Top
A successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP.

Protocol: | Top
A formal description of a set of rules and conventions that govern how devices on a network share information.

PDU - Protocol Data Unit: | Top

  • Layer 1 = Bits - 0s / 1s - on / off

  • Layer 2 = Frames - encapsulates network layer information and adds physical addresses.

  • Layer 3 = Packets or datagrams- encapsulates data; attaches header. They include the logical addresses. Packets for connection oriented and datagrams for connectionless.

  • Layer 4 = Segments - grouping of data into more manageable parts.

PRI - Primary Rate Interface: | Top
An ISDN interface to primary rate access which consists of a single 64-kbps D channel plus 23 (T1) or 30 (E1) B (Bearer) channels for voice and data.

Proxy ARP - Proxy Address Resolution Protocol: | Top
A variation of the ARP protocol in which an intermediate device (for example, a router) sends an ARP response on behalf of an end node to the requesting host. Proxy ARP can lessen bandwidth use on slow-speed WAN links.

PVC - Permanent Virtual Circuit: | Top
A virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear-down in situations where certain virtual circuits must exist all the time.

QoS - Quality of Service: | Top
A measure of performance for a transmission system that reflects its transmission quality and service availability.

RAM - Random Access Memory: | Top
Volatile memory that can be read and written by a microprocessor.
Data within RAM is only available when a device is powered-up!

RARP - Reverse Address Resolution Protocol: | Top
A protocol that provides a method for finding IP (logical-binary) addresses based a MAC (physical-hexidecimal) addresses.

Ring Topology: | Top
A network topology that consists of a series of repeater connected to one another by unidirectional transmission links to form a single closed loop. Each station on the network connects to the network at a repeater. Although logically they are rings, ring topologies are most often organized in a closed loop star.

RIP - Routing Information Protocol: | Top
RIP uses hop count as a routing metric -- distance vector.

ROM - Read Only Memory: | Top
Non-volatile memory that can be read, but not written, by a microprocessor.

Router: | Top
A network layer (3) device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network-layer information contained in routing updates.
Routers can be configured externally:

  • From the console terminal (a computer connected to the router through a console port) during its installation.

  • Via a modem by using the auxiliary port.

  • From virtual terminals, after the router has been installed on the network.

  • From TFTP server on the network.

Router - Internal Configuration Components: | Top
The following play an important part in the startup process:

  • RAM/DRAM - Stores routing tables, the ARP cache, the fast-switching cache, packet buffering (shared RAM), and packet hold queues. RAM also provides running memory for a router's configuration file while the router is powered. RAM content is lost during a power down or restart.

  • Nonvolatile RAM(NVRAM) - Nonvolatile RAM stores the router's backup/startup configuration file. NVRAM content is retained during a power down or restart.

  • Flash memory - Acts as erasable, reprogramable ROM that holds the operating system image and macrocode. Flash memory enables software updates without removing and replacing processor chips. Flash memory content is retained during power down or restart. Flash memory also can store multiple versions of IOS software.

  • ROM - Contains power-on diagnostics, a bootstrap program, and minimal operating system software. Software upgrades in ROM require removing and replacing pluggable chips on the motherboard.

Interfaces are network connections on the motherboard or on separate interface modules, through which packets enter and exit a router. When configuring the router, you must go through one or more of these external interfaces. Examples interfaces include:

  • Console port.

  • Auxiliary port.

  • Ethernet port.

  • Serial port.

Routed Protocol: | Top
A protocol that be routed by a router. A router must be able to interpret the logical internetwork as specified by that routed protocol. Examples of routed protocols are AppleTalk, DECnet, and IP.

Routing Protocol: | Top
A protocol that accomplishes routing through the implementation of a specific routing algorithm. Examples of routing protocols are IGRP, OSPF, and RIP.

Sag/Brownout: | Top
A sag is a brownout that last less than a second. These incidents occur when voltage on the power line falls below 80 percent of the normal voltage. Sometimes they are called overloaded circuits. Brownouts can also be caused intentionally by utility companies seeking to reduce the power drawn by users during peak period. Like surges, sags and brownouts account for a large proportion of the power problems that effect networks and the computing devices attached to them

Segment - Protocol Data Unit: | Top
Be careful with this term. In context - a single transport-layer unit of information.

Shielding: | Top
In cable that employs shielding, a metal braid or foil surrounds each wire pair or group of wire pairs. This shielding acts as a barrier to any interfering signals. However, as with increasing the size of conductors, using braid or foil covering increases the diameter of the cable and the cost as well. Therefore, cancellation is more commonly used technique to protect the wire from undesirable interference.

Simplex Transmission: | Top
The capability for transmission in only one direction between a sending and a receiving station. Broadcast television is an example of simplex technology.

Spike: | Top
A spike is an impulse that produces a voltage overload on the power line. Generally speaking, spikes last between .5 and 100 microseconds. In simple terms, when a spike occurs, it means that the power line has momentarily been struck with a powerful hit of at least 240 V on a power supply rated for 120 V (100 percent increase).

SPX - Sequenced Packet Exchange (Novell): | Top
A reliable, connection-orientated protocol that supplements the datagram service provided by network layer protocols.

Star Topology: | Top
A LAN topology in which endpoints on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links.

Static Route: | Top
A route that is explicitly configured and entered into the routing table by default. Static routes take precedence over routes chosen by dynamic routing protocols. Key concept - use less overhead compared with Dynamic Routes.

Surge: | Top
A surge is a voltage increase above 110 percent of the normal voltage carried by a power line. Typically, such incidents last only a few seconds; however, this type of power disruption is responsible for nearly all hardware damage that computer users experience. This is because most computer power supplies that run 120 V are not built to handle more than 132 V for any length of time. Hubs are particularly vulnerable to electrical surges because of the sensitive love-voltage data lines.

Stub Network | Top
A network that has only a single connection to a router.

Synchronous Transmission: | Top
Digital signals that are transmitted with precise clocking. Such signals have the same frequency, with individual characters encapsulated in control bits (called start bits and stop bits) that designated the beginning and end of each character.

Subnet Mask: | Top
A 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. The subnet mask has two purposes:
1. Separate the Network bits from the Host bits.
2. Determining if the destination address is on the local network or another network.

TCP - Transmission Control Protocol: | Top
A connection-orientated transport-layer protocol that provides reliable full-duplex data transmission.

Telnet: | Top
Is a standard terminal emulation protocol. It is used for remote connections, enabling users to log in to remote systems to use resources as if they were connected to a local system.

Throughput: | Top
The rate information arriving at, and possibly passing through, a particular point in a network system.

Timing Jitter: | Top
All digital systems are clocked, meaning clock pulses cause everything to happen. Clock pulses cause a CPU to calculate, data to store in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, timing jitter is the result.

Token: | Top
A frame that contains control information. Possession of the token allows a network device to transmit data onto a network.

Token Ring: | A href="#top">Top
A "token" passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to 802.5. (Deterministic - taking turns)

Trace: | Top
Uses TTL values to generate messages from each router used along the path. This command is very powerful in its capability to locate failures in the path from the source to the destination. You must understand trace command responses:

  • !H - The probe was received by the router but not forwarded, which is usually due to an access list issue.

  • P - The protocol was unreachable.

  • N - The network was unreachable.

  • U - The port was unreachable.

  • * - Timed out.

Trailer | Top
Control information appended to data when encapsulating the data for network transmission.

Tree Topology: | Top
A LAN topology similar to a bus topology, except that tree networks can contain branches with multiple nodes. Transmissions from a station propagate the length of the medium and are received by all other stations.

Time-To-Live: | Top
A field in an IP header that indicates how long a packet is considered valid.

Twisted-Pair Cable: | Top
Is a 4 or 8-conductor (pins) copper transmission medium:

STP - Shield Twisted-Pair:- A two-pair wiring medium used in a variety of network implementations. STP has a layer of shielded insulation to reduce EMI.
UTP - Unshielded Twisted-Pair:- A four-pair wiring medium used in a variety of network implementations. It falls into five grades:

  • Category 1 - good for telephone communications.

  • Category 2 - good for transmitting data up to 4 Mbps.

  • Category 3 - good for transmitting data up to 10 Mbps.

  • Category 4 - good for transmitting data up to 16 Mbps. (Token Ring)

  • Category 5 - good for transmitting data up to 100 Mbps. (Most commonly used today)

Note: There are other grades of UTP but they are not referred to within the material.

UDP - User Datagram Protocol: | Top
A connectionless transport-layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgements or guaranteed delivery, requiring that errors processing and retransmission be handled by other protocols.

UPS - Uninterruptible Power Supply: | Top
A backup device designed to provide an uninterrupted power source in the event of a power failure. UPSs are commonly installed on file servers and wiring hubs.

Virtual Circuit: | Top
A logical circuit created to ensure reliable communication between two network devices. It is defined by a VPI/VCI pair and can be either permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25.

WAN - Wide Area Network: | Top
A data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers*. Frame Relay, SMDS, and X.25 are examples of WANs.
* Qwest, Worldcom, AT&T, Sprint.

Window Size: | Top
The number of packets/messages that can be transmitted while waiting an acknowledgement.

Top