| BSIG: | Key Terms |
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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. ACL - Access Control List: | Top A list kept by a Cisco router to control access to or from the router for a number of services (for example, to prevent packets with a certain IP address from leaving a partiicular interface on the router). When configuring ACLs on a router, you must identify each ACL uniquely by assigning a number to the protocol's ACL. When you use a number to identify an ACL, the number must be within the specific range of numbers that is valid for the protocol as below:
There are three important ACLs:
Administrative distance is an integer value; a value is assigned to each source of routing information. The lower the administrative distance, the better the source of routing information. IGRP'S default is 100, OSPF's is 110, RIP's is 120, and EIGRP's is 90. The value 100 in brackets in the show ip route output signifiies that the administrative distance used for IGRP routes is 100. In other words, the default value is in use. So, if RIP and IGRP are both used, and if both learn routes to the same subnets, only IGRP's routing information for those subnets is added to the routing table. If RIP learns about a subnet that IGRP does not know about, that route is added to the routing table. This table displays the default values for route sources:
*If the administrative distance is 255, the router does not believe the source of that route and will not install the route in its routing table. 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 individdul 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. Loss of communication signal energy. AUI (Attachment Unit Interface): | Top An IEEE 802.3 interface between a MAU (Media Attachment Unit) and a network interface card. The term AUI can also refer to the rear panel port to which an AUI cable might attach, such as those found on a Cisco LightStream Ethernet access card. Also called a transceiver cable. A MAU is a device used in Ethernet and IEEE 802.3 networks that provides the interface between the AUI port of a station and the common medium of the Ethernet. The MAU, which can be built into a station or can be a separate device, performs physical layer functions including the conversion of digital data from the Ethernet interface, collision detection, and injection of bits onto the network. Sometimes referred to as a Media Access Unit, also abbreviated MAU, or as a transceiver. An AS is a collection of networks under common administration sharing a common routing strategy. Also referred to as a routing domain. The AS is assigned a 16-bit number the the Internet Assigned Numbers Authority. Part of a network that acts as the primary path for traffic that is most often sourced from, and destined for, other networks. Cabling that provides interconnections between wiring closets and the POP, and between buildings that are part of the same LAN. The retransmission delay enforced when a collision occurs. 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. 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. 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 The set of all devices that will receive broadcast frames originating from any device within the set. Broadcast domains are typically bounded by routers because routers do not forward broadcast frames. An undesirable network event in which many broadcasts are sent simultaneiously across all network segments. A broadcast storm uses substantial network bandwidth and, typically, causes network time-outs. A linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. 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 togehter, their magnetic fileds 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 conects 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 unathorized 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. 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 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. Critial point: 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. 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 Mathematically - the first octet offers a network address range of: 128.0.0.0 - 191.255.0.0. Critial point: 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. 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 Mathematically (using 8-bit binary calculations) - this offers a network address range of: 192.0.0.0 to 223.255.255.0. Critial point: 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. A node or software program (front-end device) that requests services from a server. A web browser is a client software program or application.
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. 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. 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. Traffic in access of network capacity. An access method in which network devices compete for permission to access the physical medium. 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. 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.
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. 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. 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 to decide whether to use full-duplex or half-duplex conversations between computers. 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. 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 sytems. 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. 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. Encapsulation - Cisco / Novell Encapsulation Names | Top
The process by which bits are represented by voltages. A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digitial 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, dfined 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 redunancy. 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. A device that controls who may access a private network and is itself immune to penetration. Software instructions set permanently or semi-permanently in ROM. An application layer protocol used for transferring files between network nodes. A scheme of addressing that does not use a logical hierarchy to determine location. 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. 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. Frame Relay was originally conceived as a protocol for use over ISDN interfaces. Today, Frame Relay is an industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC (High-Level Data Link Control) encapsulation between connected devices. Frame Relay uses virtual circuits to make connections through a connection-oriented service. The network providing the Frame Relay interface can be either a carrier-provided public network or a network of privately owned equipment, serving a single enterprise. A Frame Relay network can consist of computers, servers, and so on, on the user end; Frame Relay access equipment such as routers or modems, and Frame Relay network devices such as switches, routers, CSU/DSUs, or mulitiplexers. User devices are often referred to as data terminal equipment (DTE), whereas network equipment interfaces to DCE is often referred to as data circuit-terminating equipment (DCE - think clocking). Frame Relay Terminology: | Top
Full Duplex Transmission: | Top The capability for simultaneous data transmissions between a sending station and a receiving station. 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 redunancy, but because it can be prohibitively expensive to implement, it is usually reserved for network backbones.
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. 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. Control information placed before data when encapsulating that data for network transmission. 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. 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* (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 newtork layer (Layer 3) service interface. * Institute of Electrical and Electronic Engineers. See Ethernet Standards 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. Physcial 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. At the DOS prompt: - type-in either ipconfig or winipcfg - followed by Enter or Return. Either command will return information on your NIC or network settings. IGRP - Interior Gateway Routing Protocol: | Top A protocol developed by Cisco to supercede RIP and address the problems associated with routing in large, heterogeneous networks. IGRP is a distance-vector interior routing protocol. Distance-vector routing protocols call for each router to send all or a portion of its routing table in a routing update message at regular intervals to each of its neighboring routers. As routing information spreads throughout the network, routers can calculate distances to all nodes within the network. IGRP uses a combination of metrics. Network delay, bandwidth, reliability, and load are all factored into the routing decisions. Network admins can determine the settings for each of the metrics. IGRP uses either the settings determined by the administrator or the default settings of bandwidth and delay to automatically calculate the best routes. IGRP provides a number of features that are designed to enhance its stability, including the following:
IGRP uses several types of metric information. For each path through an Autonomous System (AS), IGRP records the segment with the lowest bandwidth, the accumulated delay, the smallest maximum transmission unit (MTU), and the reliability and load. By default, bandwidth is given the most importance when calculating the best path. A router running IGRP sends an IGRP update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three separate update periods (270 seconds). After seven update periods (630 seconds), the router removes the route from the routing table. IGRP uses flash update and poison reverse to speed up the convergence of the routing protocol. 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. Terminal Equipment Type 1 (TE1) - Designates a devices that is compatible with the ISDN network. A TE1 connects to an Network Terminal of either type 1 or type 2. Terminal Equipment Type 2 (TE2) - Designates a device that is not compatible with ISDN and requires a terminal adapter. Terminal Adapter (TA) - Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network. Network Termination Type 1 (NT1) - Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility. Many devices have built-in NT1s. Network Termination Type 2 (NT2) - Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating. Note: TE1 (Computer or ISDN telephone); TE2 (standard analog telephone); TA (ISDN modem). The IOS documentation focuses on two types of ISDN interfaces: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both BRI and PRI provide multiple digital bearer channels over which temporary connections can be made and data can be sent. The result is concurrent digital dial access to multiple sites. BRI and PRI features:
BRI = 2 Bearer Channels @ 64 kbps per channel + 1 D Channel @ 16 kbps Bearer Channels (B Channels) are used to transport data. B Channels are called bearer channels because they bear the burden of transporting the data. B Channels operate at speeds of up to 64 kbps, although the speed might be lower depennding on the service provider. D Channels are used for signaling.
Q.931 governs the network-layer functionality between the terminal endpoint and the local ISDN switch. This protocol does not impose an end-to-end recommendation. The various ISDN providers and switch types can and do use various implementations of Q.931. Because CPEs cover a wide variety of capabilities and require a variety of services and interfaces, the standards refer to interconnections by reference point rather than specific hardware requirements. Reference points are a series of specifications that define the connection between specific devices, depending on their function in the end-to-end connection. It is important to know about these interface types because a CPE device, such as a router, may support different reference types; the reference points supported will determine what specific equipment is needed.
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, periperals, 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 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. 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 addresssing. 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 (48 bits) long and are controlled by the IEEE. MAC addresses are always expressed as 12 hexadecimal digits. The first six hexadecimal digits (left to right), which the IEEE administers, identify the manufacturer or the vendor and are known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. There are two formats for MAC addresses:
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.
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| BSIG: | Key Terms |