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ADSL (Asymmetrical Digital Subscriber Line)

The telephone system in the United States is largely digital ,even though most telephones are analog devices. The so-called “last mile” is the local loop of wire that stretches from most homes to the telephone company’s central office (usually in the same neighborhood).This local loop is copper twisted- Pair wire That has Carried voice in a narrow band below400Hz.DSL technologies Expand The bandwidth usage of this cable. DSL technologies use line encoding to provide every high data transmission rates over the local loop.

ADTM (Asynchronous Time Division Multiplexing)

See Multiplexing and Multiplexers.

Agent, Network Management

In general, an agent is a background process that performs an action at a specified time or when an event occurs. In the realm of networking, an agent is part of a network management system that resides in workstations or other network devices (called managed elements) and Collects information to report back to a management system about those devices.

Aggregate Route-Based IP Switching

See ARIS (Aggregate Route-Based IP Switching).

Aggregation, Routing

Aggregation is the process of combining multiple contiguous network routes into a single route that can be advertised to other routers, thus reducing the amount of routing information that is advertised over the network. This is especially important on the Internet, where the number of networks has grown rapidly. Route aggregation is an important part of CIDR (Classless Inter-Domain Routing).

ANSI (American National Standards Institute)

ANSI is an organization that defines coding standards and signaling schemes in the United States and represents the United States in the ISO (International Organization for Standardization) and within the ITU (International Telecommunication Union). ANSI was a founding member of the ISO and plays an active role in its governing. It is one of five permanent members to the governing ISO Council.

Application Layer, OSI Model

The application layer is the top layer of the OSI (Open Systems Interconnection) model. he OSI model guides software developers and hardware vendors in the design of network communications products. When two systems need to communicate, they must use the same network protocols.

APPN (Advanced Peer-to-Peer Networking)

BM’s APPN was introduced by IBM in 1985 and integrated into SNA (Systems Network Architecture). It provides peer-to-peer networking services similar to but not quite the same as TCP/IP. APPN is basically link-layer independent. It can run over token ring, Ethernet, FDDI, frame relay, ISDN, X.25, SDLC, and ultra high-speed networks such as B-ISDN and ATM.APPN is based on the concept that computers on the network have enough processing power of their own to handle session management and routing.

The ARCNET (Attached Resource Computing Network) is a baseband, token-passing network system that offers flexible star and bus topologies at a low price. Transmission speeds are 2.5 Ambits/sec. ARCNET uses a token-passing protocol on a token bus network topology. While ARCNET never became popular in the LAN environment, it is used extensively for embedded and real-time applications.

ARIS (Aggregate Route-based IP Switching)

ARIS is IBM’s scheme for switching IP data grams. It is normally associated with ATM networks, but ARIS can be extended to work with other switching technologies. ARIS (and other IP switching technologies) takes advantage of integrated router switches. The idea is to map routing information to short fixed-length labels so that next-hop routers can be determined by direct indexing, rather than using the standard router packet evaluation and lookup process.

ARP (Address Resolution Protocol)

Every physical machine in a network has two addresses, an Internet Protocol address (IP) and a local MAC address (Media Access Control), when a request comes in from the internet, a protocol is required to convert the IP address into the MAC address.Address Resolution Protocol (ARP) provides this function by mapping Internet Protocol address to a physical machine address that is recognized in the local network. For example, an IP address is 32 bits long. In an Ethernet local area network, however, addresses for attached devices are 48 bits long.The ARP refers to an address table called the ARP cache, which shows the link between each MAC address and its corresponding IP address. ARP provides the protocol rules for making this correlation and providing address conversion in both directions.

ARPANET (Advanced Research Projects Agency Network)

ARPANET was a packet-switching network developed in the early 1970s. ARPANET was funded by ARPA (Advanced Research Projects Agency), which later became DARPA (Defense Advanced Research Projects Agency). The ARPANET network linked defense facilities, government research laboratories, and university sites. It evolved into the backbone of the Internet, and the term “ARPANET” was officially retired in 1990. However, MILNET (military network) was spun off from ARPANET in 1983. In addition, ARPANET spurred the development of one of the most important protocol suites available today, TCP/IP

(Transmission Control Protocol/Internet Protocol). TCP/IP is a set of communications procedures and standards that provide a basis for interconnecting dissimilar computers.

Asynchronous Communications

Asynchronous communication is the transmission of data between two devices that are not synchronized with one another via a clocking mechanism or other technique. Basically, the sender can transmit data at any time, and the receiver must be ready to accept information when it arrives. In contrast, synchronous transmission is a precisely timed stream of bits in which the start of a character is located by using a clocking mechanism.

ATM (Asynchronous Transfer Mode)

ATM is a high-speed network technology that is designed for LANs, WANs, carrier and service provider networks, and Internet core networks. It is a connection-oriented switching technology, as opposed to a connectionless technology such as IP. ATM creates a virtual circuit (dedicated path) between source and destination across its switching fabric. These circuits can guarantee bandwidth and quality-of-service.

ATM cells negotiate ATM switches with the same efficiency, providing several benefits:

* Cell switching is efficient and fast for the reasons just described.

* Traffic flow is predictable due to the fixed cell size.

* Delivery of time-sensitive traffic (live voice and video) can be guaranteed.

* ATM includes QoS (quality of service) features that can be used to guarantee bandwidth for certain types of traffic.

Attenuation is signal loss, measured in decibels, of a signal transmission over distance. The opposite of attenuation is signal amplification. On network cables, attenuation is the degradation of the digital signal or a loss of amplitude of an electric signal.

Authentication and Authorization

According toRFC 2828 (Internet Security Glossary, May 2000), authentication is “the process of verifying an identity claimed by or for a system entity.” The key word here is verify, and the correct terminology is to say that “an authentication system verifies an identity.” Authentication can provide assurance that users (or systems) are who they say they are. Authorization refers to a user’s ability to access resources on a network, usually based on user account rights and privileges.

AWG (American Wire Gauge)

AWG is a measurement system for wire that specifies its thickness. As the thickness of the wire increases, the AWG number decreases. Some common cable conductor gauges are listed here:

Gauge Diameter(mm) Cable Type

12 AWG 2.0523 Coaxial thick Ethernet cable

20 AWG 0.8128 Coaxial thin Ethernet cable

22 AWG 0.6452 S-232 serial cable, Telephone cable

23 AWG 0.5740 CAT-6 Cable

24 AWG 0.5105 CAT-3, 5 & 5e Cable

28 AWG 0.3200 Telephone cable

Access Control		Access controls are the security features that block or control the ability of a user or system to communicate and interact with another system. Access controls manage access to computers systems, networks, Web servers, extranets, and a variety of other systems and devices.

ACL (Access Control List)		See Access Control.

Access Method, Network		Local area networks (LANs) are typically shared by a number of attached systems, and only one system at a time may use the network cable to transmit data. An access method defines how a system gains access to a shared network in a cooperative way so its transmissions do not interfere with the transmissions of other systems. Simultaneous access to the cable is either revented by using a token-passing method or controlled with a carrier sensing and collision detection method.

ADC (Analog-to-Digital Conversion)		ADC, or digitizing, converts analog waveforms to digital representations that can be processed and stored in computers. The analog wave is “sampled,” or read, hundreds or thousands of times per second to map out the wave digitally. Digital music requires extremely high sampling rates (44,100 samples/sec), while it is usually acceptable to sample voice at 11,000 samples/sec or higher. There is also a factor that determines the precision of the captured signal—the more bits used to record the value of the sampled signal, the higher its resolution and the better its sound when played back. However, the more bits used, the more disk space is required for storage or bandwidth for transmission.

ADCCP (Advanced Data Communications Control Procedure)		ADCCP is a data link layer protocol that places data on a network and ensures proper delivery to a destination. ADCCP is basically an ANSI modification of IBM’s SDLC (Synchronous Data Link Control) protocol. It is the ANSIX3.66 standard. The ISO then modified ADCCP into HDLC and the CCITT, now ITU, modified HDLC for its LAPB (Link Access Protocol-Balanced) protocol.

Addresses ,Network		There are two types of network addressing schemes. One is for identifying nodes on a shared Data Link layer LAN. The other is used to identify hosts in an internetwork environment. * LAN addressing : A LAN is a shared media access system. Each node has a MAC(Media Access Control) address that is factory-programmed into its NIC (network interface card). * Internetwork addressing : An internetwork is a collection of LANs and/or other networks that are connected with routers.

ADSL (Asymmetrical Digital Subscriber Line)		The telephone system in the United States is largely digital ,even though most telephones are analog devices. The so-called “last mile” is the local loop of wire that stretches from most homes to the telephone company’s central office (usually in the same neighborhood).This local loop is copper twisted- Pair wire That has Carried voice in a narrow band below400Hz.DSL technologies Expand The bandwidth usage of this cable. DSL technologies use line encoding to provide every high data transmission rates over the local loop.

ADTM (Asynchronous Time Division Multiplexing)		See Multiplexing and Multiplexers.

Agent, Network Management		In general, an agent is a background process that performs an action at a specified time or when an event occurs. In the realm of networking, an agent is part of a network management system that resides in workstations or other network devices (called managed elements) and Collects information to report back to a management system about those devices.

Aggregate Route-Based IP Switching		See ARIS (Aggregate Route-Based IP Switching).

Aggregation, Routing		Aggregation is the process of combining multiple contiguous network routes into a single route that can be advertised to other routers, thus reducing the amount of routing information that is advertised over the network. This is especially important on the Internet, where the number of networks has grown rapidly. Route aggregation is an important part of CIDR (Classless Inter-Domain Routing).

ANSI (American National Standards Institute)		ANSI is an organization that defines coding standards and signaling schemes in the United States and represents the United States in the ISO (International Organization for Standardization) and within the ITU (International Telecommunication Union). ANSI was a founding member of the ISO and plays an active role in its governing. It is one of five permanent members to the governing ISO Council.

Application Layer, OSI Model		The application layer is the top layer of the OSI (Open Systems Interconnection) model. he OSI model guides software developers and hardware vendors in the design of network communications products. When two systems need to communicate, they must use the same network protocols.

APPN (Advanced Peer-to-Peer Networking)		BM’s APPN was introduced by IBM in 1985 and integrated into SNA (Systems Network Architecture). It provides peer-to-peer networking services similar to but not quite the same as TCP/IP. APPN is basically link-layer independent. It can run over token ring, Ethernet, FDDI, frame relay, ISDN, X.25, SDLC, and ultra high-speed networks such as B-ISDN and ATM.APPN is based on the concept that computers on the network have enough processing power of their own to handle session management and routing.

ARCNET		The ARCNET (Attached Resource Computing Network) is a baseband, token-passing network system that offers flexible star and bus topologies at a low price. Transmission speeds are 2.5 Ambits/sec. ARCNET uses a token-passing protocol on a token bus network topology. While ARCNET never became popular in the LAN environment, it is used extensively for embedded and real-time applications.

ARIS (Aggregate Route-based IP Switching)		ARIS is IBM’s scheme for switching IP data grams. It is normally associated with ATM networks, but ARIS can be extended to work with other switching technologies. ARIS (and other IP switching technologies) takes advantage of integrated router switches. The idea is to map routing information to short fixed-length labels so that next-hop routers can be determined by direct indexing, rather than using the standard router packet evaluation and lookup process.

ARP (Address Resolution Protocol)		Every physical machine in a network has two addresses, an Internet Protocol address (IP) and a local MAC address (Media Access Control), when a request comes in from the internet, a protocol is required to convert the IP address into the MAC address.Address Resolution Protocol (ARP) provides this function by mapping Internet Protocol address to a physical machine address that is recognized in the local network. For example, an IP address is 32 bits long. In an Ethernet local area network, however, addresses for attached devices are 48 bits long.The ARP refers to an address table called the ARP cache, which shows the link between each MAC address and its corresponding IP address. ARP provides the protocol rules for making this correlation and providing address conversion in both directions.

ARPANET (Advanced Research Projects Agency Network)		ARPANET was a packet-switching network developed in the early 1970s. ARPANET was funded by ARPA (Advanced Research Projects Agency), which later became DARPA (Defense Advanced Research Projects Agency). The ARPANET network linked defense facilities, government research laboratories, and university sites. It evolved into the backbone of the Internet, and the term “ARPANET” was officially retired in 1990. However, MILNET (military network) was spun off from ARPANET in 1983. In addition, ARPANET spurred the development of one of the most important protocol suites available today, TCP/IP (Transmission Control Protocol/Internet Protocol). TCP/IP is a set of communications procedures and standards that provide a basis for interconnecting dissimilar computers.

Asynchronous Communications		Asynchronous communication is the transmission of data between two devices that are not synchronized with one another via a clocking mechanism or other technique. Basically, the sender can transmit data at any time, and the receiver must be ready to accept information when it arrives. In contrast, synchronous transmission is a precisely timed stream of bits in which the start of a character is located by using a clocking mechanism.

ATM (Asynchronous Transfer Mode)		ATM is a high-speed network technology that is designed for LANs, WANs, carrier and service provider networks, and Internet core networks. It is a connection-oriented switching technology, as opposed to a connectionless technology such as IP. ATM creates a virtual circuit (dedicated path) between source and destination across its switching fabric. These circuits can guarantee bandwidth and quality-of-service. ATM cells negotiate ATM switches with the same efficiency, providing several benefits: * Cell switching is efficient and fast for the reasons just described. * Traffic flow is predictable due to the fixed cell size. * Delivery of time-sensitive traffic (live voice and video) can be guaranteed. * ATM includes QoS (quality of service) features that can be used to guarantee bandwidth for certain types of traffic.

Attenuation		Attenuation is signal loss, measured in decibels, of a signal transmission over distance. The opposite of attenuation is signal amplification. On network cables, attenuation is the degradation of the digital signal or a loss of amplitude of an electric signal.

Authentication and Authorization		According toRFC 2828 (Internet Security Glossary, May 2000), authentication is “the process of verifying an identity claimed by or for a system entity.” The key word here is verify, and the correct terminology is to say that “an authentication system verifies an identity.” Authentication can provide assurance that users (or systems) are who they say they are. Authorization refers to a user’s ability to access resources on a network, usually based on user account rights and privileges.

AWG (American Wire Gauge)		AWG is a measurement system for wire that specifies its thickness. As the thickness of the wire increases, the AWG number decreases. Some common cable conductor gauges are listed here: Gauge Diameter(mm) Cable Type 12 AWG 2.0523 Coaxial thick Ethernet cable 20 AWG 0.8128 Coaxial thin Ethernet cable 22 AWG 0.6452 S-232 serial cable, Telephone cable 23 AWG 0.5740 CAT-6 Cable 24 AWG 0.5105 CAT-3, 5 & 5e Cable 28 AWG 0.3200 Telephone cable