Encyclopedia of Networking P2

AMP (Active Monitor Present) 29 a b c d e f g h i j k l m n o p q r s t u v w x y z ▼ Alternate Route Selection (ARS) SEE ARS (Alternate Route Selection) ▼ Alternate Routing This term describes the use of an alternative communications path, such as a telephone connection, when the primary one is not available. ▼ AM (Accounting Management) In network management, a function for gathering performance and usage informa- tion from a network. ▼ AM (Active Monitor) In a token ring network, the node that is responsible for creating, passing, and main- taining the token. The performance of the AM is monitored constantly by standby monitors (SMs) to ensure that the token- passing process is not interrupted. ▼ AME (Asynchronous Modem Eliminator) An AME, also known as a null modem, is a serial cable and connector with a modified pin configuration (compared to an ordinary RS-232 cable). This cable enables two com- puters to communicate directly; that is, without modems as intermediaries. ▼ American National Standards Institute (ANSI) SEE ANSI (American National Standards Institute) ▼ America Online (AOL) SEE AOL (America Online) ▼ AMF (Account Metering Function) In the OSI network management model, the function that keeps track of every user’s resource usage. ▼ AMH (Application Message Handling) In the International Standardized Profile (ISP) model, the prefix used to identify MHS (Message Handling System) actions. ▼ AMI (Alternate Mark Inversion) A signal-encoding scheme in which a 1 is represented alternately as positive and nega- tive voltage, and 0 is represented as zero voltage. It does not use transition coding, but can detect noise-induced errors at the hardware level. SEE ALSO Encoding, Signal ▼ AMP (Active Monitor Present) In token ring networks, a packet issued every 3 seconds by the active monitor (AM) 30 Amplifier on the ring to indicate that the AM is work- ing and is still in charge. ▼ Amplifier A device for boosting an analog signal. The same service is provided by a repeater for digital signals. ▼ Amplitude The magnitude, or level, of a signal. For an electrical signal, it is expressed in volts (voltage) or amperes (current). In computer contexts, current is more likely to be expressed in milliamperes. ▼ AMPS (Advanced Mobile Phone Service) A cellular telephone service. AMPS is a wire- less analog communications service that operates in the 825 to 890 megahertz range. ▼ Analog Communication A telecommunications system that uses analog (that is, continuous, sinusoidal) signals to represent information. An exam- ple of an analog communication system is the classic voice-based telephone system (which is being replaced by the newer, digital systems). ▼ Analog Intensity Modulation (AIM) SEE AIM (Analog Intensity Modulation) ▼ Analog-to-Digital Conversion The process of converting an analog signal (one that can take on any value within a specified range) to digital form. An analog- to-digital converter (ADC) is a device that converts an analog signal to digital form. ▼ ANF (AppleTalk Networking Forum) A consortium of developers and vendors working to encapsulate AppleTalk in other protocols; for example, within the TCP/IP suite. ▼ ANI (Automatic Number Identification) In ISDN and some other telecommunica- tions environments, a feature that includes the sender’s identification number, such as telephone number, in the transmission, so that the recipient knows who is calling; also known as caller ID. ▼ Annex D In frame-relay technology, a document that specifies a method for indicating permanent virtual circuit (PVC) status. The document is part of the ANSI T1.617 standard. ▼ Anonymous FTP On the Internet, a protocol that allows a user to retrieve publicly available files from other networks. By using the special user ID, “anonymous” users can transfer files with- out a password or other login credentials. (FTP is an application-layer protocol in the Internet’s TCP/IP protocol suite.) Anti-Virus Program 31 a b c d e f g h i j k l m n o p q r s t u v w x y z ▼ Anonymous Remailer An Internet service that can be used to hide the origins of an e-mail message being sent to someone. The anonymous remailer removes any source address information from a message, substitutes any specified pen name, and then sends the message on to the specified destination. ▼ ANSI (American National Standards Institute) The United States representative in the ISO (International Standardization Organiza- tion). ANSI creates and publishes standards for programming languages, communica- tions, and networking. For example, the standard for the FDDI network architecture is ANSI X3T9.5. ▼ Anti-Virus Program An anti-virus program is used for detecting or removing a computer virus. An anti-virus program looks for suspicious activity, such as unnecessary disk access, attempts to inter- cept a BIOS or other low-level call, and attempts to format or delete files. In some cases, the anti-virus program detects a pat- tern characteristic of a particular virus. Some anti-virus programs are TSR (terminate-and-stay-resident) programs, which monitor computer activity constantly, looking for indications of a virus. In some cases, these types of programs can be extremely annoying and very processor intensive. Users have been known to remove an anti-virus TSR program from memory out of frustration. Other anti-virus programs are intended to be run periodically. When they are run, the programs look for the tell-tale signs (known as signatures) of particular viruses. These programs are minimally disruptive; on the other hand, their effectiveness is directly proportional to the frequency with which they are used. Because the coding for computer viruses is constantly changing, anti-virus programs must also be updated regularly. It is impor- tant to test anti-virus programs thoroughly, which means that every new release must be tested. Make sure an anti-virus program per- forms to your expectations before installing it on a network. Some programs can eat up a significant amount of working memory. Recently, a very different (and, conse- quently, very controversial) type of anti- virus program has become available. InVirc- ible, created by Zvi Netiv, is designed to detect viruses that have already infected a system, and to clean these up. Rather than looking for virus signatures, InVircible uses expert system rules to look for behavior characteristic of viruses: replication, use of memory, attempts to attach to the anti-virus program, etc. InVircible will even put out “virus bait” to get an existing virus to try to infect the bait. BROADER CATEGORY Data Protection RELATED ARTICLE Virus 32 AOL (America Online) ▼ AOL (America Online) America Online is a commercial online ser- vice like CompuServe and Prodigy. AOL supports both DOS and Windows users, and provides a range of services (mail, news, ref- erence, financial, entertainment, Internet access, etc.). Users pay a flat monthly fee, which allows a limited number of free hours. Additional hours are billed at a predeter- mined rate. AOL’s graphical interface is highly regarded—in fact, Apple has licensed the interface technology for use in Apple’s eWorld interface. AOL provides a very com- prehensive set of access opportunities to the Internet. FOR INFORMATION Call AOL at 800-827-6364 ▼ AOM (Application OSI Management) In the International Standardized Profile (ISP) model, the prefix for functions and services related to network management. ▼ AOW (Asia and Oceania Workshop) One of three regional workshops for imple- menters of the OSI Reference Model. The other two are EWOC (European Workshop for Open Systems) and OIW (OSI Imple- menters Workshop). ▼ AP (Application Process) In the OSI Reference Model, a program that can make use of application layer services. Application service elements (ASEs) provide the requested services for the AP. ▼ APD (Avalanche Photodiode) A detector component in some fiber-optic receivers. The APD converts light into elec- trical energy. The “avalanche” refers to the fact that the detector emits multiple elec- trons for each incoming photon (light particle). ▼ APDU (Application Protocol Data Unit) A data packet at the application layer; also called application-layer PDU. SEE ALSO OSI Reference Model ▼ API (Application Program Interface) An abstract interface to the services and pro- tocols offered by an operating system, usu- ally involving a published set of function calls. Programmers and applications can use the functions available in this interface to gain access to the operating system’s services. ▼ APIA (Application Program Interface Association) A group that writes APIs for the CCITT’s X.400 Message Handling System (MHS). ▼ APPC (Advanced Program-to- Program Communications) In IBM’s SAA (Systems Application Archi- tecture), APPC is a collection of protocols to enable executing applications to commu- nicate directly with each other as peers (without intervention by a mainframe host). AppleTalk 33 a b c d e f g h i j k l m n o p q r s t u v w x y z APPC is defined at a level comparable to the session layer in the OSI Reference Model. It can be supported in various net- working environments, including IBM’s SNA (System Network Architecture), Ethernet, Token Ring, and X.25. APPC/PC (Advanced Program-to- Program Communications/Personal Com- puters) is a PC-based version of APPC. ▼ AppleDouble In the Macintosh world, a file format that uses separate files for the data and resource forks that make up a Macintosh file. This enables the files—or at least the data por- tion—to be used on different platforms. COMPARE AppleSingle ▼ AppleShare A network operating system from Apple. AppleShare runs on a Macintosh network server, providing file and printer services. AppleShare uses the AppleTalk protocol suite to carry out its tasks. SEE ALSO AppleTalk ▼ AppleSingle In the Macintosh world, a file format that stores both a file’s contents (data fork) and its resources (resource fork) within a single file. Because data and resources are mixed in a proprietary format, such a file cannot be used on other platforms. COMPARE AppleDouble ▼ AppleTalk AppleTalk is Apple’s proprietary protocol suite for Macintosh network communica- tions. It provides a multilayer, peer-to-peer architecture that uses services built into the operating system. This gives every Macin- tosh networking capabilities. AppleTalk can run under any of several network operating systems, including Apple’s AppleShare, Novell’s NetWare for Macintosh, and Sun Microsystems’ TOPS. AppleTalk was developed in the mid- 1980s with the goal of providing a simple, portable, easy-to-use, and open networking environment. To access such a network, a user just needs to “plug in, log in, and join in.” A newer version, Phase 2, was released in 1989. This version provided some new capa- bilities and extended others. AppleTalk is a comprehensive, layered envi- ronment. It covers networking services over almost the entire range of layers specified in the OSI Reference Model. The figure “The AppleTalk protocol hierarchy” shows the organization of the AppleTalk layers, as well as the protocols in the AppleTalk Protocol Suite. AppleTalk Layers 34 AppleTalk THE APPLETALK PROTOCOL HIERARCHY Please register! AppleTalk 35 a b c d e f g h i j k l m n o p q r s t u v w x y z There are AppleTalk implementations for the following network architectures at the physical and data-link layers: ■ Apple’s 230 kilobit per second (Kbps). ■ LocalTalk architecture. LocalTalk pro- vides a media-access method and a cabling scheme for AppleTalk. The architecture uses twisted-pair cables and RS-422 connections, allows nodes to be separated by as much as 305 meters (1,000 feet), and can transmit at up to 230.4 Kbps. The term Local- Talk is sometimes used to refer to an AppleTalk network. ■ EtherTalk, Apple’s implementation of the 10 megabit per second (Mbps) Ethernet architecture. Two versions of EtherTalk exist. The earlier one, EtherTalk Phase 1, is modeled on the Blue Book Ethernet 2.0 (as opposed to the version specified in the IEEE 802.3 documentation). Its successor, Phase 2, is modeled on the IEEE 802.3 standard. Because these two variants of Ethernet define packets somewhat differently, Phase 1 and Phase 2 nodes cannot communicate directly with each other. EtherTalk has replaced LocalTalk as the default networking capability in newer Macintosh models. ■ TokenTalk, Apple’s implementation of the token-ring architecture. AppleTalk supports both the 4-Mbps version specified by IEEE 802.5 and the 16- Mbps version from IBM. The token- ring architecture is supported only in AppleTalk Phase 2. Physical and Data-Link Layers ■ FDDITalk, Apple’s implementation of the 100 Mbps FDDI architecture. For each of these architectures, a Link Access Protocol (LAP) is defined: LLAP for LocalTalk, ELAP for EtherTalk, TLAP for TokenTalk, and FLAP for FDDITalk. All AppleTalk networks use the DDP (Data- gram Delivery Protocol) at the network layer, regardless of the architecture operat- ing at the data-link layer. This protocol makes a best effort at packet delivery, but delivery is not guaranteed. Note also the AARP (AppleTalk Address Resolution Protocol) at this layer. The AARP maps AppleTalk (network) addresses to Ethernet or Token Ring (physical) addresses. For reliable packet delivery, the ADSP (AppleTalk Data Stream Protocol) and ATP (AppleTalk Transaction Protocol) are available. Each of these protocols is appropriate under different conditions. The NBP (Name Binding Protocol) and ZIP (Zone Information Protocol) help make addressing easier. NBP associates easy-to- remember names (used by users) with the appropriate address. ZIP is used mainly on larger networks or internetworks, which are more likely to be divided into zones. A zone is a logical group- ing of nodes that together make up a subnet- work. The concept of a zone was introduced to allow for larger networks with more than 255 nodes, and also to make addressing and routing tasks easier. Network Layer Higher Layers 36 AppleTalk Applications access an AppleTalk net- work through the AFP (AppleTalk Filing Protocol); they access printer services by shipping PostScript files through the PAP (Printer Access Protocol). A few protocols make use of services from more than one lower-level protocol. For example, ZIP relies on ATP and DDP services. The following protocols make up the Apple- Talk Protocol Suite (see the figure “The AppleTalk protocol hierarchy,” earlier in this article): AARP (AppleTalk Address Resolution Protocol): A network-layer protocol that maps AppleTalk (network) addresses to physical addresses. ADSP (AppleTalk Data Stream Protocol): A session-layer protocol that allows two nodes to establish a reliable con- nection through which data can be transmitted. AEP (AppleTalk Echo Protocol): A transport-layer protocol used to deter- mine whether two nodes are connected and both available. AFP (AppleTalk Filing Protocol): A pre- sentation/application-layer protocol used by applications to communicate with the network. ASDSP (AppleTalk Safe Data Stream Protocol): A session-layer protocol that is similar to ADSP but that pro- vides additional security against unauthorized use. ASP (AppleTalk Session Protocol): A session-layer protocol used to begin and end sessions, send commands from client to server, and send replies from server to client. ATP (AppleTalk Transaction Protocol): A transport-layer protocol that can pro- vide reliable packet transport. Packets are transported within the framework of a transaction (an interaction between a requesting and a responding entity {program or node}). AURP (AppleTalk Update Routing Protocol): A transport-layer routing protocol that is similar to RTMP (Routing Table Maintenance Proto- col) but that updates the routing table only when a change has been made to the network. DDP (Datagram Delivery Protocol): A network-layer protocol that prepares and routes packets for transmission on the network. LAP (Link Access Protocol): Works at the data-link layer, converting packets from higher layers into the appropriate form for the physical transmission. Each network architecture needs its own LAP. ELAP (EtherTalk Link Access Protocol): The link-access protocol used for Ethernet networks. FLAP (FDDITalk Link Access Protocol): The link-access protocol used for FDDI networks. AppleTalk Protocol Suite AppleTalk 37 a b c d e f g h i j k l m n o p q r s t u v w x y z LLAP (LocalTalk Link Access Protocol): The link-access protocol used for LocalTalk networks. TLAP (TokenTalk Link Access Proto- col): The link-access protocol used for Token Ring networks. ARAP (AppleTalk Remote Access Protocol): A link-access protocol for accessing the network from a remote location over a serial line. NBP (Name Binding Protocol): A transport-layer protocol that associ- ates device names with network addresses. If the NBP is successful, this binding process will be completely transparent to the user. PAP (Printer Access Protocol): A session- layer protocol for creating a path from the user or application to a printer. RTMP (Routing Table Maintenance Protocol): A transport-layer routing protocol for moving packets between networks. ZIP (Zone Information Protocol): A session-layer protocol used to help find a node; for example, in a large internetwork. If installed, an AppleShare server runs on top of these protocols at the uppermost (application) layer. The AppleShare server uses the AFP to provide centralized file shar- ing for its clients, and can use the PAP to provide printer sharing. In AppleTalk networks, every node has an official numerical address. In addition, a node may be part of a named group of nodes, which somehow belong together. Each AppleTalk network is assigned a unique network number, and each node in that network is assigned this number. Pack- ets addressed to a node on the network must include the network number. In addition to a network number, each node has a node number that is unique within that network. This is an 8-bit number and can be any value between 1 and 254, inclusive (0 and 255 are reserved as node numbers). However, servers must have node numbers within the range of 128 to 254, and workstations must have numbers in the 1 to 127 range. A zone is a logical grouping of nodes. The basis for the grouping can be any criterion that is useful for a particular configuration, as in the following examples: ■ Geographical, such as all machines on the second floor ■ Departmental, such as all machines in the marketing department ■ Functional, such as all machines that can provide access to printers By restricting routing or searches to machines in a particular zone, network traf- fic and work can be reduced considerably. Numbers and Zones Network and Node Numbers Zones 38 AppleTalk Accessing resources by zones also makes it easier to determine what is available for specific needs. A node may belong to more than one zone at the same time, or not be part of any zone. A zone can cross network boundaries; that is, a zone can consist of parts of two or more different networks or include multiple networks. Phase 2, an updated version of AppleTalk, was released in 1989. This version provides several improvements over Phase 1, includ- ing the following: ■ Allows more than 254 nodes per network ■ Allows a network to be assigned more than one network number ■ Introduced the AppleTalk Internet Router, which allows up to eight AppleTalk networks to be connected In AppleTalk Phase 2, a network can be assigned a range of network numbers. A particular node on this network can be asso- ciated with any one number in this range. By providing multiple network numbers for a single network, it is possible to have more than the 254 nodes allowed in a Phase 1 net- work, because each network number can support 253 (yes, 253) individual nodes. Phase 2 AppleTalk Network Numbering in Phase 2 When you are assigning number ranges, a rough guideline is to assign one network number for every 25 to 50 nodes. If you expect a lot of growth, use a smaller num- ber. For example, assigning two network numbers for a 100-node network leaves room for 406 additional nodes. When a network is part of an internet- work, there are several restrictions on what can be connected and how. These restric- tions concern routers and bridges, and the networks they can connect, as follows: ■ All routers connected to a particular network must use the same network number range for the interface with that network. For example, if a router thinks the network uses numbers 1,000 to 1,009, another router con- nected to the same network cannot use 1,002 to 1,008. ■ Routers must connect networks with different number ranges that do not overlap. This means that routers can- not connect a network to itself and that networks with overlapping net- work numbers cannot interact with each other. ■ A bridge must connect network seg- ments with the same number range. The figure “Rules for connecting AppleTalk Phase 2 internetworks” illustrates these rules. . AppleTalk networks, every node has an of cial numerical address. In addition, a node may be part of a named group of nodes, which somehow belong together at the same time, or not be part of any zone. A zone can cross network boundaries; that is, a zone can consist of parts of two or more different networks