Encyclopedia of Networking P2

Chia sẻ: Thach Sau | Ngày: | Loại File: PDF | Số trang:10

Thêm vào BST

Báo xấu

80
lượt xem 5
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

This term describes the use of an alternative communications path, such as a telephone connection, when the primary one is not available.

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Encyclopedia of Networking P2

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