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.. For reliable packet delivery, the
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▼
Alternate Route Selection (ARS)
S E E
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)
S E E
ANSI (American National Standards Institute)
▼
America Online (AOL)
S E E
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
S E E A L S O
Encoding, Signal
▼
AMP (Active Monitor Present)
In token ring networks, a packet issued every 3 seconds by the active monitor (AM)
Trang 230 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)
S E E
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.)
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▼
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
B RO A D E R C A T E G O R Y
Data Protection
R E L A T E D A R T I C L E
Virus
Trang 432 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
F O R I N F O R M A T I O N
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.
S E E A L S O
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)
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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
C O M P A RE
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
S E E A L S O
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
C O M P A RE
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
Trang 634 AppleTalk
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Please register!
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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
Trang 836 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
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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
Trang 1038 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 followinclud-ing:
■ 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