Installation and Reference Manual
Glossary
346
Installation and Reference Manual v3.2/0410/6
H.323 is a standard approved by the International Telecommunication Union (ITU) in 1996 to promote
compatibility in videoconference transmissions over IP networks. H.323 was originally promoted as a way
to provide consistency in audio, video and data packet transmissions in the event that a local area
network (LAN) did not provide guaranteed service quality (QoS). It is now considered to be the standard
for interoperability in audio, video and data transmissions as well as Internet phone and voice-over-IP
(VoIP) because it addresses call control and management for both point-to-point and multipoint
conferences as well as gateway administration of media traffic, bandwidth and user participation.
In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to
frequent fliers who travel through airport "hubs" to make connecting flights from one point to another. In
data communications, a hub is a place of convergence where data arrives from one or more directions
and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (A
product that is called a "switch" could usually be considered a hub as well.) The distinction seems to be
that the hub is the place where data comes together and the switch is what determines how and where
data is forwarded from the place where data comes together. Regarded in its switching aspects, a hub
can also include a router.
LDAP (Lightweight Directory Access Protocol) is a software protocol for enabling anyone to locate
organizations, individuals, and other resources such as files and devices in a network. LDAP is a
"lightweight" (smaller amount of code) version of Directory Access Protocol (DAP), which is part of X.500,
a standard for directory services in a network. LDAP is lighter because in its initial version it did not include
security features. LDAP originated at the University of Michigan and has been endorsed by at least 40
companies. Netscape includes it in its latest Communicator suite of products. Microsoft includes it as part
of what it calls Active Directory in a number of products including Outlook Express. Novell's NetWare
Directory Services interoperates with LDAP.
In a network, a directory tells you where in the network something is located. On TCP/IP networks
(including the Internet), the domain name system (DNS) is the directory system used to relate the domain
name to a specific network address (a unique location on the network). However, you may not know the
domain name. LDAP allows you to search for an individual without knowing where they're located
(although additional information will help with the search).
An LDAP directory is organized in a simple "tree" hierarchy consisting of the following levels:
The root directory (the starting place or the source of the tree), which branches out to
Countries, each of which branches out to
Organizations, which branch out to
Organizational units (divisions, departments, and so forth), which branches out to (includes an
entry for)
Individuals (which includes people, files, and shared resources such as printers)
An LDAP directory can be distributed among many servers. Each server can have a replicated version of
the total directory that is synchronized periodically. An LDAP server is called a Directory System Agent
(DSA). An LDAP server that receives a request from a user takes responsibility for the request, passing it to
other DSAs as necessary, but ensuring a single coordinated response for the user.
Multicast is communication between a single sender and multiple specific receivers on a network (as in e-
mail to a distribution list).
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