|
L
L2TP
The L2TP Protocol is used
for integrating multi-protocol dial-up services into existing Internet
Service Providers Point of Presence (hereafter referred to as ISP and POP,
respectively). This protocol may also be used to solve the "multilink
hunt-group splitting" problem. Multilink PPP, often used to aggregate
ISDN B channels, requires that all channels composing a multilink bundle be
grouped at a single Network Access Server (NAS). Because L2TP makes a PPP
session appear at a location other than the physical point at which the
session was physically received, it can be used to make all channels appear
at a single NAS, allowing for a multilink operation even when the physical
calls are spread across distinct physical NASs.
LAN
LAN stands for
local area network. A LAN is a group of networked computers that are hooked
together with the same type of media such as Ethernet and under the control
of a standard protocol.
LAN Driver
A LAN
driver is a software component that provides communication between the NICs
(Network Interface Card) and the redirector software running in the
computer. Each type of NIC uses its own type of LAN driver software.
LAN ID
There
are many types of addresses in data communications systems. Examples of
addresses are physical addresses, station addresses, LAN IDs, network
addresses and ports. The lowest level address in most networks is the
physical address. An example of a physical address would be a 48-bit
Ethernet address. This is the unique Ethernet NICs address which is
"burned in" to each Ethernet circuit board. Other names for physical
addresses are MAC address, station address or LAN ID. These typically
relate to the Data Link layer of the OSI model.
LAPB
LAP-B (or LAPB)
stands for Link Access Protocol Balanced. It is an HDLC protocol subset
used primarily in X.25 communications. LAP-B is the Data Link layer
sublayer of the X.25 protocol. LAP-B is responsible for point-to-point
delivery of error-free frames. It is balanced because the LAP-B standard
excludes the portions of the HDLC standard having to do with multi-drop,
"unbalanced" operation. It is very similar to LAP-D.
LAPD
LAPD or LAP-D
stands for Link Access Procedure-D. It is part of the ISDN layered
protocol. It is very similar to LAP-B. The D is for "D channel."
It defines the protocol used on the D channel to interface with the phone
company's SS7 network for setting up calls and other signaling functions.
LAT
LAT stands for
local area transport. LAT is implemented by DECservers and VAXes to
transfer character data via Ethernet. The term LAT also refers to the
protocols used by nodes that support the architecture. One important
purpose of the LAT architecture is to combine characters from several users
into one Ethernet frame. This helps to utilize the Ethernet connection more
fully. More importantly, it allows the host to handle input more
efficiently, since the host can process several characters from several
users each time it is interrupted for an Ethernet frame.
Latency
Latency is the
amount of time that it takes a network component such as a bridge or router
to transmit a received frame or packet.
Layering
Layering in the
context of data communication is the process of building complex software
communications systems through a series of software functions called
layers. Each layer serves a particular function and provides services to
adjacent layers. The OSI model is built on the concept of layering.
LDAP
The LDAP (Lightweight Directory Access Protocol.)
provides access to X.500 directories without using the DAP (Directory
Access Protocol). It is used for simple management applications and browser
applications that provide simple read/write interactive access to the X.500
directory and should complement the DAP. X.500 technology has proved to be
highly popular, and therefore led to efforts to reduce the high ?cost of
entry? associated with it. Until now methods suggested were based on
specific applications and, as such, were limited. The LDAP is also a
directory protocol alternative, but it is not dependant on a particular
application. As such it is intended to be simpler and less expensive than
existing ones.
Learning Bridges
Learning
bridges are bridges that have the intelligence to route networking frames
with only the information available in the frame itself are called
transparent bridges. Transparent bridges broadcast messages across the
network when they are first turned on to gain access to addresses within
the network. Transparent bridging takes place when the sending and
receiving nodes are not even aware that a bridge exists between them.
Transparent bridges are also called learning bridges and adaptive bridges.
Leased Line
When
the telephone network was an analog network, electro-mechanical switches
could inject a certain amount of noise into circuits. Other instruments in
the transmission path, such as multiplexers, could corrupt the signal
further. At lower data rates, say 300 or 1200bps, modems can correctly
transmit data with few if any errors even when the line is noisy, but as
data rates increase, noise causes more problems. As a practical matter, in
the analog network, data could not be transmitted reliably over long
distances on switched lines at rates over 4800bps. Therefore, it became
common practice for the telephone companies to "lease" lines to
companies for continuous, unswitched use. These leased lines are also
referred to as "dedicated circuits." A company with two computer
sites could, for example, lease a line or a set of lines to interconnect
the sites.
LEC
LEC stands for
local exchange carriers. These consist of the 23 Bell Operating Companies
(BOCs) that were created by the divestiture, the former independent
telephone companies such as GTE and Contel, and about 1500 small-town
telephone companies.
Legacy Systems
Legacy
systems refer to systems that have been with the company over a long period
of time. They are typically mainframe systems that contain programs and
information, which are vital to the day-to-day business of the company.
Link
A link is a
communications path between two or more communicating devices. Links are
also referred to as channels, lines, circuits and paths. Link Layer refers
to the Data Link Layer or layer 2 of the OSI model which is concerned with
point-to-point communications.
Link-State Routing
The
alternative to vector-distance routing is link-state routing, also called
SPF (Shortest Path First) algorithm. Whereas routers employing
vector-distance routing algorithms periodically broadcast route
information, link-state routers exchange information when specific routes
change. This is done via Link-State Packets (LSP).
LLC
LLC stands for
Logical Link Control. The IEEE 802.2 standard defines the LLC which is the
upper layer of the 802 LAN protocol suite.
Local Area Network
LAN
stands for local area network. A LAN is a group of networked computers that
are hooked together with the same type of media such as Ethernet and under
the control of a standard protocol.
Local Area Transport
LAT
stands for Local Area Transport. LAT is implemented by DECservers, VAXes
and/or clusters to transfer character data via Ethernet. The term LAT also
refers to the protocol used by nodes that support the architecture. One
important purpose of the LAT architecture is to combine characters from
several users into one Ethernet frame. This helps to utilize the Ethernet
connection more fully. More importantly, it allows the host to handle input
more efficiently, since the host can process several characters from
several users each time it is interrupted for an Ethernet frame.
Local Exchange
A
central office (CO) is a telephone company facility where local loops are
terminated. The function of a CO is to connect individual telephones
together through a series of switches. COs are tied together in a hierarchy
for efficiency in switching. Other terms for a central office are local exchange,
wiring center, and public exchange.
Logical Address
A
logical address differs from a physical address in that it is generally
implemented as software rather than a hardware entity. Some examples of
these types of addresses would be an IP address such as
"144.25.54.8," an IBM SNA LU (logical unit) address of
"02," a port number such as "23," or a SAP (Service
Access Point) number such as "E0." The most important fact to
remember concerning logical addresses is that a logical address will not
get the information "into the box." Only the physical address,
whether it's a broadcast address, multicast (group) address, or a single
destination (unicast) address, can accomplish this.
Logical Channel
A
logical channel is an X.25 term that means the logical connection (vs.
physical connection) between the user terminal and the X.25 packet network.
The DTE to DCE connection on both sides of "the cloud" has a
logical channel number assigned by the software.
Logical Link Control
LLC
stands for Logical Link Control. The IEEE 802.2 standard defines the LLC
which is the upper layer of the 802 LAN protocol suite.
Logical Unit (LU)
Each
node in an SNA network contains one or more LUs. For a cluster controller,
each device in the cluster (each terminal, printer, etc.) has one LU. LU
has several functions. In the cluster controller, it implements data
transfer between the PU (the controller) and the device (terminal, printer,
etc.). In the host, it implements data transfer between the PU and the
application. In the controller, it implements data transfer between
communication lines, or between a communications line and the CPU channel.
The
type of the LU defines the type of data that the LU is capable of accepting
and the services it provides. These are the LU types originally defined for
SNA:
-
- LU 0. This is the "catch-all" type
in nodes that don't fit any of the following descriptions. Uses
User-Defined Data Stream.
- LU 1. Used in a device such as a printer.
Uses SNA Character String Data Stream.
- LU 2. Used in an IBM 3270 or any other
similar cluster controller. Uses the SNA 3270 Data Stream.
- LU 3. Used in a printer that's not attached
to a cluster but which uses the SNA 3270 Data Stream.
- LU 4. Like LU 1, but supports direct
communications between LUs in the same cluster controller.
|
