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T
T1
In 1962, the Bell system installed
the first T-carrier system for multiplexing digitized voice signals. The
T-carrier family of systems, which now includes T1, T1C, T1D, T2, T3, T4
(and their European counterparts, E1, E2, etc.), replaced FDM systems,
providing far better transmission quality.
T1 and
its successors were designed to multiplex voice communications. Therefore,
T1 was designed such that each channel carries a digitized representation
of an analog signal that has a bandwidth of 4000Hz, as we explained
earlier. It turns out that 64Kbps is required to digitize a 4000Hz voice
signal. Current digitization technology has reduced that requirement to
32Kbps or less, but a T-carrier channel is still 64Kbps.
T3
T3 is one of the
T-carrier multiplexing standards. It operates at 44.736Mbps, the equivalent
of 672 voice circuits.
TACACS+
TACACS+ is a protocol providing access control for
routers, network access servers and other networked computing devices via
one or more centralized servers. TACACS+ provides separate authentication,
authorization and accounting services.
TCP
TCP stands for
Transmission Control Protocol. TCP is a Transport Layer protocol used to
send messages reliably across a network. It is usually paired with the
Internet protocol (IP).
TDM
TDM stands for
Time Division Multiplexing. This type of multiplexing combines many digital
bit streams with relatively low bit frequency into a single bit stream with
a relatively high bit frequency. It is, in essence, a way for many slow
communications channels to "time share" a very fast channel. The
advantage, of course, is that the cost per bit transmitted on a single fast
channel is lower than on slower channels.
TDM is
accomplished by simply interleaving data from several bit streams. This can
be done on a bit basis or on a byte basis. This is called bit interleaving
and byte interleaving. During time interval 1, 8 bits from source channel 1
are transmitted. During successive intervals, bytes from successive source
channels are transmitted on the output channel. A complete set of values
from each input channel is called a frame.
Teleconferencing
Teleconferencing
refers to holding a conference between people who are located at different
locations. It can be an audio link only or can be both audio and video.
Telephony
Telephony refers
to the transmission of voice signals over a distance (i.e., using telephone
equipment such as switches, telephones and transmission media).
Telnet
Telnet is the TCP
standard protocol for connection of remote terminals to host nodes. TELNET
is a virtual terminal protocol. Virtually all non-graphical applications
written for the TCP/IP environment use the TELNET protocol for input and
output to the user.
Terminal
A terminal refers
to an end device in a network. It is another term used synonymously with
node, host and other terms. It is often used in the context of a dumb
(unintelligent) terminal.
Terminal Emulator
A
terminal emulator program is a program that allows a computer, typically a
PC, to emulate a terminal by mimicking the protocol of the terminal. An
example of a terminal emulator is a program called tn3270. It is available
for the IBM PC and the Apple Macintosh. It makes the PC appear to the
network to be a specific model of an IBM 3270 display device. Some terminal
emulators emulate terminals that have the ability to communicate over phone
lines, so the emulator also provides the ability to connect a remote PC to
a node. For example, the tn3270 program emulates a model of the 3270 that
is connected to the CPU through a network controller, rather than directly
to the CPU's local input/output channel.
Terminal Server
A
terminal server is a device that allows multiple terminals or workstations
to be multiplexed onto a LAN. Each port of a terminal server can connect a
local or remote terminal (or a PC which is emulating a terminal), to any of
the services on the Ethernet. An example of a terminal server is a
DECserver that combines characters from several users into one Ethernet
frame for more efficient use of the LAN media.
TFTP
TFTP stands for
Trivial File Transfer Protocol. It is a TCP/IP Application Layer level
protocol used for transferring files across a network.
Throughput
Throughput refers
to the total amount of user data that can be transmitted from source to
destination over a given amount of time. It does not include the overhead
associated with protocol headers and trailers or network delays.
Time Division Multiplexing
TDM
stands for Time Division Multiplexing. This type of multiplexing combines
many digital bit streams with relatively low bit frequency into a single
bit stream with a relatively high bit frequency. It is, in essence, a way
for many slow communications channels to "time share" a very fast
channel. The advantage, of course, is that the cost per bit transmitted on
a single fast channel is lower than on slower channels.
TLS
The primary goal of the TLS (Transport Layer Security)
Protocol is to provide privacy and data integrity between two communicating
applications. The protocol is composed of two layers: the TLS Record
Protocol and the TLS Handshake Protocol. At the lowest level, layered on
top of some reliable transport protocol (e.g., TCP[TCP]), is the TLS Record
Protocol. The TLS Record Protocol provides connection security that has two
basic properties:
·
Privacy - symmetric cryptography is used for data encryption
(e.g., DES [DES], RC4 [RC4], etc.) The keys for this symmetric encryption
are generated uniquely for each connection and are based on a secret
negotiated by another protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption.
·
Reliability - message transport includes a message integrity
check using a keyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are
used for MAC computations. The Record Protocol can operate without a MAC,
but is generally only used in this mode while another protocol is using the
Record Protocol as a transport for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of
various higher level protocols. One such encapsulated protocol, the TLS
Handshake Protocol, allows the server and client to authenticate each other
and to negotiate an encryption algorithm and cryptographic keys before the
application protocol transmits or receives its first byte of data. The TLS
Handshake Protocol provides connection security that has three basic
properties:
·
The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required for at least
one of the peers.
·
The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who can place
himself in the middle of the connection.
·
The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to the
communication.
One advantage of TLS is that it is application protocol
independent.
Higher level protocols can layer on top of the TLS Protocol transparently.
The TLS standard, however, does not specify how protocols add security with
TLS; the decisions on how to initiate TLS handshaking and how to interpret
the authentication certificates exchanged are left up to the judgment of
the designers and implementors of protocols which run on top of TLS.
Topology
Topology refers
to the specific physical configuration of a network or a portion of a
network. Ring and Star are examples of different network topologies.
TPS
Transactions per
second (TPS) is probably the best measure of throughput. An example of a
TPS measurement would be 7200 TPH or 7200 transactions per hour (or 2 TPS).
Knowing the TPH is not enough to get a good handle on overall performance,
you must also know the average transaction size and the TPH over different
blocks of time.
Trace
A trace is a file
that displays network activity. It is taken using a product that can trap
frames going across a network. Examples of products that take traces are
the Network General Sniffer and the HP LAN Advisor.
Trailer
A trailer is the
last part of a data frame which carriers the error control information for
the frame. The error control information usually comes in the form of a
cyclical redundancy control bytes or frame check sequences (FCS).
Transactions Per Second/Transactions
Per Hour
See
TPS and/or TPH.
Transceiver
Transceiver is a
word that is a combination of transmitter and receiver. The physical device
that connects a host interface to a local area network, such as Ethernet.
Ethernet transceivers contain electronics that apply signals to the cable
and sense collisions.
Transfer Rate
The
transfer rate is rate of raw data movement across a network. It is most
often stated as a bit rate. The bit rate is the number of bits transmitted
per second (bps). For instance, commonly used rates for modem speeds are
300bps, 1200bps, 9600bps, 14,400bps, and 19,200bps. Another way of writing
19,200bps is 19.2Kbps. Kbps stands for kilobits per second which would mean
1000 bits transmitted per second times 19.2. 1.544Mbps is the T1 rate and
is 1,544,000bps.
Transmission Control Protocol (TCP)
The
TCP layer is responsible for providing connection-oriented, end-to-end,
error free message delivery. Since it uses the IP layer's unreliable
delivery service, its major responsibilities are error correction and
connection management.
Transmission Rate
See
transfer rate.
Transparent Bridge
Transparent
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.
Transport Layer
The
Transport Layer is the fourth layer in the OSI reference model. It is
concerned with controlling the transport of messages in a network.
Trunk Lines
Trunk
lines are the physical connections between end office of the telephone
network.
Twinax
Twinax or
twinaxial cable is cable that consists of two individually insulated coax
cables wrapped in a single cover.
Twisted Pair
A twisted
pair line, as the name implies, consists of a pair of wires twisted
together. Because the wires are twisted together, electrical interference
tends to affect both wires equally, so it does not affect the difference in
the potential between the two wires. This makes twisted pair cable less
susceptible to signal loss than if it weren't twisted. Twisted pair lines
are suitable for bit rates up to 1Mbps over short distances (less than 100m)
and lower bit rates over longer distances. More sophisticated driver and
receiver circuits enable similar or even higher rates to be achieved over
much longer distances. These are referred to as UTP or unshielded twisted
pair. UTP is used extensively in telephone networks and in many data
communications applications.
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