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| Microsoft® Encyclopedia of Networking, Second Edition |
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Chapter : T
- T1
- T1 channel bank
- T3
- T.120
- TACACS
- taking ownership
- tape drive
- tape format
- tape library
- TAPI
- T-carrier
- TCP
- TCP/IP
- TCP three-way handshake
- TDM
- TDMA
- TDR
- TechNet
- telco
- Telecommunications Industry Association (TIA)
- telecommunications services
- telecommuting
- Telephony Application Programming Interface (TAPI)
- Telnet
- terminal
- Terminal Access Controller Access Control System (TACACS)
- terminal emulator
- terminal server
- Terminal Services
- terminator
- Terrestrial Trunked Radio (Tetra)
- test equipment
- Tetra
- text file
- TFTP
- thick coax
- thicknet
- thin client
- thin coax
- thinnet
- TIA
- Time Division Multiple Access (TDMA)
- time-division multiplexing (TDM)
- time domain reflectometry (TDR)
- TLD
- TLS
- TN3270
- TN5250
- Token Ring
- top-level domain (TLD)
- topology
- tracert
- transaction
- transaction log
- Transact-SQL
- transceiver
- transceiver cable
- Transmission Control Protocol (TCP)
- Transmission Control Protocol/ Internet Protocol (TCP/IP)
- transport layer
- Transport Layer Security (TLS)
- tree
- Trivial File Transfer Protocol (TFTP)
- trunking
- trust
- T-SHARE
- tunneling
- twinax cabling
- twisted-pair cabling
- two-way transitive trust
T
T1
The lowest level of the T-carrier hierarchy.Overview
T1 is part of the T-carrier digital transmission architecture developed for the Public Switched Telephone Network (PSTN) in the 1960s. A T1 circuit (also called a T1 line) is formed from a combination of 24 DS-0 (Digital Signal Zero) channels, each having a bandwidth of 64 kilobits per second (Kbps), for a total bandwidth of 1.544 megabits per second (Mbps). These 24 DS-0 channels can either be used separately for carrying 24 separate voice circuits (called channelized T1) or aggregated into a single data stream (called unchannelized T1) for high-speed wide area network (WAN) connections.
T1 (sometimes called T-1) actually stands for T-carrier Level 1, but it is almost never referred to in this way.
Uses
T1 is the preferred technology used by enterprises for combining voice, fax, and data transmissions. This is because T1 is "trunking" technology that enables a single physical circuit to support as many as 24 separate virtual circuits, a process which is generally cheaper than provisioning 24 separate physical links. T1 lines are also typically used
- To provide enterprises with dedicated leased-line WAN links among remote locations—for example, to connect a branch office to corporate headquarters.
- To provide corporate users with high-speed access to the Internet.
Architecture
Like other members of the T-carrier family, T1 uses time-division multiplexing (TDM) to interleave multiple DS-0 channels into a single bit stream (called a DS-1 circuit). DS-0 generates 8 bits (1 byte) every 125 microseconds, or 8000 DS-0 frames per second. The bandwidth of a DS-0 channel is therefore
DS-0 = 8 bits x 8000 per second
= 64,000 bits per second (bps)
= 64 kilobits per second (Kbps)
Because T1 multiplexes 24 DS-0 channels together, a single T1 frame (or DS-1 frame) should equal 24 x 8 = 192 bps. The T1 specification, however, adds an extra bit to each frame to ensure that transceivers at each end of the line maintain their synchronization. This extra bit is added at the start of each DS-1 frame, which makes the length of a DS-1 frame equal to 192 + 1 = 193 bits. Using the same transmission rate of 8000 frames per second, this means that the total bandwidth of a T1 circuit is
T1 = 193 bits/frame x 8000 frames/sec
= 1544000 bits/sec
= 1.544 Mbps
TDM is applied to the individual DS-0 channels in such a way that each DS-0 channel is located at the same position of each DS-1 frame generated.
To package binary information into electrical signals, T1 originally used the Alternate Mark Inversion (AMI) line coding mechanism in which a voltage represents a binary 1 and no voltage represents zero. The problem with this mechanism was that it was hard to maintain synchronization between transceivers at opposite ends of the T1 circuit when a large number of successive 0s or 1s were transmitted. A scheme was therefore devised whereby bits were "robbed" from certain parts of each frame to ensure that synchronization could be maintained and to allow for control and signal maintenance information to be carried in-band within the circuit. The net result of this bit robbing was to reduce the data-carrying capacity of each DS-0 channel within DS-1 from 64 Kbps to only 56 Kbps. However, this bit-robbing scheme has no discernable effect on voice transmission.
You can work around the capacity-robbing effect of this bit robbing by replacing AMI line coding with Bipolar with 8-bit Zero Substitution (B8ZS) line coding. B8ZS substitutes a special byte if eight consecutive zero bits are detected to maintain a specific ones density to help maintain synchronization. This approach is called "ones density" and allows a T1 channel service unit/ data service unit (CSU/DSU) at the customer premises to recover the data clock reliably when synchronization is lost with the T1 multiplexer at the telco central office (CO). The result of using B8ZS is that each DS-0 channel can carry the full 64 Kbps of data. An alternative scheme to B8ZS that is also commonly used is Zero Byte Time Slot Interface (ZBTSI) line coding.
Bellcore also developed an alternate scheme whereby a 2 Binary 1 Quaternary (2B1Q) line coding scheme was employed. 2B1Q is the same signal encoding mechanism employed by Integrated Services Digital Network (ISDN) and encodes 2 bits/baud instead of the 1 bit/ baud supported by AMI. This new technology was called "repeaterless T1" because it eliminated the necessity of regenerating T1 signals every 6000 feet (1830 meters) using repeaters, a process that made original T1 deployments complex and expensive. Repeaterless T1 needed repeaters only every 12,000 feet (3660 meters) and transmitted data at only 784 Kbps over each twisted pair. Because two pairs of wires are used for T1, this new technology also carries data at T1 speed of 1.544 Mbps. This new technology is now commonly referred to as High bit-rate Digital Subscriber Line (HDSL). A telco will often provision customers with HDSL and call it T1 instead, because it is functionally equivalent in speed and framing to T1.
Implementation
T1 cannot operate over analog Plain Old Telephone Service (POTS) telephone lines. Instead, it must be deployed using specially conditioned copper twisted-pair lines, with two pairs of wires (four wires) being used for a single T1 circuit. To support full-duplex communication, two of these four wires are used for transmission (TX interface) and the other two for receiving (RX interface). T1 lines typically terminate at the customer premises with an RJ-48 connector, which looks like an RJ-45 connector but is pinned differently. T1 lines are generally unshielded twisted-pair (UTP) cabling but other media can be used, including coaxial cabling or fiber-optic cabling.
T1 usually cannot run over existing local loop wiring because:
- Bridge taps installed by telcos to trunk telephone traffic in neighborhood wiring causes distortion of T1 signals, so these must be removed to allow the circuit to carry T1 signals.
- Loading coils, which are used to reduce signal distortion for analog phone lines, have the opposite effect of increasing distortion of digital signals, and these also must be removed.
To deploy T1 as a solution for multiplexing voice traffic, a T1 channel bank is generally installed at the customer premises. This channel bank can be connected to a Private Branch Exchange (PBX), which then connects to digital telephone and fax equipment. For WAN data links the scenario is usually somewhat different, using customer premises equipment (CPE) such as
- A T1 CSU/DSU for connecting bridges or routers to T1 circuits
- T1 bridges and routers with integrated T1 CSU/ DSUs
- A T1 multiplexer (MUX), a multiplexer for aggregating several T1 circuits for even higher-speed communication
- T1 access routers, which support multiple remote access links over a single T1 line
To test T1 equipment such as channel banks and CSU/ DSUs, use a cable simulator, which is a passive device that simulates a standard 22-gauge twisted-pair T1 line that is 1310 feet (400 meters) long (the alternative is to use 1310 feet of actual 22-gauge twisted-pair wiring). Connect two cable simulators between your CPE and your T1 test equipment using the TX and RX interfaces to analyze your device's performance. A "wet" T1 line carries a small DC current of about 140 mA (milliamperes) at several hundred volts for powering the CSU/ DSU at the customer premises. "Dry" lines carry no current, so CSU/DSUs must be powered from the customer premises. Do not touch a T1 line—a wet line can give you a serious shock!
Marketplace
The cost of provisioning T1 is complex and depends on whether you are using it for high-speed Internet access (T1 local loop connections between the customer premises and the telco CO) or for building a high-speed WAN (long-haul T1 lines crossing large geographical distances). A good rule of thumb for T1 WAN links is that the long-haul cost is about $2.50 per mile, which means a 2000-mile T1 leased line would cost about $5,000. These figures were for the year 2000, and the good news is that T1 prices have been falling about 10 percent per year for the last couple of years.
The cost for a T1 local loop connection to provide your company with dedicated high-speed Internet access is generally between $1,000 and $1,500 per month. These prices seem not to be changing much, despite forecasts that Digital Subscriber Line (DSL) technologies will cut into the T1 market, the main reason being the greater reliability of T1 compared to newcomer DSL.
The primary reason T1 lines are so expensive is that they are always "on" regardless of whether they are being used. This is characteristic of leased lines and provides both the benefit of availability and the cost of underutilization. A cheaper solution for many companies that do not require full T1 capacity is to lease a fractional T1 service such as 4 x DS0 = 256 Kbps from their carrier and then have them upgrade it to higher speeds as their WAN traffic grows. Fractional T1 is usually cheaper than using individual DS0 circuits multiplexed together.
Click to view graphic
T1. Some different WAN scenarios using T1 lines.
Notes
T1 and PRI-ISDN both carry data at around 1.5 Mbps, but they are incompatible so far as their framing formats are concerned. For example:
- T1 multiplexes 24 DS-0 channels using TDM for carrying data and adds a control bit to each T1 frame (and may use bit robbing to gain additional bandwidth for control purposes)
- PRI-ISDN multiplexes 23 DS-0 channels for carrying data and adds a 24th DS-0 channel dedicated to carrying control information.
The European E1 specification avoids the bit robbing used in American T1 by adding a 16-bit control header to each E1 frame instead of the single bit added to T1 frames.
See Also: Channel Service Unit/Data Service Unit (CSU/DSU), Digital Subscriber Line (DSL), DS-0, DS-1, High-bit-rate Digital Subscriber Line (HDSL), Integrated Services Digital Network (ISDN), leased line, line coding, PRI-ISDN, Private Branch Exchange (PBX), Public Switched Telephone Network (PSTN), T1 channel bank, T3, T-carrier, time-division multiplexing (TDM), trunking
T1 channel bank
Customer premises equipment (CPE) used to terminate a T1 line and make it available across an organization.Overview
T1 channel banks are typically used to enable T1 lines to connect to
- Data terminal equipment (DTE) such as routers and access servers
- Private Branch Exchange (PBX) units that provide integrated phone/fax services
A typical T1 channel bank consists of a modular chassis unit to which you can add various expansion cards to provide digital communication services for CPE. The modular chassis allows customers to add channels and upgrade fractional T1 services to full T1 or higher. It also allows customers to multiplex several channels to provide higher bandwidth for high-speed data connections to routers, Web servers, and other DTEs. The chassis typically includes a built-in T1 Channel Service Unit (CSU) for terminating the T1 circuit at the customer premises, plus a number of slots capable of holding expansion cards for various uses.
Click to view graphic
T1 channel bank. Using a T1 channel bank to connect a router and PBX to a T1 line.
Each expansion card in a T1 channel bank typically handles either one or two DS-0 channels, which means that different channels can supply different services (such as voice, fax, or data connections). Typical types of expansion cards include the following:
- Data service cards: These usually have a dual channel format that supports two DS-0 channels and employ a serial interface such as RS-232, RS-530, or V.35. These interfaces are then used for directly connecting the unit to bridges and routers having integrated Channel Service Unit/Data Service Units (CSU/DSUs).
- High-speed data cards: These support up to 1.544 megabits per second (Mbps) in 64-kilobits per second (Kbps) or 56-Kbps increments (the speed depends on how DS-0 is provisioned by the carrier) by multiplexing DS-0 channels.
- Voice cards: These are used to connect the unit to a PBX or directly to a telephone using standard 4-wire connections.
- Modem cards: These convert the channel bank into a modem pool to support corporate remote access needs.
Some T1 channel banks can support as many as four T1 lines, which can be configured for both active and backup purposes to provide redundant wide area network (WAN) connections.
See Also: Channel Service Unit (CSU), Channel Ser-vice Unit/Data Service Unit (CSU/DSU), customer premises equipment (CPE), data terminal equipmen, (DTE), Private Branch Exchange (PBX), T1, T-carrier
T3
Part of the T-carrier hierarchy.Overview
T3 represents the "next step up" for enterprises that want to build their wide area network (WAN) connections using dedicated leased lines. Although the commonly used and relatively inexpensive T1 lines used in enterprises carry traffic at 1.544 megabits per second (Mbps), T3 lines support a much faster speed of 44.736 Mbps, well above standard 10Base2 Ethernet speeds and almost comparable to Fast Ethernet. This huge jump in speed, however, comes at a significant cost and with some associated issues:
- T3 requires fiber-optic cabling to be provisioned from the telco central office (CO) to the customer premises, because T3 cannot run over existing copper local loop wiring even if it is properly conditioned. This up-front cost of laying fiber must be factored into the cost of deploying T3 in the enterprise.
- The cost of a dedicated T3 line is generally between $25,000 and $35,000 per month, a hefty price tag compared to the $1,000 to $1,500 cost of individual T1 lines. Many companies have a difficult time justifying the cost of upgrading from T1 to T3.
- Although T3 operates over fiber-optic cabling, there is no universal specification for how physical layer signaling occurs with this system. As a result, different telcos and telecommunications equipment vendors have developed many proprietary T3 signaling schemes, and most of these schemes cannot interoperate. This means that if you want to deploy T3 you must "buy in" to equipment from a single vendor (or lease equipment from your telco).
Despite these issues, T3 has grown in popularity in the last few years, particularly for large enterprises to connect their data centers to the Internet. The main problem faces companies whose WAN or Internet access needs are too great for a T1 line to satisfy yet do not require the capacity (or cannot afford the cost) of a full T3 line. The emerging solution to this problem is for telcos to provision services that bundle multiple T1 links for greater throughput. Cable and Wireless is one provider that offers a dedicated Internet access service called NxT1 that can aggregate from two to seven T1 lines into a single fat data pipe carrying up to 10 Mbps. This system employs Cisco 7500 routers running Multilink Point-to-Point Protocol (MPPP) for link aggregation. The disadvantage of this scheme is that customers must order additional T1 port connections to the provider's network, which adds to the cost. Nevertheless, the cost of this scheme is generally less than using fractional T3, which requires a full T3 interface at the customer premises.
See Also: Multilink Point-to-Point Protocol (MPPP) T1, T-carrier
T.120
A family of standards for multiuser conferencing and collaboration over a data network.Overview
T.120 represents a series of eight International Telecommunication Union (ITU) standards that define real-time multipoint communication over a network such as the Internet. T.120 can be used for such tasks as video conferencing, data exchange, or interactive gaming. The T.120 standards define such things as
- Multipoint services for conferencing
- Standard network services
- Guidelines for defining data channels
- Whiteboard methodologies
- Application-sharing protocols
- File transfer methodologies
A related standard from the ITU is the H.323 standard for video and audio conferencing.
Architecture
The architecture of the T.120 standard follows that defined by the Open Systems Interconnection (OSI) reference model for networking. The T.120 architecture can be divided into two parts:
- Network-layer and transport-layer standards (T.122 through T.125): Allow data to be transmitted and received among conferencing nodes over a variety of supported network connections. These standards also provide platform independence and the capacity for simultaneously managing multiple participants running on different operating system platforms and conferencing software.
- Application-layer standards (T.126 through T.128): Support multiuser conferencing functions such as whiteboarding, file transfer, and application sharing across different platforms and networks.
The following table shows the details of the various standards included under the T.120 umbrella.
T.120 Suite of Conferencing Standards
| Standard | Description |
| T.121 | A required standard for T.120 applications that defines how conference nodes register themselves with a T.120 node controller. Also defines the generic application template (GAT) for building T.120 application protocols and management facilities. |
| T.122 | Defines multipoint communication services (MCS) over various topologies to enable multiple participants to send data as part of a conference. The MCS defined by T.122 are implemented by T.125. |
| T.123 | Defines flow control, error control, and sequencing mechanisms for connect, disconnect, send, and receive functions over different network connections. |
| T.124 | Defines how multipoint conferences are initiated and administered and defines the generic conference control (GCC) that manages and monitors users, address lists, data flow, and MCS resources. |
| T.125 | Defines how data is transmitted during a conference, specifying the private and broadcast channels that transport conference data. T.125 implements the MCS defined by T.122. |
| T.126 | Defines mechanisms for transmitting and receiving whiteboard information among conference nodes and managing the multiuser whiteboard workspace. |
| T.127 | Defines mechanisms for file transfer among conference nodes in either broadcast or directed mode. |
| T.128 | Defines mechanisms for application sharing among conference nodes so that users can share their local programs with others for collaborative purposes. |
Notes
T.120 also forms the basis of the Remote Desktop Protocol (RDP), which is used by the Terminal Services of Microsoft Windows 2000, Windows XP, and Windows .NET Server.
See Also: H.323, International Telecommunication Union (ITU), Open Systems Interconnection (OSI) reference model, Remote Desktop Protocol (RDP), Terminal Services
TACACS
Stands for Terminal Access Controller Access Control System, a security protocol supported by Cisco routers.See: Terminal Access Controller Access Control System (TACACS)
taking ownership
Assuming the role of an object's creator, thus having the associated rights and privileges that this role incurs.Overview
Ownership describes the highest level of permissions that can be granted to objects. On the Microsoft Windows 2000, Windows XP, and Windows .NET Server platforms, these objects can include files and folders, Active Directory directory service objects, and so on. For example, assuming ownership of an object such as a file on an NTFS file system (NTFS) volume gives one the right to share the object and assign permissions to it. Normally, the user who creates a file on an NTFS volume is the owner. Other users can take ownership of the file provided the user is either a member of the Administrators domain local group, has NTFS full control permission on the object, or has explicit permission to take ownership of the object.
Notes
Ownership can only be taken; it cannot be assigned.
See Also: NTFS permissions (Windows 2000, Windows XP, and Windows .NET Server), NTFS permissions (Windows NT), NTFS special permissions (Windows 2000, Windows XP, and Windows .NET Server), NTFS special permissions (Windows NT)
tape drive
A device used to back up data to magnetic tape.Overview
Tape drives and their larger cousins, tape libraries, form the backbone of the disaster recovery plan for most enterprises. Tape drives are distinguished from one another by a variety of factors:
- Recording technology: There are several different ways in which data can be written to magnetic tape, including linear-scan, helical-scan, or hybrids of these two basic technologies. For more information about these various recording technologies and the tape formats from different vendors that support them, see the following article, "tape format."
- Capacity: The capacity of a tape drive is the amount of data it can store on a single tape cartridge. This capacity is usually measured in gigabytes (GB) and can be expressed as either native capacity for uncompressed data or compressed capacity. Tape drives for large enterprise networks may have capacities exceeding 50 GB, but drives for departmental or workgroup use may have capacities of only a few GB. Compressed capacity is usually specified as twice the native capacity—in other words, a tape drive with 50 GB native capacity would be rated as having a compressed capacity of 100 GB. The actual capacity when compression is used, however, depends on the type of data being backed up.
- Transfer speed: This is the speed at which data can be buffered by the tape drive and written to tape. For enterprise-class tape drives, transfer speeds exceeding 25 megabytes per second (MBps) are possible, but for workgroup or small business use the capacity is often measured in megabytes per minute (MB/min) instead and is considerably less.
- Cost of media: If you are backing up large amounts of data frequently, the cost of individual tape cartridges can be a significant expense that needs to be budgeted for accordingly. Cost for tapes range from about $10 to $100, depending on the tape format used.
- Input/output (I/O) interface: Most enterprise tape drives use Small Computer Systems Interface (SCSI) as their data interface, but some cheaper drives for small business use have ATAPI/IDE, parallel, or Universal Serial Bus (USB) interfaces.
- Interoperability: Usually a tape cartridge produced by one vendor will not work in a tape drive from a different vendor. The only difference is for tapes and drives that adhere to the new Linear Tape Open (LTO) standard developed by Hewlett-Packard, IBM, and Seagate Technology. LTO drives and cartridges have only recently appeared on the market, but they are likely to become a dominant format in the years to come.
- Software driver: Before buying a tape drive for your servers, make sure that your backup software has a suitable driver for this hardware.
Marketplace
The tape drive market is basically divided into three categories:
- Enterprise: These drives have the largest capacities and best performance to meet the demanding backup needs of large companies. Some popular drives in this market include DLT 8000 drives from Quantum Corporation, which cost about $5,000 and have a native capacity of 40 GB; Exabyte Mammoth-2 and Sony AIT-2 drives, which are comparable in cost and capacity to the DLT 8000; and the new SuperDLT drives from Quantum and LTO Ultrium drives from IBM, which have higher capacity and cost.
- Departmental: These medium-capacity drives include the Tandberg SLR100, Benchmark DLT1, Ecrix VXA-1, and drives from other vendors. The cost for departmental drives is usually $1,000 or slightly higher, and capacity is measured in tens of GB.
- Workgroup: For small business settings, these are a wide range of popular drives, including digital audio tape (DAT) drives from a number of vendors, OnStream's ADR50 drives, and Ultrium 3580 from IBM. Drives for workgroup or desktop use typically have capacities of a few GB and cost several hundred dollars.
Notes
Here are some tips on getting the most from your tape drive:
- Make sure you clean your drive's read/write heads regularly, usually every 10 hours or so. Some newer drives automatically clean themselves as needed, while others display a light-emitting diode (LED) indicator when cleaning needs to be done.
- Avoid exposing both the tape drive and the tapes to stray magnetic fields such as those from computer monitors. When some tape formats are exposed to such fields, data can be lost—for others, the entire tape can be rendered unusable.
- Replace your tapes regularly according to the mean lifetime of the particular tape format you use. Most tapes can be used about 50 times before they wear out and become unreliable.
See Also: backup, disaster recovery, tape format, tape library
Last Updated: April 8, 2002
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