HDT - Host Digital Terminal

By: Preethi Ramkumar

Meaning of HDT - "Host Digital Terminal" is the interface between a cable distribution system and the telephony switch, the HDT acts as a digital multiplexer. It provides T1/E1 links to the telephony switch at 1.544 Mb/s and accepts 64 kb/s digital signals from lines on the subscriber side, usually in a T1/E1 format. Most vendors have designed the HDTs with an open interface to the telephony switch, allowing the service provider to choose different vendors for the switch and the HDT. On the subscriber side, however, the connection to the Network Interface Unit (NIU) is proprietary, requiring the cable operator to purchase both the HDTs and NIUs from the same vendor. Having an open interface to the switch also enables a cable operator to obtain telephony switching from another company through alliances or leasing agreements, so the operator doesn't need its own digital switch in the early stages of telephony offerings.

Adding circuit-switched telephony to an HFC network requires three network elements: a telephony switch, a Network Interface Unit (NIU) and a Host Digital Terminal (HDT). The functions of a telephony switch can be grouped into three categories: Call processing, Call routing and Feature provisioning. Call processing and call routing set up a path through the public network between parties when a call is initiated. Enhanced capabilities such as call screening are provided from the digital switch through the HFC telephony network. Associating these feature capabilities with an originating line is an important part of the switch's function. Switch software can also provide call forwarding, call transfer, call waiting and call conferencing. As part of the call processing function, the telephony switch can communicate with the SS7 network and its databases. This capability is a powerful enhancement to the switch software and provides for calling number identification and number portability.

Other Related Definitions:

“…MediaOne also installs network interface units (NIUs) on each home that subscribes to the digital telephony service. The NIUs provide an interface between the cable drop and the standard in-home telephony wiring. The installation of an NIU involves putting the NIU through what is called a "ranging" process - once it is installed on the house, the installer makes sure that it "talks" to the host digital terminal (HDT) in the headend and that it is activated. Following activation, the installer connects the customer's home wiring to the NIU. ” [CableLabs] 

“…The Host Digital Terminal (HDT) can be interfaced to the public switched network in the Central Office. The telephony transmitter/receiver units are located in the telephony transceiver shelf in the central office. This equipment interfaces with the coaxial transport system cables from the HDT and the fibre-optic feeder cables. Different services may require dedicated subscriber terminals (set-top-box, etc.)." [Optical Access Networks]

“…Host digital terminal (HDT): the central office terminal that connects to neighborhood nodes in SDV or HFC networks “ [Maury Wright – Technical Editor ]

“…Carrier frequencies used for telephone services may be individually assigned to particular subscriber's CNU's. Also, the telephone signal spectrum is carried on the same two fibers that carry the video and the upstream signalling between the Loop Transport Interface and the ONU. Upstream telephone signals are applied from the optical receiver 308 to a host digital terminal (HDT) 343 which provides an interface to a standard digital telephone switch 345. Downstream telephone signals from that switch pass through the HDT 343 to the RF combiner 315 for transmission in the 700-750 MHz frequency range over the fiber to the ONU 309 and the coaxial cable distribution system 311. In an alternate implementation not shown, the Loop Transport Interface 300 would dynamically allocate the DS0 channels on the coaxial cable system 311 on a time-sharing basis, in essentially the same manner as in mobile radio systems. Two additional fibers and a second optical transmitter and receiver pair would carry the two-way telephone signals to and from the ONU 309. Because of the use of the separate optical links for telephone service in this alternate implementation the HDT and telephone switch need not be closely associated or collocated. with any particular one of the Loop Transport Interfaces” [Digital entertainment terminal ] 

“…Host Digital Terminal. PSTN term for headend equipment providing access network distribution.” [PacketCable ] 

Related Links:

Flexible layout allows fast deployment of new services to HDT – CED Magazine
New Media – How telephony service work using HDT?
Cable Telephony Special Report – Communications Technology
Cable Television Systems – TV handbook

Products and Solutions:

SNET activates full-service network - Lucent Technologies
Major Elements of a VOIP system - Cable TV Marketing and Product Management

Books About:

Modern Cable Television Technology, Second Edition (The Morgan Kaufmann Series in Networking) - by Walter Ciciora, James Farmer, David Large, Michael Adams

See Also:

HDT Resources

 

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