The time interval between the transmission of signals from a

Both bus and tree topologies are characterized by the use of multipoint (71) ．For the bus，all stations attach，through appropriate hardware (72) known as a tap，directly to a linear transmission medium，or bus．Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the (73) ．A transmission from any station propagates the length of the medium in both directions and can be received by all other (74) ．At each end of the bus is a (75) ，which absorbs any signal，removing it from the bus．

（71）

A．medium

B．connection

C．token

D．resource

If practical, when shall a manned vessel being towed sound her fog signal?______.

A.Immediately before the towing vessel sounds hers

B.Immediately after the towing vessel sounds hers.

C.As close to the mid-cycle of the towing vessel’s signals as possible

D.At any time as long as the interval is correct

Telecommunications stand for devices and systems that transmit electronic or optical signals across long distances. Telecommunications enables people around the world to contact one another, to access information instantly, and to communicate from remote areas. Telecommunications usually involves a sender of information and one or more recipients linked by a technology, such as a telephone system, that transmits information from one place to another. Telecommunications enables people to sand and receive personal messages across town, between countries, and to and from outer space. It also provides the key medium for delivering news, data, information, and entertainment.

Telecommunications devices convert different types of information, such as sound and video, into electronic or optical signals. Electronic signals typically travel along a medium such as copper wire or are carried over the air as radio waves. Optical signals typically travel along a medium such as strands of glass fibers. When a signal reaches its destination, the device on the receiving end converts the signal back into an understandable message, such as sound over a telephone, moving images on a television, or words and pictures on a computer screen.

Telecommunications messages can be sent in a variety of ways and by a wide range of devices. The messages can be seat from one sender to a single receiver (point-to-point) or from one sender to many receivers (point-to-multipoint). Personal communications, such as a telephone conversation between two people or a facsimile (fax) message (see Facsimile Transmission), usually involve point-to-point transmission. Point-to-multipoint telecommunications, often called broadcasts, provide the basis for commercial radio and television programming.

Telecommunications begin with messages that are converted into electronic or optical signals. Some signals, such as those that carry voice or music, are created in an analog or wave format, but may be converted into a digital or mathematical format for faster and more efficient transmission. The signals are then sent over a medium to a receiver, where they are decoded back into a form. that the person receiving the message can understand. There are a variety of ways to create and decode signals, and many different ways to transmit signals.

Individual people, businesses, and governments use many different types of telecommunications systems. Some systems, such as the telephone system, use a network of cables, wires, and switching stations for point-to-point communication. Other systems, such as radio and television, broadcast radio signals over the air that can be received by anyone who has a device to receive them. Some systems make use of several types of media to complete a transmission. For example, a telephone call may travel by means of copper wire, fiber-optic cable, and radio waves as the call is sent from sender to receiver. All telecommunications systems are constantly evolving as telecommunications technology improves. Many recent improvements, for example, offer high-speed broadband connections that are needed to send multimedia information over the Internet.

Personal computers have pushed the limits of the telephone system as more and more complex computer messages are being sent over telephone lines, and at rapidly increasing speeds. This need for speed has encouraged the development of digital transmission technology. The growing use of personal computers for telecommunications has increased the need for innovations in fiber-optic technology.

Telecommunications and information technologies are merging and converging. This means that many of the devices now associated with only one function may evolve into more versatile equipment. This convergence is already happening in various fields. Some telephones and pagers are able to store not only phone numbers but also names and personal information abo

A．Current development.

B．Transmission of message.

C．Computer networking.

D．Government regulation.

The time interval between the transmission of signals from a pair of Loran-C stations is very closely controlled and operates with ______．

A.An atomic time standard

B.Daylight Savings Time

C.Eastern Standard Time

D.Greenwich Mean Time

As can be gathered from the passage, before 1910 the normal running time of a film was probably ______.

A．15 minutes or less

B．between 15 and 30 minutes

C．between 30 and 45 minutes

D．1 hour or more