Compression and Multiplexing


Compression makes data "smaller" so more information can be transmitted over telephone lines. It is a technique to increase the capacity of telephone lines. With compression, data to be transmitted is made smaller by removing white spaces and redundant images, as well as by abbreviating the most frequently appearing letters. For example, with a facsimile, compression removes white spaces from pictures, and only transmits the images.

Modems use compression to achieve greater throughput, or rates of transmitted information. When a modem equipped with compression transmits text, repeated words are abbreviated into smaller codes. For example, since E, T, O and I appear frequently in a text, compression sends shortened versions of 3 bits rather than 7 or 8 bits. Therefore, a page would consist of about 1,600 bits instead of 2,200 bits.

If a word processing file was 10 pages long, compression that removed white spaces, redundant characters and abbreviated characters might compress the document to 7 pages, which takes less time to transmit. Thus, a modem using compression would be able to send greater amounts of computer data in a smaller amount of time over analog lines. This increases throughput. In order for compression to work, the two sites transmitting and receiving must have matching compression.

Video Compression

Video compression works by transmitting only the changed image and not the same image repeatedly. In video conferencing, nothing is transmitted until the person being photographed moves or speaks. Fixed objects such as walls, and background are not repeatedly transmitted.Another method of video compression is to only transmit part of the image. The coder, or device doing the compression, knows that discarding minor changes in images would not distort the viewed image noticeably. Improvements in video compression in the mid-1980s gave birth to the commercial viability of video conferencing rooms. These systems made it economical to use video since they required less bandwidth, which means cheaper telephone lines. Video conferencing became affordable to a wider range of organizations since they could lease cheaper phone lines as low as $14 per hour instead of using the traditional T-1 lines at hundreds of dollars per hour.

In most cases, the cheaper lines still had acceptable video quality. New compression algorithms meant that slower speed digital lines were an acceptable choice for video meetings. As such, a new industry was created.

Compression Methods

There are various types of compression methods. Companies such as AT&T, Motorola, PictureTel and Compression Labs have all designed unique compression schemes using math algorithms.

A Codec (which is an abbreviation for a coder-decoder) encodes text, audio, video or image using a compression algorithm. For this compression to work, both the sending and receiving ends must have the same compression method. The sending end looks at the data, voice or image, then codes it, while the receiving end of the transmitter decodes it.

For devices from multiple manufacturers to operate together, compression standards have been agreed upon for modems, digital television, video conferencing and other devices. Digital television compression works much like video compression. It allows pictures to be transmitted in a highly abbreviated form.

With most video pictures, the image in one frame is similar to that in the previous frame since the background remains the same while the actors move only slightly from one frame to the next. Therefore, instead of transmitting the entire image again, a compression system sends only the parts of the picture that change. Digital compression makes it possible to represent continuous color-TV signal. This compression squeezes video and analog signals into small enough units so that studio-quality television can be sent on standard digital Television channels. The analog standard for television is 525 scan lines, or lines of images. High definition television (HDTV) will enable a television screen to display 1125 scanned lines.

A greater number of scan lines results in clearer, studio-quality pictures. Additional lines of image are seen as a denser higher resolution of detailed images on the screen. This is done through computer manipulation of the video and audio portions of the television signals.

Because of the powerful compression and decompression tool used by computers, very little of the images are lost to the viewer. The quality of digital television is high enough so that people watching television will perceive the quality to be similar to movies in theaters.


Multiplexing

Another method of manipulating data to achieve greater throughput is through multiplexing. Multiplexing is a technique used in communications and input/output operations for transmitting a number of separate signals simultaneously over a single channel or line.

To maintain the integrity of each signal on the channel, multiplexing can separate the signals by time, space, or frequency. The device used to combine the signals is a multiplexer, and the separate signals are recovered at the end by a demultiplexer.

Multiplexing combines traffic from multiple telephones and data devices into one single stream so that many devices can share a particular telecommunication path. Multiplexing makes more efficient use of telephone lines, as does compression. However, unlike compression, multiplexing does not alter the actual data sent.

Multiplexing equipment is typically located in long distance companies, local telephone companies and at end-user premises, and is associated with both analog and digital services. Examples of multiplexing over digital facilities include T-1, fractional T-1, T-3, ISDN and ATM technologies.

The oldest multiplexing techniques were devised by AT&T for use with analog voice services. The goal was to make more efficient use of the most expensive portion of the public telephone network, the outside wires used to connect homes and telephone offices to each other. This analog technique is referred to as frequency-division multiplexing, which allows multiple voice and later data calls to share paths between central offices. Therefore, AT&T would not need not construct cable connections for each conversation, since multiple conversations could share the same wire between a telephone company’s central office.

Digital Multiplexing

Digital multiplexing schemes operate at greater speeds and carry more traffic than analog multiplexing. For example, T-3 carries 672 conversations over one line at speeds of 45 megabits per second. With both digital analog multiplexing, a matching multiplexer is required at both the sending and receiving ends of the communications channel.

T-3 is used for very large customers, telephone companies and Internet Service Provider networks.

T-1 is the most common form of multiplexing for end-user organizations. T-1 is lower in both cost and capacity than T-3. T-1 allows more than two voice and / or data conversations to share a path. T-1 applications include linking organization sites together for voice calls, email, database access, and links between end-users and telephone companies for discounted rates on telephone calls. Like T-3 services, matching multiplexers are required at both ends of a T-1 link.

Frequency-Division Multiplexing

Frequency-division multiplexing is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (sub-channel) within the main channel.

With frequency-division multiplexing, each channel has its own base frequency, and its own carrier frequency. The carrier frequency can be modulated using several different methods to derive either digital or analog channels.

The modulation method and the characteristics of the information on the channel (such as the bit rate) determine the bandwidth needed per channel. The circuitry to handle a channel in a frequency-division multiplexer is quite complicated and therefore costly. For analog signals such as television signals, however, frequency-division multiplexing can still be a good choice.

Frequency-division multiplexing can be used on optical fibers by using a different frequency and thus a different wavelength of the light beam for each channel. With optical systems, the term wavelength multiplexing is used. On radio links not only different frequencies but also different polarization angles can be used. Suppose a long-distance cable is available with a bandwidth allotment of 3 Mhz. This is 3,000 kHz, so theoretically, it is possible to place 1,000 signals, each 3 kHz wide, into the long-distance channel. The circuit that does this is known as a multiplexer. It accepts the input from each individual end user, and generates a signal on a different frequency for each of the inputs. This results in a high-bandwidth, complex signal containing data from all the end users. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.

When frequency-division multiplexing is used in a communications network, each input signal is sent and received at maximum speed at all times. This is its chief asset. However, if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly.

In some systems, a different scheme, known as time-division multiplexing, is used instead.

Time-division Multiplexing

Time-division multiplexing has become a cost-effective method that is not only used on trunk circuits between switching centers but is today even starting to be used on local circuits to the customer. The basic interface of the ISDN is an example of this trend.

With time-division multiplexing, the whole bandwidth is assigned to each particular channel for a fraction of the total transmission time. This fraction can vary from one bit for bit-interleaved multiplexers, to a few thousand bits in the newest types of high bit-rate multiplexers; the synchronous time-division multiplexing (STDM) designed for the synchronous transfer mode.

Time-division can even be used to transfer samples of bits, derived by scanning the input channels with a frequency at least 3 times higher than the highest bit rate on these tributary channels. With this method digital signals from various sources with even unknown or changing bit rates can be multiplexed and reproduced (with a tolerable distortion) at the other end of the common channel (CCITT, 1988d).

All these time-division multiplexers are fixed slot time-division multiplexers, in that they assign a fixed slot to each channel in a cyclic scan of all the tributary channels. The fixed position of the slot in the cycle for each channel makes it possible to identify the destination outlet for each portion of the information received over the common channel.

This process requires synchronization in order to guarantee that the scanning of the received information at that output side was at the same speed as the cyclic scan at the input side.

All slots of one cyclic scan are arranged in a frame. In this frame, one generally finds additional information to ensure correct synchronization and frame alignment, needed to present the information from the input channels arriving at the wrong output channels as a result of being out of phase.

The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from all the end users.

As with frequency-division multiplexing, at the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. Again, as in frequency-division multiplexing, a two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.

As always, if many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of time-division multiplexing is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth.

The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour.