A Concise Summary of Public, Leased, and Private Transmission Facilities

Problem:

This report is offered primarily to clarify the sometimes conflicting terminology used to define the various classes of currently available transmission facilities and to separate the facility definitions from transmission media definitions. Facilities are generally defined

by their accessibility and by the way they are switched. The most accessible service is the public dial-up facility (telephone system). Next are the leased facilities, which can

consist of lines leased from common carrier or noncommon carrier sources. Last are the strictly private facilities in which a company builds, maintains, and completely owns dedicated communications lines and equipment. The switching classifications, at present, are line, message, and packet switching. Line switching implements a continuous point-to-point connection, from dial-up to hang-up (as in the telephone network), and is not concerned with the message content of the connection. Message switching is, as its name implies, concerned with messages. In a message-switched network, a point-to-point connection is made only for the duration of a discretely identified message, but the message length can be indeterminate. Packet switching is similar to message switching except that the "message" length is fixed, thus the term "packet"; and the original message may be physically divided among many packets. Packet switching, like line switching, is not concerned with the length of the original message. Also, packets are switched from point-to-point completely under computer control, so a physical

connection is never actually made between sender and receiver. The computer-controlled connection is called a virtual connection.

The facility media are not necessarily related to the type of serVice and switching. The most common medium consists of millions of miles of plain copper wire that inter-connects telephones in the AT&T network. The media are graded by their degree of widebandedness, which translates into transmission speed. Copper wire is the narrowest bandwidth media. In order of increasing widebandedness, the other significant media are radio, conditioned wire lines, coaxial cable, microwave, satellite (really a form of microwave), and optical fiber.

JUNE i 979 if, i979 DATAPRO RESEARCH CORPORATiON, DELRAN, NJ 08075 USA REPRODUCTION PROHIBITED

Basic Concepts

A Concise Summary of Public, Leased, and Private Transmission Facilities

Solution:

In designing a teleprocessing network the systems analyst has a number of types of communication lines that he may use in the construction of his network.

This report summarizes these types.

TRANSMISSION RATES

Perhaps the most important parameter in comparing communication lines is their transmission speed. The communication lines in use vary in transmission rate from about 50 bits per second up to more than a million. Speeds higher than those of a voice line, how-ever, are not widely available on a dial-up basis, although switched wide band facilities now exist in some cities.

Figure 1 gives an estimate of the relative popularity of different data transmission time requirements and message lengths. The digits from 1 to 9 indicate the relative usage of different areas on the chart, the most popular being real-time dialogue systems with a when AT&T introduced their 50.000-bits-per-second

"0 data transmission time requirements and message lengths. The figures from 1 to 9 indicate the relative usage of different

switched service to the first few cities in 1970, there were few subscribers for it. Given some years of development, however, this situation will almost cer-tainly change. If higher-speed lines are available at reasonable cost, the digits on this figure will migrate in a southeasterly direction.

Figure 1 was given to fifty or so systems analysts, all the data transmission industry will grow in many new directions, including some that are barely anticipated today. range, suggesting that, given appropriate transmission facilities.

a wide variety of new lIses for data transmission will emerge (see text)

CATEGORIES OF LINE

Table 1 lists the main types of leased and public communication links in order of increasing speed.

The speeds have been listed in terms of the number of data bits per second that may be sent over the line. Communication lines fall into one of three categories of speed:

1. Subvoice grade. Lines designed for telegraph and similar machines transmitting at speeds ranging, in the United States, from 45 to 150 bits per second. Some

~ 1979 DATAPRO RESEARCH CORPORATION, DELRAN, NJ 08075 USA REPRODUCTION PROHIBITED

JUNE 1979

A Concise Summary of Public, Leased, and Private Transmission Facilities

1. Public (Dial-Up) Lines

Bit Rate (bits per second) Bandwidth Type of Line

Dependent (kHz) United States United

Fixed on Modem AT&T Western Union Kingdom

Subvoice grade 45 TTY-TWX

(6 Murray and CPT-TWX

characters Up to 45 CE-TWX

per second)

50 Telex Telex

l10 TTY-TWX

(10 DIC and CPT-TWX

characters Up to 150 CE-TWX

per second) Up to 200 Datel200

The public Up to 600 Datel600

telephone Up to 1200 Datel600

network

600 to 3 Public network Public

4800 (Not all telephone

(Certain freely network

modems usable achieve because of

higher network

speeds.) signaling.)

Switched 600 (Other 2

wideband 1200 & 2400 speeds 4

networks 4800* will be

9600* achievable 8* BEX

38,400* with other 16* (Certain

modems.) 48* cities

only)

50,000 Up to Dataphone 50

50,000 (few cities

only)

*Planned but not yet available.

2. Leased Lines

Bit Rate (bits per second) Bandwidth Type of Line

Dependent (kHz) , United States United

Fixed on Modem AT&T Western Union Kingdom

jsubvoice grade Up to 45 1004

50 Tariff H

Up to 55 1002

Up to 75 1005

100 Datel100

Up to 150 1006

Up to 180 1006

200 Datel200

V oice-grade lines Up to 600 3 Datel600

Up to 1200 3 Datel600

600 to 3 3002 (Datel2000

10,500 (C1, C2, C4 and C5 refers to

con-(For the higher conditioning) ditioned or

rates, high-quality

conditioning voice lines.)

is needed.)

Wideband 19,200 24 8803 1

40,800 48 8801 II Specicd

50,000 48 8801

II

^quotation

I

230.400 240 5700 (originally TELPAK C)

1000 (approx.) i .5800 (originally TELPAK D)

I

(5700 & 5800 ""iff,

Ii

all provide "bundles" of smaller bandwidth lines.)

Table 1. Types of communication lines available

Half Duplex or Full Duplex HDX HDXor FDX HDX HDX HDX orFDX FDX FDX FDX HDXor FDX

FDX

Half Duplex or Full Duplex HDXjFDX FDX HDXjFDX HDXjFDX FDX HDXjFDX HDXjFDX FDX FDX FDX HDXjFDX

FDX FDX FDX

Basic Concepts

A Concise Summary of Public, Leased, and Private Transmission Facilities

countries have lines of higher speed than their tele-graph facilities, but still much slower than the capacity of voice lines. Britain, for example, has its Datel 200 service operating at 200 bits per second. Most indus-trialized countries outside North America have a similar 200-bit-per-second service (a CCITT stan-dard). All these lines are today commonly obtained by subdividing telephone channels.

2. Voice grade. At present, telephone channels nor-mally transmit at speeds from 600 to 4800 bits per second. Speeds of 9600 and possibly higher may become common in the near future. Dial-up telephone lines are commonly used for speeds of 1200 or 2400 bits per second today. Speeds up to 3600, however, may soon become common on the public network.

A speed of 4800 is already possible with a reason-able error rate but requires elaborate modem design.

Telephone organizations in some other countries have not yet permitted the use of such high speeds over their telephone lines. In many countries, 600 or 1200 bits per second is still the maximum.

3. Wideband. Wideband lines give speeds much higher than voice channels, using facilities that carry many simultaneous telephone calls. Speeds up to about 500,000 bits per second are in use, and higher bit rates are possible if required.

All these line types may be channeled over a variety of different physical facilities. This report and indeed the tariffs themselves normally say nothing about the medium used for transmission. It might very well be wire, coaxial cable, microwave radio, or even satellite. The transmission over different media is organized in such a way that the channels obtained have largely the same properties-same capacity, same noise level, and same error rate. The user generally cannot tell whether he is using a micro-wave link, coaxial cable, or pairs of open wires stretched between telephone poles. Only satellite trans-mission requires different data-handling equipment, and here only because a delay of one-third of a second or so occurs in travelling the great distance.

SWITCHED VS. LEASED LINES

The next most important parameter about the lines 'is whether they are public switched lines or not.

Voice lines and telegraph lines can either be switched through public excha~ges (central offices) or

per-manently connected. Facilities for switching broad-band channels are in operation in some countries, although most broadband channels today are per-,manent connections.

When you dial a friend and talk to him on the tele-phone, you speak over a line connected by means of the public exchanges. This line, referred to as a

"public" or "switched" line, could be used for the transmission of data. Alternatively, a "private" or

"leased" line could be connected permanently or semi-permanently between the transmitting machines. The private line may be connected via the local switching office, but it would not be connected to the switch-gear and signaling devices of that office. An inter-office private connection would use the same physical links as the switched circuits. It would not, however, have to carry the signaling that is needed on a switched line. This is one reason why it is possible to achieve a higher rate of data transmission over a private line.

Another reason is that private lines can be carefully balanced to provide the high quality that makes higher-speed data transmission possible.

Just as you can either dial a telephone connection or have it permanently wired, so it is with other types of lines. Telegraph lines, for example, which have a much lower speed of transmission than is possible over voice lines, may be permanently con-nected or may be dialed like a telephone line via a switched public network. Telex is such a network; it exists throughout most of the world, permitting transmission at 50 bits per second. Telex users can set up international connections to other countries.

Some countries· have a switched public network, operating at a somewhat higher speed than Telex but at less speed than telephone lines. In the United States, the TWX network gives speeds up to 150 bits per second. TWX lines can be connected to Telex lines for overseas calls. Also, certain countries are building up a switched network for very high-speed (wideband) connections. In the United States, Western Union has installed the first sections of a system in which a user can indicate in his dialing what capacity link he needs.

ADVANTAGES OF LEASED LINES

The leased voice line has the following advantages for data transmission over the switched connections:

1. If it is to be used for more than a given number of hours per day, the leased line is less expensive than the switched line. If it is used for only an hour or so per day, then it is more expensive. The breakeven point depends on the actual charges, which in turn depend on the mileage of the circuit, but it is likely to be of the order of several hours per day. This factor is clearly an important consideration in de-signing a data transmission network.

2. Private lines can be specially treated or "condi-tioned" to compensate for the distortion encountered on them. The common carriers charge extra for

JUNE 1979

A Concise Summarv of Public, Leased, and Private Transmission Facilities

ditioning. In this way the number of data errors can be reduced or, alternatively, a higher transmission rate can be made possible. The switched connection cannot be conditioned beforehand in the same way because it is not known what path the circuit will take.

Dialing at one time is likely to set up a quite dif-ferent physical path from that obtained by dialing at another time, and there is a large number of possible paths. Modems now exist in that condition dynamically and adlust to whatever connection they are used on. These-devices enable higher speeds to be ob~ained over switched circuits, but they are expensIve.

3. Switched voice lines usually carry signaling within the bandwidth that would be used for data. Data transmission machines must be designed so that the form in which the data is sent cannot interfere with the common carrier's signaling. With some machines, this operation also makes the capacity available for data transmission somewhat less than that over a private voice line. A common rate over a switched voice line in the 1960s was 1200 bits per second, whereas 2400 bits per second was common over a specially conditioned, leased voice line. As stated earlier, it is probable that up to 3600 bits per second over switched voice lines and 4800 to 9600 bits per second over conditioned, leased voice lines will be-come common in the 1970s. Already some modems transmit at higher speeds than 3600 over public voice lines.

4. The leased line may be less perturbed by noise and distortion than the switched line. The switching gear can cause impulse noise that results in errors in data.

This is a third factor that contributes to a lower error rate for a given transmission speed on private lines.

The cost advantage of switched lines will dominate if the terminal has only a low usage. In addition, the ability to dial a distant machine gives great flexibility. Different machines can be dialed with the same terminal, perhaps offering quite different facili-ties. A typewriter terminal used at one time by a secretary for computer-assisted text editing may at another time be connected to a scientific time-sharing system and at still another time may dial a computer-assisted teaching program. Machine availability is another consideration. If one system is overloaded or under repair, the terminal user might dial an alternative system. Often this dialing is done over the firm's own leased tie lines.

-LINE CONDITIONING

As has been mentioned, private leased voice lines can be conditioned so that they have better properties for

data transmission. Tariffs specify maximum levels for certain types of distortion. An additional charge is made by most carriers for lines that are conditioned.

The American -Telephone and Telegraph Company, for example, has three types of conditioned voice lines, the conditioning being referred to as Types C 1, C2, C4 and C5. A line ideal for data transmission would have an equal drop in signal voltage for all frequencies transrI1itted. Also, all frequencies would have the same propagation time. This is not so in practice. Different frequencies suffer different attenua-tion and different signal delay. Condiattenua-tioning attempts to equalize the attenuation and delay at different frequencies. Standards are laid down in the tariffs for the measure of equalization that must be achieved.

The signal attenuation and delay at different fre-quencies must lie within certain limits for each type of conditioning. The higher the conditioning number, the narrower are the limits. The result of the condi-tioning is that a higher data speed can be obtained over that line, given suitable line-termination equip-ment (modem).

Types C 1 and C2 conditioning are applicable to point-to-point and multipoint lines. Type C4 is available only on two-point, three-point and four-point lines.

Type C5 conditioning can only be applied to point-to-point lines.

TARIFFS FOR WIDEBAND LINES AND BUNDLES

The North American common carriers offer several tariffs for leased wideband lines. Some of these can be subdivided by the carrier into "bundles" of lower bandwidth. Some can be subdivided into channels for voice transmission, telephotograph, teletypewriter, control, signaling, facsimile or data. With some tariffs the user pays a lower price for the bundles than for the individual channels.

The word "TELP AK" was formally used for the

"bulk" communication services offered by the tele-phone companies and \Vestem Union. The word has now been eliminated from the tariffs but is still found in much literature. What used to be TELP AK C is now called a Type 5700 line, and what used to be TELP AK D is now Type 5800. Both can provide a wide band channel or a bundle of lesser channels.

The TELP AK customer pays a monthly charge based on the capacity of the communications channel he selects, the number of airline miles between locations, and the type and quantity of channel terminals. He has use of this channel on a full-time basis.

Basic Concepts

A Concise Summary of Public, Leased, and Private Transmission Facilities

Originally there were four sizes of TELP AK channels:

TELPAK A, B, C, and D. However, in 1964 the Federal Communications Commission ruled that rates for TELPAK A (12 voice circuits) and TEL-PAK B (24 voice circuits) were discriminatory in that a large user could obtain a group of channels at lower cost per channel than a small user, who could not take advantage of the bulk rates. In 1967 the TELP AK A and B offerings were eliminated.

The Type 5700 line has a base capacity of 60 voice channels (full duplex).

The Type 5800 line has a base capacity of 240 voice channels (full duplex).

Each voice channel in these lines can itself be sub-divided into one of the following:

1. Twelve teletype channels, half or full duplex (75 bits per second).

2. Six class-D channels, half or full duplex (180 bits per second).

3. Four AT&T Type 1006 channels, half or full duplex (150 bits per second).

There cannot be mixtures of these channel types in a voice channel. The Type 5700 line can transmit data at speeds up to 230,400 bits per second; the Type 5800 line has a potential transmission rate much higher. Line-termination equipment is provided with these links, and each link has a separate voice channel for coordination purposes.

The TELP AK channels thus serve two purposes. First, they provide a wideband channel over which data can be sent at a much higher rate than over a voice channel. Second, they provide a means of offering groups of voice or su bvoice lines at reduced rates-a kind of discount for bulk buying.

Suppose that a company requires a 50,000-bit-per-second link between two cities; together with 23 voice channels and 14 teletypewriter channels, or perhaps 30 voice channels and no teletypewriter links.

Then it would be likely to use the Type 5700 tariff. In leasing these facilities, it would have some unused capacity. If it wishes, it can make use of this capacity at no extra charge for mileage, although there would be a terminal charge.

Government agencies and certain firms in the same business whose rates and charges are regulated by the government (e. g., airliries and

railroads) may share bundled services. Airlines, for example, pool their needs for voice and teletypewriter channels. An intercompany or-ganization purchases the bundled services and then apportions the channels to individual airlines. Most of the lines channeling passenger reservations to a distant office where bookings can be made are Type 5700 or 5800 lines, and so also are the lines carrying data between terminals in those offices and a distant reserva-tions computer. There has been some demand to extend these shared facilities to other types of organizations that could benefit from them by sharing, but this is not permissible as yet.

Im Dokument COMMUNICATIONS SOLUTIONS (Seite 142-149)