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Carrier Systems

In general, carrier systems are mechanisms that provide a means to send signals from more than one source over a single physical channel. The bandwidth available to carry signals in a particular medium can be allocated in two ways: by frequency or by time intervals.

Using Frequency

The frequency spectrum represented by the available bandwidth of a channel can be divided into smaller bandwidth portions, with each of several signal sources assigned to each portion. This is the principle of frequency-division multiplexing (FDM). FDM is still used in some simple data communications systems, and, at one time, it formed the foundation for the long-haul part of the public telephone network. Since the 1980s, communications carriers have invested tens of billions of dollars in converting their infrastructure to digital technology, resulting in the replacement of essentially all carrier FDM systems by time-division multiplexing (TDM) systems. Chapter 6, "Multiplexing Techniques," covers multiplexing technology, including the operation of FDM and TDM systems. One common example of a frequency-division multiplexing system is a standard low-speed (300bps) modem, which divides the spectrum available in a voice channel into two portions—one for transmitting and one for receiving. (This concept of frequency division is discussed in more detail in Chapter 6.)

The electronic mechanisms that implemented FDM are called analog carrier systems. The carrier in an analog carrier system is a signal generated by the system, and the carrier is modulated by the signal containing the information to be transmitted. Table 3.7 shows the standard analog carrier systems in use in the public telephone network. As previously noted, however, most communications carriers have replaced their analog-based FDM equipment with digital-based TDM equipment.

Using Time

The second method of dividing the capacity of a transmission channel among several separate signal sources is to allocate a very short period on the channel in a repeating pattern to each signal. This technique is called time-division multiplexing. It is well suited to binary signals consisting of pulses representing a 1 or a 0. These pulses can be made of very short duration and still convey the desired information; therefore, many of them can be squeezed into the time available on a digital carrier channel.

Table 3.7 Analog Carrier Systems

Multiplex Level

No. of Voice Circuits

Frequency Band, KHz

Voice channel

1

0–4

Group

12

60–108

Supergroup

60

312–552

Mastergroup

600

564–3084

Jumbogroup

3,600

564–17,548


The original signal can be an analog wave that is converted to binary form for transmission (as in the case of speech signals in the telephone network), or the original signal can already be in binary form (as in the case of a business machine). The electronic systems that perform this TDM process are called digital carrier systems. As with the analog carrier systems, there is a standard hierarchy of digital carrier systems in the public telephone network, as shown in Table 3.8. Refer to Chapter 6 for specific information concerning time-division multiplexing and the operation and utilization of a T1 multiplexer that can be used with a DS1 digital carrier system.

The digital signals listed in Table 3.8 reference the type of signal that a particular digital line transports to include applicable framing. For example, a T1 line transports a DS1 digital signal. In Table 3.8, note that the lowest operating-rate digital signal is indicated as DS1, which consists of 24 voice circuits whose aggregate operating rate is 1.544Mbps. Although the DS1 digital signal is indeed the lowest operating-rate signal in the digital carrier hierarchy, it is not the lowest operating-rate digital transmission facility available for use. To understand why this is the case, you must have additional information about the DS1 signal.

Using Pulse Code Modulation

In North America, the DS1 is commonly referred to as a T1 line or circuit. That circuit was developed to relieve cable congestion in metropolitan areas by providing a transport mechanism for 24 digitized voice conversations to be simultaneously carried over one cable. To do so, each voice conversation is digitized using a technique called pulse code modulation (PCM). Under PCM, an analog voice conversation is digitized at 64Kbps. To provide information that enables one conversation to be distinguished from another and switched into and out of a group of conversations, framing bits must be added to the T1 data flow. Those framing bits operate at 8000bps and carry control information, error-detection information, and a limited data-link capability. This capability, for example, enables two private branch exchanges (PBXs) to communicate with one another while transporting 24 voice conversations on a T1 circuit interconnecting the PBXs. The 24 channels, each operating at 64Kbps, result in an operating rate of 1.536Mbps. When the 8Kbps framing information is added to the T1 line, its operating rate becomes 1.544Mbps.

Each voice channel in a DS1 digital signal, referred to as a DS0 or digital signal level zero channel, represents the lowest operating rate digital circuit marketed by communications carriers for direct connection to a channel on their T1 lines. Communications carriers also offer low-speed digital services operating at data rates from 2.4Kbps up to 56Kbps, using time-division multiplexers to group multiple low-speed digital circuits onto a 64Kbps circuit. The 64Kbps circuit, in turn, is connected to a channel on a carrier's T1 line, which represents the basic backbone infrastructure used for transporting voice, data, and video across North America.

In the telephone company infrastructure, four DS1 signals are combined by a device referred to as an M12 multiplexer to generate a DS2 signal operating at 6.312Mbps that transports 96 DS0 voice channels. At the next stage in the telephone company hierarchy, either 28 DS1 signals can be combined by a M13 multiplexer or 7 DS2 signals can be combined by an M23 multiplexer to generate a DS3 signal operating at 274.176Mbps. The resulting DS3 signal is the signal transported by a T3 circuit. Six DS3 signals can be multiplexed by an M34 device to generate a DS4 signal. However, unlike T1 and T3 circuits that are commercially available, DS2 and DS4 signals are only internally used by communications carriers, which explains the absence of commercially available T2 and T4 circuits.

Table 3.8 Digital Carrier Systems

Digital Signal No.

No. of Voice Circuits

Bit Rate, Mbps

DS1

24

1.544

DS2

96

6.312

DS3

672

44.736

DS4

4,032

274.176


Dataphone Digital Service

The T1 circuit was originally limited to use by communications carriers to relieve the cable congestion in metropolitan areas. Because of the successful use of this transmission facility, AT&T and other communications carriers tariffed its use for commercial organizations and government agencies during the mid-1980s. Slower-speed digital transmission services, such as AT&T's Dataphone Digital Service (DDS), actually preceded the public offering of T1 service because DDS was introduced during the mid-1970s.

DDS is a leased-line digital transmission service with operating rates of 2.4, 4.8, 9.6, 19.2, and 56Kbps. A switched 56Kbps offering is also available in certain cities.

The economics associated with the T1 tariff caused the monthly cost of a T1 line to be four to eight times that of one 56Kbps DDS circuit. Because a T1 line has more than 24 times the capacity of a 56Kbps DDS circuit, most organizations that required the use of multiple 56Kbps DDS lines between common geographical locations soon replaced those circuits with T1 lines. In addition to the economics associated with the use of T1 lines, their additional data-transport capacity enabled organizations to merge voice, data, and video applications onto a common circuit. In fact, by the early 1990s, the T1 circuit formed the backbone for most corporate and government networks. By the late 1990s the T1 line was the primary mechanism by which tens of thousands of businesses, academic institutions, and government agencies connected LANs to the Internet. Because Web servers are connected to LANs, the T1 circuit also became the primary access line used to connect Web servers to the Internet.

Fractional T1 Service

The difference between the maximum data rate supported by DDS and the operating rate of a T1 circuit left many organizations unsatisfied with respect to the traffic-handling capacity of communications carrier circuits. Recognizing the requirements of those organizations for the use of a fraction of a T1 line's operating rate, communications carriers introduced fractional T1 (FT1) service in the early 1990s. Today, most communications carriers offer FT1 service in operating rates from 64Kbps to 768Kbps in increments of 64Kbps. Although an organization contracts for a specific FT1 operating rate, the carrier normally installs a T1 line from the serving office to the customer's premises. The customer either installs equipment or gets equipment from the communications carrier, which places the customer's data into a group of DS0s that represents the contracted FT1 service.

Figure 3.14 illustrates an example of the digital multiplexing hierarchy used to transport FT1- and T1-based communications between cities. At the serving carrier office, data in the form of individual or groups of DS-Os from subscribers is removed (dropped) from some T1 lines and placed (inserted) onto other T1 lines. In this way, most (if not all) of the 24 DS-O slots on certain T1 lines are used. Next, groups of T1 lines are multiplexed onto a T3 or T4 line for the routing of voice and data that represents 672 or 4,032 voice-grade circuits routed between long-distance communications carrier offices.

To illustrate the economics associated with the use of FT1, Table 3.9 lists AT&T's monthly interoffice channel charges based upon a 350-mile circuit in effect in 1998 for that carrier's series of digital service offerings that provide data rates from 56Kbps to 155Mbps. The 155Mbps rate is not included in the table because it is currently priced on an individual customer basis (ICB). Also note that since 1994, AT&T converted its digital leased line pricing from a monthly charge per mile to a city-pair pricing structure. This helps explain why in many older books and articles, you will commonly see references to the monthly charge per mile for digital circuits operating at different data rates.

Figure 3.14 The digital multiplexing hierarchy.

Table 3.10 provides a monthly cost comparison between 56/64Kbps leased lines and T1 leased lines for five city pairs. If you compare the monthly cost of a 56/64Kbps leased line to a T1 line operating at 1.544Mbps, you will note that an increase in cost of approximately a factor of 7 to 8 results in an increase in data transmission capacity by a factor of 24.

Table 3.9 AT&T Accunet Spectrum of Digital Services Interoffice Channel Charges

Channel Operating Rate

Monthly Charge Per Mile

56/64Kbps

$475

128Kbps

$854

256Kbps

$1,636

512Kbps

$2,956

768Kbps

$3,992

1.544Mbps

$5,462

4.6Mbps

$17,323

7.7Mbps

$27,553

45Mbps

$52,181


Table 3.10 AT&T 56/64Kbps/T1 Private Line Monthly Pricing Based on Representative City Pair Costs

City Pair

Air Miles

56/64Kbps

T1

Atlanta–Chicago

584

$1,094.33

$8,422.69

New York–San Diego

2,415

$1,620.67

$16,390.78

New York–Atlanta

745

$1,130.51

$9,147.20

New York–Seattle

2,406

$1,655.39

$16,861.96

Atlanta–Erie

621

$1,202.36

$8,154.99


The interoffice channel charges listed in Table 3.9 and city pair pricing listed in Table 3.10 are presented for illustrative purposes. AT&T and other communications carriers offer a range of discount plans based upon multiyear contracts that can considerably reduce monthly fees for organizations that are willing to make a commitment to maintain a level of service.

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