References

[8-1] Brooks, John. Telephone: The First Hundred Years. NY: Harper and Row, 1976.

[8-2] Engineering and Operations in the Bell System. NJ: Bell Telephone Laboratories, Incorporated. 1977.

[8-3] Bates, Regis J. Introduction to T1/T3 Networking. Norwood, MA: Artech House. 1992.

[8-4] Trulove, James E. A Guide to Fractional T1. Norwood, MA: Artech House. 1992.

[8-5] Flanagan, William A. The Guide to T-1 Networking. NY: Telecom Library Inc. 1990.

[8-6] Minoli, Daniel. Enterprise Networking: Fractional T1 to SONET, Frame Relay to BISDN. Norwood, MA: Artech House. 1993.

[8-7] Nolle, Tom. “Don’t Bury Traditional T1 Just Yet.” Business Communications Review. September 1995.

[8-8] Gelber, Stan. Introduction to Data Communications: A Practical Approach. Horsham, PA: Professional Press Books. 1991.

[8-9] Doll, Dixon R. Data Communications: Facilities, Networks, and Systems Design. NY: John Wiley and Sons, Inc. 1978.

[8-10] Elbert, Bruce R. Private Telecommunication Networks. Norwood, MA: Artech House. 1989.

[8-11] Korzeniowski, Paul. “T1 Continues to Grow in Private Networks.” Business Communications Review. February 1996.

[8-12] Rhode, David. “Sprint turns on 800 Switched Data Service.” Network World. October 30, 1995.

[8-13] Briere, Daniel. “Digital 800 Era Arrives-So Does a Dilemma.” Network World. March 6, 1995.

[8-14] “Digital 800 Gains Momentum.” Network World. September 25, 1995.

[8-15] Kleinrock, Leonard. Principles and Lessons in Packet Communications. Partridge, Craig. Innovations in Internetworking. Norwood MA: Artech House. 1988.

[8-16] Heart, F.E.; Kahn, R.E.; Ornstein, S.M.; Crowther, W.R.; and Walden, D.C. The interface message processor for the ARPA computer network. Partridge, Craig. Innovations in Internetworking. Norwood, MA: Artech House. 1988.

[8-17] Spohn, Darren L. Data Network Design. NY: McGraw-Hill, Inc. 1993.

[8-18] Stallings, William. ISDN and Broadband ISDN with Frame Relay and ATM (Third Edition). NY: Prentice-Hall. 1995.

[8-19] Rybczynski, Antony. X.25 Interface and End-to-End Virtual Circuit Service Characteristics. Partridge, Craig. Innovations in Internetworking. Norwood, MA: Artech House. 1988.

[8-20] Held, Gilbert. Understanding Data Communications. Indianapolis, IN: SAMS Publishing. 1994.

[8-21] Burch, Bill. “X.25 Protocol Proves to Have Enduring Appeal.” Network World. October 31, 1995.

[8-22] “X.25 Emerges as a LAN Interconnect Option.” Data Communications. March 1995.

[8-23] Levitt, Jason. “Hold The Phone!” Information Week. May 15, 1995.

[8-24] Horak, Ray. “ISDN: To Be Delivered as Promised?” Datapro Communications Analyst. Delran, NJ: Datapro Information Services. January 1995.

[8-25] Greene, Tim. “Sprint/United, Other Small Carriers Get Into ISDN Game.” Network World. August 7, 1995.

[8-26] Buerger, David J. “National ISDN’s Third Birthday is Nothing to Write Home About.” Network World. November 13, 1995.

[8-27] Roth, Cliff. “ISDN Modems Come to Town.” NewMedia. March 11, 1996.

[8-28] Gareiss, Robin. “ISDN: Stop Singing Those Blues.” Data Communications. March 1996.

[8-29] Sullivan, Kristina B. “ISDN Moves Toward the Mainstream.” PC Week. December 11, 1995.

[8-30] Stargess, James. “ISDN D Channel Packet Service: Coming Soon to a Store Near You.” Network World. March 11, 1996.

[8-31] ISDN: A User’s Guide to Services, Applications and Resources in California. San Francisco, CA: Pacific Bell. 1994

[8-32] “ISDN and Data Networking.” Datapro Communications Analyst. Delran, NJ: Datapro Information Services. May 1995.

[8-33] Metcalfe, Bob. “The FCC’s Doubling of ISDN Rates May Finally Kick Off a User Revolt.” Infoworld. April 3, 1995.

[8-34] Case, Linda and Mulligan, John. ISDN As a Telecommuting Solution. Piscataway, NJ: Bellcore. 1996.

[8-35] Tredinnick, Ian. “X.25: A New Lease on Life with ISDN.” Telecommunications. June 1995.

[8-36] Kalman, Steve. “So You Want to Use ISDN….” Network World. December 4, 1995.

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A single BRI line can support up to eight devices, which might be in the form of telephones, facsimile machines, or computers. Additionally, up to 64 individual telephone numbers can be supported [8-31]. While BRI supports as many as three simultaneous calls, only one can be a voice conversation.

BRI uses an 8-pin connector which is defined by the International Standards Organization (ISO) in ISO 8877 and which uses a RJ-45 jack. Full duplex (FDX) connectivity is accomplished over a digital twisted-pair local loop through the application of special carrier electronics. A NT1 (Network Termination 1) device provides for compatibility with network protocols.

Primary Rate Interface (PRI)

Primary Rate Interface (PRI) also is known as 23B+D in the US and Japan. The European or ITU version is known as Primary Rate Access (PRA) or 30B+D. PRI offers 23 B (Bearer) Channels, plus 1 D (Data) channel. Both the B and D channels operate at 64 Kbps. The individual B channels can be used as discussed in the case of BRI; the D channel is reserved exclusively for signaling. As the standards provide for a D channel to support up to five PRI connections, numerous carriers have recently embraced this concept, thereby yielding additional usable bandwidth for user application. For example, the first PRI in a PBX trunking application would be provided at 23B+D; the next four PRIs would be deliver 24B+0D.

PRI provides a full duplex (FDX) point-to-point connection through a NT2-type intelligent CPE switching device (e.g., a PBX or router) device for protocol interface with the carrier CO exchange switch.

While designed for transmission over a standard T1 trunk, PRI is a significant improvement over T1, because the channels can be allocated dynamically. Each channel can act as an incoming, outgoing, combination, or DID trunk as the need arises. The nature of the channel can be determined as required or as specified, varying based on user-definable parameters. Additionally, multiple B channels can be aggregated to serve bandwidth-intensive applications, such as videoconferencing.

H-Channels

H-channels (High-speed channels), are functionally equivalent to B-channels, but provided greater aggregate bandwidth in PRI applications. H0 channel signals have a aggregate bit rate of 384 Kbps, while H1 channels operate at an aggregate of 1.536 Mbps for the North American version (H11) and 1.920 Mbps for the European version (H12). This capability of channel aggregation allows multirate communications on a dynamic basis through inverse multiplexing over multiple B-channels. It does have a drawback, however, when compared to traditional Inverse MUXs as the connection must be torn down and reinitiated when channels are added or dropped. The feature is known variously as Multirate ISDN, Nx64, Channel Aggregation and bonding. H channels find application in fast faxing (Group IV), videoconferencing, high-speed data transfer and high-quality audio transmission.

ISDN Equipment

ISDN hardware, at the end user side of the connection, includes Terminal Equipment (TE), Terminal Adapters (TAs), and Network Terminations (NTs) as shown in Figure 8.10. The carrier requires digital COs that are equipped with ISDN and SS7 software.

Terminal Equipment (TE)

Terminal Equipment (TE) is the term for a functional device that connects a customer site to ISDN services. Examples include computers, telephones, facsimile machines, and videoconferencing units. TE1 has a built-in ISDN interface, while TE2 devices do not have native ISDN compatibility.

Terminal Adapters (TAs)

Terminal Adapters (TAs) are interface adapters for connecting one or more TE2 (non-ISDN) devices to an ISDN network. TAs act as ISDN DCE, serving a function equivalent to protocol or interface converters. Applied to equipment which does not have ISDN capability built within it; the TAs must be exactly tuned to the specific CPE/DTE. TAs can be in the form of either standalone units or printed circuit boards fitting into an expansion slot of a PC.

A key function of the TA is that of rate adaption, which effectively throttles down the transmission rate from 64 Kbps to the rate at which the non-ISDN device is capable [8-32]. As an example, a non-ISDN PC might be capable of only 19.2 Kbps through the serial port. Another example might involve a connection supported at only 56 Kbps (non-ISDN or Pacific Bell “ISDN”) on the receiving end; the device would throttle down to that rate, rather than 64 Kbps. Rate adaptation is accomplished in North America through the ITU-T V.120 protocol; the European standard is V.110.

Network Terminations (NTs)

Network Terminations 0(NTs) are Network Termination devices, NT1s and NT2s.

NT2s

NT2s (Network Termination type 2) are intelligent devices responsible for the user’s side of the connection to the network, performing such functions as multiplexing, switching or ISDN concentration. A NT2 device would likely be in the form of a PABX, LAN router or switching hub.

NT1s

NT1s (Network Termination type 1) physically connect the customer site to the carrier side of the connection, performing such functions as signal conversion and maintenance of the local loop’s electrical characteristics. In a PRI environment, these functions are similar to those provided by Data Service Units (DSUs) and Channel Service Units (CSUs). In a BRI environment, these devices are TE1 devices.

Inverse MUXs,

Inverse MUXs, offered by some manufacturers, allow multiple BRIs to be bonded, or linked, for greater aggregate transmission over a BRI circuit(s). For example, four BRIs can be linked to support a 512 Kbps data transmission. Such an approach competes effectively with Fractional T1 (FT1).

D-Channel Contention Devices

D-Channel Contention Devices, offered by some manufacturers, allow as many as eight devices to share a BRI circuit, contending for access to the B-channel. The individual devices identify themselves to the network through contention for the D channel.

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Islands of ISDN were the result of these various implementations. A given carrier, using the hardware and software of a given manufacturer, could not easily achieve full connectivity with another carrier deploying another version of ISDN. Weary of delays in the standards process, some carriers (e.g., Southwestern Bell) developed and implemented proprietary versions of ISDN, further contributing to the problem. During the past several years, this problem has been mitigated through cooperation of the manufacturers and carriers, with active involvement of Bellcore [8-23] and [8-24].

Availability of ISDN was slow to develop, as the carriers were reluctant to invest in the technology unless they were convinced that a market existed for the services and/or that the technology offered internal cost savings. ISDN is not inexpensive to deploy, a fact which unfortunately has been reflected in high end-user circuit and equipment costs. Availability recently has increased to a significant extent, with the RBOC’s and GTE having made considerable commitments to its deployment. The independent telephone companies are increasingly making ISDN a priority, although they understandably lag far behind the RBOCs due to cost-of-service factors [8-25].

Regulators generally have required that the LECs pass on the cost of ISDN infrastructure to ISDN users, rather than allowing them to average those costs across the entire rate base. In other words, they have viewed ISDN as an optional service that must pay its own way, or that the carriers should absorb any associated losses. As the carriers have been unwilling to do so, ISDN rates have remained high.

Rates for ISDN access were not tariffed at attractive levels compared with the cost of basic services. Again, the regulators were, and still are, largely responsible. The carriers also bear responsibility as well, as they have not been willing to absorb initial losses in order to stimulate the growth of the service offering. Ameritech, for instance, charges $135 for installation and $34.64 per month for a low-speed ISDN BRI circuit; an additional $20 per month applies if the premise is more than three miles from an ISDN central office. Southwestern Bell is the prize winner, at $578 for installation and $57.70 per month [8-26]. In New York, NYNEX charges $14 per month in addition to the normal line charge, plus installation or upgrade charges which vary from about $50 to $312 [8-27]. Rates for PRI vary considerably as well. In some cases based on a per-channel charge and in other cases on a flat rate per PRI [8-28]. In some states, additional per-minute surcharges apply to ISDN; starting with call setup, rather than call connection. The charges typically are higher for a data call than for a voice call.

Equipment costs were high, as the manufacturers were constantly investing in R&D to maintain ISDN compliance with developing standards. Additionally, the limited demand for ISDN caused the manufacturing runs to be small-lower costs are achieved through increased volume. As an example, Terminal Adapters, which provide the interface to an ISDN circuit for non-ISDN equipment, have come down in price from about $1,000 in 1993 to an average of less than $500 today. That is an improvement, but still a substantial investment for a small business or residential user [8-29].

Marketing by the LECs was not effective. Not only were costs maintained at unattractive levels, but advertising and promotion were limited, and availability clearly was not high. Further, meaningful and cost-effective applications were not identified and stressed. With typical lack of foresight, the LECs placed heavy emphasis on the low-speed BRI version, which is suitable only for residence, small business, and SOHO (Small Office/Home Office) application. High-speed PRI was not emphasized heavily as a replacement of T1 trunking. ISDN Centrex was touted heavily, but with limited success. Centrex ISDN marketing was heavily slanted toward CO-based Local Area Networking, which was a dismal failure.

While ISDN has frustrated the industry, in general, there currently are over 200,000 ISDN access lines worldwide. Some industry pundits forecast that more than 750 million will be installed by the year 2000-the author does not share that level of enthusiasm. ISDN is much more mature in Europe and certain parts of the Pacific Rim than it is in the United States. In those regions, deployment was encouraged by the regulators and even subsidized by the governments. Additionally, marketing was much more effective, focusing on PRI, rather than BRI [8-24].

Standard Interfaces and Channel Types

The current version of ISDN is Narrowband ISDN (N-ISDN). (Broadband ISDN (B-ISDN), which is still on the drawing boards, is discussed in Chater 11.) ISDN currently is available in essentially two interface varieties BRI (2B+D) and PRI (23B+D in North America, and 30B+D in Europe and many other countries) (see Figure 8.9). In each case, the ITU specifies the electrical characteristics, signaling, coding and frame formatting. There is one variation on the theme; H-channel, designed for high-bandwidth applications, bonds multiple B channels.

Basic Rate Interface (BRI)

Basic Rate Interface (BRI) also is known as Basic Rate Access (BRA) and 2B+D. BRI provides 2 B channels (Bearer, or information-bearing channels), each operating at the clear-channel rate of 64 Kbps by virtue of SS7 nonintrusive signaling. Each B channel can carry digital data, digitized voice (PCM-encoded at 64 Kbps or a lower rate), or a mixture of low-speed (subrate) data as long as it all is intended for the same destination. BRI also provides a D (Data) channel at 16 Kbps, which is used for control, messaging, and network management. The D channel also generally is made available for packet data transmission and low-speed telemetry when not in use for signaling purposes. Cost-effective applications include credit card authorization, which involves very small bursts of data [8-30]. BRI is primarily used for residential, small business, Centrex, and telecommuting applications that are not particularly bandwidth-intensive. The B channels can be aggregated or bonded, to provide up to 128 Kbps to a given conversation, such as a videoconference; additionally, multiple BRIs can be bonded for even greater capacity. Whether bonded or not, ISDN BRI provides multiple channels over a single physical loop, which is a great advantage.

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Packet Switching Applications and Futures

The original application for X.25 packet switching was that of interactive time-sharing, which involves long connect times and low data volume. While such applications are still supported effectively by X.25, contemporary applications include online interactive processing (reservations systems), messaging (email), batch file transfer (data backup), information service access (America Online, CompuServe, and Prodigy), and Internet access.

Packet-switching offers the advantage of being a highly mature, if limited, network technology. Therefore, it is relatively inexpensive to deploy and is highly cost-effective in support of applications that require many-to-many connectivity and which involve relatively low volumes of data transport. Additionally, it is virtually ubiquitous, having been deployed in every corner of the globe. However, it is limited in terms of speed and latency. As a result, many applications and service providers are moving toward newer network technologies. For example, the airline reservations systems are rapidly moving to Frame Relay, at least for the domestic (U.S.) networks. Internet Service Providers prefer ISDN or Frame Relay. The Internet backbone network largely has shifted to Frame Relay or ATM, operating at T1 or T3 speeds at a minimum; much of the backbone is being upgraded to fiber optic facilities operating at speeds of 155 Mbps. The future of packet switching is assured because of its low cost and high availability. Additionally, low-intensity users will continue for many years to access the Internet on a dialup basis through X.25 packet switches.

X.25 is growing at record rates in Latin America, Central Europe, and other developing regions. In such areas, the poor quality of the networks make X.25′s error correction capabilities a must for data communications. Demand also is growing in Western Europe and North America, where Internet access is X.25-based, at least at the edges of the networks [8-21]. The postal administration in France recently built a X.25 LAN internetwork that eventually will extend to most of its 7,000 post offices. Bank branches in Belgium and other countries of Western Europe also are expanding their X.25 networks due to the low cost and high degree of flexibility [8-22].

Integrated Services Digital Network (ISDN)

ISDN (Integrated Services Digital Network) was first explored as a concept from 1968 to 1971 by a CCITT study group. A more focused conceptual study took place during the 1981 to 1984 CCITT study period. The first set of published standards recommendations appeared in 1984 in the form of a CCITT Red Book, which provided the basic framework for the concept, network architecture, UNI (User Network Interface) protocols and common channel signaling protocols. As a result of the 1985 to 1988 study period, a Blue Book was published, which provided description of supplementary services, rate adaptation, ISDN Frame Relay, and the initial set of B-ISDN (Broadband ISDN) recommendations. (The color of the books has no significance, other than the fact that a different color is chosen for each study period.)

ISDN is a suite of services based on a set of technologies, including transmission, switching, and signaling and control. ISDN is a set of international standards recommendations which will allow the provisioning of a wide range of services which are intended to be available on a ubiquitous basis. Additionally, the ISDN network is accessible through a standard set of interfaces-one for low-bandwidth applications and another for high-bandwidth.

The specific characteristics of ISDN include its entirely digital nature-CPE, transmission facilities and switching systems all are fully digital. Three channel types are identified, including B-channels (Bearer channels) that carry the information, D-channels (Data channels) for signaling and control, and H-channels (High-speed channels) for channel aggregation in order to accommodate bandwidth-intensive applications. The User Network Interface (UNI) protocols include Basic Rate Interface (BRI) for low-speed termination, and Primary Rate Interface (PRI) for high-speed access. Common Channel Signaling System #7 (SS7) is a fundamental requirement of ISDN.

Announced to the world with great fanfare, ISDN quickly captured the imagination of carriers, manufacturers and user organizations worldwide. ISDN offers the compelling advantages of improved bandwidth, flexibility, error performance, reliability, availability, and interconnection to a wide range of services. Unfortunately, it has stalled. For the past 25 years, ISDN has progressed at such a slow pace that it has become legendary. Among the many reasons for its slow development are slow standards development, lack of adherence to standards, lack of availability, regulatory hurdles, circuit and equipment costs, and poor marketing.

Standards development at the ITU-T is infamously slow. Standards traditionally were released every four years in monsoon fashion and with total droughts in the interim. Over time, the various committees were afforded the privilege of developing and releasing certain standards recommendations on an intermediate basis-standards now come in sprinkles and showers.

Standards from the ITU actually are in the form of standards recommendations. Individual member nations are free to implement ISDN options as they see fit, or to deviate from the standards, as long as international interconnectivity is accomplished at some reasonable level. The most notable international difference is that of the basic ISDN hierarchy. The North American version follows the T1 hierarchy, with PRI including 24 B channels; the European (ITU) version is based on E-1, providing 30 B channels. While this difference is understandable in the context of maintaining backward compatibility with existing networks, it also perpetuates issues of basic protocol incompatibility.

Systems manufacturers of CO exchanges and PBXs have a strong interest in maintaining the proprietary nature of their systems architectures. Therefore, they have implemented ISDN in distinctly different ways. ISDN compatibility became ISDN compliance -a decidedly lower level.

Additionally, carriers have implemented nonstandard versions of ISDN. Pacific Bell, for instance, offers ISDN at a rate of 56 Kbps per channel, rather than the standard 64 Kbps. This limitation is due to the fact that SS7 is not fully deployed in Pacific Bell’s carrier network; as a result, in-band signaling and control consumes 8 Kbps of channel bandwidth. Pacific Bell has announced its intention to rectify this situation in 1997, although the recently announced Pacific Bell/Southwestern Bell merger may affect that decision.

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Protocol Conversion

As an option, packet-switched networks will accomplish protocol conversion (Figure 8.8). Protocol conversion can include any protocol that is well-established, well-understood, widely-deployed and, therefore, supported by the carrier. As this process of protocol conversion adds value, packet networks (X.25) are widely recognized as the first Value-Added Networks (VANs). Protocols supported typically include asynchronous, IBM Bisync (BSC), and IBM SDLC.

Latency

Latency or delay, is a troublesome and limiting characteristic of packet networks. As each packet may take a different route though the network, each may travel a route of a different length; therefore, propagation delay may vary from packet to packet. Additionally, each packet may travel through a different number of packet nodes, each of which must act on the packet to read its address, check for errors, request retransmissions of errored packets, and so on-this fact compounds the issue of packet delay. Further, each packet may encounter different levels of congestion in the network, which may add to packet delays. Finally, additional delay is imposed on each packet if protocol conversion is required. While this process adds value, it does add to the latency factor. The end result is that some level of latency not only is assured, but also is variable and uncertain in magnitude [8-6].

While this characteristic of packet switching does not affect some applications, it renders others ineffective. For instance, e-mail communications over the Internet is not seriously impacted, although the delays may be aggravating at times. On the other hand, isochronous (stream-oriented) communications such as realtime audio, voice, or video are not effectively supported over a packet network.

Access

X.25 actually is the ITU-T standard describing the physical, link and packet level protocols between the user DTE/DCE and the network [8-17] and [8-18]. Devices capable of packetizing the data are connected over a X.25 link. In a true X.25 environment, the user accomplishes the packetizing process through DCE in the form of a PAD (Packet Assembler/Disassembler), the standard for which is X.3. The PAD also may contain intelligence for password protection and performance reporting [8-8].

Occasional or casual users typically access a packet network on a dialup basis, from asynchronous PCs through modems. In such a scenario, the actual packetizing of the data is performed at the originating network node. For example, individuals accessing the Internet through an online information service often use this approach.

Large user organizations often access the network via a dedicated, leased-line link to the closest network node. Such access often is in the form of an unchannelized T1 facility, perhaps supporting frame relay. Large users connecting substantial hosts to a X.25 network present data to the node under the terms of a protocol known as subscriber Link Access Procedure Balanced (LAPB), which ensures error-free local access and egress on a full duplex (FDX) basis. The user data is segmented into a packet by the PAD, and encapsulated in a LAPB frame before presentation to the network [8-18] and [8-20]. The frame level procedure described in X.25 can be either the ISO High Level Data Link Control Procedure (HDLC) or IBM Bisync (BSC) [8-20]. Internet Service Providers (ISPs), for instance, make heavy use of such dedicated facilities.

Network Interconnection

X.25 networks are widely available as a Public Data Network (PDN) service offering, generally using packets of 128B or 256B. However, certain applications are supported more effectively by transmission of larger packets. For example, the airline reservation systems (e.g., American Airlines’ SABRE and United Airlines’ APOLLO) have deployed custom packet networks that use packet payloads of 1,028B. As this application involves the frequent transmission of relatively large sets of data (e.g., flight schedules, fares, and seating availability), a larger packet size is more appropriate. The larger packet size improves efficiency, because the payload is very large, while the overhead information is roughly the same as in the case of a smaller packet. While a larger packet is more likely to contain an errored bit, require retransmission and, therefore, reduce throughput; the custom reservation networks employ digital facilities in order to minimize this exposure.

The interconnection of such disparate networks is accomplished through an ITU-T standard known as X.75. Through an X.75 network-to-network interface, as depicted in Figure 8.8, issues of packet size are resolved in order that information flow is not affected.

Packet-Switching Hardware

The user of a X.25 packet network may require no hardware other than a PC and modem. Occasional and casual users of the Internet through an online information service (e.g., America Online, Australia Online, CompuServe and Prodigy) fall into this category. The packetizing of the asynchronous data is performed at the local X.25 node. Larger users will install DCE in the form of a Packet Assembler/Disassembler (PAD), which is specified by the ITU-T as X.3. The PAD performs the packet assembly (segmentation) of the data for the transmitting device, disassembling the packet for the receiving device in order to reconstitute the data in its native format.

Packet carriers, of course, must invest in packet nodes, rather than circuit switches. Such packet nodes are intelligent devices capable of supporting complex routing tables, buffering packets in temporary memory, resolving packet errors, and accomplishing protocol conversions.

Packet Switching Standards

The ITU-T sets standards recommendations for packet switching. Those standards include the following [8-6], [8-8] and [8-20]:

• X.3: Packet Assembly/Disassembly functions
• X.25: Interface between DCE and DTE for public packet networks
• X.28: Terminal-to-PAD communications formats
• X.29: Host-to-PAD communications formats
• X.31: Packet-mode services over ISDN
• X.32: Defines X.25 synchronous dialup mode
• X.75: X.25 internetwork call control procedures

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The payload data is encapsulated by a beginning flag (8 bits) and an ending flag (8 bits) that serve to the distinguish each packet from other packets traveling the same path. The beginning flag also serves as synchronizing bits in order that the packet nodes (intelligent switches) and the receiving terminal equipment synchronize on the rate of transmission. As discussed in Chapter 7, this approach reduces overhead in data transmission and improves the efficiency of transmission. A packet address field of 8 bits (4 bits for the calling DTE and 4 bits for the called DTE) is prepended to the data in order that the various packet nodes might route each packet to the target device. Control data (8-16 bits) includes the packet sequence number so that the target node and terminal equipment are able to identify errored, corrupted, or lost packets, or to resequence the packets should they arrive out of order. Further, the packet sequence number allows the identification of lost or corrupted (errored) data packets in a stream of data. Additionally, the control data includes the number of the virtual circuit (4 bits) and virtual channel (8 bits) over which the data will travel, if a path has been preordained. Finally, error control data is included in the form of a CRC check (16 bits), As discussed in Chapter 7, this level of error control offers a high degree of reliability [8-17].

Packet Switching and Transmission

In a typical scenario, the transmitting terminal, equipped with a modem, dials a telephone number to gain access to a local packet node on a circuit-switched basis through the LEC central office exchange. Alternatively, a short-haul dedicated circuit might connect the user location directly to the packet node. Once the connection to the packet node is established, the transmitting device sends a control packet across the network to establish a data session with the target host computer. The originating node receives that packet, checks for transmission errors, reads the address, and forwards the packet towards the destination, across the most direct and available link. The process is repeated at the next node and so on, until the data reaches the packet node serving the target host. That node sends the packet to the target, which acknowledges its receipt and establishes a session by responding with a control packet to the originating device.

At that point, the originating device begins a stream of data, segmented into packets, with each packet numbered sequentially. Each packet is routed through the network independently, from node to node, in the direction of the target device, based on the most direct and available path at that instant. Should a reasonable route not be available immediately, the packet will be held in queue in buffer storage for a reasonable length of time, until a link becomes available. Once the communication session is complete, a control packet is sent across the network to terminate the data call.

In this scenario in Figure 8.7, each packet may take a different route from transmitter to receiver, in what is known as a datagram mode of transmission. This mode, therefore, requires that the packets be resequenced before being transmitted from the final node to the receiving device, ARPANET and its X.25 successor, pioneered the concepts of locally adaptive routing, network message segmentation, and datagram transmission mode.

The internodal links originally were dedicated analog trunks, which later were replaced by digital circuits-usually 56 Kbps DDS circuits. Over time, many of those circuits were replaced with T-carrier facilities. Currently, the facilities generally are high-speed fiber optic in nature, although all variety of analog and digital media are employed in consideration of specific network economics.

Error Control

X.25 also provides for error control through the use of a Cyclic Redundancy Check (CRC). Should an error occur in transmission, the node receiving that packet will recognize the error and correct for it through requesting a retransmission of the corrupted packet. Each node acting on the packet repeats that process. Additionally, lost packets are discovered at the destination node through examination of packet sequence numbers. Retransmissions of lost packets are then requested.

Error control was extremely important in early packet networks, as the facilities consisted of analog twisted-pair. Through a cascading error control process, the integrity of the individual packets and of the packet stream could be improved. However, this process is CPU-intensive, adding to the cost of the packet nodes. Additionally, the process is time-consuming, as each packet must be checked for errors prior to being forwarded to the next node.

Connection-Oriented

It should be noted that X.25 packet switching is a connection-oriented service. A call is set up over a shared path (virtual circuit), over which all packets may travel in support of a logical connection. On the other hand, each packet may travel over a different path, depending on the availability of the various network links at any given moment in time. Each packet of data is separately addressed and, therefore, is capable of working its way through the network independently of the other packets in a stream of packetized data. This characteristic of packet networks is a critical advantage, as the network and all of its elements are shared among large number of users-the cost of transmission across such a network is very low in the context of an appropriate application. Remember X.25 is an interface standard and does not address the internal operation of the network.

Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs)

Packet switching supports a large number of conversations over virtual circuits using the same, previously designated circuit or path. While the individual packets of the typical user may travel different paths, large users may be supported by PVCs. In this scenario, all packets will travel the same path between two computers, which path is established by routing instructions programmed in the involved nodes. Alternatively, the network may select the most available and appropriate path on a call-by-call basis using Switched Virtual Circuits (SVCs). Again, all packets in a given session travel the same path [8-19].

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X.25 Packet Switching

Packet switching was invented in the early 1960s by Paul Baran and his research associates for the RAND Corporation. Interestingly enough, the concept first was published in 1964 as a means of transmitting secure voice for military application. In the late 1960s, the U.S. General Accounting Office (GAO) issued a report suggesting that there existed a large number of data centers supported, at least in part, by the federal government. Further, the report indicated that many of those data centers were underutilized, and others were severely stressed. The imbalance was due largely to the lack of a wide area network technology that would allow the sharing of those resources on a cost-effective basis.

As a result of that study, the Advanced Research Project Agency Network (ARPANET), the first sophisticated packet switched network, was created in 1971. ARPANET was developed to link computers on a time-share basis in order to share computer resources on a cost-effectively [8-15]. Specifically, ARPANET was designed to support various defense, higher education, and research and development organizations [8-16]. In 1983 the majority of ARPANET users spun off to form the Defense Data Network (DDN), also called MILNET (MILitary NETwork), which included European and Pacific Rim continents. The United States and European locations which remained ARPANET then merged with the Defense Advanced Research Project Agency Network to become DARPA Internet [8-17].

Packet switching soon was commercialized and made widely available by Telenet, Graphnet (a facsimile-like service) and others. Packet switching was utilized very early on and extensively in Europe, as well. In fact, packet switching quickly became available in most countries, and currently is virtually ubiquitous. The CCITT (now the ITU-T) in 1976 internationally standardized X.25 as the interface for a packet-switched network.

The wide availability of packet switching has made it consistently popular over the last twenty years or so. Additionally, packet networks are highly cost-effective for applications that require many-to-many connectivity, and which involve relatively low data volumes. That popularity is growing and is ensured well into the future, largely through its historical deployment as the network technology of the Internet. It should be noted that X.25 is an interface specification, and does not define the internal operations characteristics of the data network.

The Concept of Packet Switching

The basic concept of packet switching is one of a highly flexible, shared network in support of interactive computer communications across a wide area network (WAN). Previously, large numbers of users spread across a wide area with only occasional communications requirements had no cost-effective means of sharing computer resources (time-share) from their remote terminals. In specific, the issue revolved around the fact that asynchronous communications are bursty in nature; data transmission is in bursts of keystrokes or data file transfers, with lots of idle time on the circuit between transmissions in either direction or relatively small amounts of data. Additionally, those early networks consisted of analog, twisted-pair facilities, which offered very poor error performance and relatively low bandwidth.

Existing circuit-switched networks certainly offered the required flexibility, as users could dial up the various host computers on which the desired database resided. Through a low-speed modem, data is passed over the analog network, although error performance was less than desirable. However, the cost of the connection was significant, because calls were billed based on the entire duration of the connection, even though the circuit was idle most of the time. Dedicated circuits could solve the cost issue, as costs are not usage-sensitive; however, dedicated circuits are expensive. Further, users tended not to be concentrated in locations where they could make effective use of dedicated circuits on a shared basis. Finally, large numbers of dedicated circuits would be required to establish connectivity to the various hosts.

Packet switching solved many of those problems, in the context of the limitations of the existing networks-namely, analog, twisted-pair. Packet-switched networks support low-speed, asynchronous, conversational and bursty communications between computer systems. As packet-switched network usage is billed to the user on the basis of the number of packets transmitted, they are very cost-effective for low-volume, interactive data communications. This cost advantage comes from the fact that the bursty nature of such interactive applications allows large volumes of data transmissions from multiple users to be aggregated in order to share network facilities and bandwidth [8-17]. Further, packet-switched networks perform the process of error detection and correction at each of the packet switches, or nodes, improving the integrity of the transmitted data considerably.

Understanding the concept and nature of packet switching requires the examination of a number of dimensions and characteristics of such networks including packet structure, switching and transmission, error control, connection-oriented service, latency, permanent virtual circuits versus switched virtual circuits, protocol conversion, and access techniques.

Packet Structure

Information is transported and switched through the network on the basis of packets (Figure 8.6). Each packet is of a fixed maximum size, typically containing 128B (B=byte or octet) or 256B of payload (user data); packet sizes of up to 4,096B can be accommodated in some networks. The typical upper limit of a packet is 1,024B [8-18], as is the case in many airline reservation networks.

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Even in comparison to newer LAN internetworking technologies such as Frame Relay, T-carrier holds it own. For instance, a large bank in Missouri recently tripled the size of its private T-carrier network to 30 nodes. They have plans to increase the network by another 20 nodes by 1997. The primary application is the transport of data from ATM machines and remote bank branches to IBM mainframes located at the central site in St. Louis. The T-spans are leased from a combination of IXCs [8-11].

T-carrier Developments and Futures

The most significant recent and emerging developments of digital carrier relate to equipment and applications. Many manufacturers of CSUs/DSUs have incorporated intelligence into their products, in effect turning them into commodity-level MUXs. MUX manufacturers have developed adapter cards which allow the transmission of SNA, ISDN, Frame Relay, and ATM over T-carrier. Direct connection of Ethernet and Token Ring LANs is sometimes supported, with the MUX effectively acting as a bridge or router. Some manufacturers have also released direct SONET fiber optic interfaces at speeds up to 155 Mbps. Contemporary MUXs are addressable devices that can be managed and controlled remotely. Additionally, several manufacturers of traditional MUXs have incorporated inverse multiplexing capabilities. Inverse multiplexing will spread a high-bandwidth transmission (a videoconference or file transfer) across multiple DS-0 channels or across multiple DS-1 circuits.

T-carrier certainly offers cost and performance advantages when compared to individual trunking. Additionally, it is attractive due to its ability to integrate voice, data, video, and other forms of information. Additionally, competition and deregulation have caused T-carrier costs to drop dramatically during the past ten years, although they recently have begun to increase. Yet, in a private, leased-line network application, T-carrier suffers from the same flexibility and vulnerability issues that affect alternative leased-line technologies. As a result, virtual private networks have replaced T-carrier networks in many large, voice-intensive user applications. Additionally, ISDN competes effectively with T-carrier in PBX-CO trunking applications. In data-intensive applications, T-carrier is losing ground to emerging broadband network technologies such as Frame Relay, SMDS, and ATM. Ultimately, ATM is likely to replace T-carrier, altogether.

In the meantime, however, T-carrier continues to enjoy a strong following, with T1 MUXs being the hottest-selling equipment for enterprise-wide networking. According to International Data Corporation, 1995 mux sales were $1.2 billion worldwide, an increase of 20% over the previous year. IDC forecast sales of $1.5 billion for 1996 [8-11].

Digital 800 Services

Digital 800 service is a very new offering, having been announced and trialed in 1994 by AT&T; the first general release offerings were those of AT&T and MCI in January, 1995. The service currently is offered nationwide in the United States by AT&T (800 Multimedia) and MCI (MCI 800 Digital Service) at a cost of less than 30 cents per minute. In the case of AT&T, different rates apply to voice calls (typically 6 to 10 cents per minute under contract) and data (22 cents a minute for peak calling and 19 cents for off-peak) calls [8-12]. It is intended only for medium-to-large customers subscribing to high-volume 800 service offerings. Sprint Corporation announced its Toll-Free Switched Digital Service in December 1995.

Digital 800 uses the same numbering conventions used for voice calls, based on the North American Numbering Plan (NANP), which is governed by the ITU standard dialing scheme. Therefore, carrier interconnection is provided, a problem which plagues Switched 56/64 Kbps and Frame Relay data service offerings employing proprietary numbering schemes. Additionally, the 800 numbers are portable. In fact, the same, and even existing, 800 number can be used for both analog and digital voice and data.

The carrier network distinguishes the nature of the call and routes it to the call destination via the appropriate facilities. Digital calls are routed exclusively over digital facilities. Although capabilities differ by carrier, routing considerations can include type of call (digital data versus analog voice versus videoconference), originating NPA, and time-of-day. Separate and distinct and management reports are available for each class of calling activity [8-13].

Bandwidth-on-demand, within limits, is provided as an inherent part of Digital 800. Digital bandwidth is provided in 64 Kbps increments (Nx64), up to full T1 at 1.544 Mbps. Depending on the nature of the call (voice versus data versus videoconference) the appropriate amount of bandwidth is supported.

Termination of Digital 800 requires a digital local loop, which can be in the form of ISDN, T1, or Switched 56/64 Kbps service. Termination equipment varies according to the nature of the terminating loop, although an Inverse MUX is required in order to associate multiple 64 Kbps channels where required. The carriers have suggested that the Inverse MUX requirement is temporary, to be eliminated as soon as multirate ISDN is available.

Access to the service can be in the form of an analog dialup connection, Switched 56/64 Kbps service, or ISDN. Where multiple channels are required for a high-bandwidth transmission, an inverse MUX currently is required at the originating end of the connection.

Digital 800 issues abound, as the LECs are not fully equipped at this point to handle the potential demand for the service. Therefore, users should not assume that Digital 800 termination is possible in every location. The features differ significantly between AT&T and MCI, especially with respect to sophisticated routing capabilities of analog voice versus digital data calls. Additionally, the carriers have not yet established (nor are they likely to establish) network-to-network interfaces; in other words, all incoming call centers must use the same Digital 800 carrier.

Applications for Digital 800 primarily are business-to-business. The envisioned applications include catalog ordering, software uploads/downloads, online information services, publishing, graphics editing, videoconferencing, multimedia communications, video augmentation, and image augmentation. While the carriers have a lot of work to do in order to make Digital 800 ready for mass consumption, it is expected to be a mainstream business service offering by 1998 [8-13] and [8-14].

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Channel Service Units (CSUs) and Digital Service Units (DSUs)

Discussed at length in Chapters 2 and 7, DSUs are devices which, in combination, interface the user environment to the digital network at the physical (mechanical) and electrical level, corresponding with Layer 1 of the OSI model. In contemporary systems, they generally are combined into a single device, known as a CDSU or an ISU (Integrated Service Unit), which may be under the skin of another device, such as a multiplexer (MUX). They are used in a wide variety of digital data networks, including DDS and T-carrier.

Multiplexers (MUXs)

MUXs are a significant step up from channel banks in terms of intelligence, capability, and cost. Originally based on channel banks and containing CSUs and DSUs, contemporary Statistical Time Division Multiplexers (Stat MUXs) offer a tremendous range of flexibility and capability. MUXs typically offer capabilities that include support for both channelized and non-channelized service, support for multiple medium interfaces (e.g., twisted-pair, coax, and fiber), support for multiple trunk types (e.g., DID and combination), support for superrate transmission (>64 Kbps channels), and support for subrate transmission (<64 Kbps channels). Additionally, they offer the advantage of user-definable configuration, internal diagnostics, voice compression, and T-carrier to-E-carrier protocol conversion. Intelligent MUXs also have the ability to allocate bandwidth on a priority basis for specified users and applications, and even to reserve bandwidth, perhaps for a scheduled videoconference. Finally, intelligent MUXs can allocate bandwidth on a dynamic basis, assigning channel capacity as required to meet the demands of traffic. For instance, a videoconference may require superrate capacity for a short period of time, multiple low-speed data communications may require subrate channels for a brief moment and, at other times, the entire capacity of the circuit may be in support of 32 Kbps voice conversations.

Nodal Multiplexers

Nodal MUXs, a further step up the MUX food chain, act as T-carrier network switches. In addition to serving as a traditional MUX for the resident site, they also serve as true networking devices, much as does a combined CO/Tandem switch in the voice world. Nodal MUXs provide the additional function of dynamic alternate routing (Figure 8.5), which allows them to switch traffic over an alternate path in the event of a condition of blockage or failure in the primary circuit.

Digital Access Cross-Connect Systems (DACS or DCCS)

These are nonblocking, electronic common control switches which serve to cross-connect digital carrier bit streams on a buffered basis by redirecting individual channels or frames from one circuit to another. Effectively, they provide an electronic common control means of cross-connection which replaces the traditional manual method of physical cross-connection of wires. A DAC can redirect traffic in order to better manage the capacity and performance of the T-carrier network [8-10]. Although originally developed for carrier use, DACS also are deployed in large user organizations to support private digital carrier networks. Smaller versions, residing on a PC, are available for less communications-intensive environments. Typically of significant port capacity, DACS provide support for DS-0, DS-1, and DS-3 [8-3].

Variations on the Theme: E-carrier and J-carrier

While the theme for digital carrier was set in the United States, the concept was quickly adopted by the CEPT (Committee on European Post and Telegraph). The resulting E-carrier standard is quite different in its implementation. The Japanese version, J-carrier, is similar to T-carrier, but with differences sufficient to cause incompatibility.

E-carrier is characterized by an entirely different digital hierarchy, (Table 8.2) beginning with E-1, at 2.048 Mbps. E-1 supports 30 clear information channels; 2 channels are set aside for signaling and control. This non-intrusive signaling and control convention, plus the fact that there are no 1s density rules which apply, results in E-carrier’s providing clear channel communications of a full 64 Kbps per channel. The E-carrier multiframe (16 Frames) corresponds to the T-carrier superframe (12-24 Frames).

J-carrier is quite similar to T-carrier, although the hierarchy is slightly different. Line coding and framing also vary from the U.S. ANSI approach. The advantages of these differences are questionable; incompatibility is assured, which fact should not be surprising. The J-carrier digital hierarchy begins at 1.544 Mbps, proceeding to 6.313 Mbps, 32.064 Mbps (J1), 97.728 Mbps (J3), and 397.20 Mbps [8-3] and [8-6].

Fractional T1

Fractional T1 (FT1), previously offered only in Canada, first was tariffed in the United States in 1987 by Cable and Wireless. Now offered by many LECs and IXCs, FT1 provides T1 functions and features, but involving fewer DS-0s. It is offered in fractions of T1 channel capacity, generally at 1, 2, 4, 6, 8 or 12 DS-0 channels. Subrate transmission also is available at speeds of 9.6 Kbps. FT1 is particularly applicable where remote locations are connected to a more significant location such as a regional office. At that point, they connect to a full T1 MUX or nodal processor, which aggregates the traffic with that of the larger site over a full T1/T3 backbone network. FT1 also serves videoconferencing, data communications, and other applications that require more than 56/64 Kbps, but less than a full T1 [8-4].

T-carrier Applications

The applications for Digital Carrier are many. Large user organizations find Digital Carrier services to be highly cost-effective for local loop access, typically replacing multiple, individual PBX trunks. Large corporations find T-carrier effective for private, leased-line networks or access to virtual private networks (VPNs). The ability of T-carrier to accommodate voice, facsimile, data, video, and image information on an unbiased basis and, therefore, to eliminate or reduce the number and variety of specialized circuits, offers great advantage.

Internet service providers commonly use T-carrier for access to the LEC CO, as well as access to an Internet backbone provider. Given current US tariffs, a T1 clearly becomes cost-effective in replacement of 8-10 or more individual circuits; a T3 becomes cost-effective at a level of 3-4 T1s. Notably, the relationship is far different in Europe and parts of Asia, where leased line costs are considerably higher.

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Quantizing variations are sometimes employed, although they are neither generally accepted nor widely deployed. Those variations include the following [8-5]:

• Continuously Variable Slope Delta (CVSD): Compression Ratio = 4:1 (16 Kbps), or 8:1 (9.6 Kbps)
• Vector Quantizing Code (VQC): Compression Ratio = 4:1 (16 Kbps)
• High Capacity Voice (HCV): Compression Ratio = 8:1 (8 Kbps)

Framing

T-carrier employs a very specific set of conventions to transmit information, with framing being one example. Using T1 as an illustration, each channel of input is time division multiplexed into a T1 frame, or set of data. In other words, conversation 1 might be allocated time slot 1 (channel 1), conversation 2 might be allocated time slot 2 (channel 2), and so on through conversation 24 and channel 24. That set of sampled data is inserted into frame 1, which is appended by a framing bit in order to distinguish it from subsequent frames of data. The process is then repeated for frame 2, and so on.

The combined processes of voice encoding and framing yield a total T1 bandwidth of 1.544 Mbps. Of that total, 1.536 Mbps is available for information transfer, as noted in the following calculation; the remaining 8 Kbps is required for framing.

4,000 Hz x 2
8,000 Samples/Second
x 8 Bits per Sample
64 Kbps = DS-0
x24 Channels
1.536 Mbps
+ 8 Kbps Framing
1.544 Mbps = DS-1 (T1)

There exist several generations of framing conventions, designated as D1, D2, D3, D4, and Extended SuperFrame (ESF). Additionally, the ITU-T has developed an international set of recommendations for framing digital carrier signals.

D1 Framing

Developed in 1962, D1 framing robbed the Least Significant Bit (LSB), the eighth bit, in order to insert a signaling bit in each channel of each frame; signaling bits were alternating 1s and 0s. The quality of digitized voice conversation was not affected, as 7 bits are satisfactory for reconstructing a high-quality approximation of the analog voice input. Data, however, is seriously impacted by truncating an 8-bit value. As a result, data transmission was limited to 56 Kbps. Data remains limited to 56 Kbps in many carrier networks due to similar limitations imposed in D4 framing. While D1 framing no longer is used, the least significant bit is still robbed, even in the contemporary D4 framing technique.

D2 Framing

D2 Framing was used to create a superframe, a 12-bit pattern for framing locators. Information was transmitted in a 12-frame sequence or superframe.

D3 Framing

D3 Framing, which is still in use, assumes that all inputs are analog, whether they are voice or data. It uses a superframe format and sequence bits.

D4 Framing (M24 Superframe)

D4 Framing (M24 Superframe) uses a repeating 12-bit sequence (1000 1101 1100), repeated every 12 frames, to allow robbing of the least significant bits of the sixth and twelfth frames only. Voice and data are accommodated, with data being treated as a digital input. While this improves available signal capacity and yields better voice transmission, data is still limited to 56 Kbps. Additionally, one’s density must be maintained through the insertion of stuff bits. Considered together, the twelve frames are designated a superframe.

Extended Superframe (ESF)

Extended Superframe (ESF), originally tariffed by AT&T in 1985, is now widely available. ESF superframes are 24 frames in length; signaling is accomplished in frames 6, 12, 18, and 24. The bit positions liberated by ESF are used for error checking and network management. ESF offers the advantages of nondisruptive error detection (6-bit CRC) and network management, using only 8 Kbps of overhead [8-2], [8-3], and [8-4].

ITU-T Framing Conventions

ITU-T Framing Conventions are quite different from those described above, which are used in North America and Japan (modified). ITU conventions call for Level 1 (E1) to employ 32 DS-0 channels, 30 for information and two specifically designated for signaling and control. The functional equivalent of framing bits are carried in the first such DS-0 channel, while the second carries signaling bits [8-5].

Transmission

Transmission facilities can include unshielded twisted-pair (22 or 24 gauge), shielded copper, coaxial cable, microwave, satellite, infrared or fiber optic cable. Therefore, Digital Carrier is medium-independent. A T-carrier circuit is also know as a T-span or T1 pipe. Regenerative repeaters are used to reshape and boost the signal at regular intervals, every 6,000 ft. in a twisted-pair system. The repeaters are powered by the CO exchange at levels up to 100 volts. The repeaters maintain their synchronization through the transmission bit stream. Therefore bipolar transmission is critical, as is the 1′s density rule.

Hardware

DS-1 equipment is required both for end-user organizations and for carriers. That equipment must be of the same generation in order to effect compatibility. Ideally, within the user organization, it should be of the same origin and software generic as is the carrier side in order to deliver the same functionality and feature set.

Channel Banks

Designed for voice-only service in analog applications, channel banks are used for interfacing analog switches (PBXs and COs) to DS-1 circuits. Channel banks perform two functions in sequence. First, they multiplex up to 24 (T1) analog signals on a common Pulse Amplitude Modulation (PAM) electrical bus. Second, they encode the individual PAM channels into a digital format, using PCM, for transmission over a DS-1 circuit [8-3] and [8-4].

Channel banks also will accommodate digital data. As relatively unintelligent devices, channel banks place each conversation on a separate channel; for instance, a 9.6 Kbps data conversation occupies a 64 Kbps channel, just as does a 56 Kbps data transmission or a digitized voice conversation. Therefore, channel banks do not make efficient use of available bandwidth. Combined channel banks and CSUs often are in the form of printed circuit boards which fit into PBX card slots for seamless interface to a network T1 circuit.

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