Since it is slow today let us have another lesson in one of the many facets involved in internet telephony. Todays lecture has to do with the importance of ATM.
ATM Comes of Age
The public switched telephone network (PSTN) today is very much a hierarchical system. Over nearly a century, it has evolved with a single purpose in mind-to connect millions of analog telephone subscribers together at an acceptable level of service. In this context, it was obvious that digital technology would be applied principally to enhance this role rather than to change the fundamental architecture of the network.
Thus, once they enter the network, analog voice signals are modulated as digital bit streams, and are transported and switched using time division multiplexing (TDM). Although this works well for voice and voiceband data, it has many drawbacks when it is used to transport high speed computer signals.
The reasons for this are self-evident:
The basic digital voice channel is 64 kbps. This is combined with others and is transmitted over high-speed trunks.
Circuit-switched networks use dialed numbers to connect subscribers together. These connections exist during the duration of the call, whether information is present or not.
The network operates principally at the physical layer, meaning that does not adapt well to protocol-intensive data signals.
Only in the area of signaling has the digitization of the network had significant impact on data transport. Signaling System 7 (SS7) is an internationally accepted common channel signaling system designed to speed up call processing and provide a platform for intelligent network services.
Adapting the PSTN to be a more scalable, multiservice, wide area network (WAN) has proved to be a difficult proposition. In the 1980s, a global initiative to transform the analog network into an Integrated Services Digital Network (ISDN) encountered many problems, not the least of which were switching equipment incompatibilities. During the 10 years it took to resolve these issues, the backbone network improved to such an extent that many of ISDN's benefits were realized using business-oriented, packet based solutions such as frame relay and cell relay.
Public and private networks transport data in several ways-as TDM bit streams, as packets, as frames, or as cells. Asynchronous transfer mode (ATM) is the culmination of this long and expensive process.
CELLS
ATM first came to the attention of the industry in the late 1980s as part of the International Telecommunication Union's (ITU's) Broadband-ISDN (B-ISDN) project. By 1989 the United States and Europe had agreed on a 53-byte (octet) ATM cell consisting of 5 octets of processing data (overhead) and 48 bytes of customer data (payload). Because ATM provides a common solution for local and long distance data networking, it has hastened the convergence of computers and communications.
ATM is a significant improvement over other packet technologies, for several reasons:
Different kinds of information can be transported;
The cell is fixed length and readily processed;
ATM adaptation schemes support circuit- and frame-based information flows; and
ATM scalability allows networks to grow as traffic demands require.
The advantages to the major public carriers were immediately obvious. For the first time, they had a technology that could be applied to building a seamless data network using internationally accepted protocols. This advantage translated into products that worked together using high-level protocols. Also, fast ATM cells were designed to be transported over the installed Synchronous Optical Network/Synchronous Digital Hierarchy (Sonet/SDH) infrastructure.
ATM SWITCHING
A major network element in an ATM backbone network is the ATM network switch. The ATM switch is the workhorse of the network, providing important network functions, including:
Bandwidth management; Optimized connection management; Multiservice support; And more.
FORE Systems' TNX network switches are good examples of this class of product. Designed to work in harmony with a line of multiservice concentrators, TNX products offer network operators high performance ATM switching for the core of their data and multiservice backbone networks.
Using ATM access servers, concentrators and switches, a public carrier or an Internet service provider (ISP) can build a scalable, high performance switched ATM infrastructure. Such a network architecture includes the following equipment:
Remote Access Server (RAS)-provides high density fan-in for dial-up users such as telecommuters and business travelers. The RAS equipment terminates the point-to-point protocol concentrating the data traffic onto 10/100 Ethernet or ATM network connections. RAS devices supporting ATM links can connect directly to the core ATM switch. Alternatively, an Ethernet LAN switch can be used to route traffic from a bank of RAS equipment on to ATM OC-3c connections to the ATM core.
Multiservice Concentrator (MSC)-a relatively new network element that supports multiple access services on one chassis. It provides high-density consolidation of Frame Relay, PPP, ML-PPP and ATM services on to high-capacity ATM uplinks. Traditionally, these services terminated directly on the router and/or frame relay switch with a high capital investment per port. The MSC is optimized for high-density service concentration greatly reducing the equipment cost of network access.
Network Switch (NS)-located at the core of the Internet Service Provider Network Access Point (ISP NAP). With this architecture, the ISP can construct a network that supports a logically meshed, single-hop backbone architecture, improving network performance and flexibility. The NS provides the switched connections between the RAS, the MSC, the router(s) and the Authentication Server, as well as the switched connection to the long-haul transmission network via DS3, OC-3 and OC-12 connections. In addition, the TNX-1100 can support any ATM-based Web hosting servers the ISP might deploy. Thus providers can offer multiple services over a common ATM core infrastructure. The MSC can be used to support ATM, Frame Relay, fractional T1/E1 and Internet access services to the customer locations. It concentrates these on OC-3c connections to the edge or core ATM switch, such as the TNX-110, which connects them to the appropriate service network and service providers. In this way, Internet traffic could be groomed onto ATM links that connect the local provider to an ISP. Voice traffic could be groomed onto ATM or TDM links connecting to the public switched network. Similar grooming and handoffs also could be performed for Frame Relay and ATM traffic.
SONET AND ATM
Public carriers realized early on that, to garner the maximum benefits from ATM, they must plan its deployment in harmony with the other emerging technologies. The basic building block of the backbone network today is Sonet, a transport technology that in recent years has also penetrated the access network. The power of a fiber-transported, synchronous network can be enhanced with ATM.
The cell-packaged customer payload is immediately accessible through the use of Add/Drop Multiplexers (ADMs). Because these process TDM bit streams, Sonet ADMs use overhead, require distributed maintenance, and are switched in the same manner as today's T-carrier channels. Because the ATM approach is cellular, synchronization is required only between the ports of the Send and Receive switches. Thus, traffic handled by an ATM ADM puts no additional load on the switching fabric.
The main priority of the carrier is keep network upgrade costs to a minimum. With ATM, the ADMs, switches and digital cross-connects (DCCs) are linked via existing Sonet-rate, point-to-point links. The format can be pure ATM per ITU recommendation I.432, the required overhead being absorbed by the OAM cells. Each ATM cell receives its timing from the transmit port data stream. Voice and video applications require synchronous residual time stamps for this synchronization.
In this environment, switches such as the TNX-1100 play a crucial role. They allow the carrier to connect the Sonet/SDH infrastructure that most major providers have in place to the local network that carries ATM customer data. These switches connect the backbone transport network using a variety of WAN ports at bit rates that include DS3, E3, OC-3 and OC-12.
PUBLIC ATM
During its early development, ATM was viewed as a public network backbone technology. A major issue for the regional Bell operating companies (RBOCs) was that, under the terms of the Modified Final Judgment (MFJ), they were unable to provide long distance service. Thus, public WAN coverage by each Bell company was determined by the Local Access Transport Area (LATA). Despite this restriction, several local exchange carriers instituted public ATM services on a limited geographical basis. For example:
ú Pacific Bell deployed public ATM service in a number of California cities in 1993. Initially, options were limited to permanent virtual circuits (PVCs) delivered at 45 Mbps over electrical and optical networks. A standardized user interface was an obvious priority and, by 1996, ATM UNIs enabled users to connect to the network at DS1 speed. Limited interLATA service over SVC connections was also provided.
ú In 1992, BellSouth pioneered the North Carolina Information Highway (NCIH). This is a Sonet-based broadband WAN that uses both ATM, OC-3 access lines, and core Sonet switching. The North Carolina state government became the anchor tenant for this network during its early deployment. This resulted in a heavy emphasis on applications involving medicine and education. Switching computer data and video simultaneously was a major focus of the NCIH network architects.
Public ATM services are available from many providers. In 1994, variable and constant bit rate (CBR) permanent virtual circuits (PVCs) used DS3 access. A year later, these capabilities were provided over DS1 and OC-3 facilities. Network management information became available to end users. SVC service arrived in 1996 together with frame relay interworking and circuit emulation. Multi-T1 access interfaces followed using ATM inverse multiplexing. Priority levels per virtual connection and adaptive bit rate (ABR) services probably will make their entre next year.
ENABLING TECHNOLOGY
Over the past three years, ATM has ceased to be just another set of initials and has become a multibillion-dollar industry. Why is this? The simple answer is that ATM enables both carriers and end users to integrate their networks and expand them into the backbone network using common protocols and interfaces.
Network management is inherent in ATM networks. Users can run multimedia traffic in a campus environment and carriers can transport this traffic across long distance public and private message-switched networks.
The cost savings of this network integration are only just becoming apparent. As we become an even more information-intensive society, the demands placed on our telecommunications infrastructure will be intolerable without the support of a standardized data transport mechanism. Over the years, the circuit-switched public network will be replaced by a broadband, message-oriented communications infrastructure. The long term promise of ATM lay in its role of expediting this transformation.
The original role of the public ATM WAN was to connect ATM sites together over a virtual private network (VPN). However, as we have seen, most businesses utilize multi-protocol, multi-network telecommunications environments in which they attempt to consolidate both traffic and traffic management. Because of this, subscribers would like to integrate sites that network in circuit, packet, frame and cell. Over the coming months, public telecommunications carriers will build a backbone that can connect, transport and manage these services.
The 53-octet ATM cell is alive and well-and is ignored by information industry at its peril. Tomorrow's multimedia, managed, self-healing network cannot be realized without ATM and Sonet. All segments of our industry are crafting an ATM network for the next century.
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