nvestment from telecommunications companies (telcos) in advanced digital links and enabling equipment, and international standards are replacing short-term proprietary solutions.
Two Masters
Telephone networks and LANs have traditionally served two different masters. To accommodate voice transmissions, a phone system is connection-oriented; when you place a phone call, you establish a dedicated end-to-end connection before the transmission proceeds. Once the system allocates bandwidth, the resource remains availab
le for the duration of the transmission. The two main components that determine the bandwidth, robustness, and flexibility of the telecommunications network are the broadband transport technologies (actually, the physical piping and the interfaces to it) and the protocols that define packet transmission across the wire.
In contrast to telephone networks, LANs are designed to handle the bursty (i.e., variable-rate) nature of data transmissions. Arbitration schemes ensure the integrity of data while tolerating delays for error correction and retransmissions.
These differences are the key stumbling blocks to integrated digital voice/data networks. For LANs to support voice transmissions, they need technologies that can stream voice communications across a network along an uninterrupted path. These technologies ensure that a network prioritizes data and recognizes voice data and that the voice transmission receives enough dedicated bandwidth for real-time communications. Thus, the physical link and th
e transport mechanism must be able to differentiate among different multimedia types and support a wide range of transmission requirements.
Traditional voice networks, on the other hand, must support standardized high-bandwidth interfaces to the emerging fiber infrastructure as well as modern transport protocols that can handle the large transfer of variable-rate data. Voice networks must also differentiate among different multimedia types and share bandwidth that was once dedicated to voice.
As it turns out, the requirements of voice and data transmissions actually complement each other, as long as the protocols properly handle the different media types. For example, sophisticated voice/data multiplexers can combine voice and data streams while granting priority to real-time voice transmission and dynamically allocating dedicated bandwidth to it.
For variable-rate transfer, a multiplexer can send data packets during lulls in a conversation. Variable-rate data, with its tolerance for interru
ptions, can consume open bandwidth when available and surrender bandwidth when necessary. Some companies are integrating voice/data multiplexers with dedicated T1 lines and Internet subnetworks (see the figure
"A Hybrid Solution for Voice/Data Networking"
).
Technologies for the Internet and the public-switched telephone network also look promising. Some of the best examples are the new audio technologies (for streaming real-time audio across Internet connections) that make Web phones possible (see the sidebar "Yet Another Web Phenomenon").
Digital Solutions
The above examples represent ways to combine voice and data using today's infrastructure and technologies. However, the public-switched telephone network continues to evolve. Telcos and others are improving the network with standards for fast packet switching and for interfacing to fiber-optic cable. New cell-relay transport protocols, such as asynchronous transfer mode (ATM), not only support higher tran
smission speeds but also include mechanisms for differentiating multimedia traffic, including voice, data, and video.
While fiber-optic is the high-speed link of choice, it's also helping to stall the development of integrated digital networks. High-speed fiber requires electrical-to-optical signal conversion. Asynchronous transmission services, such as T3, define a standard electrical interface but do not support a standard optical interface. The result is a number of proprietary interfaces to optical cables and little interoperability.
The answer to the confusing array of nonstandard asynchronous interfaces appears to be SONET. This standard specifies an electrical and an optical interface, enabling an interoperable delivery vehicle for broadband services.
The SONET standard is now in the third phase of its three-phase release. Earlier this year, major SONET vendors, such as Alcatel and Fujitsu, announced the third-party corroboration of interoperable SONET products from different manufact
urers. Within the next few years, SONET should finally drive the development of an interoperable, all-digital multimedia infrastructure.
ISDN provides a fast voice/data pathway to the home or office. ISDN standards currently provide for high- and medium-speed digital links. Basic rate service, also called 2B+D or Basic Rate Interface (BRI), provides two 64-Kbps channels for voice or data, plus one slower, 16-Kbps data channel. Primary rate service, also called 23B+D or Primary Rate Interface (PRI), carries 23 64-Kbps channels and one 16-Kbps channel. It's no coincidence that the number of channels and data rates for these ISDN standards is close to the number required for T1; these standards are designed to be interfaced to T1 equipment at the central office.
But deployment of ISDN has been slow everywhere except in urban areas, and business data connections over ISDN are not "flat-rate," as are leased-line and Internet connections. This has caused many once-eager customers to receive unexpectedly
high bills.
A Pickle over Packets
So-called fast-packet technologies, such as frame relay and cell relay, might offer the best solution for integrated voice/data networks. These fast-packet technologies are an outgrowth of the X.25 standard, which addresses how data is sent in discrete packets across private and public networks.
Real-time isochronous applications work best with smaller packets that reduce bottlenecks and avoid interruptions in the transmission stream. LAN transports have traditionally supported larger packets to deliver big chunks of data across a network. When X.25 was standardized in 1976, it was a watershed development, establishing a common packet-switching protocol for broadband networks. But in these heady days of multimedia convergence, X.25 transmission speeds just don't cut it.
On the other hand, frame relay requires little overhead because it leaves error recovery up to network-level protocol suites, such as TCP/IP and IPX/SPX. Frame relay can t
ransport variable-size packets at up-to-T3 transmission speeds. However, frame relay was not designed to carry voice traffic. It discards frames in the event of congestion, it's not isochronous, and it can have long and sometimes unpredictable latencies (i.e., it can be a long time before data sent into a network emerges at the other end).
Equipment that tries to route voice traffic over frame-relay connections must play tricks in an attempt to ensure timely delivery of digitized voice packets. Also, because frame-relay bandwidth is far less expensive than dedicated leased T1 lines or ordinary voice-phone service, long-distance carriers aren't eager to support a technology that could cost them revenue. So, while it's possible to buy voice equipment for frame relay, it's best used only with relatively uncongested systems that can provide a committed information rate -- that is, networks with a guaranteed minimum throughput.
The ATM Answer
Cell-relay technologies, such as ATM, out
pace frame relay and also provide a more efficient switching mechanism. Instead of sending data in variable-size packets, cell relay uses small, fixed-length (53-byte) cells to transport information. Data packets are transmitted asynchronously, but the line of cells that carries the packets flows across the network synchronously as a continuous stream. The data packets enter the stream as they become available, dropping into the first available cell. These small packets support high-speed, low-latency transport.
Unlike synchronous transfer, which preallocates bandwidth channels, ATM transports data packets whenever bandwidth is required. ATM's flexible use of bandwidth supports the continuous uninterrupted transfer of voice and video data as well as data traffic's variable bit rates. It's also data-rate-independent, supporting public networks and LAN switching alike at rates exceeding a gigabyte per second.
ATM comprises three layers (see the figure
"The Layers of ATM"
). You c
an run ATM to the desktop without bridging to another networking technology. Unfortunately, standards for voice and video transmission over ATM are still in the formative stage, and ATM equipment is relatively expensive.
The All-Digital Future
Enterprises integrating voice and data networks currently face an almost-bewildering array of choices. Many companies have bought into the obvious benefits of an integrated network, but high costs and immature standards are major deterrents.
Until prices drop and standards evolve for a variety of new digital technologies, the only sure bets are the tried-and-true voice/data technologies developed by the Bell System several decades ago -- combined with new equipment designed to make the best use of them.
Where to Find
Vendors of Web Phones
=====================================
NetSpeak
P
hone: (305) 251-4653
Internet:
http://www.itelco.com
Data's New Voice
