“As customers increasingly rely on virtual private networks, using T-1 as an access technology for high-speed packet-based networks, centralized network management will become increasingly critical.”

T-1 reinvents itself

Packet-switched technologies have stolen the limelight as the sexiest new way to provide data access. But don't think for a moment that they're stealing the show. Demand for T-1 (1.544 Mb/s) lines continues to grow. Newer services such as asynchronous transfer mode and the Internet often require T-1 lines as an access vehicle. And methods for using T-1 bandwidth more efficiently are creating opportunities for carriers.

These opportunities are based on new developments in T-1 hardware. Deploying this new access hardware can help carriers retain existing accounts and attract new ones by offering customers a better price/performance ratio. For telcos that manage large customer T-1 networks, the new T-1 access equipment can provide greater throughput without adding costly infrastructure. Additionally, wherever demand for T-3 (45 Mb/s) exceeds the ability to provide this service, carriers can use advanced access equipment with existing T-1 service to provide high-bandwidth customers with much greater throughput.

Advances in multiplexing, voice compression and grooming platforms for higher speed services are the key technologies supporting greater T-1 bandwidth usage.

One firm using T-1 as an access technology - in combination with advanced multiplexing techniques - is Oceanic Communications, a Time Warner Cable company headquartered in Mililani, Hawaii. Oceanic is incorporating its newly automated pay-per-view movie ordering system into the islands' Sonet ring (Figure 1).

The movie-ordering system will cover some 280,000 cable-connected homes linked to data processing hubs. Multiplexers located on the islands receive ordering data from the hubs over Oceanic's private Sonet network at the system's central computer. The orders are processed and programs are delivered to the customer. The multiplexers integrate data, voice, fax and LAN traffic over T-1 on copper and fiber optic links, providing fail-safe operation with complete redundancy of common logic, power supplies and main links.

This Sonet-connected Oceanic project typifies the ways in which T-1 access technology is permitting carriers to build cost-effective private networks around public networks, answering complex communications needs and linking to higher bandwidth services using existing infrastructure.

Sonet networks have a limited number of access ports for T-1. As customers require more access lines, digital cross-connect system switches can be used as grooming devices to enable private networks to use Sonet's high-speed transmission rates without installing additional infrastructure.

Advanced DCSs provide programmable, non-blocking cross-connects between 64 kb/s timeslots, and they support timeslot switching and T-1/E-1 conversion among multiple ports. T-1/E-1 conversion is increasingly critical to international network expansion.

Another route to accessing greater throughput while using T-1 bandwidth is inverse multiplexing (Figure 2). Inverse multiplexers split the high-speed traffic of T-3 and sub T-3 among two or more lower speed links. Another inverse multiplexer reconstructs the original signal at the remote site. This enables networks to use routers at faster rates, without incurring the costs of long-haul T-3 links.

As customers increasingly rely on virtual private networks, using T-1 as an access technology for high-speed packet-based networks, centralized network management will become increasingly critical.

A single management system for all network elements eliminates the need to learn new systems and train new technicians each time another box is added to the network. Simple network management protocol, the standardized network management protocol of the TCP/IP suite, allows carriers and network managers to monitor, troubleshoot and configure devices in their network from a single central management station.

One line, many voices Progress in multiplexing has been matched by similar advancements in voice compression.

T-1 was traditionally designed to provide 24 voice channels. However, advancements in voice compression technology have radically increased the throughput for both voice and data over a single T-1 link. Common voice transmission standards include pulse code modulation (PCM) at 64 kb/s, adaptive differential PCM (ADPCM) at 32 kb/s, low delay code excited linear prediction at 16 kb/s, and adaptive code excited linear prediction at 8 kb/s.

A powerful new voice compression algorithm, multipulse maximum likelihood quantization (MPMLQ), offers toll-quality voice at rates as low as 4.8 kb/s. Based on linear predictive coding, MPMLQ performs analysis-by-synthesis, using only 6.4 kb/s to generate toll-quality compressed voice comparable to ADPCM at 32 kb/s. This permits compression of up to 10 voice calls onto a single standard 64 kb/s voice connection.

Extensive tests have established the voice quality attainable with MPMLQ. Tests performed at Lucent Technologies' Bell Labs and France Telecom have shown that after two tandem encodings, the perceived sound quality of the MPMLQ signal at 6.4 kb/s - measured by its mean opinion score - is nearly equivalent to the rating for 32 kb/s G.726 ADPCM.

Implementing MPMLQ has resulted in transmitting up to 144 voice channels over a single T-1 link or compressing 30 voice circuits into five timeslots, freeing the remaining timeslots for use in local area network, Systems Network Architecture and video traffic. Additionally, a single T-1 link now can transmit two full PBX lines and 1 Mb/s of data. This is especially important for carriers that fully manage customer networks because it enables the existing infrastructure to provide maximum bandwidth.

Advancements also have been made that offer more flexibility in terms of the media over which T-1 can be delivered.

Some new systems permit T-1 users to take full advantage of the fiber's benefits. Multiplexers combine multiple T-1 lines over a single fiber optic link, allowing additional T-1 links to be deployed without the cost of deploying more fiber.

New fiber optic modems extend the range of T-1 signals over multimode or single-mode fiber up to 40 miles, enabling customers to gain the advantages of deploying fiber. These modems are integrated into T-1 multiplexers, eliminating the need for external equipment.

To serve customers with existing copper infrastructure, high-bit-rate digital subscriber line (HDSL) technology further extends the value of T-1 by connecting users via copper twisted pair to a central office three miles away, without the need for a repeater.

Companies have developed new HDSL termination units to ensure reliable transmission over noisy lines, electronically compensating for line impairment, bridge taps and mixed cabling with techniques such as equalization, adaptive filtering and echo cancellation.

Statistics compiled by Dataquest indicate that the global T-1/E-1 market is growing rapidly. Dataquest estimates that the number of local exchange carrier-installed T-1 lines in the U.S. alone has grown from 850,000 in 1994 to a projected 1.98 million in 1998 (Figure 3). The Internet and public frame relay/ATM access segments of the T-1 market are expanding at annual rates of 40%. This growth presents carriers with an opportunity to build long-term relationships with an expanding customer base increasingly in need of more bandwidth.

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