Rolling in the copper

Transport systems that send and receive Ethernet packets over copper wires at speeds up to 70 Mbps. The systems perform at quality levels comparable to fiber (10^–12 BER). Based on IEEE 802.3ah, the systems let telecom carriers and businesses boost data rates of existing copper wires.

Perform physical layer tests and interoperability tests over copper wires at lengths up to 122,000 ft with cable segments ranging from 500 ft to 10,000 ft. Simulate conditions that occur in the field by adding noise and other forms of transport signals over the same wires. Simulate interruptions in splices and with cut lines. Vary temperature to measure its effects.

• Agilent Technologies: oscilloscope. www.tm.agilent.com.
• gnubi Communications (now EXFO): multichannel test system. www.exfo.com.
• Spirent Communications: network performance analysis system, telephone line simulator, and call generator. www.spirentcom.com.
• Network equipment from Cisco Systems, Hewlett-Packard, Paradyne, and others.

Ethernet in the first mile (EFM) improves bandwidth over DSL in existing copper wires. Actelis Networks (www.actelis.com) develops EFM transport systems at its facility near Tel Aviv, Israel. To test designs for physical parameters and interoperability, the company's engineers use a test bed (see figure) containing several network elements and a "cable farm" full of copper-wire rolls that simulate field conditions.

The rolls, or "binders," consist of either 25 or 100 pairs of wires. With its line simulator, the test bed connects 25-pair binders to reach 122,000 ft while a shorter 100-pair binder can reach 30,500 ft. Engineers reconfigure the binders to simulate numerous configurations in accordance with Bellcore standards.

The line simulator also adds controlled attenuation and noise to the wire pairs, but it doesn't simulate near-end crosstalk (NEXT). Therefore, the engineers add 3000 ft of cable on either side of the line simulator. To create more interference, they use network equipment that adds DSL signals. A call generator adds analog telephone signals.

The engineers run physical-layer tests on transmitted signals and verify that a DUT can communicate with network equipment. "Depending on the test, the bed may consist of a 'golden' transport system as a transmitter or receiver or it may test two systems at once," says Ilan Rheingold, senior director of product management. In a BER test, for example, the network-performance analyzer generates PRBS signals within Ethernet packets and measures BER on the traffic sent to the receiver. In this case, the receiver is a golden unit and the transmitter is the DUT.

Each transport system must transmit its signals within a defined frequency spectrum. The network analyzer, connected at a transmitter, measures power spectral density and compares it against standard masks. Here, too, the DUT is the transmitter.

In a receiver test, the DUT must correctly interpret traffic in the presence of noise and other signals on the line. Using a golden unit as a transmitter, the Actelis engineers add interference to the transmission line. They measure BER and also test a receiver's ability to compensate for crosstalk in the cables.

Because cables must withstand temperature changes, a portion of the cable farm is under temperature control. Aerial cables are also vulnerable to wind and rain that can cause them to move, introducing short traffic interruptions. The test bed simulates those conditions, thus testing a DUT's recovery ability.

The engineers also perform interoperability tests with the test bed. They measure quality of service (QoS), queuing and priorities, and lost Ethernet packets. With an asynchronous transfer mode (ATM) generator, they add cells to the transmission and test a transport system's ability to distinguish Ethernet packets from ATM cells. The tests include protocols, security, user control, alarms, and traps.

In the past, Actelis engineers performed manual tests on their multipair copper transmission systems. Now, the automatic test bed helps them perform full regression tests in 1 to 2 days compared to 2 to 3 weeks. The temperature tests and the ability to inflict micro interruptions help the company's products pass customer acceptance tests, which the company believes gives it a competitive advantage.