Carrier Ethernet is emerging as a mobile backhaul technology, but it must overcome timing and synchronization issues

Today's Wireless Carriers have a big problem, but fortunately for them, it's a good problem to have. Thanks to the proliferation of smartphones and PDAs combined with unlimited data plans, data usage has skyrocketed.

The challenge now is for wireless carriers to backhaul all of this data traffic on networks that were designed for voice. Current wireless networks backhaul traffic from their base stations to mobile switching centers via leased T-1 lines from wireline carriers, but new IP services are poorly matched with circuit-switched infrastructure.

“As we start to grow traffic and change the mix between data and voice, we have to reexamine traditional backhaul techniques,” said John Celentano, president of Skyline Marketing Group.

Some equipment vendors are promoting carrier Ethernet, or metro Ethernet, as the ideal way to backhaul today's data-heavy wireless traffic, citing higher speeds and better rate-reach than traditional T-1 lines, all for a lower cost per bit.

But what they don't share in their marketing materials are those words that give network engineers heartburn: latency, jitter, crosstalk and dropped calls — all of which can occur using carrier Ethernet approaches without the right technology and engineering.

HYBRID NETWORKS VERSUS PSEUDOWIRE

Vendors are approaching carrier Ethernet for backhaul in two distinct ways. The first is a hybrid design that combines a carrier's current T-1s for TDM voice traffic while integrating carrier Ethernet for data traffic.

“In this scenario, the equipment does not packetize the voice signal at all, but rather leaves it in ‘native TDM’ form inside the packet channel and delivers it as a TDM voice call, so the callers do not notice any difference in quality,” Celentano said.

Glen Hunt, principal analyst for carrier infrastructure for Current Analysis, noted that this hybrid design is especially appealing to North American carriers who have already invested heavily in T-1s.

“In Europe, the equivalent of a T-1 costs significantly more than in North America, so there is financial motivation to migrate more quickly to an all-Ethernet or all-IP infrastructure in other markets than in North America.”

A second option, known as pseudowire or circuit emulation, transmits both voice and data together via packets. Critics of this approach say the voice packets don't always arrive in the same order in which they were sent, and voice quality suffers as a result.

“In this situation, clocking, timing and synchronization are fundamental to call quality because the base stations use a frequency reference to set their channel separation over the air,” said Dr. Alan Solheim, vice president of product management for DragonWave, a backhaul equipment vendor. “If they don't have a good frequency reference, then the channels start to wander and merge into each other, which causes crosstalk, interference and dropped calls.”

CARRIER ETHERNET-OVER-COPPER

With fiber unavailable to most cell sites, some vendors advocate delivering carrier Ethernet over existing copper. Gary Bolton, vice president of marketing for Hatteras Networks, noted that 90% of cell towers in the U.S. are copper-fed. Using IEEE Internet 2BasE-TL, or mid-band Ethernet, Hatteras delivers 5.7 Mb/s over each pair; two pair deliver 11.4 Mb/s, while eight pair potentially deliver 45 Mb/s.

“By delivering Ethernet-over-copper, we get two times more bandwidth than what is available over T-1s over that existing copper pair,” Bolton said.

Aktino's broadband-over-copper solutions also allow service providers to reuse existing pairs of copper with its multiple input/multiple (MIMO) output on discrete multitone (DMT) platforms, said Hossam Salib, co-founder and senior vice president of marketing for Aktino. He pointed out that while DS3 and Ethernet services are traditionally symmetrical, there are still many applications in which the downstream bandwidth is entirely consumed, while much of the upstream bandwidth goes unused.

“A prime example is IP- or ATM-based DSLAM backhaul because residential and business DSL services are typically asymmetric,” Salib said. “Cell site backhaul and residential and business video transmission are other key applications in which the ability to enable asymmetric bandwidth will be far more useful to service providers.”

The asymmetric functionality allows Aktino to double the downstream bandwidth per copper pair beyond the symmetric MIMO on DMT bandwidth, resulting in nearly 7 Mb/s per pair to the edge of the carrier serving area, Salib said.

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