Cell Backhaul Preps for Ethernet: But Old Coexists with New

Carrier Ethernet continues to be a lively topic. At Cable-Tec Expo this year, several workshops and booth exhibits placed Ethernet within the context of the evolving yet booming cellular backhaul market.

Evolving in the sense that well-established SONET and TDM technologies are coexisting with Ethernet transmission. Booming as backhaul is linked to the growth of smart phones and broadband wireless usage. Over the next five years, according to independent analyst Victor Blake, the number of required cells will increase by a minimum of four times to a maximum of fifty times, with typical increases between four and eight.

Wireless operators undergoing a technological evolution to 3G and 4G service, where multimedia applications could consume upwards of 100 Mbps of bandwidth, face competitive pressure that has kept monthly consumer bills relatively low. Thus: the now-famous delta between bandwidth and revenue growth.

"The average traffic per cellular customer is now rising markedly faster than the average revenue per user," Steve Dyck, director mobile backhaul solutions, Alcatel-Lucent, explained in his Expo paper. (For paper titles, see sidebar.)

At the same time, 3G and 4G require more capacity at base stations, where T-1 lines traditionally used for backhaul cost between $250 – $1000 to lease (the variance due mainly to competition and availability). Adding enough to meet demand would be cost prohibitive, according to Nagesh Nandiraju, systems engineering, access networks, Motorola.

Bottom line? The search for lower-cost, packet-based solutions is real; MSOs are positioned to leverage their existing infrastructure to service cellular towers; and while Ethernet is an answer, the need for efficient and secure monitoring and testing solutions is extending the life of legacy technologies.

While Ethernet (both with and without SONET) is part of Time Warner Cable’s Cell Tower Backhaul (CTBH) specification, the foundational technology for these deployments remains legacy-based. Time Warner VP Transport Network Engineering Tom Staniec was characteristically direct: "We have decided to go with SONET."

Fetch some fiber

Between 80 and 90 percent of U.S. cell sites are within cable’s HFC footprint, and 20 to 25 percent of cell sites are close to MSO fiber lines, Dyck wrote. Cable operators continue to build fiber rings and backbones and extend fiber deeper in the network to create smaller service groups. It wouldn’t be a stretch to extend fiber from this base and reach 60 to 80 percent of all cell sites, Dyck added.

A number of technologies help make this fiber more efficient for backhaul, business and residential services. Coarse wave division multiplexing (CWDM) or dense wave division multiplexing (DWDM) utilize either eight or 40 wavelengths on a single fiber by spacing them at 20nm or 0.8nm, respectively. (Time Warner’s CTBH document calls for deploying against the MSO’s Metro DWDM spec.)

"Cell backhaul will be solved with lots of technologies. There is no one that will dominate." David Foote, Hitachi

The separate wavelengths could serve different mobile operators at the same tower, Nandiraju said.

"(With) the same fiber that might be servicing residential customers, (we) might use different wavelengths to provide point-to-point business services," added John Dahlquist, vice president of marketing, Aurora Networks.

Alternatively, a carrier Ethernet switch in the optical node provides point-to-point Ethernet connections. "Traffic from all the customers is aggregated onto a single trunk port and will be carried over a single wavelength to the headend," Nandiraju said.

A third option is passive optical network (PON) technology, which uses time-shared transmission schemes to allow a single fiber to serve multiple customers. "(The MSO) can go capture cellular backhaul as the generating revenue and then look at larger businesses and offer Ethernet to them. Then as they build out, they might get a power residential customer," David Foote, CTO, Hitachi Telecom, said.

Transport and beyond

MSOs have a number of other assets in their plant they can capitalize on for backhaul, including IP/MPLS, SONET rings and WDM meshes, which they may have already built to serve business and residential customers, Foote said.

"It depends on where the cell sites are and what infrastructure is in place," Foote said. "Cell backhaul will be solved with lots of technologies. There is no one that will dominate."

There are options even if cell towers aren’t exactly close to existing infrastructure. "One or more microwave hops (e.g. daisy-chain or tree structure) can be used to extend the reach of the MSO from the nearest fiber accessible cell site or hub to outlying cell sites in the surrounding area," Dyck said. "Direct line of sight is required between towers wherever they deploy microwave systems; however, this eliminates the need for complicated right of way permissions to pull fiber long distances."

Although the evolution to 3G and 4G has begun, most cell sites will continue to hold old legacy voice TDM base stations and need a T-1 solution in addition to the Ethernet service.

"If you put fiber into the ground, you don’t just want revenue from the Ethernet base station, you want revenue from existing TDM base station radios too," Foote said. "When you deploy fiber or microwave, what equipment do you buy at both endpoints to offer both Ethernet and T-1 circuits?"

Circuit emulation can be used to connect both T-1 and E-1 to the metro Ethernet network, Dahlquist said. "They can take T-1 outputs out of base stations and (carry traffic) at 100Mbps levels to 1Gbps."

Yet, some wireless operators still prefer true T-1, which operators can offer if they have SONET rings or mesh WDM passing close to the cell site. "If you only have an Ethernet pipe, you won’t capture those base stations that need true T-1," Foote said.

Not all circuit emulation techniques have succeeded. "We tried to do pseudowire, going back three to four years," Time Warner’s Staniec said. "One of the things we learned was that it was not ready for prime time."

"One of the things we learned was that (pseudowire) was not ready for prime time." Tom Staniec, Time Warner Cable

Resources, OAMs, SLAs

Cellular backhaul is very finicky as interruptions or inconsistencies can affect the larger wireless network. Mobile operators require stringent service level agreements that cover metrics, including frame loss, frame delay, frame delay variation, committed burst size, excess burst size (EBS), MTTR, and availability.

To remain in accordance with these SLAs, providers who want to leverage Ethernet need the kinds of operations, administration and management (OAM) capabilities traditionally associated with SONET and TDM technologies.

"You’ve got to turn the circuit up right, test the service, not just the GigE pipe, but the EVC (Ethernet virtual circuit). The second thing is how you set up OAM sessions, how you manage the SLA," Jay Stewart, director Ethernet service assurance, JDSU, said.

A variety of standards address OAM. (See sidebar, page 21.) One problem is that they don’t detail how measurements should be made or how often, Stewart said. For example, availability is "loosely defined," by Y.1731, but implementation can vary. "Given this confusion, most Ethernet network equipment does not support availability measurements."

Time Warner’s Thomas Staniec explained that in lab tests of a carrier’s architecture, three different test vendors yielded three different results. "The testing done with the carrier armed us with the knowledge that standardizing to one test methodology which can be consistently replicated and defended is very important," he said.

Moreover, Staniec indicated that without automation, test procedures required to meet SLAs will not only be harder to standardize, but will also tie up numerous highly skilled technicians and engineers.

Timing and monitoring

The nature of mobile networks requires that a timing and synchronization mechanism to be incorporated into a backhaul solution. Base stations are separated by only two to five kilometers, which means they can pose interference for each other unless neighboring stations operate on guaranteed non-interfering frequencies, Nandiraju said.

There are standards that address timing and synchronization at the physical layer, Layer 2 and Layer 3. (See sidebar.) However, the Layer 3 Network Timing Protocol, which synchronizes servers, routers and network elements over the Internet, only provides a timing accuracy of 10 milliseconds.

"Most of the cellular technologies require much higher timing accuracy," Nandiraju said. "For example, in CDMA, the base station transmissions should be aligned within 3 milliseconds." Hence, IETF has formed the Timing over IP connections and Transfer of Clock working group.

In addition, packet-based synchronization techniques (IEEE 1588v2) are vulnerable to packet network impairments.

"Timing and clock recovery mechanisms become weaker with degraded end-to-end quality issues, such as jitter and packet loss," Ranga Thittai, product manager, InfoVista, said. "Enhanced monitoring of services is essential to make sure there is no degradation of service quality."

Cellular operators and businesses alike want to know that if they are subscribing to a 50 Mbps service, that’s what they are getting, said S. Tony Tam, senior product line manager, ANDA Networks. But it’s tricky.

"If you do a truck roll with test equipment to show them, indeed, they do get what they pay for, (that) is expensive," Tam said. The intrusiveness of taking down traffic to prove traffic flows argues for an automated monitoring system. If a customer pays for 50 Mbps, but only uses 5 megabits at a given time, a monitoring system sitting at the customer premise can inject traffic to test the service. That way, if traffic increased to 10 Mbps, the system would adjust in real time without affecting service, yet still demonstrate the fully contracted throughput, Tam said.

Bumpy evolution

The speed and cost structure of Ethernet recommend it as an answer to the looming backhaul crunch. The tree architecture of Ethernet PON (EPON) is a further inducement, as Victor Blake and Bright House Networks Engineer Edwin Mallette wrote in a February 2009 CT article.

But while deployed at Bright House, Ethernet faces a rocky transition industry-wide, the authors predicted: "T-1s are the gasoline and oil of cell backhaul: necessary and addictive. It will take creativity to engineer a replacement."

Some of that work has been codified in the CTBH spec. Staniec discussed additional work, especially on the OAM front. Yet some carriers remain reluctant to leave the comfort of SONET. "We are watching an evolution in real time not only for the wireless carriers moving from TDM to Ethernet, but (also) for the cable industry," he said.

Monta Hernon is contributor to Communications Technology.