November 2, 2006

Cable's T-1 Opportunity

By By Jay Junkus

If opportunity comes in windows, the bottom line at a set of morning workshops held during the SCTE Business Services Symposium on October 18 was that there is a lot of fresh air flowing to cable operators from cellular backhaul.

Dave Clough, Cisco consulting systems engineer, opened a session on mobile wireless networks by emphasizing that backhaul needs are growing at unprecedented rates. "Backhaul is the bottleneck for advanced services," he noted, as he continued with a discussion of cellular technologies and sharing of cell site locations by multiple cellular carriers.

A cellular system consists of the cell site and its air interface, a mobile telephony switching office (MTSO), and backhaul between the cell site and the MTSO. The technologies used at North American cell sites are AMPS, TDMA/GSM, UMTS/WCDMA, CDMA and EvDO. AMPS, the earliest technology, is being phased out in favor of newer digital technologies such as EvDO, which is capable of advanced services such as mobile card.

The original backhauls were conventional TDM-based T-1 circuits obtained from LECs. As technology, demand and pricing evolved, cellular operators began requesting DS-3 circuits capable of multiple T-1s and are currently moving toward pseudowire technology that provides T-1 equivalents over Ethernet.

Advanced services

With advanced services, T-1 transport needs for EvDO increase to as many as eight T-1 equivalents per operator cell site. Typically, several operators share cell sites, but each requires separate backhaul to their own MTSOs. The workshop session by Narad's Louise Wasilewski quantified the implications of this growth to cable operators. "If there are 10 to 16 T-1 equivalents per mobile operator at each cell site, and three to four mobile operators per tower, that equates to up to 64 T-1 equivalents per tower," she said. "In the half square mile typically covered by an HFC node, it is likely there are at least two tower locations. That equates to up to 120 T-1 equivalents per node." Wasilewski noted that this is dedicated capacity, although it will be divided among cable operators and other carriers competing for the business.

Cable operators have several choices to meet this need using their HFC plant. Narad offers a pseudowire solution, and Vyyo has a product that delivers T-1 over HFC using a proprietary implementation of DOCSIS technology. Plant topologies can include a coax drop, fiber drop from a coax trunk, and activation of dark fiber from a node. Clough pointed out that when fiber is used, it makes sense to evolve the fiber access to a logical ring architecture to create redundant paths.

Irrespective of the technology or architecture used to deliver backhaul, cable operators will need to meet strict criteria for service level agreements (SLAs), although these will not necessarily include "five nines" availability. Wasilewski detailed key parameters, based upon a study of cable's competition.

"You enter the ballgame by being better than the 'three nines' of reliability offered by the competition," she said. Latency performance is also important. As a mobile customer moves between the coverage areas of individual towers, the differential delay must remain within the specifications of the cellular equipment provider. CDMA 1XRTT is the most demanding technology for this parameter, requiring a maximum of 8 msec. According to Wasilewski, many cellular operators will ask for 5 msec. Also, jitter or frame delay variation must be at most 5 msec. While TDM technology typically falls below 1 msec for this parameter, Wasilewski pointed out that due to other TDM parameters, it may be easier to sell carrier Ethernet and leave it to the cellular operator to provide its own pseudowire interface.

Finally, Wasilewski pointed out the importance of support systems. "Visibility to the SLAs is an important differentiator for cable operators competing for cellular backhaul business," she said, "and typically, LECs and CLECs do not provide this information to the cellular operator." She also added the importance of methods and procedures, including end to end testing, maintenance of databases, and defined processes for circuit adds and removals.

- Justin J. Junkus