Cable Bridging CMAP's Migration Gap
The idea is to create a downstream-only version of the Access Shelf -- a key piece of the CMAP architecture -- that would end up looking a lot like a super edge QAM. That approach could help preserve modular CMTS deployments that use discrete edge QAMs for downstream traffic and the core CMTS for the upstream.
This way, the argument goes, MSOs could start to introduce elements of the CMAP before they are ready to make a complete jump to the new architecture. Vendors aren't expected to start releasing final-form CMAP products until 2012.
"We have to add equipment during the bridge years before we have a complete CMAP," says Cox Communications Inc. Senior Director of Network Architecture Jeff Finkelstein. "There's capital being spent during these bridge years."
Comcast initiated the CMAP product/architecture, and several other MSOs have since added their voices to the project. CMAP combines CMTS and edge QAM functions to significantly reduce space and power consumption requirements, and to cut the overall cost per bit. (See Comcast Proposes Its God Box , RGB Targets Comcast's CMAP, BigBand Plots Plans for Comcast's CMAP, Comcast God Box Also a Green Box, and More MSOs Back Comcast's Big Box Project .)
The specs call for a Packet Shelf (for packet processing) and an Access Shelf (for the downstream and upstream PHY and MAC layer). Each can come in modular or integrated implementations.
That's in contrast to the Modular Headend Architecture (MHA), which CableLabs developed well before the CMAP entered the picture. Essentially the technical baseline for today's modular-CMTS deployments, MHA packs the upstream capacity into the core CMTS and puts the downstream capacity in less expensive edge QAMs. (See MSOs Unite Against Telcos at the Headend.)
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The keys to the new CMAP Access Shelf approach are the CableLabs specifications for the Docsis Timing Interface (DTI) and Downstream External PHY Interface (DEPI). In an MHA, DTI keeps the clocks of the discrete upstream and downstream components synched up, while DEPI provides the IP link between the CMTS and the edge QAM.
DTI and DEPI weren't initially part of the CMAP specs but are now being considered for inclusion. That's the factor that could protect the current base of modular-CMTS deployments as MSOs develop their own CMAP strategies.
A downstream-only Access Shelf
Engineers at Comcast, Cox and other operators have been crafting an alternative use of the CMAP Access Shelf that would apply DTI and DEPI to the product specs, essentially creating a downstream-only version of an Access Shelf that functions a lot like a super-dense edge QAM. (The upstream side can be added later, by booting the device with CMAP code once the requisite upstream blades are snapped in.)
The idea here is to protect MHA/modular-CMTS deployments while adding redundancy, more sophisticated encryption, QAM replication, and other CMAP features, notes Comcast VP of Access Architecture Jorge Salinger.
"For locations where a MHA is deployed, additional and/or denser downstream M-CMTS Core modules may be added. For those cases, MSOs could deploy newer modules in CMAP equipment operating in downstream-only mode," Salinger says. "We have said all along that we don't intend to remove the Modular Headend Architecture and replace it with CMAP. CMAP does what the modular-CMTS does [in terms of creating a very dense edge], but in a different way."
This option should give operators a gradual way to migrate to the CMAP architecture in areas where MHA is deployed. Additionally, operators that use MHA today will still need to add service capacity during the months and years leading up to a full-on CMAP deployment. This newly proposed approach aims to help MSOs recoup those investments. And that incremental approach should prevent delays in future CMAP deployments. "It really simplifies that step we would have to take," Finkelstein says.
The new, high-capacity 3G60 blade from Cisco Systems Inc. (Nasdaq: CSCO) adds to the need for a CMAP migration option. Cisco is the lone CMTS vendor to develop a modular-CMTS product that works in tandem with its own edge QAMs and Harmonic Inc. (Nasdaq: HLIT)'s, and the 3G60 is extending the life of those deployments. (See Cisco Aiming to One-Up Cable's Upstream .)
Salinger and Finkelstein have coauthored a whitepaper on this topic that will be presented at the 2011 SCTE Canadian Summit, set for March 8 and 9 in Toronto.
— Jeff Baumgartner, Site Editor, Light Reading Cable