Access networks for triple-play services

The telephone industry has been around for a while, so it should come as no surprise that today, virtually every telephone company is different. The older the telco, the more upgrades and generations of technology they’ve had to integrate into their networks--starting with local voice and going all the way to the latest broadband data and video services.

The part of the multiservice delivery infrastructure with the widest variety of options is the access network; that “last mile” between the service provider’s central facilities and the subscribers themselves. “Last mile” is in quotes because in urban and suburban areas the distance from the central office might be much less than a mile, but in rural areas, it can be many.

Not only must the service provider meet the subscriber, it must build the right access infrastructure to deliver all of the required services while guaranteeing quality of service and minimizing latency. And you guessed it: Not every access solution is suitable for delivery of the “triple play” of voice, data and video, let alone the rest of the interactive digital video services such as video on demand and t-commerce, if the service provider wants to maintain a single network.

Let’s look at the different access network options:

• VDSL (very high-speed digital subscriber line). ANSI T1E1.4 standard. Ranging from 13 Mb/s (about 4500 feet) to 52 Mb/s (about 900 feet) per access line; VDSL has sufficient bandwidth to deliver two or more streams of uncompressed MPEG-2 video, with plenty of overhead for broadband Internet access and other digital services. Specialized implementations of VDSL that are also promoted for video applications include long-reach Ethernet and rate-adaptive DSL.

• ADSL (asymmetrical digital subscriber line). ANSI T1.413 standard. ADSL is probably the most mature variety of DSL and is the most widely deployed for Internet access. It can deliver two TVs if the video is compressed. ADSL offers speeds of 8 Mb/s (some suppliers claim 10 Mb/s) downstream, 640 kb/s upstream on very short loops. Some suppliers claim video reach up to 12,000 feet and data reach of 18,000 feet, but at those distances it is appropriate to question whether there is enough speed to do these applications justice. 

• ADSL+ (“ADSL Plus”). VDSL Plan 998. This is a variety of ADSL intended to address advanced service requirements, including video (up to about 8000 feet). One caveat: the spectrum used for ADSL+ overlaps with the spectrum of VDSL. Therefore, service providers should design carefully if they wish to combine ADSL+ data-only customers and VDSL triple-play customers in their networks.

• PON (passive optical network). ITU-T G.983, G.992, G.993 standards. PON utilizes ATM to create logical connections between network devices (ONUs, OLTs) over optical fiber. Bandwidth is typically OC3 (155 Mb/s) or OC12 (622 Mb/s), 1.25 Gb/s, and on the horizon, 2.5 Gb/s and even 10 Gb/s!  There are ATM PONs (APONS) and Ethernet PONs (EPONS).

• PON + VDSL. The use of a PON line to reach from a central office to subtended VDSL DSLAMs that, in turn, deliver high-speed digital service to each subscriber. Industry support is broad. The Full Service Access Network (FSAN) initiative promotes the ITU 981.2 PON standard with interoperable VDSL platforms that support voice, data and video. Please refer to my June 2002 installment of The Analyst’s Corner, on the FS-VDSL, for more details.

• Pure Ethernet all the way to the home. IEEE 802.3 standard. Ethernet enjoys ubiquity in the enterprise and has speeds from 10 Mb/s all the way to 10 Gb/s.

• HFC (hybrid fiber/coax). CableLabs standards, including OpenCable, PacketCable and DOCSIS (for data). Most cable carriers still distribute television via RF, modulating binary data into radio frequencies and back, introducing latency. Cable operators are aggressively upgrading their infrastructures to be two-way-capable and are introducing interactive TV and voice telephony (both circuit-switched and VoIP).

• Wireless. Fixed broadband wireless may be used to distribute TV over short or medium distances in campus and business park environments, and in MDU/MTU applications.

• Combination of satellite and xDSL. This requires a partnership with an outside service provider. The advantages include the reduced cost of building the infrastructure, and the short time to market. One major disadvantage is that the partnered satellite carrier will always contend with the telco for control of the subscriber.

Yes, it’s a bewildering array of options. But given the varying network legacies, geographies, subscriber bases and all the other variables that make telcos different from one another, there can be no single all-knowing access solution.   

Let’s start sorting it out by identifying some important evaluation criteria:

• Analog vs digital. Analog video is a one-way medium and requires a second line for the upstream (“return path”) communication needed by subscribers to order pay-per-view programs. Digital video can travel via analog carrier, but it’s still not two-way. On the other hand, all-digital networks pave the way to make IP possible, enabling the new services and eliminating the return path issue.

• Adequate bandwidth. Until studios and TV programmers begin to encode their content with MPEG-4, ADSL will continue to be insufficient for households demanding more than two TVs and additional overhead for data and voice services. ADSL also cannot support HDTV, which requires 19.2 Mb/s.

• Use of spectrum. Spectral compatibility is an issue. In particular, ADSL+ and VDSL both use the same portions of the spectrum, which can produce interference and degrade service. Careful attention should be paid to the different suppliers’ approaches toward support of true global standards.

• IP (Internet protocol) support. Does the access network support IP, either pure IP or IP-over-ATM? IP is has been in use since the 1960s and is fundamental to the Internet. Not only does IP allow two-way communication over a single line, it also provides the two-way communications platform for those important revenue-generating services such as video-on-demand, interactive game play and “T-commerce” (purchasing via TV remote control). Another plus: much of the media content developed for Internet use can be leveraged in video applications.

• Support for multicast & IGMP. Can the network handle the onslaught of channel-change requests from channel-surfing subscribers?  To better understand the issue, let’s take a brief detour to look at how IP-TV works. Multi-channel digital TV uses a broadcast-like technique called multicast, wherein channels of MPEG-2 television content go from the head-end to the central office and then are “pushed” simultaneously from there to routers at the edge of the network. Meanwhile, a subscriber wishing to view (“pull down”) a TV channel requests to join the “group” of subscribers receiving a specific multicast stream, by (unwittingly) sending a message upstream to the TV middleware/application server via the access line to the nearest router, using Internet Group Management Protocol (IGMP; the preferred version is IGMP-2). Then, the IP TV middleware talks to the service provider’s subscriber database and security platform to identify and authenticate the subscriber, authorize the request, and send an IGMP message back to the router, which responds by admitting the subscriber to the multicast group. When the subscriber changes channels again, he or she “leaves” that channel, and “joins” another. “Regular” (linear, broadcast) TV, pay-per-view and near-video-on-demand IP TV applications all use IGMP multicast. All of this must happen quickly: Who would wait five seconds for a channel change? Further, this process must be scalable and able to handle a large percentage of subscribers at any given moment.  Therefore, the access platform must have a high-speed network interface and must integrate well with routing capability. If the access platform did not support multicast, the central office would have to be equipped to replicate every channel for every subscriber channel request, which would cause unnecessary congestion both in the backbone network and at the access platform.

• Support for voice. If the service provider’s goal is to maximize service at the lowest possible cost, it can’t ignore what to do with telephony. The network must be able to overlay the existing POTS infrastructure or be able to support voice over IP. Does the service provider want to maintain two networks? One network with packetized voice? Again, the technology choice must be transparent to the subscriber.

• Evolution of services. As new interactive services emerge, the pressure for high bandwidth and two-way communications will continue to increase. And there’s much more. Space limitations prohibit me from getting into the important areas of systems management, OSS/BSS integration, QAM vs DMT, VDSL frequency band Plans 997 and 998 (or something else), and more.

OK, deep breath! What are my favorites in the access platform race? First of all, it has to be digital. Then, for a majority of “triple-play” implementations, for service providers that require the flexibility to serve high-density, multiple-tenant/multiple-dwellings, offices and single-family residential communities over existing lines, I still believe that VDSL is the most serious option. This gives the service provider enough speed to support multiple TVs, adequate overhead for voice, Internet access, and return-path signaling, and the option of using IP. In networks that must serve multiple communities or otherwise require longer reach, the combination of passive optical network (PON) and subtended VDSL DSLAMs is very effective.

In “greenfield” (new community) situations, in cases where the service provider plans to deliver more than 52 Mb/s to a single subscriber and for rural municipalities and public utilities forging public-private broadband partnerships, the two viable options are the PON and the pure Ethernet optical network.

While these combinations may not necessarily be the least expensive, they are all fast and digital, and can anticipate future two-way interactive functionality in any new-media application. The service provider retains the freedom to adopt any interactive application that can be implemented across an IP network, be it for consumption via TV, PC or any other “digital home” device, and leverage the incredible variety of content purposed for the Internet.

Steve Hawley is principal and consulting analyst of Advanced Media Strategies. He may be reached via his Web site, http://www.tvstrategies.com.