| Nortel IPTV解决方案 |
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| 作者：未知 来源：Nortel 点击数：1746 更新时间：2006-10-8 |
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Nortel IPTV solutions deliver fully featured, high quality television and additional interactive and revenue-additive communication services to telco subscribers by providing:
A full set of Nortel professional services that reduce implementation complexity, speed time to market, and provide a single point of contact for the IPTV solution
Pre-tested configurations of best-in-class equipment from our extensive ecosystem of IPTV alliances
Assurance of operational readiness based on integration of the IPTV solution into the existing operational infrastructure
Investment protection via support for an evolution to SIP and IMS based architectures
Nortels IPTV solutions enable wireline service providers to deploy a secure, reliable, and scalable IPTV infrastructure to deliver the entertainment and communications services that consumers value.
The Essential Components of IPTV
We know IPTV will revolutionize the way we watch television in terms of giving us access to services such as listening to our home voice mail from our TV directory, or allowing us to order a pizza from our TV, before our HD movie begins. But, with so many companies vying for position in this hot sector of telecom, and IPTV-mania sweeping the networking landscape, there is a lot of confusion about how to deliver these kinds of revolutionary new services. Many vendors are talking about a complex set of systems that many telcos, such as SBC, are spending billions of dollars to acquire. This article breaks down the complexities IPTV into four bite-size components: the IPTV head-end, the backbone network, the access network and the subscriber premises.
The IPTV Head-End
The IPTV head-end is where content (such as television channels or Video on Demand movies) is received and prepared for transmission across the operators private IP network. Perhaps the most complex piece is simply capturing the content. It comes from analog satellites, digital satellites and antennas; the content may be standard definition, high definition, or music; the format can be DVB or earlier proprietary implementations. A typical head-end will require six or more different types of systems, and multiples of each is often required.
Once the signal is downlinked or downconverted, it often needs to be altered to fit the operators network. If an analog signal is received, an encoder is required to digitize, compress and packetize the signal. If the signal is received in digital format, it may still need to be converted into the format used by the operator. After that, the operator typically will add local commercials and FCC-mandated emergency alert information. The Conditional Access (CA) system then encrypts the signal to prevent service theft or unauthorized copying. Finally, the desired signal is placed onto the network for delivery to the subscriber. Video on Demand (VoD) traffic goes through a similar process, except that the final version is placed onto a server until someone requests to view the content.
All of this happens under the control of network middleware. This software controls what the electronic program guide (EPG) displays, thereby controlling access to content and services. It logs new VoD content and adds this information into its directory. It controls the encryption process, telling the CA system when to encrypt information and when to send the key to a subscriber. It also interacts with the back end systems. For example, it checks whether a subscribers account is up to date before allowing the subscriber to view VoD or PPV content.
Each product provides vastly different functionality, comes from a different supplier, and has its own management system with a unique user interface. As a result, there is significant expertise required to create an IPTV network. And this is in addition to the cost of purchasing and maintaining the required hardware and software!
The Network Backbone
The network backbone is an often overlooked piece of enabling IPTV services. The backbone must be addressed by service providers, however, as many of todays provider backbones will likely not be able to handle the bandwidth required to offer even basic IPTV services. Consider a television service of 100 standard definition television (SDTV) channels using MPEG2, which is the existing standard for digital television. Since each channel required roughly 4 Mbps, the backbone must support 400 Mbps in total. This is certainly within reach of many existing backbones.
High definition television (HDTV) requires roughly 20 Mbps per channel. Most of todays television services offer fewer than 20 HDTV channels, but as HDTV becomes the industry standard service providers will have to increase their network backbone capacities. Using todays compression technologies, offering 100 HDTV channels requires 2 Gbps of bandwidth. This is certainly achievable with todays technology, but is beyond the reach of many existing backbones so an upgrade would be required. In the future, better compression will allow HDTV channels to require 10 Mbps per channel or less.
Video on Demand drastically changes the bandwidth requirement in the service providers network backbone. VoD essentially gives each subscriber a personal television channel of 4 Mbps (for SDTV). For a network of 10,000 subscribers, the backbone must now support 40 Gbps! Fortunately, today the number of subscribers viewing VoD content at any given time is fairly low. In addition, operators are minimizing the strain on the network by caching popular content, such as recently released movies, at a local central office closer to the subscriber. Since the movie can be distributed to these caches at low priority, at off-peak times and using multicast techniques, overall bandwidth on the backbone is greatly reduced - but at the cost of additional equipment and complexity.
As VoD grows in use and popularity with video subscribers, service providers will likely need to address their network backbones with increased capacities for their optical networks, as well as building out local video office locations in major metropolitan areas.
The Access Network
The most basic decision about the access network is figuring out how much of the existing copper loop can be used. The general guideline for offering IPTV service is that the network must support at least 20 Mbps for long-term viability of a service offering that includes HDTV service.
Today, many early adopter implementations will go as low as 10 Mbps total bandwidth to the home. This only supports 2 SDTV channels, 2 Mbps of Internet access, plus support for voice traffic. A 20 Mbps pipe to the home would enable 3 or 4 HDTV channels, with enough bandwidth left over for 4 to 8 Mbps of Internet access and support for voice traffic.
Whatever the service providers target access bandwidth, DSL throughput is highly dependent upon the length of the local loop. Often fiber must be run to DSLAM located in the outside plant, to reduce the length of the copper run. As a general rule, current ASDL2+ technology supports 20 Mbps at distances of 1000 feet (300 meters).
For new construction or overbuilding, fiber is often deployed right to the subscriber premises since laying fiber costs about the same as running copper. Passive Optical Networks (PON) are most commonly used. The only equipment in the outside plant is passive splitters, reducing network maintenance compared to putting active (powered) equipment outside of the CO. PON shares the bandwidth between 32 or more users. In denser areas requiring higher bandwidth, Ethernet may be a better solution.
The Subscriber Site
The subscribers premise is often the most costly portion of the overall solution. Each site requires a demarcation device such as a DSL modem. The PON equivalent is called an Optical Network Unit (ONU). Although powered, these typically include bypass circuitry so that the phone works even during a power failure. Both types provide connections for the subscriber to connect the end-user equipment. Connections may include telephone jacks, Ethernet, coaxial cable for television, or even T1 lines.
There are many ways to connect this demarcation device to the set-top box. Ethernet-capable wiring is the most common method. Unfortunately, Ethernet cannot run on most in-home telephone wiring, so the telco must run new wire and typically absorbs this cost. Third-party devices allow Ethernet to run on existing coax cable, allowing re-use of existing television wiring. This function is now getting integrated into DSL modems and set-top boxes. Broadband Power Line (BPL) sends the signals across the existing in-home power wiring, eliminating the need for separate packet connectors.
There are also many features available for set top boxes. The lowest cost equipment supports standard definition television service. Higher-end units support high-definition television, integrated hard disks for recording programs, digital audio outputs for connecting to the home audio system, web browsers, USB ports, and many other options. Offering one set top box is not sufficient for all subscribers, yet offering too many alternatives drives up testing and support costs.
Key Design Issues
There are several technical challenges to offering IPTV service in addition to bandwidth issues described previously. Broadcast television requires extensive use of IP multicast, and most network equipment cannot support the 100 multicast streams required to offer an IPTV service. It may be desirable to deploy emerging techniques such as MPEG4 Advanced Video Compression (AVC), which cuts the amount of bandwidth required roughly in half. VoD servers must concurrently support thousands of subscribers, each seeking to view, rewind, pause and fast forward their picture. Placement of VoD caching to minimize network usage is critical to keeping the overall cost down. Distributed head-ends may be needed to capture local channels and add that into the video stream. FCC requirements also mandate that an antenna be placed typically within 50 miles of each subscriber. Ensuring that thousands of users can order a pay per view movie minutes before it starts is another possible bottleneck.
All of this reflects that IPTV is a nascent technology. Riding the silicon technology curve will allow service providers to overcome most of these obstacles, with bandwidth and lack of standards remaining the biggest challenges. Most telcos no longer question whether they will deploy IPTV, rather question when they will deploy it.
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