PDF of this tutorialCore Networks
The function of the core network, as it pertains to digital video, is the transfer and delivery of MPEG streams: from content providers to service providers.
It also distributes to all the residential access distribution points within a service provider's domain (i.e., to all head ends and central offices [COs]). Generally, a combination of terrestrial and satellite networks are used. The service provider may own or control only a portion, or even none, of the core network. Therefore, some means of verifying the quality of the MPEG signal is needed as it arrives at the service provider's access network.
Over terrestrial core networks, ATM within SONET/SDH is the technology of choice for digital video. SONET/SDH provides the physical layer and framing functionality over optical fiber, and ATM is the payload of the SONET/SDH frame. ATM itself is a form of switched-packet transmission with fixed 53-byte packets called cells. The 53-byte cell consists of a 48-byte payload and a 5-byte header containing address and other information. The address bytes allow the cells to be routed to specific destinations via ATM switches and cross-connects. Thus, ATM is a hybrid of transmission and switching technologies. ATM is designed to carry voice, data, and video. An important function within ATM is segmentation and reassembly (SAR). SAR chops up the intended ATM payload into 48-byte chunks for insertion into cells and reverses the process when the cells have reached their final destination.
Access Networks
A key difference between core networks and access networks is that access implies that there is a physical portion of the network dedicated to each individual end user, while core network facilities are shared among many users. Cost, therefore, is a major issue in access networks, which has led to the ongoing development of a number of alternative access network technologies for the delivery of digital video and multimedia services.
The main distribution systems being deployed for digital video delivery include the following:
- direct broadcast satellite systems—These systems were the first to be massively deployed for the delivery of MPEG2. They have been very successful and allow viewers access to both high quality and quantity of digital video programs. In order to achieve early market entry, the TS of these systems vary from the ISO standard. Two-way interactivity, such as pay per view, in these systems is only possible via telephone connection from the viewer to the service provider.
- cable systems—Cable operators see digital video as the primary means to compete with the direct satellite broadcasters. With current MPEG2 compression and multiplexing technology, it is possible to squeeze at least four to six digital video programs into the spectrum of one analog TV channel. Systems using statistical multiplexing have been demonstrated, which can fit 20 or more digital programs into one analog channel. Figure 5 shows the typical architecture for delivering digital video over a cable system.
Figure 5. Digital Video Delivery Over A Cable System
The segment from the head end to the viewer—the access network—is implemented using HFC, one of several fiber-in-the-loop (FITL) technologies. HFC provides high transmission quality by reducing the number of cascaded RF amplifiers between the head end and the customer. In traditional tree-and-branch, all coaxial-cable distribution plants, there may be as many as 25–30 cascaded amplifiers from the head end to the customer. This large number of amplifiers may result in enough signal degradation to cause quality problems in digital-video services.
To remedy this situation, HFC networks reduce amplifier cascade to five and under by utilizing optical fibers from the head end to a neighborhood node. The last short stretch, from the node to the customer, is via tree-and-branch coax configuration, hence the term "fiber-to-the-neighborhood," often used when referring to HFC networks.
Another advantage of HFC over all coax systems is that the bandwidth is extended to 750 MHz in North America, and even higher in other parts of the world, providing additional analog channel capacity. As digital services are deployed, cable service providers will operate a mixture of analog and digital channels. The analog channels, of course, will use the standard analog modulation methods (i.e., 6–MHz National Television Standards Committee [NTSC] or 8–MHz public-access line [PAL])—no problem in all-coax networks. The channels which are converted to digital video will use spectrally-efficient digital modulation schemes such as 64–QAM or 256–QAM which may undergo too much degradation in all coax networks. The particular variety of QAM used is different for NTSC and PAL countries. Cable Laboratories has specified the NTSC version and DVB has specified the PAL version. Before these QAM specification were completed, various proprietary QAM modulation schemes were being proposed by various manufacturers.
Although some cable operators have extensively upgraded to HFC networks, many have curtailed their infrastructure upgrades and are planning to implement digital video on all coax networks. In this case, RF and modulation tests will all be important in the installation and maintenance of high-quality digital video which can compete with direct broadcast satellites.
For upstream transmission, required for interactive services, HFC networks have several weaknesses. The tree-and-branch portion of the network and upstream spectrum is shared among several hundred users. This results in:
- bandwidth contention
- noise (external noise is also a problem)
- security and privacy issues
SDV
Telephone companies (telcos) are planning to implement SDV, another FITL technology, networks to provide telephony, digital video, and other multimedia services over one network. Refer to Figure 6 for the architecture of a typical SDV network.
Figure 6. SDV Network Architecture
At first glance, the SDV network may seem to be very similar to an HFC network, and, in some ways, it is. SDV is also referred to as fiber-to-the-curb (FTTC), while HFC is referred to as fiber-to-the-node or fiber-to-the-neighborhood. As this implies, SDV brings the fiber closer to the end user. The key difference between HFC and SDV is the configuration and bandwidth in the link from the optical termination to the home. A star configuration is used. This link is typically a switched 51–Mbps ATM connection. This provides enough capacity for several MPEG2 programs as well as telephony, Internet, etc. Since the connection is bidirectional, the number of programs which can be provided is not limited to the capacity of the 51–Mbps link.
The user has the ability to access other programs via a set-top box which will send upstream commands to switch the program to the user's node. The physical transmission medium for SDV is either twisted pair or coax and typically uses 16–carrierless amplitude and phase modulation (CAP) or 16–QAM modulation. An advantage of SDV is that each user has a private, dedicated, bidirectional link back to the optical network termination. This contrasts with HFC in which users must share a common physical back channel with associated noise and security problems. SDV, however, may suffer crosstalk impairments when unshielded twisted pairs are used as the final link to the home.
xDSL
Various digital subscriber line (DSL) technologies were originally developed with interactive digital video in mind. Asymmetric DSL (ADSL), which allows as much as 9–Mbps downstream from the CO over a twisted pair and about 1–Mbps upstream, was envisioned as a prime digital video access network because of its reuse of the existing outside copper plant. The delay in the introduction of interactive digital video has meant that ADSL is now seen as a high-speed Internet-access technology. Telcos which conducted ADSL field trials for digital video have shifted to SDV technology.
Wireless Cable
Seemingly a contradiction in terms, wireless cable refers to wireless network architectures for delivering cable services, both analog and digital. All wireless cable networks utilize line-of-site (LOS) RF transmission from multiple transmitters to many users with tiny, house-mounted, receiving antennas. The architecture currently being implemented is multipoint multichannel distribution system (MMDS). MMDS transmitters operate in the 2–GHz microwave band. In an ideal situation, a handful of MMDS transmitters can cover an entire metropolitan area. Field trials of MMDS have shown that cities which have little vegetation and are adjacent to high mountains (for transmitter sites), such as Los Angeles, are prime candidates for MMDS. Areas on the East Coast of the United States, with high vegetation and rolling terrain, have been shown to be difficult for MMDS. At this time, it appears that MMDS will only be deployed in niche applications.
Local multipoint distribution system (LMDS) is the next-generation wireless cable architecture. LMDS utilizes the 28–GHz band with lower power and more localized transmitters. Each transmitter, therefore, serves a much smaller number of customers (analogous to micro cells in cellular phones). In theory, since a portion of the LMDS spectrum is dedicated for a back channel, interactivity is possible. Currently, LMDS is in the developmental/trial phase.
Terrestrial Broadcasting
The direction that terrestrial broadcasters in the United States will take in deploying digital video has been closely linked to the question of high-definition TV (HDTV). In 1996, the HDTV standard was approved by the Federal Communications Commission (FCC), and in 1997 the FCC mandated that all terrestrial TV broadcasts migrate to HDTV by 2006. Initially, 60 stations are required to commence HDTV broadcasts by the middle of 1999. MPEG2 compression will be used with 8–vestigial sideband (VSB) modulation.
