A CATV HFC is called an access network; it may also be referred to as a last-mile solution or (for solutions proposed by telephone companies) a local loop solution. An access network connects customers’ premises to the network termination and performs operations interfacing with the transport network, content provider, and home network elements. Figure 6 shows graphically the technological frame for the access network. All technologies share a common element, which is the network termination (NT), or network interface, in the home. The main functions of these networks are the following:
• Connection to the core transport network by switching, routing, and multiplexing
• Classification of user traffic by QoS
• Security procedures and handling of packet encapsulation
• Registration of hardware and updating of software in the household equipment
• Measurements for billing
A number of alternative cable and wireless competitive technologies are becoming feasible for the access network, so that major CATV providers have had to accelerate standardization of their products.
Competitive wire technologies are the following:
• xDSL (asymmetric, symmetric, high — and very high-bit-rate digital subscriber line)
• FTTx (fiber to the neighborhood, curb, building, etc.)
• ISDN (Integrated Service Digital Network).
Competitive wireless technologies are the following:
• MMDS (multichannel multipoint distribution system)
• LMDS (local multipoint distribution system)
• Satellite [deosynchronous earth orbit (GEO) and low earth orbit (LEO)]
In the following a brief description is given of each of these networks.
Fig. 6. Alternative technologies for access networks: wired and wireless.
xDSL (Digital Subscriber Line). xDSL refers to a series of networking technologies, comprising ADSL, HSDL, VDSL, and SDSL, that are capable of supporting high data rates over the existing telephone network. The technology bases its competitiveness on the existing extensive infrastructure and heavy capitalization associated with telcos.
Standard telecom modems establish a data stream between two arbitrary points using the entire telecom system—that is, from the sender’s local loop, through the telephone switching system (mostly digital switches now), and then to the receiver’s local loop. Standard modem connections can span continents, with one end thousands of kilometers from the other end. DSL modems, on the other hand, establish a connection from one end of a copper wire to the other end of that copper wire: the signal does not pass into the telephone switching system. Consequently, DSL modems are not limited to using the voice frequencies passed by the standard telephone system (typically 0 to 4 kHz); DSL modems typically use more than 100 kHz. To reiterate, one end of the DSL link will be at the consumer site, the other end must be at the other end of the copper cable, usually at the local telephone exchange, where data and voice are split. The voice frequencies are wired into a traditional plain ordinary telephone service (POTS) switch and enter the usual telephone switching network. The data frequencies are wired into a corresponding DSL modem, and the resulting high-speed digital data stream coming from (or going to) the consumer is then handled as ordinary data (not analog voice) and may be hooked into any number of networking technologies for further connection to the data’s destination. Thus, the data never enter the standard telephone switching system. Typically the data will be routed over a local-area network (LAN) or wide-area network (WAN) connection (10Base-T Ethernet, T1, T3, ATM, frame relay) to a business office.
Over the next five years, xDSL (and particularly ADSL) is considered to be the greatest threat to the cable modem industry.
FTTx (Fiber to the Neighborhood, Curb, Building, Etc.). The growing demand for interactivity and more bandwidth per subscriber is being satisfied by pushing fiber closer to the home and by the availability of the required electro-optic components. FTTx refers to a series of networking technologies that run optical fiber from the central office to a user’s neighborhood (FTTN), curb (FTTC), building (FTTB), home (FTTH), etc. Though the optical fiber is relatively inexpensive, optical transmitters and receivers are very costly. However, continued growth of broadband services is fueled by the clear advantages that optical fiber systems offer in cost, reliability, and performance for broadcast networks.
FTTH is not a practical solution for the moment to deliver residential data services. The present idea is to replace long copper lines with fiber optic lines (not only in the telephone network, but also in CATV). The major drawback is the cost of the replacement of the copper-based infrastructure. A critical point is what part of the network will be replaced with fiber optic lines: that is what distinguishes FTTH, FTTB, fiber to the office, FTTN, fiber-to-the-street, FTTC, etc. On the other hand, users of a CATV network again use copper lines, but all the rest of the infrastructure is fiber-optics-based. With the exception of FTTH, all the above approaches use some form of high-speed metallic access technology for service delivery to the customers’ premises.
ISDN (Integrated Service Digital Network). ISDN is a service provided by local telephone companies that modifies regular telephone lines so that they can transmit data almost five times as fast as the fastest analog modems currently available. In addition to the significant increase in transmission speed, ISDN also allows the transmission of not only data, but a combination of data, voice, and video simultaneously on one line. ISDN provides higher speeds than POTS by allowing data to be transferred digitally from end to end. In contrast, POTS converts the digital data to analog within the local loop that extends into the home or office, significantly reducing transmission speed. An ISDN line can carry up to 128 kbit/s of data. Converting to an ISDN-compatible configuration for the consumer only requires an additional piece or two of relatively inexpensive hardware. Telecommuting to an office or base requires additional equipment at the base. While ISDN usage costs are slightly higher than those for analog telephone lines, users enjoy more than commensurate benefits.
Though ISDN has a share of the high-speed connectivity market, the technology is limited to 128 kbit/s and thus is in a different class than technologies that can support megabytes per second. In particular, it is very limited for video applications; it can only be used for videoconferencing with 6 to 8 frames per second.
Satellites. There are two basic types of satellite systems being proposed: GEO and LEO.
GEOs orbit in the Clarke belt, approximately 35,000 km (22,000 miles) above the equator. With this orbit, the satellite can stay over the same area of the earth for an indefinite period of time. Each GEO serves one geographic area, and can theoretically cover about 41% of the earth’s surface. Companies proposing GEO systems are planning on using between three and fifteen satellites to deliver worldwide service. The primary advantage of GEO systems is that they are a proven technology. Most current communications satellites are GEOs. A GEO system is also far less expensive than an LEO system, and also GEO ground stations can be relatively simple because they need only target a fixed point relative to the earth. The main drawback for GEO systems is called the latency factor. In order to obtain information from an Internet server, a signal has to travel 35,000 km to the satellite, then 35,000 km back to the earth. This round trip takes approximately one-quarter second.
LEOs orbit 20 times closer to the earth, between 700 km (450 miles) and 1350 km (700 miles) above the earth’s surface. Each LEO is moving relative to the earth, covering a particular area for only a few seconds. Because of this, a network of many satellites is required to cover the world.
Teledesic plans to launch a large number of LEOs that will be capable of offering high-speed Internet access anywhere in the world. Service providers will include Teledesic, Globalstar, OrbComm, and SkyBridge and M-Star (backed by Motorola). M-Star, while a broadband LEO system, is not aimed at the consumer market like Teledesic; it is planned to offer high-bandwidth intercontinental links between network providers rather than end users.
Satellites require a dedicated piece of the spectrum. Currently, the ITU has allocated 2.5 GHz of spectrum for fixed satellite services in the 28 GHz Ka band. There are fourteen satellite applicants vying for pieces of that 2.5 GHz. Requests for single applicants range from 750 MHz to the full 2.5 GHz, with most applicants requesting 1 GHz of spectrum.
The reason the Ka band has not been used in the past is that such high-frequency transmissions are easily blocked. Buildings, trees, and other solid objects can cause a loss of signal. This makes these frequencies unsuitable for use by ground-based systems, because they would require a large number of transmitters to be able to avoid all obstacles. Satellites avoid many of the problems associated with blocking because their signals come from directly overhead. Buildings and trees do not present an impediment to signals coming from overhead satellites.
Power-Line Area Networks (Access via Utility Power Grids). There have been proposals lately for traditional power companies to provide high-speed access via their existing grids. The biggest obstacle to this technology is that data are scrambled when they pass through transformers. This obstacle is slowly being overcome as several companies continue to work towards a solution. Nortel Networks has successfully tested networks in Europe and Asia, where the transformer-to-customer ratio is 1/300. They have successfully bypassed the transformers, but their speed has been limited, being comparable with cable modems and xDSL. Data are transferred through the actual power wiring.
NIU (Network Interface Unit). NIU refers to high-speed connectivity through hybrid fiber-coax or FTTH or FTTC networks using a network interface unit at the customer premises rather than an external or PC-installed cable modem. Systems using NIUs usually support both data and telephony. Since the technology can be utilized by cable operators, it is not actually a competitor to the cable industry but to cable modems.
Multipoint Multichannel Distribution Service, and Local Multipoint Distribution Systems. Multipoint multichannel distribution service (MMDS), also referred to as wireless cable, delivers broadband services to subscribers through microwave transmitting and receiving antennas. The channels allocated to MMDS are generally used to provide a multichannel video programming service that is similar to cable television, but, rather than being hard-wired, MMDS uses microwave frequencies.
MMDS is a wireless technology for access networks that operates at frequencies 200 MHz to 2700 MHz. Its range may be 50 km to 60 km, and its main advantage is invulnerability to weather conditions. Operators that offer wireless video service can also offer data services. MMDS operators can offer telco return service and, with the emergence of LMDS (see below), will be capable of supporting bidirectional services.
The local multipoint distribution system (LMDS) is a broadband wireless technology used to deliver voice, data, Internet, and video services in the at frequencies of 25 GHz and higher (depending on the license). Spectrum in the millimeter band has been allocated for LMDSs to deliver broadband services in a point-to-point or point-to-multipoint configuration. Due to the propagation characteristics of signals in this frequency range, LMDSs use a cellularlike network architecture (normally the cells are large and a big city can be fully covered with four or five cells). The services provided are fixed (not mobile) and are seriously impaired by adverse weather.
Cable Modem versus Set-Top Box. Numerous companies are working towards high-quality, fullscreen, real-time delivery of video programming that can be delivered via a cable modem or broadband Internet connection. As the cable television networks transition from a broadcast-only network to a high-bandwidth two-way network, the importance and functionality of the set-top box increases. Currently, mixed cable modem and set-top box network architectures and services exist. Developments in the set-top box industry have been restrained by the control the cable companies have over their equipment. Currently, a set-top box includes closed, proprietary technology, which prohibits its use on other cable systems’ networks. Technology is evolving in both directions, from the cable modem to the set-top box and vice versa. Current technology development can be summarized as follows:
Cable Modem Technology.
• Cable modem from DOCSIS, IEEE 802.14, IETF, DVB, and ATMF
• EuroModem from DVB-RCC and ETS 300800
• EuroDOCSIS from DOCSIS (DOCSIS cable modem with some DVB technical compliance) Set-Top Box Technology.
A number of cable operators from across Europe have developed the Eurobox Platform. This concept is based on a common set-top box and a common application program interface API and conditional access method. The box specification is used as a reference model by the cable industry and manufacturers. Viaccess has been selected as the conditional access system, and Open TV as the API. The Eurobox Platform has been successfully implemented, for example, in France, Sweden, and Denmark. However, some cable operators, notably in the UK, do not appear to be following this platform in its entirety.
This initiative from CableLabs includes guidelines for building advanced set-top boxes including feature enrichments to support broadband applications.
MHP includes set-top boxes, integrated TV receivers, in-home digital networks, personal computers, network computers, and so on. The first specification for MHP, covering home access networks (HANs) with an active NT and based on an ATM interface operating at 25 Mbit/s or 51 Mbit/s, was approved by DVB-EBU JTC and published by ETSI as TS 101224. The MHP API consists of a software specification that will be implemented in set-top boxes, integrated digital TV receivers, and multimedia PCs. The MHP will connect the worlds of broadcast television, Internet computing, and telecommunications through these devices and their associated peripherals.