HFC

HFC

This section covers HFC arrangements, the use of cable modems, and the future of IP telephony over cable. HFC supports a wide range of services, including traditional telephony, broadcast video, and interactive broadband services. It involves the use of fiber in the backbone and in the access network. The fiber termination point (that is, the neighborhood node) can support anywhere from 200 to 2,000 homes, with 200 to 500 homes being the norm. From that neighborhood node, coax (normally 750MHz or 1,000MHz) is run to the home, in a two-way subsplit system.

In countries in which there is a history of cable TV, the cable plants have traditionally operated in one direction, for the broadcast of video programming. To handle a two-way infrastructure, they must be upgraded, and the cost of upgrading is about US$200 to US$600 per customer; companies starting from scratch may have to invest two to three times this amount. Over the past few years, this type of upgrading has been occurring in the existing cable TV infrastructure in the United States. Countries such as the Benelux countries, where there's also a rich history of cable, have also been in upgrade mode. In other parts of the world, new systems are going in with digital two-way capabilities, to support today's interactive services environment.

Figure 13.3 shows the topology of an HFC network. This figure shows a cable TV operator, and on the left side are the headends (that is, where the information is being broadcast from). Increasingly, those headends are tied together by fiber in the backbone. The cable TV operators have also made improvements in the performance of their networks, as well as the costs associated with operating them, by moving away from using coax in the backbone to using a fiber-based backbone. The backbones feed into the neighborhood nodes, or the optical nodes (that is, the access point), and from that point coax goes out to the homes. You can see in this figure that HFC has a shared infrastructure, which is one of the drawbacks of HFC.

Figure 13.3. The topology of an HFC network

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HFC Architectures

The HFC architecture uses a bus topology, meaning that it's a shared access architecture. (Chapter 8, "Local Area Networking," describes bus and other LAN topologies.) It makes use of Frequency Division Multiplexing, to derive individual channels some of which are dedicated to the support of telephony services, others of which are reserved for analog TV, and still others of which are reserved for future interactive broadband services. There is a bit of a hostile environment represented by this multiple-access coax system, in that the point at which the coax interfaces to TVs or set-top boxes is a point at which noise can be accumulated. The points where the coax connects into set-top boxes or cable-ready TV sets tend to collect noise, so the cable picks up extraneous noise from vacuum cleaners or hair dryers. If every household on the network is running a hair dryer at 6:30 am, the upstream paths are subjected to this noise, and there will be some performance degradations. Extra signal processing must therefore be added to overcome the impairment in the return channel.

The major concerns with HFC include security, privacy, reliability, and return-path issues, particularly in support of telephony. With twisted-pair, we have a private line to the local exchange. Using a shared coax system to support telephony could raise some privacy issues, so encryption of voice conversations may become very important. Also, HFC faces bandwidth constraints; as more and more homes within the neighborhood make use of their Internet access channel, everyone's downloads and bandwidth become minimized. The problem now surfacing is that cable modems have caught on and there are more subscribers to such services. Whereas a year ago people were experiencing extremely rapid downloads with cable modems, things seem to be slowing down now, which means that more users are subscribing to the services and sharing the facility, which results in lower performance for everyone. Subdividing the nodes, however, can help to alleviate bandwidth constraints, and it can also help to reduce ingress noise. If the service provider continues to subnet, performance can be kept high for the subscribers.

Cable Modems

A cable modem is needed to support high-speed data access over HFC by using the cable TV infrastructure. Cable modems function like special-purpose routers, linking the cable network's Layer 3 to another network or device. Generally this requires an external box with cable and Ethernet connections. Figure 13.4 illustrates cable modem connectivity. On the left side of the figure is a single neighborhood with users attaching to the shared coax system via their cable modems. These various coax trunks, then, come into the headend facility, where they terminate on a cable modem termination system (CMTS). The CMTSs are linked together by accessing a common Ethernet hub, which, in turn, feeds into the IP router, which then develops the optimum path to take over an optical backbone onto the ISP.

Figure 13.4. Cable modems: LAN-oriented connectivity

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CMTS functions include providing QoS, allocating bandwidth, classifying packets, policing packets for Type of Service (ToS) fields, adjusting the ToS fields as needed, performing traffic shaping, forwarding packets, converting and classifying QoS parameters, handling signaling and reserving of backbone QoS, and recording call resource usage.

Cable modems provide downstream data rates of up to 36Mbps, and the downstream rates are generally supported within the frequency band of 42MHz to 750MHz. The downstream channel depends on the QAM technique, because this is what gives it the most bits per second and, hence, the fastest data rates downstream, where rapid downloads are important. Upstream data rates are up to 10Mbps, and the upstream direction operates in the range of 5MHz to 40MHz. As mentioned earlier, this portion of the frequency band is especially subject to noise interference, so it requires modulation techniques such as Quadrature Phase Shift Keying (QPSK) and QAM 16, which transport fewer bits per second than other techniques but which also provide better noise resistance.

Many standards deal with cable modems. CableLabs is an industry leader in creating cable modem standards. Its Multimedia Cable Network Systems (MCNS) includes Data Over Cable Service Interface Specification (DOCSIS), PacketCable, and OpenCable. DAVIC is working on the Digital Video Broadcasting (DVB) standard for the EuroModem specification, which is a set of standards for digital video broadcasting that is supported by the European Cable Communications Association. (Chapter 10 discusses TV standards in more detail.) Another important standard in this area is IEEE 802.14.

DOCSIS uses either QAM 64 or QAM 256 downstream, up to 36Mbps, and it uses QPSK upstream at 2.5Mbps. (See Chapter 6, "Data Communications Basics," for information on modulation schemes such as QAM.) DOCSIS also involves an Ethernet connection to the PC, so data is transferred by using TCP/IP encapsulated in Ethernet frames between the cable modem and headend. DOCSIS includes a baseline privacy specification as well. It relies on the use of both the 40- and 56-bit versions of DES. (See Chapter 11, "Next-Generation Network Services," for more information on security.) DOCSIS is recognized by the ITU specifications and will be formalized under J112.X (where X denotes the region).

At this point CableLabs has certified about 100 modems from more than 36 companies as being DOCSIS 1.0 compliant, and it has qualified CMTSs from 8 vendors as being DOCSIS 1.0 compliant. CableLabs has traditionally focused on data, but DOCSIS 1.1 engineering improvements will facilitate voice. DOCSIS 1.1 was created because of the cable industry's desire for VoIP. DOCSIS 1.1 includes key network technologies, including dynamic QoS, which is very important to VoIP, packet fragmentation, and enhanced security. (QoS is discussed in Chapter 10.) As of mid-2001, no modems or CMTSs had passed the DOCSIS 1.1 certification, but equipment was expected to be certified in 2001. Three types of customer premises products that use DOCSIS 1.1 are expected to emerge: stand-alone cable modems with RJ-11 and Ethernet jacks for both VoIP and Internet-over-cable services; products targeted to the SOHO office market that allow for multiple IP voice and data lines; and Internet appliances (for example, Web tablets) and smart appliances (for example, smart refrigerators).

EuroDOCSIS was created because DOCSIS does not support the European cable standards, which include the 8MHz channels, a 65MHz frequency range for upstream signals, and compliance with Europe's broadcast downstream standard. EuroDOCSIS combines the North American DOCSIS standard with elements of the DVB DAVIC specification that are needed for DOCSIS to work in Europe.

The CableLabs PacketCable 1.0 specification deals with transmitting multifeatured IP phone calls over HFC and allows four independent IP voice channels through a single cable modem.

Another strong standard is the DVB standard advocated by DAVIC. This is the EuroModem specification promoted by the European Cable and Communications Association. It addresses video, audio, data, and voice services, and enables a single multiservice platform that uses ATM as the transport protocol. Standardized as ITU-T J.38 Annex A, it calls for either QAM 64 or QAM 256 downstream, and for QPSK upstream. Some think EuroModem may eventually take as much as 70% of the total European cable modem market. However, interoperability between EuroModem and EuroDOCSIS may render the argument moot. At this point EuroDOCSIS has a major head start over EuroModem. The headway being made with standards, as well as the incredible popularity of cable modems and the advantages of the cable infrastructure, make this a growing market area.

IEEE 802.14 is the Cable TV Media Access Control (MAC) and Physical (PHY) Protocol. It specifies either QAM 64 or QAM 256 downstream, and it specifies QPSK and QAM 16 upstream. For IEEE 802.14, ATM is specified as the MAC from the headend to the cable modems.

Digital Cable TV Devices

Digital cable TV devices present yet another exciting area to watch in the coming years. The goal of the CableLabs OpenCable program is to publish specifications that define digital cable network interfaces, as well as the nature of next-generation cable set-top boxes.

The CableLabs cable modem standard MCNS will be used with OpenCable set-top boxes, with advanced digital video compression circuitry to create terminals that are capable of supporting next-generation video and the entire range of current and future Internet and Web-based applications. The OpenCable effort is seen as the linchpin of the cable industry's digital future. It is processor and operating system independent. Compliant set-tops must allow both high- and low-speed bidirectional Internet service for both Internet and TV applications, and computer applications must be provided to both the television and the desktop computer through cable. Digital set-top characteristics will include expanded memory, powerful graphics engines, and support for one-way broadcasts (for example, near video-on-demand, Web browsing, Internet e-mail) as well as two-way interactive services (for example, Internet access via TV, high-definition video programming).

Another emerging area for cable TV systems is the cable-based IP telephony environment (see Figure 13.5). Thanks to deregulation and advancements in Class 5 replacement switches and IP telephony, the environment is ripe for cable providers to become competitive local exchange carriers (CLECs) or long-distance carriers. Currently, 90% of cable telephony is supported over circuit-switched networks, and this will likely be the case until late 2001 or 2002. IP telephony over cable, specifically HFC networks, is predicted to grow in the coming years. Circuit-switched telephony services are currently being offered by the likes of AT&T, Cablevision, Comcast, Cox Communications, and MediaOne Group. Operators will be able to offer VoIP or circuit-switched service exclusively or in mixed packages. Keep in mind, though, that VoIP over cable is in early stages of development. (See Chapter 11 for more on VoIP.)

Figure 13.5. Cable-based IP telephony

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The CableLabs PacketCable working group is leading cable-based IP telephony research efforts. The DOCSIS 1.1 standard addresses real-time applications such as telephony, and it includes critical measures such as dynamic QoS. The key issues in cable-based IP telephony include voice quality and how to guarantee it in terms of latency, fidelity, jitter, packet loss, and reliability at the customer end. Other issues are legacy signaling support, data security, scalability, and feature deployment at the service provider's end. Finally, there are a number of provider-specific issues, such as implementation of systems for PSTN gateways and gatekeepers, provisioning, billing, and network maintenance. Implementation of DOCSIS standards will be vital. DOCSIS 1.1 deals with enabling time-sensitive voice and multimedia packets to share in HFC networks with timing-insensitive pure data packets. DOCSIS 1.1 enables a node to recognize a nondata packet and switch to it instantaneously from whatever data packet it is working on. It requires a CMTS at the edge of the cable access network and a DOCSIS 1.1-compliant cable modem at the customer premise. Edge cable CMTSs need the intelligence to isolate traffic flows and to apply policy-based QoS treatments in real-time. Traffic flows need to be isolated by service provider, application, and subscriber so that during times of congestion, flows within the service-level agreement (SLA) are maintained and flows that exceed the SLA are discarded first. Operators then map the DOCSIS-based flows to IP specs such as DiffServ and MPLS, which are discussed in Chapter 10, to manage the handoff to the core network.

Currently, trials for cable-based IP telephony are being conducted by Lucent and High Speed Access Group; Nortel Networks and Adelphia; AT&T, which is working with both Lucent and Motorola; Scientific Atlanta, which is partnering with Net2Phone and Cox Communications; Time-Warner; and Samsung and Videotron in Canada. Cable-based IP telephony considerations include technical architecture, achieving PSTN-level reliability (that is, five nines, or 99.999%), being capable of accommodating the same PSTN-level feature sets, and regulatory issues. Operators face challenges such as how to provide detailed, sophisticated, end-to-end SLAs; how to adjust to the need to do maintenance, which will become more critical; and how to evolve from being broadband video providers to being mission-critical service providers. Stay tuned for developments in the cable-based IP telephony environment in the next couple years.

 



Telecommunications Essentials
Telecommunications Essentials: The Complete Global Source for Communications Fundamentals, Data Networking and the Internet, and Next-Generation Networks
ISBN: 0201760320
EAN: 2147483647
Year: 2005
Pages: 84

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