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Yet if we watch the same file when downloaded to a local disk, the quality is remarkably good. The problem lies with a number of bottlenecks between the origin of the content and the player. These may be between the origin and the first Internet point-of-presence, across the backbone, or the last mile to the viewer (the local loop). As broadband access extends to the home, the problems with the local loop should be solved, but there are still issues with network congestion and the costs of edge serving. Traffic will only increase; there are other demands beyond streaming and rich media voice-over IP and video-conferencing are two expanding markets. Multicasting can help ease the demands on the servers and the network for live or scheduled (simulated live) webcasts. Interactive applications, like distance learning, still provide many challenges, not the least stemming from their demands for low round-trip delays. For the mobile user or remote communities, fixed wireless and satellite offer alternatives to copper and fiber. Wireless is now widely used with mobile devices, the laptop, and the cell phone. Only time will tell if the spectrum can cope with demand. IP networks and telecommunications 39

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Wi-Fi or IEEE 802.11 The alternative to the 3G networks is Wi-Fi (wireless fidelity) local area networks. There are a number of wireless networking standards including the IEEE 802.11 family. To aid consumers, the Wi-Fi Alliance (WECA) certifies appliances to be interoperable regardless of the manufacturer as long as they are using the same version of the IEEE standards. There are three main standards: 802.11a, b, and g. 38 The Technology of Video and Audio Streaming Table 2.8 IEEE 802.11 Standards Data rate Carrier frequency band No. of channels IEEE802.11a 54 Mbit/s 5 GHz 8 IEEE802.11b 11 Mbit/s 2.4 GHz 14 IEEE802.11g 54 Mbit/s 2.4 GHz 14 The range of the wireless networks is short, up to a maximum of 100 meters, but that is more than adequate to flood your local coffee bar with wireless connectivity to the Internet. The 2.4-GHz band can be congested, as it is shared with a number of cordless appliances, phones, and Bluetooth devices, plus microwave ovens. Always-on and client security The domestic user of the Internet with a cable modem or DSL connection that stays permanently connected to the network is at a higher risk of attack by hackers than the user of a dial-up circuit who is connected only for short periods of time. As a consequence the user with permanent connection will have to take greater security precautions with software and/or hardware firewalls. The wireless networks come with integral encryption to make snooping more difficult. Summary It is a demonstration of the flexibility of the OSI layer model of network communications that a system developed for the transfer of text and data files has been stretched to carry real-time multimedia. That is not to say everything is dandy. Anyone who has watched streams will know it is not always an enriching experience. Stalling video and distorted audio do not make for good viewing.

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IP networks and telecommunications 37 cable modem coax fiber TX RX 54 - 750 MHz 5 - 24 MHz 10 baseT O/E node PC upstream splitter modulator video channels demod network termination backbone transport adaptor CATV headend Consumer s premises backbone network combiner Figure 2.10 Cable television data over cable system.

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6 8 MHz. This data is not for just one consumer but is shared between users on a node, much like an Ethernet network. The digital cable networks use Quadrature Amplitude Modulation (QAM). The upstream (consumer to head-end) channel uses the free spectrum under the video channels (5 24 MHz), with a bandwidth about 2 MHz and a data rate of 3 Mbit/s. QPSK or QAM modulation is used. Again, it is a shared channel. The telcos often make a point of saying that there is only one user on an ADSL line, so there are no issues of network contention. They cite this as an issue with cable systems. But the phone company most likely will hook up the ADSL termination equipment with an Ethernet, which also has a finite capacity. So treat these claims with care. How many cable modems are sharing a node, and how many ADSL modems share the Ethernet bandwidth? Satellite ADSL is fine if you live near the telco central office, cable modems are great if your neighborhood has cable television. If you live in rural areas, you will have to look elsewhere for broadband provision. The alternative to a copper or fiber connection is a microwave connection. This could be from a satellite or MMDS. Very small aperture satellite antennae (VSAT) offer one-way and two-way links to the Internet. In North America the DirecTV satellites offer a data service called DirecWay. A similar service is offered in Europe by the Astra satellites. Mobile The alternative to the constraints of copper or fiber is to use wireless communication, through cellular networks covering large territories or local area networks often called Wi Fi. Third-generation wireless networks The International Telecommunications Union (ITU) has developed a set of standards for a third generation of wireless networks that can provide IP connectivity to mobile devices. The second-generation digital cell phones (analog being the first generation) were limited to about 14 kbit/s data rate, clearly inadequate for video streaming. The new third-generation wireless networks (3G) offer the potential of data rates from around 300 kbit/s for somebody walking or in a moving vehicle, up to 2.4 Mbit/s for a stationary terminal. The phone operators are offering a wide range of video services, from streaming entertainment and sport, to video conferencing and videophone applications. The connection is always-on, so can be used like a conventional copper/fiber network connection. 36 The Technology of Video and Audio Streaming

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Cable modems Cable TV is the other connection in many homes. Cable operators are updating older coaxial cable systems to provide wideband fiber networks, with coax for the final link to the home. Spare capacity can be utilized to provide Internet access alongside conventional television channels. This is referred to as data over cable (DOC). The cable modem can be a PCI card fitted in the PC, or an external modem hooked up with Ethernet or USB. The modem uses one of the video channels for the downstream (head-end to consumer) connection. Between 27 and 56 Mbit/s of data can be carried over a single channel with a bandwidth of IP networks and telecommunications 35 PC 10 baseT ADSL modem telephone line low-pass POTS filter ADSL line box Figure 2.9 ADSL connections.

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The downstream data capacity is much higher than upstream, hence the descriptor asymmetric. Typical maximum downstream rates range from 1.5 up to 8 Mbit/s, and upstream up to 1 Mbit/s. The actual performance varies from supplier to supplier, and upon the local propagation conditions. ADSL is good for large file downloads and receiving streaming files. The drawback is that the client can transmit data only at a lower rate (maybe 300 kbit/s), so it is not going to be suitable for a home webcasting station. For such applications the HDSL connection is a better option, albeit at higher cost. The ADSL splitter is a passive device, so the telephone service will still operate normally, even when the modem is not powered. Rate-adaptive DSL Rate-adaptive DSL is an option where full specification DSL is not possible. The data rate adapts to the propagation conditions. The bandwidth is not guaranteed, but depends upon the distance from the CO. G.lite A key factor in providing low-cost ADSL circuits is to avoid a truck roll for the installation. G.lite was developed from full specification ADSL (called g.dmt) as a system that could be installed simply by the consumer. G.lite does not require the installation of a splitter, so it often is referred to as splitterless. Many consumer systems do not give very good results with splitterless systems. The solution is the use of a low-cost filter in the line to each device. Other DSL standards You may come across a number of other abbreviations related to DSL: Symmetric DSL (SDSL or SHDSL) This new development gives equal up and down bandwidths and offers a lower cost alternative for E1/T1 lines. It is popular with business users with servers that need a higher upstream bandwidth that ADSL offers. MSDL (moderate speed DSL) Does not achieve CSA range at T-1 rates but can satisfactorily offer lower rates (512 or 384 kbit/s), so still holds advantages over an analog modem or ISDN. VDSL (very high speed DSL) Used for a final short-distance copper link from fiber to the curb (FTTC) systems. It can achieve T-3 data rates (45 Mbit/s) over 1,000 ft. 34 The Technology of Video and Audio Streaming

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HDSL The first development was the high bit-rate digital subscriber line (HDSL). It uses the same four-level phase amplitude modulation as ISDN (referred to as 2B1Q). By applying more sophisticated DSP technology and by using a wider bandwidth, HDSL can achieve the same, symmetric rates as T1 or E1 (1.5 or 2 Mbit/s) over a range of 12,000 ft using two pairs of 24 a.w.g. wire (3 km of 0.5 mm). This range is the Carrier Serving Area (CSA); that is, the area served by the local exchange carrier or telco (in the USA the RBOC), often using Digital Loop Carrier (DLC) technology. In many areas HDSL has replaced the original T-1, which required repeaters every few hundred yards. In Europe a variant of HDSL uses a single pair, but cannot achieve the same range. This is not a problem in countries like Germany and Italy, where the local loop tends to be shorter. All the HDSL systems are loop powered (no local power requirement at the subscriber end) and they use the analog voice band for data. ADSL Asymmetric DSL or ADSL is designed to allow the plain old telephone service (POTS) to be used at the same time as the digital service. This means that the line still can be used for voice or fax while data transfer is taking place. Frequency division multiplexing is used to separate the channels. The data occupies the spectrum from 25 kHz up to over 1 MHz, leaving the low-frequency baseband clear for the POTS. IP networks and telecommunications 33 3.4 kHz 25 kHz 1.1 MHz 0 voice fax ADSL frequency Figure 2.8 ADSL frequency multiplexing.

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Leveraging existing copper The local telcos have a ready-made delivery pipe for media streams, the plain old telephone line. Copper pairs have virtually 100 percent penetration to the home, so are an attractive carrier for content. The first use has been dial-up modems over an analog speech channel. Dial-up modems The use of an analog modem over a voice circuit is not really an option for the reception of video streaming unless the pictures are thumbnail size. However, it can be successfully used for streaming audio, where the bandwidth requirement is more modest. The dial-up modem uses an analog voice circuit to the central office (CO); this is limited to a maximum speed of 56 kbit/s. In practice the best that can be achieved is more likely to be 40 kbit/s. Bear this in mind when you are encoding content. A 56 kbit/s data stream will not pass through a dial-up circuit. A lower aggregate rate has to be chosen, for example Real recommends 34 kbit/s. Much research effort has been devoted to exploiting the existing copper pair used by the analog telephone circuit. Although ISDN to the home has been successful in some countries (notably Germany), it is by no means in universal use. One drawback is that the bandwidth available (128 kbit/s) is insufficient for value-added services like video-on-demand (VOD). The digital subscriber line (DSL) ISDN could be considered the first digital subscriber line, but the focus now is on delivering higher data rates over the same copper pair used for the analog voice circuit, the plain old telephony service (POTS). This gives the bandwidth necessary for applications like streaming, but at a lower cost from the traditional ISDN/T1 provision. 32 The Technology of Video and Audio Streaming Table 2.7 ISDN Bit Rates Geographical area B-channels Data channel Total data rate 64 kbit/s BRI 2 1 16 kbit/s 192 kbit/s PRI N. America, Japan 23 1 64 kbit/s 1.544 Mbit/s Europe and rest of world 30 1 64 kbit/s 2.048 Mbit/s

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or E-1 circuit can provide a starting point, or T-3/E-3 where a higher capacity is required. Once a corporation moves to the use of streaming on a large scale, T-1/E-1 is unlikely to serve more than a handful of users. If you want to webcast, then outgoing capacity could be an issue. Serious consideration should be given to capacity planning. Will staff be allowed to watch streams from the public Internet, or will access be limited to the corporate intranet? Is streaming to be unicast or will multicast also be used? Options for small businesses Many small businesses have leased ISDN lines for telephony and data. These also can be used for connection to the Internet, but the capacity is limited for multimedia streaming to a single client. Alternatives include xDSL and T-1/E-1. Consumer connections Cost is the primary concern here, and usually takes precedence over the quality of service. Whereas business lines usually offer a guaranteed service, residential users are often provided with variable-rate access. At periods of high demand, network congestion increases and the service throttles back. Streaming media codecs incorporate design features to cope with these variable bandwidth conditions. ISDN The Integrated Subscriber Digital Network (ISDN) is a service offered by telcos that enables voice, data, and multimedia to be carried over a single line to the central office or exchange. ISDN often is introduced to a business just for voice and fax services. It has also been generally adopted for the delivery of large data files in the desktop publishing environment. Video-conferencing systems can use ISDN for H320 or H323 connections. When streaming media first rolled out, ISDN seemed a natural vehicle to deliver content. The basic service (basic rate interface or BRI) carries two bidirectional 64 kbit/s channels over two twisted pairs. The primary rate has a higher capacity of 23 or 30 channels. A separate data channel carries control information. The data rate of the BRI is more than sufficient for streaming audio, but for video is considered barely adequate. Note that the BRI includes two channels, which if used together can give a download of 128 kbit/s. This often is seen on streaming encoder configurations as dual ISDN. ISDN offers a guaranteed data rate, unlike a dial-up modem, where a 56 kbit/s throughput is the maximum achievable, and lower rates are normal, especially over long or noisy lines. IP networks and telecommunications 31

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Generalized Multiprotocol Label Switching (GMPLS) Most of the telecommunications networks have evolved to carry voice traffic and general data packets. Neither of these models is really suited to the streaming of multimedia. IP routing has evolved to use label switching, which can improve the efficiency of network communications. MPLS is a potential technology that can improve the QoS of content delivery, by allowing traffic engineering of packet routing. Generalized MPLS extends this capability to cover legacy ATM and SONET terminal equipment plus the new optical cross-connects, so that potentially IP can be transmitted directly over DWDM. The local loop There is a wide choice of connections to the Internet, split into the lower cost services for the consumer and higher capacity services for corporate systems. Services tailored to the domestic consumer, for reasons of cost, usually use existing connections. Virtually all homes have a copper pair from the telephone company, and many in metropolitan areas have a coaxial connection to the local cable television system. Both of these connections can be used to carry Internet data alongside the existing services. Wireless also offers connectivity, sometimes as a hybrid with a telephone back-channel. The enterprise user A corporate user with tens or hundreds of users will require a higher bandwidth and more reliable connection to the Internet. Traditional connections like a T-1 30 The Technology of Video and Audio Streaming IP with GMPLS fiber DWDM technology DWDM SONET ATM IP Figure 2.7 The technology migration to photonic routing.

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