Access Networks - MULTIMEDIA

An access network connects end users to the core network. It is also known as the "last mile" for delivering various multimedia services, which could include Internet access, telephony, and digital and analog TV services.

Beside ADSL, discussed earlier, some known options for access networks are:

Hybrid Fiber - Coax (HFC) Cable Network.Optical fibers connect the core network with Optical Network Units (ONUs) in the neighborhood, each of which typically serves a few hundred homes. All end users are then served by a shared coaxial cable.

Traditionally, analog cable TV was allocated a frequency range of 50 - 500 MHz, divided into 6 MHz channels for NTSC TV and 8 MHz channels in Europe. For HFC cable networks, the downstream is allocated a frequency range of 450 - 750 MHz, and upstream is allocated a range of 5 - 42 MHz. For the downstream, a cable modem acts as a tuner to capture the QAM modulated digital stream. The upstream uses Quadrature Phase - Shift Keying (QPSK) modulation, because it is more robust in the noisy and congested frequency spectrum.

A potential problem of HFC is the noise or interference on the shared coaxial cable. Privacy and security on the upstream channel are also a concern.

  1. Fiber To The Curb (FTTC). Optical fibers connect the core network with ONUs at the curb. Each ONU is then connected to dozens of homes via twisted - pair copper or coaxial cable. For FTTC, a star topology is used at the ONUs, so the media to the end user are not shared - a much improved access network over HFC. Typical data rates are T1 to T3 in the downstream direction and up to 19.44 Mbps in the upstream direction.

  2. Fiber To The Home (FTTH). Optical fibers connect the core network directly with a small group of homes, providing the highest bandwidth. For example, before reaching four homes, a 622 Mbps downstream can be split into four 155 Mbps downstreams by TDM. Since most homes have only twisted pairs and/or coaxial cables, the implementation cost of FTTH will be high.

  3. Terrestrial Distribution. Terrestrial broadcasting uses VHF and UHF spectra (approximately 40 - 800 MHz). Each channel occupies 8 MHz in Europe and 6 MHz in the U.S., and each transmission covers about 100 kilometers in diameter. AM and FM modulations are employed for analog videos, and Coded Orthogonal Frequency Division Multiplexing (COFDM) for digital videos. The standard is known as Digital Video Broadcasting - Terrestrial (DVB - T). Since the return channel (up ­ stream) is not supported in terrestrial broadcasting, a separate POTS or N - ISDN link is recommended for the upstream in interactive applications.

  4. Satellite Distribution. Satellite broadcasting uses the Gigahertz spectrum. Each satellite covers an area of several thousand kilometers. For digital video, each satellite channel typically has a data rate of 38 Mbps, good for several Digital VideoBroadcasting (DVB) channels. Its standard is Digital Video Broadcasting - Satellite (DVB - S). Similar to DVB - T, POTS or N - ISDN is proposed as a means of supporting upstream data in DVB - S.

Table Speed of Common Peripheral Interfaces

Speed of Common Peripheral Interfaces

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