SPECIALIST READING. A. Find the answers to these questions in the following text.A. Find the answers to these questions in the following text.
How many channels does an ISDN system commonly use? What types of wireless systems are named in the text? What do PCs connected to a satellite system use to send data? What types of cables are used in cable network systems? What may need to be upgraded when using a shielded DSL system? Compared to the downstream bandwidth, the upstream bandwidth in an ADSL line is: a larger b smaller с the same Which type of broadband service is the cheapest?
BROADBAND COMMUNICATIONS Integrated Services Digital Network (ISDN) ISDN services can be carried over existing telephone network infrastructure to terminal adapters (TAs) in the client machine. A common ISDN interface standard has a digital communications line consisting of three independent channels: two Bearer (B) channels, each at 64Kbit/s, and one Data (D) channel at 16Kbit/s. The D channel is used to carry signalling and supervisory information to the network, while the В channels carry the data and can be linked to provide a 128Kbit/s data channel. Wireless connections The wireless alternatives come in two forms: satellite and cellular. Satellite systems require the use of a modem to maintain the upload. Downstream bandwidth is provided via a dedicated satellite dish, connector hardware and proprietary software. Cellular systems use assigned radio frequencies and are based around a network of transmitters that are arranged in a cellular network, much like cellular mobile phone systems. The cable alternative Cable companies can also offer affordable broadband services over copper coaxial or fibre infrastructure networks. The connection is shared by several customers on a branch, so actual connection rates are variable, unlike ISDN and DSL. Digital Subscriber Line (DSL) DSL technology capitalises on the existing network of copper infrastructure, but allows digital signals to be carried rather than analogue. It allows the full bandwidth of the copper twisted-pair telephone cabling to be utilised. With splitter-based services, the DSL signal is pulled out from the phone line as it enters your premises and is wired separately to a DSL modem. This involves additional hardware and installation by the service provider at the customer site. The shielded option involves no installation, but the telephone company's equipment and some of your equipment might need upgrading. With Asymmetric Digital Subscriber Line (ADSL), most of the duplex bandwidth is devoted to the downstream direction, with only a small proportion of bandwidth being available for upstream. Much Internet traffic through the client's connection, such as Web browsing, downloads and video streaming, needs high downstream bandwidth, but user requests and responses are less significant and therefore require less on the upstream. In addition, a small proportion of the downstream bandwidth can be devoted to voice rather than data, allowing you to hold phone conversations without requiring a separate line. DSL-based services are a very low-cost option when compared to other solutions offering similar bandwidth, so they can be made available to the customer at extremely competitive prices.
B. 1. Match the terms in Table A with the statements in Table B.
2. Mark the following statements as True or False:
a ISDN can only operate over a special digital telephone line. b Two ISDN channels can be combined to give the user double the bandwidth. c Computers connected to a satellite system do not need a modem. d Cellular networks work in a similar way to mobile phone systems. e DSL systems require a special digital telephone line. f DSL systems use analogue signals. g You need a separate line to hold normal phone conversations on an ADSL system.
UNIT 8 C O M P U T I N G S U P P O R T
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