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Chapter 6 Remote Access and Communication Links. 6.1 Remote Access to a LAN Modem Dial-up Virtual Private Network 6.2 Communication Links PSTN ISDN T1/T3 DSL Cable TV network ATM. Chapter 6 Communication Links. 6.1 Remote Access to a LAN. Remote Access means
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Chapter 6 Remote Access and Communication Links 6.1 Remote Access to a LAN Modem Dial-up Virtual Private Network 6.2 Communication Links PSTN ISDN T1/T3 DSL Cable TV network ATM
Chapter 6 Communication Links 6.1 Remote Access to a LAN • Remote Access means • accessing a LAN by mobile/home users • Two means: • modem dial-up • VPN
Chapter 6 Communication Links 6.1.1 Modem Dial-up • Modem dial-up means • connection using ordinary phone lines • also called • Public Switched Telephone Network (PSTN) • Plain Old Telephone Service (POTS) • Both the remote user and the LAN must have • a registered phone number • Remote Access Service (RAS) • of the server • must be activated • RAS client program • needed by the user
Chapter 6 Communication Links 6.1.1 Modem Dial-upA. Modem (1/2) • Modem • converts between • digital • computer • analog • phone line
Chapter 6 Communication Links 6.1.1 Modem Dial-upA. Modem (1/2) • Modem (cont’) • Modulation means • the amplitude (or frequency) of analog signal is changed according to data • sender modem modulates data • Demodulation means • getting data from a modulated signal • receiver modem demodulates the analog signals • Modems are needed at both ends • for several remote users, a pool of modems are needed
Chapter 6 Communication Links 6.1.1 Modem Dial-upB. Quality • Quality is poor • error-rate can be as high as 40% • because PSTN was designed for voice communication • Re-transmission is quite often • Note: poor quality does NOT mean that data received are inaccurate!
Chapter 6 Communication Links 6.1.1 Modem Dial-upC. Bandwidth • Maximum bandwidth of PSTN is 56 Kbps • Modems negotiate the highest possible transmission rate • If one modem is 56 Kbps, but the other is 14.4 Kbps, • the highest transmission rate is only 14.4 Kbps.
Chapter 6 Communication Links 6.1.1 Modem Dial-upD. Dial-up • Dial-up is necessary (for standard line, notleased line) • the connection is not permanent • known as “dial-on-demand” • Opposite is “always-on connection” • e.g. DSL broadband
Chapter 6 Communication Links 6.1.1 Modem Dial-upE. Leased line • Leased line means • line reserved and dedicated for transmission • “Always-on connection” • higher quality • less error rate • bandwidth can be guaranteed
Chapter 6 Communication Links 6.1.1 Modem Dial-upF. Security • Security is low • data transmitted using RAS over PSTN are usually not encrypted
Chapter 6 Communication Links 6.1.1 Modem Dial-upG. Cost • Cost is • Low for local connections • High for long-distance phone line • In summary, PSTN is • slow • poor in transmission quality • needs dial-up • has security risk • ubiquitous : easy to access
Chapter 6 Communication Links 6.1.2 Virtual Private Network (VPN) (1/3) • Virtual Private Network (VPN) • remote access to a LAN • over the Internet • with security using tunnels • Tunnel • virtual dedicated channel, with following characteristics: • A. Encryption • even encrypted data are intercepted, confidentiality can be guaranteed • B. Authentication • Access control is implemented, such as login • C. Message integrity • Prevent message alteration.
Chapter 6 Communication Links 6.1.2 Virtual Private Network (VPN) (2/3)
Chapter 6 Communication Links 6.1.2 Virtual Private Network (VPN) (3/3) • IPSec (IP security) • industrial technique in VPN • e.g. In Windows: • PPTP (Point-to-Point Tunneling Protocol) • L2TP (Layer Two Tunneling Protocol) • VPN server • installed in the LAN • VPN client • installed by user • VPN is in widespread use today • e.g. access intranet from home, or • connecting LANs to form extranets
Chapter 6 Communication Links 6.2 Communication Links • Recall: MAN or WAN is constructed by connecting LANs by routers and long-distance communication links. • Long-distance communication media include: • Copper wire • Fiber-optic cable • Radio waves • Satellite links • For copper wire, WAN technologies include • ordinary phone line, ISDN, T1, T3, DSL, cable TV network and ATM. • The choice depends on • speed, reliability, cost, availability and security.
Chapter 6 Communication Links 6.2.1 PSTN • Ordinary phone line (PSTN) • is the slowest method of establishing a communication link. • but, it has highest availability • most easy to access
Chapter 6 Communication Links 6.2.2 ISDN • Integrated Services Digital Network (ISDN) • dedicated digital communication link • bandwidth 64 Kbps • provided by telephone companies • but better quality than the analog telephone system • error rate less than 1%
Chapter 6 Communication Links 6.2.3 T1 and T3 • T1 • dedicated digital communication link • bandwidth 1.5 Mbps • multiplexing 24 separate 64 Kbps channels • expensive • T3 • bandwidth 44.7 Mbps • multiplexing 672 separate 64 Kbps channels • very expensive
Chapter 6 Communication Links 6.2.4 DSL (1/3) • Digital Subscriber Line (DSL) • makes use of ordinary phone lines • plus advanced technology • broadband • much higher bandwidth than PSTN • inexpensive • Originally, the copper wire of PSTN is capable of carrying signals of very high frequency up to 1 MHz. But, the interface cards installed in the central office of the telephone company limit the frequency to 4 KHz only. DSL opens up the maximum capability by simply replacing the old interface cards in the central office of the telephone companies. • There are several versions of DSL: ADSL, HDSL, SDSL and VDSL, where the first letters in the acronyms are Asymmetric, High-data-rate, Symmetric and Very-high-data-rate respectively. The most popular version is ADSL. • Asymmetric DSL (ADSL) has a higher download speed than upload speed, for example, 6 Mbps download and 1 Mbps upload. • ADSL can reserve a small proportion (8 KHz) of total bandwidth for voice signals. Therefore, ADSL is able to transmit both data and voice at the same time. An ADSL modem is necessary.
Chapter 6 Communication Links 6.2.4 DSL (2/3) • Digital Subscriber Line (DSL) • broadband technology • makes use of ordinary phone lines • much higher bandwidth than PSTN • inexpensive • need DSL modem • Several versions: • ADSL - Asymmetric (most popular) • higher download speed than upload speed • e.g. 6 Mbps download and 1 Mbps upload • HDSL - High-data-rate • SDSL - Symmetric • VDSL - Very-high-data-rate
Chapter 6 Communication Links 6.2.4 DSL (3/3) • DSL can transmit voice and data at the same time • a small proportion (8 KHz) of total bandwidth can be reserved for voice signals • What is technology in DSL? • Originally, PSTN can carry signals up to 1 MHz • But, the telephone company limits to 4 KHz for voice • So, simply replace the equipment, and the old phone lines can have bandwidth up to 10 Mbps
Chapter 6 Communication Links 6.2.5 Cable TV Network • Cable TV network • another broadband technology • coaxial cables • bandwidth as high as 10 Mbps • needs: cable modem • can transmit data and television signals at the same time
Chapter 6 Communication Links 6.2.6 ATM • Asynchronous Transfer Mode (ATM) • very high-speed • expensive • each packet has a fixed length (53 bytes) • no need to check the end of data for each packet • multiplexed to fully utilize the network resources • media include • coaxial cable, twisted-pair, fiber-optic cables. • bandwidth: 155 Mbps to 622 Mbps