1.2 The Network Edge

  • Host

    • Clients

      • Tend to be desktop and mobile PCs, smartphones, and so on

    • Servers

      • Tend to be more powerful machines that store and distribute Web pages, stream video, relay e-mail and so on

      • Most of the servers reside in large data centers

1.2.1 Access Networks

  • Access network: the network that physically connects an end system to the first router ("edge router") on a path from the end system to any other distant end system.

  • Home Access: DSL, Cable, FTTH, Dial-Up, and Satellite

    • Two most prevalent types of broadband residential access: digital subscriber line (DSL) and cable

    • DSLAM: digital subscriber line access multiplexer

    • Hybrid fiber coax (HFC): both fiber and coaxial cable are employed

    • Fiber to the home (FTTH)

    • Optical network terminator (ONT)

  • Access in the Enterprise (and the Home): Ethernet and WiFi

    • Local area network (LAN)

      • Ethernet

    • Wireless LAN

      • Access based on IEEE 802.11 technology, more colloquially known as WiFi

  • Wide-Area Wireless Access: 3G and LTE

    • Unlike WiFi, a user need only be within a few tens of kilometers (as opposed to a few tens of meters) of the base station

1.2.2 Physical Media

  • E.x. HFC uses a combo of fiber cable and coaxial cable. DSL and Ethernet use copper wire. Mobile access networks use the radio spectrum.

  • For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium.

    • Can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path

    • E.x. twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum

    • Categories

      • guided media: the waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable

      • unguided media: the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel

    • Cost of physical link is relatively minor compared with other networking costs

  • Twisted-Paire Copper Wire

    • Unshielded twisted pair (UTP)

    • Rate: 10 Mbps to 10 Gbps

    • Dominant solution for high-speed LAN networking

  • Coaxial Cable

    • Can be used as a guided shared medium

      • A number of end systems can be connected directly to the cable, with each of the end systems receiving whatever is sent by the other end systems

  • Fiber Optics

    • Thin, flexible medium that conducts pulses of light, with each pulse representing a bit

    • Preferred for oversea links

  • Terrestrial Radio Channels

    • Radio channels carry signals in the electromagnetic spectrum.

    • Classified: short distance, local areas, span from ten to a few hundred meters, wide area, spanning tens of kilometers

  • Satellite Radio Channels

    • Links two or more Earth-based microwave transmitter/receivers, known as ground stations. The satellite receives transmissions on on frequency band, regenerates the signal using a repeater, and transmits the signal on another frequency.

    • geostationary satellites

    • low-earth orbiting (LEO) satellites

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